12-runoffprocesses
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CE 374 K – CE 374 K – HydrologyHydrologyRunoff ProcessesRunoff Processes
Daene C. McKinneyDaene C. McKinney
WatershedWatershed WatershedWatershed
Area draining to a Area draining to a streamstream
Streamflow generated Streamflow generated by water entering by water entering surface channelssurface channels
Affected byAffected by Physical, vegetative, Physical, vegetative,
and climatic featuresand climatic features Geologic considerationsGeologic considerations Stream PatternsStream Patterns
Dry periodsDry periods Flow sustained from Flow sustained from
groundwater (baseflow)groundwater (baseflow)
http://www.epa.gov/owow/watershed/whatis.html
StreamflowStreamflow Atmospheric Atmospheric
WaterWater EvapotranspirEvapotranspir
ationation PrecipitationPrecipitation
Subsurface Subsurface WaterWater InfiltrationInfiltration GroundwaterGroundwater
Surface Surface WaterWater
Atmospheric Moisture
Interception
Snowpack
Surface
Soil Moisture
Groundwater
Streams and Lakes
Runoff
RainSnow
Evaporation
Evapotranspiration
Evaporation
Throughfall and Stem Flow
Snowmelt
Infiltration
Overland Flow
Percolation
Groundwater Flow
Channel Flow
Pervious Impervious
Energy
WatershedBoundary
Streamflow HydrographStreamflow Hydrograph
Peak
Ris
ing L
imb
Recessio
n L
imb
Time
Dis
ch
arg
e,
Q
Beginning of Direct Runoff
BaseflowRecession Baseflow
Recession
Centroid of Precipitation
Basin Lag
Time of Rise
End of Direct Runoff
InflectionPoint
Baseflow SeparationBaseflow Separation
No inflow added to No inflow added to groundwater - groundwater - depletion depletion (recession) curve(recession) curve
Continuity Continuity equationequation
][constant decay
at time flow
at time flow)(
00
Tk
tQ
ttQ
Time
Dis
ch
arg
e,
Q
BaseflowRecession
ktteQtQ /)(0
0)(
)()( tQtIdt
dS
dteQdS ktt /)(0
0
)()( tkQtS
Time
Dis
ch
arg
e,
QA B
Baseflow
Direct Runoff
Baseflow Separation Baseflow Separation TechniquesTechniques
Straight – line Straight – line methodmethod Draw a horizontal Draw a horizontal
line segment (A-B) line segment (A-B) from beginning of from beginning of runoff to runoff to intersection with intersection with recession curverecession curve
Time
Dis
ch
arg
e,
QA B
D
C
2.0AN
Baseflow
Direct Runoff
Baseflow Separation Baseflow Separation TechniquesTechniques
Fixed Base MethodFixed Base Method Draw line segment Draw line segment
(A – C) from (A – C) from baseflow recession baseflow recession to a point directly to a point directly below the below the hydrograph peak hydrograph peak
Draw line segment Draw line segment (C-D) connecting a (C-D) connecting a point N time point N time periods after the periods after the peakpeak
Time
Dis
ch
arg
e,
QA B
C
E
Baseflow Separation Baseflow Separation TechniquesTechniques
Variable Slope Variable Slope MethodMethod Draw line segment (A-Draw line segment (A-
C) C) forwardforward from from baseflow recession to baseflow recession to a point directly below a point directly below the hydrograph peakthe hydrograph peak
Draw line segment (B-Draw line segment (B-E) E) backward backward from from baseflow recession to baseflow recession to a point directly below a point directly below the inflection pointthe inflection point
Draw line segment (C-Draw line segment (C-E)E)
Baseflow
Direct Runoff
Abstraction (Losses) Abstraction (Losses) EstimationEstimation
Phi – Index MethodPhi – Index Method Excess (effective) Excess (effective)
rainfall rainfall Rainfall that is not Rainfall that is not
retained or infiltratedretained or infiltrated Becomes direct runoffBecomes direct runoff Excess rainfall hyetograph Excess rainfall hyetograph
(excess rainfall vs time)(excess rainfall vs time) Abstraction (losses)Abstraction (losses)
Difference between total Difference between total and excess rainfall and excess rainfall hyetographshyetographs
Phi – IndexPhi – Index Constant rate of Constant rate of
abstraction yielding abstraction yielding excess rainfall hyetograph excess rainfall hyetograph with depth equal to depth with depth equal to depth of direct runoffof direct runoff
M
mmd tRr
1
interval time
runoffdriecttongcontributi
rainfallofintervals#
indexPhi
rainfall observed
runoffdirect ofdepth
t
M
R
r
m
d
ExampleExampleTime Observed
Rain Flow
in cfs
8:30 203
9:00 0.15 246
9:30 0.26 283
10:00 1.33 828
10:30 2.2 2323
11:00 0.2 5697
11:30 0.09 9531
12:00 11025
12:30 8234
1:00 4321
1:30 2246
2:00 1802
2:30 1230
3:00 713
3:30 394
4:00 354
4:30 303
0
2000
4000
6000
8000
10000
12000
7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM
Time
Str
eam
flo
w (
cfs)
0
0.5
1
1.5
2
2.5
No direct runoff until after 9:30And little precip after 11:00
Have precipitation and streamflow data, need to estimate losses
Basin area A = 7.03 mi2
Example (Cont.)Example (Cont.)
Estimate baseflow (straight line Estimate baseflow (straight line method)method) Constant = 400 cfsConstant = 400 cfs
0
2000
4000
6000
8000
10000
12000
7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM
Time
Str
eam
flo
w (
cfs)
baseflow
Example (Cont.)Example (Cont.)
Calculate Direct Calculate Direct Runoff HydrographRunoff Hydrograph Subtract 400 cfsSubtract 400 cfs
Total = 43,550 cfs
Example (Cont.)Example (Cont.)
Compute volume of direct runoffCompute volume of direct runoff
37
3
11
1
11
1
ft10*7.839
/sft 550,43*hr5.0*s/hr3600
n
nn
nd QttQV
Compute depth of direct runoffCompute depth of direct runoff
in80.4
ft4.0
ft5280*mi03.7
ft10*7.83922
37
A
Vr dd
Example (Cont.)Example (Cont.)
Neglect all precipitation intervals that Neglect all precipitation intervals that occur before the onset of direct runoff occur before the onset of direct runoff (before 9:30)(before 9:30)
Select Select RRmm as the precipitation values in as the precipitation values in the 1.5 hour period from 10:00 – 11:30the 1.5 hour period from 10:00 – 11:30
)5.0*3*08.220.233.1(80.41
M
mmd tRr
in27.0t
in54.0
in80.4dr
Example (Cont.)Example (Cont.)
0
2000
4000
6000
8000
10000
12000
7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM
Time
Stre
amflo
w (c
fs)
0
0.5
1
1.5
2
2.5
t=0.27
SCS Curve Number SCS Curve Number MethodMethod
Soil Conservation Service(SCS) Curve Number (CN) Soil Conservation Service(SCS) Curve Number (CN) model estimates precipitation excess as a function of model estimates precipitation excess as a function of cumulative precipitation, soil cover, land use, and cumulative precipitation, soil cover, land use, and antecedent moistureantecedent moisture
SCS developed the method for small basins (< 400 sq. SCS developed the method for small basins (< 400 sq. mi.) to "before" and "after" hydrologic response from mi.) to "before" and "after" hydrologic response from events. events.
Classify soils (60 or 70 types) into four hydrologic soil Classify soils (60 or 70 types) into four hydrologic soil groupsgroups
Method is simple enough to be used by people that Method is simple enough to be used by people that have little experience with hydrology. have little experience with hydrology.
Converts basin storage into something simpler and Converts basin storage into something simpler and more manageable (a “curve number” CN) more manageable (a “curve number” CN)
Abstractions – SCS Abstractions – SCS MethodMethod
In generalIn general
After runoff beginsAfter runoff begins
Potential runoffPotential runoff
SCS AssumptionSCS Assumption
Solve for Rainfall Solve for Rainfall ExcessExcess
Time
Pre
cip
itati
on
pt
aI aF
eP
aae FIPP
StorageMaximumPotentialS
nAbstractioContinuing
nAbstractioInitial
Excess Rainfall
Rainfall Total
a
a
e
F
I
P
P
PPe
SFa
aIP
a
ea
IP
P
S
F
SIP
IPP
a
ae
2
SCS Method (Cont.)SCS Method (Cont.)
Experiments Experiments showedshowed
SoSo
SIa 2.0
SP
SPPe 8.0
2.0 2
0
1
2
3
4
5
6
7
8
9
10
11
12
0 1 2 3 4 5 6 7 8 9 10 11 12
Cumulative Rainfall, P, in
Cu
mu
lati
ve D
irec
t R
un
off
, P
e, i
n
100
90
80
70
60
40
20
10
SurfaceSurface Impervious: CN = Impervious: CN =
100100 Natural: CN < 100Natural: CN < 100
100)CN0Units;American(
101000
CN
S
100)CN30Units;SI(
25425400
CNCN
S
SCS Method (Cont.)SCS Method (Cont.)
S and CN depend on antecedent S and CN depend on antecedent rainfall conditionsrainfall conditions
Normal conditions, AMC(II)Normal conditions, AMC(II) Dry conditions, AMC(I)Dry conditions, AMC(I)
Wet conditions, AMC(III)Wet conditions, AMC(III)
)(058.010
)(2.4)(
IICN
IICNICN
)(13.010
)(23)(
IICN
IICNIIICN
5-day antecedent rainfall 5-day antecedent rainfall (in)(in)
AMC GroupAMC Group Dormant Dormant seasonseason
Growing Growing seasonseason
II < 0.50< 0.50 < 1.4< 1.4
IIII 0.5 -- 1.10.5 -- 1.1 1.4 – 2.11.4 – 2.1
IIIIII > 1.1> 1.1 > 2.1> 2.1
SCS Method (Cont.)SCS Method (Cont.)
SCS Curve Numbers depend on soil SCS Curve Numbers depend on soil conditionsconditionsGroupGroup Minimum Minimum
Infiltration Rate Infiltration Rate (in/hr)(in/hr)
Soil typeSoil type
AA 0.3 – 0.450.3 – 0.45 High infiltration rates. Deep, High infiltration rates. Deep, well drained sands and gravelswell drained sands and gravels
BB 0.15 – 0.300.15 – 0.30 Moderate infiltration rates. Moderate infiltration rates. Moderately deep, moderately Moderately deep, moderately well drained soils with well drained soils with moderately coarse texturesmoderately coarse textures
CC 0.05 – 0.150.05 – 0.15 Slow infiltration rates. Soils Slow infiltration rates. Soils with layers, or soils with with layers, or soils with moderately fine textures moderately fine textures
DD 0.00 – 0.050.00 – 0.05 Very slow infiltration rates. Very slow infiltration rates. Clayey soils, high water table, Clayey soils, high water table, or shallow impervious layer or shallow impervious layer
Example - SCS Method - Example - SCS Method - 11
Rainfall: 5 in. Rainfall: 5 in. Area: 1000-acArea: 1000-ac Soils: Soils:
Class B: 50%Class B: 50% Class C: 50%Class C: 50%
Antecedent moisture: AMC(II)Antecedent moisture: AMC(II) Land useLand use
Residential Residential 40% with 30% impervious cover40% with 30% impervious cover 12% with 65% impervious cover12% with 65% impervious cover
Paved roads: 18% with curbs and storm Paved roads: 18% with curbs and storm sewerssewers
Open land: 16%Open land: 16% 50% fair grass cover50% fair grass cover 50% good grass cover50% good grass cover
Parking lots, etc.: 14%Parking lots, etc.: 14%
Example (SCS Method – Example (SCS Method – 1, Cont.)1, Cont.)
Hydrologic Soil GroupHydrologic Soil Group
BB CC
Land useLand use %% CNCN ProductProduct %% CNCN ProductProduct
Residential (30% imp Residential (30% imp cover)cover)
2020 7272 14.4014.40 2020 8181 16.2016.20
Residential (65% imp Residential (65% imp cover)cover)
66 8585 5.105.10 66 9090 5.405.40
RoadsRoads 99 9898 8.828.82 99 9898 8.828.82
Open land: good coverOpen land: good cover 44 6161 2.442.44 44 7474 2.962.96
Open land: Fair coverOpen land: Fair cover 44 6969 2.762.76 44 7979 3.163.16
Parking lots, etcParking lots, etc 77 9898 6.866.86 77 9898 6.866.86
TotalTotal 5050 40.3840.38 5050 43.4043.40
8.8340.4338.40 CN
Example (SCS Method – 1 Example (SCS Method – 1 Cont.)Cont.)
Average AMCAverage AMC
Wet AMCWet AMC3.92
8.83*13.010
8.83*23
)(13.010
)(23)(
IICN
IICNIIICN
in25.3
93.1*8.05
93.1*2.05
8.0
2.0 22
SP
SPPe
in93.1108.83
1000 S
8.83CN
in13.4
83.0*8.05
83.0*2.05
8.0
2.0 22
SP
SPPe
in83.0103.92
1000 S
101000 CN
S
Example (SCS Method – Example (SCS Method – 2)2)
Given P, CN = 80, AMC(II)Given P, CN = 80, AMC(II) Find: Cumulative abstractions and excess rainfall Find: Cumulative abstractions and excess rainfall
hyetographhyetographTime Time (hr)(hr)
CumulatCumulativeive
Rainfall Rainfall (in)(in)
Cumulative Cumulative
Abstractions Abstractions (in)(in)
CumulativeCumulative
Excess Rainfall Excess Rainfall (in)(in)
Excess RainfallExcess Rainfall
Hyetograph (in)Hyetograph (in)
PP IaIa FaFa PePe
00 00
11 0.20.2
22 0.90.9
33 1.271.27
44 2.312.31
55 4.654.65
66 5.295.29
77 5.365.36
Example (SCS Method – 2)Example (SCS Method – 2)
Calculate storageCalculate storage Calculate initial abstractionCalculate initial abstraction Initial abstraction removes Initial abstraction removes
0.2 in. in 10.2 in. in 1stst period (all the period (all the precip)precip)
0.3 in. in the 20.3 in. in the 2ndnd period (only period (only part of the precip)part of the precip)
Calculate continuing Calculate continuing abstractionabstraction
in50.21080
100010
1000
CNS
a
ea IP
PSF
in5.05.2*2.02.0 SIa
aae FIPP )0.2(
)5.0(5.2
)(
)(
P
P
SIP
IPSF
a
aa
in34.0)0.29.0(
)5.09.0(5.2hr)(2
aF
Time Time (hr)(hr)
CumulativeCumulative
Rainfall Rainfall (in)(in)
PP
00 00
11 0.20.2
22 0.90.9
33 1.271.27
44 2.312.31
55 4.654.65
66 5.295.29
77 5.365.36
Example (SCS method – Example (SCS method – 2)2)
Cumulative abstractions can now be calculatedCumulative abstractions can now be calculated
Time Time (hr)(hr)
CumulatCumulativeive
Rainfall Rainfall (in)(in)
Cumulative Cumulative
Abstractions Abstractions (in)(in)
PP IaIa FaFa
00 00 00 --
11 0.20.2 0.20.2 --
22 0.90.9 0.50.5 0.340.34
33 1.271.27 0.50.5 0.590.59
44 2.312.31 0.50.5 1.051.05
55 4.654.65 0.50.5 1.561.56
66 5.295.29 0.50.5 1.641.64
77 5.365.36 0.50.5 1.651.65
)0.2(
)5.0(5.2
P
PFa
Example (SCS method – Example (SCS method – 2)2)
Cumulative excess rainfall can now be calculatedCumulative excess rainfall can now be calculated Excess Rainfall Hyetograph can be calculatedExcess Rainfall Hyetograph can be calculated
Time Time (hr)(hr)
CumulatiCumulativeve
Rainfall Rainfall (in)(in)
Cumulative Cumulative
Abstractions (in)Abstractions (in)CumulativeCumulative
Excess Rainfall Excess Rainfall (in)(in)
Excess RainfallExcess Rainfall
Hyetograph (in)Hyetograph (in)
PP IaIa FaFa PePe
00 00 00 -- 00 00
11 0.20.2 0.20.2 -- 00 00
22 0.90.9 0.50.5 0.340.34 0.060.06 0.060.06
33 1.271.27 0.50.5 0.590.59 0.180.18 0.120.12
44 2.312.31 0.50.5 1.051.05 0.760.76 0.580.58
55 4.654.65 0.50.5 1.561.56 2.592.59 1.831.83
66 5.295.29 0.50.5 1.641.64 3.153.15 0.560.56
77 5.365.36 0.50.5 1.651.65 3.213.21 0.060.06
aae FIPP
Time of ConcentrationTime of Concentration
Different areas of a watershed Different areas of a watershed contribute to runoff at different contribute to runoff at different times after precipitation beginstimes after precipitation begins
Time of concentrationTime of concentration Time at which all parts of the watershed Time at which all parts of the watershed
begin contributing to the runoff from begin contributing to the runoff from the basinthe basin
Time of flow from the farthest point in Time of flow from the farthest point in the watershedthe watershed
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