Introduction to Surface Water
Hydrology Philip B. Bedient
Rice University
2006
The Hydrologic Cycle
100Precipitation on land
InfiltrationWater table
Groundwater flow
1 Groundwaterdischarge
38 Surface discharge
61Evaporation from land
39Moisture over land
385Precipitation
on ocean
424Evaporationfrom ocean
Surfacerunoff
Imperviousstrata
GroundwaterRecharge
Precipitation
Snowmelt
Major Hydrologic Processes
Precipitation (measured by radar or rain gage) Evaporation or ET (loss to atmosphere) Infiltration (loss to subsurface soils) Overland flow (sheet flow toward nearest stream) Streamflow (measured flow at stream gage) Ground water flow and well mechanics Water quality and contaminant transport (S & GW)
Recent History of Hydrology
Early 1900s saw great expansion of water supply and
flood control dams in the western U.S. - in response to
Dust Bowl and the Economic Depression of the 1930s
U.S. Dept of Agriculture began many hydrologic studies
Sherman UH and Horton infiltration theory 1930s
Theis well mechanics (1935)
Horton theory of infiltration (1940s)
Penman (1948) - complete theory of evaporation
Recent History of Hydrology Great urban expansion in 1950s and 60s - led to demand
for better water supply and prediction (after WW II) EPA formed in 1970 with a mission to clean up the rivers
and lakes of America - beginning of environmental eng. as we know it today
USGS and EPA actively involved in large-scale sampling programs at the national level - The Woodlands, TX
EPA funded development of computer models to address water quality issues in streams and lakes, and estuarine bays (1970s).
Guadalupe River Map
The Woodlands planners wanted to design the ultimate community to handle a 100-year storm.
In doing this, they attempted to minimize any changes to the existing, undeveloped floodplain.
The Woodlands
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 generate runoff to that watershed outlet.
Topographic elevation is used to define a watershed boundary (land survey or LIDAR)
Rice/TMC Area
Brays Bayou WatershedHarris Gully Area: 4.5 sq. mi.Brays Bayou Area: 129 sq. mi.
Watershed Boundary
Watershed Characteristics
Size
Slope
Shape
Soil type - LU
Storage capacity
Reservoir
Divide
Natural stream
Urban
Concrete channel
Urban runoff near Brays Bayou - moderate flow
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 in floodprone areas
No replacement of lost storage as area grows
Harris Gully Drains to Brays Bayou
Low Flow Box Culvert During Tropical Storm Frances
The Watershed Response As rain falls over a watershed area, a certain portion will
infiltrate the soil. Some water will evaporate back. Net Rainfall is available as overland flow and runs off to the
nearest stream. Smaller tributaries or streams then begin to flow and
contribute their load to the main channel at confluences. As accumulation continues, the Streamflow rises to a
maximum (peak flow) and a flood wave moves downstream through the main channel.
The flow eventually recedes or subsides as all areas drain out.
Sep 83
Jun 76
Apr 79
Mar 92
Mar 97
25,000
30,000
5,000
10,000
15,000
20,000
3 6 9 12 15 18 21 24Time, hrs
Flo
w,
cfs
Measured Flow for Brays Bayou
29,000 cfs
Problems in Hydrology:
Extreme weather and rainfall Streamflow and runoff predictions River routing and hydraulic conditions Overall water balances - local and global scales Flood control and drought measures Water supply for growing communities Watershed management for agric/urban development
Applications in Hydrology: Surface water supply and delivery systems (sewers) Ground water for supply, wells, and springs Contamination and environmental quality issue
– Lake and Coastal Bay quality studies– River quality for drinking and recreation– Hazardous waste studies for GW contamination– Waste sources from urban/industrial runoff
Land use impacts from urban development Disaster mitigation and flood control
Technology has Revolutionized the Field of Hydrology
High Speed Digital Computation Geographical Information Systems (GIS) Large Hydrologic and Meteorologic Databases GPS and LIDAR methods for ground surveys RADAR rainfall estimates from NEXRAD Advanced forecasting tools for severe weather and
flood Alert
A Note on Units
Rainfall volume is normally measured in inches or cm Rainfall rate or intensity in inches/hr or cm/hr Infiltration is measured in inches/hr or cm/hr Evaporation is measured in inches or in/hr (cm/hr) Streamflow is measured in cfs or m3/s One acre-ft of volume is 43,560 ft3 of water 1 ac-inch/hr is approx. equal to 1.008 cfs Ground water flows are measured as ft3/day or m3/day
Rainfall and Conversion to Runoff
Use either design rainfalls or historical events
Spread uniformly over a given basin area
Use Rational Method to compute peak flow for
small basin area - few hundred acres
Use Unit hydrograph to compute response for
larger basins - 10 to 100 sq miles.
Rainfall and Conversion to Runoff
Rational Method predicts peak flow
Qp = C I A in cfs
– C = runoff coefficient - fcn of land use
– I = rainfall intensity at time of concentration Tc
– A = watershed area in acres
– Tc = time for water to travel from most distant pt to
the outlet of a watershed
Rational Method uses IDF Curves
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
Rainfall (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
T.S. Allison vs. the 100-year
(Inches of rainfall)
1-hr 3-hr 6-hr 12-hr
1976 TMC 3.8 7.5 9.8 10.4
Allison (a) 4.3 10.3 12.1 14.7
100-yr (b) 4.6 6.8 8.5 10.5
Diff (a–b) –0.3 3.5 3.6 4.2
Note: Allison dropped 8.5 inches in 2 hours
RADAR Rainfall Estimates
NEXRAD provides real-time data on a ~16 km2 (6 mi2) grid
Equivalent to about 21 rain gages in Brays Bayou watershed
Each estimate represents an average rainfall amount over the entire 4 x 4 km2 area
NEXRAD rainfall estimates compare well with point rain gage measurements (r2 ~ 0.9)
FAS – NEXRAD
Midnight 1 a.m.
Hydrologic Theory One of the principal objectives in
hydrology is to transform rainfall that has fallen over a watershed area into flows to be expected in the receiving stream.
Losses must be considered such as infiltration or evaporation (long-term)
Watershed characteristics are important
Loss Rate Method:Initial and Uniform Loss Rate Method
Initial Amount Lost to Infiltration (in)
Uniform Loss at a Constant Rate (in/hr)
Example: Initial Loss = 0.5 in, Uniform Loss = 0.05 in/hr
Inch
es/H
our
Unit Hydrograph Theory The unit hydrograph represents the basin
response to 1 inch (1 cm) of uniform net rainfall for a specified duration, D.
Linear method originally devised in 1932.
Works best for relatively small subareas - in the range of 1 to 10 sq miles.
Several computational methods exist.
Synthetic UH MethodsSynthetic UH Methods
• Snyder’s Method (1938)
• Clark Method (1945)
• Nash (1958)
• SCS (1964, 1975)
• Espey-Winslow (1968)
• Kinematic Wave (1970s)
SCS Triangular UH Example
1 inch of Net Rain over D = 1.6 hr
SCS Triangular UH Example
Volume = QpTr /2 + QpB /2
Qp = 2Vol/(TR + B)
B = 1.67 TR
Qp = 484 A / TR
TR = D/2 + tp
tp = L0.8 (S + 1)/ 1900 (Y)0.5
BTR
Qp
tp = lag time
L = length to divide in ft
Y = Avg basin slope in %
S = 1000/CN - 10 (ins)
SCS Methods
Triangular UH
CN = curve number for various soil/LU
See SCS Table 2.1
Dimensionless UH
Hydrograph Hydrograph ConvolutionConvolution
1
2
3
31 2
Add up the ordinates of all three to produce storm hydrographThis add and lag procedure can be extended to large basins
FINAL STORMHYDROG
Add and Lag Method
Q
T
Flow in Pipes and Channels Rain falls over watershed A portion becomes pipe flow
(storm water). The remaining portion
becomes overland flow in streets and yards.
The total runoff reaches a stream and is the sum of both components
Total Hydrograph
Pipe Flow
Overland Flow
Out
flow
Time
Pipe Flow(SWWM)
URBAN RUNOFF
Hydraulic Calculation - Pipes
Energy Grade Line
Datum (MSL)
Flow
z1
P1
z2
P2
hL
(z + P/ + V2/2g)1 = (z + P/ + V2/2g)2 + hL
E = total energy = z + P/ + V2/2g at pts 1 and 2
Manning’s Equation Open Channels
P = Wetted Perimeter Pipe P = Circum. Natural Channel
Q=
1.49n
AR23 S
A AA
Small Watershed Response
Rice Blvd.
Shepherd
Greenbriar
Main S
t.
Fannin
Montrose
Kirby
Harris Gully
Richmond
Westheimer
Bissonnet
Rice Blvd.
Shepherd
Greenbriar
Holcombe
Main S
t.
Fannin
SW Freeway
Montrose
Kirby
Rice University
Texas Medical Center
Harris Gully
BraysBayou
Rice Blvd
Harris Gully
Digital Elevation ModelBased on 1999 Aerial Survey
DEM Used to Determine Overland Flow Connectivity and Storage
Existing Pipe Network
2-15’x15’
2-11.5’x15’
7.5’x11’
90”
60”
72”
6.5’
x10’
66”
60”
60”
72”
60”6.
5’x1
0’
66”
72”
96”
114”
54”
84”
Hermann Park
TMC
Rice
NEWPIPES
Bayou Camera - June 8-9, 2001
Provided valuable data on water levels and timing
10 p.m. 12 p.m. 11:00 a.m
Texas Medical Center - Moursund Westbound
6/10/01 - 6:44 AM
Fannin at Holcombe Overpass - TS Allison
6/9/01 - 5:58 AM
Rice Blvd at Entrance 16 looking west
Jeep indicating high water mark - inlet to Harris Gully
T.S. Allison - Houston, June 9, 2001
Southwest Freeway (US 59)
Detention Storage between Mandell and Hazard
Looking East
Looking West
Flood Warning SystemsDowntown Houston
EmergencyResponse
Flood Doors Flood Gates Facility Entrances Communications Operations Training
The Woodlands - a Totally Planned Community
The community was designed as if it were fully developed with minimal impacts on water.
Strict requirements were made about land use and natural drainage concepts were used throughout.
Mountain runoff - steep and dependent on snowmelt
Hoover Dam
Hoover Dam Facts
Hoover Dam supplied farmers withdependable supply of water in Nevada, California and Arizona.
Because of the Hoover Dam, the Colorado River was controlled for the first time in history.
Mansfield Dam Facts
Mansfield Dam sits across a canyon at Marshall Ford on the Colorado River west of Austin, Texas
Built from 1937 to 1941 Named in 1941 in honor of
U.S. Representative J.J. Mansfield
Created a 50 mile long lake that is hundreds of feet deep in lower end
Mansfield Dam Facts Mansfield Dam, owned by
LCRA, created Lake Travis Mansfield Dam and Lake
Travis are the only structures in the Highland Lakes chain specifically designed to contain floodwaters in the lower Colorado River basin
Variable level lake Cleanest in all of Texas
Agricultural runoff in California - source of chemicals