08-17-2009 Planning Commission Packet

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_.�... , , _.�_�_-__..._. ._.�..�, Historic Aerial Photo Review The purpose of the aerial photo review was to determine the origin and existence of a wet swale area located east of the • present driveway and a wet pocket in a mowed area west of the driveway. Svoboda Ecological Resources(SER) completed a wetland review of the property on September 4, 2008. Subsequently, the Minnehaha Creek Watershed District(MCWD) in its field review of the site requested further information about the swale and the open wet pocket in the mowed area. Aerial photos were obtained from the John Borchart Library, University of Minnesota. Photos were 9" x 9" black&white (b/w)excep�the 1991 photography,which was color infrared (CIR). Photo scales were one of the following— 1:9600, 1:10000, 1:20000, 1:35000 or l:40000. Photo dates covering the subject area ranged from July 5, 1937 to Apri121, 1991. The aerial photo review indicated that the open water area of Lake Minnetonka to the west of the property was a lagoon created by dredging sometime between 1945 and l 953. The lawn area was apparently used to dispose of the dredged material as the soil borings show evidence of excavated,non-native soil (see above). More recently, (exact date unknown)a 6"PVC storm sewer pipe was installed connecting the swale to the open area in the lawn. Accarding to David Fox,the PVC pipe was already in place in 1975 when they became involved with the property. Since all piping is of PVC material,the date of installation would be post mid-1960s when the use of PVC became common. The pipe ends in an open culvert with protective metal bars at an invert elevation of 928.46'. A second 8"PVC pipe exits this area in a northwesterly direction toward the lagoon opening at another catch basin(invert elevation of 927.79')on the adjoining property to the west. From this catch basin, a third 8" PVC pipe leads in a north-northwesterly direction toward lagoon and exits below the OHW(929.4'). Photos prior to 1991 do not show any evidence of the catch basin with the lawn area showing a consistent and uniform pattern of grass up to 1967. The 1969 photo does not provide clear enough detail to make an informed interpretation. There is evidence of some equipment or other objects parked on the lawn on the November 9, 1971 photo in the location of the area of the catch basin. The storm sewer piping may have been installed at � this time. The April 21, 1991 photography shows an indication of a saturated area at the location of the catch basin. Recent Birdseye view photographs posted on the Hennepin County parcel information website(P1D No. 02-117-23-43- 0023) shows clear evidence of the location of the two catch basins. Nearly half of the lower end of the swale is below the OHW of the lake therefore when the lake is at a normal water elevation, some inundation ar saturation of the lower end of the swale will occur. Closer examination of the elevations of the catch basin inverts provides an explanation of the persistent wetness of the areas. The OHW of Lal:e Minnetonka is 929.4' whereas the elevation of the catch basin on the subject parcel is 928.46', 0.94' below the OHW. The invert elevation of the catch basin on the adjoining property is even lower, 927.79', 1.61' below the OHW. The field-determined elevations were confirmed by SER with Gronberg and Associates,who provided the parcel survey data. Gronberg also indicated that it was not until the lake water levels dropped below 928' that they were able to locate the outfall point of the stormsewer running from the second catch basin into the lake. The accompanying drawing shows the various locations of the PVC pipe,the catch basins and elevations as specified in this narrative. This information indicates that the driving hydrology of the catch basin originates from a back flow of water from Lake Minnetonka any time that the lake water elevation is at or above 928.46'. Water running down the swale and under the driveway through the stormsewer pipe cannot enter the lake unless the lake elevation is below 928.46'.At all other times, water exiting the stormsewer pipe, exits the catch basin and flows overland across the lawn creating the condition of wetness. The aerial photos were scanned and are saved digitally. Copies of the photos are available on request. �