It seems like just yesterday we had another of those flashy, intense rain storms that came through the area and left some places drenched and other places needed to water the lawn. This rain cycle has been going on for some time now and those of us in the design and construction fields have taken notice. Recently, the states are starting to implement new rainfall amounts which increase the amount of rain defined as a year storm. For example- the first flush which used to be defined as the first ½” is now the one year storm.
From the design perspective, though, we are already changing how we prepare and review erosion control plans and construction sites to address the realities of the current weather patterns. Here are some examples:
• The runoff comes in bursts - It seems that having a nice day of light rain where the rain event total is around ½” for the day hasn’t happened for a while-For the most part, these afternoon rain events seem to be about ½” per 20-40 minutes. So you could have ½” of runoff every afternoon in short very intense bursts. If you are in one of those places where the storm lasted a little longer, you get between 1-2” in a thunderstorm regularly.
How does this affect your erosion control plans? The standard formula for temporary storage - 3600cubic feet per acre drainage - seems to still be working but the rain will come again before your temporary sediment basin is empty so you need to arrange to filter the water quickly to get your storage capacity back. This means that even small sites should have temporary sediment traps/basins; there should be someone on-site who can appropriately deal with pumping and filtering; and there needs to be a well-defined discharge point that won’t affect downhill neighbors, exposed slopes or other sensitive lands once the pumps get turned on.
• It is feast or famine - Are you located in one of those places that gets a storm every afternoon? If so, your storage capacity will be constantly put to the test. The construction site is covered with surface mud and work proceeds slowly. You will need to monitor your erosion control even more as water is far stronger than silt fence or berms. When it is wet and muddy, most site contractors won’t want to be on site-but today, most towns require constant vigilance of the erosion control measures. Just because you can’t get a machine safely in place is not a good reason why you continue to discharge silty water into a stream.
How does this affect your erosion control plans? The designers need to indicate contingency plans, the level of attention and frequency of inspections to clearly define the contractor’s responsibilities and to make sure these items are part of the base contract. Extras for erosion control are a lot more expensive that including them as negotiated unit prices as part of the contract.
• It is dry under the surface - The construction site will dry out very quickly between rains which means that your plant material will still need to be watered and the ground below may be much dryer than expected or wanted.
How does this affect your erosion control plans? In addition to more storage and temporary facilities, the plans should include the requirements to address dust which usually shows up when there is more than 3 days without rain.
• Require Inspections - Most municipalities and often the state (for larger projects), require regular erosion inspections. This works to every ones’ advantage. It gives the contractor a constant reminder to anticipate the weather, it allows the client to have the confidence in the contractor and it allows the Town to monitor what is happening on the site and to address issues as soon as, if not before, they happen.
How does that affect your erosion control plans? The erosion control plan itself is not a guarantee of success. No matter how detailed a plan is, site and weather conditions will create a need for changes in the field. Our experience has shown that weekly inspections are required, during seasons with rain, to respond to the ever-changing conditions of the site and prepare for upcoming weather. Regular inspections help to protect the contractor from the potentially reactionary response of the reviewer and encourage the team (designer, owner and contractor) to be on the same page with respect to the implementation of the goals of the erosion control plans (and the requirements of the town) by responding quickly to the inspection recommendations
Thinking ahead and understanding the impact the current weather cycle has on your project and the construction site makes for better erosion control plans and cleaner runoff. With the latest pressures of the MS4 regulations, the project is better off if the team controls the site and responds as needed during construction.
Terri-Ann Hahn, ASLA, CPESC, CPSWQ, LEED AP is a principal of LADA, P.C. – Land Planners, Simsbury, Conn.