Triple 18-inch De-Watering Siphons

Project Overview:

The earth dam creating Lake Holiday is an earth/rock fill structure approximately 102 foot high, 1,000 feet long with a crest width of about 45 feet.  South Lakeview Drive extends across the crest of the dam and provides access for residences situated south of Lake Holiday.  Underground utilities along this access road include a sanitary force main, waterline, electric and communications.  The original existing primary spillway included reinforced concrete training walls, six corrugated metal pipe arches and a reinforced concrete discharge chute approximately 90 feet long and 110 feet wide.  This spillway was excavated into the shale bedrock and was the only means of discharging storm flow from the dam. 

The proposed state mandated improvements included construction of a new labyrinth weir, new concrete chute structure, a new bridge for the relocated roadway and a parapet (wave) wall extending along the upstream side of the dam crest.  The concrete chute structure contains five (5) rows of baffle blocks with rip rap planned at the final discharge point.  The proposed weir and chute structure have plan dimensions of 109 feet by 150 feet and 130 feet by 150 feet, respectively.     A temporary diversion was necessary since the proposed spillway structure and bridge were to be located within the same footprint of the existing spillway structure. 

Our chosen methodology for the temporary diversion was to drawdown the lake level in advance of storm events, thus creating sufficient dry storage to attenuate the 10-year storm water runoff without overflowing into the construction area.  

Lowering the lake level was accomplished without pumps or cofferdams via a siphoning apparatus.  A siphon is a tube used to convey impounded water up over the embankment and then down to a lower level by gravity, (the water being made to enter the pipe by atmospheric pressure).   Our siphons were virtually maintenance-free, creating no air or noise emissions, and had no spill potential or pollution risk. 

Our construction personnel monitored and recorded lake levels each work day.   Siphon discharges were  monitored at the beginning and end of each work day to ensure desired lake levels were maintained and to prevent erosion.  CWH maintained a daily log of routine inspections and maintenance of the siphon apparatus, lake level, and discharges in accordance with this Temporary Diversion Plan.

 

Owner:

Frederick County, Virginia – Department of Public Works

Lake Holiday Sanitary District

Facility:

Lake Holiday Dam Spillway Upgrade

Contact:

Not Available,
Frederick County, Virginia – Department of Public Works

Contract Amount:

$6,592,290 (total Spillway Replacement project amount)

Start to Completion:

September 2011 to February 2013