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HomeMy WebLinkAboutWPO200800084 Review Comments Stormwater Management Plan 2008-09-26*-&A County of Albemarle Department of Community Development Memorandum To: Amy Pflaum, Senior Engineer From: Glenn Brooks, County Engineer Date: 24 Sep 2008 Subject: Rivanna Village, revised stormwater management concept plan (ZMA200100008, WPO200800084) This memorandum summarizes the points we discussed in review of the new Conceptual Stormwater Management Plan by the Williamsburg Environmental Group received in August from K.G. Associates. As I understand it, there were three related questions; (1) Is a variance required from the approved Zoning Map Amendment? (2) Would engineering recommend approval for such a variance? (3) Is this conceptual plan acceptable? Related to the last question, Michael Barnes of K.G. associates has requested a letter "confirming the proposed modification meets or exceeds the stormwater strategy approved during the rezoning..." (1) Is a variance required from the approved Zoning Map Amendment? No, and this would be my recommendation to the Director of Planning. This plan is not different enough from the approved rezoning plan(s) that a variance should be required. In fact, the approved rezoning plans are so confusing and disordered, that significant variations are hard to pin down. (2) Would we recommend approval for such a variance? I would ask for changes, as indicated below. (3) Is this conceptual plan acceptable? There are concerns with the new conceptual plan. It will not be possible to issue the letter requested by K.G. Associates. To begin with, there are discrepancies in the rezoning documents which make it very difficult to review. There are two conceptual plans. The approved plan from 2007 is different from the preliminary stormwater management plan referenced in the code of development, which is from 2004. Both of these are included in the approved documentation for the ZMA, together with the 2004 report and specific items listed in the code of development. The newly proposed plan contains two lists of proposed changes, one on page 5 of the new report, and the other in the summary on page 12. I will address each of these proposed changes, and then illustrate plan differences and general concerns. Proposed plan changes from page 5; 1. Replacement of 3 step -pools with wetlands and dry detention; This is not accurate. There appear to be more pools missing or replaced. It's not clear how these wetland systems will work, as the modeling and the water quality drainage areas are problematic. Albemarle County Community Development Engineering Review comments Page 2 of 7 2. Grading around the tennis courts to avoid channel impacts; There is no proposed grading plan in the new proposal. This could not be evaluated fully. This item appears to coincide with using the tennis courts for detention. Tennis courts are not usually an area of flooding, and are less acceptable than an open field. This would seem to be less acceptable to the public as well, and become a concern for maintenance. As long as there is an alternative, the tennis courts should not be used for detention. For other areas, frequency of flooding will be important. Too frequent inundation of active recreation areas will be a problem. 3. Replacement of wet pond with dry detention next to the fire station. The proposed dry pond inundation area and berm must be off of all residential lots, as shown in the approved ZMA. It is not clear how water quality is being provided for the fire station. Easements or permission must be obtained from the fire department. 4. Use of the existing Quarry ponds; This appears to be in accord with the 2007 ZMA plan, and is not a change in concept. 5. Head wall and stream relocation to reduce fill slope at area 2D. There is no proposed grading plan. There is no outlet at 2D. This could not be evaluated. 6. LID for blocks A & D; This is unclear. LID refers to so many different approaches and concepts, that it could be applied to this entire rezoning plan, or to none of it. Please indicate precisely what measures are proposed. Without more detail, this could not be evaluated. Additional plan changes from the Summary on page 12; 7. Filterra inlet systems for pre- treatment; This is in the approved ZMA plans, and is not a change in concept. 8. Potential for underground detention within park area and/or development areas on -site; This statement is too vague to be on a concept plan at this stage. The detention is preferable on -site, as shown in the approved ZMA plans and the code of development. General Concerns; 9. The need for detention to avoid downstream impacts is still a concern. A rezoning cannot proffer away or reduce ordinance requirements. It can provide more than the minimum, like the lyr storm provision. In addition, the zoning documentation is unclear with regard to approval of a detention waiver according to WPO section 314, which would be an independent action. The correspondence form Mark Graham, upon which this assumption is based, only says it will be considered. There is not enough detailed information at this point to make this determination. The MS -19 analysis for the Glenmore projects Albemarle County Community Development Engineering Review comments Page 3 of 7 downstream must also be considered, as well as MS -19 requirements for this project. 10. The plan must clarify what streams and areas are to be preserved. This is not clear, and it appears to be the justification for some of the changes. Additional stream impacts may be necessary to implement the stormwater management concepts approved with the rezoning. The concepts approved with the rezoning, and the ordinance requirements, should not be sacrificed to reduce the stream impacts and cost for the Army Corps permit. 11. The assumed 54% imperviousness for the developed areas appears low. The VSMH assumption for townhouses and commercial districts is 65% or greater (Table 4 -6a). 12. The tables and chain of treatment could not be followed. The ranges of filterra numbers are too broad, and the use of blocks is not useful. Please use only drainage areas, and narrow down the ranges or locations. 13. There was not enough detail provided to evaluate the hydraulic modeling. Some of the assumptions appear to be problematic. Plans refer to the wetlands as step pools, but they are modeled as one facility. There are discharges into different pools, which does not appear true to the model. Assuming no increases for the LID portions does not appear reasonable, especially when no information is available regarding the LID applications. It is noted that a hydraulic model is not usually needed for a true concept plan. 14. The changes in field space, and the areas expected to be flooded during larger storm events appears to be a significant change in the concept. The inundation areas for larger storms must be clarified on the plans. Active park areas should not be flooded if possible, especially areas with structural improvements, such as the tennis courts. 15. The rezoning documents mention individual stormwater management for single family lots. This was not found in the new proposal. 16. The rezoning documents indicate underground detention under parking lots. The new proposal provides detention only in the park, which is to be owned by the County. Future responsibility and maintenance are a concern. 17. Off -site impacts due to headwater at culverts will be a problem for the large off -site areas draining into the site. Overland relief must be provided, and easements for inundation areas. 18. The concept appears to run large off -site areas, and pre - treated areas, through the wetlands and ponds, when these facilities are not to be sized for these drainage areas. This would not appear to provide the intended treatment, but would more frequently inundate or flush out these systems. Large off -site areas should be treated or bypassed. Pre - treated areas should be included if pre- treatment is the concept. To point out the differences in the plans, each of the concepts has been summarized graphically. First is shown the approved ZMA plan from the 2007 package that went to the board. The stormwater features are highlighted in blue. Albemarle County Community Development Engineering Review comments Page 4 of 7 RIVANNA VILLAGE 9 TI SCHEMATIC GRADING/ STORMWATER PLAN 0C Approved ZMA plan from the 2007 package This plan is the primary reference for the approved concept. It has 9 or 10 ponds, and about 12 detention pipes placed at various locations on the site. Next is the plan referenced in the approved code of development. This is from the preliminary stormwater report of 2004. This is also an approved document, and considered the secondary source for the approved concept. The stormwater management features are highlighted in purple. Albemarle County Community Development Engineering Review comments Page 5 of 7 °° RIVANNA VIT .T .A CHM ea , , ti i i ® � 4 ' 0 � o - __ ao BMP / STORMWATER MANAGEMENT PLAN Approved ZMA plan referenced in the Code of Development This plan has about 9 ponds, a few dry detention or biofilter /wetlands areas (not quite clear), and about 10 detention pipes placed throughout the site. Also noteworthy, is that the development layout does not match the primary plan. The placement of blocks and units, as well as the layout of the park are different. Even the limits of property included in the rezoning are not the same. Both of these plans reference stormwater management on individual single family lots, underground storage below parking areas, and pretreatment, with Filterra units cited as an example in the code of development. Last is the current proposal, for which the stormwater management features have been highlighted in green. Albemarle County Community Development Engineering Review comments Page 6 of 7 b E The solid shapes are the ponds, the open shapes are the wetlands and dry detention, and the open box is underground detention. The most noticeable difference in this proposal, is that it does not contain on -site detention as in the approved ZMA documents. There also appears to be an overall reduction in ponds and stormwater treatment areas, and the use of tennis courts as detention. These differences do not look favorable. I have provided a couple of overlays of the plans on the next page for a more direct comparison. x :Y Albemarle County Community Development Engineering Review comments Page 7 of 7 i 1I �.o.• On the left is a straight overlay of the plan with the primary zoning document in black, the secondary plan in blue, and the proposed plan in green. It's rather a mess. On the right side is the proposed plan shown with only the highlighted stormwater features from the approved plans. As before, the rezoning plans are in blue and purple, with the proposed plan in green. Here it is evident that the larger ponds on the approved plan, together with the many underground detention areas, offered better management options. In fact, this could be why the review of the rezoning documents was so cursory, and obvious discrepancies in the documentation were not corrected before approval, because there appeared to be enough stormwater management proposed to work out any detention or treatment desired. In fact, a cursory review might indicate that more than the ordinance minimums could easily be provided, and were in fact offered.