Hyd_NM152.pdf

HILLSBORO, NM

Prepared by:

Megan Frye

Hydraulic Engineer

Central Federal Lands Highway Division

August 2018 ii

TABLE OF CONTENTS

Introduction

Project Background and Objectives

Site Visit

Bridge Design Criteria

Project Area Description

Hydrologic Analysis

Hydraulic Analysis

Proposed Condition Model

Summary of Recommendations

References

APPENDICIES

Site Visit Photos, Hydrology, Hydraulic Analysis Results

INTRODUCTION

This report provides a summary of the hydrologic and hydraulics analyses completed in conjunction with the proposed bridge design. The intent is to provide information to support the selected bridge type, size, and location.

PROJECT BACKGROUND AND OBJECTIVES

The Central Federal Lands Highway Division, in cooperation with New Mexico Department of Transportation and the Gila National Forest, plan to construct two new bridges along State Highway 152 between Hillsboro and Kingston, New Mexico. New Mexico 152 is a major route between Interstates 25 and 80. It also serves as a major route providing access to the Gila National Forest. Figure 1 shows the project location on Google Earth. The site can be referenced by the following geographic coordinates:

#1520: Lat. 32.919° Long. -107.650°

#1521: Lat. 32.919° Long. -107.679°

Figure 1. NM 152 Bridges– Project Location Map

Both bridges were built in 1929 and are single lane bridges. The existing structures have height and width restrictions and have major conditions rating of 4 or “poor”. The bridges have been reduced to operational rating of HS-22 which restricts the weight of vehicle allowed across the bridge.

The east bridge (#1520) is approximately 5 miles west of Hillsboro, NM and crosses Percha Creek. The proposed bridge will be constructed immediately downstream of the existing bridge and the existing bridge will remain in place. The west bridge (#1521) is approximately 7 miles west of Hillsboro, NM and crosses Middle Percha Creek, a tributary to Percha Creek. The proposed bridge will be constructed in the same location of the existing bridge and the existing bridge will be removed. The new structures will provide adequate width to accommodate 2 lanes of traffic with 5 foot shoulders.

Figures 2 and 3 show the aerials from Google Earth at each location for reference.

Figure 2. Bridge #1520 Aerial Photo

Figure 3. Bridge #1521 Aerial Photo

The objective of this project is to evaluate the hydraulic conditions and design a new bridge with sufficient capacity to meet the design criteria.

SITE VISIT

The Central Federal Lands Highway Division (CFLHD) hydraulics engineers visited the project site on December 19, 2016 to review the existing site conditions and define the scope of work for the design of the replacement bridges.

BRIDGE DESIGN CRITERIA

New Mexico 152 and the proposed bridges provide bicycle and vehicle access to the Gila National Forest and provide a critical link between Interstates 25 and 80. The average daily traffic is 256 vehicles. The CFLHD capacity design standard for bridged waterways is that the structure must convey a 50-year hydraulic design flood with a minimum of 2 feet of freeboard below the low chord. The foundation stability design standard is the 100-year scour flood with a 200-year scour check flood.

Based on discussions with the cross functional team and subsequent geotechnical investigation, the bridge abutments will be founded on competent rock. As such, there is not a potential for scour to impact the foundations and no scour analysis was required.

PROJECT AREA DESCRIPTION

Both bridges were built in 1929 and are single lane bridges. The existing structures have height and width restrictions and have major conditions rating of 4 or “poor”. The bridges have been reduced to operational rating of HS-22 which restricts the weight of vehicle allowed across the bridge.

East Bridge (#1520) The east bridge (#1520) is a simple span 100-foot steel truss bridge with concrete abutments and a treated timber deck overlaid with asphalt. The existing bridge low chord (5735.8’) provides a minimum of 21 feet of freeboard for the 50-year design event. Figure 4 shows the existing bridge from upstream viewpoint.

Figure 4. Bridge #1520

The roadway crosses the channel perpendicular to flow during the 50-year event. The channel through the bridge section is confined with tall rock walls on either side. The floodplain upstream and downstream of the bridge provides more area for floodplain flows during events higher than the 2-year event. Even with the constriction in flow through the bridge, the low chord elevation of the bridge is a minimum of 16 feet above the 200-year water surface elevation.

The channel bed material is comprised of small cobbles and boulders. The floodplains upstream and downstream of the bridge have primarily grassy vegetation with scatter mature trees throughout. The floodplains also have cobble surface material similar to the channel. The upstream floodplain is shown in Figure 5.

Figure 5. Floodplain upstream of Bridge #1520

West Bridge (#1521) The west bridge (#1521) is a simple span 100-foot steel truss bridge with concrete abutments and a treated timber deck overlaid with asphalt. The existing bridge minimum low chord elevation (5958.6’) provides over 7 feet of freeboard for the 50-year design event. Figure 6 shows the existing bridge from upstream viewpoint.

Figure 6. Bridge #1521

The channel bed material is comprised of small cobbles and boulders. The floodplains upstream and downstream of the bridge have primarily grassy vegetation with scatter mature trees throughout. The floodplains also have cobble surface material similar to the channel.

The channel through the bridge section is confined one the right side by a sloped rock face through the bridge section. Immediately upstream of the bridge, the left floodplain is confined by a rock outcrop.

Downstream of the bridge, the right floodplain is confined by rock outcrops while with left is confined by the roadway embankment. Figures 7 and 8 show the rock outcrops upstream and downstream of the bridge.

Figure 7. Rock outcrop upstream of Bridge #1521

Figure 7. Rock outcrop downstream of Bridge #1521

HYDROLOGIC ANALYSIS

A flood frequency analysis was performed using USGS regional regression equations for New Mexico Region 7 (SW Desert Region). The regional regression equations are based on drainage area. The drainage areas for each location are shown in Table 1 below.

Location Drainage Area (square miles)

Bridge #1520 21.4 Bridge #1521 9.5

Table 1. Drainage Areas

A summary of the discharges calculated using the regional regression equations for each location are shown in Table 2.

Flood Event Bridge #1520

Peak Flow (cfs)

Bridge #1521 Peak Flow

(cfs) 2-year 510 410

5-year 900 800

10-year 1,270 1,130

25-year 1,900 1,160

50-year 2,490 2,130

100-year 3,190 2,660

200-year 4,490 3,620

Table 2. Peak Discharges

The USGS collected flow data at a gage adjacent to Bridge #1520 from 1953 to 1993. While the data collected was not used to estimate peak flow using a Log-Pearson Type III analysis, the data collected at the gage was used to develop a weighted estimation using the regional regression equations. The weighted regression equation estimates use the observed peak discharges at the gage and the calculated discharge along with the number of years the gage was in operation to determine the best estimation of the peak discharges.

HYDRAULIC ANALYSIS

Hydraulic modeling of both bridges was performed using the Surface-water Modeling System, Sedimentation and River Hydraulics 2D (SMS-SRH2D) Version 12.2.10 provided by Aquaveo. At both locations, the existing bridge was modeled. For Bridge #1520, the combination of the existing and proposed bridges was modeled. For Bridge #1521, the proposed bridge was modeled in place of the existing bridge.

HYDRAULIC MODEL – BRIDGE #1520

The existing and proposed conditions at Bridge #1520 were modeled in SMS for the 50, 100, and 200-year design flows. The water surface elevations at the upstream face of the existing bridge are shown in Figure 8.

Figure 8. Upstream Cross Section – Proposed Bridge

As shown in Figure 8, the average water surface elevation for the 50-year flow is 5716.3 feet. The minimum low chord elevation on the existing bridge is 5735.8 feet. The minimum low chord elevation for the proposed bridge is 5732.85. Both bridges have adequate hydraulic capacity and allow for over 16 feet of freeboard.

A scour analysis of the bridge foundations was not completed as the abutments will be founded on competent, non-scour-able rock. Reference the Final Geotechnical Report for additional information.

HYDRAULIC MODEL – BRIDGE #1521

The existing and proposed conditions at Bridge #1521 were modeled in SMS for the 50, 100, and 200-year design flows. The water surface elevations at the upstream face of the existing bridge are shown in Figure 9.

Figure 9. Upstream Cross Section – Proposed Bridge

As shown in Figure 8, the average water surface elevation for the 50-year flow is 5950.8 feet. The minimum low chord elevation on the proposed bridge is 5957.6 feet. The proposed bridge has adequate hydraulic capacity and allow for over 6 feet of freeboard.

A scour analysis of the bridge foundations was not completed as the abutments will be founded on competent, non-scour-able rock. Reference the Final Geotechnical Report for additional information. The abutments are also set outside of the 200-year flow as shown in Figure 10.

Figure 10. Flow Depth for 200-year event

A head cut has recently moved through the bridge section to approximately 50 feet upstream of the bridge. The head cut resulted in around 4 feet of channel degradation through the bridge section. This is evidenced by the perched floodplain in front of the left abutment shown in Figure 11.

Figure 10. Perched floodplain at left abutment of Bridge #1521

Because the abutments will be founded on rock, the additional degradation downstream of the bridge, that is anticipated to potentially move through the bridge, should not be a threat to the foundations.

The potential for migration of the channel away from the rock outcrop at the right abutment and towards the left abutment exists. Based on the surrounding geomorphology and the data gathered during the geotechnical investigation, competent rock similar to the rock outcrop at the right abutment is expected to limit the extent of the channel migration and impacts to the left abutment are not anticipated.

SUMMARY OF RECOMMENDATIONS

Bridge Configuration

Bridge Span Minimum Low Chord Elevation Bridge 1520 110 feet 5718.3 feet Bridge 1521 99 feet 5952.8 feet

REFERENCES

Aquaveo Surface Water Modeling System 12.2.10 (www.aquaveo.com).

Federal Highway Administration (2008) “Project Development and Design Manual” Federal Lands Highway Division. US Department of Transportation. Chap 7.

Waltemeyer, S.D.,2008, Analysis of the Magnitude and Frequency of Peak Discharge and Maximum Observed Peak Discharge in New Mexico and Surrounding Areas: U.S. Geological Survey Scientific Investigations Report 2008-5119