From Marianne.E.Hubert at maine.gov Wed Jul 18 13:41:09 2007 From: Marianne.E.Hubert at maine.gov (Hubert, Marianne E) Date: Wed, 18 Jul 2007 13:41:09 -0400 Subject: [stormwaterengineering] Runoff detention at roofline drip edges Message-ID: <6A73A5DE6596754DB4575B71972AA460031AD22E@SOM-TEAQASMAIL2.som.w2k.state.me.us> The following question was posted to the listserve (my answer is below): I am considering using roof drip line filtration systems to assist with water quality treatment on a project. I have no issues designing the systems from a quality standpoint. However, from a quantity standpoint I was wondering how discharge from these systems should be modeled. 1. If I design a roof drip line filtration system to provide adequate storage for the direct entry of the 25-year, 24-hour storm event, how should the discharge from this system be modeled? Can the discharge from the roof drip line filtration system be disregarded due to its relatively small quantity? 2. If I design my roof drip line filtration system to provide adequate storage for the direct entry of the 25-year, 24-hour storm event, how should the discharge from this system be modeled? I assume in this case the storage reservoir would be modeled as pond with an overflow at ground surface, but can the discharge from the roof drip line filtration system be disregarded due to its relatively small quantity? Brian Pierce Project Engineer Sevee & Maher Engineers, Inc. This is my answer to the question Several options are available to design for the quantity management of roof runoff (for flooding standards) when a dripline filtration system is used. The runoff from the larger storms cannot be disregarded and the design options are the same as it would be for a roof without the dripline system. The volume may be small but it still needs to be accounted for in the model and for the application process as the dripline system is expected only to meet the treatment requirement (channel protection volume). Possible approaches: 1. Allow sufficient storage within the crush rock collection system and infiltrate the full runoff (25-year storm). Design the model as a pond with exfiltration as the primary outlet and to a pipe orifice as a secondary outlet and control structure. Take into consideration the porosity of the crushed rock. 2. Provide an overflow drain at the top of the crush rock collection system and still model the structure as a pond. Detention may still need to be provided elsewhere. 3. Allow the overflow to sheet flow and bypass the drip edge collection system filled to capacity and to enter another BMP such as a buffer, over pavement and to a pond, etc.. Hopefully, this will clear things up. Marianne Hubert Senior Environmental Engineer Division of Watershed Management Bureau of Land and Water Quality Control 17 State House Station Augusta, ME 04333 ph. (207) 287-4140 -------------- next part -------------- An HTML attachment was scrubbed... URL: http://mailman.informe.org/pipermail/stormwaterengineering/attachments/20070718/8d6b420f/attachment.html