Comments on: Percent Impervious as a Threshold Option

By: StormBlog23

StormBlog23 — Thu, 17 Dec 2009 22:53:52 +0000

I support this idea. Can we find a way to do this without requiring every property owner to do modelling to support their project? That increases the cost of the project and often makes it not feasible economically.

Trading off detention for increased BMP requirements makes sense, and we’d be accomplishing more from a water quality and stormwater management perspective.

By: StormBlog32

StormBlog32 — Tue, 24 Nov 2009 21:20:42 +0000

There are a number of different people who support the idea of requiring no detention for developments within a certain distance of a river. The basic thought is one of timing where allowing the water to runoff, get into the river and flow downstream before the peak hits will reduce the peak flow in the river.

In developments that are within a certain proximity to a river, you could argue that water quality and BMPs are more important than providing detention. Maybe eliminating or reducing the detention requirement for these sites with an increase in BMP requirements would be a fair tradeoff.

By: StormBlog32

StormBlog32 — Tue, 24 Nov 2009 20:49:19 +0000

This area of the ordinance should be revisited. The use of disturbed area as a trigger for detention does not seem appropriate for an open space type development where regrading of a vegetated area is done for soccer fields or some other vegetated improvement. Addition of impervious area is really what increases runoff. Some combination of impervious area or percent impervious with the type of development seems appropriate. After all, requiring detention for small developments is not practical. The current ordinance recognizes that.

By: StormBlog23

StormBlog23 — Tue, 17 Nov 2009 21:52:54 +0000

Please provide additional discussion of TIA and EIA. Specifically, what do you mean by this statement: “EIA is the portion of the TIA that has improved drainage area.” What do you mean by “improved”? Improved meaning “developed”, or meaning “better than it was before”? Please explain.

By: StormBlog52

StormBlog52 — Thu, 12 Nov 2009 19:57:52 +0000

Total impervious area may be part of the answer. It seems to be a simplfying change. The key is to do things that improve conditions never make them worse.
We should probably consider defining what makes things better to a degree sufficient to warrant approval of the project. Should local and or regional impacts be considered? Should quantity issues take precedence over quality issues? I think so. Adding storage in downtown areas because someone wants to redevelop their property may not be feasible at all and may kill the project economically. We should establish criteria to establish what makes exisitng condtions unacceptble. Soils are important too. If we make a site 100 % pervious when it was 100% impervious. this may mean very little if there is a clay layer below the surfacce that effectvvely traps the water filtering into the soil. It might be more useful to establish municipal sinking funds to put money into for a stormwater project that can be shown to have significatn postive impacts on the general project area rather than force detention on small fully urbanized sites. The SERZ amendment should be simplified. I would expect that the criteria have made it so difficult to achieve that very few areas have qualiifed for this exemption. Whatever we do we need to economically competitvie with surrounding counties.

By: StormBlog9

StormBlog9 — Thu, 12 Nov 2009 17:16:01 +0000

There is no doubt this section of the Ordinance that dictates when storage is required must be improved. Currently, it is very confusing and problematic. It needs to be greatly simplified.

I like the idea of basing storage on thresholds of impervious, but I think we will also need to have area thresholds in combination with percent impervious. For instance, do we want an individual residential single family quarter acre lot to provide storage if they exceed the percent impervious? On the flip side we may want a large residential development to provide storage even if they don’t exceed the percent impervious just because of the volume of runoff being generated. Much of this would depend on where we set the threshold though.

The other problem that needs to be addressed is the situation where you are inceasing the rate of runoff, but don’t meet the area threshold requirement for a particular land use and thus don’t have to provide storage at 0.1 cfs/acre. However, you do have to provide storage so as not to increase the rate of runoff. This becomes a problem because there isn’t a specified area to which ths applies. Does it apply to a 100 SF shed, 500 SF patio, 3,000 SF of sidewalk, 10,000 SF intersection improvement or a 24,000 SF commercial parking lot?

Another related problem is how we define impervious. Does it include a gravel shoulder, wetland, retention basin, pervious parking lot, flood plain, etc.? Currently DuPage County considers pervious pavement as impervious because it is usually placed on impervious clay. Exceptions are made if you can prove the insitu soil can be infiltrated. I still don’t understand this rational because all lawns are considered pervious even though almost all are placed on impervious clay.

If more detail could be provided it would help me better understand if this would be a simplier approach. I think it does hold some hope.