Civil engineers design them, landscape architects loath them, developers wonder why we need them and municipal engineers often require them - I hate them. Retention and Detention ponds have become the standard for stormwater management on commercial and residential projects throughout most of the US. They were originally intended for flood control and were later incorporated into water quality requirements as well. They serve in their capacity to attenuate peak flows from new development reasonably well, but their usefulness stops there. Here are a few of the reasons that I'm not a fan of detention/retention ponds:
They take up valuable land
Detention and retention ponds are typically designed for a storm that has a return interval of 20-100 years, so the volume is rarely, if ever, fully utilized. As a result you have a gaping hole in the ground that is an eyesore, a potential danger to children and basically useless except for those rare occasions when we receive excessive rainfall.
They don't address quantity
Detention and retention ponds rarely address stormwater quantity. Typical stormwater regulations address the peak flow rate and water quality, but do not address the actual amount of water that runs off of a site rather than soaking into the ground. Because of that, even well designed sites with detention ponds significantly reduce ground water recharge, which contributes to drought conditions, increases erosion in stream channels, and limits that amount of water available for plant life.
They offer minimal water quality
Typical stormwater regulations pertaining to detention ponds address water quality in two ways, retention time and/or water quality units. Often, regulations allow water quality to be addressed by requiring the "first flush" (usually around 1/2" of rainfall) be detained and released over a period of 24-48 hours. The idea is that most of the dissolved solids will settle out during this period and the water that is released will be reasonably clean. However, very fine and highly soluble particles will often not settle out during that period, and oils, grease, metals and other contaminates are not effectively removed either. Proprietary water quality units often perform better but have to be used in combination with a pond or other methods and are usually expensive and maintenance intensive.
Don't get me wrong, we are better off with detention ponds than without them, but there are more effective methods for managing stormwater. As with just about anything, one size does not necessarily fit all, and there are going to be times when detention ponds are the best or only solution. I have designed quite a few detention ponds, and I am sure that I will design quite a few more, but I will always look to the following methods first and exhaust those possibilities.
Call them what you like, raingardens, bioretention, bio-swales etc. they are all variations on the same basic principal - use landscaping and soil to manage and treat stormwater. Raingardens are very effective at reducing peak runoff rates, increasing groundwater recharge and improving water quality. They can also reduce potable water use for irrigation and provide for wildlife habitat. Landscaped areas are often lost opportunities for raingardens because they are raised planting beds. By depressing those planting beds the aesthetic appeal of the the landscape can be maintained and they can serve the additional purpose of stormwater management.
Pervious pavement can come in the form of asphalt, concrete, pavers or plastic grids to protect the soil. Pervious paving systems reduce peak runoff rates, increase groundwater recharge, improve stormwater quality and reduce the heat island effect. These systems are versatile and cost competitive, especially in areas where land is expensive or difficult to come by.
Wetlands are nature's way of managing stormwater quantity, quality and flow rate and they are very effective in doing so. Existing, improved, constructed and modular wetland systems can be used to manage stormwater quantity, rate and quality in addition to providing wildlife habitat. It makes perfect sense to look to the natural world for design cues and wetlands are a good fit for projects that have the space and the right environmental (soil, water source, etc) conditions.
Green roofs come in all shapes and sizes, intensive and extensive, modular and built up, sloped and flat, but they all have the same basic benefits. From a stormwater perspective, green roofs reduce peak and total runoff, improve water quality and provide for evapotranspiration. Other benefits include extended roof life, reduced cooling and heating loads, a reduction in the urban heat island effect, wildlife habitat and aesthetic appeal.
A tremendous amount of water falls on the roofs of our structures, which usually results in increased runoff. By capturing that rainfall and reusing it for landscape irrigation and/or grey-water within the building you can save money, water and energy, increase groundwater recharge and improve water quality.
The above methods are just a few of the many best management practices that can be used to manage stormwater rather than detention/retention ponds. Used alone or in any combination they can provide better stormwater quality, reduced runoff, increased groundwater recharge, peak rate reductions, less potable water use and a host of other benefits. Another great thing about these best management practices is that they can often save the owner money both in first cost and long term operations and maintenance costs. I don't know about you, but to me it sounds like a "no-brainer".
As with many green methods and technologies the biggest deterrent is often lack of knowledge. Lack of knowledge by the design engineer, the contractor, the owner and stormwater regulators. The best way to overcome that lack of knowledge is to educate yourself and others and give these best management practices a try. The best way to learn about them is to design and use them. The city of Portland and Seattle Washington have very good, readily available resources as well as Prince Georgia's County Maryland and countless other programs that have begun implementing these systems around the country.
Bob is a registered professional engineer in TN with a master’s degree in Civil Engineering and over ten years of experience in project planning, design and management. Bob was on the forefront of the green building movement and was ahead of the curve in obtaining LEED accreditation in 2005. He is passionate about sustainability and lectures and writes about green buildings and sustainability in an effort to educate the design community and the general public.