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Open Stormwater Systems

Stormwater Ponds are Ugly

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A developer said this to me recently, no not all developers think this, but this comment is fairly typical of the perception that many in the industry hold. In fact, many of the developments that I’ve been involved in over the past couple of years have made every attempt to put the stormwater treatment and conveyance underground. Not particularly, (but possibly) because of some idea that it will take up less space, or perhaps that it will be less expensive, rather just because their personal preference is that it is an unseen service, like sanitary sewer.

In some parts of the world, it’s still called storm sewer with a stigma attached to it before it’s even had a chance to redeem itself. But should we be turning around all our old school, underground storm sewer practices, and creating swales, ponds, detention basins, biofiltration systems and raingardens?

The resounding answer would appear to be “yes!” with a cautionary qualification that it only works if the local authorities are prepared to look after it. Hand in hand with that caution comes another that it really helps if the residents like it too and some level of visual and environmental amenity is improved with the project. Read more after the jump…

Stormwater Best Management Practices

Stormwater Best Management Practices, (BMPs) are known around the world by different names, in Australia we also use the phrase Water Sensitive Urban Design (WSUD), in the UK: Sustainable Urban Drainage Systems (SUDS) and generally in Europe they seem to be referred to as Sustainable Stormwater Management.

The following are some of the open or surface BMPs that you may come across in your travels.

Ponds, Wetlands and Basins

Best Management Practices often aim to be multifunctional, i.e. reduce the amount of runoff leaving a site and improve the stormwater quality of that runoff. The best devices for this multifunctional approach are wetlands and ponds, and if correctly designed for the catchment, rainfall, expected pollutants and the local topography, ponds and wetlands are extremely effective in removing organic pollutants and suspended solids from stormwater runoff.

An added benefit of ponds, (occasionally with undesirable consequences) is the enhancement of bio-diversity and the creation of habitat for birds and other wildlife. Several ponds I’ve visited around the world have been the home to large flocks of migratory birds that would otherwise have not stopped in the immediate area. This can cause consternation for human residents that are unable to enjoy the area due to the smell, noise or proximity of the birds. However in many situations the added bio-diversity enhances the urban environment, creating improved recreational and environmental areas.

Ponds and basins come in three basic configurations;

  • Stormwater Retention Basin – This is designed to store runoff for a couple of days to allow water to seep back into the groundwater. The water generally is not discharged to natural water systems, as such these are recommended for areas with soil of high infiltration capacity and a low groundwater table.
  • Dry Detention Basin – This is designed to take the peak off the storm, releasing the runoff into an adjacent water body. This can be a multiuse facility with potential use as a playing field in dry weather. A large site is often required depending on the catchment size.
  • Wet Detention Basins – These structures can be on or offline and are permanently partly full. The storm flow fills the remaining available storage, being released slowly through a discharge structure. These ponds require impermeable soil to maintain a wet profile.

Wetlands are a whole ‘nother post worth of discussion, providing much higher levels of treatment, so I’ll save that for another day.

Swales

Swales are one of the easiest forms of open stormwater system that are available in the urban designer’s toolkit. They are are simple to design, simple to maintain and their operation is straightforward too. Swales can be accept runoff directly from roads or other hardstand areas, and are able to replace kerb and gutter in this function.

These dry, often natural-shaped, vegetated channels allow water to absorb into the subsoil or convey water to other structures or waterbodies. In areas with low soil infiltration rates, rock, or shallow slopes, biofiltration media can be used to promote filtration of the runoff, with the filtrate collected in subsoil ag-pipe, then conveyed to a collection pipe network, or discharged to a water body. In areas with steep road grades, rock media can replace vegetated swales with some success to  increase bed friction and allow the runoff to filter through the bio-media.

The capture rate of many pollutants is based on the length of vegetation in the swale, so carefully manicured grass swales are not desirable from a treatment quality perspective.

Integration of Stormwater Systems into the Urban Environment

An important consideration in proposing the inclusion of stormwater BMPs is finding ways to integrate these as features in the urban environment. This can be a tricky task, and if being integrated into an existing urban area, local residents should be given opportunities to make comment on the proposal.

Aesthetic integration of stormwater systems is often beyond a civil designer and requires input from landscape architects and other designers. Innovation in subdivision design is often difficult to present to municipal bodies and regulatory authorities, however, through open discussion, ideas such as BMPs have a chance of meeting community, council and developers expectations.

Don’t be scared of these design elements, when used correctly they efficiently convey or detain runoff with the benefit of vast improvements in water quality. It is important that municipalities are aware of the ongoing maintenance costs for BMPs, and use these in commenting on proposed treatment trains.

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Well, there’s lots more to discuss on this, I’ll try to get back onto the topic shortly to fill in some of the gaps. In the meantime, if you have any pressing questions, I should be able to point you in the right direction if you drop me a line.

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Mike Thomas

Mike Thomas P.Eng. ENV SP, is the author of UrbanWorkbench.com and Director of Engineering at the City of Revelstoke in the Interior of British Columbia, Canada. If I post something here that you find helpful as you navigate the world of engineering, planning and building communities, that’s wonderful. But when push comes to shove: This is my personal blog. The views expressed on these pages are mine alone and not those of my employer.