carbonate geology

[turbo170]

I am designing stormwater detention structures for a site with extensive carbonate geology. It is evident because runoff from the entire 220+ acre drainage area passes through an existing 15" pipe without problem. There are also some sinkholes on the site. The problem I have is accounting for this carbonate geology in the stormwater runoff calculations. I have used PSU IV for other projects but the minimum drainage area for that method is 1 sq. mile. Any valid methods out there for accounting for carbonate geology when performing stormwater calculations?

 

[cvg]

a large part of hydrology requires applying "engineering judgement". However, I would investigate what others have done in the past before just dreaming up some new loss coefficient. You may find however, that the subject has been studied by others and subsequently accepted by the regulating agencies. I would suggest you discuss this with your agency reviewer, indicating that you feel an adjustment of loss parameters is a reasonable approach and at least get some concurrence what an appropriate value of adjustment might be.

 

[francesca]

You need to delineate your site into its various drainage basins. I'd wager most of it drains to sinkholes and only a small section to the 15" pipe. When you do your development, you'll use the normal hydrology methods to calculate your runoff and you need to make sure that your retention basins where your sinkholes are are large enough to hold the design storm volume.

Plan to install Class V injection wells in each sinkhole to assist the water in draining and prevent spectacular fall-outs. As the water percolates into the soil, it forms a vortex. If that vortex isn't contained in a pipe, it is very erosive and can have some pretty dire consequences for any near-by structures. Changing thet land use increases the amount of water draining to a sinkhole and this is when the sinkholes become unstable and fall out all over the place.

 

[turbo170]

cvg - Thanks for your response. I have talked with several agencies, including the agency reviewer, and they did not suggest any accepted calculation methods for this situation. This is part of my investigation to determine what others have done in the past instead of dreaming up a reduction factor.
francesa - Thanks for your response. The pre-development drainage area totals 220+ acres which all drains to the existing 15" pipe. There are sinkholes on the site but none in the primary drainage course through the site. Therefore it can be concluded that runoff does not necessarily enter the ground through visible sinkholes. In fact there is a spring on the site which surfaces for a couple hundred feet of overland flow and then spreads out and disappears. There is no visible sinkhole, it just percolates through the soil. I have delineated the site into several post-development drainage areas which all drain to detention basins. The problem arises when I try to meet the pre-post flow reduction for this large area (much of which is not disturbed).
I am unable to create a reasonable pre-development model of the site which shows what runoff is doing on the site.

 

[LHA]

The 1 sq mi min. is just an arbitrary number PennDOT uses. If it is not a PennDOT job, you can use it, although 220 acres (less than 0.5 sqw mi) is pretty small for a regression model. Section 5 of PSU IV allows correcting output less than 1.5 sq miles, by PennDOT rainfall region.

Engineering is the practice of the art of science - Steve

 

[francesca]

Have you done a soils analysis? The water can only get into the sinkhole as fast as it can get through the soil, and it sounds as if your site has some particularly porous soil.

When you say "visible sinkholes" do you mean open throated sinkholes or depressions with no outlet?

I agree that the site is a bit on the small side for a regression model, but if you want to go that way, Kentucky Region 7 regression equations were developed for a Carst landscape. The soils generally are type B & C, and by the sounds of it you're dealing with a type A soil, so the equations may not be applicable.