FLOODS

Hydrology and Water Resources RG 744Institute of Space TechnologyDecember 18 2013

Flood: Peak in a hydrograph Response of river to precipitation An unusually high stage of a river May fill up the stream up to its banks and

often spills over to the adjoining flood plain

Movement of Flood Wave

Flood may be considered as a wave that propagates downwards

In an ideal channel with frictionless fluid, flood wave may be considered traveling with no change from its point of origin

In natural channel energy is lost due to frictional forces

As a result magnitude of flood wave reduced or attenuated as it travels downstream

But discharge may also increase in downstream reaches due to increase in watershed area

Flood Estimates Estimate of extreme flood flow is required for

the design of hydraulic structures Proper selection of design flood value is of

great importance A higher value results in an increase in the cost of

hydraulic structures, An under-estimated value is likely to place the

structure and population involved at some risk Magnitude of flood may be estimated in

accordance with the importance of the structure

Design Flood Design flood may be defined as

The maximum flood that any structure can safely pass

The flood considered for the design of a structure corresponding to a maximum tolerable risk

The flood which a project (involving a hydraulic structure) can sustain without any substantial damage, either to the objects which it protects or to its own structures

The largest flood that may be selected for design as safety evaluation of a structure

Routing

Routing is the process of predicting temporal and spatial variation of a flood wave as it travels through a river (or channel) reach or reservoir

There are Hydrologic and Hydraulic Routings – here we will study Hydrologic Routing

Flood/Channel Routing

Once overland flow arrives at a stream it becomes channel flow

Routing To know how outflow from a reservoir/stream is

related to its inflow? What the downstream hydrograph (outflow) will

be if upstream hydrograph (inflow) is known?

Hydrologic Routing

Known parameters: you have a hydrograph at one location

(I) you have river characteristics

Need: a hydrograph at different location (O)

Flood Routing

2

4

1

3

Hydrograph computed at outlet of each subarea

Hydrographs routed to the outlet of the watershed

Hydrographs routed thru one reach of the watershed

I – Q = dS/dt

Routing Impact: Inflow, Outflow Hydrographs

Assume no seepage, leakage, evaporation or inflow other than main inflow

Hydrologic Routing In hydrologic routing techniques, equation of

continuity and an analytical/empirical relationship between storage and outflow in a river or reservoir are used

Continuity Equation:I – O = ΔS/Δt

Where:I = Inflow rateO = outflow rateΔS/Δt = rate of change of storage

Application of Flood Routing Flood prediction and flood warning Design of hydraulic structures (dams,

spillways, etc.) Evaluation of flood control measures Etc.

Two Flood Routing Problems

Reservoir Routing (storage routing): Study of effect of a flood wave entering a

reservoir River Routing (channel or stream flow

routing): change in shape of hydrograph as it

travels down a channel is studied

River vs. Reservoir Routing

During the advance of a flood wave, inflow exceeds outflow producing a wedge of storage

During the recession, outflow exceeds inflow, resulting in a negative wedge

Also, there is a prism of storage which is formed by a volume of constant cross-section along the length of prismatic channel

Assumption: X-sectional area of the flood flow is directly proportional to the discharge at that section S = KQ

Level pool reservoir

Flood Routing in River

Predicting temporal and spatial variation of a flood wave as it travel through a river reach

Flood waves passing down a river have their peaks attenuated due to storage characteristics of the stream reach if no lateral inflow is added

Muskingum Method

Estimates the transformation of flood wave as it moves through a river channel

S = KO + KX(I-O)S = storageO = OutflowI = inflowX = weighting factor that varies between 0

and 0.5K = storage time constant (T)

Muskingum Method

Muskingum Method

Muskingum Method

Muskingum Method

Homework: show derivation of the above simplified equations

Estimation of K, X and Δt

Example:

tp = 4 hr L = 2 miles Vavg = 2.5 ft/s K = ? X = ? Δt = ? Co =? C1 =? C2 =?

Example:

Flood Routing in Reservoir

Reservoirs are important in flood control because of large storage capacity

Downstream hydrograph peaks are smaller in magnitude and delayed in time

Peak flow at the upstream side of the reservoir is controlled in such a way that the flow at the downstream side is reduced to safe discharge

In figure below upstream hydrograph has higher peak with shorter base

Flood waves passing through a reservoir have their peaks attenuated and time base enlarge due to storageDownstream hydrograph with lower peak and broader base achieved by detaining flood water for some time by closing the spillways and then gradually releasing it

Flood routing involves Fixation of maximum reservoir level up

to which the structure is completely safe

Implementation of outflow pattern from the reservoir so that it may not create any danger in the downstream side

Inflow = outflow + change in storage

Zones of Storage and Levels

Reservoir Routing

Two relationships specific for reservoir

Reservoir Routing

Data required Inflow hydrograph Starting elevation above spillway

Storage-Indication Method Consider a reservoir having an ungated

spillway (weir, outlet discharge pipe) or gated spillway with fixed position

Use relationship between outflow (Q) and elevation head (H) for a sharp crested rectangular weir

Q = CLH3/2 Q = Discharge at the outlet (cfs) C = Discharge coefficient of weir (cfs) L = Length of crest (ft) H = Depth above spillway

Reservoir Routing

Reservoir Routing

Reservoir Routing

Routing Steps

Reservoir Routing

Estimation of Δt Δt < tp/5

Example

Example

Example

Flood Estimation

Rainfall-runoff modeling Flood Frequency Analysis (Statistical

methods)

Review: Flood Frequency Analysis There may be floods exceeding the design

specification of a structure What is a probability of those floods

occurring in any given year The probability of a flood exceeding or

equaling a given magnitude? Highest or peak discharges (floods) in each

year used for calculation Procedure for frequency calculation: Refer

lecture 3 under “ Recurrence Interval of a Storm”

Estimate the 100 year Flood?

Flood: Over topping of banks

History of Floods

Floods occurring along rivers, streams and in coastal area are natural events that have always been occurring throughout the history

Due to excessive rain runoff increases and streams or rivers overflow

Natural Floodplain Features

Floodplain: Normally dry land area adjoining rivers, streams, lakes, bays or ocean that is undated during flood events

Flood Losses and Damages

Lots of damages and death due to river flooding each year throughout the world

Damages caused by floods are sometimes aggravated by manmade factors e.g. Increasing urbanization Deforestation Uncontrolled development restricting waterways Floodplains used for agriculture or settlement

purposes

Flood Mitigation/Control Flood control is an important issue

throughout the world Measures to reduce or alleviate the

negative consequences of flooding Range of options to be considered in flood

protection schemes Both structural and non-structural approaches These approaches must be weighted in terms

of costs and benefits

Structural Approaches to Flood Control

Levees (dikes or flood embankments) Detention/retention Ponds, Dams/reservoirs Diversions Channel widening/modification

Due consideration to be given to the design of hydraulic structures to prevent from collapsing

Collapsing may also cause further damage by the force of water released from behind the structures

Involves huge sums of capital investment

Levees Earthen banks along the river course To confine river into a limited cross-

sectional width Heights of levees are higher than the

design flood level with sufficient free board

Flood Control Operation of a Reservoir

Non-Structural Approaches to Flood Control Floodplain zoning and Management Preservation of natural wetlands Flood forecasting and warning

system

Floodplain Management Corrective and preventive measures for

reducing flood damages. Some of the measures are

Floodplain management program Emergency preparedness plan Flood control works Floodplain management regulations

Defines flood hazard area Investigates problems arisen in developed

areas and potential problems due to future development

Floodplain ZoningZoning features of a regulated floodplainThe flood hazard area is generally defined

at the 100-year floodplain.

Development and Flooding Increasing urbanization leads to

increased overbank flows (floods) More runoff and more stream runoff

Floods of Pakistan

Flood season 15th June- 15th October

Ref: Flood Frequency Analysis

Notes1. Most modern dams are designed so that they can afford to

lose some storage capacity without their performance being impaired – the part of a reservoir known as "dead storage" which lies beneath the elevation of the dam’s lowest outlet. However sediments do not build up evenly along a horizontal plane, so that some "live storage" is usually lost long before the dead storage is filled. At Tarbela Reservoir in Pakistan, for example, 12 per cent of the live storage had been lost by 1992 (after 18 years of operation) while 55 per cent of the dead storage was still empty of sediment.

2. Absolute control over flood is rarely feasible either practically or economically. What we seek to do is to reduce flood damage to a minimum consistent with the cost involved.

River Management

Barriers: levees, floodwalls, storage basins, riprap

Adjustment: floodplain regulation (i.e. zoning)

Redesign: channelization (e.g. Trinity River)

Riprap

To add erosion-resistant material to a stream bank

Detention Ponds

Provide flood control Treat urban runoff Recreational spots

Dam Effect on Streams

Weir