Is RWSA Changing Measurement Metrics?
August 13th, 2009
Q. “Do you know when or why RWSA has stopped using percentage of total capacity as the measure for water levels? People may still remember that percentages were used in the “drought” of 2002 (and “01, ‘00). Changing the metric makes it difficult for people to compare today’s water levels to those of past years. Percentages were used in 2002 so you couldn’t compare the drought severity to the days of supply used in the record drought of 1977. Is this the reason for constantly changing the units for measuring water supply?
A. Based on the information below (see Tom Frederick’s reply), the metrics have been expanded but still includethe total capacity number.
Q. I see when you talked about Buck Mountain Reservoir, you made it sound like we knew South Rivanna 1966 would eventually silt in and got around to dealing with this in 1983. Was there a drought between ‘66 and ‘83 that may have prompted Buck Mountain?
A. 1976 (based on Dr. Palmer’s answer below)
Do you have any info on switching from percentages to the meaningless feet below top of dam? “Why is water supply measured in feet now instead of percentages as in 2002?”
A. No. While the feet below the top of the dam information is provided, I have always heard the other statistic of “useable water supply” expressed as a percentage (See Tom Frederick’s note below)
CvilleWaterAnswers.com posed these questions to Tom Frederick, Executive Director of the Rivanna Water and Sewer Authority and Liz Palmer, a long time water advocate and now a Board Member of The Albemarle County Water and Sewer Authority.
As we paraphrased their answers above, their complete answers appear below.
Mr. Frederick’s reply:
RWSA continues to use percentages as well as depth below top of dam in its daily reports to the public and has not interrupted the use of percentages at any time that any of our employees can recall. To the best of the knowledge of our current staff, there has only been one time that the calculation method for the percentage of capacity has been adjusted: in November 2006 the basis for capacity was changed from total reservoir capacity to useable storage capacity. During the public outreach meetings for the Community Water Supply Plan in 2004-06 useable storage capacity was widely used in public discussion and is the most reliable engineering definition of the quantity of water reasonably available for treatment and human consumption. We were not able to find a written record of the decision to change from total reservoir storage to useable storage in November 2006, but the use of useable storage would be an improvement to more accurately reflect the percentage of water storage available for the public system.
Early decisions to pursue a reservoir at Buck Mountain Creek were made either in the late 1970s or early 1980s, and due to the passage of time none of the present management staff at RWSA were in a position at that time to know the details of to what degree, if any, a drought event may have contributed to this idea being pursued. It might also be reasonable to conclude that a role in the decision to pursue a new water supply is the responsibility of RWSA to the City and ACSA to provide adequate water supply, a responsibility that continues even today.
Dr. Palmer’s reply:
We now have a much more sophisticated drought management plan than we did in 2002. Hydrologics I believe did the risk management evaluation and modeling. It is called the Oasis Model. The drought management plan now takes into consideration stream flows (Mechum gauge), weather, time of year and management of the system in addition to the percentage of water in the reservoirs. So now if the levels in the reservoirs get low Tom Frederick can run the model or eyeball it (since he has run the model several times before and often can guess the outcome).
If you remember 2008 was also a dry year. People were questioning RWSA and asking why they weren’t declaring a drought watch. We barely escaped a formal Drought declaration. RWSA ran the OASIS model several times in August coming close to but never triggering a Drought Watch. A rain event on August 30 brought sufficient relief to avoid the Watch. RWSA also maximized the South Fork Plant and minimized the Observatory Plant in August, causing alarm to some citizens who watched the South Fork Reservoir drop 4 feet, but the model indicates had RWSA not minimized Observatory the Drought Watch trigger would have been reached.
This works so much better than before! The decision making process over when and for how long to declare restrictions in 2002 was less precise and less organized than the decision making process is today. Also decisions in 2002 were harder to defend against those who were not happy with the restrictions.
Buck Mountain was purchased because we knew the SFRR would silt in and we knew population would grow. Elected officials wanted to be prepared for the future. There was a drought between 66 and 83. I was told that drought restrictions were in place in the fall of I believe 1976 but I do not know how bad it was. It did not replace the 1930 drought as the drought of record.
The Spinney Muscle was known already as an indicator species. It would have been better if they checked at that time. More importantly it was not anticipated in the 1980’s that the regulatory environment would change such that run- of -the river reservoirs like the SFRR or the future Buck Mountain Reservoir would fall out of favor with the EPA, DEQ and other government regulators. We are lucky that we have a reservoir like Ragged Mountain which sits in a natural bowl with a 1.8 square mile watershed and only dams a couple of small streams. It is essentially a pump storage reservoir. This type can be easier to permit