Grading - Stormwater pond closure.

[jjv2006]

I am working on the grading for the closing of an existing storm water facility with fill and stone (6 inches) of almost 1 acre size in area and 10 feet deep. The closure should provide positive drainage to a regional detention basin
Downstream. Because of the enormous size of fill involved I kept the top of the berm and grades on the other side of the embankment intact (3:1) intact and graded the whole area at 3% close to where the outlet structure of the old detention basin was located. I did not want to create a high point anywhere in the middle of the pond to avoid an increase in fill required for the same. Also, there is a large pipe 48" which is located on one side of the pond. I am not sure on how the drainage from that area will be transmitted downstream. Should I design a pipe extension to the downgrade area?
In summary, all I want to do is close the pond, fill it with stone and fill, take care of the drainage from one of the inflow points and provide a positive drainage flow to the downgrade area (i.e. old outlet structure area).

Any comments and suggestions will be greatly appreciated on the concept based on the information provided above.

 

[LHA]

Why not just demo the berm and outlet structure? That reduces the amount of fill and eliminates any potential drainage problems, including the 48" pipe. Just grade in a swale through the basin bottom and throught the demo'd berm.

With an acre to play with, and only 10 feet drop across, you should be able to keep all final grades inside under 10%, so you can re-use the land area.

Engineering is the practice of the art of science - Steve

 

[jjv2006]

Thanks for the input. The client wants to keep the berm intact. I did grade by keeping the berm intact on both sides of the iregular shaped pond to provide a positive drainage (2-4%) to the middle of the pond to a inlet and then connect it to a existing pipe/sump system to the downgrade reservoir.

Is this a right approach...


Thanks.

 

[BelspringKen]

I don't see why that would not work.

Why is the client so eager to fill the pond and not demo the berm and outlet structure? This client must have a use for the area in mind.

 

[TDAA]

Is the inlet existing or do you have to add it? If adding, look to tie in the 48" into that. That should save some piping. Perhaps even move the inlet closer to the 48".

 

[jjv2006]

Thanks for the input.

The idea of keeping the berm intact is to keep the drainage pattern on the other side of the berm intact and not modify anything since there is a rail road on one side of the berm. The use of the area after the basin is closed is to have a open space area to satisfy local officials.

I am planning to use a doghouse manhole or some new drainage boz near the endwall which has the 48
" pipe and tie in to the proposed inlet and then connect it downstream. Because of cover conditions, i am planning to tie the existing 48" pipe to a new inlet box and then use a equivalent elliptical pipe on the other side of the proposed inlet to handle existing flow and also drainage from the filled in area of the basin to the downstream area....


Again thanks for the input and let me know if the above is a good approach.

 

[TDAA]

It seems OK form a geotech perspective. If the site is to be used for anything in the future, I would be sure to maintain good documentation on the fill type, and methods of placement. I would hat to see someone recommend pulling it out in the future because there is insufficient record of what is really there.

Also, consider the migration of soil through the rock fill. You may see some soil loss on the top, if the materials differ significantly (as in: doesn’t act as a filter to keep it in place). Also consider that water infiltrating into the stone will be trapped. Do you still have an outlet for it at the bottom of the fill?

 

[jjv2006]

The fill will be documented with the local municipality and DEP as part of the closure. I am planning to provide of 6 inch stone buffer to account for compaction and all other soil loss. I don't there will be any other outlet at the bottom of the fill than the closure and the proposed drainage pipes. Is there a need for a outlet..

 

[Ron]

What are the soil conditions at the bottom of the pond? Since it was apparently used for some time, there is a likelihood of organic material buildup (muck), that shouldn't be filled over...it should be removed to competent material and then filled properly....it's likely that the area could be built upon in the future, so don't build in a soil problem.

 

[jjv2006]

The soil conditions is silty loam and probably in a bad shape. But, I think the geologist is looking into the groundwater issues. I don't foresee any use to this area because it is going to be designated as a open space area. Also, a environmental site assessment will be done to account for the soil conditions and other related issues.
Thanks for your message Ron.