Final_Hydraulics_Recommendations-FortOrd_Aug_2014.pdf

FINAL HYDRAULIC REPORT

GUIDOTTI PEDESTRIAN BRIDGE REPLACEMENT OVER EL TORO CREEK

FORT ORD NATIONAL MONUMENT, CA

CA BLM 19041(2)

Prepared by:

Laura Girard

Technical Services Hydraulic Engineer Central Federal Lands Highway Division

August 2014 ii

TABLE OF CONTENTS

Introduction

Project Background and Objectives

Site Visit

Bridge Design Criteria

Project Data

Project Area Description

Hydrologic Analysis

Hydraulic Analysis

Proposed Condition Model

Scour Analysis

Abutment Protection

Summary of Recommendations

References

LIST OF TABLES

Table 1. El Toro Creek Peak Flows (Monterey County FIS) Table 2. HEC-RAS Input Values for proposed Bridge Configuration Table 3. Hydraulic Modeling Results for the Proposed Bridge Table 4. Scour depths for the proposed bridge. (*RESULTS NOT FINAL)

LIST OF FIGURES

Figure 1. Guidotti Bridge – CA BLM 19041(2) Project Location Map Figure 2. Guidotti Bridge – CA BLM 19041(2) Close-up Location Map Figure 3. Approximate 4 foot headcut downstream of Guidotti Bridge (February 2010 Figure 4. Plan view of HEC-RAS hydraulic model cross section locations Figure 5. Water Surface Profiles for Existing and Alternative Bridge Spans Figure 6. Proposed Upstream Bridge Cross Section

APPENDICIES

Site Visit Photos, 2010 Fort Ord Roadways Drainage Report, Hydraulic Analysis Results

INTRODUCTION

This report provides an overview of the key project elements and summary of the hydrologic and hydraulic analyses that resulted in the selected proposed bridge design. The intent is to provide information that supports the selected bridge alternative and alignment for design.

PROJECT BACKGROUND AND OBJECTIVES

The Central Federal Lands Highway Division with cooperation with the Bureau of Land Management, plan to replace the existing Guidotti Bridge. The bridge is located on Guidotti Road, approximately 1/4 mile from the Badger Hills Trailhead visitor parking lots. Guidotti Bridge is a 125 foot long, 3 span, filled concrete spandrel arch structure that crosses El Toro Creek. The middle span was constructed in 1908 and 2 exterior spans constructed in 1911, as indicated by the stamps imprinted on the fascia of the arch.

The arches are supported by shallow spread footings that have been intermittently exposed along the bottom along the piers. The bridge railing consists of solid concrete vertical parapets. The width of the structure is approximately 22’; the structure is skewed about 15 degrees to the 50-year flow event. At the ends of the bridge, concrete wingwalls extend from the arch faces at varying skews and lengths. The bridge has failed due to scour and subsequent settlement of the shallow spread footings. The 2009 bridge inspection report mentions seismic activity as a possible contributor to failure of the bridge. Refill has occurred below the footings to restore grade where settlement “dropped” the arch segments.

The existing bridge is currently a one-lane bridge used by pedestrians, bikers, and equestrians to access recreation trails of Fort Ord National Monument. The inspection report classifies the bridge as critically deficient and recommends replacement; as the condition of the structure continues to deteriorate it will become necessary to close the structure to all traffic.

A previous drainage study was conducted in 2010 for this site by CH2MHill on behalf of CFLHD. This earlier study included a hydrologic analysis to determine peak flows at the existing bridge. Additionally, a hydraulic model was developed to assess the capacity constraints of the existing bridge. The hydrologic analysis and model was utilized as part of this study to identify potential bridge replacement alternatives to meet the design criteria. The 2010 Fort Ord Roadways Drainage Report by CH2MHill is included in Appendix B.

The objective of this project is to evaluate the existing hydraulic conditions and design a new bridge with sufficient capacity to meet the design criteria. For reference, the project is located in Fort Ord National Monument approximately 6.7 miles east of Del Rey Oaks, California. Figures 1 show the project location and surrounding area and Figure 2 presents an aerial map of the site provided by the BLM. The site can be referenced by the following geographic coordinates:

Lat. 36.582981°, and Long. -121.716017°.

Figure 1. Guidotti Bridge – CA BLM 19041(2) Project Location Map.

Figure 2. Guidotti Bridge – CA BLM 19041(2) Close-up Location Map.

Guidotti Bridge

SITE VISIT

The Central Federal Lands Highway Division (CFLHD) project team visited the project site on February 19, 2014 to review the existing site conditions and define the scope of work for the design of a replacement bridge. Photos from the site visit are included in the Appendix A.

BRIDGE DESIGN CRITERIA

Guidotti Road serves as a hiking, equestrian, and biking trail which is gated at the Badger Hills Trailhead parking lot from public vehicular use. CFLHD design standards for bridged waterways are a 50-year flood design, with a minimum of 2 ft of freeboard below the low chord. Stability design is for the 100-yr flood with a 500-yr check flood (SF≥1).

PROJECT DATA

Detailed mapping information of the project reach was collected as part of the earlier 2010 study. IN 2014 the CFL survey team provided supplemental ground surveys near the bridge that was supplemented into the older hydraulic model. The mapping products were provided by the survey team.

PROJECT AREA DESCRIPTION

The bridge is located on Guidotti Road, approximately 1/4 mile from the Badger Hills Trailhead visitor parking lots. Guidotti Bridge is a 125 foot long, 3 span, filled concrete spandrel arch structure that crosses El Toro Creek. The middle span was constructed in 1908 and 2 exterior spans constructed in 1911, as indicated by the stamps imprinted on the fascia of the arch. The arches are supported by shallow spread footings that have been intermittently exposed along the bottom along the piers. The 2014 field site visit noted undermining of existing bridge footings at the middle arch span and large settlement of this section.

The roadway crosses the channel at about a 15 degree angle during the 50-yr event and constricts flow through the existing bridge opening. For smaller events the skew is upwards of 45 degrees. At the ends of the bridge, concrete wingwalls extend from the arch faces at varying skews and lengths.

The active, ephemeral channel is contained in the middle arch section. The channel in the south and north arches contain a floodplain bench that is approximately 6 ft higher than the main channel.

Main channel bed material appears to be coarse sand. North arch also has silt/sand sediment in bed.

Side arch channel beds are composed of sand and silt. High water marks were observed one the outside of the side arches at about 1.5 ft higher than the bed.

There is heavy vegetation in the channel immediately upstream of the bridge with 6-8 saplings directly at the upstream face of the bridge and in the active channel. Downstream has less severe vegetation in the channel (mostly 12-18 inch plants).

A 4 foot head cut downstream of bridge is about 300- 350 downstream of bridge; much closer than the 1000 ft indicated in the 2010 Geomorphic Report. At present debris jam is holding head cut, see Figure 3.

Figure 3. Approximate 4 foot headcut downstream of Guidotti Bridge (February 2010).

HYDROLOGIC ANALYSIS

Previous hydraulic studies completed by consultants (CH2MHill 2010) evaluated the existing bridge capacity. The previous study included a review of published hydrology data for the drainage basin from the Monterey County Floodplain Management Plan and published FEMA flow data. Ultimately, they concluded that the FEMA flow data was higher and produced the most conservative results. A summary of hydrologic methods and results is included in Appendix B and a summary of the FEMA discharges are below in Table 1.

Flood Event Peak Discharge (cfs)

2-year 126 10-year 356 50-year 1,033 100-year 1,642 500-year 2,844

Table 1. El Toro Creek Peak Flows (Monterey County FIS).

HYDRAULIC ANALYSIS

Hydraulic modeling of Guidotti Bridge was performed using the Hydrologic Engineering Center River Analysis System (HEC-RAS) version 4.1.0 published by the US Army Corps of Engineers. Existing conditions of Guidotti Bridge were assessed in 2010, details can be found in the attached drainage study.

PROPOSED CONDITION MODEL

Based on coordination with the CFL bridge design team, the proposed replacement bridge is an approximately 100 feet long single span structure. A single span structure is preferred to minimize construction within the stream channel while maximizing available flow area. The 2H:1V sloping abutments are aligned normal to the bridge centerline. Total bridge width of 16’-0” is proposed. Based on a 2014 field survey, cross sections in the 2010 existing conditions model were modified immediately upstream and downstream of the proposed bridge. The existing bridge was removed from the model and the proposed conditions entered including 2H:1V sloping abutments. The proposed bridge configuration was modeled using a proposed low chord value provided by the bridge team and summarized in Table 3 below.

Bridge Low Chord Bridge Width (ft)

220.7 16.0

Table 2. HEC-RAS Input Values for proposed Bridge Configuration.

Values from Table 1 were used to model a mixed flow regime of the proposed bridge configuration. The main channel was modeled with a Manning’s n value of 0.035 while the overbanks are modeled with a Manning’s n value of 0.05 and the upper floodplain with a value of 0.08. Boundary conditions for upstream and downstream were determined by normal depth calculations with slopes of 0.01 and 0.013 feet per foot, respectively

The model limits extended roughly 150 ft upstream of the bridge and 200 ft downstream. A total of 13 cross sections were extracted from the project topographic mapping to represent the channel geometry through the reach. The locations of the cross sections are shown below in Figure 3.

Figure 4. Plan view of HEC-RAS hydraulic model cross section locations.

Table 3 presents the hydraulic modeling results for the cross section immediately upstream of the proposed Guidotti Bridge. The. The resulting water surface elevation plots for each flow are shown below in Figure 4 and the 50-yr and 100-yr design water surface elevations at the bridge are shown below in Figure 5.

Flow Profile Flow Rate (cfs)

Water Surface Elevation

Energy Gradeline Elevation

Channel Velocity (ft/sec.)

2 Year 126 212.50 212.67 3.32

50 Year 1,033 215.96 216.44 5.58

100 Year 1,642 217.39 217.87 5.85

500 Year 2,844 219.63 220.14 6.32

Table 3. Hydraulic Modeling Results for the Proposed Bridge.

Figure 5. Water Surface Profiles for Existing and Alternative Bridge Spans.

Bridge low chord elevation = 220.7

Figure 6. Proposed Upstream Bridge Cross Section.

The proposed bridge configuration will safely pass a 100-yr flood event and provide 2 ft of freeboard in the 50-yr event, based on a low chord elevation of 220.7 ft. By removing the existing concrete arch bridge, the two wide pier elements and the spandrel from the flow path hydraulic capacity is greatly improve, allowing for a shorter bridge than the existing.

SCOUR ANALYSIS

A scour analysis was performed using the results from the HEC-RAS model, and the 2014 survey information. Bed material was conservatively assumed to be 2 mm sand. Hydraulic analysis indicates that the hydraulic capacity of the new bridge will be adequate to pass the 50-year flow event and will be stable for the 100-year flow event.

Scour analysis for the proposed Guidotti Bridge was estimated using three methods presented in HEC 18 (2012):

1. Clear water contraction scour based on Laursen’s equation (1963)

2. NCHRP abutment scour approach

**TBD The total scour depth estimates are summarized below in Table 4. The total scour depth is estimated at 15 feet at an elevation of ** feet.

Event Contraction Scour (ft)

Abutment Scour (ft) Left Right

100-yr 0 10 4 500-yr 0 14 15

Table 4. Scour depths for the proposed bridge. (*RESULTS NOT FINAL)

Proposed berm elevation for 190 ft span bridge (5669.0 ft)

Existing berm

ABUTMENT PROTECTION

Riprap, to be placed along the abutments, was selected based on the methodology presented in HEC-23 Volume 2, Design Guideline14.

The riprap around the bridge abutments should be toed into the channel bed by extending the veneer 3 feet below the bed elevation and extending it horizontally into the channel 8 feet. Due to environmental concerns the horizontal extent of the riprap was decreased form recommended. The decrease in extent is accounted for by an increase in depth, thereby allowing for the same volume of rock to be placed.

SUMMARY OF RECOMMENDATIONS

Hydraulic capacity of the new structure was based on the following criteria:

• Q50 design flood with at least two feet of freeboard

• Passing Q100

By removing the existing concrete arch bridge, the two wide pier elements and the spandrel are removed from blocking the flow path, thus improving hydraulic capacity and allowing for a shorter bridge than the existing. Based on preliminary hydraulic analysis a proposed structure length of 100’ will allow for 2 feet of freeboard for the 50-yr event and will pass the 100-yr event. A shorter bridge could potentially meet the above hydraulic requirements but may not be preferable from an environmental impact due to the increased foot print of the fill areas in the riparian areas adjacent to the structure.

It is recommended that the existing bridge be removed and spill-through, riprap abutments be designed and constructed in conjunction with a proposed 100 ft single-span design. The abutments would be designed to provide up to 100-yr scour and erosion protection and Guidotti Bridge would have the capacity to safely pass the 50-year flood event with 2 ft of freeboard.

Bridge Configuration

• Recommended bridge span – 100 ft (distance between abutments)

• Minimum low chord elevation – 220.7 ft

Riprap Abutment Protection

• 2H:1V Sloping abutment protection

• Class III riprap

Foundation Design

• The 100-year design scour elevation is ** ft.

• Bridge pier and abutment foundations should be designed to account for an unsupported length above elevation ** ft.

REFERENCES

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

Federal Highway Administration HEC-18, Evaluating Scour at Bridges, Fifth Edition, April 2012.

Federal Highway Administration HEC-23, Bridge Scour and Stream Instability Countermeasures:

Experience, Selection, and Design guidance-third Edition, Volume2, September 2009.

US Army Corps of Engineers, HEC-RAS v 4.1.

US Geological Survey (1992) “Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Flood Plains” USGS Water Supply Paper 2339. US Dep. of Interior. Denver, CO.

APPENDIX A

Site Visit Photos

Figure A.1. Upstream bridge face.

Figure A.2. Downstream main channel.

Figure A.3. Scoured and failed shallow footing pier.

Figure A.4. Downstream headcut.

APPENDIX B

2010 Fort Ord Roadways Drainage Report

APPENDIX C

Hydrologic Analysis Results