07-09-2003 Council Agenda - Laserfiche WebLink

Home Browse Search

1992 Minnesota Wetland Conservation Act (required 2:1 replacement) 1997 Wetland Buffer Pretreatment Drainage Patterns The exact drainage patterns near and thru the wetland prior to the construction of the berm are difficult to determine due to the fact that the available topography is limited in detail and accuracy. The topography in the 1965 aerial survey has a 4 foot contour interval, generally the accuracy of contour data is one half the contour interval or plus or minus 2 feet in this case. There also appears to be some variation in the datum the topography is referenced to. The topography prepared in 1965 shows the entire area within the pond to be 861.0 which does not agree with the 1981 Markhurd topography elevation of 862.8. The information analyzed suggests that construction of the berm had minor effects on the drainage pattern that existed prior to its construction. A field survey was completed by HRG to determine the berm, ditch, and wetland elevations in the vicinity of the berm. Areas of importance include the elevation of the top of the berm (approximately 868.0), the intersection of the northwest ditch and the ditch along the berm edge (Point A — 860.8), the end of the ditch along the berm at its south end (Point B — 859.2), and the elevation of the wetland at the pond outlet (Point C — 859.8). These points are shown on the attached Exhibit 1. The distance between points A and B, which is the current drainage path along the berm, is 350 feet and the slope of this ditch is 0.47 %. The condition, which may have existed prior to construction of the berm, appears to be direct drainage from point A to the area of the outlet pipe for the pond at point C. The distance between points A and C is 400 feet and the slope between these points is 0.25 %. This indicates that the existing drainage path along the berm could convey more water than the assumed drainage path prior to the berm construction if the same ditch cross - section were used. However, it is unknown what the pre -berm channel dimensions were or if any channel even existed prior to the berm being constructed; and therefore, an actual comparison of the two drainage paths is nearly impossible to accurately complete. A site visit of the property was completed with the Sculley's and meetings were held with both the Sculley's and Mr. Progreba to discuss their drainage concerns and to determine if any other hard data was available. Both expressed that the area is "wetter" than was in the past, but neither had any additional topography to review or hard evidence to aid in the study. Our office also spoke with Tim Fell with the Army Corps of Engineers on August 19, 2002 and discussed the materials that were reviewed and asked if they had any additional materials such as topography that we could look at. He indicated that the Corp does not have any other information about this site. Berm Drainage Ditch The existing drainage ditch on the west side of the berm was constructed to direct drainage from the north and northwest along the berm to Kohlman Lake. The drainage area of approximately 94 acres was modeled in Hydrocad. The model indicates the ditch has a maximum capacity to convey approximately 300 cfs as compared to the calculated flow of 150 cfs in the 100 yr event. These calculations were completed assuming a normal water level of Kohlman Lake of 859.0 as provided by the Ramsey Washington Metro Watershed. The capacity of the ditch along the berm is dependent on the lake level. The Watershed has C: \WINDOWS \Temporary Internet Files \OLKF1A3 \Sculley Drainage Findings.doc 3