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i. DATE: July 26, 2004 TO: Sandy Brown ~~ FROM: Dan Mathews RE: Pond Survey Relative Elevations at Roadside and Fruita Mines Last week I tried out the self-leveling rotary level survey instrument for the first time. I checked a couple ponds at Roadside Mine, and the sediment pond at the Fruita Mine bond forfeiture site. Once you get the hang of it, its pretty simple and quick for one person to survey elevations. For general backfilling and grading compliance checks, it is probably over-kill, but it may have applicability for checking reclaimed channel dimensions and profiles. One thing I became aware of upon re-reading the manual (after I did the surveys) is that the receiver unit has an adjustment for "coarse", "medium" and "fine" level of resolution. The default setting is apparently "coarse", so my readings may not really have been accurate to hundredths of a foot. The next chance I get, I will repeat the same elevation checks using the "fine resolution" setting. Anyway, my survey data and summaries are enclosed. Note that, for the Roadside ponds, my surveyed sediment elevations (relative to emergency spillway elevation) varied from Jim Stover's 2nd Quarter pond inspection values by around 5 to 7 inches. I'm pretty sure Jim just measures the height of the sediment marker with a tape measure, but the top of the sediment marker is supposed to correspond to the elevation of the spillway invert, so the surveyed elevation difference between base of sediment marker and the spillway should correspond to the measured height of the sediment marker. I'll check this again using the fine resolution setting, and see what I get. Another consideration to be aware of when surveying is that the higher the survey rod has to be extended to intercept the level plane projected by the laser, the lower the precision of the reading. This is because, after the telescoping survey rod has been extended more than a couple lengths, the top of the rod where the receiver is attached gets pretty wobbly, making it harder to be sure of the exact elevation of the laser "hiY' (which is indicated by an audio tone). Based on the survey I did, there is an apparent problem at the Fruita Pond, which I mentioned to you last week, It appears that the pond bank may be a few inches lower than the open channel emergency spillway in a couple locations. The pond was not originally constructed with an open channel spillway, just the riser pipe culvert type spillway with a smaller diameter primary pipe nested inside the larger emergency spillway culvert. But when AML did the flood repair remediation/reclamation a few years ago, we had the contractor put in an open channel "backup" emergency spillway, that would function in the event that the riser spillway became clogged. Good idea but apparently poor execution. Paul and I had both satisfied ourselves using hand levels that the spillway cut was lower than the low bank areas along the south end of the pond, but that is not what the laser survey results indicate. There is less than one foot of free-board between the top of the culvert spillway riser and the existing open channel spillway invert, so the best "fix" would appear to be build up of the elevation of the south bank. I believe that BLM contracts occasional road maintenance on the nearby road that provides access to the bike trails in the area. They might be willing to push up a berm a couple feet high, to protect their road in the event that the riser pipe spillway were to malfunction.