Implementing low impact commercial development in Black Creek

Christy Perrin, Water Resources Research Institute, NC Sea Grant; NCSU

Bill Hunt, PhD, PE, NCSU Dept. of Biological and Agricultural Engineering

Sarah Waickowski, PE, NCSU Dept. of Biological and Agricultural Engineering

Chris Mitchell, Daly Seven, Inc.

Gary McCabe, PE, Red Line Engineering

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June 29, 2022

Black Creek

• 3.3 mi²
• Urban/suburban mix
• Crabtree Creek-Neuse River

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Beechtree Neighborhood

LID hotel site

Black Creek assets drive continual progress

People

• A core team stays involved (since 2006)
• Educated, interested, engaged population
• Developer partners willing to step up to the plate
• Town of Cary staff supportive
• Multiple people lead projects, we meet ~ annually

Cause of Black Creek’s Impairment:

• Excessive stormwater flows and volume are washing stream

bottom habitat away

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• Secondary causes are nutrients and toxics (oil, pesticides) in runoff

Watershed Plan- 2009

Goal: Increased diversity and abundance of aquatic animals in Black Creek and tributaries by improving water quality

• Reduce volume and velocity of stormwater reaching Black Creek and tributaries
• Improve in-stream habitat

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HOW? Disconnect Impervious Surfaces from Creeks

• ~30% impervious cover
• 24% is “effective” impervious cover (pre-Neuse rules)
• How close to 11% will it take to see in-stream improvement?

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Projects in Ground/in Progress

• 24 residential “rainscapes”
• 3 rainwater harvesting cisterns
• 6 large bioretention areas (schools, church, park)
• West Cary Middle (2), Northwoods Elementary, Harvest Church, Kingswood Elementary
• Permeable walkways
• 2 stream restoration/stabilizations (via SWCD)
• Dam removal/stream restoration (via Beechtree HOA/Piedmont Conservation Council lead-in progress)

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Kingswood Elementary School GSI projects

Volunteers are active and ready to engage!

(2022 West Cary Middle School rain garden care event)

Benthic macro-invertebrate sampling indicated additional strategies may be needed

• Sampling occurred 2006 and 2016
• EPT taxa richness values consistently within a 3-6 taxa range suggesting Poor water quality conditions using DWR criteria.

http://penrose.consulting/

Existing Conditions (0% Impervious)

Original N. Harrison Hotel Proposal

Low Impact Development Plan

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6/23/2022

Underground infiltration

Bioretention area

Sun decks-educational opportunity

Low Impact Development Hotel: Goals

• Post-devpt. hydrology regime = pre-devpt. hydrology
• Monitor and document effectiveness of approach
• Share example with developers, Town of Cary, professionals state-wide

Low Impact Development: �Hotel Design/Construction

• Compare traditional design (stormwater pond) with LID design (use SWMM)
• Construct site using “hydrology protecting” SCMs (infiltrates/evapotranspirates)
• 0 - 10% of pre-development runoff from site

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Low Impact Development: �Hotel Design/Construction

• Runoff volume match is how DEQ defines “Low Impact Development.”
• 15A NCAC 02H .1003 (44) "Runoff volume match" means the annual runoff volume after development shall not be more than 10% higher than the annual runoff volume before development

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-NCDEQ Stormwater Design Manual

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Stream Impact Monitoring

• Compare traditional design (stormwater pond) with LID design (use SWMM)
• Pre-construction stream assessment
• Hydrology monitoring pre-construction, post-clearing, during/post construction
• Post-construction stream assessment

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Stream Impact Monitoring

Stream Impact Monitoring

• Per DEQ standard must discharge runoff in manner that reduces hydrologic impacts to streams
• Lack of technical information available to minimize impact

Stream Impact Monitoring

• Improve creek and research existing issue
• Continue hydrologic monitoring of stream through 1 year post-development
• Classify flow regimes to identify impacts of SCMs
• If promising, develop design criteria to meet regulations

Note- Stream impact monitoring is being conducted by NCSU BAE as unofficial cost share, it is not paid for by grant.

Infiltration Device Monitoring

• Infiltration devices under-studied in North Carolina
• Evaluate design standards
• Identify maintenance regimen

Who pays for what?

Cost of a wet-detention pond that meets requirements - $51,000

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Costs of LID SCM series:

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SCM#1 - Infiltration System: $238,700

SCM#2 – Bioretention: + $54,600

Total LID SCM Cost: $293,300

Difference LID SCMs - Conventional Wet-Detention Pond: $293,300- $51,000 = $242,300

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EPA 319 Grant request: $242,000/2= $121,000

Developer cost share amount: $242,000/2= $121,000

Total amount developer pays: $51,000+ $121,000= $172,000

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Educational activities and audiences

• Audience- developers and regulators
• Share results at Town of Cary Council & Environmental Advisory Board meetings
• Share results with design consultants, municipalities, state agencies via WRRI conference presentation and online story, social media, BAE workshops

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• Audience- watershed residents
• Three annual volunteer/service events with team
• BCWA Facebook & email list posts
• Celebration/tour after completion

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Additional questions

• How will maintenance be assured over time?
• The hotel owners will be required to inspect and maintain the SCMs annually, per the O&M Agreement. The owner has agreed to maintain LID SCMs and other stormwater elments.

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• Did a smaller SCM footprint result in more impervious surface planned for hotel (as a trade off?)
• Nope. 1) SCM footprint did not shrink as two large SCMS were used rather than many small integrated.
• 2) Developer requested a reduction in required parking spaces, reducing site imperviousness. This is another LID technique used on the site.

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Adaptive watershed management means trying and testing new strategies as we learn what is working and what needs to improve.

Questions:

Christy_Perrin@ncsu.edu

919-513-1152