Green building alternatives to rainwater harvesting in Colorado

By Jane Clary, Chuck Haines and Jonathan Jones, P.E., Wright Water Engineers, Inc.

“Green Building,” “Low Impact Development,” “Better Site Design” and “Sustainability” movements are gaining increasing popularity across the nation and in Colorado.  In some cases, however, recommendations that are well suited to other regions of the country may conflict with basic principles of Colorado water law.  Use of cisterns to capture rainwater (a practice which has been used successfully elsewhere) is one of these controversial topics that has prompted much discussion in Colorado, including the recently introduced Senate Bill 08-119, which proposes allowing limited collection of rainwater under certain conditions.  In the context of Green Building and Low Impact Development, rainwater harvesting is viewed as a tool not only to conserve potable water resources, but also to help reduce potential adverse effects of urban runoff.

Rainwater harvesting is the process of intercepting storm-water runoff and putting it to beneficial use. Rainwater is usually collected or harvested from rooftops, concrete patios, driveways and other impervious surfaces. The diversion and use of rainwater is subject to the Constitution of the State of Colorado, state statutes and case law.

Water rights in Colorado are unique compared to parts of the eastern United States. The use of water in this state and other western states is governed by what is known as the prior appropriation doctrine. This system of water allocation controls who uses how much water, the types of uses allowed, and when those waters can be used. A simplified way to explain this system is often referred to as the priority system or "first in time, first in right." An appropriation is made when an individual physically takes water from a stream or well (when legally available) and puts that water to some type of beneficial use. The first person to appropriate water and apply that water to use has the first right to that water within a particular stream system. This person, after receiving a court decree verifying their priority status, then becomes the senior water right holder and that water right must be satisfied before any other water rights are filled. In Colorado, the State Engineer has the statutory obligation to protect all vested water rights. The process of allocating water to various water users is traditionally referred to as water rights administration, and is the responsibility of the Division of Water Resources.

Of course, the appropriation system is much more complicated than this. Some priorities on major stream systems in Colorado date back to the 1850's, and most of the stream systems have been over-appropriated, meaning that at some or all times of the year, a call for water by a senior appropriator is not being satisfied. Practically speaking, it means that in most river drainages, a person cannot divert rainwater and put it to a beneficial use without a plan for augmentation that replaces the depletions associated with that diversion.

In a nutshell, the issue is that capture and storage of small amounts of storm runoff by many individual property owners, for the purpose of putting the water to a consumptive beneficial use outside of the priority system, can have a significant aggregate effect on senior water rights downstream.  This is why the use of rainwater harvesting cisterns in Colorado is not currently allowed. 

In the context of stormwater management, it is important to recognize that there are less controversial approaches to managing site runoff that are equally as important from a flood control and water quality protection perspective, as long as the developer’s objective is not “the irrigation of landscaping” or some other beneficial use.  In essence, a site developer also needs to be thinking about how to balance these issues:

• Peak Runoff Rate and Volume:  “How much additional runoff volume and peak flow rates are being generated from my development and how can I best minimize adverse impacts downstream?  Does the runoff from my site ‘do no more harm than formerly’?”
• Water Quality: “How dirty is the water coming off my site—can I improve the quality?  Is the quality of the water as good as that to which downstream water users have ‘historically been accustomed’?”

Downspout at Koelbel Public Library, in Centennial, (top) drains across a grass buffer prior to entering a vegetated detention area (bottom). Photos provided by Jane Clary

These questions are important because before development, most of the rain that falls on the ground soaks into the soil or is captured by vegetation; relatively little rainfall runs off and flows downstream. However, after development, rain that falls on roofs and pavement may contribute to increased rates of surface runoff.  Whereas one runoff event per year may be typical prior to development, about 30 runoff events per year may occur after urbanization (Urbonas et al. 1989).  If appropriate methods for managing urban storm drainage are not implemented, increased peak flows and volumes of runoff can be environmentally harmful, causing erosion in stream channels and generating greater pollutant loading downstream.

At the simplest level, one way to manage runoff quality and quantity, while avoiding the actual “harvesting” of rainwater, is simply to direct runoff toward pervious areas rather than directly into concrete gutters.  Using this approach, the water is not being “diverted, captured and stored” for later use; it is simply being slowed down and allowed to infiltrate back into the ground.  While some uptake by plants occurs incidentally in this process, “landscape irrigation” is not and cannot be the objective from a water rights perspective.  Some reduction in immediate runoff volumes will occur, but downstream water rights owners also benefit due to better water quality, less damaging peak flow rates and gradual groundwater return flows to the surface stream.  In essence, development can be designed to follow the principle of reducing runoff rates and volumes to more closely mimic natural conditions.  (Note:  other vital flood control measures are typically required on developments in addition to these techniques, but are not described in this short article.)

One effective stormwater quality Best Management Practice (BMP) is managing urban  runoff in a manner that seeks to mimic natural conditions. The Denver Water Quality Management Plan (WWE et al. 2004) and the Urban Drainage Flood Control District Storm Drainage Criteria Manual, Volume 3 Best Management Practices (UDFCD 2008) describe several techniques that can be used to achieve this goal; a few examples include:

• Place stormwater in contact with the landscape and soil. Instead of routing storm runoff from pavement to inlets to storm sewers to offsite pipes or concrete channels, an approach is recommended that places runoff in contact with landscape areas to slow down the stormwater and promote infiltration.  Porous pavement areas also serve to reduce runoff and encourage infiltration.
• Apply the principle of “minimizing directly connected impervious area” (MDCIA).  This involves breaking up areas of imperviousness and directing runoff from roofs and paved areas to grass buffers, swales, and other landscape areas prior to being conveyed off the site.  Fragmenting impervious areas with even small pervious areas can have a significant impact on reducing runoff and improving water quality.
• Select treatment areas that promote greater infiltration.  Porous landscape detention, porous pavement detention, and sand-filter detention promote greater volume reduction than extended detention basins, since runoff tends to be absorbed into the filter media or infiltrate into underlying soils. 

By employing these techniques, and assuming that long-term questions related to maintenance, appearance and continued functionality are addressed, projects can reduce the increase in runoff and related stream degradation and pollutant loading that comes with conventional development.  For now, the controversial issue of rainwater harvesting and storage in cisterns is thereby avoided, downstream properties are better protected from erosion and flooding damage, water quality is improved, and plants still receive some incidental benefit from the runoff. 

For More Information:

For more information, contact the authors at Wright Water Engineers, Inc., 303-480-1700, (clary@wrightwater.com, chaines@wrightwater.com, jonjones@wrightwater.com) and/or see the following resources:

Colorado Division of Water Resources, 2003. Graywater Systems and Rainwater Harvesting in Colorado (http://water.state.co.us/pubs/policies/waterharvesting.pdf).

Urban Drainage and Flood Control District, 2008.  Urban Storm Drainage Criteria Manual, Volume 3 Best Management Practices. (www.udfcd.org).

Wright Water Engineers, Wenk Associates, Muller Engineering, Matrix Design Group and Smith Environmental 2004.  Denver Water Quality Management Plan, Chapter 6 Stormwater Quality BMP Implementation Guidelines (http://www.denvergov.org/tabid/396037/Default.aspx?). 

Originally published in Colorado Building Green Newsletter, April/May 2008.

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