It is not easy to separate “policy” from “technical” in something like a stormwater ordinance, but as a bunch of scientists and engineers we want to make sure that we understand where the line between the two is.  The technology behind this blog imposes some limitations,  and we now understand that it is our choices of topics that have not adequately given voice to issues of “policy” or “procedure”.  So, thank you for your patience, forgive us, and please help us by commenting on this post with any topic you think needs to be its own discussion thread in this blog.  We will then post on each topic separately, which will give anyone interested a chance to engage on the issue.

After today’s Municipal Engineers Meeting,  there are a few revisions and tweaks to the proposed water quality control volume (wcqv) and Site Stormwater Storage (detention) approach.  This is introduced below in the previous post.

 First the applicability of the “roadway” category needs clarification.  Roadway was intended to apply to projects within publically owned rights of way under the jurisdiction of a unit of government.  It was not intended that the new roadways that are part of a site development proposal would fall under this category, but instead be considered as part of a larger development.

 The “thresholds” embodied in “note 4.” Of the blog were discussed at some length.  The current ordinance requires a permit to be submitted for any development that disturbs more than 5000 sq. ft. of area.   The WCQV-Detention proposal  is that developments that add 500 sq. ft to 5000 sq. ft. of additional impervious area are required to provide BMP’s.  This could lead to situations where a submittal and some form of permit would be required in cases that under the current ordinance are exempt.  While true, this is justified by the number of drainage complaints that occur between neighbors, even with projects at this small of a scale.  One suggestion was to create a very simple and “light” permit for these small projects, focused strictly on the installation of an appropriate BMP.  It was discussed and generally agreed that “policing” these BMP’s could be a bit of a challenge as projects change ownership, but even if all that is really accomplished is the initial installation that will be an improvement over continuing to ignore the cumulative effect of small projects.

We have discussed now on several occasions using Total Impervious Area (TIA) to determine what requirements apply for Site Runoff Storage (detention) and Water Quality Control Volume (WQCV).

The table or attached is our first specific proposal on how that might work.  Recall that the “water Quality Control volume” is targeted to smaller but more frequent events and is effective in reducing the volume of stormwater runoff, while detention is what we do in the current ordinance in specifying a release rate for peak discharge in an extreme event.  This proposal balances expenditures for stormwater infrastructure between frequent events which are associated with volume and pollutants and extreme events associated with peak discharge.   Stream stability would increase  and stream bank erosion would be reduced to some degree as well with the WCQV requirement.

Proposed Site Runoff Storage Table

Please take notice in the table of the notes, particularly the 4^th one.  This note sets the threshold at which the requirements would apply,  please review these thresholds and let us know what you think of them.

Consider these three single family lot scenarios.  Imagine these scenarios with very small lots and very large lots.

• A vacant lot in an already platted development with no detention.
• A built lot where the home is adding an addition, or a patio, or completely replacing the driveway, or some combination of the usual “building permit” situations our residents bring to us.
• A previously built lot, where a “teardown” is proposed and a new very large home is to be built.

With these rules, detention would apply only if the vacant lot was large enough that the combination of new impervious (home, garage, driveway, patio, sidewalk) was over 5000 sq.ft. and the total area disturbed was over 25,000 sq. ft.  If the lot is smaller, say 10,000 sq.ft., and the new impervious was 5000 sq ft (which seems unlikely on a 10,000 sq. ft. lot) then since the finished project is 50% impervious, the WCQV is 0.75 inches multiplied by 5000 sq. ft, or a total of 312.5 cubic feet.   A 15” rock blanket placed under a 10’x16’ patio and 20′x25′ driveway would hold the volume required.  It would also hold all of the runoff from more than half of the rainfall events that occur in a year-which would go a long way in relieving the inevitable Neighbor vs. neighbor drainage complaints when “the last vacant lot on the block” gets built.

Do you agree with the direction this is heading:  frequent event volume control?  “Zoningless” thresholds?