L5_ FloodRouting

8/3/2019 L5_ FloodRouting

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Prediction of Flood Regimes(contd.)

flood routing and prediction of inundation

remote sensing and other forms of inundation predictions

Flood Routing

Intense runoff from a watershed supplies large volumesof water to the valley floors of a river basin.

In some time interval (hour, day) the inflow volume into areach of valley floor (channels and floodplain) goes into

an increase in volume stored in the reach (i.e. an increase in thedepth of the flow

the outflow from the reach

Input = Output +/- Rate of change of storage

So, if I know (or can predict) the rate of inflow to a reachfrom its watershed including upstream reaches, and I cancalculate the resulting rate of change of storage, I cancompute the rate of outflow to the reach downstream

This process is known as flood routing

Two types of flood routing

Hydrologic Routing

Based only on the continuity equation: I = O +/-S/t

O = I -/+S/t

Reservoir Routing: combine cont. eq with an outflow-storage fn.

Channel Routing: combine cont. eq with an I-O-Sfn.

Hydraulic Routing

Based on continuity and hydraulic principles

Flow velocity in a reach of channel or floodplain depends onaverage flow depth and flow resistance

Mannings equation:

Water level in reservoir low: outflow rate small

O S


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Water level in reservoir higher; outflow rate is

higher

SO

Reservoir routing rule

Outflow =f( stage) i.e.

Outflow =f(volume stored)

Calculate relationship between stage and volume storedfrom shape of the reservoir

Need to specify a functional relationship, which willdepend on the shape and size of the outflow control

(pipe, spillway)

E.g. O = aSb

For each interval compute S and Oin a book-keepingformat

Routing of a flood wave through a reservoir with aspecific area and outlet geometry

See Water in Environmental Planningby Dunne and Leopold for details of computation scheme

Reservoir routing:

S = gOh

S = kO

Channel routing (e.g. Muskingum approximation):

- S = K[xI + (1-x)O]

O SO


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Muskingum routing of a hydrograph through a channel reach

See Water in Environmental Planningby Dunne and Leopold for details of computation scheme

IRAQ

KUWAIT

SAUDI ARABIA

JORDAN

SYRIA

TURKEY

IRAN

Gulf

Baghdad

Euphrates

Tigris

TURKEY

KebanKarakayaAtaturk

SYRIATabaqa

IRAQHaditha DamRamadi BarageHabbania Lake

(Warrar &Deban) ThartharOutletFeluja BarageHindya Barag

Turkeys GapProject

Ongoing

Tabaqa Dam

Haditha Dam

RamadiBarrage

Warrar Regulator,

Habbaniya Lake, DibanLake, Mujjarah

Regulator, Abu DibbisDepression/Razzaza

Tharthar LakeOutlet

Feluja Barrage

Hindiya BarrageKarkhah

Ataturk Dam, Turkey R. Euphrates hydrograph


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Hydraulic routing through a reach of

channel/floodplain

Based on continuity equation and hydraulicprinciples

Flow velocity in a reach of channel orfloodplain depends on average flow depth andflow resistance

Mannings equation:

Hydraulic Routing with Mannings Equation

for steady uniform flow

wdvQ =

Can apply to whole cross section of channel or to some increment of width

n

sdv

21

32

=

Metric

In the units formerly known as British

n

sd5.1v

21

32

=

Steady uniform flow downstream in achannel-floodplain system

Q is the sum of three channels coupled by a horizontal water surface

Floodplain A Floodplain B

Channel

Back-calculated n-value 0.10; flow depth 0.7 m

Arcement, G.J., Jr. and V. R. Schneider, Guide for selecting Mannings roughness coefficients for

natural channels and floodplains, US Geological Survey Water-Supply Paper 2339


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n = 0.11; flow depth = 0.9 m

Arcement, G.J., Jr. and V. R. Schneider, Guide for selecting Mannings roughness coefficients fornatural channels and floodplains, US Geological Survey Water-Supply Paper 2339

n = 0.13; flow depth ~1m


n = 0.20; flow depth 0.9 m


Gradually varied flows:step-backwater calculation

Downstream control

Q

H1

H2

H3

H4

Values of H and velocity at each cross section computed in an upstream-moving sequence beginning at some downstream control (e.g. a majorriver or sea level) where the bed elevation and water surface are known.

Method is known as a step-backwater calculation

Requires surveyed cross sections and bed long profile and estimates ofMannings nfor each cross section and flow component


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Gradually varied flows

Downstream control

Q

H1

H2

H3

H4

HEC-RAS does step-backwater calculations for gradually varied f lowthrough a sequence of cross sections across a channel and floodplain.

Can incorporate a lot of richness in representation of landform geometryand basin hydrology

Overbank flood in Merced R

.

Overbank flood in Merced R, Mar. 31, 2005.

Scenario exploration with hydraulic routing:setting back levees for risk reduction and

floodplain habitat


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Scenario exploration with hydraulic routing: Re-

vegetation of floodplain and riparian zoneGradually varied flows

Downstream control

Q

H1

H2

H3

H4

HEC-RAS does step-backwater calculations for gradually varied f lowthrough a sequence of cross sections across a channel and floodplain.

Can incorporate a lot of richness in representation of landform geometryand basin hydrology

Two-dimensional flow modeling on a floodplain requires high-resolution digital elevation data (1990s)

1 meter contour elevation map

AnimationScenarioA


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Two types of flood routing

Hydrologic Routing Reservoir Routing

Requires knowledge of inflow rate, reservoir geometry (stage-volume relationship), and outlet geometry(stage-outflow relationship)

Doesnt require previous gauging

Software packages such as HEC-HMS, HEC-5

Channel Routing (Muskingum)

Requires estimation of bulk parameters of flow in a reach, back-calculated from measurements atgauges

Software packages such as HEC-HMS

Hydraulic Routing

Requires large amounts of data on channel cross section geometry, gradient, hydraulic roughness ofchannel and floodplain

If these static data are available, the method doesnt require gauging, though direct measurements of

stage-discharge relationships (from a gauge or post-flood survey) are useful for estimation of hydraulicroughness

Can be used to explore design scenarios (e.g. setting back levees; altering floodplain and channelroughness during re-vegetation; altering channel geometry or roughness through dredging)

Software packages such as HEC-RAS

See http://www.azwater.gov/dwr/Content/Publications/files/ss9-02FloodplainModeling1.PDF for adetailed review of applications to complex situations

Relation between river leveland area of inundation in

tropical wetlands (Hamilton,Melack, et al.,

Documents

L5_ FloodRouting