Transcript StormCAD Capabilities

Introduction to Surface Water
Hydrology and Watersheds
Lecture 1
Philip B. Bedient
Rice University
November, 2000
The Hydrologic Cycle
P
r
e
1
c
0
i
0
p di
t
a
t
i
o
n
o
n
39
Moisture over land
l a n
61
Evaporation from land
385
Precipitation
on ocean
Snow
melt
Surface
runoff
Precipitation
424
Evaporation
from ocean
Infiltration
Groundwater
Recharge
Wat
er
t ab l
e
Groundwater flow
Impervious
strata
38
Surface discharge
1 Groundwater
discharge
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 Watershed
Harris Gully
4.5 mi2
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
 Net
Plotted as a Bar Chart
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).
remaining portion
becomes overland flow
in streets and yards.
 The total runoff is the
sum of both components
Hydrograph
Outflow
 The
Overland Flow
Pipe Flow
(SWWM)
Pipe
Flow
Time
Harris Gully Watershed
Brays
Bayou
Major Causes of Flooding
(Excess Water that Inundates)
 Highly
Developed (urbanized) Area
 Intensity
 Flat
and Duration of Rainfall
Topography with Little Storage
 Poor
Building Practices - Floodplains
Tailwater Effects
Bayou Water Level 3
Bayou Water Level 2
Bayou Water Level 1
Receiving Bayou
Manning’s Equation
2
149
.
V
R 3 S
n
V = Velocity of Flow, ft/s
n = Manning’s Roughness Coefficient
S = Slope of Channel, ft/ft
R = A/P, where
A = Cross-sectional Area of Flow
P = Wetted Perimeter of Channel
Backwater Calculation
z1 + hp1 + hL = z2 + hp2
hL
hp2
hp1
z2
z1
Datum (MSL)
HEC-HMS Model
 Hydrologic
Eng Center Army Corps of Eng.
Losses Removed
1
Losses Removed
2
 Converts
Input rainfall
into runoff hydrographs
Surface Runoff
Q
t
Surface Runoff
Routing
 Uses
either historical
data or design storms
the total outflow
hydrograph for a basin
Q
+
t
Q
t
Combination
 Predicts
Q
t
HEC-HMS Theory
 Input
Rainfall
Losses Removed
1
 Loss
Losses Removed
Rate Function
2
Surface Runoff
 Unit
Hydrograph
Q
t
Surface Runoff
Routing
 Computes
Runoff
Q
+
t
 Flood
Routing
 Combination
Q
t
Combination
Steps
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)
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
Q
T
Clark Unit
Hydrograph Computation
Surface Runoff Theory:
Clark TC & R Method
L1
channel
L = channel length (mi)
S = slope of channel (ft/mi)
LCA = length to centroid (mi)
S
LCA
L2
S
outlet
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  2.2( L S ) .706
1.06
LCA 
TC  D 
 S
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/mi
So = watershed slope (ft/mi)
Effects of Stream Flow Routing
Routing Theory 1:
Muskingum Method
A
K
= Travel Time
Through the Reach
B

x = Weighting Factor
(Storage Coefficient)
A

N = Number of
Steps in
Computation
Outflow
B
Time
Routing Theory 2:
Modified Puls Method
 Based
A
on Manning’s

Storage vs Outflow

Numerical Procedure
B
A

Inflows Converted to
Outflows from Reach
Outflow
B
Time
Modified Puls (Storage)
 Storage-Indication
 Solved
Relationship:
Numerically in Model
Obtaining Storage-Discharge
Data
Combination Step:
Superposition
Design Rainfalls
 Design
Storm from
HCFCD and NWS
 Based on Statistical
Analysis of Data
 5,
10, 25, 50, 100
Year Events
 Various
Durations
Harris Gully Watershed
Brays
Bayou
Rainfall and Runoff Response
Flow Measured
from USGS Gage 403
Inside Harris Gully
Rainfall Measured
from USGS Gage 400
at Harris Gully Outlet
February 12, 1997
Calibration Results
Average Errors:
Peak Flow: 6%
Volume:
4%
Peak Time: 0.9 Hours
Verification Results
 Volume
Error
Could Be Due
to Unmeasured
Quantities
 Calibration
Successful
Urban Basin - Low Flow
Rainfall
Pattern
Pipe
Elevations
and Sizes
Junction
Locations
Inlets to Pipes
Bayou Level
Urban Basin - Flood
Flooding Areas
High Bayou
Level
Backflow
at Outlet
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
Total
Runoff
Diversion
to Pipes
capacity
Remaining
Overland
Runoff
Harris Gully at Brays Bayou
50
Brays Bayou Station 51128
Upstream of Harris Gully
Ground
Elevation
44.4
45
41
40
38
35
40
35
Box Culver t
30
Bo tt om of B ox
25
23
20
8.64
Location
Profi l e f rom 1973 D atum
Overland Flow for Rising Brays Bayou
Rainfall: 1.5 in/hr for 3 hours
Detention Pond Locations
7
6
8
9
10
11
5
4
3
Ric e Unive rs ity
2
Her ma nn
Pa r k
Br ay s
Bayou
1
Tex as
Medic al
Center
Detention Pond Options
1)
50 Acre-ft in Hermann Park
2)
100 Acre-ft in Hermann Park
3)
100 Acre-ft in Hermann Park
AND
50 Acre-ft on Rice Campus
Pond Impact on Overland Volume
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!