Catie George wrote:Lots of questions (mostly for you to think about). I am personally not a fan of small unengineered dams, so this colours my response.
First... What is downstream of this dam if it were to break? Even a 4' dam breaking can cause an amazing amount of damage if there is enough of a pond behind it.
Second- what are your local laws regarding dam construction, and is this a year round stream? Here a dam like this may require an environmental assessment and most likely a permit.
Are you intending for the dam to overtop or how does the water escape? What are the slopes like (of the valley, and grade of stream, and flow volumes)? What is the foundation soil type?
Why quickcrete? Would it not be easier to just build an earth embankment with a bit of a spillway? Also, you would want to embed the concrete dam in the ground a foot or two to prevent the water just piping beneath it and eventually toppling it/rapid release of water, and likely would want 2-3 courses of bags in width... this sounds expensive. I also anticipate the bag joints and corners becoming places for frost jacking to eventually break the concrete and ruin it- earth bags with reasonably silty soil (20-30% silt and clay) and some sort of protective cover might be cheaper/less backbreaking and about as water tight.
If you do use quick crete, I would suggest making sure it is in the dry season, so water doesnt wash away the cement in the concrete before it cures (cant remember the technical term for this).
Safest (less prone to breaking due to lower pressures, less risk if break due to less of a pond) and possibly more effective/less time consuming/less material probably would be if you were to make a series of weirs rather than one 4 ft tall dam. A couple 1-2' tall structures would definitely increase groundwater penetration without some of the risks of a taller structure. Plus, they are less disrupting to habitat and less prone to fill up with silt.
Catie George wrote:Those gullies look like they may see a fair amount of water during big storms to me. You definitely want to figure out how you plan for the dam to empty once filled, even if it is a very rare occurance. if I recall, over topping is the most common method of dam failure.
I once saw the result of a catastrophic failure of a beaver dam (which caused a cascade of beaver dam failures), which included moving 6' boulders and scouring out a 6' channel in the hillside, removing 2 old bridge foundations, and washing out a road, and completely removing an island from a small river. Water is powerful stuff.
I suppose, if I was determined go make a 4' tall dam on my own property, I would probably consider building a 8'-10' wide at the crest homogeneous earth structure, with 2.5h:1V to 3H:1V slopes with a layer of filter cloth and large gravel to 6" rock covering the upstream side, and a spillway made of a concrete channel drain (long piece of concrete with 2 raised sides) about 1 ft lower than the crest and more filter cloth and rock to protect the downstream slope below the spillway. Or a lined rock fill dam, with liner on the upstream face trenched in at top and bottom. Good liner with UV protectants should have a 20-100 year lifespan. This is not engineering advice, I have never seen the site, no idea about seismicity, soil types, rainfall, catchment, etc,etc, but these are the dam types I have seen be both relatively easy to build and long term successful.
If you used earth bags, you could cover with gravel.
Seriously though, I would have nightmares about dam failure and liability. I have spent far too much time swearing about previous small dam builders to be comfortable with them. IMO half assing a dam is rather like half assing electrical systems. Either you regret it, or the next person regrets it.
Oh- and if you decide to build a concrete dam, it doesnt work, and you decide to later build an earth embankment, make sure to take the concrete out before you build the earth embankment, as the concrete will likely make the earth dam more likely to fail.