How High is Glory Hole

How High Is Glory Hole (and Lake Berryessa)?

Photographic and video evidence may show the lake spilling over the rim of Glory Hole. However, the official measurement provided by the Solano County Water Agency at that exact moment is 439.9 feet - low by 1 inch.

The rim of the structure is very flat - as can be seen from the photos and videos showing lake water spilling over at exactly the same height all around its circumference. Those of us who have ever used a bubble level to simply hang a picture frame should be immensely impressed by the this feat of engineering from 1956.

The Lake Berryessa water level is actually measured on the Monticello Dam in a “stilling well” equipped with a float tape attached to a digital rotary encoder that measures precisely to 0.01 foot or 0.12 inch. There is a small discrepancy with the encoder that measures the lake level. It is also difficult to get the measured lake level dialed in to this exact level when there are any wind waves that cause small fluctuations in the stilling well.

Stilling well

The lake elevation is calibrated by measuring the distance from a known elevation to the water surface in the stilling well. This known elevation was determined during the original survey of the Berryessa Valley when the dam was built. The fact that the calibration between the measuring device and the actual lake level is only off by only .04 feet (1/2 inch) is amazing. But how accurate is that measurement and how important is it? 

Precision and accuracy are often used interchangeably, but in science they have very different meanings. Measurements that are close to the known value are said to be accurate, whereas measurements that are close to each other are said to be precise. So a good machine may measure precisely to a half inch every time it takes a lake level reading, but if the calibration point is off in accuracy by one foot, what does it matter? The ideal situation is when the measurement is both accurate and precise. But the accuracy of any measurement relies on how the measurement is made - and all measurements have some variability. 

Variability is the tendency of the measurement process to produce slightly different measurements on the same test item, where conditions of measurement are either stable or vary over time, temperature, operators, etc. 

Short-term variability due to to the precision of the measuring device. Since most of the survey measurements on which Lake Berryessa data depends were performed in the 1950s with good optical equipment but in rough, irregular terrain, how variable in accuracy was the final data set - what was the level of the statistical standard deviations of the fundamental measurement results?

Mean Sea Level: Is There Such A Thing As Accurate Height?

Mean sea level (msl) is an average level of the surface of one of Earth's oceans from which heights such as lake and mountain elevations may be measured. Height above mean sea level is the elevation (on the ground) or altitude (in the air) of an object, relative to the average sea level datum. For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm (0.07 in) per year - 4.2 inches since Monticello Dam was  completed.

At present, the maximum cumulative uncertainty for operational Vertical Datum regions in San Francisco Bay Area, which includes Lake Berryessa, is 4 inches. So what does a measurement of 439.96 feet mean in that context? We can precisely measure the lake level to 439.96 feet within a tenth of an inch, BUT we only know the accuracy of that measurement to plus or minus 2 inches!

There are several methods of measuring the height of a point above msl. The choice depends on the accuracy required and the equipment available. Satellite altimeters have been making precise measurements of sea level since the launch of the TOPEX/Poseidon satellite in 1992. But there were no satellites in 1955 when the dam was being built. Surveyors used precise optical instruments to do the job.

A surveying instrument known as an optical level is used to project a horizontal line in two directions (forwards and backwards) so that differences in height can be measured.  At each end of the line being measured the value is noted where this horizontal line intersects a vertical staff (basically just a big ruler). 

This is the equipment you see when you drive by surveyors planning a new road. The difference between the values at each end of the line is the difference in height.

Provided the line of sight is kept relatively short (usually less than 150 feet) the measurements are quite accurate. By repeating this process in a leap-frog manner and adding up all the differences in height, the total difference between two very distant points can be calculated.

Over long distances this is a tedious technique, but it produces accurate results.  It depends on the instrument used and the care taken, but for the typical standard of optical leveling the accuracy is about 1 inch over 6 miles, or 3 inches over 62 miles.

Since Lake Berryessa is approximately 15.5 miles long and 3 miles wide with approximately 165 miles of shoreline, surveying the whole area was a major design task. But who cares how high above mean sea level the lake actually is? Basically no one.

Lake Berryessa is a reservoir providing agricultural irrigation water and drinking water to Solano County (recreation was not a goal). So the important measurement is the volume of the lake at any given time - not its absolute height above sea level. 

And the volume of a lake is measured by knowing the relative heights of the valley in which it sits. Lake Berryessa could be a hundred feet higher than it is and it would not matter - because the important measurement is its volume. 

The surface area of the lake is about 20,700 acres (plus or minus what?). If the height of the lake level is off by 2 inches that’s an uncertainty of about plus or minus 3.5 acre-feet - or about .02% - a very small number.  On the other hand, an average California household uses between one-half and one acre-foot of water per year for indoor and outdoor use. From that perspective it may seem like a lot..

How Much Water Does Lake Berryessa Hold?

Heights are usually shown on maps by contour lines which give the height above msl. The volume of water is a function of the difference in depth between two successive depth contours where the area of the lake within the outer depth contour is compared to the area of the lake within the inner contour line under consideration. 

The procedure consists of determining the volumes of successive layers of water, and then summing these volumes to obtain the total volume of the lake. Making these calculations over an area the size and shape of the Berryessa Valley before the dam was built clearly includes a lot of uncertainty in the final volume determination.

The relationship between the lake level and its storage capacity is not linear. The lake is shaped roughly like a bowl (with peaks and valleys and inlets and large flat areas), which means that the higher the water level gets the more rain is needed to raise it.

The published data defines the surface area of Lake Berryessa when it is at a lake level of 440’ msl (the Glory Hole spillway height) as 20,700 acres with a total volume of 1,602,000 acre feet.

However, the world-wide attention focused on Lake Berryessa’s Glory Hole Spillway overflowing in 2017 for the first time in a decade raised some questions from sharp-eyed observers. Using the published standard capacity of 1,602,000 acre-feet (AF), they noticed that as the level approached the 440 foot mark (100%), the capacity was less than that figure. In fact, at 440 feet (100%) the official storage value was only 96.2% of capacity - a 3.2% off.

 1,602,000 AF is a number that was derived from the original Area-Capacity curve developed from surveys of the empty lake bed when the project was being built. In 2007 the Solano County Water Agency (SCWA) performed a comprehensive bathymetric survey of Lake Berryessa to look at sediment accumulation over the last 50 years of the project. Another goal was to verify the accuracy of the Area-Capacity curve. 

In the Spring of 2007 the Solano County Water Agency performed a survey of the underwater landscape of Lake Berryessa. They wanted to accurately determine the capacity of the lake using the latest technology. Siltation from a lake’s shoreline is often a factor in lowering the total water-carrying capacity of reservoirs. SCWA wanted to investigate the sedimentation rate and establish a baseline as well as create accurate bathymetric maps.

The data used to make bathymetric maps today typically comes from an echosounder (sonar) mounted beneath or over the side of a boat, "pinging" a beam of sound downward at the lake bottom. The amount of time it takes for the sound or light to travel through the water, bounce off the lake bottom, and return to the sounder tells the equipment the depth.

The sedimentation in the lake was determined to be minimal considering the 50 year life of the project. The scuba divers who went down to the old Putah Creek Stone Bridge found only a few inches of fine silt on the bridge surface.

Bathymetric - Lake from south

A new lake level versus capacity was officially adopted by Reclamation and SCWA in 2009. The new curve shows a lake capacity of 1,551,292 AF at a level of 440 feet - 50,708 AF less than previously.

The difference between the present and former AC curves is a combination of some sedimentation and the difference in technology used to derive the new curve. Obviously another traditional survey of the dry lake bed will not be possible. The bathymetric survey was the only option that could be used to provide a data that could be used in future studies.

The new AC curve has been officially accepted by Reclamation, but the old 1,602,000 AF is still used by most agencies. That figure remains in public descriptions of lake capacity and is proving to be very hard to change since it’s been used for so long.

The Glory Hole Is A Circular Waterfall

A review of photos and videos shows that laminar (smooth) flow over the lip of Glory Hole and down the spillway is the same as the flow over a waterfall.

Horseshoe Falls1 copy 2                       © Peter Kilkus 2018