HomeMy WebLinkAbout1995-10-09 Barr Engineering to BRAA Re SAHS Temporary Flood EasementsOctober 9, 1995
EngineerThg Company
Mr. Joe Anderlik, City Engineer
City of Oak Park Heights
Bonestroo, Rosene, Anderlik & Associates
2335 West Highway 36
St. Paul, Minnesota 55113
RE: Temporary Flood Easements, Stillwater Area High School
Dear Mr. AnderIlk:
8300 Norrrn Center Drive 555 West 27t Street
Minnepalis, MN 554,37 -1026 Hibbing, MN 55746
Phone: (612) 832 -28 7 Phone: (218) 2624465
Fax 832 -2601 Fax: (218) 262 -3460
Due to the current flood elevation of Cloverdale Lake, the Valley Branch Watershed District is
requesting a temporary flooding easement for property owners surrounding the northwest and
south ponds located in the City of Oak Park Heights. The District is investigating installing
restrictions at the outlets from the northwest and south ponds on the Stillwater High School
property. Michael Robertson asked that all pond modification requests be submitted through you.
The attached figure, produced by )3RA & Associates shows the location of the northwest and south
ponds.
The existing pond outlet structure for the northwest pond is not functioning as designed. The
bottom of the stop-log skimmer device is approximately 6 inches above the outlet pipe elevation,
rather than at the pipe outlet elevation as designed. Modification of the stop -log skimmer is
needed for it to function as designed. Further modification could easily be accomplished to raise
the ponds normal level from Elevation 924.0 to 926.0.
The proposed restriction of the south pond involves installation of a 5 foot high stop -log structure
within the manhole immediately downstream of the south pond. No runoff would then exit from
the high school site until the south pond rose 5 feet. Once the pond has risen 5 feet, outflow from
the pond would overtop the stop -logs and exit the pond at essentially the same rate as currently
exists. This use in the normal ele vation of the south pond will not effect the normal level of the
northwest pond.
I have used the previous hydrologic modeling information presented by BRA & Associates to
analyze the effects of raising the normal levels of these two ponds. If a 100 -year frequency rainfall
event were to occur, the resulting flood elevations for the northwest and south ponds would
increase by 1.7 and 3.6 feet, respectively. These increases would be from Elevation 929.0 to 930.7
for the northwest pond and Elevation 921.0 to 924.6 for the south pond. A 1.7 foot elevation
increase in the predicted flood elevation will remain within the fenced area surrounding the
northwest pond. An increase in flood elevation at the south pond to Elevation 924.6 is
approximately 5.4 feet lower than the lowest existing septic system and 9.4 feet lower than the
basement floor elevation of the lowest home on the south side of the pond.
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