flood level calculation

[mize_zl]

Good Evening, Dr Quek

I want to ask you, if there have any method or calculation to find a flood level for the proposed crossing bridge after we calculate the Peak discharge using rational method. normally we use HEC RAS, or is it posible to Manning ?

I appreciate if you can explain.

Thank You.

[drquek]

Hello mize,

To calculate the flood level for a bridge crossing, it is recommended that you use a backwater computation software to compute the backwater profile. This is because most rivers have constrictions which causes the flow to be non-uniform normal depth flow. If you use Manning's equation, the result will not be accurate as it assumed uniform normal depth flow.

On the method for computing the design flow, you should not use the Rational Method if the catchment area is more than 0.8 km2 or 80 ha. You should use the Hydrograph Method e.g., Time-Area Method or Runoff Routing Method.