FW-WHRI 13(1) Hydraulics Memo.pdf

FHWA: W.Hiciano (6218) :zc 3/7/2018 M:\PROJECTS\_FWS\WHRI\13(1)\Techserv\Hydra\Reports\WHRI 13(1) Hydraulics Recommendations Memo.docx 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: FWS-WHRI 13(1): Updated Hydraulics Memo

TO MESSAGE/COMMENT FROM/DATE

Ramesh Kotadia Project Manager

Shoukat Nawaz Highway Design Manager

We have updated our hydraulic analysis for this project per the Fish and Wildlife Services (FWS) request at the Plans-In-Hands (PIH) meeting. The FWS had requested to increase the size of the culvert for easier maintenance access to the culvert, due to the presence of beavers in this area. Beavers are a serious problem for culvert pipes as they create dams that impound water against the roadbeds, which may flood or wash out roads. Increasing the rise of culvert would facilitate better maintenance access.

We have completed the analysis for the existing 36-inch double barrel culvert located within the Dale Bumper White River National Wildlife Refuge in the Arkansas County, Arkansas. The culvert crossing, located at Station 60+26.04, had been identified by the FWS as a point with frequent overtopping. The culvert crossing is located on a two-way aggregate road that provides access to a camping area and a boat landing at the end of Jack’s Bay Landing Road. There are wetlands on both sides of the road. The area has been designated as high risk area with high incident of floods by the Federal Emergency Management Agency (FEMA) in their Flood Insurance Rate Map (FIRM). Due to the frequent flooding (usually from 8-10 feet of inundation at times) the Refuge requested increasing the size of the culverts to mitigate overtopping at this location. Please refer to Attachment 1 for a FIRM designation zone map.

Hydrologic and Hydraulic analysis were performed to determine the adequate size for the proposed culvert. A drainage area was delineated using Bentley Microstation with geo-referenced USGS quad maps into the software for the project site. The calculated drainage area for the site was

3.48 square miles and was verified with the United States Geological Survey (USGS) StreamStats application Version 3. Hydrology was performed using USGS StreamStats application Version 3, due to the size of the drainage area.

For a summary of flowrates please refer to Table 1.

Table 1: Flow Rates

Site 2 yr (cfs)

10 yr (cfs)

25 yr (cfs)

50 yr (cfs)

100 yr (cfs)

Sta.

60+26.04

336 723 942 1110 1280

Wendy Hiciano Hydraulics Engineer

THROUGH

Abhishek Kapoor Acting Hydraulics Team Leader

March 7, 2018

FHWA: W.Hiciano (6218) :zc 3/7/2018 M:\PROJECTS\_FWS\WHRI\13(1)\Techserv\Hydra\Reports\WHRI 13(1) Hydraulics Recommendations Memo.docx CC: Hydraulics, Geotech, Env, HD Engineer, TS Engineer, Bridge

Hydraulic design was based on the 25-year storm in accordance the PDDM Chapter 7 for low standard roads, given that the posted speed is less than 50 mph. The hydraulic analysis for the existing structures were performed using HY-8, version 7.3

Sta. 60+24: 36 inch Double Barrel Culvert

Background and Observations:

The existing structure is a double 36 inches CMP culvert. The culvert is in an area with wetlands located on both sides of the road and several miles of drainage drains directly through the structure causing overtopping during periods of rain. The culvert receives flow from the upstream wetland, roadway ditches, and the flow from McPete Branch.

Given that this is a low speed and low volume road, replacing the culvert to completely prevent overtopping would require excessively increasing the size of the culvert. Increasing the size of the culvert would require increasing the road grade elevations, would impact the adjacent wetlands and additionally it would increase final project costs.

Recommendations:

Our recommendation will require a design exception due to minimum design standards not being met per PDDM. Recommendations provided on this memo are still based on the 25-year storm;

taking into account that overtopping will still occur. It is recommended to install a warning sign and water depth indicator to indicate possible flooding of the road and the water depth.

Riprap is not necessary for this site because current sites conditions do not show any signs of stream instability and/or any potential scour. We recommend replacing the existing structure with a 10 feet span by 5 feet rise Reinforced Concrete Pipe (RCP), and replacing the existing square edge headwall with 45 degrees skew wingwalls. See Table 2 for a summary of the proposed culvert data.

Table 2: Proposed Culvert Summary

Station Size (ft-in) Material Inlet El.

(ft)

Outlet El.

(ft)

Length (ft)

60+26.04 10’ span x 5’ rise RCP 148.190 147.87 25

The proposed recommendations will accommodate a 2-year storm event, with a headwater elevation of 153.41 feet, without any overtopping. Please refer to the Table 3 for a summary of the allowable headwater elevations.

Table 3: Allowable Headwater Elevations Event (Yr) Flow (cfs) AHW el. (ft) HW el. (ft)

2 336 153.55 153.33 25 942 153.55 155.84

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

wendy.hiciano Oval