Flood Peak Flow Rate

Gents

I am in the gun again because the local hydrologists havent got the balls to make a call.
Catchment area = 3.2 sq km
Peak 24 hr rainfall = 112mm
Rainfall runs into dam/sump 32000 m^3 vol full, 5000m^3 effectively empty.
Catchment area is approximately part cone shaped. Average distance to dam/sump = 700m
Average fall to dam/sump = 60m
Climate is subtropical or 45 deg S.

Terrain: bare earth with some tussock,say 25% cover.

I need to size a pump and pipeline to stop the dam from overflow. Peak pumping rate is required.
If I know the peak hourly inflow to the dam, then the answer can be derived from a series of simple sums. The inflow is what I normally expect to be provided by hydrologists, but the clients hydrologists prefer to analyse historical data rather than do anything useful. Unless I receive better information, or some guidance, I am going to assume that 85% of the entire 112mm reports to the dam, evenly spread over a 4 hour period, so I need pump with a peak capacity of :

(0.85*(3.2 * 10^6*0.112))-(32000-5000))/4 = 70,000m^3/h = 20m^3/s Which seems ridiculously high.

Am I out by a factor here?

The "4" is the wild guess. If the local hydrologists disagree, I will probably punch them on the nose.

Can anyone help me out here, or does anyone have an associate who can?

Hope I have made a mistake. My footie team just lost a big game, and I have been out commiserating (XXX) so my arithmetic may be less than perfect.

Cheers

Steve McKenzie Received on Mon Mar 31 06:13:00 2003