NC 2009- SSc6.1 Storwater Quantity Acceptable Methods

You describe a scenario that is common in the west. We have the same issue in Colorado with our clay soils. Look at my comments, posted on October 11, 2010.

Also, our most successful approaches have been to use the filtered water as a source for irrigation. Of course, this only works if you are using a raw water source for irrigation. If you are using a potable water source, it is difficult to use two sources for irrigation. However, if you want to look hard at this option, it does, potentially, provide you with some credit on the water conservation issue (WEc1). This seems to work best if you are proposing a native landscape pallete that can survive with less water, and the source of the water becomes your stormwater that has been filtered. You may have to supplement the filtered stormwater during longer dry periods, and you may have to over apply water if you have stormwater that you need to dispose of.

The other option for this water is to direct it to a cistern that may also collect gray water from your building and rooftop (depending on your water laws). This gray water/stormwater can be used for toilet flushing or irrigation and the combined sources will allow for a smaller cistern.

Lastly, the filtered stormwater can be applied to raw land and allowed to evaporate or supplement natural rainfall to grow native grasses. This is similar to a land application approach for treated wastewater disposal.

Bottom line, this is not easy with the soil conditions we have, and the best option is to look for a good use for the filtered stormwater, and not just view this as an isolated problem of disposal. I don't have a suggestion for an alternative if you cannot use the water on site or infiltrate or evaporate it.

I have used bioretention facilites with underdrains several times for storm water quantity peak reduction. These projects were in Virginia where there is a lot of clay in the soil which makes infiltration not feasible. Your civil engineer can route the bioretention facility similar to a dry pond with a "grate" (some routing programs have underdrains, but a grate will also work). This will show that the water is exiting at a lower peak flow rate over a longer period of time which is the requirement for <50% impervious area. We typically use a 40% void ratio for the media within the facility when calculating the volume within the facility. It shouldn't be a problem to send this water directly into an existing storm sewer system and is common in urban areas.

If your site had greater than 50% imperviousness, then you would have to find someplace else to use 25% percent of the water.

I am having the same issue as Lucy on one of our current projects, and the comments above were helpful but I still have some questions.
Scenario:
-existing imperviousness is less than 50%
-new buildng footprint will increase imperviousness
-we will build a new detention pond/swale with an outlet to city storm system
-there will be a restrictor to control the rate of outflow from the pond/swale
-we do not plan to use the water for reuse in the building or landscaping, etc.

1.) I thought that showing the restrictor outflow rate would be enough for the calculations but we got the following comment from the GBCI: "peak rate from a detention pond is typically a function of the outlet structure configuration, outliet pipe size, and the depth of water in the pond. Peak rate does not necessarily decrease at a constant rate with a decrease in runoff volume. Please revise and resubmit calculations for the peak rate leaving the pond..." So, can someone please illustrate how to calculate the peak rate if we have a restrictor (at the outlet from the pond) which allows a maximum flow of 0.5cfs/acre?

2.) The second issue is as follows: if we are catching storm water in the pond, the restrictor will solve the issue of the "rate", but not the problem of the "volume". Does someone know how to overcome this issue?
Thank you in advance!

This is an older post, and I assume you have resolved, but I will post a reply for others:

1) By "restrictor" I assume you are referring to some form of outlet control structure, with circular orifice and weir or some for of combinations. Unless you are using a pump, your basin will never discharge at a constant rate as you state. The discharge is a function of the head above the outlet. Consult with a civil engineer trained in stormwater management to determine the peak rate discharge.

2) Consult with a civil engineer trained in stormwater management to recommend and design the best BMP for your project.