How To Survey A Reservoir

Pond In Mountains, Photo Credit: Pixabay
Pond In Mountains,  Credit: Pixabay

If you have a reservoir, the Water Board requires you to measure and report: how much you divert to it, how much water you store in it by month, and how much water you release if the pond has a controllable outlet.  It is usually not possible (or at least not feasible) to measure the inflow, so what is actually reported is the monthly positive change in storage, the amount that fills it up.

If your reservoir is less than 10 acre-feet (AF) per year, you only have to report it.  Measurement is not required.

How do you know how much is stored in your reservoir?  Each pond needs an elevation-storage table or curve, as the Water Board calls it.  Engineers call it an area-capacity table or curve – that’s what I created during part of my career as a water bureaucrat.  I’ll use the Water Board’s terminology here since water diversions and storage are reported to them.

First check to see if the Water Board or Division of Safety of Dams has an inspection report for your reservoir.  If not that, then an elevation-storage table or curve may be available.  You can have it emailed as a PDF.  If there is no information for your reservoir, then you have to create the table and curve yourself.

First a reservoir has to be surveyed, so you know how full it is for any given elevation of water.  The elevations start at zero storage.  The figure below shows a cross section and the contour map for a reservoir with a minimum elevation of 86 feet.

Pond Contours. Photo Credit: civilblog.org
          Pond Contours,  Credit: civilblog.org

How do you survey a reservoir?  You could hire an engineer or surveyor to survey it.  Depending on your budget and your need for accuracy, the elevation-storage table from a survey could be anywhere from +/- 5%, to +/- 10% accurate.  The Water Board requires +/- 10% for larger reservoirs, and +/- 15% for 100 acre-foot (AF) or smaller reservoirs.  I recommend aiming for +/- 5% in case you have significant errors elsewhere in the measurement system.

On the high end, the survey could be done using GPS survey instruments, so the result could be accurate and could overlay other digital maps.  Almost as high a cost is to have a transit with EDM (electronic distance measurement).  Robotic units only require one person acting as the rodman, and the instrument “follows” by keeping pointed at the laser prism reflector.

At the bottom end of the scale, a survey level or possibly a hand level, and a couple of 100′, 200′, or 300′ tapes can be used.  Many survey levels can read horizontal angles within a degree or two, so the instrument person can note angle and distance to every point.  If angles are not used, then two tapes are used, one to measure the distance along a side, the other to measure perpendiculars out to the rodman.  In the boat, the rodman measures depths, and on land he has a telescoping level rod to get elevations above the Photo_0188water level.  This way you measure X and Y distances that are plotted on a grid along with each point’s elevation or depth.

With plotted points, now you can draw contours for every foot, or every two feet, or every five feet, depending on the size of your reservoir.  Then calculate the area for each contour, and the volume between each set of contours.

Here’s where Google Earth can be your best friend.  Navigate to your property and reservoir in Google Earth.  Then take a digital photo of your contour map, and import it into Google Earth as an image overlay.  Make it 50 percent transparent, and move and resize the image until it fits over your reservoir.  Now you can use the polygon tool, trace over your contours, and let Google Earth calculate the areas for you!  Make sure to get those areas in square feet or acres and not square miles.

An alternative way of doing this is to print out a map of your reservoir from Google Earth, then draw your survey points and contours right on that map.  Then when you import the scanned or photographed, edited map, it will be a lot easier to overlay on your reservoir.

Make an elevation-storage table and draw an elevation-storage curve like the one below.

Let’s look at an example.  To get the reservoir volume, add the areas of two adjacent contours, say, the 90-foot contour and the 92-foot contour, divide by 2, then multiply by the elevation difference (in this case, 2 feet).  If the 90-foot contour has an area of 6.1 acres, and the 92-foot contour has an area of 7.6 acres, the calculation is [(6.1 + 7.6)  / 2] * 2 = 13.7 AF.

190-AF Reservoir Capacity Chart

Here’s the great thing about simple methods: anyone can measure his own reservoir by reading up on it first (Google, Bing, or DuckDuckGo) and then taking some care (and good notes) to do the job well.  If the topography is difficult, or the pond is too large for tapes, or you are just too busy doing the work you have to get done, then talk with an engineer and negotiate cost vs. quality and accuracy.

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Great AB 589 Self-Certification Training!

I attended the Flow Measurement Devices and Methods course in Cottonwood yesterday, for diverters to become a “qualified individual” per AB 589.  What a great class!  All my appreciation and applause to Larry Forero, Allan Fulton, and Khaled Bali of the UC Division of Agriculture and Natural Resources, who taught the course.  They laid out the requirements and the details of several ways to comply with SB 88, including weirs, flumes, water level loggers (pressure transducers with data collectors), in-line meters, in-line differential pressure, how to determine and track reservoir volume, and how to report changes in volume.

There were specific examples of how to size a weir to install the correct device, how to convert measured flow rates to the volumes that must be reported to the Water Board, and how to select an inline meter if your diversion is piped.  There were detailed examples of how to comply with the Water Board’s reporting requirements, and discussion of the most relevant parts of SB 88.

I hope that Larry and Allan will make their Powerpoint presentation available online for public use.  It is well done and really helps understand how to comply with SB 88, both in the field and online at the Water Board.

Update:  Allan Fulton contacted me and let me know that this course IS accepted by the Water Board!  So sign up, take the 3-4 hour course, and you will be certified to install your own measurement device(s).

I heard a week ago that it isn’t a 100 percent lock that completion of the course will be accepted by the Water Board.  However, the course is more than adequate in my estimation, and I don’t think the Water Board has any alternatives to comply with AB 589.  It is going to be accepted!

AB 589 says, in part:

“…any diverter who has completed an instructional course regarding the devices or measurement method included in the course administered by the University of California Cooperative Extension, including passage of a proficiency test before the completion of the course, shall be considered a qualified individual when installing and maintaining devices or implementing methods of measurement that were taught in the course for the diverter’s diversion.”

Thanks to the Shasta Livestock Auction Yard for providing the location – my guess is that there were 140 people attending, a good crowd for this narrow subject.  The Cattlemen’s Association got the word out and provided refreshments.  It was hundreds of hours of work among 8 people or so to pull this off.  Job well done, everyone!

Converting water logger pressures to depths to storage or flows

How are water pressure logger measurements converted to diverted flows or reservoir storage?  Why does anyone even have to have an electronic pressure logger?  PMC, Onset Computer, In-Situ, , and other manufacturers sell data loggers and water level loggers, not pressure loggers, so why is this post talking about measuring pressures at all?

Most “loggers” record pressure, because that is the easiest physical attribute to measure.  A data logger in water does not know how deep it is, and it does not know how much flow is going by, or how much water is being stored in a

 

reservoir.  Pressures relate directly to static (standing) water depths, and then equations convert the depths to flows, or to reservoir storage volumes.

 

How is pressure converted to depth?  It’s an easy calculation – water that is one foot deep has a pressure of 0.4335 psi at the bottom.  So, if your logger measures 7.00 psi, then the calculation to get depth is 7.00 psi / 0.4335 psi per foot = 16.15 feet of depth.

Now that you can calculate any depth, how do you convert depths to reservoir storage?  That requires an Area-Capacity curve, also known as an Elevation-Storage curve.  The points can be picked off the curve.  For example, in the curve below, a depth of 8.5 feet would correspond to an elevation of 2,802.5 feet, and a reservoir storage volume of 30 acre-feet.

An owner of a reservoir with a capacity over 10 acre-feet must collect monthly storage values.  That’s easily done by hand.  However, a reservoir with a capacity of 50 AF requires weekly measurement; over 200 AF requires daily measurement; and over 1,000 AF requires hourly measurement.  That is really tedious to do by hand.

This is where an Excel spreadsheet can make the task a whole lot easier!  The spreadsheets shown below are just for this.  The first sheet helps translate a graph into a table of elevations and storage volumes.  The second sheet translates collected pressure values into depth and storage values, for as many data points as needed.

How are pressures converted to flows?  The logger is in a stilling well, usually a pipe connected to the inside or outside wall of the weir, flume, or orifice.  It measures pressure, which is easily converted to depths.

As with reservoirs, Excel spreadsheets make the conversion process a whole lot easier.  The sheets below have the rating curve for a suppressed weir, and the second sheet converts pressure to actual water depths over the weir boards.  Even for thousands of hourly readings, the hourly flow volumes are quickly calculated and are ready to send to the Water Board: