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HomeMy WebLinkAboutFDP201900006 Letter of Map Revision 2023-03-10 (4)® COLLINS ENGINEERING February 08, 2023 To: Albemarle County and Federal Emergency Management Agency Re: FDP201900006 - Pleasant Green Floodplain Study — CLOMR-F FEMA Case Number: CLOMR 21-03-0459R 200 GARRETT ST, SUITE K CHARLOTTESVILLE VA 22902 434.293.3719 PH 434.293.2813 FX www.collin5-engineering.wm Subject: Project Narrative for CLOMR-F for the Pleasant Green Residential Development (FDP201900006) — Re -Submittal 4 Attachments: Attachment A— Work Map Exhibits Attachment B— HEC-RAS Model Results Attachment C— Water Surface Elevation Comparison Table Attachment D— Community Acknowledgment Form Electronic Files: Shapefiles (.shp), coordination system: NAD1983 State Plane Virginia South FIPS 4502 Feet: Proposed 0.2 Percent Annual Chance Flood Hazard Proposed 1 Percent Annual Chance Flood Hazard Proposed Regulatory Floodway Revised Powells Creek Centerline Effective 0.2 Percent Annual Chance Flood Hazard Effective 1 Percent Annual Chance Flood Hazard Effective Regulatory Floodway Effective Powells Creek Centerline HEC-RAS version 4.1.0 zip files: Duplicate Effective Existing Proposed Overview: The Pleasant Green residential development, located in Crozet, Virginia just north of Jarman's Gap Road, consists of seven (7) development parcels and fifty (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 in Crozet, Virginia. 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, Page 1 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 16, 2020. The new stream crossing also requires an amendment to the previously 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-F was approved on February 1, 2018. The box culvert and fill proposed with the 2018 CLOMR-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 new crossing, and a new location for the stream crossing over Powell's Creek is being proposed with this 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-F is proposing an approximately 500 linear -foot bridge and minor fill for the abutments and riprap at the bridge abutments to mitigate scour. These scour countermeasures were calculated by Endesco, and the corresponding impacts were entered into the floodplain model as part of this effort. 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 bridge span. The affected area is located on FEMA Map 51003C Panel 0229, effective date 02/04/05. The datum for the FEMA study is NAVD 1988. The approved CLOMR-F 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-F 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 similar parameters (topography, cross -sections, and hydrologic model) as the 2017 approved study. However, ultimately this study utilized the most recent LOMR model (affecting Cross Sections E and F) in addition to the survey/LiDAR data updated as of the 2017 study to complete the HEC-RAS modeling. As part of the approval of Special use permit (SP2019-00002) for the new stream crossing and bridge crossing, 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. When comparing the Existing and Proposed Models, it can be seen that the maximum difference in water surface elevation in the 100-year storm is 2.27 feet immediately downstream of the proposed bridge crossing. The rise is slightly higher downstream of the bridge than upstream of Page 2 the bridge since the higher velocities through culvert are inundating the downstream area. These high velocities flowing into the downstream cross-section cause a high peak stage and a tighter construction than the Con/Span. As shown in the modeling outputs in Attachment B, the 100-year floodplain elevation with the new crossing matches the existing floodplain elevations approximately 1040 feet upstream of the crossing and 600 feet downstream of the crossing. Through the modeling completed, it was found that there is a zero -rise in the Base Flood Elevation (BFE) associated with the proposed fill and bridge crossing on the adjacent properties. In addition to the Special Use Permit, the new location for the road crossing has been permitted through the United State Army Corps of Engineers (USACE). FEMA Hydraulic Analysis: The CLOMR-F study that was approved on February 1, 2018 by FEMA was initially used as the existing conditions (i.e. duplicate effective model) for this amendment with the relocation of the stream crossing. However, the effective model for LOMR Case No. 02-03-1533P effective June 23, 2021 was used instead. The entire model —Cross Sections A through N was used in the updated analysis. (The LOMR mostly impacted the culvert crossing at Jarman's Gap Road). Duplicate Effective The received LOMR model was copied, and a duplicate effective model was used in HEC-RAS version 4.1.0. The model was not truncated but the work map in Attachment A Figure 1 was truncated for easier viewing of the effective and proposed floodplain boundaries. The datum was confirmed to be in feet, NAVD88. The duplicate effective steady flow analysis file was provided by FEMA and further modifications to the hydrologic parameters were not performed as part of this study. The reach boundary conditions were provided with the model, as well. The boundary conditions were "Known Water Surface Elevations", which were retained in the analysis. Existing Model The geometry of the existing model is the same as the Effective model in this case. For the Existing model, the following revisions were made: Added Cross Sections 4345, 4253, 4064, and 3950 to the model that would, along with effective Cross Section 4149, form the bounding Cross Sections for the Proposed Model's Pleasant Green Phase 2 crossing. Proposed Model The Proposed Model used the Existing Model as a base. The only difference between the Existing and Proposed Model is the addition of a bridge at River Station 4149. This bridge spans the left and right overbank of Powells Creek tying in at the proposed elevations of the site development. The certified bridge design is contained in Attachment A Figure 3. The hydraulic opening of the bridge was modeled as a culvert in HEC-RAS with standard Con/Span dimensions and a rise/span Page 3 that match Attachment A Figure 3. The bounding river stations were adjusted to be at the appropriate distance to the proposed bridge per HEC-RAS guidance. Attachment A Figure 1 shows the proposed conditions model parameters overlaid on the aerial work map. Attachment A Figure 3 is the certified design of the bridge structure. Results: Attachment A Figure 1 shows the model results overlaid on aerial imagery with topographic contour data. Attachment A Figure 2A and 2B show the model results overlaid on FEMA Effective FIRM effective 02/04/2005 and the LOMR FIRM effective 06/23/2021, respectively. Figures 1, 2A, and 2B all show the 100-year (base flood), 0.2-percent chance annual flood and the floodway (encroached). The proposed development and location of the crossing is shown overlaid with the results. In Figure 1, the existing and proposed contours are shown for reference. Attachment B provides the HEC-RAS output for the Existing and Proposed Models including summary tables, profiles, and section views. The profile, summary tables and Cross Sections of the floodway results are also included in Appendix B . The tie-ins are shown at the upstream and downstream ends of the subject area. The bounding cross -sections are 4253 and 4064. The tie-ins when comparing the Proposed and Existing models is less than 0.1 vertical feet as seen in the model results in Attachment B. When comparing the water surface elevations between the Existing and Proposed there is a maximum rise of 1.99 feet upstream and 2.27 feet downstream due to the proposed bridge crossing. The hydraulic constriction of the Con/Span creates a rise upstream of the bridge, and the high velocities through the Con/Span create a high peak stage downstream of the bridge. A full comparison of the water surface elevations at each Cross Section for both models can be seen it Attachment C. Conclusions: • This floodplain study is an amendment to the previously approved 2018 FEMA floodplain study • The hydraulic opening was modeled as a Con/Span culvert in HEC-RAS with a rise and span to match the plans provided by Collins Engineering • The riprap scour mitigation measures provided by Endesco for Stanley Martin Homes were added as modeled obstructions to the proposed bridge bounding cross -sections • When comparing the existing to the proposed conditions, the obstructions created a maximum rise of 1.99 feet upstream of the bridge and 2.27 feet downstream of the bridge The 100-year floodplain elevation with the new crossing matches the existing floodplain elevations approximately 1,040 feet upstream of the crossing and 600 feet downstream of the crossing. • The annotated FIRM with the Existing and Proposed delineations overlaid on the latest FIRM for the development area can be found in Attachment A Figure 2A. • The annotated FIRM with the Existing and Proposed delineations overlaid on the LOMR 20-03-1533P FIRM for the development area can be found in Attachment A Figure 2B. • The signed Community Acknowledgment Form can be found in Attachment D. 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