L5_ FloodRouting
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Transcript of 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
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 = 0.20; 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
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.,