Agenda - Council - 04/12/1994 - Laserfiche WebLink

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_Wetlands: There are no wetlands affected by the proposed development. Drainage and Gradin_~ Plan; The drainage and grading plan prepared by Kenneth Gust, P.E., as revised December 9, 1993, has been submitted in conjunction with this preIiminary plat. In general, the 35 acres included within the boundaries of this plat we conveyed to a two-pond storm water storage system which proposes an outlet from the northeast comer of the northern pond which would discharge at a controlled rate into an existing storm sewer on Tungsten Way N.W. which had been installed in conjunction with the Cedar Hills 2nd Addition plat. There are two small exceptions within the plat which do not drain to the two-pond system. On the extreme northwest corner of the plat, six tots convey their backyard drainage to a low spot which is located near the southern border of Lot 9 of Block 4 in the Hunter's Hill subdivision. Within 100 feet of this depression a culvert conducts drainage to the west side of Sunfish Lake Boulevard N.W. Tim amount of mn-off being conducted to this depression should be less following development because of the fact that the contributing drainage area is being reduced from more than five acres to less than two acres. During construction, however, the lots along the boundary with Hunter's Hill should be protected with silt fence to prevent sediment from washing on to these properties. In addition, it is recommended that during construction a sump area of approximately 250 square feet be constructed on Lot 3 adjacent to the sump area on Lot 9 of Hunter's Hill. This sump will allow for sedimentation which can be anticipated from the erosion from construction and will also decrease the velocity of the water impinging upon the silt fence. The second area which does not discharge into the pond .system includes back yards of nine lots in Block 3 of the subdivision which lies adjacent to Chestnut Ponds. Drainage from the house pads flows eastward into the back yards in Chestnut Ponds. The back yards of the lots in Chestnut Pond appear to be graded such that flow would be directed toward the common lot line of Lot~ 3 and 4 of Block 1. Based upon the information provided at this point, we believe the"drainage of these backyards could be problematic. I would recommend that a backyard drain be placed to intercept this drain and conduct it to tile storm sewer located in the su'eet. This would in turn require that the entire drainage system be two feet lower than currently proposed, and that additional pipe be installed to reach a point Iow enough in the Cedar Hills 2nd Addition storm sewer system to allow this discharge. With the contingency described above, the drainage plan as submitted provides adequate inlet and ponding storage to serve the preliminary plat. Outflow from the pond storage system should be limited to four cubic feet per second by means of an orifice plate or other suitable control device. There is a note on the grading plan that the designated multi-family area will be used as a borrow area. If material is taken from this area for grading, the City would want some assurance that the borrow areas were restored with tuff and some reasonable grades. This may be done with some language in the development agreement, but preferably a mass balance for the dirt work could be done and the finished.grades extended to include this area. Finally, with regard to grading, it has previously been noted that a benn should be used to provide screening along the six lots which border County Road #116. A culvert should be placed through this berm to transmit drainage from the backyards of these lots into the