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HomeMy WebLinkAboutFDP201900006 Correspondence 2020-12-30® COLLINS ENGINEERING December 02, 2020 To: Albemarle County and Federal Emergency Management Agency Re: Pleasant Green Floodplain Study 200 GARRETT ST, SUITE K CHARLOTTESVILLE VA 22902 434.293.3719 PH 434.293.2813 FX www. col I i ns-engi nee ring.com Subject: Project Narrative for CLOMR-F for the Pleasant Green Residential Development Overview: The Pleasant Green residential development, located in Crozet just north of Jarman's Gap Road, consists of (7) development parcels and (50) existing residential lots. The total acreage of the development is 41.4 acres and proposes a total of approximately 250 units. The parcels are located along Powell's creek, between Cling Lane and Jarman's Gap Road. All the proposed residential development is located outside of the stream buffer and 100-year floodplain along Powell's Creek. The overall development of the property requires a connection (stream crossing) across Powell's Creek to Orchard Drive. A stream crossing was approved in 2018 for a road crossing that connected a portion of the property to Orchard Drive. Since this approval of the stream crossing, the subject property area has increased to include the development of the adjacent land to the east, which is now part of the 41.4 acres on the subject properties. With the additional property, a new location for the stream crossing is being proposed that reduces the overall impacts to the stream buffer, floodplain, and preserved slopes on the property. The relocation of the stream crossing required an amendment to the special use permit that was approved by the Albemarle County Board of Supervisors on September 161h, 2020. The new stream crossing also requires an amendment to the approved FEMA CLOMR-F. Floodplain Narrative: This amendment to the FEMA floodplain along Powell's Creek is a revision to the recently approved Conditional Letter of Map Revision. The previous case number is 17-03-1753R, and the CLOMR was approved on February 1, 2018. The box culvert and fill proposed with the 2018 CLORM-F was for a road crossing approximately 1,896 feet downstream of Cling Lane. This crossing was approved but not installed. The property that was utilizing the crossing has been added to the adjacent property downstream of this crossing, and a new location for the stream crossing over Powell's Creek is being proposed with the development. This new stream crossing, which is a more direct crossing of Powell's Creek, reduces the floodplain and stream impacts. The new crossing is approximately 2,856 feet downstream of Cling Lane along Powell's Creek. This amendment to the CLOMR is proposing 3 — new 10'Wx8'H Box Culverts and Fill for a road crossing over Powell's Creek. On September 16"', 2020, Albemarle County approved a Special Use permit for the relocation of the stream crossing on Powell's creek for the purpose of a road connection to Orchard Drive. An updated HEC-RAS model of Powell's Creek has been completed between Cross Sections F and M for the new location of the stream crossing with the box culverts and fill. The affected area is located on FEMA Map 51003C Panel 0229, effective date 2/4/05. The datum for the FEMA study is NAVD 1988. The approved CLOMR and HEC-RAS modeling from 2017 determined that the existing GIS topography along Powell's Creek in this section of the FEMA Mapping was not accurate. The approved CLOMR updated the modeling and topography between cross sections F and M on this portion of Powell's Creek, creating an effective model that used the FEMA-approved Hydrologic Model for peak flows and cross-section locations. The amendment to this approved modeling utilizes the same parameters (topography, cross -sections, and hydrologic model) as the 2017 approved study. The only revision with this study is the relocation of the box culvert and fill for the stream crossing approximately 960 feet downstream. As part of the approval of Special use permit (SP2019-00002) for the new stream crossing and box culverts, there must be a zero -rise in the 100-year floodplain elevation on all adjoining properties. As shown in the modeling, flood plain study, and reports, there is a zero -rise on all the adjoining properties. The floodplain is not increased on any of the adjoining properties. There is a maximum rise of 2.59 feet in the 100-year flood elevation immediately upstream of the crossing and slight rise downstream; however, the floodplain is located completely on the subject properties in the location of the 100-year elevation rise. As shown on the modeling, the 100-year floodplain elevation with the new crossing matches the existing floodplain elevations approximately 192.7 feet upstream of the crossing and 404.6 feet downstream of the crossing. The floodplain is entirely on the subject properties for this portion of Powell's Creek. The fill associated with the road crossing occurs within the floodplain fringe only. There is a zero -rise in the BFE associated with the proposed fill. There is a nominal expansion of the floodway limits due to the fill on the fringes of the floodplain, but this does not impact the BFE. In addition, the new location for the road crossing has been permitted through the USACOE. FEMA Hydraulic Analysis: The CLOMR-F study that was approved on February 1, 2018 by FEMA was used as the existing conditions or effective model for this amendment with the relocation of the stream crossing. A new HEC-RAS hydraulic model was created for both the existing conditions and the proposed conditions to analysis the effects of the stream crossing on the 100-year base flood elevation and floodplain along Powell's Creek. A terrain model was created with the updated topographic survey data from the approved 2018 CLOMR-F report and was used to provide the elevation data for the HEC-RAS model. In addition, (6) new cross -sections were created in the location of the stream crossing to accurately measure the effects of the crossing on the floodplain in this location. The results from the model estimated a maximum rise of 2.59 feet in the 100-year floodplain elevation immediately upstream of the proposed crossing. There was a slight rise downstream of the proposed crossing as well, mainly due to the modeling hydraulic jump with the decrease in velocity and geometry of the stream channel with the culvert crossing. As noted above, the stream crossing has a no -rise effect on any of the adjacent properties. Attached is the overall HEC-RAS report. Summary: This floodplain study is an amendment to the previously approved 2018 FEMA floodplain study. The stream crossing, which has been relocated downstream approximately 960 feet, has essentially the same effect on the floodplain along this portion of Powell's Creek. There is a slight increase in the floodplain upstream of the crossing, and within 200 linear feet upstream of the crossing the floodplain matches the existing floodplain along the stream. The is no change to the BFE with the stream crossing, and there is no -rise to the floodplain on any adjacent properties. A LORM will be submitted after the culvert and fill have been installed to the complete the FEMA map revision.