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Chapter-6�Hydrology of Floods

6.1 Definition, causes, effects and mitigation measures of floods

Flood:- Flood is an unusually high stage (or water level) in a river, in which, normally, river overflows its banks and inundates the adjoining area.

Causes of floods:- Floods are caused by natural phenomena but may be increased by human intervention.

A. Natural

  • Continuous rainfall and cloudburst.
  • Landslides: blocking of river by landslides
  • Dam outburst: sudden release of huge amount of water stored in the dam due to failures.
  • Glacier lake outburst snow and glacier melt.
  • Synchronization of peak flow of rivers.
  • Sea storm
  • Earthquake in sea (Tsunami)

B. Human intervention:

  • Land use changes e.g. deforestation, urbanization.
  • Drainage congestion caused by uncoordinated development activities.
  • Structural failures e.g. dam, embankment failure.

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Effects of flood:-

  • Loss of life.
  • Loss of property
  • Destruction of physical infrastructure.
  • Disruption of social and economic development
  • Damage to agriculture.
  • Damage to hydraulic structures such as bridge, embankment.
  • Sometime effects on transportation.
  • Damage to reservoirs and dams.

Mitigation measures of floods:-

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A. Structural measures

B. Non Structural Measures

Storage reservoir:- Flood water is stored in the reservoir and released in a controlled manner over an extended life.

Land Use Regulation:- This involves regulation of land reclamation and regulation of development

Detention reservoir:- It is an obstruction in river in the form of small structure. It is used for storing water temporarily and restricting the outflow rate.

Flood proofing facilities.

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��Mitigation measures of floods:- contd….�

Flood embankment (Levees, dikes):- It is an earthen bank constructed parallel to river course to confine it to fixed course and limited cross-sectional width. The heights of embankment will be higher than design flood level with sufficient free board. This is the most common methods of flood protection works. The cross section of embankment is designed like an earth dam.

Flood pain Zoning:- Flood pain zoning is a map which shows the location and extent of areas likely to be inundated due to floods of different return periods. Development plans of these areas are prepared in such a manner that the resulting damages due to flood are within acceptable limits of risk.

Floodways:- They are natural or manmade channel to divert flood.

Flood forecasting and warning:- flood forecasting is an expanding area of application of hydrologic techniques.

Soil conservation:- Soil conservation measures increase infiltration and evaporation, reduce soil erosion and reduce the runoff

Evacuation and relocation:- Evacuation of people and goods in the flood affected area and relocation of them in a nearby area is also a non structural measure of flood management.

Channel improvement:- Widening or deepening channel, reduction of roughness, short circuiting of meander loops.

Flood insurance:- flood insurance reduces the impact of loss burden.

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Training and education about flood awareness.

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Design flood and its efficiency:-

  • A flood used for the design of structure on considerations of its safety, economy, life expectancy and probable damage considerations is called as “design flood”.
  • Small structures such as culverts and storm- drainages can be designed for less severe floods as the consequence of higher than design flood will not be very serious.
  • Larger structures such as dams demands greater attention to the magnitude of floods used in the design, because of failure of these structures causes large loss of life and property on the downstream of structures
  • Choosing appropriate magnitude of flood as
  • “design flood” depends upon;
    • Types of structure
    • Importance of the structure
    • Economic development in the surrounding area

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��Different types of design floods are;�

1. Frequency-based flood (FBF): mostly used for designing spillways (also called as Spillway design flood).

      • design flood estimated using flood frequency analysis (e.g., 10 yr, 20 yr, 50 yr, 100 yr floods)

2. Probable maximum flood (PMF):

      • the extreme flood that is physically possible in a region as a severe-most combinations. It is
  • estimated based on unit hydrograph & PMP (probable maximum precipitation)

3. Standard project flood (SPF):

      • flood computed from standard project storm occurred over the project area or on the adjoining
  • areas with similar hydro-meteorological and basin characteristics
      • Generally 40-60% of PMF for the same drainage basin.
  • Frequency refers to the number of occurrences of a variate (i.e., an individual
  • observation or the value of any variable)
    • A plot of frequency against the variate is called as frequency distribution

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Return Period, Frequency & Risk

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Estimating Flood Peak – Rational Method

  • It’s the most widely used method for the analysis of runoff response from small catchments
  • Though it’s simple, a reasonable care is necessary to apply it effectively.
  • It’s particular application is in urban storm drainage  to estimate peak runoff rates
  • for the design of storm sewers and small drainage facilities.
  • The hydrologic characteristics or processes that Rational method accounts for are;
    • Rainfall intensity
    • Rainfall duration
    • Rainfall frequency
    • Catchment area
    • Hydrologic abstractions
    • Runoff concentration,
    • Runoff diffusion: a measure of the catchment’s ability to attenuate the flood peaks.

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Contd…..

  • If a rainfall of uniform intensity and very long duration is occurring over a catchment, the runoff rate gradually increases from zero to a constant value (see figure), then the peak value of runoff at the outlet, a per Rational Method, is given by;
    • QP = C * i * A (for t ≥ tc) ; where, C is the coefficient of runoff (= runoff/ rainfall), i = intensity of rainfall, A = catchment area; tc = time of concentration.
    • In SI unit, with Q in m3/s, “i” in mm/hr, and “A” in km2, the equation is modified as; QP = 1/3.6 * C * i * A .
  • Limitations of Rationale Method
    • Applicable for small-sized catchments (< 50 km2)
    • Rainfall intensity must be of constant over the entire basin during tc. And, duration of rainfall intensity > tc
    • Gives only peak, but not a complete hydrograph
    • C assumed to be small for all storms

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  • The rational method does NOT take into account these characteristics or processes;
    • Spatial or temporal variations in either total or effective rainfall
    • Time of concentration much greater than storm duration
    • A significant portion of runoff occurring in the form of streamflow
  • The Rational Method also does NOT explicitly account for the catchment’s antecedent moisture content, however it may be implicitly accounted for by varying the runoff coefficient.
  • Limit of catchment area:
    • Upper limit: There is no consensus on upper limit of catchment area to apply Rational Method. However, current trend is to use area <=2.5 km2 as upper limit.
    • Lower limit: There is no theoretical lower limit. Catchments as small as 1 ha or less can be analysed using rational method.

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�Time of Concentration (tc):�

    • Time required to a drop of water to flow from the farthest part of the catchment to reach
  • the outlet. If rainfall continues beyond tc, the runoff will be constant.

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    • Number of empirical equations are available for the estimation of tc.

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    • The most commonly used method is, Kirpich Equation (1940) tc = 0.01947 * L 0.77 * S – 0.385; where, tc is the time of concentration (in minutes), L is the maximum length of travel of water or the longest flow path (in meters), S is slope of the catchment = ΔH/L, in which, ΔH is the difference in elevation between the most remote point on the catchment and the outlet.

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  • Runoff Coefficient (C):
    • Value of C depends upon integrated effects of the catchment losses, therefore, depends on
      • Nature of surface
      • Surface slope
      • Rainfall intensity
    • Value of C ranges from 0.1 (heavy forest) to 1.0 (rocky and permeable soil). Please refer
  • Table 7.1 in K. Subramanya book for value of C for various types of areas
    • If the catchment is non-homogenous (which is usually the case!), value of C for each sub- basin can be calculated using weighted average C, where Ci, Ai are runoff coefficient and catchment area of iTH sub-basin.

∑(C1 A1 + C2 A2 + C3 A3 + ..)

C = A1 + A2 + A3 + ..

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