Conclusions

Conclusions:

The maximum carrying capacity of the CCC ditch is currently 45 cubic feet per second. This capacity is limited by cross sections that have become degraded since the ditch’s closure in 1982. If the ditch were, in fact, reopened and repaired, the carrying capacity could potentially double, in effect leading to a full volume flow of 90 cubic feet per second. This would be advisable if the homeowners want to capture more available volume to be diverted to the lake.

When examining Table 13, the maximum volume added when including runoff from the adjacent watershed (100-Year storm) allows 489,000 cubic feet of water to enter the lake. If the ditch is closed, only 373,000 cubic feet will contribute to lake volume. If we correlate these values to Tables 14 and 15, this translates to a rise of 4 feet in elevation when the ditch is open, and 3.2 feet when the ditch is closed. Existing lake elevation, and property location will determine potential for any flood risks. May 2, 1980 marks the highest level of lake elevation on record, and was measured at 6730 feet¹³. If existing lake elevation were at 6730 feet, and the total volume from a 100-Year event storm were allowed to enter the lake basin, this would sum to a total elevation of 6734 feet, and this lake level is illustrated in Figure Q.

The Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map¹⁴ places the 100-Year Flood Boundary at 6730 feet. When determining the floodplain, FEMA states that the floodplain was defined using statistical methods based on average lake elevation (with the ditch closed). If you analyze Malcolm Pirnie and ADEQ’s report, the results may be somewhat contradictory – they state that a bathymetric survey determined average lake depth to be 6 feet. However, they state the basin is dry at an elevation of 6714 in one place, yet in another section, they state the lake is dry at 6717. If we assume the starting elevation (dry) is 6717, and then add 6 feet to determine an average elevation of 6723, an additional 4 feet from a 100-Year event will cause the 100-Year floodplain to be located at 6727 feet. Additionally, the USDA Forest Service estimates the hundred-year lake level to range between 6732 and 6735 for a two-year budget process using a Log Pearson Type III Frequency analysis¹⁵. They also state the most reasonable 100-year floodplain to be at an elevation of 6734 feet for the Stoneman Lake Watershed alone (not including the ditch). All of these different reports raise a question as to the legitimacy of the 100-Year floodplain. The correct value is up for interpretation. The most likely value would arise by examining statistical values to determine average lake elevation, and then add 4 feet of rise for the 100-year event.

Precipitation:

The gages placed at Stoneman Lake have a +/- 5% error, which is depicted by error bars in Figure N. Stoneman Lake precipitation values are probably not accurate for July because data was lost during the week of July 17-25 due to mechanical failure – the tipping bucket got stuck. December precipitation is also questionable, because these types of gages cannot accurately measure snowfall. The remainder of data seems to be accurate because August through November values agree very closely between Gages 1 and 2 (total monthly values), and no snowfall occurred during this time. In addition, Table 9 shows a strong correlation between Stoneman Lake and Happy Jack precipitation (Gage 1 was used to compare data). The average difference between values was 23%. The average difference between Flagstaff and Stoneman Values was 30%.

It is important to note that Malcolm Pirnie’s study was based on Happy Jack precipitation. This is an acceptable method to use, as long as one recognizes that precipitation values can vary as much as 70% from one location to another.

At the start of this project, it had been my intent to calibrate rainfall to watershed runoff by examining the resulting lake elevation. However, the lake was dry during the entire course of my study. I wish that I had possessed the foresight of installing a runoff gage in the ditch, because then I could have calculated a more accurate runoff coefficient. For the future, it would be of value to place a runoff gage in the CCC ditch, as well as retaining gages onsite at Stoneman Lake.