Dry Fire Hydrant Design #12 - Shallow Water Installation - Oxford, MD
 
By President Mark Davis
December 7, 2017
 

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.

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The pond during our July site assessment visit. The pond is mostly empty. The permanent pool can be seen in the distance. When full, the water reaches the vegetation demarcation line.
The pond during our July site assessment visit. The pond is mostly empty. The permanent pool can be seen in the distance. When full, the water reaches the vegetation demarcation line.
Checking the water depth near the permanent pool. This spot will be where the suction strainer will be located.
Checking the water depth near the permanent pool. This spot will be where the suction strainer will be located.
 
Because the suction pipe is also the drain pipe, and a stream strainer was used, a backwater valve was also installed so that the system could be pressure-backflushed to blow out aquatic life and "crud" on an annual basis.
Because the suction pipe is also the drain pipe, and a stream strainer was used, a backwater valve was also installed so that the system could be pressure-backflushed to blow out aquatic life and "crud" on an annual basis.
The plan included using 100 feet of 8-inch PVC to reach the permanent pool. The pipe would rest on the pond bottom and be anchored in place using an auger dock system. A mini-excavator was used to create a shallow trench.
The plan included using 100 feet of 8-inch PVC to reach the permanent pool. The pipe would rest on the pond bottom and be anchored in place using an auger dock system. A mini-excavator was used to create a shallow trench.
 
The pond bottom was hard clay which allowed the mini-excavator to access the pond without worry.
The pond bottom was hard clay which allowed the mini-excavator to access the pond without worry.
Moving the 8-inch PVC into position. The process was a "glue and float" process.
Moving the 8-inch PVC into position. The process was a "glue and float" process.
 
Because the berm had to be cut through completely, an anti-seep collar was used. The 6-inch gate valve is seen near the top of the photo.
Because the berm had to be cut through completely, an anti-seep collar was used. The 6-inch gate valve is seen near the top of the photo.
Another view of the anti-seep collar.
Another view of the anti-seep collar.
 
The suction outlet. Dry except when the gate valve is opened...then it is gravity fed.
The suction outlet. Dry except when the gate valve is opened...then it is gravity fed.
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The water intake end of the system. A 6-inch ETT stream strainer was used due to the shallow water. A 4-inch back-water valve was used as a means to backflush the system when needed.
The water intake end of the system. A 6-inch ETT stream strainer was used due to the shallow water. A 4-inch back-water valve was used as a means to backflush the system when needed.
A laser level was used to control digging operation depth.
A laser level was used to control digging operation depth.
 
Gluing the 8-inch pipe that came in 20-ft sections.
Gluing the 8-inch pipe that came in 20-ft sections.
The 8-inch pipe was glued one section at a time and then floated out into the water where eventually it was sunk and secured to the bottom.
The 8-inch pipe was glued one section at a time and then floated out into the water where eventually it was sunk and secured to the bottom.
 
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The wooden cover is the access panel to the 6-inch gate valve which controls flow to the suction head/drain outlet.
The wooden cover is the access panel to the 6-inch gate valve which controls flow to the suction head/drain outlet.
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The drain WORKS!
The drain WORKS!
The finished product.
The finished product.