Percent Impervious as a Threshold Option
Filed in Stormwater Management |
It appears that the majority of the effects on stormwater quantity and quality can be tracked either explicitly or implicitly through the measurement of the percentage of impervious area on a zoning parcel of land. In reviewing literature and studies done in this regard, a few observations may prove useful in the ordinance rewrite.
- Total Impervious Area (TIA) is often distinguished from Effective Impervious Area (EIA) when tracking impacts. EIA is the portion of the TIA that has improved drainage area. It is similar to concepts like “Directly connected Impervious Area” found in TR-55. Unfortunately, it takes quite a bit of investigation to separate EIA from TIA on a project parcel, and often generalized ratios dependent on description of the land use are used rather than direct measurement.
- An on-going challenge in stormwater Ordinance language is finding language for and descriptions of “Development” that make meaningful distinctions technically, practically and administratively. One potential way out of this on-going problem is to use the % impervious of a project as a threshold trigger for certain mitigation requirements. Here is an example:
- Define any project that is less than 10% TIA as not requiring a site runoff storage facility.
- Define projects that increase the % impervious area of a project site as requiring a site runoff storage facility to mitigate the change, if the TIA is greater than 10%.
- If a project can be shown to significantly reduce the EIA, then a site runoff storage facility may not be required.
- These are only examples, but consider something like the holdings of the Forest Preserve District, or the trend toward Low Impact Design.
- The overwhelming majority of studies show that percent impervious can be correlated with the “health” of streams, with nearly all studies beginning to register stream impairments as low as 10% impervious. While that is the “bad news”, it also tells us what is and is not likely to further degrade water quality. Many studies site the percent impervious threshold for characterization of land use as rural/undeveloped less than 10% impervious, suburban as up to 50% impervious and urban as greater than 50% impervious. Is a site that has been urban for a number of years, likely to degrade or improve conditions if a new project on the site is approved as an urban land use but at a reduced percent impervious? Certainly a trend toward a watershed scale reduction in impervious surfaces has already been recognized as a goal by the EPA and as a valid approach to meeting NPDES requirements.
- What about flooding? Don’t all developments, of any kind, increase flooding? The answer is: not necessarily if one considers that the drainage of an existing land use has often been accomplished by existing drainage infrastructure for years. Consider many of our older “downtown” areas. While localized drainage problems may exist, drainage infrastructure out of the downtown area is usually “mature” at this stage, and a new outlet or modification of the existing outlet into a stream is probably not proposed. If an urban project is discharging to the same storm sewer as the previous urban land-use, is not causing its neighbors additional damages, and has actually reduced the volume of pollutants and runoff volume, doesn’t that embody the trend we desire? Considering the current push towards denser and more intense downtown land use, Does a pond or an underground tank really sound like wise use of infrastructure dollars?
Where is this discussion going? I am wondering if we can’t eliminate the often confusing pages and pages of ordinance language with something more direct, like percent impervious, that is not only more universally recognizable but more directly associated with the impacts we wish to continue regulating. Can we replace the references to single family residential, multifamily, commercial, industrial, institutional land use with thresholds of percent impervious? Is that an improvement, or just a new complication? What do you think?
Thursday, November 12th 2009 at 11:16 am |
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.
Thursday, November 12th 2009 at 1:57 pm |
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.
Tuesday, November 17th 2009 at 3:52 pm |
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.
Tuesday, November 24th 2009 at 2:49 pm |
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.
Tuesday, November 24th 2009 at 3:20 pm |
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.
Thursday, December 17th 2009 at 4:53 pm |
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.