Some doubts regarding culvert design

[c2h2u2]

Hi Dr Quek,

Thanks for answering my questions regarding Hydrograph Method.

I would like to ask you some questions here.

1. The greater the time of concentration, then longer time is needed for the rainfall to reach the outlet, and therefore the lesser the rainfall intensity. Do you got any more detail explanation to relate the time of concentration with rainfall intensity?

2. Appendix 27.B Worked Example 27.B.1 (Pipe Culvert_Inlet Control)

How to determine the Acceptable upstream flood level & Estimated downstream tailwater level in the above example? Why the minimum height of pavement above water level is taken as 0.30m?

I thanked you in advance for answering my questions.

Best regards,
c2h2u2

[drquek]

1. The greater the time of concentration, then longer time is needed for the rainfall to reach the outlet, and therefore the lesser the rainfall intensity. Do you got any more detail explanation to relate the time of concentration with rainfall intensity?


What you stated above is true, but there are some flaws in your reasoning.
Rainfall intensity decreases not because of an increase in the time of concentration of a catchment.
Rainfall intensity decreases with increase in the storm duration, and storm duration increases with increasing time of concentration.
Hence your observation that the greater the time of concentration, the lesser the rainfall intensity.
In the Rational Method, the critical storm duration is assumed equal to the time of concentration.

2. Appendix 27.B Worked Example 27.B.1 (Pipe Culvert_Inlet Control)

How to determine the Acceptable upstream flood level & Estimated downstream tailwater level in the above example? Why the minimum height of pavement above water level is taken as 0.30m?
Acceptable upstream flood level can be determined based on the maximum level of flooding which will not impact lives or property.
Estimated downstream tailwater level can be associated with the water level of ponds or water bodies downstream. This is usually determined from site inspection. 0.3 m is the freeboard requirement. This is common for roads or bridges.