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GEOG415 Lecture 11: Storm Runoff

Runoff

Runoff occurs when part of rain or snowmelt water flows overland – 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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Dunne and Leopold (1978, Fig. 9-1)

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.

Dunne and Leopold (1978, Fig. 9-4)

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 (Ks).

Ks 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 withmore rain?

Dunne and Leopold (1978, Fig. 9-7)

rain

Pre-storm soil moisture profile

infiltration front

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Dunne and Leopold (1978, Figs. 9-7 and 9-9)

rain

infiltration front

Saturation overland flow

The water table finally reaches the ground surface.

The surface is not only saturated, but also has upward flow of groundwater.

Consequences?

Return flow (RF): subsurface water “returns” to surface.

Direct precipitation onto saturate areas (DPS): no infiltration.

These two processes are collectively called saturation overland flow. → Most important runoff mechanism in

undisturbed basins during unfrozen period.

Where do RF and DPS occur?

Timing? Intensity?

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

Saturation overland flow quickly responds to rain, and has a high intensity.

Subsurface storm flow has a slower response and a lower intensity.

Variable source concept

The area of saturation expands during a storm.

Consequence?

Dunne and Leopold (1978, Fig. 9-11)

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

disc

harg

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?

Dunne and Leopold (1978, Fig. 10-4)

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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)

Dunne and Leopold (1978, Fig. 10-5)

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 the US 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?

Dunne and Leopold (1978, Fig. 10-8)

Stor

m ru

noff

, Q(in

ches

)

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

Dunne and Leopold (1978, Table 10-6)

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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, Qpk is given by:

Qpk = 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 Qpk?

Dunne and Leopold (1978, Table 10-9)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 (tc hr) is commonly estimated from:

tc = L1.15 / (7700H0.38)

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 the outlet. Note that this equation is only valid when specified units are used.

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GEOG415 Lecture 11 Appendix

Dunne and Leopold (1978, Fig. 10-8)

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