I have talked to a few hundred people about reservoirs over the last year. How is a reservoir different from a pond? What size has to be measured? If you have a reservoir, how do you measure it? Can a 15 acre-foot pond be measured the same way as a 1,500 acre-foot reservoir? I have been estimating the storage for years, can’t I just keep doing that?
A reservoir is man-made, and may or may not have a pipe outlet. “Pond” is a more general term and does not signify whether the body of water is natural or man-made. Reservoirs include everything from less than 1 acre-foot stock ponds with a dam a few feet high, to Shasta Reservoir (Lake Shasta).
Every reservoir over 10 acre-feet has to have its storage volume measured monthly or more often, up to hourly for 1,000 acre-foot and larger reservoirs. How is that done? We’ll get to that in a few paragraphs. Here is the table from the Water Board that specifies diversion amounts, storage amounts, compliance deadlines, accuracy, monitoring frequency, and who can do the work. Personal gripe: Hey, Water Board web design folks, your web pages are dang hard to see for folks that are over 50! I had to modify the screenshot just so folks can see the rows and columns.
NOTE: if you take the new class that the U.C. is putting together, you can install and certify your own measurement device(s)!
How can you comply, and what has to be done? There are thousands, or tens of thousands of ways considering every permutation of each option, but I am going to list the four main ones that I use, from cheapest to most expensive:
- Alternative Compliance Plan – This is doing something besides standard compliance and still coming up with accurate-enough storage values at the
necessary frequency. Within an ACP are thousands of possibilities, but the simplest case is a 10 acre-foot reservoir 3 driving hours away from the ranch house or first gate, not accessible in the winter. An ACP for this often boils down to this: it is a landscape feature, no action of man is needed to fill or drain it, livestock and wildlife rely on it, it takes most of a day to get there and back, and a data collector is likely to get stomped on if cattle or elk can ever get to it. This is the “I’m just going to keep estimating it and that meets everyone’s needs” option, and I think it is valid in a fair number of cases. Just see some of the ACPs I have filed. An ACP is harder to justify at the large end of reservoirs, but it might include things like occasional drone flights, using CIMIS data for evaporation, perhaps only measuring the outlet some distance downstream of the reservoir while estimating storage, and so on.
- Run a tape measure or long plastic tape from the edge of the dam road, down to the water’s edge, once a month or whatever the necessary frequency is. This requires getting the inspection report, or at least the area-capacity curve from the Water Board. You need two pieces of information: the slope of the water-side dam surface, and the storage volume at a few intervals of depth. Most small reservoirs that I have seen have a water-side dam slope of 3 feet horizontally to 1 foot vertically. Area-capacity curves can vary, and in the rare case where the Water Board or the Division of Safety of Dams does not have a curve, you’ll need to do a survey of some depths to generate contours and make the curve.
- Install staff gages. This involves driving or setting lengths of 2″ galvanized pipe that overlap vertically with the next pipe up-slope or down-slope. Then 2″ x 6″ lumber and 3′ or longer gages are bolted to the pipe, using a survey level to make sure the 5-foot mark, for example, at the top of one gage matches the 5′ mark at the bottom of the next-higher gage. This can cost a few hundred or a few thousand dollars, depending on how much work you do vs. hiring work out, access, depth of the reservoir, and so on. Oregon Rule is one good company to get the staff gages.
- Install water level loggers to collect pressure data, and convert the data to depths, then to elevations, and then to storage volumes, after the data is downloaded. This is standard compliance. What if you don’t have power lines to the dam? There are many data loggers that run on batteries, for times of 2 years up to 12 years. I mostly use the Onset Hobo and PMC Versaline water level loggers. Hobos are put in protective, short PVC lengths, with a moderate weight, a rope, and buoy, and then setting them at the deepest part of the reservoir using a boat. Data is downloaded by boating out, hauling up the Hobo, and putting it back in the same place. This is easy if a heavy weight is attached by a second rope to the same or a different buoy to keep the location constant. The PMC Versaline keeps the data collector on shore, with a cable to the water level logger. It is installed in whatever length of PVC pipe is needed to get from the dam at a point above where any water gets, down to the logger end. Then, no boat is needed to download data. Which option is better? See some of my earlier posts on the specifics of water level loggers.
Those are brief explanations, but you can find the detail in earlier posts on my blog, and some other blogs. Does this help you? Please let me know, whether yes, no or maybe!