How to Choose small dam spillway design?

OP Joined: Posts: 5 Dear Forum,

I�m new to this forum and new to ponds but have done enough research to realize that even with my little project it still takes some savvy engineering and competent construction techniques to produce a desirable result. My pond will have 10+ acres of watershed, approximately an acre of surface area, will be eight to ten feet deep, and have a dam on one end that will be approximately six feet high on the downstream side including the freeboard.

My concern that I would appreciate some feedback on is in regard to designing the primary spillway / outflow. I don�t have the correct spillway dimension requirements yet but once obtained I wonder if the following approach would seem sound to the experienced members of this site.

I am thinking of leaving a small section of the dam unfinished as a �temporary construction drain� to allow inconvenient rainfalls to temporarily bypass the primary spillway construction area while I build it. I would like to use 24� plastic drainpipe to create the primary spillway, the number of pipes to be placed through the dam will be determined by the properly engineered flow requirements. This pipe is double walled, corrugated outside, smooth inside and has �O� ring seals to join and seal them together. 90-degree sealed elbows are available for this pipe. I would install this pipe in the dam, close to level with only a slight down slope to the outlet end, and to the proper depth so that a 90-degree elbow placed on the inlet end, when turned up, will create an upright standpipe spillway inlet, basically a downfall spillway design.

I intend to excavate a single trench through the new dam for the pipe(s), and dig two, 12� wide �anti-seep trenches� across and at 90 degrees to the pipe trench that extend 3�-4� feet to either side and below the pipe trench. Using the earth trenches as a form I believe I can pour a shallow bed of mildly �soupy� concrete in this �trench complex�, roll in the pipe, put a pre-made inlet standpipe form and pre-made forms for the top of the �anti-seep wings� in place (the outlet apron form will already be in place) and finish as a single concrete pour.

Since I am not building rocket parts, any shape �defects� resulting from a very mildly �soupy� concrete mix should be entirely livable, the �soupy� consistency being desirable to eliminate as many voids as possible. Re-rod will already be in place where possible and pre-cut pieces added as necessary in the forms during the pour. All pipe joints will be sealed in concrete. Except for some of the standpipe inlet area, the concrete covering the pipe will be buried in �dam-quality� compacted earth. The spillway pipe inlet will be near the dam edge but sufficiently into the pond to allow for proper pipe filling and to avoid erosion issues associated with high, localized escaping water velocities. The area between the standpipe inlet and dam will be armored with rip-rap for insurance.

The wooden standpipe inlet form will allow for a protective concrete and re-rod tower to be made around the upturned plastic standpipe upon which a debris catch can be mounted. A 4�-6� PVC drainpipe will be installed in the concrete bed along with the primary spillway pipe as my topography will only allow me to gravity drain to the depth of the bottom of the spillway pipe.

Separate from this spillway complex I will install a small diameter, height / depth adjustable, flexible siphon tube with a downstream shutoff valve, and bury the siphon tube in the upper freeboard area of the dam. The siphon tube is to provide a means of slightly (albeit slowly) adjusting the final water level set-point, but is primarily intended to provide a small but nearly continuous supply of water during non-freezing months to a number of downstream waterscapes in my yard, just enough to keep them wet.

There are several reasons I am considering this approach. First, I believe this design lends itself to a �hidden�, unimposing and relatively natural look. Second I only want to do this once so I want the structure to be relatively permanent. Since plastic and concrete should last a lifetime or more, I believe undesirable seepage that leads to erosion then becomes the primary concern. Pouring mildly �soupy� concrete into appropriate and well compacted soil that has been excavated to leave rough walls would seem to lend itself to a very good seal between the earth dam and the penetrating drain structure, particularly considering the imposition created by the large, concrete, �anti-seep wings� in this design. The seal between the concrete and the plastic pipe(s) should be excellent, but if not is of absolutely no consequence.

As this spillway structure is at most 6� below the water level there is not much head pressure on the spillway system. Still, I think it might be best to not complete the unfinished �temporary construction drain� area of the dam that has kept the pond off the spillway until the partially complete spillway structure has seen at least several rains to help it settle and seal. After a few rains, then compact more dam above the exposed concrete spillway structure and finally finish the �temporary construction drain� area of the dam in time to seed the entire dam and emergency spillway. This would allow the dam and spillway structure see a few mild rains and then a good winter freeze / thaw cycle before the heavy spring rains hit. I am also thinking of leaving the pond drain open until after receiving several good spring rains to further keep the load off the dam and spillway and allow for more rain induced settling-in before subjecting these new structures to a permanent, full pond load. Some of this may seem like overkill but I am new to this and would rather put in a little more time and effort up front if it might increase my odds of success.

I have only recently added this 5 acres to my backyard and cannot believe how excited and absorbed I suddenly am in this project. I have dreamt of this for years but only recently have I been able to pry loose this land from my farmer neighbor. I only add this explanation to let you know I believe a may be a �pond guy� in the making and as such I truly would appreciate any good advice.

Thanks in advance, Michael
PS Love the site by the way!




Last edited by Mino; 02:52 PM. Reason: paragraphing OP Joined: Posts: 5 By �� pouring a shallow bed � � I meant pouring only 3-4� of concrete in the trench first, then immediately putting the pipe in the trench and sitting it on top of this 3-4� of fresh concrete and then immediately continuing the pour from there.

However, having thought over the helpful comments made so far I am rethinking some things if I go the concrete route. I had thought I would use the front bucket on my tractor to hold the pipe down during the pour, but now think I would also like to chain the pipe to some preset anchors in the bottom of the trench for additional insurance. If the pipe floated out during the pour it would be a major disaster. For this and other reasons I now think it would be far better to properly locate the pipe in the trench, sitting in on concrete blocks to keep it off the bottom, before beginning any concrete pouring and hope the concrete fills all voids beneath the pipe. I may need to ensure a vibrator is used during the installation.

Thinking over this floating issue also made me realize that the wetter the mix the more likely the floating problem. I have no training or experience in concrete work other than watching the professionals that have done some jobs on my property before and know they vary the water content depending on what they are trying to achieve. I think now I will want to ensure they not make it any wetter than necessary, a �proper� mix if you will, as scott69 recommends.

What got me thinking about concrete in the first place was not being able to imagine how one could possibly get good compaction under the �radius overhang� of a 24� diameter pipe, it would seem impossible to me. From what I have read compaction is critical, and not fully achieving it somewhere, especially around a pipe penetrating a dam, seems quite scary. But given your comments Essup I will look into the pogo-stick compactor you mentioned, as it would be far simpler if I could minimize any unnecessary concrete work.

Now I am beginning to wonder what extending an earth dam peninsula into the pond and locating the downfall spillway pipe in that peninsula would achieve. Oh-oh, this jelly of mine that passes for a brain is starting to have visions again �
Thanks for your patience with my inexperience and the quality feedback gentlemen, much appreciated. Merry Christmas, Michael