Floodplain Management D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 Water Resources Systems Modeling

Floodplain Management

D Nagesh Kumar, IIScWater Resources Planning and Management: M8L5

Water Resources Systems Modeling

Objectives


To discuss flood control measures

To estimate damage caused by floods

To discuss HEC flood damage analysis package

Introduction


Floodplain: Land adjacent to the river or stream that is subjected to occasional or

periodic flooding

Acts as a conveyance or temporary storage for flood flows

Main objective of flood control is to reduce the negative consequences of floods

Flood control is normally done

By constructing structural measures such as reservoirs, diversions etc or

By nonstructural measures such as flood warning, flood proofing etc

Floodplain management comprises all engineering, nonstructural and

administrative measures to minimize the losses due to flooding

Flood Control Measures…


Flood control measures can be broadly classified into

1. Structural and

2. Nonstructural measures.

Three functional relationships are assessed to determine the value of these

flood control alternatives

1. Stage-damage relationships which give the flood severity in terms of damage

cost at various stages;

2. Stage-discharge relationships (or rating curves) and

3. Flood frequency relationships which give the recurrence interval vs. flood

magnitude

Flood Control Measures


Functional relationships without any control measures are as shown

Flood control measures modify one or more of these relationships to reduce

the damage due to flood

Damage cost Discharge Exceedance frequency

Functional relationships for flood assessment (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)

Structural measures


This includes

Flood control reservoirs

Diversion structures

Levees and

Channel modifications

Structural measures…


1. Flood control reservoirs Store flood waters and release it subsequently after the flood event Reducing the magnitude of peak discharge Frequency relationship is modified due to the decrease in the peak event

Long term use modify the stream regime and can result in channel degradation at

downstream sections, thereby changing the rating curve.


Effect of reservoir (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)



2. Diversion Structures

Divert flood flows from damage centers

Reduce the peak flows at those places

These structures also modify the frequency curve

Other two relationships are not altered.

Effect of diversion structures (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)




3. Levees Keep flood flows from flood plains damage centers Reduces the damage in protected areas from water surface stages in the main channel Modifies the stage-damage relationship by truncating it at a stage equal to the design elevation

of levee Conveyance area of the flood water is reduced due to the obstruction from levee This results in increased stages for discharges Hence, stage-discharge relationship is shifted upward This will also result in increase in downstream peak discharge, thereby shifting the frequency

curve upward

Effect of levees (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)




4. Channel Modifications Improve the conveyance characteristics by lowering the stage for a particular discharge This lowers the stage-discharge relationship Peak discharges are passed through at low stages decreasing the effect of natural valley

storage during a flood This results in high peak discharges downstream which causes an upward shift in the

frequency curve

Effect of channel modifications (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)


Nonstructural measures


These include Flood proofing, Flood warning and Land use controls

Flood proofing

Modify the damage potential of structure exposed to flood damage

This includes elevating structures, water proofing exterior walls, rearranging the

structural work space etc

It shifts the stage-damage relationship upward

Effect of channel modifications (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)


Nonstructural measures…


Flood warning

Reduces the damage by providing prior notice to the flood plain inhabitants

The lead time provides time to do minor flood proofing, or to remove the property

susceptible to flooding.

This needs real time forecasting and communication facilities.

Land use controls

Include many administrative actions such as zoning, building ordinances, building

codes etc.

Estimation of Flood Damage


Damages from floods may be classified as:

1. Direct damage from properties

2. Indirect damage from lost services and businesses, rerouting traffic etc.

3. Secondary damages from hardships to those depend on the outputs from the

damaged properties or hindered services

4. Intangible damages like environmental quality, social well-being and aesthetic

values and

5. Uncertainty damages due to uncertainty in flooding

Most common method for flood damage assessment is through empirical depth-

damage curves

Estimation of Flood Damage…


Individual or aggregated estimates of stage vs. damage curves for the crops,

structures, roads, utilities etc. in the floodplain need to be prepared through a

property suvery of the whole floodplain

Stage – damage relationship is then related with the stage – discharge relationship to

derive the damage – discharge relationship.

This is further related with the discharge – frequency relationship to derive the

damage – frequency relationship (illustrated in the next slide)

Estimation of Flood Damage…


Damage cost Discharge

Discharge Exceedance frequency

Stage – Damage Relation Stage – Discharge Relation

Damage – Discharge Relation Discharge - Frequency Relation

Exceedance frequency

Damage - Frequency Relation

Estimation of damages (Source: U.S. Army Corps of Engineers Hydrologic

Engineering Center)

HEC Flood Damage Analysis Package


Hydrologic Engineering Center Flood Damage Analysis (HEC-FDA) by U.S. Army

Corps of Engineers

A package of hydrologic, hydraulic and flood damage programs linked to data

storage system

Data storage system called HECDSS transfers data automatically between the

programs linked to it

Package contains

Three hydrologic and hydraulic programs

Five flood damage analysis programs

Three data management programs and

A library of data management software.



(i) Hydrologic analysis programs:

(a)HEC-1:

Flood hydrograph package

Simulates rainfall-runoff, simple reservoirs and hydrologic channel routing

Develops existing, without and modified flow frequency curves.

(b)HEC-2:

Water surface profiles

Computes steady-state, uniform flow profiles

Used to develop elevation-flow rating curves

(c)HEC-5:

Simulates flood control and conservation systems, complex reservoirs

Develops existing, without and modified flow frequency curves.



(ii) Flood damage analysis programs:

(a) Structural Inventory for Damage analysis, SID: Processes inventories of structures in the flood plain Develops elevation – damage relationships

(b) Structural Inventory for Damage Analysis Edit Program: Edits the structure inventory and damage function files for SID program

(c) Damage Reach State-Damage Calculation, DAMCAL : Same as SID, except develops elevation – damage relationships for a geographic

unit.

(d) Expected Annual Damage, EAD: Computes expected annual damage and compares flood damage mitigation plans

(e) FDA2PO: Computes the reference flood elevation at structure and stores stage-flow rating

curves



(iii) Data management programs

(a) Paired-function Input Program, PIP:

Inputs paired function relationship to a data storage system (DSS) data file.

(b) DSSUTL:

Performs utility functions on data stored in the HEC-DSS data file.

(c) DSPLAY:

Displays tables and plots in HEC-DSS data file.


Thank You

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Floodplain Management D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 Water Resources Systems Modeling