Posted on 21 December 2009 · Written by Jeff Walker
Stormwater management has become an increasingly challenging issue for urban communities. In many cases, rainwater can no longer follow its natural patterns, which increases the volumes of runoff and its associated problems.
In a natural environment, runoff is minimized since rainfall for the most part is dealt with naturally through infiltration, evaporation or plant transpiration. As a result of urbanization, however, we’re seeing added stress on natural watercourses and related environments, simply because the increased imperviousness reduces the ability for rainfall to be absorbed into the soil, detained or treated. This effect is changing the natural water balance.
The water balance is an expression of the flow of water in and out of the system and represents the proportion of precipitation that infiltrates, evapotranspirates, or runs off. If one or more of the elements goes out of its predevelopment balance, it generates consequences to the natural system such as increases in volumes, peak flows and durations, temperatures and sediment. Excessive runoff in particular can lead to flooding and altered stream flow, erosion, algae growth, compromised water quality, threats to aquatic habitat and changes to groundwater resources. Managing the water balance therefore is important in terms of flood protection, sustaining water quality, erosion control and improving infiltration.
There’s a growing need to implement stormwater management systems that control the volume and quality of runoff by other than traditional pipe-based means. The low impact approach, which uses landscaping and other onsite techniques to mimic pre-development conditions, will be especially critical to sustaining the surface and groundwater regime.
The evolution of stormwater practices
Prior to the 1990s, the major focus was simply controlling quantity. This prompted widespread installation of piped drainage systems that were designed to convey the runoff to downstream watercourses as quickly as possible.
In the early 1990s, the focus expanded to include water quality and erosion. At this juncture, we saw the early iterations of ponds as a stormwater measure. Today, as concern grows about protecting fisheries, preserving stream morphology, and protecting groundwater resources, stormwater treatment has come under the spotlight.
Management objectives are now being accomplished through a variety of source and conveyance control measures that more closely simulate conditions prior to development. There are a number of these measures in use today, including
- Attenuation techniques that slow the release rate of stored water,
- Infiltration processes for recharging groundwater,
- Filtration systems for improving water quality, and
- Phytoremediation to remove contaminants using plant material processes.
These source and conveyance measures do not preclude the use of traditional end-of-pipe approaches, which continue to be a factor in stormwater planning. Rather, the objective of these measures is to minimize and treat runoff before it reaches the end of pipe elements.