NE FTFW VALE 13(1) 100(1) Drainage Memo.pdf

CFLHD | NE FTFW VALE 13(1) & 100(1)

Drainage Memo hdrinc.com 1670 Broadway Suite 3400 Denver, CO 80202-4824

(303) 764-1520

Memo Date: Monday, July 06, 2020

Project: NE FTFW VALE 13(1) & 100(1)

To: FHWA – Central Federal Lands Highway Division. Dorothy Eisenbraun, Project Manager

From: Colleen Koenig, HDR

Subject: Hydrology and Hydraulics Recommendations

Introduction

This memo summarizes the hydrologic and hydraulic analyses performed for the Valentine National

Wildlife Refuge NE FTFW VALE 13(1) & 100(1) Auto Tour RTE #013 & RTE #100, located in Cherry

County, Nebraska.

The drainage design for this project was prepared in accordance with the Federal Lands Highway

“Project Development and Design Manual” (PDDM), Chapter 7 Hydrology and Hydraulics, December

2012.

The project includes 10 miles of roadway improvements, however the drainage analysis and improvements are limited to a single culvert installation at Little Hay Valley Trail. The proposed roadway improvements include adding the gravel to the existing 10-mile road surface to make the route more attractive to refuge visitors and to provide safe and accessible features for refuge visitors to utilize.

The drainage improvements for Little Hay Valley Trail includes the installation of an 18” RCP culvert proposed at an existing low point (STA 175+43) to relieve water currently ponding on the north side of the road during normal conveyance conditions and to mitigate overtopping during high water conditions on Dewey Lake.

Criteria

Drainage design takes into consideration the design criteria in the PDDM for Low-Standard roadway.

Criteria is outlined below in Table 1.

Table 1 Criteria Summary Table

Criteria was met for hydrologic calculations. However based on discussions with the Refuge and Central

Federal Lands an 18” RCP is proposed as a compromise between the 12” RCP recommended by the

Refuge and FHWA standard minimum size of 24”. The exception to the 24” minimum pipe size is being proposed due to the fact that the 18” culvert will allow both the 25-yr and 100-yr storms to drain without overtopping during the normal conveyance conditions and will improve existing conditions during high water situations while mitigating the required cost and roadway disturbance associated with the installation of a 24” culvert.

Hydrology

The rational method was used to develop peak flows. Soil information was acquired from the Natural

Resources Conservation Service (NRCS) and consist of Hydrologic Soils Group A. Point Precipitation

Frequencies were acquired from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14, volume 11, Version 2. Mapping has been included in Appendix A.

Basins were delineated using Lidar from the USDA Geospatial Data Gateway for Cherry County, NE. The basin tributary to the proposed culvert (Basin 1) has been delineated and analyzed. Basin Maps and hydrologic calculations have been included in Appendix B.

Hydraulics

The proposed culvert has been analyzed using developed flows and Bentley Flowmaster CONNECT for hydraulic capacity based on the 25-yr design storm and the 100-yr check storm.

Design Feature PDDM Criteria

Peak Flow Methods Rational Method

Design Storm Frequency Design: 25-yr

Check: 100-yr

Roadway Spread

Required Clear Width Limit the spread to half of one travel lane for gutter flow, both on-grade and in roadway sags.

Allowable depth Flow depth at the curb should not exceed the curb height or the allowable spread for the design discharge.

Culverts

Capacity Design and Stability 25-year flood

Minimum Size 24” or equivalent for cross road culverts

18" for parallel culverts in ditches and channels

Pipe Material RCP: Class II, Metal 0.064"

Outlet Protection Riprap

Hydraulic results of the culvert are summarized below in Table 2.

Table 2 Hydraulic Summary Table

Culvert Size

IN

Contributing

Basin

Storm Event Basin Runoff

CFS

Culvert Capacity

CFS

CV1 18” Basin 1 25-yr 0.47 10.50

CV1 18” Basin 1 100-yr 1.31 10.50

Flowmaster output reports have been included in Appendix C.

Summary

The proposed culvert will convey the 25-yr and 100-yr design storms without overtopping during normal conveyance conditions. The culvert, in combination with associated profile raise, will also mitigate overtopping during high water conditions at the location of concern. Existing drainage patterns will be perpetuated for the remainder of the project.

Appendix A

Maps

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