Truesdale Lake  

South Salem, New York

June 13, 2019 – Lakewide meeting – Drawdown Initial Results & 2019 Plan

On Thursday, June 13 starting at 8:15pm at the South Salem Presbyterian Church, we will be holding a meeting open to all lake association and lakefront owners about the drawdown of Lake Truesdale.

The Pond and Lake Connection (the company that has managed Truesdale Lake weed and algae control for the past two years) attempted to draw down Truesdale Lake in the Fall of 2018 but they ran into technical, material, and weather difficulties that ultimately doomed the project for this past winter.

The unusually wet summer and fall of 2018 pushed the initial siphon pipe installation back several weeks from the original plan of mid-October. Once the siphon pipes were in, we received several very significant rainfalls that ended up filling the lake up faster than the pipes were able to drain it. The storms also disconnected the siphon pipes and interrupted the drawdown on several occasions.

2018 was a learning experience for us and Pond & Lake Connection and we will be doing an analysis of what went wrong and how to correct and account for it.

Some of the factors that need to be taken into account to correctly model the inflow to Truesdale Lake include:

That’s the easy stuff.

When the lake is at a higher level over the spillway, it drains faster. Every inch it goes higher increases the rate of outflow. The reverse is true as well.

Once the water level goes below the spillway height, the lake ceases to drain. The aim of the drawdown is to get the lake to a point 2-3 feet lower than the spillway height. How is this accomplished? Siphon pipes are what we are using.

To model the outflow in this situation, we need to know:

  • diameter of the pipe (6″ used in 2018, recommend 12″ pipe for 2019)
  • number of pipes used (4 used in 2018, 6-8 recommended in 2019)
  • speed of water flow
  • length of pipe
    • 12″ pipe is significantly more expensive than 6″ pipe
    • length of pipe impacts ability to siphon effectively
    • need more siphons and longer lengths of pipe

There are a number of other factors that impact flow rate that we will discuss. Using these, we can calculate estimated volume of flow over the spillway through the pipes. To be effective at lowering the lake level, this volume must be higher than the inflow rates. Complicating the real world results, the inflow rates can and do change on a daily basis due to rainfall and runoff.

Assuming we can get a volume of water out of the lake significantly exceeding the actual inflow into the lake, we will be able to achieve a drawdown. Once this has been achieved the questions that remain are:

  • how many days do we drain for to get to desired drawdown height
  • how long do the siphons stay in place for optimal drawdown impact
    • soil compaction,
    • freezing weed roots and seeds,
    • time to work on exposed waterfront {permits may be required!},
    • …etc
  • how long will the lake take to re-fill up to spillway height for spring

The meeting in June will be a chance for Pond & Lake Connection — along with the volunteers who are helping with the project — a chance to speak and answer questions about these calculations and plans for the upcoming season.

Hope you can come!

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