PROBABLE MAXIMUM PRECIPITATION (PMP)
In the design of major hydraulic structures such as spillways in large dam, hydraulic engineers should try the failure probability as low as possible.
In the design and analyses of such structures, probable maximum precipitation (PMP) that may reasonably be expected in a basin is adopted.
This PMP is defined as the greatest or extreme rainfall of a given duration that is physically possible over a basin.
PMP is the critical depth area duration (DAD) relation for a given basin, which would call from a storm due to the most critical meteorological conditions that are probability of occurrence.
The development of PMP for a given basin is quite complicated and requires the help of an expert hydrometeorologist.
The PMP is usually estimated by mauming the effect of various meteorological factors
humidity, temperature, and wind
There are two approaches for the estimation of PMP :
1. Meteorological approach
2. Statistical approach
1. Meteorological approach:
In this approach, PMP is derived from the depth area duration (DAD) analysis of precipitation that have occurred or would have occurred in the basin.
These DAD curves are adjusted for maximum moisture changes which are probable.
The DAD curves of PMP are then obtained by enveloping curves of the adjusted values for all storms
If the precipitation data of a basin is not available to develop DAD curves but the same is available for an adjacent basin having similar meteorological conditions, storm transposition can be done.
In this process, the storm of the adjacent hasin is assumed to occur over the hasin under consideration.
In other words, the DAD curves of the adjacent basin are used for the basin under consideration.
However, the transposed storm is modified for factors such as elevation, latitude and distance from moisture source so as to make it representative for the basin.
2. Statistical approach:
PMP of a basin can be estimated by the statistical approach if data is available for a long period.
The record of maximum annual rainfalls at the rain gauge station is analysed using the probability theory.
The mean of annual maximum precipitation and the standard deviation are determined. PMP is then estimated from the relation :
where K ~ frequency factor depends upon the statistical distribution of the annual maximum precipitation series, number of years of record and the recurrence period. Its value varies between 5 and 30. An average value of 15 is usually taken.
USE OF PMP :
PMP is required for the estimation of probable maximum flood (PMF) for the design of spillways of large dams. Because the failure of such dams would cause heavy damage to life and property.