25 Aug “Greening” the Stormwater Component of Water Management

A caption for this picture could read – with apologies to Jimmy Buffett – they’re ugly and they’re square and they shouldn’t be there. Here’s why.

A resolution to “direct the City Manager to convene a green roof stakeholder group that will develop policy recommendations” is on council’s agenda this week (8/27). This highlights that the stormwater component of our water management strategy is – just like the more obvious opportunities to more effectively and efficiently manage our water resources – in need of some attention. Green roofs are really just one component of a larger strategy which has been termed “volume-based hydrology”. The aim of this strategy is to retain and infiltrate, or store for later use, a considerable portion of the rainfall, rather than to focus on “efficiently” draining it to that magical place we call “away”, considering only the peak rate at which it goes “away”.

A highly interesting article, entitled “Volume-Based Hydrology”, in the current edition of Stormwater magazine (see Volume-Based Hydrology) introduces the concept: “Recent discussion by stormwater opinion leaders is now pointing to a convergence on what we will call volume-based hydrology (VBH) and movement away from the peak-flow-based version. Many states and local communities are considering how to accommodate or understand this idea.” Unfortunately, to date Austin – despite its self-proclaimed “green” credentials – is lagging on this. I have been asserting for some years now that it should be all over this concept, and this council resolution can be a significant step in that direction.

The article states the essence of the issue: “VBH starts with the premise that it is the increased volume of runoff due to urban development that is causing a set of problems, and that any other focus-variable (velocity, peak flow, impervious percent, event mean concentration [EMC] reduction, etc.) is mostly a one-off approach from the real problem: increased volume. If we focus first on volume, then the other variables will fall more readily into line.”

In more layman-friendly terms, our stormwater management strategy should be heavy on methods like bioretention, rainwater harvesting and – yes – green roofs, with the end-of-pipe pond being viewed as the strategy of last resort, once we’ve done everything we can to incorporate enhanced infiltration into the project design. This is the essence of the low-impact development (LID) concept, under which various aspects of site design are employed as part of the stormwater management strategy, rather than simply appending on a device at the end of a drain pipe.

The sad state of affairs in Austin is that the end-of-pipe pond strategy is pretty much the ONLY thing that the city expects to see. In fact, it appears that it would not even allow the use of a bona-fide LID strategy, according to a recent discussion I had with a person who is quite knowledgeable about their rules and procedures. I pointed out how one can show with an “LID hydrologic analysis” that runoff volumes and rates (and thus pollution) that get to the “end-of-pipe” would be significantly reduced by employing strategies such as bioretention – distributed about the site, designed in to the landscaping. She said that does not make any difference, that “THE hydrologic analysis” required by the city considers only percent imperviousness as the sole determinant of the water volume that needs to be captured at the end of the pipe, and they indeed expect a facility at the end of the pipe to be designed on that basis. Do anything else and you’re in variance territory, and no developer wants his engineer to go there.

It is also a sad fact that a regulation the city issued for the bioretention bed (which they tellingly term “biofiltration” instead, apparently exactly because they do not understand or expect it to perform a bioRETENTION function) addresses this strategy solely as an alternate to a sand filter, at the end of the pipe. The rule requires it to be sized and designed in an essentially identical manner, apparently failing to recognize that the rich ecology and enhanced moisture holding capacity of the bioretention bed would logically provide superior treatment AND some control of flow volume.

This capability is in fact a central feature of volume-based hydrology. As the article notes, “Various standard stormwater treatment practices designed to remove pollutants … have been shown to also play a role in runoff volume reduction, though the significance of this was not realized early on because most investigators simply measured pollution concentration in and out without regard to lost mass of pollution due to runoff reduction.” The article shows a table listing the “estimated annual percent volume reduction” for various practices. The value for sand filters – the method practically required by the city – is zero! For bioretention, values listed are 40% for the “average” design and a whopping 80% for an “enhanced” design. For green roofs, these values are 45% and 60%.

For those of you so inclined, an example of how volume-based hydrology could be applied using bioretention is on my web site at Bioretention redesign. It indicates that the runoff from about 75% of all storms could be fully retained, even on a site that is 78% impervious! This would have a major impact on the amount of pollution exiting this site, even without regard to whether the bioretention bed is inherently more effective at pollution reduction than an end-of-pipe pond.

It seems this whole approach to stormwater management is being “missed” by the city, and local practice indeed centers on making runoff go “away”. Instead, we should be doing everything we can to hold it on the land – to increase long-term soil moisture storage, which can hold down irrigation demand for the project landscaping – or to store it, to provide a portion of the water supply on the project, directly conserving our water supplies. These methods – in particular the green roof, since it displaces a heat reflector with a heat sink – will also help to blunt the heat island effect and the “desertification” of densely developed areas.

There is also a very practical reason why the city should immediately reconsider the strategies it imposes. According to the article, “… rumblings out of EPA lead us to believe that a volume-based approach will begin to replace other National Pollutant Discharge Elimination System (NPDES) Phase II approaches in future permits. Such prescribed conditions might include mandated green approaches with measurable goals that specify acreage treated with green designs; mandated reduction of impervious area; total maximum daily load (TMDL) implementation through green infrastructure; and mandated capture and treat depths … with a hierarchy of controls: infiltration, evapotranspiration, and capture and reuse.” In short, if this supposedly “green” city wishes to indeed be in the forefront of green, rather than having to play catch-up after the fact, it should start the “greening” of its stormwater management strategy ASAP.

There are a number of issues and details impacting on how we choose and govern stormwater management practices which are too complex to deal with in this forum. The main point is that we need to begin a re-examination of them NOW. So please contact City Council and support the green roof committee resolution. But don’t stop there. Please urge them to expand the focus to consider the whole range of volume-based hydrology strategies, of which green roofs are only one. It’s well past time that this city actually became a “green” leader in water management, and stormwater is a good place to start.