Archive for March 2014

Stormwater Management Can Be “Green” Too

March 16, 2014

Meet Dr. Katherine Lieberknecht. She is a professor in the University of Texas School of Architecture who proposes the revolutionary idea that stormwater runoff can – and should – be managed as a water resource, rather than as nuisance to be drained “away” as “efficiently” as practical. This is “revolutionary”, of course, only to the conventional mindset, whose nuisance-centric mental model of stormwater management unfortunately continues to hold sway over much of the regulatory machinery and design community. Coming to understand the management strategy that Katherine advocates and to move that to the fore of mainstream practice is a deep conservation strategy; it is how we can make stormwater management “green” too, helping to move us toward sustainable water.

Katherine suggests capturing natural cycles of water movement through the landscape at the neighborhood scale, asserting this would lead to cost savings – including, notably, energy savings by minimizing irrigation needs – and would enhance habitat value of the landscaping. Medians, rights-of-way, parks – just about any greenspace can be made multi-functional, designed to hold onto stormwater instead of to shed it, serving as beautification and accomplishing water quality and quantity management goals. In this vein, Katherine suggests that a community attitude should be fostered. Shared, collaborative and cooperative “storage” of stormwater is urged, utilizing otherwise relatively hydrologically functionless areas – for example church grounds – to hold more water on the land. Throw in green roofs and the areas covered by buildings can also be made more hydrologically functional, holding water on the site instead of making it flow “away”. Or harvest the roof runoff and store it in cisterns to provide irrigation supply, which also effectively holds more water on the land.

The landscape-based strategy can be manifested in two basic ways. One is to design the greenspace as “formal” water quality management devices, such as bioretention beds, that infiltrate the water that flows into them. The other is to use a type of landscaping, such as wildflower meadows or restored native prairie, that naturally hold more water on the land, and perhaps to enhance that by utilizing permaculture techniques that create micro-ponding areas to further increase the amount of runoff that gets infiltrated. Consulting a table of curve numbers (CN) – a parameter that determines the propensity to shed or infiltrate runoff, the higher the number, the more runoff – shows that for a Group D soil (the class that produces the most runoff), a conventional turf landscape would have a CN of about 84 while a wildflower meadow or native prairie would have a CN of about 73. This would create a very significant reduction in runoff. And result in a whole lot more water held on the land, contributing to deep soil moisture and so maintaining the landscape better through drought periods.

The native landscape would also demand far less routine maintenance – very little mowing, no fertilization, little or no irrigation. Bringing up … A critical aspect of this, Katherine stresses, is follow-through with appropriate O&M, to assure that the hydrologic function of this distributed system is maintained. In setting forth this overall concept to the local regulatory system, it is indeed a concern for the O&M costs of many small installations that is their major objection to “mainstreaming” this concept. This highlights that we must use lower O&M strategies, like “passive” infiltration rain gardens and low-maintenance native plantings, as the mode of implementing this distributed concept.

But considering this concern for O&M also highlights the inherent resilience of this decentralized concept of stormwater management. The “failure” – however one might define that – of any one distributed component – e.g., “clogging” of a rain garden – impacts on only a very small part of the overall system. And so that overall system would continue to provide good overall performance, assuming there is sufficient O&M provided so that the occasional such failures would be detected and corrected before they become so widespread as to meaningfully impact on the overall system. Again, by choosing practices which would entail low O&M to begin with, such as choice of landscaping and passive infiltration devices, that “sufficient O&M” would entail a fairly low liability.

Another component of keeping O&M manageable would be education, so that the property owners would understand what that landscaped depression in the corner of their lot is and what function it provides, so that it would be less likely that they might do something “stupid”, like fill it in or radically alter the landscaping. The literature on Low-Impact Development (LID) – of which this distributed “green” approach is an exponent – universally notes that education is a fundamental component of that strategy. The concerns of the regulatory system could be blunted by requiring that the educational component be part and parcel of implementing that strategy.

This whole idea of rendering the landscape more hydrologically functional and lower maintenance, along with allied practices, can yield a number of benefits. To quantify some of those benefits, let me introduce Tom Hegemier. Now working in the private sector, Tom produced some estimates of water savings potential from pursuing various distributed strategies when he worked on water supply issues at the Lower Colorado River Authority, the agency that manages the lower part of the Colorado River in Texas. Tom’s calculations are not belabored here – I can provide the methodology to anyone who is interested – but they indicate there is huge potential for water savings.

Basically, Tom asked what if half of all new housing built in Travis County – where Austin, the largest city in the lower Colorado basin, is located – between now and 2040 utilized one or more of a suite of water management options. These include:

  • The “Hill County Landscape Option”, which minimizes turf in favor of native plants and emphasizes improving the soil so that its water holding capacity is enhanced. This would result in significantly decreased demand for landscape irrigation water.
  • Building-scale rainwater harvesting to capture water to provide landscape irrigation water.
  • Wastewater reclamation and reuse to defray landscape irrigation water demands.

Tom’s estimates of water savings were as follows:

  • Application of the Hill Country Landscape Option – water demand reduction ranging from 12,000 to 15,000 acre-feet per year.
  • Application of the Hill Country Landscape Option plus rainwater harvesting – water demand reduction ranging from 17,000 to 19,000 acre-feet per year.
  • Application of the Hill Country Landscape Option plus wastewater reuse to defray landscape irrigation demands – water demand reduction ranging from 20,500 to 24,000 acre-feet per year.
  • A combination of all three strategies – water demand reduction ranging from 25,000 to 28,500 acre-feet per year.

As a point of comparison, the total water use in Austin was about 170,000 acre-feet in 2011, and it is projected to be about 300,000 acre-feet in 2040. So a 50% penetration of just these site-based strategies would accomplish almost a 10% reduction in demand by 2040. Tom went on to ask, what if in addition to these strategies, LID practices like those advocated by Katherine plus rainwater harvesting were more universally employed as the manner in which stormwater is managed as sites are developed, opining that this would move development a long way toward being “water neutral”. Or what I set forth in the previous post to this blog as Zero Net Water.

An underappreciated facet of the savings potential is the reduction in peak water demands these strategies can offer. In the climate of Central Texas, annual peaking is driven by irrigation demands. So when irrigation demand is reduced, peaking is reduced. And when peaking is reduced, the sizes of all manner of water supply infrastructure can be reduced, or their implementation can be put off further into the future.

None of this is inherently difficult to accomplish. It hinges on the choice to view rain falling on the site as a water resource to be husbanded to the maximum practical extent, instead of as a nuisance to be shed and made to go “away”. The means to do this, in the physical sense, are readily available and are largely cost efficient. Institutionally, it is “merely” a matter of making that choice and setting regulations and accepted best practices to husband that water resource. Particularly in areas like Central Texas, where water supplies are becoming stressed by growth and that stress is being exacerbated by chronic drought, it is high time that our controlling institutions make that choice.