Hi,
Thanks for the suggestions on our dry lot. I have a lot more pics and or can take more to help us try to figure out what to do. What info do you need?
Julie
Hi Julie,
The hard part about runoff management is that 90% of the time, you need a small-ish capacity system. But to prevent damage the other 10% of the time, the capacity you need skyrockets. Also, water always wins. Since it sounds like you’d like to divert everything from the smallest storm to Noah’s Flood so that you don’t lose your footing several times a year, it might be a lot of capacity.
If you’re comfortable sharing it, send along your address and I can take a look at your property on aerials and hopefully find some topo maps to help understand how much area runs off to your paddock. Totally understand if you’re not comfortable doing that though!
Others on the thread had great overall thoughts - if your paddock is in the low area, the water will come for it, no easy way to get around it. In the end, it is most likely cheapest and easiest to make the paddock smaller. Maybe move your footing along with the fence to make the paddock a different shape and not lose as much space? Also, yes, the drainage inlets need to be the lowest point in their imediate area. But if their inlet capacity is not sufficient, runoff will still pond up over the drain and overflow anyway.
Alternatively, here are some general thoughts and a basic process you might use to figure out how much water you’re dealing with (“rational method” for computing runoff):
- You need the (approx.) size of the area that runs off the point at which you’re trying to catch it and divert it. 1/2 acre? 10 acres?
- You need to quantify the land cover of the runoff area. Pavement (coefficient = 1) sheds water way faster than grass (coefficient = 0.35) does, so the same overall amount of rainfall on a parking lot needs much higher capacity inlets than the exact same area covered in trees.
- You need to decide how much rain (specifically, the intensity of rain) that you want to control. A system that handles 4" per hour is much different than a system that handles 11" per hour.
You might then go to these handy tools for simplified calculations of what you need:
https://hdsc.nws.noaa.gov/hdsc/pfds/ to get rainfall (click on your state, make sure to select precipitation intensity on the top left drop-down, enter your address, then you can play around with the in/hr that are listed in the table below)
https://www.ndspro.com/drainage-pipe-calculator to estimate a pipe diameter. Unfortunately, this calculator only goes up to 3"/hr. But it is a starting point and it’s user-friendly. For example, if I enter 1 acre drainage area (43560 sf), 0.35 coefficient of runoff, 3 inches/hr, and 2% slope of pipe, I see at the right that you need at least a 10" corrugated pipe to handle that. Your rate of runoff is approximately 1.05 cubic feet per second or 471 gallons per minute. That’s a lot!
For grate capacity, it really depends what the grate looks like, but- something like this product sizing chart might be comparable (something like pg 12?) - https://www.google.com/url?q=http://www.nyloplast-us.com/sites/default/files/Nyloplast%20Grate%20Inlet%20Capacity%20Charts%20June%202012.pdf&sa=U&ved=0ahUKEwiRhvaM_MncAhULylMKHTCDD3MQFggEMAA&client=internal-uds-cse&cx=004573352478797342373:mvrprxg7hh8&usg=AOvVaw0O6_mv8tWGdRd7WBbZP3Nx
The chart is saying that if water ponds up over the grate (creating “head”), for example 0.5 ft deep, then the grate will convey a bit more than 2 cfs, or more than 900 gallons per minute.
Regarding a swale with larger rock, I would suspect that gravel would be carried away as well, depending on how much flow you actually get through there. But you might be willing to give it a try anyway, depends on your tolerance for trial and error.
Regarding digging a ditch around or through the paddock, again the feasiblity depends on how much flow you decide you want to divert. Here’s a simple-ish website to play with: http://onlinecalc.sdsu.edu/onlinechannel15.php.
Sorry if it’s all overwhelming!! I think this stuff is fun but it can be a lot to deal with. And my disclaimer - this is work that a civil engineer does for a living and this isn’t pro engineering advice :=). All of this is just to play around with, to give you a sense of how much water you’re talking about and how far away you might be from your goal right now.
