CHARACTERISTICS OF RUNOFF

CHARACTERISTICS OF RUNOFF

• Peak Discharge

• Time Variation of Runoff - Hydrograph

• Stage versus Discharge for Stream Channels

• Total Volume of Runoff

• Frequency of Runoff - Statistics

• Return Period

RUNOFF

• Excess rainfall is the rainfall remaining after all of the hydrologic abstractions have been subtracted. Excess rainfall becomes runoff and eventually streamflow.

• A hydrograph is a curve describing the variation of discharge with time as a result of a storm. The main features are the rising and falling limbs, the peak and the base duration.

RUNOFF

• The most important characteristics of runoff are: peak discharge, total runoff volume, and frequency of peak flows.

• The peak discharge (peak flow) is the maximum flow a given point as a result of a storm.

RUNOFF

• For a given storm, the resulting peak flow determines the size of the drainage structures.

• Peak flows depend on rainfall characteristics, and watershed size and other features.

• The total runoff volume is equal to the area under the hydrograph. It can be measured in cubic meters or, cubic feet or, dividing by the watershed area in, in mm or inches.

RUNOFF

• The volume is used in design of storage facilities which may be related to highway design.

• Similar to storm events, the exceedance probability of peak flows is the probability that, in any one year, a peak of given magnitude will be equaled or exceeded.

• The return period is calculated as one over the probability.

EFFECT OF BASIN CHARACTERISTICS

• In addition to the spatial and temporal characteristics of rainfall, the physical features of the watershed also control the shape of the runoff hydrograph.

• •Factors affecting the hydrograph include: drainage area, slope, roughness, storage, drainage density, channel length, and antecedent moisture conditions.

EFFECT OF BASIN CHARACTERISTICS

• Features that facilitate runoff removal produce high peaks and short hydrographs. Features that delay runoff removal produce low peaks and long hydrographs.

BASIN EFFECTS

Time in hours

Flo

w in

cfs

Drainage Area - Watershed Area

• The drainage area of a basin is the single most important factor affecting the magnitude of peak flows.

• In general, a large drainage area implies a large peak flow; however, urbanization can modify this behavior.

Slope

• In general, mild slopes produce less runoff volume and smaller peaks

• The slope of the terrain affects the total runoff volume. If the slope is mild, the velocity of overland flow will be low and there will be more time for water to infiltrate thereby reducing the amount reaching the stream.

• The slope of the channel affects the magnitude of the peak and the duration of runoff. A steep channel produces greater velocities and allows faster removal of the runoff from the watershed; therefore, shorter times to peak take place.

Slope

• The slope of the channel affects the magnitude of the peak and the duration of runoff. A steep channel produces greater velocities and allows faster removal of the runoff from the watershed; therefore, shorter times to peak take place.

Roughness

• Roughness affects the velocity of overland flow and stream flow. A rough channel will cause smaller peaks than a smooth channel.

• For a given discharge, stage levels (water surface elevations) in a stream are higher for rough channels.

Storage

• Storage can take place within the streambed, flood plains, marshes, lakes, or in artificial features such as stormwater management facilities and reservoirs. Storage also takes place in the impoundments created upstream of road crossings (e.g., bridges, and culverts).

• In general, storage reduces and delays peaks and increases the duration of runoff.

Storage

• The total runoff volume may be reduced by the increased effect of abstractions. For instance, a reservoir provides a large surface area exposed to evaporation and may retain a portion of the runoff in the permanent pool volume.

Drainage Density

• The drainage density is a measure of the total length of well defined channels that drain the watershed (sometimes measured as the blue lines representing the streams on a topographic map).

• Drainage density is defined as the sum of the lengths of all of the channels (km or miles) divided by the total watershed area (km2 or ft2). This ratio can be determined from topographical maps.

Drainage Density

• Drainage density affects the response of the watershed to rainfall. High densities usually allow fast runoff removal. Therefore, greater peaks and hydrographs with shorter durations are expected for watersheds with higher drainage densities.

• The effect of drainage density on runoff volume is associated with the time during which the runoff remains in the watershed. Low densities allow for long residence times; therefore, abstraction mechanisms have more time to remove water.

Drainage Density

• Drainage density can be increased by urbanization. Because of the faster response facilitated by high densities, the frequency distribution of streamflow approximately follows that of precipitation events.

Channel Length

• The effective length of a channel depends on flow magnitude. Large flows overtop the banks and fill the floodplain whose length is usually shorter than that of the meandering streambed.

• A long drainage channel usually indicates a long runoff removal time. Therefore, longer channels cause a response to rainfall slower than for shorter channels.

Channel Length

• Long channels also cause more attenuation of peaks due to storage and hydraulic roughness. Consequently, long channels cause low peaks and hydrographs of long duration.

Antecedent Moisture

• Antecedent Moisture is the amount of water present in the soil at the beginning of a storm.

• If the moisture content of the soil is high, more runoff will be produced because a smaller amount of water is required to saturate the soil.

Other Factors

• Other special watershed conditions can affect the distribution of runoff. Examples are channel rerouting, vegetation type, hydraulic structures, and urbanization.

• In summary, if the result is to facilitate runoff removal, the effect will be increased peaks and shorter hydrograph durations. If the result is to slow the flow of runoff, the opposite effects will take place.