posted 8 years ago
Joseph,
Great start on a plan!
Swales:
1. Easy to plan for perfectly level on paper, but much harder to pull off once equipment begins moving dirt. You will have high and low spots in the swale, but they will tend to "level" out over time.
2. Bottom profile will also be determined by which equipment is used to move the dirt. Some people use motor graders, others backhoes/tractors, others excavators, other bulldozers, etc.. Each will have a "sweet spot" where work can be performed effectively. Slope of your land will also determine if some equipment cannot be safely used.
3. Are placed to soak water into the ground. A flat bottom profile gives a lot more sq. ft. to soak water in compared to a more rounded channel.
4. Downslope wall height is the determining factor in how much water is held back by each swale.
5. Used to collect and move water (like to a pond) will need a slope directed toward the collection point. A flat bottom in these swale types is very desirable because you don't want the water to scour material as it drains toward your pond.
Imprinting:
1. More effective in areas with "soil" rather than "rocks", and flatter rather than steeper slopes.
Keyline ripping:
1. Each rip does in fact act as a "mini" swale once grasses begin to grow in the rip.
2. Can be performed on flat ground or slopes, and depending on what you use to perform the rip, can handle varying degrees of rocks and hardpan.
The plan:
A one inch rainfall event equals ~ 28,000 gallons per acre.
28,000 gallons per acre x ~1,000 acres in your watershed (not on your property) = 28,000,000 gallons .
How much soaks in vs. how much runs off:
10% soaks in; 90% runs off (very quick downpour and/or soil with no grass cover, etc.) = 28,000,000 x .1 = 2,800,000 gallons soak into the watershed and (28,000,000-2,800,000)gals is runoff = 25,200,000 gallons coming generally your/your neighbors way.
50% soaks in; 50 % runs off = 14,000,000 gallons each
Lets say your first swale only gets 50% of the runoff from the watershed (rest goes to neighbors):
1. In the first scenario, your first swale at the N side of the property would see 12,600,000 gallons of water hitting it. (1/2 of 25,200,000 gallons = 12,600,000 gallons)
2. In the second scenario, your first swale at the N side of the property would see 7,000,000 gallons of water hitting it. ( 1/2 of 14,000,000 gallons = 7,000,000 gallons).
This is the reason in Google Earth you can see the "delta" formation on your/your neighbors land. Water has and will continue to rush out of the confined watershed above your property, rush down the deeper channels and spread out over the "delta" as the slope of the land flattens. As the water spreads out, it also slows down which is why there is deposition of gravels, sands, and fines throughout the delta.
This is what Tyler Ludens is referring to in her post. Your must respect the power moving water has.
I have a very good friend who is sited on a similar 100acre situation as you. His drainage watershed is a little over 600 acres. It has very little grass and a lot of bentonite clays and shales. The runoff coefficient is very high for him. All the water comes down a single 150 foot wide draw that runs through the northeast part of his land. The bottom profile of the draw is flat. When they get a 1" type rain his draw will go from bone dry to 2+ FEET deep in a few minutes. It will run like that for 15-30 minutes and then begin to subside. In 45-60 minutes he can walk across the draw (mud now -- just small channels with some water still flowing). About halfway down his property the draw starts widening out and the delta formation begins. By the time your reach his south property line it is all delta and no draw.
My friend had observed his draw running before he ever started digging and knew there was no way he could build something that could even come close to catching what he observed flowing down it in even moderate rainfall events. He built smaller ponds where the side of the draw formed part of the dam wall. The dam height is 5-6' higher than the water flow line in the draw. The remainder of the dam is then aimed upstream in the draw and continues further upstream than the anchor point to the wall of the draw. Kind of like a boomerang. The pond fills with water coming down the draw, and when full, the flow into the pond stagnates, and water then flows past the whole pond and continues downstream. The next pond would anchor off the opposite side of the draw, etc. As the draw opened out into the delta he made a boomerang shaped pond that was not attached to a side, but continued to offset them so water would flow past/around a dam when full to the next pond downstream. He currently has 12 ponds that hold ~ 1,000,000 gallons when they are all full. Some years he can hold water in them all year, others only 8-9 months. It depends on how many runoff rains he gets during the year. He captures all of the runoff in smaller rain events, but in the big rain events he only captures a small % of what flows through.
I believe you are going to have to continue thinking about your earthworks in light of a large rain event because I don't think your swale design can come close to handling the big events that eventually come in the desert. Think about what can happen when we get a Pacific hurricane coming up through Mexico and it rains for a week straight. You can spend a lot of time and money and have your efforts destroyed in the blink of an eye. A swale will simply blow out the downslope wall and release the water on its way. Build a 2 acre pond 15' deep in the path of that breach and you will collect a lot of sediment in your expensive pond.
As far as siting your house you will have to put a critical eye to that. I don't think I would try to hide it behind swales. I would look for a rise that is at least 10' above water flow lines. Keep in mind that as trees and grasses establish, they will trap sediment and raise grade a little bit each time water floods onto you. Give yourself a good safety factor for important and expensive things.
I really like your thoughts on paddocks. Once you figure out how to tame your water a little bit and store more in/on the land you can grow a lot of browse.
Kevin