Additional information and definition of important terms
Free board
The marginal distance that is providing above the maximum reservoir level to avoid the possibility of water spilling over the dam is known as the free board.
Hydro meteorologically homogeneous subzones
This indicates a partition of the country in terms of similar hydrological and meteorological areas. There are, in all, 26 subzones in the country. This has been done together by the Central Water Commission (CWC), Research Designs and Standards Organization (RDSO), and India Meteorological Department (IMD).
Channel routing
The outlet of each subcatchment is located many km upstream of the outlet of the main catchment. The outflow of a subcatchment will pass through the channels before finally reaching the catchment outlet. The inflow hydrograph to a channel will get modified by the temporary storage of channel; hence it is necessary to estimate the outflow hydrograph of the channel to in order to find the flow at the outlet of the catchment outlet by a process is known as channel routing.
Reservoir routing
The hydrograph of a flood entering a reservoir will change in shape as it emerging out from the reservoir. This is due to volume of water stored in reservoir temporarily. The peak of the hydrograph will be reduced, time to peak will be delayed and base of the hydrograph will be increased. The extent up to which an inflow hydrograph will be modified in the reservoir will be computed by the process is known as reservoir routing.
Data requirement for PMF/SPF studies
Steps for evaluating PMF/SPF
Limitation of PMF/SPF calculations
Normal Distribution
The Normal distribution is one of the most important distributions in statistical hydrology. This is used to fit empirical distributions with skewness coefficient close to zero. The probability density function (PDF) of the distribution is given by
− ∞ < x < ∞
Where, μ is the location parameter and σ is the scale parameter. The cumulative distribution function (CDF) of the normal distribution is given by:
Log – Normal Distribution
If the logarithms, lnx, of a variable x are normally distributed, then the variable x is said to be log normally distributed so that
Where, μ_{y} and σ_{y} are the mean and standard deviation of the natural logarithm of x. Log normal distributions can be applied to a wide variety of hydrologic events especially in the cases in which the corresponding variable has a lower bound, the frequency distribution is not symmetrical and the factors causing those are independent and multiplicative.
If the variable x has a lower boundary x_{0}, different from zero, and the variable z= x  x_{0} follows a lognormal distribution, then x is lognormally distributed with three parameters. The probability distribution function of the lognormal distribution with parameters is
Where, μ_{y}, σ_{y} and x_{0} are called the scale, the shape and the location parameters respectively. Parameter x_{0} is generally estimated by trial and error.
Pearson Type III Distribution
Pearson type III is a three parameter distribution, also known as Gamma distribution with three parameters. The PDF of the distribution is given as
The CDF of the Pearson Type III distribution is given by
Where x_{0}, β, and γ are location, scale and shape parameters respectively.
Gumbel Distribution
Gumbel distribution is a member of family of Extreme Value distributions with the value of parameter k = 0. It is a two parameter distribution and is widely used in hydrology. The PDF is given as
And CDF is given as
E ( X ) = μ + 0.5772α
Where, u and α are location and shape parameters respectively.
Lag Time Lag is the time between the peak flow and the centroid of rainfall.
Travel time The time taken by the water to reach the basin outlet, from the different points in the basin, is called the travel time.
Evaporation The process of extracting moisture is known evaporation.
Infiltration Infiltration is defined as the slow passage of a liquid through a filtering medium.
Interception Interception is the act of catching the precipitation by the trees or buildings without reaching to the ground surface.
Base flow Base flow is the portion of the stream discharge that is derived from natural storage (e.g., groundwater outflow and the draining of large lakes and swamps or other source outside the net rainfall that creates surface runoff).
Base period The time between the first watering of a crop at the time of its sowing to its last watering before harvesting is called base period. Base period is always less than crop period.
Unit hydrograph A unit hydrograph is defined as the hydrograph of runoff produced by excess rainfall of 1cm occurring uniformly over the entire drainage basin at a uniform rate over the entire specified duration.
Time of concentration The time of concentration of a drainage basin is the time required by the water to reach the outlet from the most remote point of the drainage area.
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1. What is flood estimation in civil engineering? 
2. How is flood estimation conducted in civil engineering? 
3. What are the key factors considered in flood estimation? 
4. How accurate are flood estimation methods in civil engineering? 
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