Storm Water Flows on Developed Sites

  Ron Duncan – SENTINEL CORRESPONDENT Article Launched: 12/13/2008 01:34:57 AM PST

  The traditional solution to deal with storm water flows on developed sites has been to move the water offsite as rapidly as possible. Engineering solutions that view rainwater as a resource and runoff as the problem are gaining acceptance through the Low Impact Development LID movement, which seeks to advance environmentally responsible site development.

 Impervious surfaces such as roads, driveways and buildings contribute significantly to storm water runoff, and cover up to 50 percent of the urban areas in Santa Cruz County. Storm water from these surfaces must be managed to protect public safety, property and the environment. The runoff flows through street drains to our streams and ultimately ends up in the ocean. To accommodate peak flows, surface water impoundments, like the one at 38th Avenue and Brommer in Live Oak, are sometimes needed to prevent flooding.

 Newer permeable materials

 The formulas for traditional impermeable materials, such as concrete and asphalt, are being reworked to allow these materials to be permeable. New permeable construction materials, such as paving blocks and open-celled grids, are also being introduced. Design elements like bio-swales, rain catchment systems and living roofs can also decrease runoff.

 Permeable concrete is made by using less fine-grained materials in the "recipe." The result is in a more open structure within the concrete.

 Asphalt can be more porous by changing to permeable ingredients, such as certain types of coal.

 Paving blocks, which are like super bricks, are installed with joints between the blocks which allow water to infiltrate. Open grid cells are frameworks made of plastic or concrete in paver-like patterns that can be filled with a porous material such as gravel or plants.

 Issues and benefits

 Environmental issues caused by storm-water runoff include: degraded water quality of our local streams and ocean, increased flooding and erosion and reduced groundwater recharge. Various studies have shown that using permeable surfaces can significantly reduce runoff and pollutants flowing to local water bodies. A few innovative folks are even developing natural substances that can be applied to permeable surfaces to "digest" and neutralize certain contaminants.

 Erosion can be partially mitigated by using permeable surfaces. Some studies show a 90 percent runoff reduction at sites using permeable pavers. Allowing storm water to infiltrate onsite can also help recharge our stressed groundwater aquifers.

 Permeable surfaces even help with safety issues. Mosquito abatement is enhanced by lessening the amount of standing or pooling water. Oregon and Georgia are requiring state highways be made with porous asphalts to increase vehicle traction and reduce runoff pollution.

 Tom Ralston of Tom Ralston Concrete has seen a steady increase in requests for permeable driveways and patios. He notes they are useful to help meet building requirements that limit lot coverage of impervious surfaces. Ralston said the cost for permeable concrete is about the same as regular concrete, ranging around $10 to $12 per square foot. The cost for pavers is higher, $13 to $22 per square foot.

 Local government agencies and nonprofit entities are beginning to recognize the multiple benefits resulting from the increased use of permeable hardscapes. Water suppliers appreciate enhanced aquifer recharge, fish and environmental advocacy entities like healthier streams and oceans, and planning authorities appreciate less flooding and lower capital costs related to engineering storm water management systems.

 Residents and business owners should consider using permeable materials when installing or replacing walkways, driveways and patios. Pavers and open grid cells are especially popular for residences and businesses. Re-using broken concrete as pavers can create an attractive permeable surface, and flagstone patios can be made to infiltrate water if the spaces among the stones are filled with a permeable material such as pebbles or a groundcover. Crushed granitic walkways are another permeable and aesthetically pleasing option.

 The Resource Conservation District is publishing a "Home Drainage Guide" that includes many LID tips.

 Ron Duncan writes a biweekly column for the Sentinel on water-related issues. He is a manager for the Soquel Creek Water District. The district offers free home/business water survey visits by calling 475–8500. He can be reached at rond@soquelcreekwater.org