By James W. Gallagher, Jr., P.E.
Floods, New Hampshire’s most common natural disaster, occur in every season. Floods caused by tropical depressions, like Tropical Storms Sandy and Irene, occur in the summer and fall hurricane season. Winter floods can also occur due to unseasonable thawing conditions and heavy precipitation combined with ice jams, like those that occurred in January of this year in locations largely south of the White Mountains. But it is the spring when the state is most likely to experience floods.
Unlike other parts of the country where precipitation is higher in the spring, New Hampshire’s spring precipitation is not much different than the precipitation that falls on it over the other three seasons. On average, New Hampshire receives 3½ to 4 inches of rain each month. However, in the early spring, when the ground is typically saturated from the melting of the winter’s snow, and before the vegetation, which absorbs large amounts of water, has budded out, the amount of runoff from that rainfall is much greater than it is during other seasons of the year. This seasonal pattern of rainfall and runoff is shown on the figure below, which illustrates the monthly rainfall and streamflow into Lake Waukewan in Meredith, New Hampton and Center Harbor.
The high April flows are also the result of the melting of the snowpack that is usually present at the beginning of April. Based on approximately 50 years of snow data collected by the New Hampshire Department of Environmental Services (NHDES), there is usually between 4½ and 5 inches of water content in the snow pack in New Hampshire at the beginning of April, which is greater than the amount of precipitation that normally falls in a month.
NHDES is responsible for operating 211 state-owned dams. To help reduce damages due to spring flooding, NHDES lowers the water level on the 50 largest lakes impounded by state owned dams, to depths ranging from one to 7½ feet to provide storage to capture these high spring flows, providing a level of protection to both property along the shoreline of the lakes as well as downstream of the dams. These drawdowns also can reduce erosion of the shoreline along the lakes. High water conditions, in combination with heavy winds, cause erosion of the shoreline above the normal water line. Drawdowns lower the water level so that the erosive forces of waves are acting below the normal shoreline. Drawdowns also provide vertical space so that if water levels rise as a result of high runoff, they come up to normal levels rather than overtopping the dam as could be the case if the waterbody were kept full through the winter and spring flood season.
The lakes are also drawn down to reduce winter ice damage to shoreline properties. Lake ice can reach a thickness of two feet or more. The force of massive ice is exerted in three ways. Under the warming spring sun, as the lake ice expands, it can exert considerable force on anything in its path including docks, walls, and the natural shoreline. Should lake levels fluctuate when the ice is frozen onto an object, that object will be moved accordingly up or down. As the near shore areas thaw in the spring, the remaining ice sheet can be driven by the wind onto the shore. Drawdowns are effective at transferring the location at which these forces are exerted away from the natural shoreline and structures built there. To protect shoreline structures, many municipal and private owners of dams also drawdown their lakes for the winter.
The drawdowns generally occur around Columbus Day each year so that they occur before amphibians have begun hibernating and before fish that spawn in the fall begin laying their eggs. Over the past several years there has been an increased interest from boaters to postpone the drawdown date to lengthen the boating season, but because of the potential negative impact to aquatic species these efforts have been resisted.
Should there be a very dry spring, it may be difficult to refill the lakes completely before the summer recreation season, while still maintaining instream flow needs downstream. Spring refill requires careful attention to current and forecasted conditions to refill the lakes by the end of May, while still providing some level of flood protection during the spring flood season.
James Gallagher is the Administrator and Chief Engineer of the Dam Bureau in the New Hampshire Department of Environmental Services. Jim may be reached by phone at 603.271.1961 or by email at firstname.lastname@example.org.
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