Is Sediment Basin Needed? 1

[jsullivan52]

Is a sediment basin needed with the site described below.

1) All flow is directed to a single point (culvert), that has inlet protection in the form of rip rap being placed to a height of 1' foot above the top of culvert.

2) Silt fence has also been required for the perimeter of the site.

3) 15 acres of disturbed area.

4) A total of two weeks from time of breaking ground to final seeding.

In my mind a sediment basin will provide an unneeded third measure of redundancy.

Can someone please explain.

Thanks for your help!

 

[cvg]

the inlet protection probably does not trap any sediment, so your ONLY line of defense is the silt fence. if a "flat" site than perhaps you are ok. however, if this is a steeper site than you have very little protection against releasing sediment.

 

[sam74]

It would depend on what your state/local government requirements are. Here anything with over five acres of disturbed area that drains to an existing impaired stream requires a sediment basin. Any project that has more than 10 acres of disturbed area requires a sediment basin.

What does your state (assuming your in the states) Erosion and Sediment Control Manual specify?

 

[jsullivan52]

Thanks for your comments.

The state DEQ approved it without one, but the city is requiring one, stating that it is needed. I was just wondering on the reasoning why. The state manual states to have one on a site where 10 or more acres will be disturbed at any given time and also have the capacity of 133 cubic yards per disturbed arce, but by the time the contractor builds the levee for a basin that size (which will take up about half the site) he will have already moved the dirt he needed to move and the project will be over and the site will be seeded. This is just a grading job. No building construction, parking lots, roads, utility, Etc... I guess I need to call the state and ask them why they let it go, so I may plead with the city to allow the basin to be ommited or is there a logical reason to have a basin this large for this short of a period?


Also when i ran through the post developed and pre developed hydrograph, I got less flow on the post-developed. The pre-developed land is wooded, lightly brushed (CN = 77), slopes of 6%. The post-developed will be grassed (CN = 74), slopes of 5%. The city engineer said that he didn't see how the post-developement was less than that of the pre-development... Why?

 

[cvg]

first of all, the earthwork cannot begin until the bmp's are in place,

secondly - it could rain hard during your two weeks in which case you will discharge sediment

third - it will take a month or more for the grass to grow and realistically about two months before the site is fully stabilized per your estimated CN = 74. Until that time your CN will be much greater than 74.

The size of the basin is unrelated to the time it takes to grade your site. It is determined by an estimate of the amount of runoff and sediment that the basin will see during the design storm.

Around here, anything other than a single residential lot is required to construct full - permanent stormwater retention for the 100-year storm.

 

[jsullivan52]

Thanks cvg,

Are there any alternatives such as some form of trap located at the culvert to control sediment runoff? Is it normal practice to leave the sediment basin until you have reached full stablization (i.e. grass fully developed) or just until construction is finished and ground has been seeded?

 

[cvg]

you may want to have a discussion with the city since they are apparently administering NPDES in your area. They can answer your questions better

 

[francesca]

Your basin has to remain until the site is fully stabilized, likely in its final stabilization - i.e. constructed in the final purpose. NPDES permits cover the life of the project and while the contractor will sign off after grading and stabilization is complete, the building contractor and anyone else who disturbs ground will sign onto it. In a residential subdivision, for instance, every home builder should sign the NPDES permit, though few do.

Yes, a basin is definitely required on sites like yours in most parts of the country. Here, the requirements are similar to Sam's, but it's 5 acres drained, regardless of disturbed area. You can get around it on certain sites. For instance, I designed a subdivision near Kentucky Lake that had roads along the ridgetops and large draws down to the lakeshore. We called for silt fence on contour all along the road and sediment traps at every pipe outfall, of which there were few. The argument was that building a sediment basin would disturb vast areas of ground which was intended to be left wooded.

 

[HawaiianMarine]

I am going to assume that there is a ditch running to the culvert. Your BMP plan can be custom fit for the jobsite so I would reccomment placing several check dams in the ditch. They are much easier to maintain and most agencies will let you leave them in place at the end of the project. Sediment detention basins, which are very effective, are somewhat ill suited for a short duration project. I would try to sell the city on providing some permanent BMPs such as straw bale check dams, SC-150 ditch liner, etc. Be creative, there are a ton of products out there.

 

[cfj104]

Just an idea - but you may want to consider an 'above ground basin'. Basically this is just large wattles or coir logs arranged in a baffle configuration to trap sediment and increase the flow path. These can be installed easily, however, they can not be moved so be sure to locate outside your sonstruction area.

I have presented this system to the conservation district in the past and they have accepted the design.

Filtrexx is the manufacturer I specified in the past - just be aware that they require an approved contractor install the system.