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<TITLE>Runoff detention at roofline drip edges </TITLE>
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<P><B><I><FONT SIZE=2 FACE="Arial">The following question was posted to the listserve (my answer is below):</FONT></I></B>
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<P><FONT SIZE=2 FACE="Arial">I am considering using roof drip line filtration systems to assist </FONT>
<BR><FONT SIZE=2 FACE="Arial">with water quality treatment on a project. I have no issues </FONT>
<BR><FONT SIZE=2 FACE="Arial">designing the systems from a quality standpoint. However, from a </FONT>
<BR><FONT SIZE=2 FACE="Arial">quantity standpoint I was wondering how discharge from these systems </FONT>
<BR><FONT SIZE=2 FACE="Arial">should be modeled.</FONT>
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<P><FONT SIZE=2 FACE="Arial">1. If I design a roof drip line filtration system to provide </FONT>
<BR><FONT SIZE=2 FACE="Arial">adequate storage for the direct entry of the 25-year, 24-hour storm </FONT>
<BR><FONT SIZE=2 FACE="Arial">event, how should the discharge from this system be modeled? Can the </FONT>
<BR><FONT SIZE=2 FACE="Arial">discharge from the roof drip line filtration system be disregarded </FONT>
<BR><FONT SIZE=2 FACE="Arial">due to its relatively small quantity?</FONT>
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<P><FONT SIZE=2 FACE="Arial">2. If I design my roof drip line filtration system to provide </FONT>
<BR><FONT SIZE=2 FACE="Arial">adequate storage for the direct entry of the 25-year, 24-hour storm </FONT>
<BR><FONT SIZE=2 FACE="Arial">event, how should the discharge from this system be modeled? I </FONT>
<BR><FONT SIZE=2 FACE="Arial">assume in this case the storage reservoir would be modeled as pond </FONT>
<BR><FONT SIZE=2 FACE="Arial">with an overflow at ground surface, but can the discharge from the </FONT>
<BR><FONT SIZE=2 FACE="Arial">roof drip line filtration system be disregarded due to its relatively </FONT>
<BR><FONT SIZE=2 FACE="Arial">small quantity?</FONT>
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<P><FONT SIZE=2 FACE="Arial">Brian Pierce</FONT>
<BR><FONT SIZE=2 FACE="Arial">Project Engineer</FONT>
<BR><FONT SIZE=2 FACE="Arial">Sevee & Maher Engineers, Inc.</FONT>
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<P><B><I><FONT COLOR="#000000" SIZE=2 FACE="Arial">This is my answer to the question</FONT></I></B><I></I>
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<P><FONT COLOR="#000000" SIZE=2 FACE="Arial">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</FONT><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">.</FONT><FONT COLOR="#000000" SIZE=2 FACE="Arial"> 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). </FONT></P>
<P><FONT COLOR="#000000" SIZE=2 FACE="Arial">Possible approaches:</FONT>
<BR><FONT COLOR="#000000" SIZE=2 FACE="Arial">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.</FONT></P>
<P><FONT COLOR="#000000" SIZE=2 FACE="Arial">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.</FONT></P>
<P><FONT COLOR="#000000" SIZE=2 FACE="Arial">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..</FONT></P>
<P><FONT COLOR="#000000" SIZE=2 FACE="Arial">Hopefully, this will clear things up.</FONT>
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<P><FONT COLOR="#000000" SIZE=2 FACE="Arial">Marianne Hubert</FONT>
<BR><FONT COLOR="#000000" SIZE=2 FACE="Arial">Senior Environmental Engineer</FONT>
<BR><FONT COLOR="#000000" SIZE=2 FACE="Arial">Division of Watershed Management</FONT>
<BR><FONT COLOR="#000000" SIZE=2 FACE="Arial">Bureau of Land and Water Quality Control</FONT>
<BR><FONT COLOR="#000000" SIZE=2 FACE="Arial">17 State House Station</FONT>
<BR><FONT COLOR="#000000" SIZE=2 FACE="Arial">Augusta, ME 04333</FONT>
<BR><FONT COLOR="#000000" SIZE=2 FACE="Arial">ph. (207) 287-4140</FONT>
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