HomeMy WebLinkAboutWPO201300067 Review Comments Stormwater Management Plan 2013-11-21� OF AL
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road, Room 227
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: Hollymead Town Center WPO plan
Plan preparer: Collins Engineering, [scott @collinsengineering.com]
Owner or rep.: Route 29 LLC [ulcwww @embarqmail.com]
Plan received date: 28 Oct 2013
Date of comments: 21 Nov 2013
Reviewer: Glenn Brooks
Stormwater Management Plan (WP0201300067)
1. Provide as -built surveys of the basins and any features to remain in place. Volumes and structures must
be accurate. Structures to remain in place must be in new condition.
2. This is a residential area, and none of the basins should have 2:1 slopes leading to water. To avoid
safety hazards, a 3:1 slope or flatter should be provided, or ample (10' min.) safety shelves above the
water level.
3. This entire area was wooded prior to clearing and development, and pre - development values should
reflect this.
4. It is noted that since Hollymead Town Center, Abington, and Willow Glen zoning amendments and
preliminary plans were originally approved, the Water Protection Ordinance has changed in the
application of stream buffers. Now, streams like the one shown on this plan are assessed to determine
if they are perennial and a buffer should be applied. There is a likelihood that this is a perennial
stream. This does not mean these basins, which already exist, cannot remain here. This comment is
for information only at this point, and as long as this question remains open, additional disturbances in
the stream area will be carefully considered.
5. An adequate channel analysis must be provided, and provisions for adequate channels or channel
protection provided.
6. Facility #1:
a. The geometry is short- circuited, with outlet and inlet adjacent. One of them should be moved.
b. The emergency spillway is over the fill slope and must be armored and have an outlet channel to the
stream.
c. Lowering basin #1 geometry, which appears to have multiple rock outcroppings, will be
problematic. Please provide a current estimate for blasting to achieve this grade, and include a plan
provision for sealing any leaks caused by fissures in the rock, as the basin must hold water in this
proposal. Also, where plants are to be placed, the rock must be lowered well below grade to allow
root growth.
d. The lowest invert is higher than the bottom, and the riser is shown on a slope. Detail the riser
footing to avoid instability.
e. Please update the topography to reflect the Willow Glen development and areas draining to this
basin.
f. The routing is relying on flow through the top of the riser in the 10yr storm, which is unusual. The
routing only appears to work because of a curve you have used as input. Please provide this curve
with explanation. (Most designs set the riser top at the 10yr elevation.)
7. Facility #2:
a. The drainage area does not appear to be accurate in the area of the basin. It is not clear in the area
Engineering Review Comments
Page 2 of 2
of the mobile home park. Please include the mobile home park drainage system and outfalls.
b. Show depth zones on the plan to assure required marsh areas.
c. Please provide a routing to ensure the system works and does not overflow. This could also help
with the routing of facility # 1.
8. Facility #3:
a. The plans are requesting credit for detention and treatment of 110,000 square feet of impervious
area over 11.44 acres. This is not workable. The basin must be designed for the ultimate build -out
(which can include other facilities in the watershed). For facilities 1 and 2, this appears to be an
assumption of about 65% imperviousness. For this basin, it is 22 %, which is not realistic. With
urban development typical of Hollymead, at least 75% is more realistic. The basin simply doesn't
work with flow from that ultimate buildout, and it is difficult to parcel out the flow as is
theoretically done with water quality credits in series.
b. Sheets 6 and 7 appear to show the same thing, drainage areas. Please remove areas not pertaining
to this plan.
c. Please show the drainage system on the Abington side. Lockwood Drive appears to drain to this
basin.
d. The 100 year storm will likely overflow the Abington inlet collection system and the public roads,
adding drainage to this basin. Storms of this magnitude tend to follow grade divides when the inlet
system tries to drain against, at least partially.
e. Provide stabilized inlet flow channels into the forebay.
f. This basin, which appears too steep and too small for the drainage area and location, appears to
require a different conceptual approach.
Please be aware that the Water Protection Ordinance is currently under revision to meet new state
mandates. These plans do not guarantee that future developments in these watersheds will not have to
meet new state mandates which go over and above current requirements.
File: E1 swm GEB HTC- TownCenterDrive- basins.doc