Feb 16, 2006

Managing Sediment Storm Water Run Off

There is no one BMP for any given situation or location

Sediments contribute to pollutants by silting up and smothering stream beds and killing fish and fish eggs, which is why it's important to have best management practices for sediment runoff.
Sediments contribute to pollutants by silting up and smothering stream beds and killing fish and fish eggs.

Here are many Best Management Practices (BMPs) for controlling storm water sediment runoff, and there seem to be new products popping up every day, which can be confusing.

In 1972, an EPA study found that erosion and sediment contribute greatly to surface water pollution. With the implementation of Phase II regulations, federal law required storm water discharge permits with five or more acres of land disturbance (March 2003). In the case of a point discharge to surface waters, only 1 to 5 acres will trigger the regulations. In both cases, a plan is required to minimize the erosion and sediment pollution.

Pollution includes products of erosion, sediment, soil and organics (bits of leaves, grass, etc). Sediments contribute to pollutants by silting up and smothering stream beds and killing fish and fish eggs. Sediments also act as carrying agents for heavy metals, bacteria and other pollutants.

Fortunately, sediment particles can be trapped fairly easily by using a variety of methods that slow the flow of water, causing the particles of sediment to deposit. Oils, metals and soluble pollutants can be more difficult to trap, often requiring the use of adsorbents such as fossil rock, charcoals, ion exchange media and other means. This article will look at the phases of construction to see where different BMPs work best.

 

Design phase for storm water sediment runoff management 

NEPDS-friendly drainage systems can cost up to 50% less than the cost of a retrofit to install. By including the right storm water management solution in the design, one can save money and avoid EPA or local agency fines. Examples of eco-friendly drainage systems include grass pavers and infiltration systems.

Grass pavers such as the EZ Roll and Tuff Track prevent soil compaction and reduce erosion. They allow grass to thrive in areas where hardscape would normally be used, such as rights-of-way and emergency vehicle access points. Grass pavers protect grass from permanent damage and protect roots from vehicle traffic. The better types are soil filled, rather than sand or light gravel filled, as soil will allow for the healthiest grass growth. Support beneath the grass pavers should be road base to accommodate heavy vehicles. Maximum load should be enough to handle the weight of a fire truck.

Infiltration systems such as the Flow-Well keep storm water on site. Down spouts and surface drains can be connected to the Flo-Well to infiltrate storm water directly into the soil, eliminating runoff completely.

 

Build phase for storm water sediment runoff management 

The main concern during the build is sediment control. Until recently, this was little more than an afterthought. Today, silt fences and sand bags are common around any construction site. In many areas, to prevent sediment tracking, heavy vehicles have to be washed down and driven over steel plates with ridges to clean off mud. With several high profile big box retailers being fined recently, the writing is on the wall; the middle tier is next. Local municipalities have no choice but to enforce the rules or face fines themselves.

That was the experience of Roland Serna, principal of Roland Design Systems of Orange County, Calif. He was working on a single family residence in San Diego County and was required to upgrade his sediment control measures.

Serna’s silt fence was inspected from end to end several times. He was required to modify it three separate times to comply with local rules and avoid having the project delayed. One measure he took was to leave existing vegetation in place wherever possible. He also kept the footprint of the disturbed soil limited to just the area needed for the foundation and driveway. He waited until after the structure was completed and the concrete was poured before working on the landscape. This kept the area of disturbed soil to a minimum.

Gravel bags are not all equal, and sun resistance plays a big part. Some materials are sun and rot resistant, and others fail quickly. A good benchmark in order to plan properly is 2,000 hours, which is 166 days of sunlight (at 12 hours of sunlight a day). Sand bags are set around the perimeter, usually to hold down a silt fence and to filter runoff. A snake bag works better for curbs and along gutters. They are usually 4 to 6 ft long and are placed perpendicular or at a slight angle to the curb, which slows down the water, causing sediment to accumulate behind the bags and preventing it from entering the storm water drain. Drain donuts work very well on catch basins to keep out larger debris and to slow the flow of water, helping to keep sediment out of the drainage system. Clearing pipe of sediment is expensive and time consuming.

When choosing drain donuts, the material and configuration is crucial. Much like gravel bags, a 2,000-hour sunlight exposure life is appropriate. There are numerous types of catch basin covers, and many are quickly overwhelmed by leaves and sediment. Select one that has a large surface area and is easy to clean without having to remove the catch grate. One of the most important features to look for would be a high-flow bypass so the water doesn’t form a huge puddle with the drain donut acting like the cork in a bathtub. Finding and cleaning it under muddy water would be an unpleasant task.

It is important to choose a drain donut with a high-visibility cover to keep traffic from running over it, causing it to fail. Often, drain systems require removal of the grate and shovels or suction trucks to suck out the debris. Depending on the type of system, when full of wet debris, some will require the use of heavy machinery to lift it out of the catch basin. This is an additional maintenance cost.

 

Post build phase for storm water sediment runoff management 

In many cases, once construction of a property is finished and the occupant moves in, NPDES is quickly forgotten. This could be a big mistake. For example, several municipalities in California have been aggressively pushing storm water mitigation at the source. In the Lake Tahoe area, homeowners are required to retrofit their property prior to sale. Some cities in Orange County, Calif., require that all water that falls on the property stay there, a zero runoff policy.

In the post build phase, the current focus is on road oils. There are many solutions on the market today. Most absorbents are low maintenance, requiring change out only once or twice a year and sediment removal between two and four times a year. Most are designed to fit into existing catch basins without modification. Some are low cost and have life spans of three to five years; others are heavy duty, designed to have long lives of up to 20 years. Most residential users will probably opt for lower cost units. Municipalities will do better with heavy-duty units with a longer life.

Choosing the right BMP will depend on which phase a project is in and local conditions. Clean water is to everyone’s benefit, and regulations are getting stricter as time passes. By using good practices, one can avoid costly fines and delays and promote clean water conditions.

About the author

Phil Sallaway is product manager for NDS Inc. He can be reached at 818/710-4009, or by e-mail at [email protected].

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