earthworks question

Discussion in 'Designing, building, making and powering your life' started by Richard on Maui, Feb 1, 2005.

  1. Richard on Maui

    Richard on Maui Junior Member

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    We are looking at putting in a "top dam" which would gravity feed most of the 14 acre property we are working on. One idea is to cut a driveway off the road, to provide access for the earthmovers and also to divert rainwater runoff into the dam. (Our neighbours downhill of us should love this as they all pay for the costly maintenaince of the road which gets washed out regularly)...

    The existing road runs along the spine of the ridge, and is cut into the landscape so that it has six feet vertical walls on either side... so, to get water off the road and into the dam, we would have to cut a similar depth trench into our land, at least at the top of the property. The height of the walls of this miniature gorge would have to start at about 6ft and would taper off gradually as the slope descends to the point where the land is at the same level as the road or lower; this would be the site of our main excavation.

    Does this sound too invasive? Is there a better way? The ridge itself would provide some catchment area, though probably not enough to fill the dam in drier times when we are using a lot of water. We could look into pumping water up there using various methods, and forget about the road runoff. But it seems a waste to just let it run down the road.

    Never having engaged in such earth moving activity before I would be grateful for any ideas and perspectives.
     
  2. mossbackfarm

    mossbackfarm Junior Member

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    Hi Richard

    I'm far from a hydrologist, but I know one well, and he would ask the following....

    What's the catchment of the pond without your trench? ie Do you need that extra area to fill the pond?

    What's the pond size?

    What are the soils that you'd have to dig the trench into? Would a 6 ft 'gorge' be stable, or would it's angle of repose require it to be wider (45deg angle would be 12' wide....)

    This is a hard one without a picture, but an answer to those questions would get the thoughts flowing.....

    Cheers
    Rich
     
  3. Richard on Maui

    Richard on Maui Junior Member

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    Rich, thanks for your considered response and thoughtful questions.

    The catchment for this site (chosen mainly for the fact that it will provide gravity feed to the rest of the property) isn't great. It is on top of a ridge with only a small neighbouring property above it. They have a nice house (from which I assume the majority of the rainwater runoff goes into tanks) and then a small patch of lawn, maybe about 100 ft wide by perhaps 50ft deep. Other than that the runoff is diverted by the road that we are discussing tapping into...
    I haven't done the calculations, but I am thinking that if we were actually using the water a lot for irrigation and domestics we would probably be draining the thing a lot between rains.
    HOw big is the pond we are proposing? Well, partly this depends on how long it takes the excavator to make the hole I suppose, and how much money we have to spare when he is here! We still have to work out how far down the ridge we need to go to get that trench to run in, if you know what I mean. At a rough guess this will give us a surface area of about 80ft across and something like 300 ft longways. Then I guess we would make it as deep as the excavator has time to dig/can dig and still get out again! I suppose we could make a bigger hole if we didn't use the road for runoff, but then we would have an even bigger hole with no catchment. We could then I suppose use solar pumps or whatever to pump water up from lower cathments pools, but that seems like a hard way to go when a bulldozer can change things forever in an afternoon.
    As for the soil type and how stable it is? Well, it is a young, volcanic soil obviously. It seems high in clay content, but then it drains quite rapidly and dries out like sand! I guess the sugar and pineapple farming didn't enhance the soil structure much... But, the road itself that we are talking about diverting the rainwater from, it has absolutely sheer vertical walls of over six feet in places that don't appear too susceptible to collapse - the only thing is that the road keeps getting deeper over time.
    Perhaps we need to make sure that the pond doesn't end up catching the gravel and silt off of the road somehow too?
    Jeez, I wish my Designers Manual wasn't in another country in my friends garage!
    Hopefully you can make some sense of this now. I woiuld really appreciate any feedback.
     
  4. mossbackfarm

    mossbackfarm Junior Member

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    Hi Richard

    Here's a couple of back-of-the-envelope calculations that sound relevant....

    Your pond area will be ~2/3 acre or so
    1 inch of rain over 1 acre is ~25,000 US gallons
    If your pond is 6' (72inches) avg depth, it will hold (very roughly) 1.2M gals
    Your upstream catchment seems to be ~1/8 acre (not including road).
    So....1" of rain will give you (2/3acre + 1/8 acre=.79ac * 25000gal) ~20,000gal, assuming 100% runoff (generally, a bad assumption....)

    So the question that leads from that is are you on the windward side of Maui? How long is your dry season? What's the rainfall pattern (rain every afternoon, flashy thunderstorms, or steady drizzle???) Using my assumptions and heavy runoff, you'll need 60" of rain to fill the reservoir. Pretty wet.

    Cutting the road could be a good option unless you're in the dumping trade wind zone...things to consider could be

    -your point about silt and sediment...I would look into the possiblity of diverting the water at multiple locations, if possible, to prevent flows from cutting into the road, as well as slow it's capture of sediment from the road.

    -does the existing road cut deepen because of slumping (fail from below), or surface erosion (fail from above?). If the latter, you'll have to consider that someone someday may regrade the road and/or add gravel to the roadbed...you'd want to design to keep that from choking off your water supply.

    -the final thing I'd ponder seriously is pond sealing...have you done any perc tests in the area? I know lots of the islands are pretty porous and hard to seal. There may not be enough bentonite in the Pacific if you're on one of those sites...

    I'm in the process of rehabilitating a 1/3 acre pond...rodents have degraded the dam wall and a failed deepening attempt sometime in the last few decades caused deep gullying upstream and sediment buildup in the pond. I've replaced the spillway, and am in the process of patching the rodent holes with bentonite. Unfortunately, it's been such a dry winter that the runoff into the pond isn't enough to fill it, so I may have to wait another winter to complete this one.

    If bentonite doesn't go, I'l going to try the hay-pig-cattle gley effect, probably a years hence.

    As if you didn't already have enough to think about......:?

    Rich
     
  5. Richard on Maui

    Richard on Maui Junior Member

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    Rich, thanks for continuing to take the time to help me get my head around this. Your calculations are a handy and invaluable help, and I think roughly correspond with my lazy guesstamites... :lol:

    To answer some of your questions: We are on the windward side of Maui here in Huelo, and although we aren't in the very wettest part of the island. The wettest months typically occur in the winter, with the rest of the rainfall being pretty well distributed throughout the remaining months. But you know, like everywhere it seems, the weather and climate are becoming more erratic and unpredicable. Last year they had a really big wet here. This year we have been quite dry. Hence our interest in creating more water storages.
    As for why the road is so deep - I believe it is a result of erosion from above, as you put it. The rain runoff digs out potholes deeper and deeper until finally the local community agrees to get a grader out who solves the problem by digging the new road surface down to the level of the deepest pothole... It is possible that the raod would be reagraded up a little bit, and it is a good point you mention, but we will actually be tapping into the flow from a gutter that runs down the side of the road (an afterthought by the local road builders that may have saved a lot of road base if they had thought of it sooner).
    Your suggestion to catch the water in multiple locations to prevent flows sounds good, but I don't think the layout of the land lends itself well to this, unfortunately. I am wondering if there are other ways to build sediment traps into the driveway that will slow down the action of silting up the pond itself. Some sort of plantings between the driveway and where it feeds into the pond perhaps?

    Sealing the pond as you point out is probably our biggest quandry. Wer have a big hole (as opposed to a pond) at a lower level on the property that doesn't hold water for long at all, and according to what I have been able to discover from talking to neighbours they did run dozers over the surface and the wall of the pond for a good day or so trying to comapct the thing. I don't think they tried anything like bentonite though...
    Bentonite is available in the islands, for $10 per 50 pound bag, and you have to buy it by the pallet load unless you go to the Big Island and pick it up yourself... If it works it would work out to be a fair bit cheaper than a synthetic pond liner which is about 25c per square metre. I think a natural finish would be superior to plastic in many respects too...
    But we have been getting conflicting advice from the various people we speak to at various extension services etc. Some say our soils are too porous to compact sufficiently, whereas others suggest that bentonite won't work because the soil is too clay and it is too difficult to work the bentonite in sufficiently. Also, the presence of a lot of rocks could cause problems too.
    So we are thinking to try and get our hands on one 50 pound bag and try to seal a little duck pond and if that works we can try something bigger.

    Problem is, our neighbour is getting a bulldozer and an excavator out here in just under two weeks and if we get him to do the work while he is here we can save a fortune on the delivery cost of the machines!

    Interesting to hear about your own dam challenges. Keep us informed of how they work out!
     
  6. SueinWA

    SueinWA Junior Member

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    I hope you don't mind my jumping in here, late as it is.

    I developed a mental picture from your descripton of your drainage problem. I may have it wrong, or it just may not suit, but here's my idea, which is less invasive than ditch-digging (esp for the depth you have to go).

    Years ago, I read a description of a handy way to tunnel under a concrete walkway or driveway without destroying the asphalt or cement, which possibly you could adjust to your need.

    Get a length of pipe (plastic or metal) long enough to cross the area (I would use 2" / 50 mm), at least to start.

    Line it up (plastic pipe may be more flexible if that is needed), insert a garden hose, turn on the garden hose.

    The water washes out the soil, and as it does, you push the pipe further and further until it comes out the other end. You could do this to form a series of small tunnels (lined with pipe). If the amount of runoff would warrant it, and you had the water pressure available, maybe a larger pipe would be better, like 4"/100mm.

    I have used this for only a driveway about 8 ft (
     
  7. Richard on Maui

    Richard on Maui Junior Member

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    Hi Sue! Thanks for your suggestion. I think you have gotten the gist of what I am trying to describe. Unfortunately we don't have the water pressure up there, unless we borrowed from the neighbours or something like that... but it is a pretty ingenious idea! I'll definitely be storing that one in the back of my head and hope to try it out one day.
    By the way, when I first read your username I assumed you were from West Australia! :lol: Nice to know there are some US permies frequenting this board!
     
  8. mossbackfarm

    mossbackfarm Junior Member

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    Hey Richard & Sue

    Sue's approach would be great in some applications...not for high volume ones, though, since you'd only get 2 (or 3 or 4") of water through there.

    Re: sediment traps, depending on the layout of the site, creating detention basins shouldn't be too difficult...if they fill up, you just shovel them out. If there's room, you can hook them up in series, one spilling into the next. They work pretty well in urban runoff areas, so should be great for you. Even just strawbale dams to slow the water would help...every time water slows, it drops its sediment load, so you just want to stop it from racing or (worse) picking up more sediment.

    I find it funny (in an unfortunate sort of way) that some folks say your soil is too porous to seal, and some say it's too clayey for bentonite. Opposite ends of the spectrum of problems. Not much to say, except try it out on the duck pond....

    Good luck!

    Rich
     

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