Problem 10.1. You are hired to assess the flash flood potential of a given basin

Question

Problem 10.1. You are hired to assess the flash flood
potential of a given basin, where “flash floods” are generally
associated with short duration (high intensity) storms that
generate infiltration excess runoff. The basin is composed of a
homogeneous silt loam soil. To comply with regulations for
the region, different downstream infrastructure must be
designed for various “return-period” storm events, where the
return-period is associated with the probability (and therefore
magnitude) of the storm event. The 1-hour duration design
storm events for the 10-year and 25-year return periods in this
region are:

a) For this particular basin, what is the expected (if any)
flash flood stormflow (i.e. infiltration excess runoff) for each
return period design storm? Design regulations dictate that it
should be assumed that 75% of the soil pore space is filled
prior to the storm. The Philip model can be used as an
infiltration capacity model (as needed).
b) A previous analysis of the basin found the 0.5-hr unit
hydrograph to be given by:
Construct the 1-hr unit hydrograph from the known 0.5-hr
unit hydrograph. Your answer should be in tabulated form

STORM RETURN PERIOD
10-year 25-year
PRECIP. INTENSITY
2 cm/hour 7 cm/hour TIME FLOW (ma/s/cm) (HOUR) 0.5 10 1.5 8 2.5 3.0 c) Based on the constructed 1-hr unit hydrograph, what is the predicted peak flash flood stormflow (i.e. design flow) for the two return period storms analyzed in part a). The infiltration excess runoff values computed above can be treated as the effective (excess) precipitation.

Explanation / Answer

Infiltration is the process by which water from the rainfall enter into the soil. Maximum infiltration capacity is measured in mm/hr or inch/hour. It depends on various factors like type of soil ,vegetation, initial moisture present in the soil, angle of slope. The maximum infiltration capacity of the loamy soil is 0.50inch/hr for the slope 0-4%.

infiltration rate of soil=0.50 inch/hour=1.27cm/hour

Therfore infiltration excess runoff= precipitation-infiltration capacity

precipitation intensity for the 10 year return period flood=2cm/hr

Now infiltration excess runoff for the 10 year flood=2-1.27=0.73cm/hr

For the storm return period of 25 years

precipitation intensity=7cm/hr

infiltration capacity=0.50 cm/hr

Therefore precpitation excess runoff= 7-0.5=6.5cm/hr

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