FinalScourMemo.pdf

FHWA:A.Kapoor:3549:ZC:M:\PROJECTS\maca\13(1)_14(2)\Techserv\Hydra\Correspondence\NP_MAC A 13(1),14(2)_Scour Memo CC: Hydraulics, Geotech, Env, HD Engineer, TS Engineer, Bridge

Form FHWA-201 U.S. DEPARTMENT OF TRANSPORTATION (Rev. 11-87) FEDERAL HIGHWAY ADMINISTRATION

MINUTE - MEMO

Use this form in lieu of transmittal slips within Dept. of Trans. when message comment is to be retained as file material. Do not prepare carbons. Not to be used in lieu of Form FHWA-121 for informal correspondence.

SUBJECT: MACA 13(1)14(2) Scour Analysis Memo

TO MESSAGE/COMMENT FROM/DATE

Michael Tessitore Project Manager

Marc Carruthers Design Manager

The project is located at the Green River ferry crossing within Mammoth Caves National Park, located in Edmonson county, KY. The scope of work involves extending the approach ramps into the Green River to accommodate safe operations of the ferry, since the water levels have receded due to a dam failure approximately 15 miles downstream, in Brownsville, KY. Our analysis included studying the effect of the ramp extension on the 100-year water surface elevation (WSEL), and computing scour for foundation design of the walls at the end of each ramp. We have completed our analysis for the subject project. Hydraulic analysis was performed using HEC RAS v5.0.3. Scour analysis was performed using Hydraulic Toolbox v4.3, per the equations for contraction scour as described in Chapter 6 of the Hydraulic Engineering Circular (HEC) 18.

For Hydrology, Stream Stats – state application for KY was used to compute the peak flows initially. However, the drainage area for the project exceeds the range of drainage areas applicable for regression equations for the state of KY. The drainage area for the project is 1980 square miles and the maximum area applicable for regression equations is 1299 square miles. Therefore, the results obtained from Stream Stats could not be used. The existing flood insurance study (FIS) - 21061CV000A was consulted to obtain flows for the project, but flows were not available for our project site. The closest data available was for the Brownsville dam, which is approximately 15 miles downstream of the Green River ferry crossing. The USGS gage # 03309000, just downstream of the ferry crossing, could not be used either since it did not have enough data points available to perform a Log – Pearson type 3 analysis; the gage was decommissioned in 1950. Per a conversation with the project manager, it was decided to use the flow data available for the Brownsville dam to come up with conservative estimates for the analysis.

Several alternatives had been developed to extend the ramp into the river.

The park selected alternative #2 where the ramps would be extended down to an elevation of 416.85 feet. This alternative was used to study its impact on the 100 year WSEL. A known WSEL of 470 feet was used as the downstream boundary condition for the hydraulic model. This WSEL was obtained from the FEMA flood insurance rate map (FIRM), panel # 21061C0190B. The difference in WSEL between existing and proposed conditions is 0.01 feet indicating negligible impact from the proposed conditions on the river.

Abhishek Kapoor Hydraulics Engineer

THROUGH

Libby Gress Hydraulics Team Leader

July 05, 2017

FHWA:A.Kapoor:3549:ZC:M:\PROJECTS\maca\13(1)_14(2)\Techserv\Hydra\Correspondence\NP_MAC A 13(1),14(2)_Scour Memo CC: Hydraulics, Geotech, Env, HD Engineer, TS Engineer, Bridge

Since no guidance exits for selecting a design storm for scour computation for a ferry project, a decision was made to use the 100-year storm. Scour computations for contraction scour resulted in a negative value indicating a no scour condition for the 100-year storm. However, we recommend coordinating with the park, and determining the depth to which they will be dredging the channel to facilitate the ferry operation. The foundation of the walls should be designed to extend at least a couple of feet below the dredged channel thalweg to account for long term degradation of the channel. Also, per our conversation, we recommend using gabion baskets to stabilize the eroded section of the banks adjacent to the ramps. The use of gabion baskets will limit the amount of disturbance into the stream. Please consult with Geotechnical team for the design of gabion baskets.

Please let us know if you have any questions or concerns.