Dams in flat country

Discussion in 'Designing, building, making and powering your life' started by Kyle Tengler, Apr 5, 2013.

  1. Kyle Tengler

    Kyle Tengler Junior Member

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    Hi all. I live in pretty flat country in the panhandle of Texas. Generally our slopes are 2% or less. When reading about dam construction in the Designer's Manual (pg. 159 on the bottom) there are illustrations of contour dams in flat country that look like trapezoids from aerial illustrations. With a dam constructed this way, is the keyway excavated and compacted just on the lower wall, or are the sides also excavated and compacted making a U shaped keyway? Does anyone know of illustrations or photographs of the construction of such dams? We're typically very flat and often very dry (avg. rainfall 480 mm/year, with variations from 127 mm like last year to 1016 mm in our 100 year flood like in 1940) so I feel like we need more small dams on any property we can build them on. We have dark red high clay soils, so having enough clay content shouldn't be an issue. Any advice would be fantastic!
     
  2. gardenlen

    gardenlen Group for banned users

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    g'day kyle,

    first is your soil suited for dams? that is it must hold the water collected, a lot put dams in and they don't hold water over the time between rains, lots of them where we live now, those who build dams (dozer owners only want teh money they make) so talk to local farmers with dams they may share their wisdom and knowledge. some people here have gone as far as using bentonite to make the dam work, damn expensive, and may not be guaranteed. for me my advice would be put in rain water tanks of 25k/l capacity at least we have 2 in app' 800mm av/fall area, would like 3 or even 4. plus dams take up food growing areas on blocks less than 30 acres.

    other down sides of dams is evaporation, and not advisable to grow trees on the wall or adjacent, go for teh tanks once full that is how they stay until you use the last drop.

    in low rainfall areas like yours and ours you need good water management routine we use all of our used water at least twice, this saves fresh water and the electric pump. heaps of mulching around trees and on gardens heaps 8"s on gardens 12"s around trees clear of trunk X 4"s and out to just beyond the drip line. if you do go the dam check your local gov' for regulations and possible license, all gov's say they own 90% of the rain. i wee in a bucket as well. buckets in showers for toilet flushing etc.,. never flush urine.

    len
     
  3. Kyle Tengler

    Kyle Tengler Junior Member

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    My question was really about the keyway for this particular kind of dam as pictured in Mollison's Manual. Tanks are great and I will be using them, but it would be prohibitively expensive to build them in the capacity and numbers to completely replace dams.
     
  4. ecodharmamark

    ecodharmamark Junior Member

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  5. 9anda1f

    9anda1f Administrator Staff Member

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    Mark's reference is a good one. Thanks Mark!

    Kyle: As the contour dam is constructed by building up walls on three sides (as opposed to building only one wall on the downhill side for most dam types) I would say the key must be dug in the "U" shape for all three walls. The key excavation is to prevent leakage and ensure a continuous compacted bond between the dam walls and the earth below. It seems prudent to me to take the extra time to make this bond on all three built-up dam walls.
     
  6. gardenlen

    gardenlen Group for banned users

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    those illustrations of marks are just regular dam shape determined by geography so to say. an earth dam is a dam is a dam. seen dams like that built beside flood creeks to take flood overflow only. best way he could do it if he tried blocking creek the flood would wash his dam away.

    but anyway a dam won't hold water no matter how much compaction if the sub soil is not suited. also where walls are going to be pushed up be sure to remove all top growth and top soil if it of a different soil to the sub-soil, seen many dams leaking where walls meet original surface.

    anyhow just words to the wise.

    len
     
  7. matto

    matto Junior Member

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    Check your subsoil for sodicity, Kyle, but otherwise it sounds good.

    Keyline Planning is well suited to flat land, and there have been plenty of examples in Western NSW, which is close to your rainfall. Try and get a copy of Water for Every Farm, by Yeomans.

    Wetlands, if suited to your landscape, can be another way to catch water. www.earthintegral.com have some great examples in this field.
     
  8. camwilson79

    camwilson79 Junior Member

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    Hi Kyle

    In my opinion, with the high evaporation rates you'd experience, coupled with infrequent rainfall at times, your main focus is on getting as much depth as possible as a ratio to surface area. With the high clay content you mentioned, you should be able to achieve this as you'll be able to get away with quite steep sides on the below-ground reservoir (as long as the clay doesn't slake or disperse too much).

    Because of your very minimal slope, the majority of your storage from runoff is going to be basically an excavated tank (i.e. a hole in the ground), with a little extra storage gained by building a contour dam on the lower edge with the spill. A 20 tonne excavator will rip through a bunch of deep holes for you in a day.

    Because of the low slope, the pressure on the wall will be very minimal so you won't need to get too fancy with the keyway. Just make sure you scrape back to clay (which you'll do anyway when pushing the topsoil aside for top-dressing the wall later), then give the clay a good rip to loosen the surface and make sure you get a good bond with the wall material. Pretty obvious, but if the clay content varies, use the better stuff for any of the wall that's going to be below water level.

    Not sure about over there, but Turkey's Nest Dams are used very commonly in the flat rangelands of Australia. Water is pumped from bores (or creeks or other dams) by a windmill into these storages which are situated above ground level. This allows low pressure gravity feed to stock troughs.

    All the best, Cam
     
  9. gardenlen

    gardenlen Group for banned users

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    well put cam,

    lots of earth tanks in the flattish lands around the mt isa area

    my best bet still is water tanks (poly ones) you are guaranteed not to lose valuable water through evaporation of soakage, don't take design too much on board, key line system was built on land with some fall maybe around 6%? and it was on a large acreage. in your case water is especially valuable resource.

    do what is really best for you, for your dollar and for your property, don't be seen to be different, be different. not sure but our new neighbour over the back might be building a dam where all dams leak except where bentonite has been used, doesn't make sense but to put a 1/4 acre dam on a 1 acre block, especially in the most arable soil.

    anyhow good luck with your project please keep us informed
     
  10. gardenlen

    gardenlen Group for banned users

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    can't edit my last post?

    i have forgotten to say in our system we sue 1st flush diverters each on collects about 7-10 litres of water and with heavy dews out of 3 diverters we can get up to 2 buckets a day, the diverters a simple as system, but then we get around 880mm per annum.

    in your case diverters may hold out too much water from your tanks, it takes about .2mm to fill a diverter, we have 117 meters of roof collection corrugated iron roof. so diverter water for us is an extra. you could be collecting around 40k/l of rain per annum, that's 2 tanks per year. 1mm of rain collected is around 1 litre of water from a 1 sq/mt collection area. we also use buckets and wheel barrows when it rains.

    len
     
  11. floot

    floot Junior Member

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    Hello Kyle,

    There is a remarkable section as part of Bill Mollison's presentation called 'GLOBAL GARDENER' on exactly your type of situation, you can find them on youtube. He discusses both the issue of water capture and also water incorporation into the soil. It is all about maximising the amount of precipitation you can harvest annually. It is also done using video as a medium which for many is easier to understand.

    Watch all of it but especially the sections on the Kalahari and the US southwest. The US Southwest features innovations put down after the 'Dust Bowl' period in the USA.

    Cheers,
     
  12. Edensgardener

    Edensgardener Junior Member

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    This is an excellent thread - many of us dream/wish for land that is full of rolling slopes - but have flat, desert-like conditions. A bit more rain here in N. TX than you have up in the panhandle Kyle, but I'm with you on the flat. I have a pond (thank God) that was here since homestead was founded I think - it gets close to dry during super dry summers b/c I irrigate my 2 acre annual crops from it. I have a low area to the run off side of it I'd like to dam up and make a 2nd pond with a swale on the side, but we have super sandy soil and I'm afraid it will give way and flood neigbors' homes - (I'm urban farm - 14 acres smack dab in the midst of residential - or is that they're smack dab up against my farm. :) )

    VERY limited resources and no formal training yet but absorbing like a sponge. Keep this thread going and shoot some pics of your progress, please!
     
  13. songbird

    songbird Senior Member

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    Edens,

    i would try to look uphill further and see if you could put the added swale(s) in so that if it(they) failed it would empty into the pond already there. this might help even out the pond level during the drier periods, but your overall pond level may be lower for a few years as the uphill ground water recharges.

    as a failure of a large system would be disastrous, i would talk to an engineer to make sure of anything larger than a puddle in your situation, when surrounded by residential... i wish my neighbors were as thoughtful/responsible.
     

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