Erosion Control BMPs
What is a BMP?
BMP stands for Best Management Practice. The term refers to practices that keep construction sites from being a problem for the people living nearby. Practices such as properly installing silt fences containing trash will help your neighbors see construction as something that benefits their neighborhood and reduce the complaints that people make about construction sites. Proper use of BMPs can also drive business. When people drive by your site and see your sign, they are more likely to think approvingly of your business if the site is well-managed.
Sediment is one of the most serious pollutants on a construction site. Excess sediment eroding from a site into a stream causes all kinds of problems. It can silt up streams and storm drains, or the erosion that carries sediment off a site can increase the force of flowing water and create a vicious cycle of worsening erosion. Often both problems exist at the same time, in different places. In order to prevent this vicious cycle of erosion from occurring, the city has various standard BMPs for construction sites. Below are some of the most common types of BMPs, including examples of typical installation practices and the criteria the city uses when inspecting sites.
Construction sites need to have a way for vehicles to enter and leave the site without tracking mud onto the surrounding road. The standard construction entrance is a path made of 3” gravel long enough to clean mud off the wheels and tracks of vehicles before they leave a site. Some sites use the concrete driveway as a construction entrance. The standard is that there must not be muddy tracks in the public road.
Erosion control refers to mechanisms designed to keep rainwater from washing dirt off the site into the road or into nearby waterbodies. The most common form of erosion control is silt fences, but silt fences only function as erosion control if they are installed properly. Too many contractors merely staple the silt fence fabric to posts and call it a day. This does not stop sediment at all. Water can just flow under the fabric! The silt fence fabric must be buried at least 3” underground so that it can filter the water. Simply placing clods of dirt on top of the filter fabric is also not sufficient. Silt fences also need maintenance. They are not sturdy! When the posts start to lean or break, they must be straightened or replaced. When the sediment builds up to 1/3 of the fence's height, it needs to be removed. If the filter fabric tears, that section of the fence needs to be replaced. Straw bales can be used to reinforce silt fences but can’t replace them: they don’t filter effectively enough on their own. Some sites use tubes of filter fabric wrapped around mulch known as “Erosion Eels”, which can be a low maintenance way of achieving the same result. Erosion Eels need to be placed in such a way that water must flow through them instead of going around, and they must be replaced or repaired if damaged. The standard for erosion protection is that sediment does not wash off the site over the edges or curb.
Good site design is essential for erosion protection. For example, a 50 ft pile of bare soil cannot be contained merely by silt fence. The grade of a site needs to be designed to keep steep slopes to a minimum. If water channels start forming on the site, the ground needs more stabilization (such as rock, grass, or straw mats) to reduce erosion. Infill sites will need more stabilization than just a silt fence at the bottom. A lone silt fence will simply be crushed by the subsiding soil. I’ve personally seen this happen many times. Instead, the grade and materials of infill need to be chosen to reduce the tendency of soil to subside, and the soil needs to be stabilized before it bears the weight of equipment and a house. Large sites will require more advanced BMPs, such as sedimentation basins and rock check dams.
The most important requirement of erosion & sediment control is that loose sediment does not end up in streams or other waterbodies. Inlet protection is an important part of this. Larger inlets need riprap and may need a silt fence or other filtering mechanism to protect the water downstream. On smaller sites, a fabric tube filled with mulch (sometimes called a “sediment sock” or “gutter buddy”) placed over the storm drain may suffice. On sites without curb erosion or track out, inlet protection can be enough sediment control all by itself. Inlet protection needs to be maintained by removing excess accumulated sediment and repairing or replacing damaged socks. It’s especially important to check on inlet protection during the winter, when plow trucks may damage or remove curbside inlet protection. It’s also important that inlet protection completely covers the inlet, and that all inlets that receive runoff from the site be protected. The inspection standard is that inlets be completely covered, and that the type of protection used be sufficient to keep trash, dirt, and any other pollutants out of streams and storm drains.
The most direct and effective way to combat erosion is by stabilizing the soil. Turf is the most common form of stabilization, but other methods can be used as well. More severely eroded areas may need straw mulch or even erosion control mats to prevent erosion from washing away the plants while rooting is still being established. Areas with channelized flow may need riprap, grass swales, or other methods of slowing down flows to enable stabilization. Grass seed or native plant seeds can be used for soil stabilization, but it’s important to make sure that the seeds don’t wash away before they can take root. Stabilization is an often-forgotten BMP, but Land Disturbance Permits cannot be closed until stabilization is complete. The standard is 90% stabilization over 100% of the disturbed area.
The most important way to prevent pollution by hazardous waste is to store materials properly. All containers of chemicals other than water (including, but not limited to, paint, asphalt, caulk, oil, and gasoline) need to be stored inside a building or trailer, not exposed to the elements. If chemicals spill on the ground, they need to be cleaned up with an absorbent (such as cat litter or paper towels). Any contaminated soil needs to be thrown away. If you record that a spill happened on an inspection sheet and properly clean it up; that is NOT a stormwater violation. Instead, it shows us that you are acting in good faith and taking care of your own site. This is exactly what we want to see from self-inspections.
Sanitary Facilities: They’re fairly simple from a stormwater perspective: just make sure they don’t spill and clean up the spilled liquid if they do. Since they are a potential source of pollution, Land Disturbance Permits can’t be closed while they’re still on the property (unless somebody else officially takes responsibility for them).
The City of Moberly has had a serious problem in recent years with cement trucks washing out illegally into steams, storm drains, and manholes. This is not only illegal, but it also causes the storm drains and streams to become clogged with accumulated thin layers of cement. As part of a multi-pronged effort to combat this practice, we require all Land Disturbance Sites to provide a cement washout for cement trucks and to make sure the cement trucks use it. A cement washout needs to have an impervious material (such as waterproof plastic tarp) completely lining it, and it needs to be big enough to hold all the wash water without leaking or overflowing. It also needs to be above ground so that any leaks can be seen quickly. The most common type of washout is a frame built of straw bales and lined with a plastic sheet. The cement in the washout needs to be removed when the washout is 75% of the way full. If cement is washed out onto the ground, or if the washout is not used when cement is poured, that is a stormwater violation. Make sure the washout is clearly labeled, easy to see, and that crew and subcontractors know where and what it is.
Very large sites sometimes have temporary detention basins built during the grading stage of construction in order to allow water that runs off of sites to settle out most of the sediment. Sometimes these temporary basins are removed, and sometimes they’re converted into permanent stormwater detention. They need to be designed by a certified engineer and approved with the initial plans if they’re used. If they’re converted into long-term detention, they also need to have a long-term maintenance agreement so that they aren’t effectively abandoned once construction is complete. Single-home sites in new subdivisions almost never have these.
Rock Check Dams: This is another BMP commonly found on very large sites, but almost never on smaller ones. Rock check dams are mini dams of gravel or other rock built across on-site drainage ditches in order to slow the water (preventing it from scouring out the drainage ditch) and remove some of the sediment. During inspections, these need to be effectively slowing the water; meaning that there shouldn’t be erosion or accumulated sediment immediately downstream. These are different from riprap around an inlet but serve a similar purpose. They also need to be designed by an engineer.