Ordinary high water mark

[calculor]

I am working on project which bound by a swale on one side (which cannot be used to drain this site) and a river. My problem is that I can only drain the site to the river which is located 45 feet from the nearest catchbasin. The ordinary high water mark (OHWM) for the river is 7.42', the top of my catchbasin is 11.5'. With 2' cover at the catchbasin for the lead and an 18" dia. lead at 1% I will be approx. 6" above the OHWM. In addition I also have to install a stormceptor before discharging in the the river. Does anybody have comments or suggestions?

 

[WILDWINGS27]

DO YOU HAVE TO RUN THE PIPE AT 1.00%, OR CAN IT BE LESS TO CARRY THE FLOW?

 

[calculor]

.5% would be adequate. I am thinking I may reduce cover at catchbasin to 1.5' and reduce grade to .5%, this would put me around 16" above OHWM.

 

[cvg]

You don't have much freeboard, what about 100-year flows? Would you consider filling the site? If not, install a check valve in the outfall - flap gate or tide flex

 

[calculor]

can't fill the site because of the height restriction for the building. I have specified a tideflex check valve at the outfall. My only concern is for the 100 year flow, if the river level rises the and the check valve closes the stormceptor and catchbasin will be surcharged.

 

[cvg]

a good solution (which is always required around here) would be to construct a retention basin. The basin can be slowly drained into the river after the water level drops. It will also trap sediment which might allow you to eliminate the stormceptor

 

[calculor]

I don't have enough area to retain the water, my 100 year flow is around 4cfs. Since I don't know how high the river will rise or how quickly it will recede I can't really get a feel for how long of a period I will be accumulating water without any discharge, an hour, 4 hours?

 

[Maury]

I've never heard the term "ordinary high water mark". Can someone tell me what it means and where it is used?

 

[cvg]

stormceptors aren't typically designed to handle 100-year flows and maybe you won't be required to treat this flow rate. Can you design a smaller basin that will overflow through a spillway to the river during the 100-year flood and then retain the smaller storms?

 

[calculor]

It really just occurred to me that the pipe will actually be under pressure as long as the river elevation does not rise higher than the top of the catchbasin. Therefore I really shouldn't be overly concerned if the invert of the pipe is below the OHWM. I also agree that the stormceptor will not be able to had 100 year flow, therefore I will install a bypass around the stormceptor at a higher elevation for the 100 year flow. Does this sound logical?

 

[francesca]

4 cfs? Seriously? Q = AiC. You didn't say your land use, but if water quality is required, it's at least residential (C = 0.5). Around where I am i = 10.8 for the 100-year, 5-minute storm. Solving backwards, A = 4/10.8/0.5 = 0.046 acres = 2016 sq ft. That's hardly a large enough lot to put a house onto.

If it's a parking lot (C=0.95), half that size.

So are you sure about that 100-year flow? Because I believe you now about not having enough room for a detention basin.

 

[calculor]

I believe based on your example A=4/(10.8*.5)=0.74ac.

In my area i=6.6in/hr, area of grass=.4ac, roof=0.34ac, parking lot/driveway=0.312ac, therefore total = 1.05ac, C=0.6....Q=4cfs. In the parking lot I have maximum elevation diff of 1 foot...very level.

 

[cvg]

in my area, 100-year, 2-hour retention is required for 2.7 inches of rainfall...

volume of runoff is approximated by V = C * (I/12) * A

assuming total storm is 6.6 inches, then
V = 0.6 * 6.6/12 * 1.05 = .35 ac-ft

depress your grass by 1 foot and you have room to retain the entire thing