HomeMy WebLinkAboutSP201000045 Review Comments Special Use Permit 2010-12-28*-&A
County of Albemarle
Department of Community Development
Memorandum
To: Megan Yaniglos, Current Development Project Planner
From: Phil Custer, Current Development engineering review
Date: 28 December 2010
Subject: Granger Subdivision Stream Crossings (SP- 2010 -00044 and SP- 2010 - 00045)
Engineering has reviewed the applications for both of the stream crossings for the Granger Property.
Comments for both applications have been provided in one review letter because the analysis package was
submitted as one document:
1. The stream crossings on the exhibit appear to be mislabeled. The culvert on the Fontaine - Sunset
Connector is labeled as Stream Crossing 2, but the calculations appear to refer to it as crossing 1.
Please clarify and adjust applications if necessary.
2. On the exhibit, please modify the limits of the existing FEMA Floodplain (red circles) to match
the site topography. For instance, at cross - section 693.2 the FEMA floodplain line is shown at an
elevation of 414 in the study. Given the existing topography shown by the applicant, the width of
the floodplain should be close to 70ft, rather than 145ft.
3. Table 1 did not print correctly and does not show the 50 or 100 year discharges used by the
applicant in the model. Please correct.
On the summary table near the end of the submittal package, the 100 -year discharges used for each
cross - section are provided. However, these values appear to be too low when compared to the
discharge used in the previous FEMA study (4400cfs at Section Y -Y). Because of the general
development within the watershed in the last 35 years, this discharge has surely increased. Please
clarify.
Also, please explain why the discharge used to analyze the cross - sections downstream of the
confluence with Morey Creek (0 thru 426.3) is the same as the discharge used to analyze cross -
section 693.2 and all other upstream cross - sections.
4. The differences between elevations that were the result of the applicant's model and those
provided by FEMA are not insignificant. (Please note that the 414 elevation in the FEMA map is
closer to cross - section 693.2 rather than 566.6.) What are the reasons for this discrepancy? These
issues must be identified as early in the process as possible in order to trust the accuracy of the
new model.
5. The cross - sections that are analyzed with this plan should use the stationing of the existing FEMA
study.
6. Page 2 of the Stream Crossing Analysis states that the crossings assume no attenuation of peak
flows due to the upstream railroad culverts, but the study appears to include the railroad culvert on
Morey Creek (916.2 and 1024.7). Please clarify.
7. Please provide all HEC files to engineering review upon the next submittal.
8. The stream cross - sections with culverts indicate a manning coefficient of 0.045. Please confirm
that this manning coefficient was not used for the culvert.
9. In three instances, the application shows a trail underneath a road crossing. However, at each
crossing all culverts are proposed at the same elevation. For the culverts used by the trails in dry
weather conditions, please elevate the invert at least 3ft. Engineering review will likely
Current Development
Engineering Review Comments
Page 2 of 2
recommend that the trail culverts be lighted if they continue to be shown as a pedestrian crossing
of the connector road. A crossing at street level may be more acceptable to the community. This
aspect of the plan should be discussed further at the Planning Commission. Access from the road
sidewalks to the trails should be provided throughout the plan.
10. The VDOT structure and bridge office must tentatively approve the design of in order crossing for
county engineering review to recommend approval to these Special Use Permit applications
because a recent VDOT comment has noted that the design of the crossing may change based on
their review.
11. Please note that the Thomas Jefferson Planning District Commission is in the process of updating
the traffic model for the county and the city. They expect to have results from their study by the
end of March. Engineering review recommends that the application be deferred until input from
the Thomas Jefferson Planning District Commission can be considered by staff.
File: El_ sp_PBC_SP- 2010 -00044 SP- 2010 -00045 Granger Stream Crossings.doc