I am a new member here. I am from Namibia in Africa, I own 15,000 hectare farm and I am a farmer. I have a smallish river that runs through my farm, it is a non perennial river and I estimate it floods about 5-10 times on average each year if the rainfall is good. I don't have any idea on the flow rate but I have a video of it in flood if that may help. It is a tributary to a larger river. I was wondering the other day why can't I dam this river, nobody uses this water except it feeds my boreholes. The neighbour's farm which the river also flows through doesn't have any boreholes close to it as far as I know. The location I am thinking of for the dam is downstream of all of my boreholes so the river will still feed them, the location of the proposed dam is in a valley about 400m wide I estimate and the river is about 200m wide here. That may be a bit wide, but the other location where the valley is narrower like 200m and river 30m is upstream of my boreholes and on the lower location there is also 3 other smaller rivers which flows into the main one. Earth filled would probably be the best option I guess as concrete would be expensive and difficult to build. I don't yet have a idea on what capacity I am looking for as I don't want too small or too big of a dam, that's why I am looking for advice from you guys. I played around with Google Earth a bit using elevation, if I build the dam wall 10m high I will have a surface area of 30 hectares or 74 acres roughly. I don't know how to calculate the capacity as the ground is irregularly shaped. Depending on the capacity I will be using the dam as a recreational area and probably put in a few fish but if it's large enough I will maybe irrigate a few hectares from it. If I can take a guess I would say the capacity of the 74 acres surface area would be 1,000,000 cubic metres. I also need to mention that I will most likely be buying earthmoving machines to do the job myself and do training somewhere. If I must pay someone to do it it will get quite expensive. I know there is a lot to consider when building a dam of this size and if it feasible. This is still in the planning or considering stages. Thank you.
Rod has some good background on this topic and hits on some good points but I want to add a few of my own.
1. The cost vs value figure to me is something you can only put a number on if it's non-recreational.
2. Water has a funny way of speeding up through narrow passageways so I would stick to the consideration of wider is better for the purpose of slower flow in high rain events which are less likely to erode or "cut" a new channel.
3. For this size of project, (as Rod mentions above) it needs to be very well engineered because money is money.. doing it right once is ALWAYS cheaper than having to go back and redesign.
4. And least important, if I had a 70-75acre pond, I would move to Africa to make it happen... The fact you have this opportunity is nothing short of amazing.
Placing a dam in a river is a serious engineering project. Every single flood will try its best to destroy your dam.
If you read posts on Pond Boss to get more background information, most of them concern much smaller dam projects to collect surface rainwater that is partially funneled into the pond site by the topography of the land. Even then, some dams get blown out just by heavy rains on the 20 acres of land upstream of a 1 acre pond.
It might be a $1,000,000 (U.S.) project to build a lasting dam for the size and location you are describing. Will you be getting a benefit of more than a million dollars?
If not, my advice would be to scale back the project to more appropriately match your exact needs. Is irrigation your main benefit? If so, will you lose valuable crops if the river recedes during your growing season? In that case, you might calculate your annual water needs and design a pond to match.
Do you have a much smaller waterway on your land that has a narrow constriction between two hills? That might make a better pond site. Especially if you could place that dam a little higher than the flood elevation of your river. (You don't want the floodwaters gnawing at the backside of your dam, but there are measures you can take if your best site for a smaller pond does fall within your floodplain.)
Even if your pond does not quite capture enough surface water for your annual irrigation needs, you could probably pump water from the river and top off your pond at any time prior to summer drying up your river.
Of course, you have LOTS of other options! You could just dig out some deep pools adjacent to the river course during the dry season. That would be an inexpensive option, but you could capture a lot of water. Not great for your to raise your own fish, since they will travel during the next flood. However, if the river has ample fish, then they should collect in your pools whenever the water level in the river recedes.
What exactly are you describing as your "boreholes"? Are those your irrigation wells? (That term can mean a lot of different things in the U.S.)
I am NOT a dam expert, but I am a geologist. If you start getting some samples of your subsoils on your property for a dam site, I can at least give you some decent advice in that area.
First off, my figure of $1,000,000 is a WAG (wild a$$ guess). You will need a lot more information to get a more accurate estimate.
However, if that number does NOT scare you, then time for more discussion!
One of the most important considerations for building a large dam on a river, is the potential loss of human life and property damages in the event of a dam failure. A 10 meter high dam would release a massive wall of water downstream if it fails catastrophically. Hopefully, there are no villages or even small homesteads in the floodplain downstream of your dam for a considerable distance.
The most common way for an earthen dam to fail in a flood is to be overtopped by the flood waters. Once a little channel starts eroding through the flat top of the dam, then all of the water tries to go through that "chute" and it can quickly cut through the entire height of the dam. The best defense against that failure scenario is to have a very large "emergency" spillway for floodwaters to pass around your dam. I assume with your large landholding that you can create a nice flat area at a slightly lower elevation than the top of the dam? A dense cover crop to use the root systems to prevent erosion would probably work well.
Also, do you have heavy machinery on your farm? (Like dozers and excavators?) Using your own equipment and your own skilled operators could save you a lot of money.
Thank you Boondoggle, I'll have a look at that. Yes I am going to hire a engineer, I also have a friend who is a engineer that may help me with it. I am rethinking if I should do this project as many of you suggest if I am going to earn my money back. The main reason for my this project is irrigation as I have said but do be honest I will not be able to irrigate much hectares from it. Also if I build the dam at the proposed location there is a lot of trees that will also need to be removed and I am not so keen to destroy all the trees and wildlife in that area. Thank you everyone.
I understand that a million dollars is just a rough guess.
There is a homestead about 8km downstream where some people live but I think it's safe to say that their house is also about 10 meters higher than the river. There is some other outposts of the neighbour's farm on the flood plain of the river which definitely would get flooded in the event of dam failure but luckily no people live there and if the dam ever fails I will pay the neighbour if any of his outposts are damaged.
I wouldn't want the dam to fail anytime. That would be quite a loss of money for me, I guess that is why it must be well engineered dam. Putting grass or something on the dam wall would also look a bit more natural than pouring it with cement. What do you mean by the "nice flat area" that I must create?
I am working on getting me all the heavy equipment to do the job myself as you said it will be much cheaper. I'd rather invest the money I saved from doing the job myself in heavy equipment as I have quite a few other jobs I would like to do with them aswell. I already have a bulldozer and wheel loader but I think I will also need a excavator, 2 rock trucks and a padfoot compacter. Please let me know if I need anything else. I will personally be very involved in the project as no mistakes can be made and would probably do some training with at least 2 of my workers somewhere else. I hope by doing it this way the project would work out a little cheaper and still be very well build.
What type of soil is needed for building a earth dam? Mostly clay?
I uploaded the video of the river in flood on YouTube and I will share the link here. Thanks alot for the advice.
