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