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.
If you are going to try and do this work yourself, you may want to look at some of the books about building ponds offered by Pond Boss.
There are many critical steps to building a dam and/or pond, and if you miss or skip a step, the whole project can fail.
Good luck!!
Excerpt from Robert Crais' "The Monkey's Raincoat:" "She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."
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.
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.
Wow, thank you for all this great advice! So I need a very very strong dam to hold up a river I see. I may have underestimated the power of this river until now. I will send you a video of the river in full flood.
Yes irrigation would be the main thing I will be using the dam for but for irrigation as you may know I need a lot of water so that is why the dam must be large enough so that it will be worth it. The crops which I am interested in growing is alfalfa/lucerne, corn and wheat. Corn and wheat doesn't have such high water requirements as alfalfa but they are also worth a bit less. I might also be interested in fish farming. I would say if I try very hard with a lot of different things like the irrigation, fish farming, tourists maybe, camp sites and fishing competitions it would take me about 10 years or more before I'll start earning profit from the dam if it costs 1 million US dollars. So it is definitely a long term investment but if everything works out and the dam receive good rainfall it will eventually pay off I think.
Yes there is locations where the river is like 20m wide but like I said in my original post I would also like to catch the water from the tributaries of this river if I am going to build a large dam. I saw some earth fill dams gets rocks placed on the inside so that the water doesn't erode away the wall, if I do that and my wall is very strong I shouldn't have any problems don't you think? I would also need a way to release water if the level gets too high. I think they call it a riser, when the level gets to the point it flows down the riser into a pipe and then out at the other side of the dam but they usually also have a emergency spillway too consisting of concrete I think. My worst fear would be not being able to release the water fast enough, that could be catastrophic. The safest but most expensive way would probably be to get a stepped spillway made out of concrete and at the same time you could probably also pour the rest of the wall with concrete aswell instead of the rocks.
Usually there is only a small stream running in the river for about 1-4 weeks or so after the river flooded and there isn't really much water to pump out of the river either as most of it is seeped into the ground quickly. The river bed is made out of fine sand pebbles and stones so the water dissappears quickly.
The river unfortunately doesn't have any natural fish so I would probably not be doing that option as my fish would dissappear in the next flood as you said. Wouldn't the holes also be filled with sediment in floods?
By boreholes I mean holes drilled in the ground by a drill rig. Submersible pumps is then inserted to extract the water. I currently only use them for giving water to livestock and some gardening and self usage.
Thank you for offering. I will have a look if I can get it sampled. What type of soil is needed for the dam build? Mostly clay?
I hope I answered all of your questions so that you can give me some more advise. Please also note that I don't know much about dams so please correct me on anything if I am wrong.
Yes I think I will be sticking to my original location because of that reason and I want to catch the main river's tributaries aswell as they will also definitely help.
Thank you for pointing that out. I will probably need a professional to help with the planning and designing of this project.
Thank you, I am really excited about this project and really hope it works out.
The easiest way to get a video up here is to put the video on a 3rd party picture-hosting site like imgur.
Then put the link in a post, or embed it in a post.
Excerpt from Robert Crais' "The Monkey's Raincoat:" "She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."
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.
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.
NADam, welcome to the forum. For something of that size and scope, along with the potential of affecting other landowners downstream in event of a failure, I would contact an engineering firm to design and provide design parameters for construction. You just can't pile dirt and make a dam, you need the proper dirt, proper moisture content and proper compaction, along with the correct design. During the design, 50, 100 and 500 year rain events have to be taken into consideration. Like was said earlier, build it once and build it right, or you just threw all that money away.
A customer in Texas has a reservoir that was constructed by our government between 1935 and 1943 It retains somewhere between 90 and 130 acres of water, depending on how wet the year was, but typically it's around 120 acres in size. The dam is roughly 1 kilometer long, roughly 30 meters wide at the top and at the highest point it is roughly 18 meters high. So, something that you are thinking of doing is possible with the correct engineering and the correct construction practices.
Here is some reading for you that might help you get started in the correct direction:.
The forum has been giving me issues this morning so to get these to show up I had to add a space between the https and the :
Copy/paste each one in your browser and remove the space then hit enter. That should take you to the site and open the information for you. Al or someone that has better computer skills than I do, if you can fix what the issue is, please do so. I give up. I've been fighting with this for the last 1/2 hour and I'm throwing in the towel.
Thanks a lot for this information. I am also beginning to think that I must contact a engineering firm as this project isn't your normal 5 acre pond. Thanks again.
Welcome to PondBoss! That youtube video of the river in a flood stage is a eye opener, it will take a massive amount of free board and a massive overflow structure to handle that kind of water volume and speed, to the point of I'm not sure that's doable without some serious engineering expertise. That may be able to be dammed up with an earthen dam but the overflow structure will have to be made of concrete or some other means, the speed and velocity of the water in the video will be multiple times faster and more erosive when its headed down the elevation of a 10m tall dam in the few meter distance to the toe of the dam.
Water is an extremely powerful force and will need to be handled as such.
It can be done but it will not be an easy or inexpensive task, damn failure is almost not an option, as it would take a vast amount of your dirt with it when it goes, making rebuilding a vastly more expensive project.
Good Luck! keep us posted and let us know how it goes, would love to see some pictures of the work being done as you are building it.
All the really good ideas I've ever had came to me while I was milking a cow.
What do you mean by the "nice flat area" that I must create?
That raging river in your video must be tamed for your dam to survive. Generally there are two ways to do it.
1.) You can build an earthen dam, but you must have a concrete chute to pass the entire amount of floodwater at maximum flood stage. If the flood in your video is 50m wide and 2m deep, then you need a chute of 100 square meters plus a safety margin. That is a big concrete project!
2.) You can build an earthen dam, but use an emergency spillway to direct floodwaters AROUND the dam. Imagine a "nice flat area" about 10.5m above the river bed and beside your dam. If you had an area 500m wide, then the same floodwater flow would only be running 0.2m deep. That is much less force than what is shown in your video which literally can rip trees from the ground.
For a normal lake water level 10m above the river bed, you would probably need a dam that is 12-13m tall. The extra 2-3m is called the freeboard (as gehajake mentions above). As the floodwaters rise, the water level will move up the freeboard of your dam and the water will continually get deeper flowing through your emergency spillway. You still must prevent the water from rising over the top of your dam at the peak of the flood. That is the purpose of the freeboard.
You also will need a concrete spillway with a large pipe through the dam to safely pass "normal" river flows through your dam, such that the emergency spillway does NOT get regular use.
A big flood will probably do some damage to your emergency spillway. However, it should be eroding across a wide plain rather than just cutting through a narrow dam. Usually there is lots of stone placed against the wing of the dam that is adjacent to the emergency spillway, so that any erosion does not reach the dam. You can then repair the damage in the emergency spillway, make it as flat as possible again, and replant your vegetation.
I personally, am still worried about the size of the project required to dam a river that wild.
One option would be to build some pond basins on higher ground and pump water from the river to fill them.
If you have some dry waterways (low ground) running down to the river banks, another option would be to build some dams adjacent to the river. You could run pipes at the elevation of your desired pond surface water level into the river. That way the river waters would flow into your ponds at flood stage. If you widen the banks of the main river there, that will calm the flow and you can put rock on your dam faces to prevent erosion.
Do you have any sandy ground adjacent to the river? I have some "groundwater" ponds on my property that are in very sandy areas adjacent to a small creek. Even when the creek goes "dry", that sand is still full of water. I have dug out a pond that is BELOW the creek bed, and it has water in it even though we are in a 3 year drought.
If your sandy layer is extensive and contiguous, you do not need to dig out a pond immediately adjacent to the river. You might be able to excavate a pond beyond a bank that is not over-topped when your river is at flood stage. You can test this with a large backhoe or even a small excavator by digging a test pit and observing how fast it fills with water. The overnight water level should very closely match the water level in the river if the sand readily allows for subsurface water flow.
Good luck and keep learning on this project! The more you know, then the better ideas and questions you can have for the engineer during the design phase.
There is a number out there for feet or meters per second for the speed of flowing water where below that number it does NOT cause erosion. Do you know what that number is? I used to know it but it has had so much other stuff piled on top of it that I can't seen to remember what it is.
That's how you determine how wide to make an earthen emergency spillway. Calculate the amount of water coming in, then run the numbers to make the emergency spillway wide enough so that volume of water is running less than "X" fps.
Looks like a very large project. If it were me in your shoes I would hire an engineer or at least consult with one on site first. Engineer should be able to give you some sort of fee proposal for the work and likely save you some huge dollars on a project of this scale. The money you would save may not be noticed until or if there was ever a failure in the dam but rest assured.....that never discovered savings would be much better than an oopsie at the start of the project that wouldn't be known until there was a problem later on.
I do NOT know any of the regulatory requirements in Namibia. In the U.S., if you hired an engineer specialist, they would also inform the client of any unknown regulatory requirements.
There is a number out there for feet or meters per second for the speed of flowing water where below that number it does NOT cause erosion. Do you know what that number is? I used to know it but it has had so much other stuff piled on top of it that I can't seen to remember what it is.
That's how you determine how wide to make an earthen emergency spillway. Calculate the amount of water coming in, then run the numbers to make the emergency spillway wide enough so that volume of water is running less than "X" fps.
Below is the link for water flows thru channels. (I hope this is the data you remember, esshup.) I use this data since an emergency spillway is a form of a channel. The relevant chart is on page 6.
However, anyone utilizing this data also needs to read the technical discussion to use it correctly.
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.
