Transcript CHARACTERISTICS OF RUNOFF

CHARACTERISTICS OF
RUNOFF
CHARACTERISTICS OF
RUNOFF
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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.
Flow in cfs
BASIN EFFECTS
Time in hours
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.