lecture 11 storm runoff

8/8/2019 Lecture 11 Storm Runoff

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GEOG415 Lecture 11: Storm Runoff

Runoff

Runoff occurs when part of rain or snowmelt water flowsoverland – runoff generation.

Runoff water eventually reaches streams and causes the

discharge to increase – storm flow.

Where in a basin is runoff generated?

How does it reach streams?

Significance for environmental planning?



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Baseflow and storm runoff

Dunne and Leopold (1978, Fig. 9-1)

This diagram shows a typical storm runoff

“hydrograph”. Note the relationship between

rainfall and storm flow.

Baseflow – Dry period flow, sustained by groundwater

Main objective of engineering hydrology is to predict the

storm runoff hydrograph when precipitation data are given.

How?

(1)

(2)

We need to understand the mechanisms of runoff generation.



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1. Horton overland flow

2. Groundwater (baseflow)

3. Subsurface storm flow

4. Saturation overland flow

Horton overland flow

Runoff occurs when infiltration capacity is exceeded by

rainfall intensity, and depression storage is filled up.


depression

storage

surface retention



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Is Horton overland flow common? Does it occur everywhere?

Recall: Infiltration capacity is always greater than saturated

hydraulic conductivity ( K s).

K s of most undisturbed soils > 100 mm/hr

Rainfall intensity?

Conclusion:

Subsurface storm flow (SSSF)

The water table rises near a stream during a storm.

Consequences?

Intensity?

What will happen with

more rain?


rain

Pre-storm soil

moisture profile

infiltration

front



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Saturation overland flow

quickly responds to rain,

and has a high intensity.

Subsurface storm flowhas a slower response

and a lower intensity.

Variable source concept

The area of saturation expands

during a storm.

Consequence?


Saturated area

pre-storm

end of storm



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Factors affecting runoff generation

steep slope gentle slope

concave slope flat slope

thick soil thin soil

initially wet initially dry

undisturbed disturbed (compaction etc.)

Implication on environmental protection/management?

Effects of urbanization

More opportunities for Horton overland flow.

“Efficient” transmission of storm water to major rivers by

storm sewers – smaller flow resistance than natural drainage.

Combined effects?



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Transmission and storage of storm water

Stream discharge increases during a storm – storm hydrograph.

Downstream migration of a hydrograph peak is often called

“flood wave”, even though the discharge may not be large

enough to cause actual flooding of the flood plain.

Translation: Simple lateral

migration of peak without

attenuation.

Attenuation: Damping of the peak height caused by

storage.Dunne and Leopold (1978, Fig. 10-1)

time

d i s c h a r g e

Main objective of flood analysis is to forecast the hydrograph

when the amount of rainfall or snowmelt is given.

Total discharge volume

Peak discharge

Peak height and velocity

Arrival time and duration

This requires:

(1) Estimates of runoff generation

(2) Channel storage and transmission characteristics



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Flood records

Estimates of runoff are usually based on empirical relation

between rainfall and runoff depending on a number of

parameters (vegetation, soil, slope, etc.).

Empirical relations are derived from historical records of

floods. In Canada, daily stream discharge data are available on

a CD-ROM database called HYDAT, published by the Water

Survey of Canada.→MADGIC section in the library (2LT).

Baseflow separation

For the analysis storm runoff, the baseflow contribution needs

to be subtracted from total stream discharge.

How?

Is it an accurate representation of baseflow?




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Runoff ratio

Once runoff volume is estimated by baseflow separation, we

can define the runoff depth for each individual storm:

Runoff depth = (total runoff volume) / (basin area)


Runoff ratio = (runoff depth) / (total precipitation)

Average runoff ratio in Fig.10-5?

Runoff ratio in fact depends on many parameters.

Antecedent soil moisture condition

This is a particularly important parameter for storm runoff.

Why?



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Curve Number method

It is impossible to characterize all parameters in a large basin.

As a practical tool for rough estimation, hydrologists

commonly use the curve number method put together by theUS Soil Conservation Service (SCS).

Curve number (CN) represents integrated effects of soil

texture, antecedent moisture, land use, vegetation, slope, etc.

Storm runoff is estimated from precipitation and curve

number.

e.g. Given CN = 80 and P = 4 inches, what is storm runoff?


S t o r m

r u n o f f , Q

( i n c h e s )

Rainfall, P (inches)



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Estimating curve number

Dunne and Leopold (1978, Table 10-1)

Hydrologic condition (DL, Table 10-5): depends on land use

practice. For example, in crop land, fallow results in “Poor”hydrologic condition.

Soil group (DL, Table 10-4): depends on soil texture. For

example, fine texture soils belong to Group C.

Therefore, a typical prairie soil in summer-fallow condition

will have a curve number of 91.Antecedent moisture condition is represented by Classes

determined by antecedent rainfall. For example, if the fallow

soil has received 10 mm of rain in the last five days, it will be

classified as Class I. The curve number of this soil needs to be

adjusted to 80 (see DL, Table 10-7).




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Rational method

This method is commonly used in urban areas for estimating

the capacity of storm sewer systems. The method should

ideally be used for small basins (< 200 acres).

Assumptions:

- Uniform intensity storm over the entire basin.

- Runoff ratio is fixed over the entire basin.

- Steady state is reached a short time after the onset of runoff.

→ all parts of the basin contributes runoff at a constant rate.

The peak intensity of runoff, Q pk is given by:

Q pk = CIA

where C is a dimensionless “rational” coefficient, I is rainfall

intensity, and A is basin area. Note that consistent units must

be used in the equation.

Example: A thunder storm having an intensity of 50 mm hr -1

falls on a 400-m2 parking lot. What is the expected Q pk ?


C



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Time of Concentration

Time required for runoff water to travel from the most distant

part of the basin to the outlet.

In rational method, it is the time required to reach steady state.Time of concentration (t c hr) is commonly estimated from:

t c = L1.15 / (7700 H 0.38)

where L (ft) is the length of the basin along the main stream

from the most distant ridge to the outlet, and H (ft) is the

elevation difference between the most distant edge and theoutlet. Note that this equation is only valid when specified

units are used.



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GEOG415 Lecture 11 Appendix




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lecture 11 storm runoff

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