Similitude of events + outlet device

[SMIAH]

I'm working on a retention basin proposal for a 75 acres semi-urban development.
The design criteria is Q_peak pre = Q_peak 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!

 

[cvg]

I have seen 100-year storm drain outfall into river flowing at 10-year level commonly accepted here to account for the fact that the watershed for the river is mostly in the adjacent counties and controlled by several dams and so the 100-year peak flows are unlikely to ever occur at the same time. your situation is similar although not quite so extreme. I think you could justify a lower flow in the river for your tailwater and 10-year flood levels might be reasonable. But it depends on the type of storm, duration and rainfall distribution. You may need to convince the reviewer what a reasonable assumption of that tailwater is.

 

[SMIAH]

I'm almost thinking of doing a complete analysis to waive off the detention criteria. Erosion control and sediment control for frequent storm.
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.

 

[cvg]

might be possible. detention can cause unintended harmful consequences downstream. Retention for water quality and infiltration may be a better alternative.

 

[SMIAH]

I'll try to explain this to the non-hydrologist in charge of the review.

 

[beej67]

If I were you, I'd do an unsteady state HEC-RAS model, figure out my variable tailwater in the creek/river, and utilize that variable tailwater in my pond routing.

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

 

[Drew08]

You can probably push beej67's unsteady-state method past a reviewer. I've done it. But, the catch is real storms don't happen in nice 24-hour durations, and 100-year floods don't happen during 100-year rainfall. It is more typical that there are exceedingly wet conditions for a week, a month, that brings larger rivers up to the 100-year flood stage, then a short, high intensity spurt happens that has little impact to the large river, but drives the small tributary to its 100-year flood, and magically you have matching peaks of very different times of concentration.

 

[beej67]

Agree Drew. That's why Florida DOT, for instance, makes you model a bunch of different duration storms when you're doing jobs down there, to see which one is the worst for your watershed's particular geometry. I haven't found anyplace else in the southeast that goes to that length to check hydrograph timing.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

one could argue that the "joint probability" of a 100-year flood stage in the river coinciding with 100-year peak discharge from your subdivision would be statistically something greater than 100-years. Again, convincing the reviewer would be difficult but you might be able to convince the floodplain manager who hopefully is an engineer.

 

[SMIAH]

That's want I want to do next week : perform an hydrological modeling using different storm intensity/duration and different loss method/value. But I think it will come up to the transform method/value...

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).

 

[SMIAH]

Read you Drew about matching peaks of very different times of concentration.
Hmm!