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HomeMy WebLinkAboutWPO201000009 Review Comments Stormwater Management Plan 2010-02-24� OF AL ,. vIRGI1`IZP COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 Project: Crozet Elementary School Dropoff Area Minor Amendment SDP - 2010 -00012 and WPO- 2010 -00009 Plan preparer: Mr. John Hash, PE; Timmons Group Owner: School Board of Albemarle County Representative: Neale Craft; Office of Facilities Development, County of Albemarle Plan received date: 08 February 2010 Date of comments: 24 February 2010 Reviewer: Phil Custer A. Minor Site Plan Amendment Comments (SDP- 2010 - 00012) 1. The Chief of Current Development has granted a waiver of the travelway width to allow the current proposed configuration. 2. Drainage computations for the new pipe and inlet are needed. The computations should include a map with the watershed's area, tc, and C value specified. 3. Please provide a channel computation for the newly created and existing channel downstream of the 15" pipe outlet. Will this channel need a lining? B. Stormwater Management Plan Comments (WPO- 2010 - 00009) 1. The depth of the biofilter mix must be 2.5ft. 2. The biofilter landscape plan must contain at least three species of trees. The ratio of shrubs to trees should be at least 2:1. [VSMH, DM] 3. Please make sure the weir elevation is specified on Sheet C4.0 on the next submittal. 4. A sediment forebay is needed within the facility. A forebay composing of 10% -20% of the bed area created by a horseshoe ring of stone is acceptable. The stone ring would be comparable to the filter weir on a sediment trap with stone of a smaller diameter on the inlet side. 5. No SWM bond will be necessary since this is a County project. C. Erosion & Sediment Control Plan Comments (WPO- 2010 - 00009) 1. Engineering review has concerns with the concept of the ESC plan. As the fill operation proceeds, the earthwork will encroach upon the sediment trap. At the same time, the fill will direct runoff around the west side of the mulched play area which is not protected by sediment control measures. If possible, the biofilter should be lowered and designed as a trap for phases I and II of construction to filter runoff once final grades are reached. Also, the proposed drainage plan is dramatically increasing the discharge in the depression west of the mulch area. This flow path does not appear it would meet the state's definition of an adequate channel. Despite the mild grades, engineering review is concerned that increased intensity and frequency of storms will cause lasting erosion. Our concerns would be diminished if the existing watersheds were maintained and the offsite flow was directed into the existing channel between the school and the play area. If warranted, this concept would be coupled with a few upgrades to the channel downstream of the discharge point. If the current bypass concept is maintained, the applicant must either prove downstream adequacy or create a new channel to the stream. Albemarle County Community Development Engineering Review Comments Page 2 of 2 I recommend devising a few concept alternatives and meeting with me to discuss how the concerns mentioned above can be dealt with. 2. Diversion dikes appear to be used as "cleanwater" diversions which is not a state approved standard. Diversions should only be used to direct runoff to sediment traps and basins. Please remove these diversions from the plan. 3. Please show the soil map within the plan. A separate detail or layer in the ESC plan are both acceptable. [DM] 4. Inlet protection is generally adequate only for drainage areas less than 1 acre. Remove the IP symbol from the DI -7 and wrap the silt fence around the south and east sides of the structure. 5. Please remove all notes about a temporary slope drain from the plan. The slope drain would get in the way of the grading operations and would be impractical to maintain. 6. In the construction sequence specify that the mobile classroom units must be removed before the bypass storm system is installed. 7. The construction entrance should be shown graphically as 70ft. 8. No ESC bond will be necessary since this is a County project.