For 37 yrs LAW, Inc has shined in the more difficult jobs…the jobs no one else wants to deal with. One such project was the Dorrs Dam rebuild in Manchester, NH. It showcased new technology (equipment) meshing with one of man’s oldest techniques (siphoning water) in some very non-conducive conditions.
Originally printed in the Union Leader on Monday, February 20, 2012
Five times in the past seven years rainfall in Manchester has been intense enough to cause water levels in Dorrs Pond in Livingston Park to rise above dam level, bringing flow crashing down onto the rocks below. Repeated overtopping can compromise the integrity of a dam by washing away soil necessary to its stability. As a result, the city has recently completed a project intended to fortify the area downstream of the dam to withstand overtopping at a rate more than twice that expected during a 100 year storm.
Leighton A White Inc. of Milford was contracted by the Parks, Recreation, and Cemetery Division of the Highway Department to install some 2500 sf of curbing on a concrete base to resist the forces of moving water. The $100,000 project was designed by TFM Inc. of Bedford. It was reviewed and approved by the NH State Dam Bureau and was funded through grants from the NHDES and the EPA. Dam specialists HTE Northeast Inc. provided inspection services.
Dorrs Pond Dam is classified as a high hazard dam by the state because people live immediately downstream and failure may result in loss of life. As a result, state requirements include the ability to withstand 2.5 times a 100 year storm. Rains corresponding with such a storm would result in water overtopping the dam by two feet; hence the use of robust granite curbstone pieces (minimum size of 3 feet x 1.5 feet x 6 inches). Physically the dam is made up of two walls, a 1.5 foot wide concrete wall along the pond side, and a stone masonry wall capped with concrete along the downstream side. The nine feet of space between them consists of compacted earth fill, and its length is approximately 125 feet. The dam passes pond flow via a 6.5 foot wide spillway, and it last saw work during a rehabilitation conducted in 1986.
Like the previous work, current tasks focused on minimizing erosion on the downstream side to prevent conditions that could lead to dam failure. In just over a month of work at the dam, the contractor removed two feet of soil, built the ground back up with compacted gravel, and placed approximately 200 pieces of used granite curbing on top creating an unerodable surface. The curbstones were laid on their side, and were grouted together with concrete. The spillway was doubly armored, first with two feet of 4000 psi concrete, and then with granite slabs. Granite was chosen not only for its hardness but because it was readily available, relatively inexpensive, and aesthetically pleasing.
In order to dig in the ground in the most favorable conditions Leighton White employed the use of four six-inch diameter siphons to lower the pond level to approximately three feet below the spillway. Siphons, depicted in Egyptian reliefs as far back as 1500 BC, use atmospheric pressure to pull water out of the pond, pass it over the dam, and discharge it below the worksite. The use of siphons was a perfect application on the project because once in operation they required no external power and ran 24 hours a day. Lowered pond levels made for easier and more stable excavation for the contractor. With the project complete, the Dorrs Pond Dam is fully compliant with NHDES regulations. Patrons of Livingston park, whether in the adjacent playground or out on the mile-long trail around the pond can admire the stonework for decades to come.