HomeMy WebLinkAboutSP201500003 Review Comments Appeal to BOS 2015-06-01*-&A
County of Albemarle
Department of Community Development
Memorandum
To: Claudette Grant, Planning
From: Glenn Brooks, Engineering
Date: 27 Feb 2015
Rev. 1: 20 May 2015
Subject: North Point amendment (SP201500003, amending SP200700003)
The special use permit application and floodplain development permit for amending the middle entrance
crossing of Flat Branch at the proposed North Point development has been reviewed. There is not enough
information provided to make a full review of the floodplain impacts and changes to the special use permit.
The following comments and requests for information are provided for Planning and the applicant;
Rev. 1: Please see the comments for revision 1 below. There are too many issues with the computer model
and the proposed design to recommend approval with this revision.
1. Please provide a comparison of the approved arch span and the proposed amendment using box
culverts. Show the differences to the stream and to the floodplain, as well as the hydraulic models.
Rev. 1: The comparison shows an unexpected result. It was expected that the culverts would have less
hydraulic efficiency, and that both the arch and culverts would have a headwater. The revised
application documents and model seem to propose that the culverts (and arch) will have no impacts,
and actually reduce, rather than increase the floodplain limits upstream. This is highly unusual. In the
case of the box culverts, it seems to have been done by proposing four culverts where two might
suffice. This is not likely to stand up to final plan review through VDOT. Please provide a
preliminary review or approval from VDOT that the extent of culverts proposed will be acceptable for
the public road. Typically, VDOT will require that the number of culverts be minimized, and lengths
be reduced, and the fill over the culverts as low as possible, to allow for maintenance.
2. Please provide the HEC -RAS analysis on disc. The computer files are needed for adequate review.
Rev. 1: Thank you for providing the model. Please address the following issues with the model;
a. The ineffective flow areas upstream and downstream of the culverts do not appear correct.
b. Additional cross - sections are required upstream and downstream of the culverts to model the
expansion and contraction correctly. Refer to HEC -RAS modeling instructions on -line.
c. Modify the channel sections to model the enlargement and armoring of the channel on either side
of the culverts.
d. Provide hydrologic computations to adjust flows for proposed development. FEMA values
typically only reflect existing conditions at the time the model is developed. This is part of a
development proposal and should reflect the build -out year also.
e. The extent of the culvert in the model appears incorrect. On the plan drawing section "O" at
station 5105 is shown in the culverts. On the model this station is upstream, although section "O"
has been removed.
f. The model indicates a hydraulic jump in the culverts. This would seem to require inlet control,
which when specified in the model changes headwater results. The worst case scenario should be
used. In smaller storms, the hydraulic jump would likely occur downstream, necessitating more
Albemarle County Community Development
Engineering Review comments
Page 2 of 3
structured energy dissipation measures, which should be shown.
3. The HEC -RAS model appears to be independently generated. The FEMA model must be used.
Please obtain the FEMA model for Flat Branch, and update the existing model with the surveyed
cross - sections to establish the base flood. Use the existing model to generate the proposed model.
Rev. 1: Provide verification this is the most current model from FEMA. The current FEMA Flood
Insurance Study for Albemarle gives an elevation of 423' at section "O ", and 1950cfs at the North
Fork Rivanna and 1290cfs at the study limit. This data does not appear to match the model.
4. Please provide data to demonstrate compliance with 18- 30.3.13B and 18- 30.3.14. Specify the
floodplain rise, how obstruction or fill is minimized and protected against erosion or pollution.
Rev. 1:
a. It is not clear that anything has been done to reduce the amount of fill in the floodplain. For
example, if the road were lowered and crossed more perpendicular to the stream, it could
significantly reduce grading and culvert lengths.
b. The culverts entrances are too skewed in relation to the creek, with the potential to push water into
the left bank. This appears to be caused by both the angle of the crossing, the grading for the
multi -use path, and the placement of wingwalls. The culvert entrance should be more
perpendicular, with the wing -walls arranged to accommodate the fill slopes and protect the banks.
5. Much of the grading activity shown on the plans is not for the crossing. Please clarify that this fill area
is not part of this permit.
Rev. 1: A note may not be sufficient. It would be better to remove this from the plan or separate it
somehow. The WPO does not allow encroachment more than 50ft into the buffer on the sides, which
is not related to the stream crossing, but appears to be for the commercial area and stormwater basin,
and for the multi -use path. It should be clear that the road layout and grading are not part of the plan
for the special use permit, and the extent of the grading for the commercial area and stormwater
management, and the roadway features and intersection are not included in any action.
6. The plans show only one floodplain line beyond the fill area, implying there is no change. This is not
usually the case in front of a culvert constriction. Please show lines before and after the proposal, so
the changes to the floodplain are clear.
Rev. 1: This has been shown on a reduced size plan sheet. Please provide a full sized sheet, and include
stationing and all sections used in the model in the vicinity of the crossing.
7. Condition #1 should be revised to reflect new ordinance requirements. 18- 30.3.13 requires a
Conditional Letter of Map Revision prior to construction. A Letter of Map Amendment or Revision is
required after construction. It is recommended that plan and plat approvals be conditioned on both
these FEMA approvals.
Rev. 1: Please reflect the distinction between a conditional letter of map revision, and a letter of map
revision. The former is obtained prior to construction, and the latter after.
8. Please show appropriate transition between the stream channel and the box culverts, clarifying the
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Engineering Review comments
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length of stream necessary to be disturbed. This will be for comparison with the arch.
Rev. 1: Please provide information on the channel transitions.
9. Conditions 2, 3, 4, and 9 are unnecessary, as these are required by ordinance. As an example, in
condition #9, the county no longer has a Natural Resources Manager position, and in addition to
mitigation and erosion control plans, a VSMP /SWPPP may be required.
Rev. 1: Regarding revised conditions;
a. New condition 4 implies that the county engineer can permit disturbances closer than 50 feet to the
channel bank. Buffer requirements in the Water Protection Ordinance do not appear to give the
county engineer this authority.
Condition 4 also implies permission to build the multi -use path within the buffer. Where the path
encroaches into the buffer, such permission will need to be separate from the SP for the crossing.
Nature trails are a by -right use in the buffers, as they involve no clearing and grading. This path is
shown on exhibits as a multi -use, graded and surfaced path, which is a different item. It would
need to follow the normal Water Protection Ordinance requirements for development.
10. Please provide a copy of the wetland and stream disturbance documentation submitted to the state.
The difference in stream impacts at this crossing is pertinent to the review, as it appear the prior
approval emphasized minimizing impacts to the stream in the use of the arch.
Rev. 1: This has been received.
file: E2_sp_GEB_NorthPoint_amendment _ middle _ crossing.doc