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• Ms. Michele McPherson May 23, 2002 Page Two 4. The drainage calculations have been reviewed, and the following comments are pertaining to them: • The drainage pond needs to be moved out of the drainage and utility easement that is intended for watermain looping. • The outlet for the pond is shown to have three 3-inch orifices. Elk River standards allow a minimum orifice 4 inches in diameter. The outlet structure should be revised to include an orifice at least 4 inches in diameter. • No drainage area map was submitted. • The grading plan shows the top of the berm to be at 925. The High Water Level (HWL) of the pond is 924.3. Elk River standards require 1 foot of freeboard from the HWL to the top of the berm. The grading plan should be revised to show the top of the berm at 925.3. Additionally, the drainage calculations state the Emergency Overflow (EOF) for the pond is at an elevation of 925. The EOF should be shown on the grading plan and should be detailed to insure it is constructed properly. • There is an area along the south property line that is hatched on the grading plan, but there is no indication what this hatched area represents. This hatching should be referenced in the legend. • The low point in the parking area near CBMH 102 appears to overflow onto the property to the south. An emergency overflow for this lowpoint should be provided to route this area to the proposed pond rather than off the property to the south. The EOF for this low point should overflow at an elevation no greater than 1 foot above the elevation of the low point. • • The ponding calculations show the proposed weighted Cn for the fully developed site with 2.34 ac. of impervious to be 63. Using 2.34 ac with a Cn of 98 and 3.60 ac. with a Cn of 58, we calculate the weighted Cn to be 73. The drainage calculations should be revised to show the correct Cn. • A pair of dashed lines is shown crossing the pond. It is unclear as to whether this is an easement, and if so, whether a pond can be constructed over it. • The grade of the parking area near the southerly handicapped parking space is 3.13%. 2% is the maximum grade permitted. • The grades in the area along the north side of the future additions are extremely flat, and the pavement at the northwest corner of the easterly parking lot is higher than the proposed floor elevation. • The casting for CB 101 is called out to be a beehive type; Elk River's standard casting in this case would be a stool type R-4342. • Additional catch basins are needed in the parking lot to collect runoff. 5. The area for future expansion is very flat and will not drain properly. Even after the addition is built, the area will be too flat. This area should be looked at and reworked. If you have any questions regarding any of these comments, please call. Sincerely, Howard R. Green Company / / / ee,<) • Terry J. aurer, P.E. Howard R. Green Company Ltr-052302-McPherson.doc CONSULTING ENGINEERS