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HomeMy WebLinkAboutSP201000045 Review Comments Special Use Permit 2011-08-30*-&A County of Albemarle Department of Community Development Memorandum To: Megan Yaniglos, Current Development Project Planner From: Phil Custer, Current Development engineering review Date: 30 August 2011 Subject: Granger Subdivision Stream Crossings (SP- 2010 -00044 and SP- 2010 - 00045) Engineering has reviewed the first revision to 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. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Comment has been withdrawn. 3.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. (Rev. 1) Table now currently prints correctly. 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. (Rev. 1) The 100 year discharge used in this analysis is not acceptable. The FEMA study performed 30 years shows a discharge -10% higher than the number currently used by the applicant. In those 30 years much of the watershed has developed so the actual discharge is likely much higher. In 1996, Albemarle County and the City of Charlottesville sponsored a joint study of the watershed which calculated an existing 100 year peak flow for this approximate point along Moore's Creek of 7,273cfs. The study also looked at future land uses in the watershed and determined that with the existing comprehensive plan at that time the likely 100 year discharge would be 7,837cfs. These numbers appear to be more reasonable than 4,050cfs. Please adjust the calculations accordingly based on the latest accepted comprehensive plan. 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. (Rev. I) This is acceptable. 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 Current Development Engineering Review Comments Page 2 of 2 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. (Rev. 1) Please refer to Comment 7. 5. The cross - sections that are analyzed with this plan should use the stationing of the existing FEMA study. (Rev. 1) Comment has not been addressed. 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. (Rev. I) Comment has been addressed. 7. Please provide all HEC files to engineering review upon the next submittal. (Rev. 1) Engineering offers the following comments after reviewing the first submittal of the HEC-RAS file: a. As mentioned previous in Comment 3, the county had a study done for Moore's Creek in 1996, which has a HEC2 model that appears more recent that the existing FEMA model. It has different flow values, and updated details. This HEC2 file should probably be the basis for further analysis within this watershed. b. The calculations should be updated using a higher, more realistic 100 year discharge. (Please see comment 3.) c. When opening the applicant's file, immediately there is an error message stating that files StreamCrossings.g02, StreamCrossingsf01, and StreamCrossingsp02 could not be found. Please explain the ramifications of this error message. d. The county engineer is currently reviewing these calculations and additional comments may be sent to the applicant. 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. (Rev. 1) Comment has been addressed. 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 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. (Rev. 1) Trails no longer are placed through culverts underneath roadways. However, the alignment of the paths shown on the plan is of concern to staff in the following ways: a. The paths run perpendicular to steep contours throughout the property. This path will be located in one of the county's more urban areas and it is expected to be highly utilized. The alignment of the path should be designed to meet Class A Type II which includes specific grade restrictions. b. The construction of the trail cannot fill in the floodway. Please remove the crossing of Moore's Creek downstream of Culvert 1. For the trail crossings of the secondary tributaries (southeast of Sunset Court and downstream of pond 4) move the trail crossing outside of the floodway. All other areas of the trail that are located within the floodway must be constructed using cut. c. The trail terminates before the property line southwest of Sunset Court. Please extend this trail to the existing pavement. 10. The VDOT structure and bridge office must tentatively approve the design of in order crossing for Current Development Engineering Review Comments Page 3 of 3 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. (Rev. 1) Comment has been acknowledged by the applicant. 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. (Rev. 1) The Thomas Jefferson Planning District Commission appears to have dropped the update to the regional traffic model so additional input will not be given. Absent of this technical input, I recommend that the applicant request to schedule a work session for the project to discuss all remaining concerns and discretionary items. This should be done, however, once all third parties have signed off on the application (Dominion Virginia Power and the owner of the property to the northeast). File: E2_sp_PBC_SP- 2010 -00044 SP- 2010 -00045 Granger Stream Crossings.doc