Sunday, January 17, 2010

Plan for a Rainy Day

Wondering what you can do when heavy rains are forecast to hit your construction site? There are numerous erosion and sediment control methods, practices, and materials available to the contractor, but there are a few usual suspects when things go wrong when it’s raining (yes, sometimes it actually rains in San Diego). Here are the five biggest oversights or mistakes that we’ve witnessed on sites when it comes to construction BMPs. Avoiding them may mean the difference between an uneventful rainfall and a costly cleanup later.

1.) Silt fences that don’t have a chance of containing any silt:
If you can see daylight at the bottom of your silt fence, then the only purpose it serves is that of a flimsy 2-foot high perimeter construction fence; alerting neighbors where the edge of your construction site is. A properly-installed silt fence should be embedded at least 4 inches (6 inches is standard) into the soil below, and the base of the fabric below the soil should be ‘keyed-in’. To achieve this, cut a 6” x 6” trench at the base of your future fence, run the fabric along the bottom of your trench and then up the side, so that at least 8-12 inches of the fabric lies below grade once you fill your trench back in.

2.) Limited or No Monitoring of the Site:
Even with the best-laid protection devices on a construction site, without monitoring by a person delegated with the responsibility, a small breach or channel created by a single rainstorm can render the performance of your materials worthless. You may have constructed a fine desilting basin at the outflow point, and then the water carves its way around or beneath the gravel bags or earth berm, sending a deluge of silt downstream over the course of hours (or days). The site should be inspected after each significant rainfall. When the storm is particularly heavy, lasts a long time, or storms follow one after the other, put on your best boots and check out the storm while it is raining to survey the installed devices in action, and take action on the spot if need be.

3.) The Right Materials Installed at the Wrong Location:
Silt fences and straw wattles are very effective sediment control devices, but they are nearly worthless at points of concentrated flows. Any low points or other places where you expect a stream of water during a storm need heavy devices such as gravel bags, rock check dams, or earthen berms to keep from tracking sediments off site. And putting something intended for flatter areas (like loose mulch or straw) on steep slopes just creates more debris to wash downstream when it does finally rain.

4.) No Backup Plan:
A well-protected job can fall apart quickly in a bad storm if there are no materials stockpiled on site. Unless you have a really good relationship with your erosion control supplier, you’re not going to get them to show up with an hour’s notice in the middle of a stormy week. The best single item to keep at hand? Gravel bags, or at least a pile of gravel, bags to put it in, and the manpower to assemble them. Gravel bags can be used to create a strong desilting basin or barrier, as small checkdams in a drainage swale, and can also be used in place of a breached section of silt fence or straw wattle. Once things dry out, the gravel can be re-used in numerous other capacities on site.

5.) Size Matters:
Installing a small check dam or desilting basin on a postage-stamp size lot isn’t the same as controlling the flows from a 10-acre construction site. Larger sites need special attention to the size of the installed features. Temporary dams and basins may need to be excavated out. Their size should be calculated from expected flow rates and types of soil on the site.

Thankfully in the dry climate of San Diego, attention to erosion and sediment control on the construction site is usually not a problem. Some projects can be started and finished over the summer, or during a particularly dry winter. But we can’t let that lull us into a false sense of security when we get a bad storm or series of storms. The risks of damage to downstream properties and facilities, not to mention the fines and claims that may come with them, are too great to just ignore.

John S. Coffey, PE, is founder and President of Coffey Engineering, Inc. in San Diego. He’s contributed to over a thousand civil engineering, surveying, and planning projects in San Diego and surrounding communities over the past 14 years. 858-831-0111