Similitude of events + outlet device
Similitude of events + outlet device
(OP)
I'm working on a retention basin proposal for a 75 acres semi-urban development.
The design criteria is Qpeak pre = Qpeak post-conditions for 2-year to 100-year (usual day at the office there).
The basin outlet has to be in stream draining a watershed of approx. 750 acres. And the slope available is relatively flat for the whole project so that submergence effects is a concern.
I first did a HEC RAS steady flow analysis to evaluate the water level in the stream for 2-year to 100-year flood (using peak flow).
So I got the WSE for these conditions taking in effect the downstream conditions.
Checking the results : the retention pond has to be able to release it's flow rate (attenuated) with some submergence effects.
Questions:
1) Should I use the 100-year flood level in the stream (750 acres with a time of concentration of 2-3 hours) AND the calculated (routed) water level of the 100-year flood in the retention pond. Or, can I presume that both events aren't going at the same time (routed peak level in the pond is approx. 45 min). e.g. using the 25-year condition in the retention basin and 10-year in the stream.
2) If using similar events, the outlet pipe will be flowing in charge as the WSE in the stream isn't very far from the routed WSE in the pond (check valve working). What kind of outlet structure would you use to control the released flow (Perforated riser with barrel was my choice... but it's kinda hard to check the orifice flow conditions for each storms).
Thanks!
The design criteria is Qpeak pre = Qpeak post-conditions for 2-year to 100-year (usual day at the office there).
The basin outlet has to be in stream draining a watershed of approx. 750 acres. And the slope available is relatively flat for the whole project so that submergence effects is a concern.
I first did a HEC RAS steady flow analysis to evaluate the water level in the stream for 2-year to 100-year flood (using peak flow).
So I got the WSE for these conditions taking in effect the downstream conditions.
Checking the results : the retention pond has to be able to release it's flow rate (attenuated) with some submergence effects.
Questions:
1) Should I use the 100-year flood level in the stream (750 acres with a time of concentration of 2-3 hours) AND the calculated (routed) water level of the 100-year flood in the retention pond. Or, can I presume that both events aren't going at the same time (routed peak level in the pond is approx. 45 min). e.g. using the 25-year condition in the retention basin and 10-year in the stream.
2) If using similar events, the outlet pipe will be flowing in charge as the WSE in the stream isn't very far from the routed WSE in the pond (check valve working). What kind of outlet structure would you use to control the released flow (Perforated riser with barrel was my choice... but it's kinda hard to check the orifice flow conditions for each storms).
Thanks!





RE: Similitude of events + outlet device
RE: Similitude of events + outlet device
The drainage area ratio is 10%.
The stream is almost uncontrolled but the hydraulic length is almost 2 miles on a flat slope (0,5%).
Not easy to shod without performing a detailed analysis.
RE: Similitude of events + outlet device
RE: Similitude of events + outlet device
RE: Similitude of events + outlet device
That's the most complete way to do it.
There are times when I've done that, and been able to show to a reviewer that detention actually harms more than it helps.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Similitude of events + outlet device
RE: Similitude of events + outlet device
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Similitude of events + outlet device
RE: Similitude of events + outlet device
I know that the unregulated 100-year peak flow of the urban development will be equal to the ±15-year peak flow of the stream. If this 100-year flow is released before the 15-year peak flow of the stream, then there's shouldn't be a problem (i.e. erosion, exceeded capacity, flooding) immediately downstream (as the 25-year flow isn't causing such problem).
There's a culvert 150 ft downstream and then a couple of houses 20 to 30 ft from the stream before flowing onto a undeveloped land (woods).
RE: Similitude of events + outlet device
Hmm!