Test: Floods - 2 - Civil Engineering (CE) MCQ

Gumbel’s distribution has the property which gives T = 2,33 years for the average of the annual series when N is very large. Thus the value of a flood with T = 2.33 years is called the mean annual flood.

Kirpich equation is an empirical relation used for the estimation of the time of concentration. It is given as,
t_c = 0.01947 L^0.77 S^-0.385
where, t_c = time of concentration in minutes 
L = maximum length of travel of water (m)
S = slope of the catchment = ΔH/L
ΔH = difference in elevation between the most remote point on the catchment outlet.

Risk = Probability of flood being exceeded at least once over a period of n years.

Probability corresponding to 10 year return period is,
P = 1/10 = 0.1
The probability of flood occurring at least once in 4 years is,
= 1 - (1 - P)⁴ = 1 - 0.9⁴ = 0.3439 ≈ 35%

The Probable Maximum Precipitation (PMP) may be defined as the maximum depth of precipitation for a given duration that may possibly occur on a given catchment at any time of the year. Hence such a precipitation would result from the possible severest storm that may result from the worst possible combinations of hydrological conditions in the area.
Probable Maximum Flood (PMF) is the extreme flood that is physically possible in a region as a result of severe most combinations, including rare combinations of meteorological and hydrological factors. The PMF is used in situations where the failure of the structure would result in loss of life and catastrophic damage and as such complete security from potential floods is sought.

The flood that would result from a sever combination of meteorological and hydrological factors that are reasonably applicable to the region. Extremely rare combinations of factors are excluded.