Mary and Sherry of the Sierra Wildlife Coalition posted some great questions about estimating pipe size for their next installation on the beaver management facebook forum and I thought the discussion this morning would be appreciated.
Sharon Brown of Beavers: Wetlands & Wildlife. You could also use a USGS topo map of the area to estimate the size of the drainage basin (find the largest continuous line surrounding the site and then use the map’s scale to estimate the size). An 8″ diameter flex pipe normally drains about one sq. mile, but sites with severe slopes or much development may require more pipes.
Mike Callahan of Beaver Solutions: When installing pond leveler pipes this question of sizing is often asked. Sharon Brown was correct in her recent BMF post about the value of using the USGS topographical maps to calculate the size of the watershed upstream of the beaver dam. I use these maps on nearly every pipe installation that I do.
As a general rule of thumb for single wall pipe I will use a 10 inch diameter pipe for watersheds up to one half square-mile, a 12 inch diameter pipe for watersheds up to one square-mile, and a 15 inch diameter pipe for watersheds of up to two square miles. If only double wall pipe were used then I would increase the watershed area by 50%. However, since I do not use double wall pipe very often, maybe someone else with more experience with double wall pipe such as Skip Lisle could comment on this.
Keep in mind that the hydrology of every watershed is different, so two watersheds with the same area can have different stream flows. Some variables that can affect stream flows are the amount of man-made or natural impervious surfaces, as well as natural springs in the watershed. With experience, field observations of the stream flow at the time of the site assessment combined with local knowledge of recent weather events are also helpful in determining what size pipe to use. For example, in the summer if there hasn’t been much rain I will observe how much water is flowing and anticipate that during wetter periods of the year the normal stream flow will be significantly greater than what I am observing at that time and larger pipes will be needed. By the same token if I’m doing a site assessment during a prolonged rainy period or large storm event the reverse is true.
A great way to learn how much water a certain diameter pipe can move is to observe the amount of water flowing downstream of an installed pond leveler pipe. When the pond leveler pipe is flowing at near capacity I have found that observing the amount of water moving in the stream channel downstream of the pipe is a great learning experience.
One does not have to be a well-trained hydrologist, or need expensive hydrologic studies in order to determine the best pipe size for a given dam. However, pipe sizing is definitely a skill that is developed over time. When I first started doing this work it was one of the most difficult aspects of flow device installation, and was the cause of some early failures as I underestimated how much water could be moved through certain diameter pipes. In general, if the water course is called a river chances are it cannot be cost-effectively piped. Fortunately though, almost all beaver dams are located on smaller order streams which are amenable to being piped.
One final comment, when sizing pipes it is also important to consider what human development surrounds the beaver pond to determine how much water level fluctuation can be tolerated. When human development is very close to the beaver pond then not much fluctuation can be tolerated in the water level. When that is the case I’ll err on the side of installing slightly larger pipes that I think are needed. The extra pipe capacity will move water past the beaver dam quicker during periods of high water flow giving the beavers less chance to hold back the extra water to cause pond level fluctuations.
From Skip Lisle of Beaver Deceivers International:
You have to look at the characteristics of the dam, too.I shoot for optimum efficiency, so I use smooth-walled pipes, often including pvc. I also put them in quite straight with little “humping” at the dam or “snaking” elsewhere. This, as well, increases capacity and is more resistant to the “valley-sedimentation” process (inside pipes) that gravity is known to produce. All of this translates to more gallons per minute in a given pipe size.
“Sizing for dams” is a complex subject that is necessarily “fuzzy” and, if you will forgive me, requires some long-windedness. There are numerous variables and factors to consider in addition to watershed size and pipe efficiency. For instance, the nature and the length of the dam, the behavioral tendencies of beavers, and the elevation that the pipe will be set relative to the dam.
I don’t need to see numbers on flow rate or watershed size to make decisions, but I am unusually experienced. I can usually tell the rough pipe requirements with a quick, in-person visual analysis of the site and the stream. I also know that beaver dams are restricted to a certain flow regime. With a “regular” beaver dam (not one “braced” by a manmade structure like a culvert), the watershed can only be so big. Beyond that, dams do not endure floods, and thus don’t need to be controlled with flow devices. Furthermore, I recognize that there are only a couple available, practical pipe sizes to choose from. For example, in a big watershed it is 12″ or 15″. I think 18″ is a little too unwieldy to be generally practical. If it is a really big watershed, and assuming there is space, I may start with two systems. Then I observe the site over the months and years to see how things develop, or, if far away, respond to feedback from my client if its provided. I am quite open to adding additional systems if there is an apparent need, and the money is available. But sites that big are rare.
Also, the goal of a pipe system should be to control the vertical growth of the dam by holding the water down, and eliminating “spill-over” stimuli, for some significant portion of the relatively low-flow damming season. To do this, a system does not need to carry all the flow all the time. This, of course, also affects the pipe-size decision. When big flows come, water is expected to flow over the top of the dam, but over a dam that is at a lower level, possibly a far lower level, than otherwise would have been the case.
Another thing to consider is that the beaver dam is part of the system. There has to be a predictable damming plane for one to pierce with a pipe. Furthermore, the size of that dam is a function of the length of time beaver’s had been damming there before the flow device was installed—a factor completely unrelated to the flow device builder, or the installation process.
And if the beavers are killed, or discouraged from working the site? Then no dam maintenance will occur and, ultimately, the dam, in whole or part, will wash away. You will be left with an expensive pipe system that’s doing nothing. The danger of over-piping a beaver dam, particularly if one’s goal is partly to maintain the wetland, is that it increases the likelihood that beavers will abandon the effort at that point (they do not continue long with obviously futile activities), start a new dam just upstream or downstream (neutralizing the flow device), or leave the area altogether. Dam control represents a delicate, complex, and nebulous equation, which a client often doesn’t fully understand. It is why I enjoy the more straightforward nature of culvert sites. The beauty of beaver dams and wetlands is their dynamism, but the human brain seems to seek simplicity, certainty, and specificity. Part of the remedy is getting people to think different, as the folks at Apple would say, and which you have been trying to do for a long time.
Well this should get you all started. Now we need installers in California. Anyone like to play in the mud as a child? We’ll set you on a career path and get you the best possible training! Send your application now!
