SIZE OF FLOODS
8.1 SIZE OF FLOODS
A flood is an unusual high stage of a river due to runoff from rainfall and/or melting of snow in quantities too great to be confined in the normal water surface elevations of the river or stream, as the result of unusual meteorological combination.
The maximum flood that any structure can safely pass is called the ‘design flood’ and is selected after consideration of economic and hydrologic factors. The design flood is related to the project feature; for example, the spillway design flood may be much higher than the flood control reservoir design flood or the design flood adopted for the temporary coffer dams. A design flood may be arrived by considering the cost of constructing the structure to provide flood control and the flood control benefits arising directly by prevention of damage to structures downstream, disruption communication, loss of life and property, damage to crops and under- utilisation of land and indirectly, the money saved under insurance and workmen’s compensation laws, higher yields from intensive cultivation of protected lands and elimination of losses arising from interruption of business, reduction in diseases resulting from inundation of flood waters. The direct benefits are called tangible benefits and the indirect benefits are called intangible benefits. The design flood is usually selected after making a cost-benefit analysis and exercising engineering judgement.
When the structure is designed for a flood less than the maximum probable, there exists
a certain amount of flood risk to the structure, nor is it economical to design for 100% flood protection. Protection against the highest rare floods is uneconomical because of the large investment and infrequent flood occurrence.
In the design flood estimates, reference is usually made to three classes: (a) Standard Project Flood (SPF). This is the estimate of the flood likely to occur from
the most severe combination of the meteorological and hydrological conditions, which are rea- sonably characteristic of the drainage basin being considered, but excluding extremely rare combination.
(b) Maximum Probable Flood (MPF). This differs from the SPF in that it includes the extremely rare and catastrophic floods and is usually confined to spillway design of very high dams. The SPF is usually around 80% of the MPF for the basin.
(c) Probable Maximum Precipitation (PMP). From the observations of air moisture from the maximum due-point and temperature recorded and air-inflow (from the wind speed and barometric pressure recorded), the moisture inflow index in the storm is determined. The best known upward adjustment to be applied to the historical and hypothetical major storms is the
FLOODS-ESTIMATION AND CONTROL
maximisation with respect to moisture charge. The adjusted storm rainfall is assumed to bear the same ratio to the observed storm rainfall, as the maximum moisture charge over the basin to the moisture charge of the observed storm. From the critical combinations of storms, and moisture adjustment the PMP is derived which, after minimising losses, when applied on the design unit hydrograph for the basin, will produce the MPF. Occasionally when enough storm data for the given basin is not available, PMP can be estimated by adopting a severe storm over neighbouring catchment (which is meteorologically homogeneous) and transposing it to the catchment under consideration.
(d) Design Flood—It is the flood adopted for the design of hydraulic structures like spillways, bridge openings, flood banks, etc. It may be the MPF or SPF or a flood of any desired recurrence interval depending upon the degree of flood protection to be offered and cost eco- nomics of construction of structures to the desired flood stage; the design flood is usually se- lected after making a cost-benefit analysis, i.e., the ratio of benefit to cost may be desired to be the maximum.