The New Hampshire Department of Transportation’s award-winning work managing construction stormwater to protect two water bodies adjacent to Interstate 93 has been a successful demonstration of innovative techniques under real-world conditions.
NHDOT took a completely new approach, especially for the temporary stormwater controls, when widening and reconstructing I-93 south of Manchester where it passes between Canobie Lake and Cobbett’s Pond, according to NHDOT’s Director of Project Development Peter Stamnas.
The proposed work on the segment, a nearly three-mile stretch at Exit 3, prompted significant public concern over the potential for turbidity and sediment affecting the nearby water bodies, especially Canobie Lake which provides local drinking water.
I-93 between Manchester and the Massachusetts border had not been significantly upgraded since it was built in the early 1960s, according to project documentation. NHDOT was faced with performing major construction while still meeting the New Hampshire Department of Environmental Services’ stringent water quality standards and the public’s demand for the cleaner construction.
Knowing that the entire 20-mile project, and the Exit 3 construction in particular, was constrained by the natural geography, NHDOT “really had to start from scratch from a design standpoint” to create a solution for the construction stormwater that would meet NHDES standards and receive public support, according to Stamnas, who joined the project in 2006.
NHDOT used a “new design approach that would be similar to a traffic control plan” but applied to construction stormwater, Stamnas said. The agency reviewed the entire length of the project, analyzing the runoff risk and calculating runoff volumes. Prior to the construction phase, staff identified the areas that would require stormwater controls, resulting in a comprehensive stormwater management plan.
The agency faced constraints within the right of way regarding the surface area for stormwater storage, the hilly terrain, and the nearby water bodies, and had to devise innovative solutions. “There was just too much water and not enough real estate,” Stamnas said.
NHDOT maximized the limited land available within the right of way by collecting water wherever possible and using a system of pumps and pipes to get the water “where it needed to go” for storage and treatment, Stamnas said. This had the added benefit of reducing the number of treatment cells that needed to be constructed.
Treatment with Polyacrylamides
Canobie Lake and Cobbett’s Pond created significant challenges since they are the low geographic points along Exit 3 and “everything went there,” Stamnas said. NHDES standards for class B waterbodies (Cobbett’s Pond) require down stream flows not be increased by more than 10 nephelometric turbidity units (NTUs) after the introduction of the stormwater flows. For Class A waterbodies (Canobie Lake) the standard is further reduced to a 0 NTU increase above background levels.
To address this, the agency pioneered a program for linear highway projects which included flocculent system designs, contract bid items, and procedures for the construction use of anionic polyacrylamides (PAMs) as a flocculant to induce the suspended solids in the runoff to form larger flakes. These particles then could be filtered out or allowed to settle, turning sediment-clouded runoff into clear water that would not disrupt local water quality.
NHDOT started out using self-dosing blocks that control the release of PAM into the runoff water. The flocculant was added to a flocculant dosing tank system. The water was then run through in-ground settling basins and clarifying structures that used filtration baffles.
In 2008 and 2009, “we were on the cutting edge” of PAM use from a linear construction standpoint, Stamnas said, and the dosing and filtration methods evolved over time during the project. At the beginning, NHDOT had “difficulties” receiving regulatory approval to use PAMs but “we finally worked through it,” Stamnas said.
The regulators’ concern was that NHDOT would use too much PAM, causing it to be a pollutant itself. Stamnas said eventually they were able to prove that they could make effective use of the flocculants without creating a hazard to water quality. The agency found that by switching from self-dosing blocks to PAM powders, the dosage could be controlled more accurately.
Techniques also were used to divert water that was coming into the site, directing it through bypass pipes so that it did not pick up sediment. This lowered the amount of water that required treatment, Stamnas said.
In addition to treatment, PAMs were used as a soil stabilizer for disturbed slopes.
Contracting and Costs
The significant permanent stormwater retention structures and the locations for construction stormwater controls were included up front in the original contract documents, Stamnas said.
NHDOT provided base plans, required contractors to provide more detailed stormwater control plans, and set up regular meetings to review progress. This allowed the contractors and the field staff to be a lot more prepared, Stamnas said. Conventional erosion and sediment control best management practices (BMPs) and typical management approaches to construction were determined to be inadequate and unnecessarily expensive, Stamnas said.
Also, agency planners and designers created strategies and tools they knew would work in various situations and that could be applied to address field conditions. It provided the contractors and the field staff with options from which they could make decisions based on the situations they were presented with, Stamnas said. Examples of these tools included hydraulically applied mulch systems, unit water diversion items, standard pump and pipe systems, and a packaged flocculent treatment system. NHDOT provided the proposed collection of tools early on so that contractors could bid on them up front rather than including them later through change orders or creating a situation where there was a sole-source supplier.
The construction work at Exit 3 cost approximately $150 million. The agency wanted to place the stormwater controls into the bidding environment to achieve some level of economy. NHDOT was looking for ways to reduce costs, Stamnas said, “and I think we did that.”
According to NHDOT, completing a construction stormwater assessment during the design phase of large projects pays dividends. It identifies potential risks early and allows items to be included in the contract to minimize costs and mitigate risk potential. Also, contractors are more prepared, and it reduces their time to prepare stormwater protection plans, so work can start sooner.
Diverting water around the active construction zone is critical and constructing temporary sedimentation basins as early as possible reduces treatment efforts.
Also, the construction site is constantly changing, and strategies that work one month may not work as well the following month. The matrix of tools allows for solutions to be implemented quickly and cost-effectively.
Additionally, PAMs are very effective in reducing turbidity in construction runoff and are safe for the environment when used properly. They reduce soil loss, reduce phosphorous levels in treated stormwater, have negligible effect on water pH, and lower biochemical oxygen demand in runoff.
Traffic was placed in the final lane configurations through the Exit 3 area over the summer and this segment of the corridor project was declared substantially complete in November of 2016. Four more years of construction lie ahead along the remaining segments of the 20-mile corridor project.
The Exit 3 stormwater control project earned NHDOT the “Best Use of Innovation” award in the medium-sized project category from the Northeast Association of State Transportation Officials (NASTO) for its efforts to protect the natural resources.
“It’s a new way of looking at linear construction and it’s working really well,” Stamnas said. Based on the size and complexity of future projects NHDOT now has a comprehensive list of strategies to draw from for use around the state.