Based on the discussions during February’s Steering Committee (SC) meeting, we looked into what the communities covered in the National Ordinance Review Technical Memorandum require in terms of BMPs for small or single family residential properties, and how they enforce BMP maintenance. It came out in this SC meeting that there was interest in knowing what other progressive communities are doing to deal with this issue, but the original Technical Memorandum didn’t investigate this in great detail. Discussion topics included how small projects and individual homeowner activities (additions, teardowns, etc.) are managed across the Nation, and if BMPs must be implemented in the projects, as well as who is responsible for the maintenance. Thus, we took the initiative to do some investigation and create the attached summary document, for the Steering Committee’s review and consideration.

National Ordinance Review Additional Investigation: BMP Requirements and Enforcement for Small Properties

We welcome any comments or questions.

Application of the ordinance as currently written to typical roadway projects has proved challenging over the last 18-years.  Publicly maintained roadways are generally in land holdings that reflect the absolute minimum land necessary to accomplish the primary mission of transportation.  As is often pointed out by those charged with that mission, “the purpose of a transportation project is not stormwater management”.   In this day and age, however, no responsible public official suggests that transportation projects can, or should, ignore stormwater impacts.  We do though need to evaluate our approach to stormwater management for public roadways and make sure we get maximum benefit from the infrastructure and tax dollars available to build and maintain it.

A workable application of the current Ordinance has relied heavily on interpretation and intent.  A sub-group of the Steering Committee met recently to discuss roadways, and more broadly “linear Transportation Projects” in the context of the Ordinance update, and has a few recommendations that will begin our discussion.

• As we have been discussing throughout this Update, keep the focus on Impervious areas, and changes to impervious areas.
• The changes in impervious surfaces from a roadway project should not cause an increase in flood flows from the right-of-way.   This is within the control of the roadway agency, and is a different case than the flow from a cross culvert draining areas through the Right-of-way.  Often, an undersized cross-culvert turns the roadway into a low-level dam.  The dam attenuates peak flows downstream by “backing up” flood water.  Just like a dam, at some event the roadway will overtop, which can cause “traffic damages” in the form of delays and slowed emergency response, and sometimes even more hazardous conditions.  So what are the options?  If the size of the culvert is increased, is it the roadway agencies responsibility to create a flood control reservoir to mimic action of flood attenuation of the undersized culvert?  Different jurisdictions have answered that question differently, but the implications of that answer are enormous for roadway agencies who own very limited land.  Very often, a “problem culvert” becomes an institution because of the inability to replace the “dam effect” elsewhere than at that culvert.  While no specific recommendation is offered, the issue needs to be further discussed.
• Roadways are key factors in the pollutant loading to a stream, and like all impervious surfaces a contributor to the total runoff volume.  Would it be a better focus of the funds spent on stormwater management for roadway projects to require that the “Water Quality Control Volume” be addressed (see earlier post for a discussion)?  One of our conclusions in the subgroup was that if we could get strict compliance with the “Water Quality Control Volume”, and the changes to impervious area did not increase discharges, we don’t really need roadway stormwater infrastructure to do much else except of course drain the road.
• Simple engineering applications, like the unit area detention nomagraph already used for roadway projects, are usually as technical as we need to get for the limited real impact roadways have.

So, what are your thoughts on roadways?

We have posted before regarding an approach to integrating water quantity regulation with water quality regulation.  We have reviewed other programs nationally and surveyed the literature and trends in this regard, and believe that designating a “water quality control volume” would meet the goal of integrating the Counties strong water quantity regulatory program with water quality.

Many studies already have identified that pollutants accumulate on impervious surfaces, such as parking lots, roofs, driveways and roads.  Drainage from these surfaces is almost always “improved” by addition of storm inlets and storm sewers.  Unlike pervious surfaces, impervious surfaces begin to discharge stormwater in much smaller events, contributing to annual Runoff volume significantly as well as washing the accumulated pollutants into the water column in what we generally have called the “first flush”.  As we have been discussing, the impervious surfaces of the developed site are a “marker” for stormwater quantity and quality, and  in concept then, whatever is the “right” water quality control volume should be applied to the impervious area of a site.

Designing for water quantity in terms of flood damages, we typically target much larger storms.  Our small release rates tend to mimic the effect of measures which are explicit for “runoff volume control”, but do not capture and treat the first flush.

Discussing this issue at the Ordinance Update Steering Committee, we agreed that a re-focusing and change in emphasis was needed to better meet these dual quantity and quality challenges.  The current ordinance demands a high level of stormwater infrastructure to meet it already, and it is not really necessary to think of adding new costs to meet these goals.  Instead, we would redistribute the emphasis to fully acknowledge the “water quality control volume” as a high priority, with a corresponding de-emphasis on the total volume of detention storage required.

How much is “enough” for a water quality control volume?  Again, we can turn to some analysis of rainfall statistics that compare total rainfall in each storm to the total numver of “rainfall events” in a year.  We find the following:

• 70% of all rainfall events in a year have a total rainfall depth of 0.54 inches or less
• 85% of all rainfall events in a year have a total rainfall depth of 0.85 inches or less
• 90% of all rainfall events in a year have a total rainfall depth of 1.06 inches or less

In the above analysis, “events” are separated from each other by a minimum of 3-hours.  How would this be applied?  The above list thought of another way says, I need to design to capture and treat 1.06 inches of rainfall, applied over the impervious area of a site (assuming it is basically all runoff) if I wish to treat 90% of all the storms that occur.  We are ignoring the pervious areas in these “small events, which is a simplification but is balanced by the differences in impervious surfaces runoff characteristics when not all of the drainage from impervious surfaces is “improved”.

So if a site came in that was 10 acres in size and in its developed state it would be 50% impervious, and we wanted to capture 90% of all the events that occur in a year, a simplified approach would be to apply 1.06” rainfall depth to 50% of 10acres, for a total “water quality control volume” of 0.42 acre-feet that would need to be intercepted, held and treated by the use of BMP’s.  Corresponding to that calculated volume would be the simplified approach of “subtracting” that volume from any  calculated site runoff storage (detention) volume requirement that applies.

So here are some questions for feedback.

• Is this a good idea?
• What percentage of storms, or what rainfall depth, should be used to define the water quality volume?  85% of events-0.85” of rain? More? Less?
• How should it be applied?