Stormwater management in & around Coastal Regions

[SACPEengineer]

We are doing a new residential subdivision in a coastal region. My supervisor is thinking of using ditches to convey some of the stormwater with some ponds here & there. However the land is very flat where the average elevation is 6-7 feet above sea level. Is this a wise thing to do?

Have anyone of heard of the stormwater modeling program Supra ?

Does anyone have an idea to what would be the best stormwater modeling program to model stormwater for coastal regions? Constructed pond storage and conveyance routing?

Thanks much in advance

 

[cvg]

surface conveyance may be your only option, sea water, groundwater and tides will easily surcharge a storm drain pipe.

 

[Twinkie]

Backflow preventers are your friend.

 

[TerryScan]

Surface drainage utilizing ditches and biofiltration swales is a useful tool in coastal areas. If one needs to deal with water quality, one will need to create small watersheds for bio-swales so that one does not exceed treatment capacity.

While ponds end up being a practical solution as well, be aware that they may take up a lot more area than in other regions, since one must make up for the lack of the vertical component of volume -horizontally. Pipe surcharges may be an unavoidable reality too.

I have not heard of Supra and could not find it in a quick web search.

It may be highly relevant to use a program which utilizes dynamic equations - handling interconnected pojnds, backwater and reverse flows. These dynamics are common in coastal stormwater design. (SWMM, ICPR, Pondpack...)

Backflow preventers MAY be your friend -or an unneccessary expense. I have seen designs (created with a non-dynamic model) which called for expensive Tideflex valves (which I have used where appropriate) - where water levels in a fully dynamic model were no different without the valves due to tailwater (there was not a fresh/salt water concern).

 

[SACPEengineer]

Thank you all much for the responses,
I like the idea of utilizing backflow preventers for the pipes. I will make sure to pay close attention to surcharges.

TerryScan
The other day I pulled the records for the proposed site. Apparently some other engineers had previously done some preliminary work on it, but nothing came of it. It had some stormwater calcs & modeling that was done in Supra. Just never heard of it and neither has my supervisor. We have ICPR, but I didn't know if Supra was more appropriate for coastal regions.

Anyway some soil borings illustrated that the seasonal high water table was approximately near 4 ft.
Since they did this some time ago, would the soil boring results be applicable or is it commonly known that the shwt fluctuates throughout the years?
Would the high water table be very problematic for bio-retention swales? If we go this route, is standing water to be normally expected during the rainy season?

thanks

 

[Twinkie]

You ideally should keep your swales 2' above the SHWT. As to whether you should re-rest the soils, it's a judgement call. Are we talking 2 year or 20 year old tests?

 

[TerryScan]

ICPR was developed by an engineer in a coastal region (Florida). I believe ICPR is quite appropriate for your application(one can model tidal tailwater, for example).

I am still surprised there are no search hits for Supra. I can find mention of other civil software that has been dead for decades. Is there any indication of who the publisher of Supra is/was?

 

[SACPEengineer]

Twinkie
They are more closer to 20 year old.


TerryScan
The most I can find is that Supra was made by Supra Engineering Software. I located a couple engineering companies that listed it as part of their modeling packages that they use on their websites. Which had originally led me to believe that it was used for a very specific purpose (coastal regions).

 

[cvg]

SHWT levels are controlled by source, topography and geology.
for instance, if controlled by a groundwater aquifer, than they can vary due to amount of pumping and the geologic profile and can be very deep. if close to a river or lake, than the lake level will exert influence. Being close to the ocean, they are likely tied closely to sea level. so your groundwater is probably always just a few feet above sea level. So I would not expect the 4 foot above sea level measurement to vary too much.
standing water is to be expected because of the high water table and one reason why underground pipes are problematic. the pipes will likely always be partially full of water which will either enter from the drainage outfall, infiltrate through pipe joints and manholes, or will be draining out of your wet pond or swale.

 

[beej67]

Some initial thoughts:

ICPR is ideal for what you're doing, because it was developed to do exactly what you're talking about.

I have also never heard of Supra.

In every coastal region I've worked in (Gulf Coast states primarily) ditches were the preferred method of stormwater conveyance unless you were forced to curb/gutter/pipe by the client or municipality. In some areas, you can't get them above the SHWT, and they just stay wet. Depends on the site and the area of the country.

In many areas, if you've got a network of ditches, you can count the volume in the ditches in your stormwater routing for detention / retention.

Check your local and state regs. Stormwater management in low, flat, coastal areas is almost always extremely different from places that actually have topography. In South Florida, for example, you don't route the 100 year storm at all. You draw an invisible glass box around your site, treat it as one giant pond with no discharge, dump the 100 year storm on it, and see what elevation it comes to. Your FFE has to be higher than that elevation, but you have to grade your site to have enough site storage that your 100 year 'glass box' stage is less than the BFE. (FIRMs down there are hilarious too, they're just one big grey blob with a number on it.) You only route the 25 year storm or below. Their rationale is if it's worse than the 25 year storm, it's a Cat 5 hurricane, and they're watching it on TV from Valdosta anyway.



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