Worked with our original builder and another lake designer/builder that works the larger lakes in our area. After doing more detailed on the ground review of neighboring properties along with up-to-date LiDAR mapping, we recognized just at 100 acres of drainage area. We shaved another 14 acres off by redirecting drainage by repurposing some trails on our property to become new water channels to push the water from those acres downstream of the pond.
Our final drain line we calculated was a 24" pipe, and at just over 10' of total drop and 110' long, assuming the worst possible runoff rates from the drainage area, we calculated that would handle a 500-year 1-hour rainfall for our area with 10% extra. We added another 2' in depth to the pond, 2' in freeboard, and another 2' for the new emergency spillway, which had to be moved.
Given the amount of water coming out of that pipe - downstream erosion at the exit point was a concern, so we built a basin for it to go into and then lined that basin with 8 dump trucks worth of broken concrete (in this case, the foundations of two houses recently demo'd). The inlet to the pipe is a 3'x3' concrete storm drain box with a metal grate, and for maintenance/clearing purposes, the depth from the dam to the box on a straight line is 2', enabling walking in to clear it off should it get covered in debris. The dam is actually extended all the way to the box with compacted clay, so the box itself is the entry barrier to the dam.
The pipe was quite a bit of work. Sections were connected with rubber gaskets, clamped with coated steel bands, and then several hundred pounds of cement-stabilized sand was added on the outside of each joint, then one of the crew crawled in through the pipe and sealed each joint with polymer on the inside.
We harvested all of the clay needed to raise the dam from a hill right next to the pond, and created another section of the pond in the process - taking advantage of an existing channel through a bedding area at this end of the pond to connect the new area. Overall, the work moved a ton of earth, and left us with a much better pond in the process, but it wasn't cheap
I'm attaching a few photos of the work and the finished result. I've seeded everything with native cool and warm season plants, now we just have to wait for everything to grow in and stop all of the erosion!
