Dewatering

[martin888888]

We have a 4ac construction site consisting of mass excavation, utility installation, etc where dewatering techniques are required. The plan is to discharge to an existing roadside ditch which has been designed with bio-bags for ESC.

What is normally the proper technique to accomplish this? Are sediment removal products such as dewatering bags always used? I have suggested to use dewatering bags and add bio bags at smaller intervals within the ditch. Contractor also needs to put this in their SWPPP. Any other hints of what the best techniques are to avoid erosion and contain sediment when dewatering?

 

[sam74]

I have never seen the results of the process you are describing with the products you have specified.

I have seen the results of one of the methods listed in the publication linked below using polymer enhanced BMP applications in a manmade trench where a sediment pond was pumped. It seems pretty effective but the polymers used are site specific so you would have to send a sample to the vendor to verify which polymers to use which would take additional time. Also this publication is not really a design guide in the sense that it does not tell you how long of a trench or how far to space floc logs or how many. Possibly the vendor could assist with this.

http://www.siltstop.com/altbmpguide/closedloop/WT.pdf

 

[Twinkie]

Are you using a pump?

Do you have the ability to design a perforated riser or use a skimmer?

 

[DDBPE]

De-watering bags are used quite often for pumping water from sites that discharge offsite. The bags can be sized to the pump and expected volume of silt. Sometimes smaller bags are used, you just have to monitor how full they get and switch them out when they are at capacity.
I have used ones that are 10'x10' up to 25'x100' (we used 8 of the larger ones at a single site,let them drain out typically into a sediment trap, and then buried them in a non-structural fill area). Several environmental supply companies in the states make them and have the literature.

 

[bimr]

It depends on your NPDES discharge requirements. Some agencies have more restrictive construction discharge requirements than others. What are the NPDES requiremens?

The best management practice will probably be a filtration system. The bags may not be adequate.

http://www.clearcreeksystems.com/temporary/best.htm

 

[cvg]

besides trench dewatering for utilities (and not counting stormwater runoff) - is there anything else? seems like you could retain the water pumped on-site in a temporary retention basin and just allow it to infiltrate back into the ground.

usually dewatering bags, polymer treatment and floc etc is used when the discharge is potentially running back into a stream.

 

[beej67]

Most of my dewatering experience is in South and Central Florida, and in those areas you typically can't discharge the water at all, you've got to infiltrate it back into the ground on your site in a periphery swale, so you don't impact the water tables of neighboring land.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[MarcoSG]

Depends on the contract requirements. But could place a Rock Check Dams and Silt Fence Barriers in the direction of flow of the ditch and make sure these are cleaned regularly. This would be the least costly and pretty effective.

 

[Twinkie]

Check dams are fine in a roadside ditch, I would shy away from using silt fence in an area of concentrated flow such as this.

What are you doing for permanent SWM? Is there any reason why you aren't doing a sed basin or trap?

 

[Phidelt218]

I am a SWPPP Preparer and also a storm water inspector. But first i will say read the project SWPPP they typically line out how to dewater the project. If not, i would read your state regulations for dewatering as sometimes there are exceptions to needing BMPS, say if you are only pumping over a certain rate. If it is not in the SWPPP then an addendum would be needed to the SWPPP to add it.


It is a BIG NO NO to pump water from dewatering into a ditch without any BMPs since a ditch is classified as a conveyance system. I have seen many sites where the contractor leaves out those controls. That is why it is typical to have several control in case one fails. Problem with direct pumping into a ditch is you can over-top your other controls if pumping more water then can be handled. Bio-Bags, are you referring to something similiar to a wattle, coir log or along those lines, used to make a check dam?

In my SWPPPs i typically call out a dewatering bag that then drains onto the controls site that goes into a vegetated swale and through the perimeter control, let is sheet flow into a ditch from the dewatering bag with wattles, check dams, ect., or if in area with curb/gutter, place wattles in curb line and then place drop inlet protector (witches hat).

Do not place a silt fence in a concentrated flow, they are not designed for that. If you have a detention pond onsite you can pump to that as well.