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Introduction to Surface Water Hydrology and Watersheds
Lecture 1
Philip B. BedientRice University
November, 2000
The Hydrologic Cycle
100 Precipitation on land
Infiltration
Groundwater flow
1 Groundwater discharge
38 Surface discharge
61 Evaporation from land
39Moisture over land
385Precipitation
on ocean
424 Evaporationfrom ocean
Surface runoff
Impervious strata
Groundwater Recharge
Precipitation
Snow melt
Major Hydrologic Processes
Precipitation (measured at rain gage) Evaporation or ET (loss to atmosphere) Infiltration (loss to subsurface) Overland Flow Stream Flow (measured at stage gage) Ground Water Flow
The Watershed or Basin
Area of land that drains to a single outlet and is separated from other watersheds by a drainage divide.
Rainfall that falls in a watershed will create runoff to that watershed outlet.
All other rainfall falling outside a basin will not affect the runoff response.
Brays Bayou Watershed4.5 mi2
Keeg an s
Keeg an s Bayou
610
45
288
8North
Harris Gully
The Hydrograph
Graph of Discharge vs Time at a Single Location
Rising Limb, Crest Segment, Falling Limb,and
Recession
Base Flow is Usually Subtracted to yield DRO
Peak Gives the Maximum Flow for the Event
The Hyetograph
Graph of Rainfall Rate vs Time at a Single Gage
Usually Plotted as a Bar Chart
Net Rainfall is Found by Subtr. Infiltration Losses
Integration of Net Rainfall in Time yields the Total
Rainfall Vol (DRO) in inches over a Watershed
Runoff in an Urban Basin
A portion becomes pipe flow (storm water).
The remaining portion becomes overland flow in streets and yards.
The total runoff is the sum of both components
Hydrograph
Pipe Flow
Overland Flow
Out
flow
Time
Pipe Flow(SWWM)
Harris Gully Watershed
Ri ce Blvd.
Sh
e ph
e rd
Gree
nbri
ar
Main
St.
Fan
nin
Mon
tros
e
Kirb y
Harris Gully
Richmond
Westheimer
Bissonnet
Ri ce Blvd.
Sh
e ph
e rd
Gree
nbri
ar
Holcombe
Main
St.
Fan
nin
SW Freeway
Mon
tros
e
Kirb y
Rice University
Texas Medical Center
Harris Gully Brays
Bayou
Major Causes of Flooding(Excess Water that Inundates)
Highly Developed (urbanized) Area
Intensity and Duration of Rainfall
Flat Topography with Little Storage
Poor Building Practices - Floodplains
Tailwater Effects
HGL: Gravity Flow
HGL: Surcharged Flow 1
Ground surface
Flow
Incoming Culvert
Bayou Water Level 1
Receiving Bayou
HGL: Surcharged Flow 2
Top of Culvert
Bottom of Culvert
Bayou Water Level 2
Bayou Water Level 3
Manning’s Equation
V = Velocity of Flow, ft/sn = Manning’s Roughness CoefficientS = Slope of Channel, ft/ftR = A/P, where
A = Cross-sectional Area of FlowP = Wetted Perimeter of Channel
VnR S
149 23
.
Backwater Calculation
Hydraulic Grade Line
Datum (MSL)
Flow
z2
hp2
z1
hp1
hL
z1 + hp1 + hL = z2 + hp2
HEC-HMS Model Hydrologic Eng Center -
Army Corps of Eng. Converts Input rainfall
into runoff hydrographs Uses either historical
data or design storms Predicts the total outflow
hydrograph for a basin
Routing
Surface Runoff
+
Combination
Q
t
Surface Runoff
t
Q
Losses Removed
Losses Removed1
2
Q
t
Q
t
HEC-HMS Theory Input Rainfall
Loss Rate Function
Unit Hydrograph
Computes Runoff
Flood Routing
Combination Steps
Routing
Surface Runoff
+
Combination
Q
t
Surface Runoff
t
Q
Losses Removed
Losses Removed1
2
Q
t
Q
t
Loss Rate Method:Initial and Uniform Loss Rate Method
Initial Amount Lost to Infiltration (in)
Soil is Saturated.
Uniform Loss of a Constant Rate (in/hr)
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4 5 6 7 8 9 10 11 12 13
Time (hours)
Rai
nfal
l (in
/hr)
Rainfall Excess Losses
Example: Initial Loss = 0.5 in, Uniform Loss = 0.05 in/hr
Unit Hydrograph Theory
The unit hydrograph represents the basin response to 1 inch of uniform net rainfall for a specified duration.
Linear method originally devised in 1932.
Works best for relatively small subareas.
Several computational methods exist.
Unit Hydrograph Method
Snyder’s Method (1938)
Clark TC & R Method (1945)
Nash (1958)
SCS Method (1964)
Espey-Winslow (1968)
Surface Runoff Theory:Clark TC & R Method
T
Q
Clark UnitHydrograph Computation
Surface Runoff Theory:Clark TC & R Method
LCA
L1
L2
chan
nel
So
S
S
outlet
L = channel length (mi)
S = slope of channel (ft/mi)
LCA = length to centroid (mi)
So = watershed slope (ft/mi)
Surface Runoff Theory:Clark TC & R Method
TC = Travel time of overland runoff from most remote point to the outlet
R = Routing Coefficient; Relates Storage and Outflow
TC R L S
TC DLSCA
2 2 706
1 06
. ( ).
.
where: L = channel length (mi)S = slope of channel (ft/mi)LCA = length to centroid (mi)D = 0.94 for developed watersheds of So<20 ft/miSo = watershed slope (ft/mi)
Effects of Stream Flow Routing
Routing Theory 1:Muskingum Method
K = Travel Time Through the Reach
x = Weighting Factor (Storage Coefficient)
N = Number of Steps in Computation
Out
flow
Time
A
B
A
B
Routing Theory 2:Modified Puls Method
Based on Manning’s
Storage vs Outflow
Numerical Procedure
Inflows Converted to Outflows from Reach O
utflo
wTime
A
B
A
B
Modified Puls (Storage)
Storage-Indication Relationship: Solved Numerically in Model
Obtaining Storage-Discharge Data
Combination Step:Superposition
0
50
100
150
200
250
300
350
400
450
500
0 2 4 6 8 10 12 14
Time (hrs)
Out
flow
(cfs
)
Total Hydrograph(Summation)
Hydrograph 1
Hydrograph 2
Design Rainfalls
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0:30
1:00
1:30 2:00
2:30
3:00
3:30 4:00
4:30
5:00
5:30 6:00
Time
Rai
nfal
l (in
)
100 Year Storm8.4 inches
10 Year Storm5.6 inches
25 Year Storm6.6 inches
5 Year Storm4.8 inches
Design Storm from HCFCD and NWS
Based on Statistical Analysis of Data
5, 10, 25, 50, 100 Year Events
Various Durations
Harris Gully Watershed
Ri ce Blvd.
Sh
e ph
e rd
Gree
nbri
ar
Main
St.
Fan
nin
Mon
tros
e
Kirb y
Harris Gully
Richmond
Westheimer
Bissonnet
Ri ce Blvd.
Sh
e ph
e rd
Gree
nbri
ar
Holcombe
Main
St.
Fan
nin
SW Freeway
Mon
tros
e
Kirb y
Rice University
Texas Medical Center
Harris Gully Brays
Bayou
Rainfall and Runoff Response
0
200
400
600
800
1000
1200
1400
1600
1800
2000
6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00Time
Outfl
ow (c
fs)
0.00
0.10
0.20
0.30
0.40
0.50
0.60
Rain
fall (in
)
Rainfall Measuredfrom USGS Gage 400at Harris Gully Outlet
February 12, 1997
Flow Measuredfrom USGS Gage 403Inside Harris Gully
Calibration ResultsFebruary 12, 1997
0200400600800
10001200140016001800
4:00 8:00 12:00 16:00 20:00 0:00Time
Out
flow
(cfs
)
HEC-1 Hydrograph
Measured
April 4, 1997
0
200
400
600
800
1000
1200
1400
0:00 4:00 8:00 12:00 16:00 20:00Time
Out
flow
(cfs
)
HEC-1 Hydrograph
Measured
April 25, 1997
0
500
1000
1500
2000
8:00 12:00 16:00 20:00 0:00 4:00 8:00Time
Out
flow
(cfs
)
Measured
HEC-1 Hydrograph Average Errors:Peak Flow: 6%Volume: 4%Peak Time: 0.9 Hours
Verification ResultsJanuary 21, 1998
0
500
1000
1500
2000
2500
3000
12:00 16:00 20:00 0:00 4:00 8:00 12:00Time
Out
flow
(cfs
)
Measured
HEC-1 Hydrograph0.6% Peak Flow1.5 Hr Peak Time
15% Volume
Volume Error Could Be Due to Unmeasured Quantities
Calibration Successful
Urban Basin - Low Flow
Bayou Level
Inlets to PipesPipe Elevations and Sizes
Junction Locations
Rainfall Pattern
Urban Basin - Flood
Backflow at Outlet
High Bayou Level
Flooding Areas
Pipe Capacity
Harris Gully Models Determine Pipe Capacities at Six
Different Tailwater Elevations
DIVERT this Amount from Total Runoff Computed in HEC-HMS
HEC-HMS Model: Computes Total Runoff, Subtracts Amount Diverted to Pipes, Remaining is Overland Flow
HEC-1 Diversion Operation
Diversionto Pipes
RemainingOverlandRunoff
TotalRunoff
capacity
Harris Gully at Brays Bayou
20
25
30
35
40
45
50
Location
GroundElevation
Bo ttom of Box
Brays Bayou Station 51128Upstream of Harris Gully
44.4
8.64
Profile from 1973 Datum
23
35384041
Box Culvert
Overland Flow for Rising Brays BayouRainfall: 1.5 in/hr for 3 hours
41'40.5'
40'
38'
35'23'
80
100
120
140
160
180
200
220
240
22 24 26 28 30 32 34 36 38 40 42
Brays Bayou Elevation (ft)
Ove
rland
Vol
ume
(Ac-
ft)
Brays BayouElevation
(ft)
Overland FlowVolume(acre-ft)
23 9035 96
38 115
40 149
40.5 226
41 241
Detention Pond Locations
6
9 10
5
21
4
11
8
Ric e Univers ity
Tex as Medic al Center
Bray s Bayou
3
7
Herma nn Pa rk
Detention Pond Options1) 50 Acre-ft in Hermann Park
2) 100 Acre-ft in Hermann Park
3) 100 Acre-ft in Hermann ParkAND
50 Acre-ft on Rice Campus
Pond Impact on Overland Volume
0
100
200
300
400
500
600
700
800
0 2 4 6 8 10 12Time (hrs)
Ove
rland
Flo
w a
t Out
let (
cfs)
1.5 in/hr for 3 hrsTailwater = 41 ft
Overland Flow With No Pond
Option 3100 Ac-ft Pond in Hermann Park
(only 67 Ac-ft used)AND 50 Ac-ft Pond on Rice
Pond Option Volume % Reduction No Pond 241 Ac-ft 0%1) 50 Ac-ft in Hermann Park 191 Ac-ft 21%2) 100 Ac-ft in Hermann Park 175 Ac-ft 27%3) 100 Ac-ft in Hermann ParkAND 50 Ac-ft on Rice
125 Ac-ft 48%
HEC-HMS Output Tables
– Summary– Detailed (Time Series)
Hyetograph Plots Sub-Basin Hydrograph Plots Routed Hydrograph Plots Combined Hydrograph Plots Recorded Hydrographs - comparison
Viewing Results
Hydrograph
HEC-HMS Output Sub-Basin
Plots– Rainfall – Hydrographs – Abstractions– Base Flow
Conclusions for Harris Gully
67 Acre-ft Pond in Hermann Park AND 50 Acre-ft Pond at Rice University would have prevented 48% of Overland Flow in March 1997 Storm
Ponds are imperative for Flood Relief!