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 |
Structure
|
Recommended design flood
|
1.
|
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 |
2.
|
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. |
3.
|
Pickup weirs
|
Flood with a return period of 100 or 50 years,depending on the
importance of the project
|
4.
|
Aqueducts
(a) Waterway (b) Foundations and free board |
Flood with T = 50 years
Flood with T = 100 years |
5.
|
Project with very scanty or inadequate data
|
Empirical formulae
|
