Earth dam made weak by rotting tree stumps
By Lynn R. Parks
Early analysis points to hollows in the earthen dike at Hearn's Pond as the cause of the dike's failure after heavy rainfall two weeks ago. According to John Hughes, director of soil and water conservation for the Department of Natural Resources and Environmental Control, the hollows in the 89-year-old dike were possibly caused by the stumps of long-ago cut trees, left in the soil to rot and allow the earth to settle.
"The presence of high rainfall forced water through the hollows," he said, eroding the soil and causing the dike to fail.
Hughes added that the dike probably would have collapsed even if the flood gates at the dam had been open. A state employee visited the dam the night of the rain and found that the breach had already occurred.
"There is no indication that opening the spillway would have saved the pond at all," he said. "The dike was in an antiquated state. It was not built to modern specifications at all."
The dike was constructed of earth, reinforced with pockets of refuse such as bottles.
Hughes added, however, that inspection of the dam would have detected the hollows. Delaware has no dam inspection program. In 1999, the state received $45,000 from the federal government to inspect dams. The state used that money to check all its "high hazard" dams, or dams whose failure would lead to human fatalities, and a few "significant hazard" dams, of which the Hearn's Pond dam was one. But the money ran out before the Hearn's dam, last inspected in 1973, was checked.
A program to inspect all dams is typically provided for by legislation, Hughes said. Such legislation is "in discussion" now, he added.
Hearn's Pond, created over 100 years ago to power a grain mill, covers an area of 53 acres and when full has a mean depth of 4.4 feet; at its deepest, it is 9 feet. It is fed from the west by Buck's Branch and spills into Williams Pond. Its dam broke Saturday, Aug. 11, when the Seaford area received more than 8 inches of rain.
When the dike failed, its 2,500 cubic yards of earth was carried by the rush of water downstream into Williams Pond. In addition, silt from the bottom of the pond was carried into Williams Pond.
Quantities of that pond-bottom silt, measuring "probably in the 1000s of yards," Hughes said, will continue to flow into Williams Pond and on into the Nanticoke River with each new rainfall, carrying with it nutrients, primarily phosphorus, that have run off into the pond. When Hearn's Pond holds water, it acts as a sediment trap, containing rainfall until the excess nutrients which it carries settle out.
Without that trap, the nutrient flow and consequent concentration of phosphorus could lead to algae blooms and then to fish kills in Williams Pond and the Nanticoke, Hughes said. "Nutrients are only a problem when there are too many," he said. "But most of our waterways already have too many," including the Nanticoke River.
The flow of water and silt could also carry with it PCBs, toxic waste chemicals that are found in practically all waterways. Those PCBs will add to the PCBs already in Williams Pond and the Nanticoke, possibly poisoning life in the river. "Consequences of that will not be immediately evident," Hughes said. Heavy metals such as mercury that are often found on river bottoms are not a concern. "You would find those mostly in industrial watersheds, or downstream from historical mining areas," Hughes said.
Erosion of the banks is also not a concern; their "angle of repose is set and there will be no sliding," Hughes said. However, the absence of water can present a hazard. "Deep mud is not the safest place to allow children," he added.
Hughes said that repair of the dam is "better to do quickly," in order to reinstate the pond as a sediment trap as well as because the pond is a "living resource."
"Many people use the pond and it is certainly an amenity to the people who live there," he said.
The state is in the process of requesting proposals for design of a new dam. Once an engineering firm is selected and a design finalized, bids will go out for construction. "This is not an overnight process," Hughes said. "Fast track, I would guess that it might be completed by mid-fall."
He was unable to give an estimate of the cost, other than to say that it will be in six or seven figures.
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