Grande Hills Lake Dam (J. Bennett Johnston Waterway)

Field Descriptions

Dam Name: Grande Hills Lake Dam
NID ID: LA00351
Longitude: -89.9855
Latitude: 31.85
Map Section: S27,T5S,R12E
County: ST TAMMANY
River: TEN MILE BRANCH
State: LA
Nearest City: POWHATAN,LA(NOT AFFECTED)
Distance: 7.00 miles
Owner Name: GRANDE HILLS ESTATES HOMEOWNER'S ASSOC.
Owner Type: Private
Private Dam? No
Dam Type: Earth
Core: Unlisted/Unknown (Position)
Earth (Type)
Known (Certainty)
Foundation: SZ
Purposes: Recreation
Dam Length: 600 feet
Dam Height: 15 feet
Structural Height: 12 feet
Hydraulic Height: 15 feet
Normal Storage: 894 acre-feet
Surface Area: 59.6 acres
Drainage Area: 1.5 square miles
Hazard Potential: Significant
Emergency Action Plan? Not Required By Submitting Agency
Inspection Date: 2011-11-16
Inspection Frequency: 5
State Regulated Dam? Yes
Spillway Type: Uncontrolled
Spillway Width: 20 feet
Outlet Gates: X
Source Agency:

Dam Safety For Boats

A large amount of water can be released from a dam without any warning at any time and by any means. For example, when the demand for electricity is high, the turbines at a dam may be turned on automatically, resulting in a significant increase in the downstream flow of water in only a matter of seconds.

If there's a need to release water through the sluiceways (outlets at the base of the dam), this operation can also create a great swell of discharged water downstream.

During flood operations, any or all spillway gates across the width of a dam can be opened to release upstream flood water that needs to pass to the next downstream reservoir. Upstream or downstream, even the most experienced boater with the strongest motor is no match for this strong flow of water plunging over a spillway of a dam. Even if you're boating far downstream of a spilling dam, recirculating current can pull a powerful boat upstream toward plunging water that could shred any boat.

Some dams equipped with navigation locks create turbulent water as well. When vessels pass through, strong flow is released near the exhaust ports of the wing wall of the lock.

Warning Systems At Dams

To warn reservoir users of potential danger, warning devices are installed at many dams:




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