Rainfall-Runoff modeling Rainfall-Runoff modeling • Forecasting and predicting Forecasting and predicting – Flood peaks Flood peaks – Runoff volumes Runoff volumes Due to Large rain and snowmelt events Due to Large rain and snowmelt events ***especially important when we have no ***especially important when we have no prior recorded experience. prior recorded experience. 1. 1. extreme flood producing rains extreme flood producing rains 2. 2. Major land-use changes Major land-use changes 3. 3. Altered climatic regimes Altered climatic regimes • Physical and empirical based models Physical and empirical based models
Rainfall-Runoff modeling. Forecasting and predicting Flood peaks Runoff volumes Due to Large rain and snowmelt events ***especially important when we have no prior recorded experience. extreme flood producing rains Major land-use changes Altered climatic regimes - PowerPoint PPT Presentation
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Rainfall-Runoff modelingRainfall-Runoff modeling
• Forecasting and predictingForecasting and predicting– Flood peaksFlood peaks– Runoff volumesRunoff volumesDue to Large rain and snowmelt eventsDue to Large rain and snowmelt events
***especially important when we have no prior recorded ***especially important when we have no prior recorded experience.experience.
– Inputs: hyetographsInputs: hyetographs– Produces an identifiable resultProduces an identifiable result
• Important pts.Important pts.1.1. Hyetograph represents only a portion of the total input in the Hyetograph represents only a portion of the total input in the
associated response of the hydrographassociated response of the hydrograph2.2. Hydrograph represents only the identifiable response to the Hydrograph represents only the identifiable response to the
current hyetograph.current hyetograph.• Does not include baseflow (earlier events)Does not include baseflow (earlier events)• Or portion of the streamflow after the event response.Or portion of the streamflow after the event response.
3.3. Determination of event-flow volume based on hydrograph Determination of event-flow volume based on hydrograph separation.separation.
4.4. Volume of effective water inputs=volume of event flowVolume of effective water inputs=volume of event flow5.5. Based on reasonable assumptions rather than hydrologic Based on reasonable assumptions rather than hydrologic
processes.processes.
implicit assumption
• Water that appears as output in the same water that is identified from the hyethograph.– In reality-substantial portion of the water
appearing is “old water”. Water that has entered the watershed from a previous event
Effective water input
• Weff=W-”losses” DSETlosses c
Design Floods v. Floods
• Models are used to generate design floods from actual storms.
– Design of culverts, bridges, flood retention basins, levees, dam spillways, or floodplain management.
1. Forecast flooding from in-progress storms.
2. Calibrate models.
3 models for design flows
1. Rational method• Urban areas
2. SCS method• Sub-urban and rural areas
3. Unit hydrograph• Generate design flows for large watersheds
Rational Method
SCS Method
• Most widely used method for design purposes.
Unit Hydrograph
• Most widely used transfer function for systems modeling.
• Definition: basin outflow resulting from 1”(1 cm) of runoff generated uniformly over the drainage area at a uniform rate.
Unit Hydrograph TheoryUnit Hydrograph Theory
13
Unit Hydrograph “Lingo”Unit Hydrograph “Lingo”
• Duration• Lag Time• Time of Concentration• Rising Limb• Recession Limb (falling limb)• Peak Flow• Time to Peak (rise time)• Separation• Base flow
14
Graphical RepresentationGraphical Representation
Lag time
Time of concentration
Duration of excess precipitation.
Base flow
15
Methods of Developing UHG’sMethods of Developing UHG’s
• The hydrograph that results from 1-inch of excess precipitation (or runoff) spread uniformly in space and time over a watershed for a given duration.
• The key points :1-inch of EXCESS precipitationSpread uniformly over space - evenly over the watershedUniformly in time - the excess rate is constant over the
time intervalThere is a given duration
UHG’sUHG’s
• 2 key assumptions for the development.2 key assumptions for the development.
1.1. Linearity-Linearity-Given a UHG, a hydrograph for a Given a UHG, a hydrograph for a runoff depth other than unity can be obtained runoff depth other than unity can be obtained by multiplying the UHG ordinates by the by multiplying the UHG ordinates by the indicated runoff depth. Therefore, the time indicated runoff depth. Therefore, the time base of the hydrograph is the same as the time base of the hydrograph is the same as the time base of the UHG.base of the UHG.
2.2. Superposition- Superposition- summation of corresponding summation of corresponding hydrograph or UHG ordinateshydrograph or UHG ordinates
Rules of Thumb :… the storm should be fairly uniform in nature and the excess precipitation should be equally as uniform throughout the basin. This may require the initial conditions throughout the basin to be spatially similar. … Second, the storm should be relatively constant in time, meaning that there should be no breaks or periods of no precipitation. … Finally, the storm should produce at least an inch of excess precipitation (the area under the hydrograph after
correcting for baseflow).
21
Deriving a UHG from a StormDeriving a UHG from a Stormsample watershed = 450 mi2sample watershed = 450 mi2
0
5000
10000
15000
20000
25000
Time (hrs.)
Flo
w (
cfs)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Pre
cip
itat
ion
(in
ches
)
22
Separation of BaseflowSeparation of Baseflow
... generally accepted that the inflection point on the recession limb of a hydrograph is the result of a change in the controlling physical processes of the excess precipitation flowing to the basin outlet.
In this example, baseflow is considered to be a straight line connecting that point at which the hydrograph begins to rise rapidly and the inflection point on the recession side of the hydrograph.
the inflection point may be found by plotting the hydrograph in semi-log fashion with flow being plotted on the log scale and noting the time at which the recession side fits a straight line.
23
Hydrograph & BaseflowHydrograph & Baseflow
0
5000
10000
15000
20000
25000
0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105
112
119
126
133
Time (hrs.)
Flo
w (
cfs)
24
Separate BaseflowSeparate Baseflow
0
5000
10000
15000
20000
25000
Time (hrs.)
Flo
w (
cfs)
25
Sample CalculationsSample Calculations
• In the present example (hourly time step), the flows are summed and then multiplied by 3600 seconds to determine the volume of runoff in cubic feet. If desired, this value may then be converted to acre-feet by dividing by 43,560 square feet per acre.
• The depth of direct runoff in feet is found by dividing the total volume of excess precipitation (now in acre-feet) by the watershed area (450 mi2 converted to 288,000 acres).
• In this example, the volume of excess precipitation or direct runoff for storm #1 was determined to be 39,692 acre-feet.
• The depth of direct runoff is found to be 0.1378 feet after dividing by the watershed area of 288,000 acres.
• Finally, the depth of direct runoff in inches is 0.1378 x 12 = 1.65 inches.
26
Obtain UHG OrdinatesObtain UHG Ordinates
• The ordinates of the unit hydrograph are obtained by dividing each flow in the direct runoff hydrograph by the depth of excess precipitation (Weff).
• In this example, the units of the unit hydrograph would be cfs/inch (of excess precipitation).
27
Final UHGFinal UHG
0
5000
10000
15000
20000
25000
0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105
112
119
126
133
Time (hrs.)
Flo
w (
cfs)
Storm #1 hydrograph
Storm#1 direct runoff hydrograph
Storm # 1 unit hydrograph
Storm #1 baseflow
28
Determine Duration of UHGDetermine Duration of UHG
• The duration of the derived unit hydrograph is found by examining the precipitation for the event and determining that precipitation which is in excess.
• This is generally accomplished by plotting the precipitation in hyetograph form and drawing a horizontal line such that the precipitation above this line is equal to the depth of excess precipitation as previously determined.
• This horizontal line is generally referred to as the -index and is based on the assumption of a constant or uniform infiltration rate.
• The uniform infiltration necessary to cause 1.65 inches of excess precipitation was determined to be approximately 0.2 inches per hour.