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4.2.3 Echo Lake Water Management Plan 4.2.3.1 Location and General Description Echo Lake is a small, shallow lake (DNR #82-135W) located in Subwatershed ECO1, approximately 1/2 mile northwest of Long Lake, in the City of Mahtomedi. It has a surface area of about 30 acres and is situated along the east boundary of the 916 Vocational -Technical School property. The DNR's ordinary high water elevation (OHW) for Echo Lake is 996.3. The Echo Lake tributary area is 190 acres, which is comprised of Subwatersheds ECQ1 through EC06. Figure 2-3 shows the tributary area. Runoff from a portion of the Vocational -Technical Institute buildings and parking lots passes through a small detention/sedimentation pond before reaching Echo Lake. Residential development is occurring in the south and east portions of the tributary area. The remainder of the watershed is expected to develop in the next 5 years. 4.2.3.2 Drainav-,e _Patterns and Stormwater issues The existing outlet from Echo Lake is a deformed corrugated metal pipe approximately 15 inches in diameter, with an upstream invert at Elevation 995.5. The pipe is located on Lot 5, Block 2 of Echo Lake 2nd Addition, under old Echo Lake Avenue, on the south side of the wetland which comprises the southwest side of the lake. Prior to construction of the Echo Lake Addition development, the surface overflow from Echo Lake was at Elevation 996.6. As a result of the development, the emergency overflow was relocated to a point near the outlet pipe and raised to Elevation 997.8. Water discharged from the Echo Lake outlet makes its way south and east through a series of wetlands to Weber Pond. There are three ponding areas in the Echo Lake Addition development which discharge j to Echo Lake but are also designed to be part of the Echo Lake flood plain. Water from Echo Lake will overflow to these ponding areas only when the Echo Lake water level is equal to or greater than its outlet elevation (995.5). Prior to raising the surface overflow elevation, the calculated 100-year (full development) flood level of Echo Lake was Elevation 996.8, which includes the flood plain/wetland area east of the lake. Under these conditions, the 100-year flood elevation of Long Lake increases 0.5 foot above the elevation stated in the previous VBWD plan (from 943.2 to 943.7). The flood elevation in the previous VBWD plan was based on restricted outlets from both Long Lake and Echo Lake. As a result, additional storage is needed on Echo Lake (and Weber Pond) to prevent this increase in the Long Lake flood elevation. This was anticipated in VBWD's previous plan, which stated: "A 100-year flood level" (Elevation 998) "3 feet above the normal level is proposed for Echo Lake. The increase in storage volume is proposed to be done by the City or developer at the time when development of this subwatershed is occurring. Storage in the uneven terrain northwest of the lake should be preserved during development and the lake itself should have maximum utilization of its storage volume during development to keep outflow rates very low." However, the previous VBWD plan did not include the Echo Lake outlet improvements or easement acquisition as part of the Capital Improvement Program. As a result, the existing outlet remains in place and easements still need to be acquired for the proposed higher flood level on Echo Lake. In addition, VBWD must obtain a DNR permit to construct a new outlet from Echo Lake. 23\82\045\SEC4.RPT\KMH 7 DRAFT: June 23, 1994