sroane,

I saw your various spillway pics from your other post. It appears they did some excellent work back in the day!

Is something getting close to failure, or are you just trying to add one more level of safety in case a hurricane remnant parks in your watershed?

The hydrograph data were generally created by people that were very good at their jobs. However, they are "theoretical" estimates of the water flows. I agree that you should try to get any info that your NRCS office can track down.

Depending on your concerns, you can also gather actual field data for your location. See how much a 3" rain at your property changes the inflow to the lake, and then compare that to a 3" rain that covers the entire watershed. After a few years of careful data collection you should have some excellent data records. "Experimental" data always trumps "theoretical" data. Of course, your experimental data will certainly be sparse and have some gaping holes, but it could still provide you with some valuable info to evaluate your concerns?

The bigger the watershed, the longer it takes for the inlet stream at your location to crest. Do you live at the property? You might be able to open your two flood pipes after a surprise thunderstorm begins and let out enough water to make a significant difference at your lake prior to peak discharge arriving (for example) 7 hours later.

Have you examined "automating" your flood control system? I have seen flood diversion systems with pipes around the same size as your pipes. The outlets are covered with big swing gates. The gates are weighted. For example, they have enough weight that they stay closed when there is 6" of water in the bottom of the pipe. However, if the water level rises a little, they are cracked open and start releasing a little water. If the water level keeps rising, then they continue to open a little farther and pass even more water in response. It is impressive to see the gates fully open during flood stage and banging against the top of the raging stream of water that is passing through!

In Kansas, you can hire a pond contractor (or DIY) for ponds up to a certain size, or dams of a certain height or volume of water impounded. Above those exclusions, every project must be designed by a certified civil engineer. Our state regulatory agency provides a list of independent engineers for lakes of that size. Perhaps you can look in the Texas regulations for building a large dam and see if they list some engineers or engineering firms that perform those projects. Obviously, those engineers would have to be able to design a proper chute type spillway when it was needed for the designed dam. That might get you to an experienced engineer for your project.

Good luck on your project of keeping your beautiful lake happy and safe!