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!