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

Objectives D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 2  To discuss flood control measures  To estimate damage caused by floods  To discuss HEC flood damage analysis package

Introduction D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 3 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… D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 4 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 D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 5 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 costDischarge Stage Discharge Exceedance frequency Functional relationships for flood assessment (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)

Structural measures D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 6 This includes Flood control reservoirs Diversion structures Levees and Channel modifications

Structural measures… D Nagesh Kumar, IISc Water Resources Planning and Management: M8L 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. Damage costDischarge Stage Discharge Exceedance frequency Effect of reservoir (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)

Structural measures… D Nagesh Kumar, IISc Water Resources Planning and Management: M8L 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) Damage costDischarge Stage Discharge Exceedance frequency

Structural measures… D Nagesh Kumar, IISc Water Resources Planning and Management: M8L 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) Damage costDischarge Stage Discharge Exceedance frequency

Structural measures… D Nagesh Kumar, IISc Water Resources Planning and Management: M8L 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) Damage costDischarge Stage Discharge Exceedance frequency

Nonstructural measures D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 11 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) Damage costDischarge Stage Discharge Exceedance frequency

Nonstructural measures… D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 12 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 D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 13 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… D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 14 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… D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 15 Damage costDischarge Stage Damage Discharge Exceedance frequency Stage – Damage Relation Stage – Discharge Relation Damage – Discharge Relation Discharge - Frequency Relation Damage Exceedance frequency Damage - Frequency Relation Estimation of damages (Source: U.S. Army Corps of Engineers Hydrologic Engineering Center)

HEC Flood Damage Analysis Package D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 16 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.

HEC Flood Damage Analysis Package D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 17 (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.

HEC Flood Damage Analysis Package D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 18 (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

HEC Flood Damage Analysis Package D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 19 (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.

D Nagesh Kumar, IISc Water Resources Planning and Management: M8L5 Thank You 20