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January 12, 2017 Ms. Georgia Gray, Chairperson Village of Voorheesville Planning Commission 29 Voorheesville Avenue Voorheesville, NY 12186 RE: Stewart’s Shops – 112 Maple Avenue Floodplain Evaluation CHA Project No.: 31753 Dear Ms. Gray: CHA Consulting Inc. (CHA) performed a Floodplain Evaluation in response to the following comment raised in a letter dated December 22, 2016 from Lauren Sherman of C.T. Male Associates: “The Applicant shall provide a quantitative analysis of any potential impacts the proposed fill in the floodplain (outside of the floodway) could have on adjacent properties.” This letter summarizes the scope and methodology utilized in CHA’s evaluation along with our findings and recommendations. Background The proposed project includes the construction of a new Stewart’s in the Village of Voorheesville (Village), Albany County, New York (See Figure 1 – Location Map). The proposed site is located along Vly Creek just southeast of the intersection of Route 85A (Maple Ave) and Altamont Road. A review of the effective Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) for Albany County (March 2015), indicates that Vly Creek has been studied by detailed methods (Zone AE), and as such a base (100-year) flood elevation (floodplain) and floodway have been established through the project site (See Figure 2 – FEMA Map). Although the most recent hydrologic and hydraulic analysis for Vly Creek was completed in 1980, the floodplain was remapped on updated topography during the most recent countywide revision. For the purposes of the National Flood Insurance Program (NFIP), the concept of a floodway is used to assist in the matter of flood management. Under this concept, the 100-year floodplain is divided into a floodway and a floodway fringe (floodplain). The floodway is the area of the channel, which must be kept free of encroachment so the 100-year flood can be conveyed while limiting the increase to flood elevations. As a minimum standard, FEMA limits this increase to one foot. The requirements pertaining to floodplain management contained in the FEMA Flood Damage Prevention Guidelines require that the following conditions be met: Any structures and/or fill associated with the proposed work cannot raise upstream 100-year flood elevations more than a foot at any point. All encroachments including fill, new construction, substantial improvements, and other development are prohibited within the limits of the floodway unless a technical evaluation demonstrates that such encroachments shall not result in any increase in floodway water surface elevations during the occurrence of the base flood. Ms. Georgia Gray, Chairperson January 12, 2017 January 9, 2017 Village of Voorheesville Planning Commission Page 2 Page 2 The proposed project includes the placement of 4,000 cubic yards (cy) of fill within the 100-year floodplain. Since no fill is being placed within the floodway, a detailed floodplain analysis is not typically warranted; however, CHA conducted an evaluation in response to a comment made by C.T. Male Associates regarding potential flooding impacts to adjacent landowners. A detailed explanation of the floodplain evaluation is provided below. All elevations provided below are based on unobstructed flow and are referenced to the North American Vertical Datum of 1988 (NAVD88). Hydrology The peak discharges for Vly Creek were developed for the FIS using the United States Army Corps of Engineers (ACOE) Regional Frequency Study. Based on the FIS, the 100-year peak discharge at the Route 85A Bridge, located immediately upstream of proposed site was 2,330 cubic feet per second (cfs). Since this flow was used in the development of the effective FIS, CHA utilized this value for the floodplain evaluation described below. Supporting files from the FIS can be found in the Technical Appendix. Hydraulic Model Since FEMA studied Vly Creek by detailed methods, CHA contacted the New York State Department of Environmental Conservation (NYSDEC) and requested a copy of the FIS backup data. Although the 1980 backup data could not be obtained, CHA believes that the original model is likely outdated. According to bridge inspection reports, both the Route 85A and Stonington Hill Road Bridges were replaced in 1998 and 1999, respectively. Additionally, detailed topographic data (LiDAR) was released for Albany County in 2008. Therefore, CHA developed a hydraulic model using the ACOE Hydrologic Engineering Center River Analysis System (HEC-RAS) software (Version 5.0.3). The modeled reach begins approximately 500 feet upstream of the Route 85A Bridge and extends downstream approximately 1,200 feet, ending near the Saint Matthew’s Church (FIS Section H). The cross section geometry for the model was developed from the 2008 Albany County LiDAR dataset using the HEC-GeoRAS extension for ArcMap (See Figure 3 – Cross Section Map). The LiDAR was supplemented with survey data of the channel and bridge, which was collected by GYMO Architecture, Engineering and Land Surveying, P.C. in December of 2016. Using the HEC-GeoRAS extension, CHA modeled all structures within the floodplain using blocked obstructions. In order to account for the potential losses associated with the Route 85A Bridge, the contraction and expansion coefficients for cross sections around the bridge were updated to 0.3 and 0.5, respectively. Finally, a known water surface elevation from FIS Section H (Floodway Data Table) was utilized as the downstream boundary condition. Using the existing condition model, CHA compared the 100-year water surface elevations from the model with those published in the FIS. In general, the water surface elevations from the model were found to be similar (within 0.5 feet) to those published in the FIS. Since the original study was completed in 1980, CHA believes that the minor differences in the water surface elevation are likely the result of updated station elevation data and localized changes to the channel and overbanks (development) over the past 36 years. CHA then developed the proposed condition model by revising the existing condition to reflect the proposed grading shown in the 2016 Design Plans. Changes were limited to cross-sections 614 and 732 and represent the fill Ms. Georgia Gray, Chairperson January 12, 2017 January 9, 2017 Village of Voorheesville Planning Commission Page 3 Page 3 associated with the installation of a retaining wall and the expansion of on-site parking. Detailed modeling output and a comparison of the existing and proposed cross sections can be found in the Technical Appendix. Model Results Based on review of recent aerial imagery and the LiDAR dataset, 2 residential structures were identified and used in to evaluate potential incremental flooding impacts. The structures are located on the opposite bank, immediately upstream of the project site and are shown as blocked obstructions in model cross-sections 833 and 896. Based on the results of the existing and proposed HEC-RAS model (floodplain), both of these structures are located above the 100-year (floodplain) water surface elevation. Additionally, the maximum increase in water surface elevation associated with the addition the proposed fill is less than 0.1 ft, which is attenuated to 0.0 ft at the upstream limits of the model. As such, CHA does not anticipate any incremental impacts due to the proposed project. A summary of the water surface elevations and modeled finished floor elevations is provided in Table 1 on the following page. Supporting documentation and model output can be found in the Technical Appendix. Table 1 - 100-year Floodplain Results Water Surface Elevation Modeled Finished Floor 100-year Floodplain Existing (ft) Proposed (ft) Elevation (ft) XS 1292 (School) 332.2 332.2 328.8 XS 896 (House) 330.7 330.7 331.9 XS 833 (House) 330.7 330.8 331.9 XS 732 (House) 330.7 330.6 331.7 XS 614 330.5 330.5 N/A Although the proposed fill is to remain outside of the regulated floodway, CHA utilized the existing and proposed model, explained above, to develop a floodway analysis. Since the original backup data was not available, CHA determined the effective floodway limits in ArcMap using the National Flood Hazard Layer (NFHL) and maintained them in both models. Based on a comparison of the existing and proposed condition, the fill associated with the proposed project does not increase the floodway elevations through the modeled reach. Table 2 provides a hydraulic summary for the 100-year floodway evaluation. Detailed results can be found in the Technical Appendix. Table 2 - 100-year Floodway Results Water Surface Elevation Modeled Finished Floor 100-year Floodway Existing (ft) Proposed (ft) Elevation (ft) XS 1292 (School) 333.0 333.0 328.8 XS 896 (House) 331.4 331.4 331.9 XS 833 (House) 331.4 331.4 331.9 XS 732 (House) 331.3 331.3 331.7 XS 614 331.2 331.2 N/A Ms. Georgia Gray, Chairperson January 12, 2017 January 9, 2017 Village of Voorheesville Planning Commission Page 4 Page 4 Conclusion As explained above, the proposed project includes the addition of approximately 4,000 cubic yards of fill within the 100-year floodplain of Vly Creek. Although the fill is located outside of the regulated floodway, a floodplain evaluation was conducted to determine if there are incremental impacts to adjacent properties. Based on the results of the analysis, the maximum increase in water surface elevation associated with the addition the proposed fill is 0.1 ft, which is attenuated to 0.0 ft at the upstream limits of the model. As such, the proposed project is not expected to result in incremental impacts to adjacent properties and is therefore consistent with FEMA Flood Damage Prevention Guidelines. If you have any questions regarding the results of the analysis please do not hesitate to contact me at (518) 453- 4540. Sincerely, Kris Detlefsen, P.E.