On the Job: Dewatering Operations Are Critical
June 1, 2011
Construction site dewatering is not a new phenomenon. However, over the last decade, water resource management – at nearly every level – has become an extremely high-profile environmental issue. In particular, the protection of lakes, streams and estuaries are becoming a greater concern as the conversion of surrounding land into commercial and residential habitats continues to creep closer to these delicate water ecosystems.
But what is good for the environment is not always good for the contractor, because today, the water not only has to be removed, it now has to be clarified before it can be discharged. This adds time, cost and, in many cases, aggravation to nearly every major construction project located in these sensitive areas. That is why it has become paramount for the contractor to take the right steps to properly remove and treat the ground water.
A recently completed highway expansion project along a section of I-75 in Florida is a prime example of the right way to dewater a site located close to a body of water designated as an outstanding Florida water (OFW), by the Florida Department of Environmental Protection (DEP).
The water clarity criteria established by the Florida DEP for this particular project stated the maximum allowable turbidity level of the discharged water could not exceed 29 NTU (nephelometric turbidity units) above background, which, in this case, was 5 NTU. Pre-construction water clarity test results indicated the untreated water from the site was ranging from 100 NTU to 950 NTU. It was apparent the contractor had his work cut out for him.
At this point, the contractor contacted R.H. Moore & Associates, a local distributor of erosion-control and soil-stabilization products, and asked the firm to come up with a solution. After assessing the situation, R.H. Moore & Associates proposed a portable, above-ground, water-clarification treatment channel to bring the site’s turbid ground water into compliance.
First, a 1,400-foot trapezoidal-shaped ditch/channel lining system, called SmartDitch, was assembled along the roadside near the source of the water runoff. Each channel section was 9 feet long and attached, at each gasketed joint, with removable screws. Once assembled, the 18-inch-deep-by-48-inch-wide channel would be able to handle up to 13.5 inches of water, at velocity of nearly 10-feet-pre-second at a 5-percent slope. Then, Floc Logs, designed to remove the fine particles and reduce NTU values, were placed every 9 feet inside the SmartDitch channel. At the end of this clarification channel, approximately 100 feet of all-natural woven, biodegradable jute matting was laid on top of construction film to further capture loose particles and fine sediment.
Once in place, submersible pumps and hoses were engaged at the upstream end to extract and direct the turbid ground water through the channel for clarification. Due to the high ground water in this particular area, and frequent storm events, the contractor ran the dewatering/clarification system 12 hours per day.
DEP inspectors then tested the water daily to make sure the contractor was meeting the water clarity criteria. “Without a doubt, this system saved the contractor time and money,” states Jeff Peterson, an R.H. Moore engineer. “Without this system in place, even the most environmentally conscious contractor could not have stayed in compliance, and would have paid extensive fines.”