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Saltwater Impoundment Design

Saltwater Impoundment Design

Saltwater Impoundment Design

Our firm has been asked to design a saltwater impoundment (pond) along an existing saltwater creek that will be used to hold crabs/shrimp. The pond will be approximatley 1.7 acres and 11 ft. deep. The pond will fill and drain through a single culvert connected to the creek. We'd like to maximize water exchange with the tide change and need to size the culvert accordingly. I'm thinking design would be similar to designing a manmade wetland.

Before we start digging too deep, I thought I would check on here and see if we can get pointed in the right direction. Any references, guidance, or pointers would be most welcome.

RE: Saltwater Impoundment Design

How are you planning to keep the crabs/shrimp from going out the culvert with the tide? The challenge seems to be retaining one while allowing the other.

Peter Smart
HydroCAD Software

RE: Saltwater Impoundment Design

It will be mature crabs and there will be a screen in front of the inlet/oulet that will keep the crabs in.

The inlet/outlet will consist of 36" pipe open to the tidal creek and with a concrete inlet structure in the pond. The invert of the pipe will be 4' below Mean High Water and 2' above Mean Low Water. The bottom of the pond will be 2' below the invert of the pipe so that 1 to 2' of water is maintained.

I need to make sure that the 36" pipe will allow the pond to rise and fall with the tides? Would hydrocad help me to model this or is there a formula out there that will help?

RE: Saltwater Impoundment Design

The WSE elevation in the pond is always going to lag the tides by some time interval and the variation in the pond WSE will be somewhat less than in the creek. The bigger the pipe the smaller the difference - but there will always be a difference. So the question is how much tidal variation is required in the pond? Then you can size the pipe accordingly.

But you also mentioned an "inlet structure" within the pond, which will presumably effect the behavior of the system compared to a bare culvert.

Peter Smart
HydroCAD Software

RE: Saltwater Impoundment Design

I agree with the points put forward by "psmart".
moreover if we observe the size of the pond and culvert levels as mentioned in the above comments, a simple culvert with no control structure will provide you only 2' water level in the pond. please correct me if am wrong.

RE: Saltwater Impoundment Design

If you're in the USA, be aware that there are often rules against connecting aquaculture operations to state waters that stem from the Clean Water Act. Typically aquaculture ponds must be off-line, and their water must be treated in some way before discharge to state waters.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Saltwater Impoundment Design

Thank you all for your responses.

psmart - The elevation difference between the WSE creek and WSE pond is what I am trying to calculate. This should be a function of the resistance in the pipe. I also need to make sure that the pond will drain down to the invert of the 36" pipe. We are looking for approximately 4' of water exchange with the tides. I'm thinking a hydraulic profile with the pond, outlet structure, 300' of pipe, then creek. Wondering what flow I'd put in for the tidal exchange?

mdrehansk - Why only 2' pond level? As the tide comes in the pond will fill and the pond level will match the creek level or will be close. At least that is what we are after. At high tide as the tide retreats the level at the creek and at the pond will lower with the pond WSE slightly higher until the tide recedes below the outlet pipe invert.

beej67 - Thanks for the heads up. The client is handling the environmental permitting and we are strictly hydraulics.

The outlet structure will be a concrete structure, 4' square with one side completely open. The open sides will have channels so that boards can be installed if they need to keep the pond level low.

RE: Saltwater Impoundment Design

you need to do a dynamic flow routing analysis. depending on the stage/discharge relationship of the culvert (which will be continuously variable), length and timing of tides and stage/storage relationship of your pond, your pond may or may not fully equalize with the creek level. it seems like one 36 inch culvert may be questionable if it has the required capacity. I'm thinking you might need two pipes

RE: Saltwater Impoundment Design


Thanks. As I've gotten deeper I don't believe that (1) 36" pipe will work. I don't think the the pond level and creek level will ever equalize. The tides vary from 0' to 7' over 6 hours and it is a pretty good size pond; 1.7 acres. The volume of water that we'd like to exchange is approximately 242,000 cf (4' of tide). Is there a good program out there that you'd recommend that will allow me to model the scenario and run the dynamic analysis?

RE: Saltwater Impoundment Design

HEC-HMS, HEC-RAS, HEC1, SWMMM, Hydrocad, probably others. HEC-RAS might be the best bet

if you had the stage / storage data for the pond and the stage / discharge data for the culvert, you could probably do a rudimentary analysis with Excel, at least enough to get comfortable with the pipe size

RE: Saltwater Impoundment Design

Rudimentary calculations: I’ve calculated the total volume to be exchanged through the tide 242,000 cfs, which basically accounts for 4’ of tide. I’ve determined the stage/storage data for the pond and the stage/discharge data for the culvert. For the culvert stage/discharge data, I assumed a WSE difference of 0.5’ between the creek (headwater) and the pond (tailwater) and came up with the following for 200’ of 36” pipe with no slope:

headwater/tailwater/flow (cfs)
7’/6.5’/25.8 cfs
6.5’/6’/25.8 cfs
6’/5.5’/25.8 cfs
5.5’/5’/21.7 cfs
5’/4.5’/20.2 cfs
4.5’/4’/12.6 cfs
4’/3.5’/6.3 cfs
3.5’/3’/1.9 cfs

For the stage/storage of the pond for the 4’ tidal exchange we have:

Elevation/Volume (CF)/Delta (CF)/Flow (Req’d) CFS
2.92’/0 CF/4,234 CF/1.18 cfs
3.00’/4,234 CF/55,092 CF/15.30 cfs
4.00’/59,326 CF/58,797 CF/16.33 cfs
5.00’/118,123 CF/62,498 CF/17.36 cfs
6.00’/180,622 CF/60,907 CF/16.92 cfs
6.92’/241,529 CF//

The flow required is calc'd as the volume change between elevations divide by 3600 (1 hr.). Based on this information it appears that the 36” pipe will do the job. Welcome all comments/suggestions.

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