By far one of our more challenging designs was the dry fire hydrant/pond drainage system that we helped install in Oxford, Maryland in November 2017. We were approached earlier in 2017 about helping out the Oxford VFD with the development of additional water supply sites - specifically, the installation of dry fire hydrants. One of the locations chosen was a 5-acre, manmade pond that is part of a large, privately-owned wildlife management area. The pond was made by pushing top soil off a corn field and building a 30-ft wide berm all around the pond. Since no stream or spring supplies the pond, accumulation of water in the pond is dependent on rainfall and on a groundwater well. The main challenge during the design process was the depth of pond water...which was generally under 3 feet in most places. A plan was developed to use a 6-inch stream strainer on 8-inch PVC pipe. The strainer would be located in the pond's permanent pool area which was clear of aquatic vegetation. The pipe would rest on the pond bottom for the most part and this would place the strainer a few inches off of the bottom.
During the design process, the property manager asked if the dry fire hydrant system could be used to drain the pond; something that is done every couple of years so that aquatic growth can be managed. The FD wished to honor the design request so a design change was made so that the DFH suction pipe could also be used to drain the pond down to the level of the permanent pool. This design change resulted in the need to install a control valve in the suction pipe so that the suction head area could be left dry during freezing conditions. A 6-inch gate valve was installed mid-berm. When the FD needs to use the DFH, they connect to the suction head, open the gate valve, and take suction on the system. (Head pressure in the wet system is <2 psi.) When the property manager needs to drain the pond, he removes the suction head cap and opens the gate valve.
One concern that had to be worked out was the ability to back flush the pipe to rid the system of any "crud" or small fish. Since the stream strainer did not have a back flush feature, a 4-inch, PVC backwater valve was installed at the end of the system so that the FD can pressurize the system (<50 psi) and blow out the pipe. Under drafting conditions and when the pond is being drained, the backwater valve remains closed; it only opens under pressure from the suction head end of the system.
While the system installed in this pond is an atypical design, it is a design that meets the needs of both the FD and the property owner while providing access to 5 acres of water.
Many thanks to the folks at the Oxford VFD for helping install the system.