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NATURE-BASED SOLUTIONS FOR� DISASTER RISK MANAGEMENT

River flood hazards

Photo credit: Flickr/Sergio Tittarini

This presentation is part of a suite of information and training materials prepared for the World Bank on Nature-based Solutions (NBS). These resources are designed for the World Bank and its network of partners to understand how NBS can enhance their project portfolio, and how to consider and utilize these solutions. resources include:

A flagship report “Integrating Green and Gray: Creating Next-Generation Infrastructure” (Browder et al. forthcoming 2019): Featuring guidance to evaluate and design NBS, with more than a dozen NBS case studies
A booklet about key considerations for using NBS for DRM
A factsheet about the World Bank NBS program
A series of PowerPoints featuring NBS for DRM objectives, including an overview, coastal flood risk management, river flood risk management, and urban flood risk management.

For more information and resources, visit https://naturebasedsolutions.org/.

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References:

World Bank. Forthcoming. “Natural Hazards - Nature-Based Solutions.” Project Platform. Natural Hazards - Nature-Based Solutions. https://naturebasedsolutions.org/.
Image source: https://www.flickr.com/photos/8603336@N05/15030285561/

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MANY TERMS FOR “NATURE-BASED SOLUTIONS”

Source: Cohen-Shacham et al. 2016; UNEP et al. 2014; EC 2015;

Lo 2016; WWF 2017; USACE n.d.; EcoShape 2018; WBCSD 2017

This presentation describes a set of nature-based solutions (NBS) that can protect, restore, and manage watersheds and riverine ecosystems to safeguard communities and enhance development project performance. Throughout the presentation, we will also discusses the challenges and limitations of this solution set.

“Nature-based solutions” (NBS) are strategies that can protect, sustainably manage, and restore natural and modified ecosystems. They harness natural processes and ecosystem services for functional purposes, such as managing the impacts of natural hazards and climate change impacts. NBS can address pressing societal challenges and risks effectively and adaptively.

What we call “NBS” is synonymous, or closely related, to a number of other terms used in common practice to describe the same or similar measures, listed on the screen. These include:

Natural infrastructure (WBCSD 2017)
Green Infrastructure (UNEP et al. 2014, Soz 2016)
Nature-based flood management (WWF 2017)
Ecosystem-based approaches (Lo 2016)
Engineering with nature (USACE n.d.)
Building with nature (EcoShape 2018)

While DRM and development objectives conventionally involve built engineering measures, the application of NBS have increasingly emerged as viable alternatives or complements to traditional approaches.

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References:

Cohen-Shacham, E., G. Walters, C. Janzen, and S. Maginnis, eds. 2016. Nature-Based Solutions to Address Global Societal Challenges. IUCN International Union for Conservation of Nature. doi:10.2305/IUCN.CH.2016.13.en.
Ecoshape. 2018. “Building with Nature.” Building with Nature. https://www.ecoshape.org/en/.
Lo, V. 2016. “Synthesis Report on Experiences with Ecosystem-Based Approaches to Climate Change Adaptation and Disaster Risk Reduction.” Technical Series, no. 85.
Soz, S.A., J. Kryspin-Watson, and Z. Stanton-Geddes. 2016. “The Role of Green Infrastructure Solutions in Urban Flood Risk Management.”
UNEP, IUCN, TNC, WRI, and U.S. Army Corps of Engineers. 2014. Green Infrastructure Guide for Water Management: Ecosystem-Based Management Approaches for Water-Related Infrastructure Projects. http://www.unepdhi.org/-/media/microsite_unepdhi/publications/documents/unep/web-unep-dhigroup-green-infrastructure-guide-en-20140814.pdf.
US Army Corps of Engineers. n.d. “Engineering with Nature.” https://ewn.el.erdc.dren.mil/.
WBCSD. 2017. “Incentives for Natural Infrastructure Review of Existing Policies, Incentives and Barriers Related to Permitting, Finance and Insurance of Natural Infrastructure.” WBCSD. https://www.wbcsd.org/Clusters/Water/Natural-Infrastructure-for-Business/Resources/Incentives-for-Natural-Infrastructure.
WWF. 2017. Natural and Nature-Based Flood Management: A Green Guide. WWF. https://c402277.ssl.cf1.rackcdn.com/publications/1058/files/original/WWF_Flood_Green_Guide_FINAL.pdf?1495628174.
World Bank. 2017. Implementing Nature-Based Flood Protection: Principles and Implementation Guidance. Washington, D.C: World Bank Publications. doi:10.1596/978-1-4648-0484-7

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PRESENTATION STRUCTURE

Photo credit: Flickr/Bureau of Land Management OR/WA

Context
The solutions:

Floodplains and bypasses
Inland wetlands
Stream banks and beds
Upland forests

Wrap-up

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RIVER FLOODING

Photo credit: Flickr/Amir Jina

Average 5,900 lives lost annually
2.3 billion people have been negatively affected in last 20 yrs.
Average annual flood losses exceed $23 billion

River flooding is essential:

Productive and diverse ecosystems
Food for hundreds of millions of people

River flooding is a specific type of flooding that occurs when a river overtops its banks and flows onto surrounding lands. Causes range from heavy upstream rainfall and rapid snowmelt to dam and levee failure, ice jams, and channel migration.

This presentation focuses specifically on riverine flooding and its associated risks, while other PowerPoints in this series address NBS for urban and coastal floods – see https://naturebasedsolutions.org/.

Unfortunately, flood damage statistics often lump together a variety of floods – such as urban, coastal, and river floods, making it difficult to understand the exact impact of flooding that occurs when rivers overtop their banks.

That said, flooding is the most damaging natural disaster in the world, accounting for nearly half of all weather related disasters worldwide since 1995.

Flooding killed an average of 5,900 people each year from 1980-2012 (Jongman et al. 2015) and negatively affected some 2.3 billion others from 1995-2015, such as being displaced or suffering economic losses (UNISDR and CRED 2015). The majority of people affected by floods (95%) live in Asia. (UNISDR and CRED 2015).

Between 1980 and 2012, annual reported losses from flooding exceeded US$ 23 bn.

Flood damage examples in 2017 (Park 2017):

Floods in South Asia (India, Nepal and Bangladesh) affected over 40 million people, killing 1,200 people, while millions were forced from their homes and 18,000 schools shut down across the region.
A massive mudslide sparked by heavy rains and flooding in an area of Freetown, Sierra Leone, killed around 500 people and left hundreds more missing. The flash flood and the subsequent mudslide has displaced 20,000 people.
Sixteen people died after flash monsoon flooding in Karachi, Pakistan's largest city. Hurricane Harvey caused $125 billion (2017 USD) in damage, caused by catastrophic rainfall. The massive flooding killed 106 people in the U.S., displaced over 30,000 people and damaged or destroyed over 200,000 homes and businesses. An estimated $100m is necessary to rebuild Texas infrastructure.

While people often think of floods as rare, destructive events, it’s important to remember that flooding is an essential and common natural process; Under ”natural” conditions most rivers would overtop their banks every 1 to 2 years (Opperman 2014).

The connection between rivers and floodplains is one of the most important ecological processes on Earth, helping to make river-floodplain systems among the most productive and diverse ecosystems on the planet (Bayley 1995; Junk et al. 1989).
River-floodplain systems also support productive fisheries and agriculture, providing a primary food source for hundreds of millions of people in rural communities, in Africa, Asia and Latin America (UNEP 2010).

Maintaining or restoring these processes through NBS can effectively mitigate flood risk to humans, while providing a plethora of additional benefits.

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References:

Bayley, P.B. 1995. “Understanding Large River: Floodplain Ecosystems.” BioScience 45 (3): 153–158.
Jongman, B., H.C. Winsemius, J.C. Aerts, E.C. de Perez, M.K. van Aalst, W. Kron, and P.J. Ward. 2015. “Declining Vulnerability to River Floods and the Global Benefits of Adaptation.” Proceedings of the National Academy of Sciences 112 (18):E2271–E2280.
Junk, W.J., P.B. Bayley, R.E. Sparks, and others. 1989. “The Flood Pulse Concept in River-Floodplain Systems.” Canadian Special Publication of Fisheries and Aquatic Sciences 106 (1): 110–127.
Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.
Park, M. 2017. “It’s Not Just Harvey: August Marked by Deadly Floods around World.” CNN, September 1, 2017. https://www.cnn.com/2017/09/01/world/deadly-world-floods/index.html.
UNEP. 2010. “Blue harvest: inland fisheries as an ecosystem service.” WorldFish Center, Penang, Malaysia.
UNISDR and CRED. 2015. “The Human Cost of Natural Disasters: A Global Perspective.” Centre for Research on the Epidemiology of Disaster (CRED).

Image source: https://www.flickr.com/photos/amirjina/3747836979

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FLOOD RISK

Photos/Figure (L-R): Wikipedia/NOAA; Wikipedia/KennyOMG; Wikipedia/Kumarrakajee

Hazard

Vulnerability

Exposure

The risk that humans face from flooding is general defined as a function of three factors:

1) The natural probability of a flood event, typically based on the historic hydrological record.

2) Exposure, described as the population and value of assets subject to flooding. Flood management infrastructure such as dams, levees, and bypasses, decrease the amount of lands, assets, and people exposed to floods. Alternatively, development of flood prone areas puts more people and assets in harm’s way should a flood occur, increasing “exposure” (Koks et al. 2015).

More than half of the world's population lives closer than 3 km to a surface freshwater body (Kummu 2011).
In the U.S., floodplains are the home to some 30 million people (NYU Forman 2017).

3) Vulnerability, defined as the capacity of a society to deal with the event (Koks et al. 2015). Poor populations almost always lose a greater share of their assets and income when hit by a flood and have less ability to cope and recover (Hallegatte et al. 2017). Indirect and often long-term effects, such as disease, malnutrition, reduced education, and loss of livelihoods, can also decrease community resilience and the ability to meet other development goals.

Flood risk faced by many areas of the world has resulted in massive and increasing damages, which not only harms poor people more, but can also increase poverty.

At the municipal level in Mexico, floods and droughts increased poverty levels by 1.5 to 3.7 percent between 2000 and 2005 (Rodriguez-Oreggia et al. 2013).
In Bolivia, the poverty incidence rose 12 percent in Trinidad City following the 2006 floods (Lopez-Calva and Ortiz-Juarez 2009).

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References:

Hallegatte, S., M. Bangalore, J. Rozenberg, and A. Vogt-Schilb. 2017. “Unbreakable - Building Resilience of the Poor in the Face of Natural Disasters.”
Koks, E. E., B. Jongman, T. G. Husby, and W. J. W. Botzen. 2015. “Combining Hazard, Exposure and Social Vulnerability to Provide Lessons for Flood Risk Management.” Environmental Science & Policy 47 (March): 42–52. https://doi.org/10.1016/j.envsci.2014.10.013.
Kummu, M., H. de Moel, P.J. Ward, and O. Varis. 2011. “How Close Do We Live to Water? A Global Analysis of Population Distance to Freshwater Bodies.” PLoS ONE 6 (6). https://doi.org/10.1371/journal.pone.0020578.
Lopez-Calva, L.F., and E. Ortiz-Juarez. 2009. “Evidence and Policy Lessons on the Links between Disaster Risk and Poverty in Latin America.” Munich Personal RePEc Archive (MPRA) Paper No. 18342. UNDP Regional Bureau for Latin America and the Caribbean.
NYU Forman Center. 2017. “Report: More Than 30 Million People Live in U.S. Floodplains.” December 18, 2017. http://furmancenter.org/news/press-release/report-more-than-30-million-people-live-in-u.s.-floodplains.
Rodriguez-Oreggia, E., A. De La Fuente, R. De La Torre, and H.A. Moreno. 2013. “Natural Disasters, Human Development and Poverty at the Municipal Level in Mexico.” The Journal of Development Studies 49 (3):442–455.

Image/graphic sources (left to right):

https://en.wikipedia.org/wiki/Hydrograph#/media/File:Stream_hydrograph.gif
https://en.wikipedia.org/wiki/Srinagar#/media/File:Srinagar_pano.jpg
https://commons.wikimedia.org/wiki/File:NDRF_in_Bihar_Flood.jpg

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INCREASING FLOOD RISK

Source: Alfieri et al. 2016

With 2°C increase, 170% increase in damages and affected population

Global Projections of River Flood Risk in a Warmer World

Development, climate change, and aging infrastructure
Population in floodplains increased by 114% (1970-2010)
Economic losses increasing 6.3%/ yr

Flooding risk is on the rise due to population growth and economic development in flood-prone areas, climate change contributing to more intense storms, and the aging of flood-management infrastructure (Opperman 2014).

From 1970 to 2010, the world population grew by 87 percent, while the population in floodplains grew by 114 percent, increasing exposure to flood events (Hallegatte et al. 2017)

Due to both increased flood frequency and growing populations in flood-prone areas of the U.S., a study by the U.S. Federal Emergency Management Agency (FEMA) predicted that the land area vulnerable to a “100-year flood” will increase by 45% in by 2100 (Lehmann 2011).

Subsequently, economic losses from river floods have been increasing on average by 6.3% per year (between 1960 and 2013) (Tanoue et al. 2016) (based on EM-DAT data).

Depending on the data source, the number of flood-related fatalities may be declining over time, largely due to flood management efforts, increasing income and better governance in lower income countries (UNISDR and CRED. 2015, Jongman et al. 2015); another study found that the annual number of reported fatalities has increased at 1.5% per year (between 1960 and 2013) (Tanoue et al. 2016) (based on EM-DAT data).

On top of these factors, climate change is projected to increase the probability of flooding (i.e., hazard) in many parts of the world, by increasing the magnitude and frequency of extreme rainfall:

With a 2°C temperature increase, both the affected population and the related flood damages would rise by 170% compared to present levels (Alfieri et al. 2016).
At 4°C warming, river flood damage in 15 countries in Central Europe, South Asia, South America, and Japan would exceed 1000%, as compared to that in 1976–2005 (Alfieri et al. 2016, see figure).
Earth experienced 12% more record-breaking downpours between 1980 and 2010 than might have been expected, had the climate not been changing (see figure) (Lehmann et al. 2015).

The increasing and disturbing flood risk trends that have occurred despite flood management efforts, suggests the need for more innovative and resilient approaches to flood management.

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References:

Alfieri, L., B. Bisselink, F. Dottori, G. Naumann, A. de Roo, P. Salamon, K. Wyser, and L. Feyen. 2016. “Global Projections of River Flood Risk in a Warmer World.” Earth’s Future 5 (2):171–82. doi:10.1002/2016EF000485.
Hallegatte, S., M. Bangalore, J. Rozenberg, and A. Vogt-Schilb. 2017. “Unbreakable - Building Resilience of the Poor in the Face of Natural Disasters.”
Jongman, B., H.C. Winsemius, J. Aerts, E.C. de Perez, M.K. van Aalst, W. Kron, and P.J. Ward. 2015. “Declining Vulnerability to River Floods and the Global Benefits of Adaptation.” Proceedings of the National Academy of Sciences 112 (18): E2271–E2280.
Lehmann, E. 2011. “Flood-Prone Land Likely to Increase by 45%: A Major Challenge to Federal Insurance Program.” Climate Wire, July 22.
Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.
Tanoue, M., Y. Hirabayashi, and H. Ikeuchi. 2016. “Global-Scale River Flood Vulnerability in the Last 50 Years.” Scientific Reports 6: 36021.
UNISDR and CRED. 2015. “The Human Cost of Natural Disasters: A Global Perspective.” Centre for Research on the Epidemiology of Disaster (CRED).

Graphic source: Alfieri, L., B. Bisselink, F. Dottori, G. Naumann, A. de Roo, P. Salamon, K. Wyser, and L. Feyen. Global projections of river flood risk in a warmer world, Volume: 5, Issue: 2, Pages: 171-182, First published: 26 December 2016, DOI: (10.1002/2016EF000485)

Average change in expected damage (d, f) per country at SWLs. Hatching indicates countries where the confidence level of the average change is less than 90%.

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STRUCTURAL STRATEGIES

Source: World Bank 2017

	Nature-based Solutions (NBS)
Built	Hybrid	Natural
Hard, gray, engineered structures built to address development and DRR objectives	Combination of ecosystem elements and hard engineering interventions for addressing development and DRR objectives	Creation, protection or restoration of only ecosystem elements for addressing development and DRR objectives

To address this ongoing and rising risk, flood management strategies almost always include a portfolio of structural (i.e. dams, levees, bypasses, etc.) and non-structural measures (i.e. warning systems, evacuation plans, etc.). This presentation focuses on a subset of structural measures – NBS to reduce the risk from river flooding hazards.

This slide presents a typology developed by the World Bank and WRI of the structural infrastructure strategies included in flood management projects.

NBS can be classified as completely ‘natural’ (i.e., ecosystem elements only) or ‘hybrid’ (i.e., combination of ecosystem elements and built infrastructure). They can be used as alternatives, complements, or safeguards to traditional built infrastructure approaches (World Bank 2017).

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References:

World Bank. 2017. Implementing Nature-Based Flood Protection: Principles and Implementation Guidance. Washington, D.C: World Bank Publications. doi:10.1596/978-1-4648-0484-7.

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WORLD BANK INVESTMENT PORTFOLIO � DISASTER RISK MANAGEMENT (DRM)

Invested ~US$ 53 billion in more than 680 DRM projects globally

(FY2012-2018)

Source: World Bank 2018

Invested ~US$ 1.2 billion in 34 projects targeting river flooding with NBS

(FY2012-2018)

The Word Bank’s invests in a portfolio of structural and non-structural, built, and nature-based strategies to reduce the risk from river flooding hazards.

Between FY 2012 -2018, the World Bank invested US$ 52.87 billion across 681 disaster risk management projects globally – largely using built infrastructure solutions.

However, for the past decade the World Bank has financed projects with NBS components around the world, and there is a growing evidence base of successfully implemented projects both in and outside the World Bank portfolio.

From FY 2012-2018, the World Bank invested US$ 1.2 billion in about 34 projects that targeted riverine flooding with NBS.

Managing increasing flood risk into the future requires the application of innovative and resilient management strategies, including a greater emphasis on NBS.

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References:

For more information, visit www.naturebasedsolutions.org

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CONVENTIONAL: ‘BUILT’ INFRASTRUCTURE

Photo credit: Wikimedia/Ssbertuccelli

ADVANTAGES

Essential role in preventing floodwaters from damaging assets and harming people
Deep industry knowledge
High performance certainty and control

EXAMPLES: Dams, levees/dykes, flood walls, channel modifications

CHALLENGES

20% of freshwater fish species at risk
Can increase flood risk over time
Massive investment gap in flood infrastructure

ADVANTAGES

Countries have made massive investments in built flood-management infrastructure, and this infrastructure has played an essential role in preventing floodwaters from inundating cities, towns, and flood-vulnerable crops.

Examples include: dams, levees / dykes, flood walls, channel modifications, etc. In general, built flood infrastructure attempts to control floodwaters within a river’s banks, preventing water from damaging assets or harming people.

Flood control dams cross rivers to create a reservoir capable of storing floodwater, with outlets capable of controlling how much water flows downstream. In addition, flood control dams are often multi-purposed (e.g., water supply, irrigation, power generation, flood control…) and most of these cannot be easily replaced.
Levees, dykes, and floodwalls all perform similar functions by paralleling rivers, holding floodwaters within an artificially created channel
Other channel modifications to address flooding include straightening channels and replacing riparian vegetation with armored banks of concreate, stones, or other materials designed to quickly move floodwaters out of the system and prevent the river from meandering, or moving across the landscape and causing damages.

Flood control practitioners are well versed and extensively trained in the use of built infrastructure, the use of which dates back over 5,000 years to the reign of DaYu and Gun in China (Luo et al. 2015).

The wide breadth of experience and data surrounding the use of built infrastructure, along with the engineered components, provide a high level of performance certainty and control – for example, engineers can accurately determine how much water a reservoir can hold and can precisely control how much water is released downstream to ensure that levees or dykes are not overtopped.

Importantly, this doesn’t mean that built infrastructure will never fail, but rather, that we can accurate determine how well built infrastructure will perform and whether it is likely to fail.

CHALLENGES

Despite the many benefits of built flood infrastructure, its extensive modification of natural flows regimes and riverine environments has come at a cost.

Successful performance of built infrastructure necessarily eliminates the connection between rivers and floodplains - one of the most important ecological processes on Earth – as well as causes other damage to riverine ecosystems (Opperman 2014).

As a result, freshwater species are endangered at higher rates than either terrestrial or marine species, in large part to the fragmentation and changes in flow resulting from built infrastructure (Richter et al. 1997).

Twenty percent of the world’s freshwater fish are endangered, vulnerable or extinct (International Rivers 2012).
Missouri River commercial fish harvest declined 80% from the 1880s to the 1990s due to flow regulation and levee construction (Galat et al. 1998).

Of great concern, built infrastructure can also unintentionally increase flood risk in several ways (Opperman 2014):

Dams and levees cannot stop all floods; new development behind levees or below dams increases exposure to potential damages if, and when, a flood does occur
As climate changes imposes greater pressure on aging infrastructure designed for historic conditions, the potential for failure and potentially catastrophic damages increases.

Further, some built infrastructure only shifts, rather than alleviates, flood risk. For example, a tight levee that moves water quickly to reduce flood risk in one area, will increase flood height and flow velocity downstream, increasing the chance of downstream flooding.

Lastly, investment needs for water infrastructure, including for flood control, are enormous - on a global basis they even exceed investment needs for energy and transport infrastructure (Woetzel 2016).

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References:

Galat, D.L., L.H. Fredrickson, D.D. Humburg, K.J. Bataille, J.R. Bodie, J. Dohrenwend, G.T. Gelwicks, J.E. Havel, D.L. Helmers, and J.B. Hooker. 1998. “Flooding to Restore Connectivity of Regulated, Large-River Wetlands: Natural and Controlled Flooding as Complementary Processes along the Lower Missouri River.” BioScience 48 (9):721–733.
International Rivers. 2012. “World Rivers Review - June 2012: Focus on Rio+20.” Vol. 27/ No.2. https://www.internationalrivers.org/world-rivers-review/world-rivers-review-june-2012-focus-on-rio-20.
Luo, P., B. He, K. Takara, Y.E. Xiong, D. Nover, W. Duan, and . Fukushi. 2015. “Historical Assessment of Chinese and Japanese Flood Management Policies and Implications for Managing Future Floods.” Environmental Science & Policy 48 (April): 265–77. https://doi.org/10.1016/j.envsci.2014.12.015.
Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.
Richter, B.D., D.P. Braun, M.A. Mendelson, and L.L. Master. 1997. “Threats to Imperiled Freshwater Fauna.” Conservation Biology 11 (5):1081–1093.
Woetzel, J., N. Garemo, J. Mischke, M. Hjerpe, and R. Palter. 2016. “Bridging Global Infrastructure Gaps.” New York, NY, USA: McKinsey and Company.

Image source: https://commons.wikimedia.org/wiki/File:L.A._River_Under_Clouds.jpg#/media/File:L.A._River_Under_Clouds.jpg

Image details: Los Angeles, CA

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NATURE-BASED SOLUTIONS

Photo credit: Flickr/Keith Ewing

Allow watersheds to function naturally, with beneficial flooding
Slow and attenuate floodwaters
‘Hybrid’ solutions integrate and enhance the benefits of natural and built solutions

Examples: Floodplains, inland wetlands, stream beds and banks, and upland forests

In contrast to the approach used in built infrastructure to control floodwaters within river’s banks, nature-based solutions promote natural watershed functions, including the allowance of beneficial flooding in areas where water won’t cause significant damages (ex. The Mississippi River flood-management system allows floodwaters to spread out onto the historic floodplain, relieving pressure on levees and sparing cities New Orleans).

Nature-based solutions include: floodplains, inland wetlands, stream beds and banks, and upland forests, each discussed in detail in the following section of this presentation.

Floodplains and bypasses are relatively flat lands adjacent to rivers that store and convey water when floods exceed the capacity of a river channel.
Inland wetlands are permanently or seasonally saturated lands (marshes, wet meadows, peat forests, bogs, etc.), usually comprised of a depression that can store and filter runoff during moderate floods.
River beds and banks form the channel network that collects runoff from the land and determine how fast floodwaters are conveyed downstream.
Upland forests grow in the upper watershed where precipitation falls before being intercepted, evaporated, transpired or conveyed to a river.

These NBS have a critical role to play in slowing and attenuating floodwaters to decrease flood risk. Protecting these areas from development also reduces the number of people and assets in flood-prone areas and subsequent consequences should a flood occur.

Managing floodwaters is a formidable challenge with serious consequences of failure; purely natural solutions are often insufficient to manage flooding due to the sheer volume of flood waters, space limitations, and inherent uncertainty of natural systems.

Alternatively, hybrid solutions match the benefits of natural infrastructure with the security and greater performance certainty associated with built infrastructure (Opperman 2014). For example, weirs or other built diversions are often used to control the timing and volume of floodwaters entering a flood bypass or polder, rather than uncontrollably allowing water to overtop a river’s bank at any random point.

Other examples of hybrid solutions include:

Upland forests and dams: Upland forests can be managed to slow the flow of floodwaters entering a flood management dam as well as reduce erosion and sedimentation that could reduce the flood storage capacity of the dam.
Constructed wetlands: Artificial constructed wetlands for flood management are typically engineered with built components to improve performance and provide greater certainty.

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References:

Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.

Image source: https://www.flickr.com/photos/kewing/9505153040/

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ADVANTAGES OF NATURE-BASED SOLUTIONS

Provide wide range of additional co-benefits, beyond flood risk reduction
Can be more cost-effective
Can be designed as resilient, flexible, climate adaptation measures
Have capacity to adapt and regenerate

Photo credit: Wikimedia/Nicholas A. Tonelli

NBS are newer and not as broadly-tested or established as built infrastructure, but a wealth of evidence indicates that NBS can contribute significantly to achieving flood management goals. NBS also offer several additional advantages.
NBS are multi-purposed solutions, and provide a wealth of valuable co-benefits, such as habitat creation, recreational opportunities, and productive agriculture and fisheries. In contrast, built infrastructure generally provides a single benefit (dams being an exception), such as a dyke that prevents floodwaters from inundating a community (Opperman 2014).
By working with nature rather than against it, NBS can present lower construction and maintenance costs than built solutions. A growing repository of economic valuation data is contributing to strengthening the business case for using NBS for flood management.
When designed properly, NBS also have the capacity to adapt to future conditions and regenerate naturally after disaster events. NBS represent “no-regret” climate adaptation strategies, in that they provide benefits no matter what climate scenario materializes in the future.

For example, expanding the size of flood bypasses in anticipation to increased flood magnitudes, can provide agriculture and recreation benefits and leave options open for future development. In contrast, investments in raising the height of a dam or levee would likely be wasted if flood magnitudes don’t increase in the future.

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References:

Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.
Image source: https://commons.wikimedia.org/wiki/File:Riparian_Zone_(29344954721).jpg

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WORDS OF CAUTION

Photo credit: Pxhere

Greater variability and uncertainty
Disconnect between upstream sources of river flooding and downstream communities at risk
Data and capacity limitations
Challenging to make the economic case
Land requirements and social equity

The inherent variability of natural systems results in higher levels of performance variability and uncertainty for river flooding NBS than built solutions.

Some NBS - reforestation, constructed wetland, riparian restoration - may require a run-in period for vegetation to fully establish itself and to perform as designed.

Even when fully functional, NBS alone are often insufficient to manage riverine flooding – integrating a variety of nature-based and built solutions can help build redundancy and reduce uncertainty for a more sustainable and resilient approach to flood-risk management (Opperman 2014).

NBS effectiveness will vary with location and the size of the river or watershed - a method that may reduce flood risk for a small watershed in Poland will not necessarily be equally effective, or scalable, for the Yangtze River. Tools and case studies are available to help decisionmakers determine the appropriateness of NBS in a particular context.

NBS for riverine flooding may also face political challenges - while the consequences of riverine flooding are typically concentrated in downstream lowlands, river floods originate in upstream locations. Communities in lower portions of the basin that experience flood damages may have little control over upstream land and river management, adding complications to implementing NBS. Developing collaboration relationships between stakeholders may require upfront time and money, but will likely lead to more sustainable long-term NBS projects.

On the technical side, sophisticated hydrologic models exist to assess NBS benefits; however, applying these models may require overcoming data, human, and financial capacity limitations – a role for development agencies to help mainstream NBS in flood management.

NBS for river management often entail large land requirements. These can contribute to large upfront costs related to land acquisition or compensation for the use of private lands and restriction of rights.

Since poor and rural populations are disproportionally affected by flooding, efforts must be made to provide equitable flood protection. Investments in nature-based and other structural solutions should be complimented by non-structural interventions to compensate for any losses that occur and to increase their ability to cope with flood disasters.

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References:

Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.

Image source: https://pxhere.com/en/photo/1349872

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NBS FOR RIVER FLOODING

Image: Flickr/WRI

Floodplains and bypasses
Inland wetlands
Stream beds and banks
Upland forests

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1. FLOODPLAINS, (BYPASSES AND POLDERS)

Photo: Pxhere

Relatively flat areas between rivers and uplands
High levels of spatio-temporal variability and species diversity
Variety of ecosystems
Hybrid provide added control, but often less diversity

Floodplains refer to the relatively flat areas between rivers and uplands that naturally flood when a river overtops its banks (slide image).
They contain high levels of spatio-temporal variability, which maintains a diversity of lentic (stagnant, standing water, i.e. wetlands) and lotic (flowing water, i.e. riparian areas) habitat types.
Floodplains can include wetlands, forests, and/or riparian grasses, rushes, and sedges (use slide image to point out examples of stagnant isolated waterbodies, moving waters connected to the river, and unsaturated lands). This diversity of ecosystems and ecotones between those systems, helps make floodplains one of the most species-rich environments in the world.
This portion of the presentation discusses floodplains generally and doesn’t distinguish between specific habitat types. Since not all floodplains contain wetlands and wetlands can be found outside of floodplains, more specific information related to wetlands is discussed in the next section. Riparian areas are discussed in the section on stream beds and banks.
Flood bypasses and polder systems are hybrid features adjacent to rivers, that mimic that function of natural floodplains and are often sited on historic floodplains. These hybrid NBS, usually use weirs or other built diversions to control the timing and volume of entering (and exiting) floodwaters.
The integration of built features provide engineers with greater control over the floodwaters in the system and allow for more strategic utilization of the bypass/polder when not flooded (i.e. for agriculture, recreation, etc.).
However, as a trade off to utilization of bypass or polder land for agricultural or other beneficial uses, diversity is often not as high as would be found in a natural floodplain.

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References:

Image source: https://pxhere.com/en/photo/83776

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RISK REDUCTION BENEFITS

Photo: Flickr/Roger Price

Integrated with rivers to slowly, convey water and sediment
Capture large proportion of upstream water
Successful applications across the globe

Natural floodplains, and hybrid bypasses or polder systems, all function similarly by storing and slowly conveying water and sediment that exceed a river’s channel capacity.

A key benefit of floodplains is their ability to capture a very large proportion of runoff from upstream basins, providing significant water storage capabilities.

Yolo Bypass in California, USA is able to capture and convey more than 80% of the Sacramento River’s flows (Opperman et al. 2014).
The Mississippi Flood of 2011 had the greatest volume of floodwater ever recorded for that river. The flood-management system was able to safely convey that flood in large part because several massive floodways allowed water to spread out onto the historic floodplain, away from cities (Opperman et al. 2014).

Floodplain systems can also store water for large periods of time, releasing it over the course of hours, days, or weeks, significantly reducing peak flow.

Flood managers around around the globe have deliberately allowed floods to spread out on floodplains, bypasses, and polders to successfully reduce risk to cities and farms — including on the Mississippi and Sacramento (USA), Rhine (Netherlands), Danube (Bulgaria, Romania, Ukraine and Moldova), Msimbazi River (Tanzania), Odra River (Poland and Czech Republic), Yangtze (China), and many others.

Examples of flood benefits:

A study of the Illinois River found that reconnection of 8000 hectares (ha) of floodplain would improve protection for 26,000 ha of farmland by halving the probability of inundation from major floods (Opperman et al. 2009).

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References:

Opperman, J., G. Galloway, J. Fargione, J. Mount, B. Richter, and S. Secchi. 2009. “Sustainable Floodplains Through Large-Scale Reconnection to Rivers.” Science Magazine, Policy Forum, 326: 1487–88. https://doi.org/10.1126/science.1178256.
Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.

Image source: https://www.flickr.com/photos/_stefano_/14204263083/in/photostream/

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ADDITIONAL BENEFITS

Photo credit: Wikimedia/Basile Morin

Biodiverse habitat
Improved water quality
Groundwater recharge
Productive agriculture and fisheries
Carbon sequestration
Recreation

Markets exist for some services

Floodplains provide a wealth of additional benefits beyond flood mitigation. Based on these benefits, floodplains are the second ranked ecosystem type, behind only estuaries, in terms of their per-hectare value to society (Costanza et al. 1997).

Despite representing <2% of Earth’s terrestrial land surface area, floodplains provide approximately 25% of all ‘‘terrestrial’’ (i.e., non-marine) ecosystem service benefits (Opperman 2014).
Replacing the services provided by functioning floodplains (e.g. through built infrastructure) would cost approximately $150,000 per hectare (Sheaffer et al. 2002).

River-floodplain systems are among the most productive and diverse ecosystems on the planet creating a combination of terrestrial and aquatic habitat for fish, birds, and other wildlife (Bayley 1995).

Floodplains can also help improve downstream water quality. Flood waters can contain high sediment and nutrient loads, but when a river overtops its banks and spreads out on floodplains, much of the sediment and nutrients in transport are deposited, improving downstream water quality (Noe and Hupp 2005).

During overbank flooding, floodwaters can infiltrate into soils and recharge groundwater. Groundwater recharge on floodplains can support higher plant productivity than the surrounding land, particularly in arid or semi-arid climates (Opperman 2014).

Groundwater recharge is often one of the most important ecosystem services provided by floodplains, supporting much of the agriculture in parts of Africa and along major rivers in Asia and Latin America.

The deposited nutrient-rich sediments also create fertile soils for sustenance and market-based agriculture. Floodplains also support fisheries by providing beneficial conditions for fish, including slow velocity water, extensive cover, and abundant food sources (Opperman 2014).

The productive fisheries and agriculture supported by floodplains provides a primary food source for hundreds of millions of people in rural communities, in Africa, Asia and Latin America (Dugan et al. 2010).
For example, the Mekong River supports the largest freshwater fishery in the world supported by extensive floodplains and lakes inundated annually. Food derived from the river and provides the primary source of protein to tens of millions of people in South-east Asia (Dugan et al. 2010).

In addition, productive floodplains can support rapidly growing forests that effectively sequester carbon.

Lastly, healthy floodplains attract recreational users to: fish, kayak or canoe on their waters; view migratory birds; or simply wonder within an attractive, biodiverse landscape.

Importantly, markets exist for many of the mentioned services, which can provide revenue to floodplain landowners. This includes revenue from: recreational user fees, carbon sequestration sales, habitat banks that compensate landowners for providing desirable habitat, or payments for improved water quality through pollutant trading programs (Opperman 2014).

Revenue from these services may allow landowners to receive considerable income from land in natural floodplain vegetation, particularly if revenue from multiple services can be “stacked” together (Opperman 2014).

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References:

Bayley, P.B. 1995. “Understanding Large River: Floodplain Ecosystems.” BioScience 45 (3):153–158.
Costanza, R., R. d’Arge, R. de Groot, S. Farber, M. Grasso, B. Hannon, K. Limburg, et al. 1997. “The Value of the World’s Ecosystem Services and Natural Capital.” Nature 387 (6630):253–60. doi:10.1038/387253a0.
Dugan, P., A. Delaporte, N. Andrew, M. O’Keefe, and R. Welcomme. 2010. “Blue Harvest: Inland Fisheries as an Ecosystem Service.” The WorldFish Center Working Papers. UNEP (United Nations Environment Program).
Noe, G.B., and C.R. Hupp. 2005. “Carbon, Nitrogen, and Phosphorus Accumulation in Floodplains of Atlantic Coastal Plain Rivers, USA.” Ecological Applications 15 (4):1178–1190.
Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.

Image source: https://en.wikipedia.org/wiki/Si_Phan_Don#/media/File:Six_children_in_the_Mekong_with_buffalos_and_boat.jpg

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CONSIDERATIONS FOR USING FLOODPLAINS

Photo credit: Flickr/UW News

Large scale interventions – up to 10,000s of hectares
Land costs and competition
Floodplain and water development
Environmental justice and social equity
Most effective during short duration floods

Floodplains can range in size from only a few meters wide to tens of thousands of hectares. In general, they tend to require more land than other NBS or built approaches.

In areas experiencing rapid urbanization and economic development, acquiring land can be prohibitively expensive, but the high upfront cost is often recoverable in the long-term, especially when considering ancillary benefits (Opperman 2014).

Other consideration include whether or not structures already exists in the floodplain. It is difficult, costly, and nearly impossible to reverse existing structures. Preventing encroachment in floodplains at an early state of development (i.e., through zoning regulations) is the most cost-effective way to maintain the ecosystem’s flood mitigation function.

Another suitability factor is how much the river is relied upon for development and infrastructure. For instance, water-intensive dam operations may prohibit managers from allowing sufficient water flow to inundate and reconnect floodplains.

Sixty-percent of the world’s largest rivers are disconnected from floodplains due to dams (International Rivers 2012). Reconnecting floodplains on these rivers means intentionally allowing rivers to overtop their banks – presenting an array of technical, legal, and social challenges.

Efforts to restore floodplains should also be mindful of how changes may disproportionately advantage or disadvantage people on the basis of race, ethnicity, gender, or class.

Lastly, floodplains are most effective at reducing flood peaks for floods of short duration. During long-duration flooding, the storage capacity will reach its limit (Opperman 2014).

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References:

International Rivers. 2012. “World Rivers Review - June 2012: Focus on Rio+20.” Vol. 27/ No.2. https://www.internationalrivers.org/world-rivers-review/world-rivers-review-june-2012-focus-on-rio-20.
Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.

Image source: https://www.flickr.com/photos/uwnews/10540914803/

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WHAT DO FLOODPLAINS AND BYPASSES COST?

Photo credit: Flickr/US Army Corps of Engineers

Dependent on land prices - often largest cost
Variable: $10,000 – $700,000/ha
Operations and maintenance costs are typically low (0.5-1.5%)

For floodplain lands not currently under protection, the purchase of the land or rights to restrict development on the land typically serves as the largest cost of floodplains.

Due to the variable cost of land, floodplain restoration spans a large range from US$ 10.000 – 800,000/ha, depending on whether the land serves an agricultural or residential/urban purpose (respectively) (European Environment Agency 2017).

Like capital costs, operations and maintenance costs of floodplains are highly variable, but are typically relatively low and estimated at 0.5% to 1.5% of total investment costs (European Environment Agency 2017).

As an example of the costs and benefits of floodplains, in the 1970s, the U.S. Corps of Engineers concluded that acquisition and protection of floodplain wetlands along the Charles River could provide similar protection as new levees and a flood-control reservoir for approximately 1/10 the cost. The Corps acquired 3,440 ha in the floodplain for approximately $10 million. Total annual cost of the project is $477,000 and the Corps estimates that annual benefits are $722,000, with $125,000 of those benefits derived from recreational and environmental benefits (e.g., open space, habitat, recreation) (Opperman 2014).

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References:

European Environment Agency. 2017. “Green Infrastructure and Flood Management: Promoting Cost-Efficient Flood Risk Reduction via Green Infrastructure Solutions.” Publication EEA Report No 14/2017. https://www.eea.europa.eu/publications/green-infrastructure-and-flood-management.
Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.

Image source: https://www.flickr.com/photos/usacehq/15818110098/

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CASE STUDY: DANUBE GREEN CORRIDOR

Photo credit: fFickr/chris lovelock and WWF 2010:

2006 floods: US$ 464 million in damages

Dikes cut off floodplains
80% of wetlands lost

Danube Green Corridor

Restore 224,000 ha of natural floodplain
Cost: US$ 214 million
Expected ecosystem services earnings: US$ 100 million/ yr

NOTE TO PRESENTER: This case study could also be used as an example of floodplain reconnection or wetland restoration

The Lower Danube is one of the last free flowing stretches of river in Europe and home to 29 million people (WWF 2015).

The river serves as an important migration corridor for fish, fowl and other fauna. The lower Danube and Delta are especially important breeding and resting places for some 331 species of birds and 45 species of freshwater fish (WWF 2015).

The Danube River basin has also been the site of many disastrous floods over the centuries. Most recently, severe flooding occurred in 2005 and 2006, with the 2005 flood resulting in US$ 464 million in damages (WWF 2014).

Dykes built as part of the flood management system have cut off the river from the majority of its floodplains. About 80% of Danube’s floodplain wetlands have been lost in the past century because of human intervention (WWF 2014).

In 2000, the governments of Bulgaria, Romania, Ukraine and Moldova pledged to work together to establish a green corridor along the entire length of the Lower Danube River (~1,000 km) in recognition of the benefits for flood management, water purification and the environment (WWF 2014).

The four governments committed themselves to preserve a total of 935,000 ha of natural areas and to restore 224,000 ha of former floodplain wetland areas.

Most of the floodplain restoration has been achieved by removing sections of dykes at an estimated to cost of US$ 214 million (WWF 2015).

In addition to the avoidance of damages due to floods (e.g. the 2005 flood resulted in US$ 46 million in damages), the restored floodplains are expected to provide US$ 100 million (US$ 584 /hectare) per year in ecosystem services (e.g. fisheries, tourism) and help to diversify the livelihoods of local people (WWF 2014).

Importantly, proper attention to the issue of land ownership was noted as a key for success of the project. It was important to ensure landowners didn't lose the property rights and that stakeholders understood the benefits deriving from changing the unproductive arable land into wetlands (WWF 2014).

As an early indication of the benefits of floodplain reconnection, no damages were reported during the 2013 flood along the lower Danube, although the water level was above average (WWF 2014).

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References:

WWF (World Wildlife Fund). 2014. “Lower Danube Green Corridor: Floodplain Restoration for Flood Protection — Climate-ADAPT.” 2014. https://climate-adapt.eea.europa.eu/metadata/case-studies/lower-danube-green-corridor-floodplain-restoration-for-flood-protection.
WWF. 2015. “Green Infrastructure for Europe: The Lower Danube River Corridor.” http://wwf.panda.org/?255978/Green-Infrastructure-for-Europe-The-Lower-Danube-Green-Corridor.

Image source (left to right):

https://www.flickr.com/photos/131046771@N02/37941169594
WWF 2014.

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2. INLAND WETLANDS

Photo credit: Flickr/Ed Dunens

Complex, integrated systems of water, plants, animals, and microorganisms
Require specific environmental conditions
Wide variety of wetlands and flood attenuation potential
64–71% of the world’s natural wetland area have been lost

Wetlands are complex, integrated systems consisting of water, plants, animals, and microorganisms that require very specific environmental conditions.

By definition, wetlands require permanent (e.g., perennial) or intermittent saturation. As such, they occur in isolated, stagnant water basins, such as lakes, reservoirs, potholes, marshes, ponds, and stock ponds, which can be located in a floodplain or in uplands.

Each of these wetland types and locations can have a different hydrological function. An important distinction for flood management exists between river-fed wetlands located in floodplains and rain-fed upland wetlands, disconnected from a river channel and floodplain (Acreman and Holden 2013), discussed on the next slide.

Since the year 1900, an estimated 64–71% of the world’s natural wetland area has been lost due to human activity (Davidson 2014). This loss of ecosystems removes important water storage areas, that would also filter pollutants and recharge groundwater.

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References:

Acreman, M., and J. Holden. 2013. “How Wetlands Affect Floods.” Wetlands 33 (5):773–786.
Davidson, N.C. 2014. “How Much Wetland Has the World Lost? Long-Term and Recent Trends in Global Wetland Area.” Marine and Freshwater Research 65 (10):934–41. doi:10.1071/MF14173.
Image source: https://www.flickr.com/photos/blachswan/13343871183/

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RISK REDUCTION BENEFITS

Photo credit: Flickr/Eustaquio Santimano

“Act like a sponge”
A hectare of wetland can store up to 9,400 -14,000 m³ of floodwater
Type and location determine function
Floodplain river-fed wetlands greater potential to reduce floods

Wetlands are often said to “act like a sponge,” storing water during wet periods and releasing it during dry periods (Acreman and Holden 2013).

At the high end, a hectare of wetland can store up to 9,400 -14,000 m³ of floodwater (US EPA n.d.)

Depressional wetlands in the Devils Lake basin of North Dakota, USA can store 72% of total runoff volume from a 2-year frequency rainfall event (Ludden et al. 1983).
Reconnecting the river-lake wetlands system in the Yangtze River has resulted in the restoration of 44,800 hectares of wetlands and 285 million m³ of floodwater retention capacity (6,361 m^3/ ha⁾ that can help protect vulnerable downstream areas from flooding. This has also benefited fisheries production, which increased by more than 17% once the sluice gates were reopened seasonally (Yu and Jiang 2009).

However, the type and location of wetlands determine more specifically how they function and affect floodwaters.

In general, floodplain river-fed wetlands have a greater potential capture, slow down and process floodwater to reduce floods than upland rain-fed wetlands.

Upland rain-fed wetlands tend to remain saturated year-round, often lacking the capacity to store significant amounts of precipitation. This can lead to overland flow occurring for long periods after rainfall has ceased, with the potential for upland wetlands to generate floods (Acreman and Holden 2013).

A major review of wetlands found that around 80% of relevant studies suggested floodplain wetlands reduced flooding, but 41% of studies on headwater wetlands indicated that those wetlands enhanced flooding (Bullock and Acreman 2003).

While upland wetlands may not have significant flood management capacity they regulate water flow on an annual and interannual basis, by storing water during times of excess and releasing it in times of scarcity (Stella Consulting 2012).

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References:

Acreman, M., and J. Holden. 2013. “How Wetlands Affect Floods.” Wetlands 33 (5):773–786.
Bullock, A., and M. Acreman. 2003. “The Role of Wetlands in the Hydrological Cycle.” Hydrology and Earth System Sciences Discussions 7 (3):358–389.
Ludden, A.P., D.L. Frink, and D.H. Johnson. 1983. “Water Storage Capacity of Natural Wetland Depressions in the Devils Lake Basin of North Dakota.” Journal of Soil and Water Conservation 38 (1):45–48.
Stella Consulting. 2012. “Costs, Benefits and Climate Proofing of Natural Water Retention Measures (NWRM).” Final Report to DG Environment, Contract 07037/2010/581332/SER/D1. Brussels: European Commission.
US EPA (U.S. Environmental Protection Agency). n.d. “Functions & Values of Wetlands.” EPA 843-F-01-002c. https://nepis.epa.gov/Exe/ZyPDF.cgi/200053Q1.PDF?Dockey=200053Q1.PDF.
Yu, X., and L. Jiang. 2009. “Freshwater Management and Climate Change Adaptation: Experiences from the Central Yangtze in China.” Climate and Development, November. doi:10.3763/cdev.2009.0023.

Image source: https://www.flickr.com/photos/eustaquio/10944390564/

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ADDITIONAL BENEFITS

Photo credit: Flickr/NPS, Patrick Myers

Effectively filter sediments and pollutants
Hydrologic connectivity and water security
Wildlife and biodiversity
Recreation, tourism and education opportunities
Ecosystem services averaged $26,000 /ha/yr in 2011

Wetlands have benefits to society far beyond flood risk reduction.

Wetlands can address water quality challenges due to their ability to effectively filter and capture sediments and a range of pollutants, including nutrients, hydrocarbons, and even certain pharmaceuticals and metals.

Wetlands also play an important part in the freshwater cycle, storing water during wet periods and releasing it during dry periods, improving water security by serving as a source of freshwater during times of scarcity.

Wetlands are some of the most biologically productive natural ecosystems in the world, comparable to tropical rain forests and coral reefs in their productivity and the diversity of species they support (US EPA 2015).

The aesthetic beauty and biodiversity of wetlands attracts humans who want to recreate in these areas, for hunting, fishing, bird-watching, and photography.

In 2001, wetland-related ecotourism activities attracted more than 82 million Americans, adding approximately $108 billion to the national U.S. economy (US EPA n.d.)

Aggregating these benefits, in 2011, the global value of ecosystem services provided by wetlands was estimated to average 25,681 2007$/ha/yr (WWAP/UN-Water 2018)

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References:

US EPA (U.S. Environmental Protection Agency). 2015. “Why Are Wetlands Important?” Overviews and Factsheets. US EPA. April 9, 2015. https://www.epa.gov/wetlands/why-are-wetlands-important.
US EPA. n.d. “Economic Benefits of Wetlands.” EPA 843-F-01-002c. https://nepis.epa.gov/Exe/ZyPDF.cgi/2000D2PF.PDF?Dockey=2000D2PF.PDF.
WWAP (United Nations World Water Assessment Programme)/UN-Water. 2018. “The United Nations World Water Development Report 2018: Nature-Based Solutions for Water.” Paris, UNESCO

Image source: https://www.flickr.com/photos/greatsanddunesnpp/12640970503/

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CONSIDERATIONS FOR USING INLAND WETLANDS

Photo credit: Flickr/Franck Zecchin

Site-specific environmental conditions
Seasonal and conditional variation in performance
Potential for moderate flood management benefits
Justification may require evaluation of additional benefits

To function properly and achieve the intended benefits, wetlands require very specific and site-dependent environmental conditions (temperature, nutrients, soils, gases, and inflow conditions). These factors influence the services wetlands provide and must be considered in design and planning.

In general, wetland restoration projects are best suited for areas in which wetlands naturally occur(ed).

Often, the restoration of a former wetland entails the removal of water retention measures such as a dike or a dam to inundate an area that was previously kept dry. However, simply flooding lands is insufficient for developing ecologically functional wetlands; as such wetland restoration often requires adaptive management to ensure proper function.

Further, flood storage benefits vary seasonally in response to changing environmental conditions. In particular, the flood attenuation capacity of wetlands depends upon the saturation level of the wetland and antecedent conditions (e.g. time since last rainfall) A heavy flow of incoming water can generate floods in a wetland, while a dry spell can damage plants and severely limit wetland function (Acreman and Holden 2013).

Temperature also affects wetland function and can cause a wetland to display inconsistent contaminant removal rates. Colder conditions, in particular, slow the rate at which the wetland is able break down contaminants.

Overall, the potential benefits of wetland restoration in terms of flood protection are somewhat more moderate in individual terms compared with larger scale NBS, such as floodplain reconnection. Obtaining significant flood risk reduction and pollutant filtration benefits from wetlands requires a relatively large amount of space.

As such, the use of wetlands for flood management requires careful consideration and site-specific research.

However, what wetlands lack in flood management benefits they make up for in additional benefits. A holistic accounting of wetland benefits is necessary to accurately evaluate the value of wetlands and justify their application in relation to other flood management strategies.

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References:

Acreman, M., and J. Holden. 2013. “How Wetlands Affect Floods.” Wetlands 33 (5):773–786.

Image source: https://www.flickr.com/photos/distant_trails/3106350703/

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WHAT DO INLAND WETLANDS COST?

Photo credit: Flickr//DraconianRain

The construction cost of wetlands can vary greatly depending on the configuration, location, site-specific conditions, etc.

The scale of wetland restoration projects is typically smaller (<100 km²) than restoration of floodplains or re‑meandering projects. However, the cost per area of inland wetlands – estimated at US$ 33,000 – can exceed that of other NBS (Strassburg and Latawiec 2014).

Land acquisition costs are a significant factor in the cost-efficiency of wetland restoration projects (European Environment Agency 2017).

Operation and maintenance expenses are low, but necessary. One study estimated operation and maintenance costs of wetlands at US$ 410/ha per year (Stella Consulting 2012).

Typical maintenance includes: monitoring and tending plant growth (more often during establishment, then less frequently), removing debris or trash (multiple times each year), removing accumulated sediment (yearly to multi-yearly), and inspecting and repairing structural features such as embankments, inlets and outlets (yearly).

Example costs and benefits:

The US Army Corps of Engineers found that acquiring and protecting floodplain wetlands could reduce Boston’s flood risk from the Charles River for 1/10 the cost of a plan that included new engineered infrastructure, saving $17 million (Opperman 2014).

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References:

DiMuro, J.L., F.M. Guertin, R.K. Helling, J.L. Perkins, and S. Romer. 2014. “A Financial and Environmental Analysis of Constructed Wetlands for Industrial Wastewater Treatment.” Journal of Industrial Ecology 18 (5): 631–640.
European Environment Agency. 2017. “Green Infrastructure and Flood Management: Promoting Cost-Efficient Flood Risk Reduction via Green Infrastructure Solutions.” Publication EEA Report No 14/2017. https://www.eea.europa.eu/publications/green-infrastructure-and-flood-management.
Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.
Stella Consulting. 2012. “Costs, Benefits and Climate Proofing of Natural Water Retention Measures (NWRM).” Final Report to DG Environment, Contract 07037/2010/581332/SER/D1. Brussels: European Commission.
Strassburg, B.B., and A.E. Latawiec. 2014. “The Economics of Restoration: Costs, Benefits, Scale and Spatial Aspects.” presented at the CBD Meeting - Linares, International Institute for Sustainability, March. https://www.cbd.int/doc/meetings/ecr/cbwecr-sa-01/other/cbwecr-sa-01-iis-en.pdf.

Image source: https://www.flickr.com/photos/draconianrain/22989846335/in/photolist-B2wYmX

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Photo credit: World Bank/Andrina Fernando (top)

Sri Lanka Ministry of Defense and Urban Development (bottom)

CASE STUDY: BEDDAGANA WETLAND PARK, SRI LANKA

2010 Flood: 36,000 families homeless, US$ 50-100 million damages
Wetlands capture 39% of the flood waters during storms
Degraded at 1.2% (23 ha)/ yr
Without wetlands 1% lost of GDP/yr

Built on a low-lying river estuary, the Sri Lanka metropole, Colombo, is highly vulnerable to flooding. Like many developing urban cities, flood risk is on the rise due rapid economic growth, haphazard land use, and reclamation of the city’s unique natural lands.

Floods in 2010 paralyzed the city, leaving 36,000 families homeless, marooning traffic and submerging the country's Parliament in four feet of water (Athas 2010). Losses due to the May 2010 flooding were initially estimated at US$ 50 million, although actual losses may have reached $100 million (Rozenberg et al. 2015).

Wetlands of the Colombo area play a key role in flood management and are able to capture 39% of the flood waters during a storm event (Rozenberg et al. 2015). However, they have been degraded and lost at a rate of 1.2% (23 ha) per year (World Bank 2016). Without wetlands, Colombo would lose 1% of its GDP on average each year due to flood damages similar to that experienced in 2010 (World Bank 2016).

To address these challenges, the Government of Sri Lanka developed a Wetland Management Strategy in partnership with the World Bank, designed to preserve wetlands, educate the public, and reduce the social and economic impacts of floods in Colombo.

Part of this project invested ≈US$ 1.2 million in restoring and protecting the 18 ha Beddagana Wetland Park within the heart of Colombo.

The wetland park contributes to the flood management goals and demonstrates how a wetland can be preserved while promoting eco-tourism and improved livability in the city (World Bank 2016). The income potential of recreational opportunities is estimated at around $13.6 million annually (World Bank 2016), more than ten times the initial investment.

The park also provides a refuge for flora and fauna and moderates the temperature of the immediate surroundings (World Bank 2016).

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References:

Athas, I. 2010. “Record Rains Flood Sri Lanka, Leaving 36,000 Families Homeless.” CNN, November 11, 2010, sec. World. http://www.cnn.com/2010/WORLD/asiapcf/11/11/sri.lanka.floods/index.html.
Rozenberg, J., M. Simpson, L. Bonzanigo, M. Bangalore, and L. Prasanga. 2015. “Wetlands Conservation and Management: A New Model for Urban Resilience in Colombo.” Washington, D.C.: World Bank Group. https://collaboration.worldbank.org/servlet/JiveServlet/downloadBody/21539-102-5-27686/final_report_wetlands_Colombo_mtdf.pdf.
World Bank. 2016. “Preserving the Beddagana Wetland for Flood Protection, Conservation Education, and Improved Quality of Life.” Text/HTML. World Bank. June 17, 2016. http://www.worldbank.org/en/news/feature/2016/06/17/preserving-beddagana-wetlands-flood-protection-conservation-education-improved-quality-life.

Image source:

http://www.worldbank.org/en/country/srilanka/brief/beddagana-wetland-park-fact-sheet (top)
http://www.defence.lk/MCUDP/pdf/Environmental_Management_lan_Beddagana_Biodiversity_park_20140521.pdf (bottom)

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3. STREAM BEDS AND BANKS

Photo credit: Wikipedia/US Bureau of Reclamation

Vegetated banks along meandering streams slow floodwaters
The majority of large rivers have been modified
Modifications fight against natural processes

Interventions: Re-meandering, setting back levees, de-armoring and revegetating banks.

Naturally flowing rivers meander along a curved path, which increases the river’s length and therefore slows down the river flow. Vegetation along this path provides hydraulic roughness, or resistance to flow, which also reduces flow velocity. By slowing floodwaters, these natural processes reduce downstream flood risk.

On the other hand, conventional water management strategies often entail modifications to streambeds and banks, including:

Straightening and dredging channels to increase the speed and volume of water at which a flood travels down the channel;
Tightly lining rivers with levees/dykes to prevent rivers from changing course (e.g., meandering), while maximizing the amount of land protected by the levees; and
“Armoring” of banks with concrete or other built infrastructure to reduce the erosion of stream banks and prevent meandering.

These modifications fight against natural riverine processes (e.g. meandering, sedimentation, and deposition), which can result in higher maintenance costs.

For example, levees that greatly narrow the area available to transport floods, rapidly flush floodwaters through the system – but this means that the levees are exposed to high-velocity water along their “wet” side, that can weaken their structure. Repair costs of up to $500 million have been estimated for levees in the Middle Mississippi River (Dierauer et al. 2012).

Adjusting these modifications to restore more of the natural functions of rivers can lead to a long-term reduction in flood risk. This includes: re-meandering, setting back levees, de-armoring and revegetating banks, as described next.

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References:

Dierauer, J., N. Pinter, and J.W.F. Remo. 2012. “Evaluation of Levee Setbacks for Flood-Loss Reduction, Middle Mississippi River, USA.” Journal of Hydrology 450–451 (July):1–8. doi:10.1016/j.jhydrol.2012.05.044.

Image source: https://ca.wikipedia.org/wiki/Riu_Sacramento#/media/File:Redbluffdivdam.jpg

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RISK REDUCTION BENEFITS

Photo credit: Flickr/Marcos Molina

Re-meandering the River Skjern extended stream length 36%

Mississippi River levee set backs could reduce expected annual damages by 55%

Restoring streambed delayed flood wave by two hours

Restoring modified rivers usually requires a combination of strategies in response to existing modifications and characteristics of the natural river and watershed.

One strategy is to re-meander straightened channels, reestablishing the curves in the river and increasing its length.

Re-meandering the River Skjern (Denmark) extended the length of the river by 36%, from 19 km to 25.9 km (Blackwell and Maltby, 2006).

Another strategy is to setback levees, moving them further away from the river, to increase the magnitude of flood that the river is able to convey between levees.

Setting back levees along the Middle Mississippi River, USA would decrease expected annual damages by 55% in urban areas and 93% in agricultural areas. The optimal strategy includes buying out structures outside the new levee protection area (Dierauer et al. 2012).

Lastly, de-armoring and revegetating banks can reduce the velocity of floodwaters.

Restoring 4 km of the Cerný Potok stream in the Czech Republic and restoring 4.3 ha of marsh on the significantly delayed the flood wave - up to 2 hours for the 1-year flood compared to the modified stream and 20 minutes for a 100-year flood. Restoration also resulted in a slight reduction in peak flow (<5%) (Jongepierová et al. 2012).
Different types of land cover can have very different values for hydraulic roughness, making them more or less effective at reducing flood risk. For example, short grasses provide much less resistance than a diverse riparian habitat with long grasses, bushes and trees.

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References:

Blackwell, M. S. A., E. Maltby, A. L. Gerritsen, M. Haasnoot, C. C. Hoffmann, W. Kotowski, EJTM Leenen, T. Okruszko, W. E. Penning, and H. Piórkowski. 2006. “How to Use Floodplains for Flood Risk Reduction.” Luxembourg, Belgium: European Communities.
Dierauer, J., N. Pinter, and J.W.F. Remo. 2012. “Evaluation of Levee Setbacks for Flood-Loss Reduction, Middle Mississippi River, USA.” Journal of Hydrology 450–451 (July):1–8. doi:10.1016/j.jhydrol.2012.05.044.
Jongepierová, I., P. Pešout, J.W. Jongepier, and K. Prach, eds. 2012. Ecological Restoration in the Czech Republic. Prague: Nature Conservation Agency of the Czech Republic.

Image source: https://www.flickr.com/photos/larou/13335253903/in/photolist-mjoEoK-86DpFv-dN78tt-7691io-aFbxk-9imthR-dH9hSN-dGyaeJ-8qmDiq-5jFEa1-7691F7-dGsGDe-76913A-ax27cn-9GizKx-jdNTfd-ghQQ72-74uq4Y-dorFbo-kmXT8-23jssGh-fLC1CC-dgT4gA-bmPMJ4-txD9j-GqevNe-23J4tFi-261v32L-fLk3Ne-x8jXj-PUCQY3-tsFph-6AidV1-awKvud-einAmN-74qvPH-759zH1-hfFZpH-tt8ej-74uryW-9bP8Ey-4m6Uwn-txGrw-4WpQk8-7qJZK4-YvCJ7b-bWdzs1-75yR2K-rU1RoL-9aY2ms

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ADDITIONAL BENEFITS

Photo credit: Flickr/WRI

Improved riparian biodiversity
More diverse fish habitat
Decreased water temperature
Erosion control
Recreation and aesthetic value

Natural river banks in Belgium evaluated at US$ 27,000/km – 60,000/km per year

Stream restoration improves both riparian and aquatic habitat. In addition to increased biodiversity, riverbank vegetation can provide shade, decreasing water temperature, and providing more favorable conditions for fish species.

Stream modifications such as armoring banks can help control erosion where the are located, but this can create sediment “starved” rivers and also increase stream velocity, thereby causing more erosion downstream (Kondolf 1997). Restoring riparian vegetation creates natural erosion control and helps to restore the river’s sediment balance.

A survey of public willingness to pay for these benefits, valued more natural riverbanks in Flanders, Belgium at US$ 27,000/km per river per year for improvements improved conditions from bad to moderate and an additional US$ 33,000/km (US$ 60,000/km total) per river per year for improvement from moderate to good (Jongepierová et al. 2012).

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References:

Jongepierová, I., P. Pešout, J.W. Jongepier, and K. Prach, eds. 2012. Ecological Restoration in the Czech Republic. Prague: Nature Conservation Agency of the Czech Republic.
Kondolf, G. M. 1997. “Hungry Water: Effects of Dams and Gravel Mining on River Channels.” Environmental Management 21: 533–552.

Image source: https://www.flickr.com/photos/worldresourcesinstitute/41260025634/in/album-72157668749248648/

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CONSIDERATIONS FOR RESTORING STREAM BEDS AND BANKS

Photo credit: Flickr/ William Veerbeek

Reference state and objectives
Requires solid understanding of current and future hydrologic regimes
Dynamic river systems versus anthropogenic constraints

Stream restoration projects face a serious challenge in determining the target state to which the system should be restored to, or whether a new state is needed. Many rivers have undergone significant modifications, such as: dams that alter the natural flow regime, withdrawals that decrease discharge, and infrastructure that cuts off rivers from floodplain. The meander patterns and natural vegetation found under a "natural" pre-disturbance condition, may no longer be functional or sustainable.

Projects can fail if attempts are made to restore back to an unachievable state and/or if anthropogenic modifications (i.e. land use change, water withdrawals, dam development) and climatic changes are not taken into consideration.

In particular, successful and resilient restoration of stream beds and bank requires careful planning and design taking into consideration both current and future hydrologic regimes (ex. Designing projects based on natural conditions, prior to river regulation and development, can lead to projects unable to perform desired flood management functions due to the changing conditions).

Stream restoration processes that aim to restore natural, dynamic river processes, such as meandering and migration, may need to purchase lands, relocate assets, and/or compensate landowners to move out of newly flood-prone areas.

Land uses with large potential damages (e.g. residential or commercial areas) and high costs of relocation/removal, will likely still require protection using built infrastructure.

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References:

Image source: https://bit.ly/2SDJ2aH

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WHAT DOES IS COST TO RESTORE STREAM BEDS AND BANKS?

Photo credit: Flickr/USFWS, Pacific region

High construction costs
Lower land acquisition and compensation costs

Bank stabilization: $29,000 to $137,000/km
Channel rehabilitation: $25,000 to $85,000/km

Restoration costs depend upon the existing modifications to the stream, surrounding land use, and restoration objectives.

A large portion of stream restoration costs go towards design and construction (or deconstruction of built infrastructure). In some areas legal and regulatory expenses can also be significant.

Compared to other NBS, such as floodplain reconnection and upland forest, little additional land is typically needed for streambed or riverbank transformations, resulting in lower land acquisition and compensation costs (European Environment Agency 2017). However, in urban areas and where development abuts a river (ex. Room for the River, next slide), land and compensation costs can be significant.

Stream restoration costs vary widely. As an example, bank stabilization for a project in the U.S. ranged from $29,000 – 137,0000 per kilometer of river; while channel rehabilitation ranged from $16,000 - $53,00 per river kilometer (Bair 2000).

Other example restoration costs:

Re-naturalization of the Seymaz River (Switzerland) costs EUR 12,671/ha (EUR 15 million construction costs and EUR 22 million capital investment costs. Operation and maintenance costs were also envisaged, but that approximate cost figure is not available. (European Environment Agency 2017)
Marsh and streambed restoration of the Cerný Potok stream (Czech Republic) costed EUR 27,009/ha (European Environment Agency 2017)

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References:

Bair, B. 2000. “Stream Restoration Cost Estimates.” Proceedings of the Salmon Habitat.
European Environment Agency. 2017. “Green Infrastructure and Flood Management: Promoting Cost-Efficient Flood Risk Reduction via Green Infrastructure Solutions.” Publication EEA Report No 14/2017. https://www.eea.europa.eu/publications/green-infrastructure-and-flood-management.

Image source: https://www.flickr.com/photos/usfwspacific/5616612013

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CASE STUDY: ROOM FOR THE RIVER, THE NETHERLANDS

Photo credit: Wikipedia/Roger Veringmeier

55% of housing in flood-prone areas
Higher dykes no longer sufficient due to climate change

Room for the River - Nijmegen

US$ 460 million to push dyke 350 m inland
Local participation and compensation
New island and river park

The Netherlands forms the low-lying delta of North-Western Europe, where 55 percent of housing is located in areas prone to flooding (ClimateWire 2012).

Over the past 1,000 years the rivers have been harnessed between higher and stronger dikes (ClimateWire 2012).

After the floods in the 1990s, and in consideration of climate change projections, the Dutch government decided it no longer made sense to raise the dikes (ClimateWire 2012).

Instead the country decided to embark on the colossal task of moving dozens of dikes back to make “Room for the River” – the signature national flood management program.

At more than 30 locations, measures are being taken to give swelling rivers more space reduce flood risks (ClimateWire 2012).

One of these sites lie in the City of Nijmegen, where high water levels in 1993 and 1995 prompted the evacuation of 250,000 people (ClimateWire 2012).

The US$ 460 million (€359 million) project involved pushing a dike 350 meters inland, and digging a new channel for the river Waal (150 to 200 meters wide and 3 kilometers long). The channel created a new island and an urban river park in the heart of the city (ClimateWire 2012).

Throughout the design and planning process, many discussions were held with people living in the areas to understand their needs and concerns. These discussions led to the inclusion of space for walking, biking and numerous cafes with outdoor seating and views of the river, along with a new marina for rowing and sailing (ClimateWire 2012).

Completion of the project entailed demolishing around 50 houses within the setback dyke zone. This required negotiations and compensation for the property owners under the Netherlands' eminent domain laws (ClimateWire 2012).

The Room for the River Program serves as a model for developed countries, which are beginning to recognize that built infrastructure of the past is no longer sufficient to manage future flood risk.

It also demonstrates the high costs of retroactively moving built flood infrastructure, which could be avoided if sufficient room is allocated for rivers as countries develop their urban areas and flood management programs.

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References:

ClimateWire. 2012. “How the Dutch Make ‘Room for the River’ by Redesigning Cities.” Scientific American. January 20, 2012. https://www.scientificamerican.com/article/how-the-dutch-make-room-for-the-river/.

Image source: https://en.wikipedia.org/wiki/Nijmegen#/media/File:Nijmegen_Panorama_Wolken_met_water_en_eiland_2016.jpg

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4. UPLAND FORESTS

Photo credit: Flickr/Peter Steward

Upstream watershed characteristics influence downstream river floods
Upstream forests slow and retain runoff
Land use changes increase flood runoff

A flood in a river is generated by heavy rainfall and/or snowmelt in the upstream watershed, which eventually accumulates and drains downstream. The characteristics of a watershed, including geology, topography, and land cover, all influence flood risk at a given point (Opperman 2014).

Upstream forests with deep soils tend to slow and retain runoff, resulting in lower flood peaks with a longer lag time to reach downstream areas.

Forests intercept and store of precipitation on the leafy canopy, before it can run off the land (Opperman 2014).
Forests generally have a thick layer of organic matter on the ground, such as leaves and wood, that serves as a sponge to hold water and also protects the soil from the direct impact of raindrops, reducing erosion (Opperman 2014).
Forests also tend to have porous soils that allow water to infiltrate and much of the precipitation moves into the shallow groundwater, a much slower path to the stream than via surface runoff (Opperman 2014).
Lastly, letting dead wood from the forest block the stream where it will creates pools that can temporarily store and slow floodwaters.

Conversely, impermeable surfaces, such as pavement and compacted agricultural fields, prevent infiltration of water in the soil. This produces rapid and high levels of runoff and thus higher flood peaks.

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References:

Opperman, J. 2014. “A Flood of Benefits: Using Green Infrastructure to Reduce Flood Risks.” Arlington, Virginia: The Nature Conservancy.
Image source: https://www.flickr.com/photos/pete_steward/10462964526

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RISK REDUCTION BENEFITS

Photo credit: WRI/James Anderson

Most risk reduction evidence from North American and European temperate forests
Most effective during moderate floods of short duration

82% of studies reported a decrease in peak flow after restoration
Reforesting areas over 25-40% of a UK catchment could decrease the flood maximum by 20%

Many studies have demonstrated that clear cut timber harvesting and road building increase peak flows, while forested regions experience lower peak flows. For example,

In a review of forest restoration studies, 35 out of 43 (82%) reported a decrease in peak flow after forest restoration (Filoso et al. 2017).
A UK study found that foresting the areas around tributary streams over 25-40% of the main catchment can decrease the flood maximum by 20% (Dixon et al. 2016).
Peak flow increases averaged approximately 13–16% after clear cut silviculture in the northwestern US, for 1-yr recurrence interval events, and 6–9% for 5-yr recurrence interval events (Beschta 2005).
In Coalburn, England, peak flows immediately downstream of timber operations in a conifer forest were increased by about 15% over the short to medium time scale (Robinson et al. 2003).
Clear cutting of eucalyptus plantations in Southern Europe resulting in an increase of about 50% in peak flows after the cutting, and this effect lasted for 1–2 years (Robinson et al. 2003).
In southern Chile, clear cutting of a Pinus radiata plantation generated a 32% mean increase in peak flows (Iroumé et al. 2005).

However, the complexity of watershed and land use dynamics prohibits simple generalization of the impact of upland forest management on flood risk reduction (Calder and Aylward 2006).

Most of the scientific studies of forest impacts on stream flows have been conducted in small temperate forest watersheds in North America and Europe, providing a very limited understanding of impacts in other regions and forest types (Robinson et al. 2003).

We do know that forest management is most effective at retaining and slowing moderate floods of short duration (Bathurst et al. 2011). During very large or very long floods, soils become saturated with water and thereafter have limited or no further influence on the flood (Pitlick 1997).

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References:

Bathurst, J.C., A. Iroumé, F. Cisneros, J. Fallas, R. Iturraspe, M.G. Novillo, A. Urciuolo, B. De Bièvre, V.G. Borges, and C. Coello. 2011. “Forest Impact on Floods Due to Extreme Rainfall and Snowmelt in Four Latin American Environments 1: Field Data Analysis.” Journal of Hydrology 400 (3–4):281–291.
Beschta, R. L., M. R. Pyles, A. E. Skaugset, and C. G. Surfleet. 2000. “Peakflow Responses to Forest Practices in the Western Cascades of Oregon, USA.” Journal of Hydrology 233 (1): 102–120.
Calder, I.R., and B. Aylward. 2006. “Forest and Floods.” Water International 31 (1):87–99. doi:10.1080/02508060608691918.
Dixon, S.J., D.A. Sear, N.A. Odoni, T. Sykes, and S.N. Lane. 2016. “The Effects of River Restoration on Catchment Scale Flood Risk and Flood Hydrology.” Earth Surface Processes and Landforms 41 (7): 997–1008.
Filoso, Solange, Maíra Ometto Bezerra, Katherine C. B. Weiss, and Margaret A. Palmer. 2017. “Impacts of Forest Restoration on Water Yield: A Systematic Review.” PLOS ONE 12 (8): e0183210. https://doi.org/10.1371/journal.pone.0183210.
Iroumé, A., O. Mayen, and A. Huber. 2005. “Runoff and Peak Flow Responses to Timber Harvest and Forest Age in Southern Chile.” Hydrological Processes 20 (1):37–50. doi:10.1002/hyp.5897.
Pitlick, J. 1997. “A Regional Perspective of the Hydrology of the 1993 Mississippi River Basin Floods.” Annals of the Association of American Geographers 87 (1):135–151.
Robinson, M., A. -L Cognard-Plancq, C. Cosandey, J. David, P. Durand, H. -W Führer, R Hall, et al. 2003. “Studies of the Impact of Forests on Peak Flows and Baseflows: A European Perspective.” Forest Ecology and Management 186 (1): 85–97. https://doi.org/10.1016/S0378-1127(03)00238-X.

Image source: James Anderson, World Resources Institute

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ADDITIONAL BENEFITS

Photo credit: Flickr/Mathias Appel

Water and air purification
Carbon storage
Soil production, reduced erosion and sedimentation
Timber, food, and fuel
Habitat creation
Recreation

Sustainable forest management supports healthy forest ecosystems that provide a wealth of benefits, likely exceeding those related to flood risk reduction.

Forests cycle nutrients and filter pollutants, purifying both water and air.

Among those nutrients, forests uptake and sequester carbon, reducing the amount of greenhouse gases in the atmosphere.

Forests aid in the production of rich soils, while strong roots systems hold those soils in place, reducing erosion and sedimentation in downstream waterways and reservoirs.

Forests can also be sustainably managed to provide several marketable products, including timber, food, and fuel.

The habitat created by healthy forests supports a wide range of plants and animals.

These forests are an ideal environment for recreation, and they provide a wide range of outdoor opportunities that contribute to human well-being.

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References:

Image source: https://www.flickr.com/photos/mathiasappel/38576869000/

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CONSIDERATIONS FOR UPLAND FOREST RESTORATION

Photo credit: Flickr/WRI

Not all forests are created equal
Some studies demonstrate negligible flood impacts
Problems of scale and cost
Importance of data and monitoring

The role of upland forests as an NBS for flood management has been debated due to a lack of robust evidence into its effectiveness in reducing flood risk during extreme events, particular outside temperate forests in North America and Europe.

Not all forests are created equal – many studies have found minimal impacts of upland forests on downstream flooding, reflecting the need for site-specific project evaluation, design, and monitoring plans.

Forty (40) years of data on the Ganga-Brahmaputra-Megha lowlands found that, “ there is no reason to believe that floods in the lowlands have intensified as a result of human impact in the highlands” (Hofer 1998).
Field data at 22 stream crossings in central Nepal, “demonstrate that 82 percent of the variation in bank-full discharge can be explained as a function of drainage area alone; forest cover did not add explanatory power” (Marston et al. 1996).

Further, unsustainable forest management practices may negatively affect forests’ ability to slow and retain flood waters. In particular, plantation forests without natural understory vegetation and intrusive forest management activities (i.e. road construction, site preparation, increased drainage, and logging) may decrease forests’ ability to slow and retain flood waters.

These activities can also contribute to increases in rates of erosion and transportation of sediments into watercourses

Studies undertaken by the U.S. Forest Service and others have also highlighted the problem of scale. In small scale catchments, increased water and sediment runoff due to deforestation can be identified empirically and demonstrated with hydrological models. However in larger catchments, increases in flood flows are not as easily discerned or modeled (Calder and Aylward 2006).

Relatedly, to provide substantive flood risk reduction benefits, a large proportion of the upstream watershed needs to be forested – undertaking small scale restoration projects will likely yield negligible flood risk reduction benefits, at best.

At the low end, foresting 10-15% a UK catchment could prevent some flooding (Dixon et al. 2016), but insufficient funds are available to attempt the scale of works (Harrabin 2016).

These challenges and the lack of a wide breadth of scientific evidence linking forest management with flood management, support the need for site-specific data, modelling and monitoring when developing and implementing forest management activities.

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References:

Calder, I.R., and B. Aylward. 2006. “Forest and Floods.” Water International 31 (1):87–99. doi:10.1080/02508060608691918.
Harrabin, R. 2016. “Tree Planting ‘Can Reduce Flooding.’” BBC News, March 11, 2016, sec. Science & Environment. https://www.bbc.co.uk/news/science-environment-35777927.
Hofer, T. 1998. “Do Land Use Changes in the Himalayas Affect Downstream Flooding?-Traditional Understanding and New Evidences.” MEMOIRS-GEOLOGICAL SOCIETY OF INDIA, 119–142.
Marston, R., J. Kleinman, and M. Miller. 1996. “Geomorphic and Forest Cover Controls on Monsoon Flooding, Central Nepal Himalaya.” Mountain Research and Development, 257–264.

Image source: https://www.flickr.com/photos/worldresourcesinstitute/32435852994

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WHAT DOES FOREST RESTORATION COST?

Photo credit: Flickr/WRI

Lower per hectare cost than other NBS: US$ 3,450/ha (tropical), 2,390/ha (other)
Significant compensation and transaction costs in catchments with large private landownership
High aggregate costs

Forest restoration costs per hectare (US$ 2,390-3,450/ha) are often less than other NBS strategies (up to $9,000/ha) (Strassburg and Latawiec 2014).

However, to effectively reduce peak flows, forest restoration must take place on a large proportion of a catchment.

Undertaking forest restoration in catchments containing large upland areas of private landownership may require significant compensation and transactions costs.

As such aggregate costs for effective forest restoration can be quite high and strongly depend upon the spatial scale of the project.

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References:

Strassburg, B.B., and A.E. Latawiec. 2014. “The Economics of Restoration: Costs, Benefits, Scale and Spatial Aspects.” presented at the CBD Meeting - Linares, International Institute for Sustainability, March. https://www.cbd.int/doc/meetings/ecr/cbwecr-sa-01/other/cbwecr-sa-01-iis-en.pdf.

Image source: https://www.flickr.com/photos/worldresourcesinstitute/14244906307/

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CASE STUDY: UPLAND FOREST RESTORATION AS PART OF INTEGRATED FLOOD MANAGEMENT FOR DAR ES SALAAM

Photo credit: Flickr/Eunheui

Suite of NBS, including forest restoration, outperformed other strategies
Pays for itself in less than 10 years
Net benefits of US$80 million over 20 years

7 disastrous floods since 1995
Avg of 14 people die annually
Main cause of cholera outbreaks
Charcoal use severely degraded upland forests

Tanzania’s commercial capital of Dar es Salaam is home to over 4.5 million people, and the population is expected to increase by nearly 60 percent by 2025. In many places, the growing need for housing has resulted in informal settlements that push right up to the river (Kroeger 2018).

Large-scale flooding occurs on an almost annual basis, with 7 disastrous flood events since 1995 (Kroeger 2018).

Flooding inundates large areas of informal housing and the central business district, leading to widespread damages to homes, bridges, roads, massive transport delays and business interruption losses (Kroeger 2018).

During 2011-2017, an average of 14 people died annually as a direct result of flooding (Kroeger 2018).

Moreover, a recent study found that flooding also is the main cause of cholera outbreaks in the city (Picarelli et al. 2017).

To address these flood risk challenges, a stakeholder group formed, which identified the severe degradation of upland forests to support the widespread use of charcoal as a contributing factor to the flood challenges, among other factors.

The group was comprised of representatives the metropolitan and national government, citizen groups, universities, non-governmental organizations, and multi-lateral institutions and formed In December 2014.

A suite of potential flood mitigation interventions were developed to address these challenges and help mitigate flooding, including:

Restoration of forests in the upper catchment,
Rehabilitation and enhancement of riparian and floodplain areas in the middle catchment,
River cleaning in the middle catchment,
Floodplain rehabilitation in the lower catchment, and
Construction of a built stormwater retention basin in the lower catchment (White et al. 2017).

Changes in the expected annual losses under different combinations of the interventions were estimated based on the expected effects on the flood hydrograph, as well as the change in structures at risk (De Risi et al. 2018).

More specifically, the scenarios were evaluated and compared on three decision criteria: Life Cycle Cost, a common measure that describes the budget needs of infrastructure projects over their life time; Return on Investment, which is the ratio of a project’s benefits and costs; and Net Present Value, or the discounted sum of the future stream of annual costs and benefits of a project (De Risi et al. 2018).

The scenario that combined a suite of NBS throughout the catchment outperformed all others. This included: reforestation in the upper catchment, the rehabilitation of river buffers in the middle catchment, and the reconnection of floodplains in the lower reaches.

Upland forest efforts included restoration and protection of 776 hectares of the Pugu Forest Reserve for at an initial cost of US$ 845,000 and US$170,000 /yr in maintenance (White et al. 2017).

This scenario would pay for itself within less than 10 years in terms of reductions in flood damages; Over a 20-year period it would generate net benefits with a present value of US$ 80 million.

Importantly, all measures would be expensive because of the fact that people have settled illegally in the forest reserve, riparian reserve and floodplain areas, with the full combination amounting to US$138.5 million (White et al. 2017).

The World Bank is currently conducting a follow-up engineering study to fill in data gaps and assess the feasibility of specific intervention options (White et al. 2017).

The World Bank also is also planning a participatory design process to work with stakeholders on the development of a strategic open-space plan for the basin. The goal is to combine the engineering studies and open space plan in a process that designs a comprehensive flood mitigation and open space project in the city (White et al. 2017).

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References:

De Risi, R., F. De Paola, J. Turpie, and T. Kroeger. 2018. “Life Cycle Cost and Return on Investment as Complementary Decision Variables for Urban Flood Risk Management in Developing Countries.” International Journal of Disaster Risk Reduction 28: 88–106
Kroeger, T. 2018. “Rehabilitating Watershed Natural Infrastructure in Africa: A Smart Investment to Reduce Urban Flood Damages.” Cool Green Science (blog). February 23, 2018. https://blog.nature.org/science/2018/02/23/rehabilitating-watershed-natural-infrastructure-smart-investment/.
Picarelli, N., P. Jaupart, and Y. Chen. 2017. “Cholera in Times of Floods.”
White, R., J. Turpie, and G. Letley. 2017. “Greening Africa’s Cities.” The World Bank.

Image source: https://www.flickr.com/photos/eunheui/9683715695/in/photolist-fKHzir-fEjrBG-fEq4H3-facco3-f9Xa6R-f9WQ5R-f9XktZ-eHFzBd-eHFoiW-eFdCfA-ef2bpX-bNNfQZ-bNMUkv-bNNgJk-bNMXUv-bu1tir-bu1sZT-yygYd-yygYh-yt97k-bF4NSz-bzTWCQ-bu1sEM-7ZiwaN-enQs6M-epyAbt-ejFWEN-ejG2rf-ef2b7i-em31xb-edutJR-ef7Vk9-edvMLT-e9HobA-e1jqFL-e1jsKE-dYnmr4-dys4Tp-dAM7pq-dct4yS-d21frb-d1Wq1G-aFuNyr-9THg2Q-9TEs1B-8izkgE-8iqtdg-8iqBQ8-8itQjb-7YREv4

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KEY CONSIDERATIONS FOR INTEGRATING NBS INTO � RIVER FLOOD MANAGEMENT

Photo credit: Flickr/International Rivers

Natural versus current conditions
Watershed risks and additional benefits
Integration with built infrastructure
Spatial footprint and land cost

NBS to address river flood management largely rely on protecting or restoring the processes and functions that historically occurred in natural ecosystems – floodplains, wetlands, streams and forests. Examining pre-modified landscape conditions and what ecosystems and processes have been lost due to environmental degradation in that particular location can help determine the potential benefits NBS could provide (i.e. have floodplains been disconnected by levees, wetlands filled, streams armored and straightened, or forests cut for timber).

To realize these diverse benefits, cities must first protect and manage the natural defenses they already have. We can’t keep repeating the mistakes of the past as unplanned growth obliterates wetlands and floodplains, erasing their protective functions, and sending higher volumes of floodwater toward homes and businesses.

While each of the NBS presented here have the potential to reduce flood risk, they vary in their effectiveness and the suite of additional benefits they can provide.

Floodplains maintain the greatest potential capacity to store and slow down floodwaters, followed by wetlands. Flood risk reduction for both of these strategies is proportional to the spatial scale of the intervention.
Actions to restore stream beds and banks may only provide significant risk reduction benefits on smaller segments of river, but they can provide significant instream habitat improvements.
Lastly, the connection between upland forests and downstream flood risk is the least well understood flood NBS presented. Flood risk benefits alone may not be sufficient to initiate a forest restoration project, but including water quality and quantity benefits can tip the benefit-cost ratio towards forest restoration.

Importantly, NBS provide a multitude of additional ecosystem services beyond flood management, and these benefits differ among solution type and design. Additional benefits may even outweigh the primary purpose of the NBS, making them an important consideration in project selection, planning and design.

Built flood infrastructure has saved countless lives and protections billions of dollars of assets from flooding each year. Once built infrastructure is in place, it is often difficult or costly to alter or remove (ex. setting back a levee). This infrastructure will inevitably continue to play a role in future flood risk management, making it important to understand how NBS can most effectively integrate with built infrastructure.

Land and space near rivers are often limited, making it difficult to create room for new NBS, such as reclaiming a large swath of land to restore a former floodplain. Protecting valuable nature-based flood infrastructure requires strategic land use planning and zoning.

Considering these factors when planning and designing nature-based solutions will help ensure that the intervention performs optimally and cost-effectively in relation to the local conditions and needs.

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References:

Image source: https://www.flickr.com/photos/internationalrivers/39935554395/

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THANK YOU

Photo credit: Flickr/bmalarky

For more information, contact:

Denis Jordy: djordy@worldbank.org

Brenden Jongman: bjongman@worldbank.org

Brenden Van Zanten: bvanzanten@worldbank.org

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CASE STUDY: NATURAL FOREST CONSERVATION PROGRAM (NFCP), CHINA - METRICS OF SUCCESS?�

Photo credit: Flickr/CIFOR

$26 billion in loses (1998 floods) due to deforestation and steep cultivation
NFCP meant to reduce flood risk, but no flood metrics formulated
3.3 times less forest loss
0.84 million increase in forest employment
However, questionable net benefits

The story of the Natural Forest Conservation Program (NFCP) in China provides both evidence of success in meeting China’s goals, as well as illustrates many of the challenges and potential failures of NBS initiatives.

In 1998, devastating flooding attributed to overlogging and steep cultivation in the upper Yangtze, Songhua, and Nenjiang Rivers in China (Tianjie, 2008), caused a total loss of 166.6 billion yuan (US$ 26 billion).

In response, China introduced the Natural Forest Protection Program (NFPP), a logging ban to help protect against erosion and rapid runoff. The Grain to Green program was introduced at the same time to reconvert agricultural fields in steep slopes into forests.

Although these forest programs were motivated by flood risk, the goals and metrics used to evaluate the programs do not assess flood risk – This illustrates a common challenge with NBS monitoring programs, which tend to evaluate metrics which are “easier” to evaluate (i.e. hectares reforested), rather than those that directly link actions with outcomes (i.e. reduction in expected annual damages).

Despite this, benefits of the Natural Forest Conservation Program have been demonstrated.

Forest‐loss in provinces enrolled in the Program is 3.3 times lower than in the non‐NFCP provinces (0.62%, versus 2.7% forest loss) (Ren et al., n.d.).

The NFCP was expected to expand and the annual output of the forest sectors by 5.8 billion Yuan and increase employment in the forest sector by 0.84 million by 2010 (Ren et al. 2015).

Other theoretical benefits include decreased soil erosion and biodiversity loss (Ren et al. 2015).

Importantly, critics of the Natural Forest Conservation Programs point to the fact that China has become one of the leading timber importers in the world as a result of the timber restrictions (Tianjie 2008). In essence, the program exported deforestation to other parts of the world, some of which are now losing high biodiversity forests at alarming rates.

Further, many of the projects undertaken as part of the program entail monoculture plantations, planted with trees that are not native, don’t tolerate local conditions, and don’t provide quality habitat for wildlife (Luoma 2012).

As such, the Natural Forest Conservation Program illustrates the challenges of NBS. On a positive note, China’s State Forestry Administration has begun collaborating on projects aimed specifically at restoring native species and is working with the Climate Community and Biodiversity Alliance (CCBA), whose members include Conservation International, the Nature Conservancy, and the Rainforest Alliance (Luoma 2012).

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References:

Luoma, J. 2012. “China’s Reforestation Programs: Big Success or Just an Illusion?” Yale E360 (blog). January 17, 2012. https://e360.yale.edu/features/chinas_reforestation_programs_big_success_or_just_an_illusion.
Ren, G., S. S. Young, L. Wang, W. Wang, Y. Long, R. Wu, J. Li, J. Zhu, and D. W. Yu. 2015. “Effectiveness of China’s National Forest Protection Program and nature reserves.” Conservation Biology 29 (5): 1368–77. https://doi.org/10.1111/cobi.12561.
Tianjie, M. 2008. “Interconnected Forests: Global and Domestic Impacts of China’s Forestry Conservation.” In Wilson Center China Environment Forum.

Image source: https://www.flickr.com/photos/cifor/36882323944/

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CASE STUDY: YOLO BYPASS, USA

Photo credit: Flickr/USFWS, Steve Martarano

Multi-purpose advantages of hybrid infrastructure
Conveys 80% flood flow
200 bird species, and highest salmon population in CA
2/3 are in private agriculture
Multi-billion dollar investment

Early flood management plans (1911) for the State of California, U.S., included a system of levees, weirs, and bypasses, which protected a portion of the natural floodplains from development.

Yolo Bypass is the largest of these – a 240 km² area (65 kilometers long) within a natural floodplain depression along the west side of the Sacramento River (Opperman et al. 2009).

The Bypass is a prime example of the multi-purpose advantages of hybrid infrastructure.

A series of weirs control the rate and volume of water entering and leaving the bypass, which can hold more than 4.5 times as much water as the channel of the Sacramento River (Smalling et al. 2007), conveying 80 percent of its floodwaters during large events (Opperman et al. 2009).

Although flood management is the Bypass’s main purpose, it also provides valuable groundwater recharge to the drought stricken state and fosters abundant wildlife habitat, and recreation.

The Bypass is home to nearly 200 species of birds and sustains the highest salmon population in California (Sommer et al. 2001)

Yolo Bypass encompasses a 64 km² Wildlife Area managed by the state’s Department of Fish and Wildlife (Smalling et al. 2007; Opperman et al. 2009).

Further two-thirds of the Bypass is privately owned and used for productive agriculture, when it is not flooded (Opperman et al. 2009).

While Yolo Bypass is considered a greatly successful NBS with a wealth of co-benefits beyond flood control it is also a very expensive solution, requiring multi-billions of dollar of investment to date and recurring costs to ensure its performance and stability.

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References:

Opperman, J., G. Galloway, J. Fargione, J. Mount, B. Richter, and S. Secchi. 2009. “Sustainable Floodplains Through Large-Scale Reconnection to Rivers.” Science Magazine, Policy Forum, 326: 1487–88. https://doi.org/10.1126/science.1178256.
Smalling, K.L., J.L. Orlando, and K.M. Kuivila. 2007. “Occurrence of Pesticides in Water, Sediment, and Soil from the Yolo Bypass, California.” San Francisco Estuary and Watershed Science 5 (1). https://escholarship.org/content/qt9716926g/qt9716926g.pdf?nosplash=3e54395fafe4c3dd81565c6e96307d7d.
Sommer, T., B. Harrell, M. Nobriga, R. Brown, P. Moyle, W. Kimmerer, and L. Schemel. 2001. “California’s Yolo Bypass: Evidence That Flood Control Can Be Compatible with Fisheries, Wetlands, Wildlife, and Agriculture.” Fisheries 26 (8):6–16.

Image source: https://www.flickr.com/photos/usfws_pacificsw/32970121911