Description of Core Hydrological Terminologies in Flood Control Analysis

By Engineer John Cee Onwualu (FNSE)

Rainfall Intensity

This term is used to define the amount of rain falling during a specific time within the most intense period of the rain.  This value is then converted into the amount of rain that will fall in one hour.  The rainfall intensity is expressed as:

Rainfall Intensity, I = Maximum Rainfall Depth (mm)/Duration (hr) ………………           Eq. 1

Rainfall Intensity has an important effect on runoff proportion, as it determines the rate at which rainfall runoff arrives at the soil surface and, consequently, when infiltration rate of the soil is low to allow absorption, flooding of the environment does occur. 

Return Period

Since this study involves the design of hydraulic structures for the management and control of high flood levels without failing, a knowledge of design frequencies (Return Periods) of these maximum rainfall intensities must be obtained.  This Return Period (T) is the average number of years during which a flood of a given magnitude will be expected to be equalled or exceeded once.  It is computed using Weibull’s method of ranking and expressed as:-

Return Period, T = (n+1)/m                          ……….………….…………………….                         Eq. 2


            n = number of years

            m = event ranking

            T = recurrence interval

The derived data were used to generate the rainfall Intensity-Duration-Frequency (IDF) Curves for Asaba and Warri for the various rainfall intensities used in the computation of peak flow discharge (Qp) at different durations or time of concentration (Tc).