HomeMy WebLinkAboutSDP201600025 Study 2016-04-27 ',NW?
O WATERWAYS
Ecological Restoration Design - Civil Engineering - Natural Resource Management
Hydrologic and Hydraulic Analysis
for
Briarwood Development, Charlottesville VA
Prepared for Collins Engineering
April 27, 2016
BACKGROUND
The proposed project is located along the North Fork Rivanna River adjacent to St. Ives Road and
approximately 1,100 linear feet upstream of the Route 29 crossing. The proposed project calls
for fill within Zone AE of the FEMA Flood Insurance Rate Map(FIRM)for Albemarle County and
the City of Charlottesville(Map#: 51003C0145D). The proposed fill area is located outside of
the regulatory floodway. Waterways Consulting conducted a basic hydraulic analysis of the
North Fork Rivanna River in the vicinity of the project to determine impacts, if any,to the 100-
year base flood elevations.
HYDROLOGY
Table 3 of FEMA Flood Insurance Study(FIS)for Albemarle County and the City of Charlottesville
(Study#51003CV000B)states that the 100-year peak discharge for the North Fork Rivanna River
is 30,000 cubic feet per second upstream of Preddy Creek. This value was used for the
hydraulic analysis.
HYDRAULICS
A HEC-RAS hydraulic model was developed to compare base flood (100-year) elevations for
existing conditions and proposed grading conditions. The following model components were
used in this analysis.
Cross Sections
2-foot contours, provided by Collins Engineering,were used for the north side of the river.
This data was supplemented with 4-foot contours, acquired from the County of Albemarle's
on-line GIS website,for the south side of the river. Cross section locations were selected
based on channel topography and the proposed project location (See Figure 1). Eight cross
sections were used to develop the hydraulic model. The following table shows the model
cross sections which correspond with FEMA cross sections.
Model Cross Section FEMA Cross Section
XS-2 AE
XS-6 AF
XS-8 AG
509A Swift St,Santa Cruz,CA 95060, Ph:831-421-9291 // 1020 5W Taylor St.,Ste 610,Portland,OR 97205, Ph:503-227-5979
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WATERWAYS
CONSULTING, INC.
The Route 29 crossing downstream of the proposed project was not modeled as part of this
analysis. Roughness values (Mannings n)for existing conditions were set at 0.06 for within
the channel and 0.06 for overbank areas, per Table 4 of the FIS. These roughness values
were maintained for proposed conditions; except at fill locations where the roughness was
set at 0.03 to match anticipated vegetation conditions (grass).
Flows
The 100-year flow value of 30,000 cfs was used for the entire reach of the model. Upstream
boundary conditions were set based on a channel slope (Normal Depth)equal to 0.3%.
Downstream boundary conditions were set as a known water surface of 394.5'.
RESULTS
The results of this analysis show no appreciable rise in the 100-year base flood elevations due to
proposed fill within the floodplain. HEC RAS flood profiles,cross sections, and output table have
been provided in Appendix A.
Ecological Restoration Design — Civil Engineering — Natural Resource Management
',�► WATERWAYS
CONSULTING, INC.
APPENDIX A
Ecological Restoration Design — Civil Engineering — Natural Resource Management
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