Attachment 4: GLCA 6914 Predesign Material-Hydraulic Tech Memo

Technical Memorandum

GLCA Hydrologic and Hydraulic Memo
Date:

04/17/2025

Project name:

Glen Canyon Low Water Ramps - Hite

Task Order no:

140P2024F0155

Contract no:

140P2023D0006

Attention:

Mike Will, PE, Denver Service Center

Company:

National Park Service

Prepared by:

Douglas P. Stewart, PE

Reviewed by:

Thom Rutledge, PE, CFM

Copies to:

Aaron Swafford, PE

OBJECTIVE

This memo has been prepared to summarize the hydrologic analysis, parameters, and assumptions that were used to determine the peak flow rates to prospective culvert crossings and ditch sizing along the roadway within the Hite boat ramp area. Major basins were delineated to potential cross-culvert locations placed at natural low points along the roadway alignment. Other minor basins were delineated to calculate the amount of flow in roadside ditches tributary to the prospective culvert crossings.

HYDROLOGIC ANALYSIS

The offsite hydrology was completed using the SCS unit hydrograph method applied within the HEC-HMS model version 4.10. Hydrologic analysis for the ditch sizing was completed using the Rational Method. Subbasin Delineation and Topography
Major basins were delineated to estimate the concentrated flow at a potential culvert location. Other minor basins were delineated to estimate the flow that would drain directly towards the proposed roadway alignment and will need to be conveyed via ditches to one of the potential culvert locations. Major basins and ditch subbasin boundaries are shown on the Hydrology Map in Figure 1 included as Attachment 1 with this memo. The topography data used for the subbasin delineation included local LiDAR data flown for the project and the 10meter USGS DEM data. Using these contours, the basins were manually delineated to adjust for the proposed roadway alignment.
Rainfall Data (Frequency, Depth, Distribution)
The hydrologic calculations are completed for the 5-year, 10-year, 50-year, and 100-year storm events. The rainfall data is referenced from National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Volume 1, Version 5 Point Precipitation Frequency Estimate for Lake Powell, Utah, located at latitude 37.8806 and longitude -110.3709. The full NOAA report can be found in Attachment 2.

Technical Memorandum
The rainfall values corresponding to each storm event are provided in Table 1 below: Table 1: Rainfall Data
Return Period
(Yr)

Rainfall Intensity*
(inch/hr)
2.12
2.66
4.33
5.24

Rainfall Depth**
(inch)
1.21
1.43
1.99
2.26

*Rainfall Intensity corresponding to 5-minute time of concentration (tc) is listed and the rainfall intensity corresponding