Letter from Terracon – Reservoir Draw-down

Terracon logo (PRNewsFoto/Terracon)

Letter from our engineer at Terracon:

I understand there is a discussion regarding drawing down the reservoir ahead of the potential arrival of Hurricane Irma. I offer the following points for your discussions:

My understanding is the concrete sidewalk is located at the edge of water for Lake Wackena. A concrete “seawall” supports the upstream edge of the sidewalk. The seawall reportedly has a concrete footing at the bottom of the wall, which is approximately 2.5 feet below the normal reservoir surface elevation. The earthfill embankment upstream slope of the dam then continues down from the footing. It is unknown if there is drainage behind and through the seawall.

The reason for the drawdown would be an attempt to prevent some of the flooding that occurred during Hurricane Matthew, and also an attempt to minimize additional damage to the previously damaged emergency spillway. The concern of drawdown is the potential for damage to the dam embankment and the seawall.

Advantages to Drawdown

  • Allows additional storage volume prior to the arrival of the storm.
  • Drawdown of Spring Lake would help store part of the storm runoff from approx.. 1/6 of the drainage area.
  • Providing additional storage for rainfall inflow can sometimes prevent the overtopping of the dam
  • Perception of the public is that owner was “proactive” in trying to prevent dam overtopping in the event of an historical rainfall event.
  • Plenty of notice of an impending storm allows a significant drawdown prior to the arrival…drawdown should not exceed 1 foot per day of drop in the lake level.

Disadvantages to Drawdown

  • The drainage area is very large for the dam, so the drawdown would need to almost empty the lake to provide much benefit. For example, the peak inflow for the design storm (10 inches of rain in 24 hours) is 3200 cfs. If this peak inflow into Lake Wackena occurred for 1 hour, this would be equivalent to 87 million gallons of water coming into the lake in 1 hour, which would fill the lake back up to normal pool if you drew the lake down 1.5 feet. That only accounts for 1 hour of inflow for 24 hours of rainfall.
  • Drawdown of the lake within 6 inches to 1 foot above OR below the wall footing would likely cause erosion to the soils supporting the seawall footing due to wind and wave action in the early portion of the storm arrival.
  • A drawdown much lower than the seawall footing could potentially cause erosion of the embankment slope due to wind and wave action. This would be likely due to the soils being saturated and especially if the surficial embankment soils are sandy with not much clay content (I am unfamiliar with the soil composition of the dam embankment).
  • Hard surface protection like concrete is not desirable on a dam because it hides erosion that has occurred beneath it and the erosion usually only shows up when there is a significant failure, or collapse, of the concrete.
  • It is unknown if the seawall has drainage behind and through the wall. If there is no drainage system for the wall, the wall could bulge or fail in areas if the lake is drawn down below the bottom of the footing.
  • The dam handled hurricane Matthew, with damage to the emergency spillway. The damage was mostly due to flaws in the design of the energy dissipation of the outlet chute spillway.
  • The sidewalk and seawall did not reportedly exhibit distress from hurricane Matthew. If the lake had been drawn down, it may have caused damage to the upstream slope of the dam in addition to the emergency spillway damage that occurred.
  • If you draw the lake down far, and the storm goes a different direction, the public might complain that the lake is too low to use in the event there is not much rain in the area in the weeks/months following the drawdown.

I know this isn’t an answer or recommendation to draw the lake down or not. If the drainage area were much smaller, it would probably be much more clear what to do. But the total amount of rainfall runoff that comes into the lake is very difficult to store with the configuration of spillways and top of dam elevation you have. One of the most obvious ways to protect your main asset should the emergency spillway activate is to install some rock riprap against the earth and structure where the erosion occurred at the emergency spillway.

I will send you a simple sketch of the area of riprap should you want to protect the structure from headcutting/erosion. Please feel free to call, or set up a conference call with others if needed.



Russell Bendel, P.E.
Senior Associate | Geotechnical Group
7327-G West Friendly Ave. I Greensboro, NC 27410
D (336) 365-7031 I F (336) 365-7020 I M (336) 944-2484
russ.bendel@terracon.com | terracon.com