The flood adopted for design of hydraulic structures is called Design flood. Any hydraulic structure is generally made safe against  the maximum flood possible in the catchment. Smaller structures such as culverts, storm drainage systems can be designed for relatively small floods (i.e., more frequent floods) as the consequences of a higher than design flood may only cause temporary inconvenience and some repair works without any loss of life and property. However, failure of structures such as spillway would cause huge loss of life and property and, therefore, such structures should be designed for relatively more severe floods having relatively larger return period. The table below  provides guidelines for selecting design floods. The terms PMF and SPF in the Table below have the following meaning...

PMF or probable maximum flood is the extreme large flood that is physically possible in a region as a result of severe most (including rare ones) combination of meteorological and hydrological factors.

SPF is the standard project flood that would result from a severe combination of meteorological and hydrological factors. Usually, SPF is about 40 to 60% of PMF.

General Guidelines for designing floods is as following…

Sl No
Recommended design flood
Spillways  [storage more than 60 Mm3]
(a) PMF determined by unit hydrograph and probable maximum precipitation (PMP)
(b) If (a) is not applicable or possible, flood frequency method with T = 1000 years to 5000 years
Permanent barrage and minor dams [capacity less than 60 Mm3]
(a) SPF determined by unit hydrograph and standard project storm (SPS) which is usually the largest recorded storm in the region
(b) Flood with a return period of 100 years.(a) or (b) whichever gives higher value.
Pickup weirs
Flood with a return period of 100 or 50 years,depending on the importance of the project
(a) Waterway
(b) Foundations and free board
Flood with T = 50 years
Flood with T = 100 years
Project with very scanty or inadequate data
Empirical formulae

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