5.2. SR 09-22-1997 - Laserfiche WebLink

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Ce COMPARISON OF 1970 AND 1996 DNR LAKE SURVEYS DRAFT Based on fieldwork completed on May 5, 1970, the Minnesota Department of Conservation (now the DNR), mapped the depth to sediment and produced a lake map for Lake Orono. At the request of the City of Elk River, Lake Orono was remapped on August 28, 1996, for the purpose of comparing the two surveys such that any changes in lake depth could be observed for the 26- year time period between surveys (see Appendix F). The 1970 survey was scanned into a computer and, using the computer-aided design package Intergraph, the 1970 survey was compared to the 1996 survey electronically and the volume differential computed. To compare the two surveys in the above manner, the following adjustments were made to the survey data: The lake outlines on the two surveys were drawn from two different aerial photographs taken 28 years apart. The later survey omitted the very shallow upper portion of the lake; as a result the areas were approximately 10 percent different, and the lake outlines between the two surveys varied and could not be directly compared. During the analysis, the volumetric comparison was based on only the lake area included in both surveys and does not include lake area upstream of the limits of the 1996 survey. To normalize the lake outline differences, the 1996 lake outline was utilized due to its higher level of detail. The two lake surveys do not reference the same benchmarks. The survey benchmarks were investigated and the surveys adjusted to account for the varying lake elevations on the two survey dates. Benchmarks referenced in both surveys were surveyed together and it was confirmed that their relative elevations are correct. The 1970 survey references two benchmarks. Benchmark #2 is a brass monument on top of the concrete wingwall on the south side of the Lake Orono Dam west of Main Street. The benchmark was established in 1970 by the N:\0598\01LELKKIVEP~P, PT-rlb 8