Stormwater management is an important element of any building or major renovation project, particularly at urban sites. Low Impact Development (LID) approaches are become much more common, and there are a number of LID options available when selecting stormwater management techniques. Three popular LID are porous pavement, bio-retention ponds, and underground infiltration systems.
For a project applying LID techniques, each option should be evaluated based on its suitability for the site, performance, overall cost, and the time and cost of maintenance. This post describes each option and how it could work for a site.
Porous pavement is a permeable pavement constructed on top of a stone reservoir to allow water to naturally filter and infiltrate the ground. This technique works well for a site that has low usage levels with a slope of less than 5%. Due to the nature of the pavement, the bedrock should be more than four feet below the base of the system, the seasonal high water table more than three feet, and the site should be protected from wind-blown sediments.
Maintenance for porous pavement is more costly and requires more time than other LID options. Vacuum sweeping and or pressure washing is recommended along with the use of environmentally benign deicers in lieu of salt, which can damage the pavement. Inspections to check for deterioration are recommended on a monthly basis and after storm events to check functionality.
On the other hand, porous pavement reduces the amount of stormwater infrastructure and the need for plowing and salting and increases groundwater recharge. But be aware that porous pavement systems have a lower load-bearing capacity than traditional pavement systems.
Bio-retention areas mimic natural landscapes to filter stormwater using soils and vegetation in shallow landscaped depressions. Bio-retention ponds or basins are used for small sub-drainage areas, generally less than five acres, with a seasonal high water table more than two feet from the pond invert. The site slope should be less than 20% for best results. Maintenance should be done routinely on landscaped areas surrounding the ponds, along with removal of accumulated debris and sediment. Inspections should be done to observe the health of the plants for system functionality.
Bio-retention ponds provide pollutant treatment, increase groundwater recharge for unlined systems, and reduce urban heat effects. Sites with a steep slope or limited space should perform adequate due diligence to see if bio-retentions the best option.
Underground infiltration systems capture stormwater and slowly allow it to infiltrate the ground. These systems need to meet more site requirements than the other two LID techniques. Distance from on-site sewage disposal systems, community and private wells, property lines, structures, and surface water bodies all need to be considered. Maintenance includes bi-annual inspection of pretreatment devices and chambers, cleaning of pretreatment devices as necessary, and removal of accumulated sediment over three inches.
Underground infiltration increases groundwater recharge and allows for flexibility in the system location and configuration to best suit the application. The system restricts accessibility for maintenance, requires a piping system infrastructure, and requires pretreatment to minimize maintenance; all things to consider before choosing underground infiltration.
Every project is different and all alternatives should be evaluated based on site needs and design. Costs range based on the project, but for large scale projects porous pavement tends to have the lowest capital expense followed by bio-retention ponds then underground infiltration.
Contributed by Janice Greenwood of Woodard & Curran's Rhode Island office
Posted by Blog Contributor at 12/10/2009 02:56:16 PM