Flash Flood Risk Management – A Training of Trainers ... - ReliefWeb

Flash Flood Risk Management: A Training of Trainers Manual
Figure 14: Slope area method
Session 13
Flood level
Day Three
Flow area = A
Wetted perimeter = P
Nature of the bed = n
Slope = S
Source: Shrestha (2008)
where n is the Manning coefficient, R is the hydraulic radius, and S is the longitudinal slope (see Figure 14).
The steps for estimating discharge using the slope area method are as follows:
Step 1: Select as straight a section of the river with as uniform a slope, cross-section, and roughness as
possible.
Step 2: Conduct a detailed survey of the river reach, and estimate its Manning roughness coefficient, n. The
Manning coefficients are given in Table 9. The highest flood mark should also be recorded.
Step 3: Calculate the flow area A and determine the wetted perimeter P using the survey data. The
longitudinal slope also needs to be taken into account. The hydraulic radius, R is calculated using R = A/P.
The values obtained are used to calculate the flow velocity during the flash flood using the Manning equation.
Then the discharge, Q, is calculated using Q = A x V.
RM 13.4: Flood routing and calculating hydrographs at different locations
Flood routing
Flood routing is a procedure used to determine the time and magnitude of flow at a given point on a water
course; it uses known or assumed flood parameters at one or more points upstream. The methods used to
calculate the parameters are described above. Flood routing can provide information such as the impact of
the flood at the locations of communities and settlements downstream of the catchment outlet. It is a highly
technical procedure and various computer software programs are available to calculate this complicated
flood routing.
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