Introduction to Surface
Water Hydrology
Philip B. Bedient
Rice University
2006
The Hydrologic Cycle
P
r
e
1
c
0
i
0
pd i
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 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
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 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)
Brays Bayou Watershed
Harris Gully Area:
Brays Bayou Area:
4.5 sq. mi.
129 sq. mi.
Rice/TMC Area
Watershed Boundary
Watershed Characteristics
Divide

Size
 Slope
 Shape
Reservoir
Natural
stream
 Soil type - LU
 Storage capacity
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.
Measured Flow for Brays Bayou
30,000
29,000 cfs
Flow, cfs
25,000
Jun 76
Apr 79
20,000
Sep 83
Mar 92
15,000
Mar 97
10,000
5,000
3
6
9
15
12
Time, hrs
18
21
24
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
 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
Lost
to Infiltration (in)
Inches/Hour
 Initial Amount
 Uniform
Loss at 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 (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 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
Qp = 2Vol/(TR + B)
TR
B
B = 1.67 TR
Qp = 484 A / TR
tp = lag time
L = length to divide in ft
Y = Avg basin slope in %
S = 1000/CN - 10 (ins)
TR = D/2 + tp
tp = L0.8 (S + 1)/ 1900 (Y)0.5
SCS Methods
Triangular UH
CN = curve number
for various soil/LU
See SCS Table 2.1
Dimensionless UH
Hydrograph Convolution
1
2
Add and Lag Method
3
1
Q
2
FINAL
STORM
HYDROG
3
T
Add up the ordinates of all three to produce storm hydrograph
This add and lag procedure can be extended to large basins
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
URBAN RUNOFF
Overland Flow
Total Hydrograph
Outflow

Pipe Flow
(SWWM)
Pipe
Flow
Time
Hydraulic Calculation - Pipes
(z + P/g + V2/2g)1 = (z + P/g + V2/2g)2 + hL
E = total energy = z + P/g + V2/2g at pts 1 and 2
hL
P1
P2
z1
z2
Datum (MSL)
Manning’s Equation Open Channels
A
P = Wetted Perimeter
A
Pipe P = Circum.
2
1.49
Q=
AR 3 S
n
A
Natural Channel
Small Watershed Response
Brays
Bayou
Digital Elevation Model
Based on 1999 Aerial Survey
DEM Used to Determine Overland
Flow Connectivity and Storage
Existing Pipe Network
60”
7.5’x11’
NEW
PIPES
Rice
TMC
Hermann
Park
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
T.S. Allison - Houston,
June 9, 2001
Rice Blvd at Entrance 16
looking west
Jeep indicating high water
mark - inlet to Harris Gully
Southwest Freeway (US 59)
Detention Storage between
Mandell and Hazard
Looking West
Looking East
Flood Warning Systems
Downtown Houston
Emergency
Response






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 with
dependable 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