HomeMy WebLinkAboutGore Creek Revegetation Project 11.4- Floodplain compliance.pdf • VER
AES TOHAT/0N.OAG
PO Box 248 Carbondale,CO 81623 (970)947-9568
September 14, 2016
Thomas Kassmel, PE
Town of Vail
Department of Public Works
Town Engineer
1309 Elkhorn Drive
Vail, CO 81632
Phone 970-479-2235
RE: Gore Creek Bank Revegetation Project 11.4, Floodplain Compliance
Dear Tom,
RiverRestoration has been working with the Eagle River Watershed Council and the
Town of Vail on the proposed bank revegetation on the north bank of Gore Creek, just
downstream of the skier bridge at Lion Square. The proposed bank restoration was
designed to convey future flooding events up to the 100 year flood event without
increasing modeled Proposed Conditions Water Surface Elevations (WSE) above the
modeled corrected effective Base Flood Elevations (BFE) within the construction reach.
This letter describes how the proposed project complies with Town of Vail and Federal
floodplain development guidelines.
TOV: 12-21-10: DEVELOPMENT RESTRICTED: (E) The Administrator may
require any applicant or person desiring to modify the flood plain by fill,
construction, channelization, grading, or other similar changes, to submit for
review an environmental impact statement in accordance with Chapter 12 of this
Title, to establish that the work will not adversely affect adjacent properties, or
increase the quantity or velocity of flood waters. (Ord. 16(1983) § 1: Ord.
12(1978) §4)
Hydrology
No additional hydrologic investigation was performed as part of this analysis. Hydrologic
input to the hydraulic models was taken from the Eagle County Federal Insurance Study
(FIS). No significant development has occurred in the Gore Creek watershed upstream of
the project since the most recent FIS was published. A 100-year discharge of 2,310 cubic
feet per second (cfs) was used to calculate water surface elevations through the Project
reach for existing and proposed conditions.
1
• VER
AES TOHAT/0N.OAG
L_ PO Box 248 Carbondale,CO 81623 (970)947-9568
Existing Conditions
The Effective HEC-RAS model for Gore Creek was obtained from the Town of Vail.
This model contains the geometry from the most current FEMA flood model for Gore
Creek within the project reach. Additional survey information was added to the Effective
HEC-RAS model to develop the Corrected Effective model. Four additional cross
sections were added to the Effective model in the immediate vicinity of the proposed
bank work to add the detail needed to more accurately model the proposed conditions.
The four cross sections were added between FEMA sections 148 and 149. These sections
included topographic survey data collected by RiverRestoration in 2016. These additional
sections captured important topographical information about the channel in the vicinity of
the project.
During the hydraulic evaluation of the site, we uncovered a discrepancy between the
FEMA Effective Base Flood Elevation line and the Corrected Effective Base Flood
Elevation line. The Effective FEMA 100-year inundation line extends to the edge of the
bike path but not over the bike path. However, the lawn on the north side of the bike
path, adjacent to the swimming pool at Lion Square, is at an elevation between two and
three feet lower than the bike path. Because the bike path does not qualify as a certified
flood levee for FEMA mapping purposes, the 100 year BFE inundation line should
extend onto the lawn. All areas of land north of the bike path and below the elevation of
the predicted 100-year water surface elevation should be delineated as FEMA flood zone
AE.
The 100-year BFE predicted from the Corrected Effective model is approximately 0.5
feet higher at two of the added cross sections than the interpolated water surface elevation
of the Effective model. This discrepancy is due to limited survey data resolution of the
Effective model in the area of the proposed project. The topography represented in the
Effective model is based on aerial and ground topography developed over long distances
in 1995. The Corrected Effective model includes a greater amount of local detail about
the existing bike path and the Gore Creek channel in the project reach. Although the
Corrected Effective BFE is slightly elevated above the Effective Model at two of the
added cross sections, the predicted water surface elevations still remain below the
elevation of the bike path. Therefore, whether or not the inundation limits of the 100-
year flood are changed to consider the bike path as an un-certified levee, the limits of
inundation of the 100-year flood resulting from the Corrected Effective model would
remain unchanged within the accuracy of the topographic survey (one foot). See Figure 1
for a comparison of 100-year limits of inundation.
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• UVER
ESTORATION.OAG
PO Box 248 Carbondale,CO 81623 (970)947-9568
Proposed Conditions
A Proposed Effective model was developed by modifying the Corrected Effective model
to represent the channel under proposed conditions. RiverRestoration surveyed the
completed project in August 2016 to collect the topographical and hydrographic data
required for this effort. The survey performed included updating cross sections
throughout the project reach along with detailed survey of the area of proposed bank
restoration.
To demonstrate compliance with Town of Vail and Federal floodplain development
guidelines, 100-year flood elevations calculated from the Proposed Effective model were
compared to the Corrected Effective model and the Effective BFEs published by FEMA.
The calculated proposed 100-year water surface elevations are less than Corrected
Effective values; both the Corrected Effective and Proposed Effective models predict
slightly increased BFEs resulting from increased survey detail when compared to the
Effective model. See Table 1 and Figure 2 for a tabular and graphical comparison of
these water surface elevations.
Table 1 Base Flood Elevation Comparison-100 year Flood Elevations(2,310cfs)
Corrected Proposed Modeled Difference
Cross Effective Effective Effective Between Proposed Effective
Model BFE
Section Model BFE Model BFE Model and Effective Model
(feet) (feet) (feet) (feet)
149.5 8103.76 8103.76 8103.76 0.00
149 8095.52 8095.52 8095.52 0.00
148.7(1) 8094.82(2) 8094.77 8094.76 -0.06
148.6(1) 8094.21(2) 8094.71 8094.69 0.48
148.5(1) 8093.65(2) 8094.05 8094.02 0.37
148.4(1) 8093.02(2) 8092.73 8092.73 -0.29
148 8087.96 8087.96 8087.96 0.00
(1)Cross sections added to represent increased detail for existing conditions
(2)Water surface elevations interpolated between sections
Conclusion
Three conclusions were able to be determined from the comparison of BFEs predicted by
the three models referenced in this letter (Effective, Corrected Effective, and Proposed
Effective).
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• VER
AES TOHAT/0N.OAG
PO Box 248 Carbondale,CO 81623 (970)947-9568
1) The Effective FEMA floodplain model under-predicts the extent of inundation of
the 100-year water surface within the project reach due to water surface elevations
being elevated above ground elevations adjacent to the bike path, which is not a
certified levee. The limits of the 100-year floodplain from the current FEMA
Effective model should inundate the lawn north of the bike path, adjacent to the
pool at Lion Square Lodge. These limits of inundation would remain unchanged
when compared to the Corrected Effective model within the accuracy of the
topographic survey (one foot).
2) The current FEMA floodplain model under-predicts 100-year water surface
elevations within the project reach of approximately 0.5 feet at two of the added
cross sections due to limited survey data resolution of Effective model.
3) The proposed project is not anticipated to increase 100-year water surface
elevations above the Corrected Effective model.
The results of this analysis demonstrate that the Proposed Conditions is not anticipated to
adversely affect adjacent properties or increase the elevation or quantity of flood waters
over existing conditions. RiverRestoration recommends that the Town of Vail leave the
Base Flood Elevations for the project reach at their current elevations and consider re-
zoning the lawn north of the bike path to floodway zone AE unless the elevation of the
lawn is raise above the Base Flood Elevation predicted at that location.
If you have any additional questions or comments in regard to this Floodplain
Compliance Report,please feel free to contact me at your convenience.
Sincerely,
_�c
41,
1/
Scott Prins P.E.
River Engineer
scott.prins@riverrestoration.org
(970) 947-9568
Attachments:
Gore Creek Bank Revegetation Project 11.4— 100-year Water Surface Inundation Limits
Comparison
Gore Creek Bank Revegetation Project 11.4— 100-year Water Surface Profile
Comparison
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Figure 1:Gore Creek Bank Revegetation Project 11.4—100year Water Surface Inundation Limits ComparisoN
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Figure 2:Gore Creek Bank Revegetation Project 11.4— 100year Water Surface Profile Comparison
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