J.11_2018_06_14_Thurman_Arroyos_Design_Report.pdf

United States Section of the International Boundary and Water Commission

Design for the Construction of Channel Maintenance Alternatives within the

Rio Grande Canalization Project Doña Ana County, New Mexico

Contract No. IBM15D0003

Order No. IBM16T0018

Design Summary Report Final Submittal

June 14, 2018

Prepared by:

9400 Amberglen Blvd

Austin, TX 78729

Design of Channel Maintenance Alternatives Contract No. IBM15D0003 Doña Ana County, NM Order No. IBM16T0018

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Table of Contents

1.0 PROJECT DESCRIPTION .............................................................................................. 1-1

1.1 Historical Background ......................................................................................... 1-1

1.2 Project Background .............................................................................................. 1-1

1.3 Objectives ............................................................................................................ 1-3

2.0 REVIEW OF PREVIOUS REPORTS AND DRAWINGS ............................................. 2-1

2.1 Previous Reports and Drawings Obtained ........................................................... 2-1

2.2 Sediment Trap System Design Considerations .................................................... 2-1

3.0 TOPOGRAPHIC SURVEY............................................................................................. 3-1

3.1 Survey Control ..................................................................................................... 3-1

4.0 UTILITIES STUDY ........................................................................................................ 4-1

4.1 Overview .............................................................................................................. 4-1

5.0 HYDROLOGIC AND HYDRAULIC ANALYSES ....................................................... 5-1

6.0 GEOTECHNICAL DESIGN ........................................................................................... 6-1

6.1 Overview .............................................................................................................. 6-1

6.2 Summary of Existing Information ....................................................................... 6-1

Previous Geotechnical Studies ................................................................. 6-1 6.2.1 Groundwater Information ........................................................................ 6-1 6.2.2 Groundwater Conditions .......................................................................... 6-2 6.2.3

6.3 Site Surface Conditions........................................................................................ 6-4

6.4 Subsurface Characterization ................................................................................ 6-4

Geology .................................................................................................... 6-4 6.4.1 Near-Surface Soil Mapping ..................................................................... 6-5 6.4.2 Properties of Arroyo Sediments ............................................................... 6-5 6.4.3 Subsurface Stratigraphy ........................................................................... 6-5 6.4.4

6.5 Seismic Design Considerations ............................................................................ 6-7 Site Seismic Classification ....................................................................... 6-7 6.5.1 Site Coefficients ....................................................................................... 6-7 6.5.2 Liquefaction Potential .............................................................................. 6-7 6.5.3

6.6 Arroyo Channel Design Considerations .............................................................. 6-8 General Description ................................................................................. 6-8 6.6.1 Stability of Permanent Slopes .................................................................. 6-8 6.6.2 Erosion Protection .................................................................................... 6-9 6.6.3 Excavation and Temporary Slopes ........................................................ 6-10 6.6.4 Dewatering ............................................................................................. 6-11 6.6.5 Perimeter Berms..................................................................................... 6-11 6.6.6

6.7 Sediment Basin Design Considerations ............................................................. 6-11 General Description ............................................................................... 6-11 6.7.1 Foundation Support Type Considerations ............................................. 6-12 6.7.2 Foundation Design Recommendations .................................................. 6-13 6.7.3

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Lateral Earth Pressures .......................................................................... 6-14 6.7.4 External Surcharge Loads ...................................................................... 6-15 6.7.5 Seismic Loads ........................................................................................ 6-15 6.7.6 Hydrodynamic Loads ............................................................................. 6-15 6.7.7 Uplift and Buoyancy .............................................................................. 6-15 6.7.8

6.8 Dewatering Design Considerations .................................................................... 6-16 Overview ................................................................................................ 6-16 6.8.1 General Design Criteria ......................................................................... 6-16 6.8.2 Design Groundwater Levels .................................................................. 6-17 6.8.3 Design Hydraulic Conductivity ............................................................. 6-18 6.8.4 Permitting Requirements ....................................................................... 6-18 6.8.5 Dewatering Plan ..................................................................................... 6-18 6.8.6 Dewatering Pilot Study .......................................................................... 6-19 6.8.7

6.9 Construction Considerations .............................................................................. 6-19 Site Preparation ...................................................................................... 6-19 6.9.1 Excavations ............................................................................................ 6-19 6.9.2 Subgrade Preparation ............................................................................. 6-20 6.9.3 Embankment Fill .................................................................................... 6-20 6.9.4 Drainage Fill .......................................................................................... 6-21 6.9.5 Structure Backfill ................................................................................... 6-21 6.9.6 Quality Control / Quality Assurance ...................................................... 6-21 6.9.7

7.0 SEDIMENT BASIN END WALL STRUCTURAL DESIGN ........................................ 7-1

7.1 General ................................................................................................................. 7-1

7.2 Concrete Design Criteria ...................................................................................... 7-1

Concrete Reinforcing Steel ...................................................................... 7-1 7.2.1

7.3 Steel Design Criteria ............................................................................................ 7-2

7.4 Design Loads ....................................................................................................... 7-2

General Loading....................................................................................... 7-2 7.4.1 Wind Loading .......................................................................................... 7-3 7.4.2 Seismic Loading....................................................................................... 7-3 7.4.3

7.5 Design Load Parameters ...................................................................................... 7-4 Vertical Loads .......................................................................................... 7-4 7.5.1 Horizontal Loads ...................................................................................... 7-4 7.5.2 Seismic Pressure Coefficients .................................................................. 7-4 7.5.3 Erosion Control Protection ...................................................................... 7-5 7.5.4

7.6 Design of End Walls ............................................................................................ 7-5 Wall Sections ........................................................................................... 7-5 7.6.1 Load Cases ............................................................................................... 7-5 7.6.2

8.0 RIPRAP DESIGN ............................................................................................................ 8-1

8.1 Data Collection .................................................................................................... 8-1

Site Visit................................................................................................... 8-1 8.1.1 Pebble Count ............................................................................................ 8-1 8.1.2 Relation to River Flood ............................................................................ 8-1 8.1.3

8.2 Riprap Design ...................................................................................................... 8-1 Stone Size................................................................................................. 8-1 8.2.1

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Riprap Placement ..................................................................................... 8-3 8.2.2

8.3 Limitations ........................................................................................................... 8-4

9.0 SEDIMENT REMOVAL................................................................................................. 9-1

9.1 Overview .............................................................................................................. 9-1

9.2 Sediment Removal Plan Development ................................................................ 9-1

10.0 RIGHT-OF-WAY/EASEMENTS ................................................................................. 10-1

11.0 CONSIDERATIONS AND ENGINEERING DECISIONS ......................................... 11-1

11.1 Summary ............................................................................................................ 11-1

11.2 Sediment Basin Sizing and Maintenance Interval ............................................. 11-1

11.3 Compensatory Mitigation Considerations ......................................................... 11-2

12.0 REFERENCES .............................................................................................................. 12-1

Appendix A Hydrologic and Hydraulic Sediment Trap-Basin Analysis

Appendix B Geotechnical Information

Appendix C Operations and Maintenance Plan

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List of Tables

Table 6-1. Summary of Available Groundwater Information by Others .................................. 6-22

Table 6-2. Summary of Nearest River Gage Station Data and Water Level Estimates for Project Site ..................................................................................................................... 6-23

Table 6-3. Summary of Available Groundwater Information by Others .................................. 6-24

Table 6-4. Summary of Published Engineering Properties of Near-Surface Soil Units near Thurman I Arroyo .................................................................................................. 6-26

Table 6-5. Summary of Published Engineering Properties of Near-Surface Soil Units near Thurman II Arroyo ................................................................................................. 6-27

Table 6-6. Approximate Gradation of Arroyo Sediments ........................................................ 6-27

Table 6-7. Subsurface Stratigraphy and Measured Properties near Thurman I Arroyo (Upstream) ..................................................................................................................... 6-28

Table 6-8. Subsurface Stratigraphy and Measured Properties near Thurman I Arroyo (Downstream) ................................................................................................................ 6-28

Table 6-9. Subsurface Stratigraphy and Measured Properties near Thurman II Arroyo (Upstream) ..................................................................................................................... 6-29

Table 6-10. Subsurface Stratigraphy and Measured Properties near Thurman II Arroyo (Downstream) ................................................................................................................ 6-29

Table 6-11. Seismic Design Coefficients (Site Class E) ........................................................... 6-29

Table 6-12. Calculated Slope Stability Factors of Safety (FOS) .............................................. 6-30

Table 6-13. Allowable Axial Capacities for Drilled Piers ........................................................ 6-30

Table 6-14. Lateral Resistance Parameters for Deep Foundations ........................................... 6-31

Table 6-15. Lateral Earth Pressure Recommendations ............................................................. 6-31

Table 6-16. Construction Elevations ......................................................................................... 6-32

Table 6-17. Design Water Levels for Thurman I and II Arroyos ............................................. 6-33

Table 6-18. Recommended Hydraulic Conductivity Design Parameters ................................. 6-34

Table 7-1. Unit Weights .............................................................................................................. 7-2

Table 7-2. Load Combinations for Structural End / Retaining Wall Design .............................. 7-9

Table 8-1. Modeled Hydraulic Parameters in Thurman I and Thurman II Arroyos Downstream of Proposed Sediment Basins ..................................................................... 8-2

Table 8-2. Riprap Sizing Analysis Results ................................................................................. 8-2

Table 8-3. D50, 1.5*D50 and D100 Values for Thurman I and Thurman II Arroyos .................... 8-4

Table 9-1. Tetra Tech's Excavation Parameters for the Channel Excavation Long Sediment Removal Alternative at Problem Location 2 ................................................... 9-2

Table 9-2. URS Designed Excavation Parameters for the Channel Excavation Long Sediment Removal Alternative at Problem Location 2 ................................................... 9-2

Table 11-1. Sediment Trap vs. Basin Comparison ................................................................... 11-3

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List of Figures

Figure 1-1. Project Location Map ............................................................................................... 1-2

Figure 4-1. NPMS Public Map Viewer Near Project Site .......................................................... 4-2

Figure 6-1. Plan of Boring Locations ....................................................................................... 6-35

Figure 6-2. Plan of Groundwater Well Locations ..................................................................... 6-36

Figure 6-3. Plan of River Gage Locations ................................................................................ 6-37

Figure 6-4. River Gage and Groundwater Monitoring Wells – Water Surface Elevation Variations (1994 to 2016) ............................................................................................. 6-38

Figure 6-5. River Gage and Groundwater Monitoring Wells – Water Surface Elevation Variations (2010 to 2016) ............................................................................................. 6-39

Figure 7-1. Proposed Pier-Supported Floodwall ........................................................................ 7-6

Figure 9-1. Pre-Sediment Removal Thalweg vs. Post-Sediment Removal Thalweg ................. 9-2

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1.0 PROJECT DESCRIPTION

1.1 Historical Background

The Rio Grande Canalization Project (RGCP), which extends 105.4 miles from Percha Dam in Sierra County, New Mexico to American Dam in El Paso, Texas, was constructed between 1938 and 1943 as authorized by the Act of Congress approved June 4, 1936 (Public Law 648, 49 Stat 1463) to: "facilitate compliance with the convention between the United States and Mexico concluded May 21, 1906, providing for the equitable division of the waters of the Rio Grande, and to properly regulate and control, to the fullest extent possible, the water supply for use in the two countries as provided by treaty." The Act authorizes the United States International Boundary and Water Commission (USIBWC) to construct, operate and maintain the RGCP in accordance with the plan in the Engineering Record of December 14, 1935. The USIBWC objectives for the RGCP can be summarized by: Flood Conveyance and Flood Protection, Channel Conveyance Reliability, Delivery Efficiency, Compliance with U.S. Regulations, and Minimizing Costs.

1.2 Project Background

There is ongoing sediment inflow from the tributary arroyos, resulting in sediment deposition forming sediment plugs at arroyo confluences along sections of the Rio Grande. Sediment inflow also results in island formations and raising of river beds. Sediment accumulation prevents draining of irrigation return flow to the Rio Grande and may result in increases in water surface elevations, which could impact levee freeboard and increase the flooding risk to adjoining communities. A study entitled Channel Maintenance Alternatives and Sediment Transport Studies for the RGCP Final Report was completed in 2015 by Tetra Tech, Inc.

(hereafter "Tetra Tech 2015"). The report identified nine (9) representative problem locations experiencing sediment accumulation along the 105.4 miles of the RGCP that were evaluated in the study. The report then evaluated, scored, and ranked various Channel Maintenance Alternatives (CMAs) for each of the nine (9) problem locations. The report presented a conceptual sediment trap as one of the CMAs, and because of the high benefit-to-cost consequence of the sediment trap as determined in the report, it was recommended as an alternative to be used at all of the problem locations.

USIBWC contracted URS Group, Inc. (URS) to perform design of one or more of the CMAs at each of two (2) selected locations within USIBWC's ROW. The two selected sites are referred to as "Thurman I Arroyo" and "Thurman II Arroyo" and are located within Problem Location 2, which extends a distance of approximately 3.3 miles from the Salem Bridge at NM Highway 391 downstream to the confluence with Placitas Arroyo (see Figure 1-1). The Tetra Tech 2015 study found that during recent monsoon season tributary flow events (i.e., 2006 and 2013), Thurman I and II Arroyos each delivered significant quantities of sediment to the RGCP, and appear to have delivered additional sediment since that time. After the 2006 events, USIBWC removed sediment from the river, reconstructed the opposite bank and excavated the mouth of the Thurman I Arroyo. Evidence of bank protection along the right bank opposite Thurman II Arroyo suggests that similar activities were undertaken at this tributary. Islands have formed along the downstream portions of both of the Thurman Arroyo fans, along with numerous other islands and vegetated bars along the reach.

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The intent of the design project is such that the constructed alternatives will improve conveyance efficiency, hydraulic capacity, drainage return flows, and levee infrastructure, and will decrease flood risk and reduce the overall operations and maintenance at each site.

Figure 1-1. Project Location Map

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1.3 Objectives

The design objectives for both sites addressed under this scope of work are as follows:

1. Provide design for a solution that permits removal of localized sediment within the main channel to ensure adequate hydraulic capacity for each design and allow for routine maintenance within the sediment trap(s).

2. Coordinate all work with impacted stakeholders.

3. Analyze potential drainage impacts along to adjacent properties so such drainage improvements are incorporated into the design.

4. Design any modifications to affected structures that may be necessary due to the proposed modifications.

5. Provide any alternative recommendations in lieu of the sediment traps for effectively trapping/collecting the sediment prior to reaching the Rio Grande confluence.

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2.0 REVIEW OF PREVIOUS REPORTS AND DRAWINGS

2.1 Previous Reports and Drawings Obtained

The following documents were provided by USIBWC and reviewed by URS for information:

1. Tetra Tech, Inc. Channel Maintenance Alternatives (CMA) and Sediment Transport Studies for the Rio Grande Canalization Project: Final Report. Tetra Tech, Inc.;

October 20, 2015.

2. USIBWC. Canalization River Management Plan – Part 4 – Channel Maintenance Plan, Draft. USIBWC; August 2016.

2.2 Sediment Trap System Design Considerations

The recommended channel maintenance alternative for Thurman I and II Arroyos, as presented in Tetra Tech 2015, was a screen-based sediment trap system. The sediment trap system functions by filtering (capturing) sediment from incoming arroyo flows to prevent sediment accumulation in the Rio Grande. Large particles are captured at the upstream end of the channel, and progressively finer sediments are captured at intermediate locations as flows travel downstream. Based on conceptual design drawings presented in Tetra Tech 2015, each arroyo sediment trap system is a collective of several independent structures ("trapping features") as described below.

1. Upstream Debris Rack. The debris rack captures large particles (cobbles, gravels, etc.) and other debris near the beginning of the channel that could detrimentally affect downstream trapping features. The debris rack consists of structural steel sections connected to and supported by deep foundations (e.g., driven piles or drilled piers). A conceptual drawing of the debris rack is provided in Graphic 2-1.

2. Intermediate Sediment Traps. Intermediate sediment traps are intended to capture sand, silt, and clay size fractions of the incoming flows, and typically consist of either rock check structures or mesh screens. For this project, only mesh screen options were considered in design. In this case, the sediment trap consists of a mesh screen of rebar or welded wire fabric (WWF) connected to and supported by vertical angle sections driven into the subgrade. Preliminary information indicated five intermediate sediment traps would be required at each Thurman I Arroyo and Thurman II Arroyo. A conceptual drawing of the sediment trap is provided in Graphic 2-2.

3. Downstream Embayment. The downstream embayment would be constructed at the confluence of the arroyo and the Rio Grande. The purpose of the embayment is to provide habitat benefits as a lower velocity, off-channel refuge area with vegetative cover off the Rio Grande.

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Graphic 2-1. Conceptual Drawing of Upstream Debris Rack (Tetra Tech 2015).

Graphic 2-2. Conceptual Drawing of Sediment Trap Screen (Tetra Tech 2015).

As part of this project, URS performed engineering evaluation and comparison of different sediment trap systems for the Thurman I and II Arroyos, including a sediment basin design.

Detailed discussion of the findings and conclusions are provided in Section 5, Hydrologic and Hydraulic Analyses.

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3.0 TOPOGRAPHIC SURVEY

Topographic surveys of the two arroyo sites were not performed for this design effort. To perform the design, create the necessary grading, and develop construction drawings of the sediment basins, URS used LiDAR data provided by USIBWC. The LiDAR data were created in 2011 by Watershed Sciences, Inc. for Tetra Tech, Inc., the contractor for USIBWC.

USIBWC also provided surveyed cross-section data of the two arroyos. Nine cross-sections were provided at Thurman I Arroyo – three cross-sections between the Rio Grande and the north USIBWC right-of-way boundary; five cross-sections between the north right-of-way and Interstate Highway 25; and one cross-section on the north side of IH25. Nineteen cross-sections were provided at Thurman II Arroyo – six cross-sections between the Rio Grande and north USIBWC right-of-way boundary; eleven cross-sections between the north right-of-way and Interstate Highway 25; and two cross-section on the north side of IH25. The cross-section data were collected by USIBWC personnel in September 2016.

3.1 Survey Control

Horizontal and vertical datums used for the design drawings are based on:

Horizontal Control:

New Mexico State Plane Coordinate System, Central Zone, North American

Datum of 1983 (NAD 83)

Vertical Control:

North American Vertical Datum of 1998 (NAVD 88)

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4.0 UTILITIES STUDY

4.1 Overview

URS conducted a utility study encompassing the area of the proposed sediment traps to determine if existing utility conflicts were present. A site visit was made by URS employees to both arroyos on October 20, 2016. During the site visit, observations were made to determine if there were any visible signs of utilities, either overhead or underground. There were no visible indications that utilities are present at the two sites. After the initial site visit, the New Mexico 811 Damage Prevention Center (NM811) was contacted to facilitate location of utilities in the project area. NM811 generated a design conference ticket that notified utility companies which have utilities in the area. The purpose of the design conference ticket was to open a line of communication between the utility owners and URS to identify if any conflicts exist. Utility owners of the area include: CenturyLink, El Paso Electric, and The Village of Hatch.

CenturyLink, the telecommunications utility and internet service provider of the area, was the first utility owner to contact URS. CenturyLink informed URS that none of their utilities exist in the area. After discussion with CenturyLink, a technician from United States Infrastructure Corporation (USIC), a large underground utility locator, contacted URS. URS provided the project location map to the technician, and the technician stated that no utilities exist in the area except for the possibility of a small telephone line. After informing the technician that CenturyLink cleared the area of their utilities, this concern was relieved.

To identify water utilities, The Village of Hatch Public Works Director (PWD) was contacted.

The design conference ticket listed Garfield Mutual Domestic Water Consumers Association (Garfield MDWCA) as the water utility owner of the area, but upon contacting Garfield MDWCA, URS was instructed to speak with The Village of Hatch Public Works. Initial conversation with the PWD revealed that Hatch does not have water utilities extending to the site, but the project location map was sent to reaffirm, and the PWD did confirm this.

El Paso Electric utility locations were determined by conversation with USIC. URS was informed that all electric utilities in the area are aerial, so no buried conflicts would be encountered.

To determine the presence of gas lines, the National Pipeline Mapping System (NPMS) Public Map Viewer was used and confirmed that gas lines do not exist near the project area (see Figure 4-1). According to the NPMS, the Public Map Viewer is a web-based mapping application designed to assist the general public with displaying and querying data related to gas transmission and hazardous liquid pipelines, liquefied natural gas plants, and breakout tanks that are under Department of Transportation (DOT) Pipeline and Hazardous Materials Safety Administration (PHMSA) jurisdiction. The NPMS also notes that the application does not contain distribution or gas-gathering pipelines.

Although no evidence of utilities was found at either of the two sites, this does not guarantee that no utilities are present. An extensive Subsurface Utility Engineering (SUE) investigation was not part of this contract and was not performed at the two sites. The Construction Contractor will need to contact the utility providers at least 48 hours prior to the commencement of any

June 2018 4-2 construction work. It is the responsibility of each individual utility owner to remove, relocate, and protect their respective utility.

Figure 4-1. NPMS Public Map Viewer Near Project Site

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5.0 HYDROLOGIC AND HYDRAULIC ANALYSES

Extensive hydrologic and hydraulic (H&H) analyses were performed as part of Task Order IBM16T0018, Channel Maintenance Alternatives and are documented in a separate report entitled Hydrologic & Hydraulic Sediment Trap-Basin Analysis. The H&H report is included in Appendix A of this report.

In summary, the H&H report concluded that a sediment trap system consisting of a basin-based trap would likely have superior performance to that of a mesh-based trap. This conclusion was based on predicted sediment trapping efficiency, structural design considerations, ease of maintenance, and scour potential. Based on H&H findings, the URS team determined that a sediment trap basin is the preferred channel maintenance alternative for both Thurman I and II Arroyos.

The sediment basin system consists of deepening the arroyo channel to construct a basin for sediment collection. The basin is designed to provide sufficient time for sediment to settle out of arroyo flows and to be deposited at the bottom of the channel. The basin is sloped towards the river so that the larger size sediments are deposited at the beginning of the basin, and progressively finer particles are deposited further downstream. The sedimentation basin side-slopes are laid back at a minimum of 3H:1V to maintain long-term stable slopes. The downstream termination of the basin is accomplished by constructing a reinforced-concrete retaining wall ("basin end wall") near the confluence with the Rio Grande. The end wall will be approximately 4.4 to 5.7 feet above the basin finish elevation to provide freeboard for the 100-year storm flows in the arroyo. The end wall serves as an overflow weir when higher flows occur, or when significant volumes of sediment have already been collected in the basin that permit less stormwater storage. Scour protection would be provided on both the upstream and downstream sides of the end wall.

URS proposed the change to a sediment basin system to USIBWC at the 60% design submittal, and USIBWC concurred with the design change. Consequently, the sediment trap basin system is the selected channel maintenance alternative for both arroyos on this project. Details of the sediment trap basin designs are provided in the subsequent sections of this report.

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6.0 GEOTECHNICAL DESIGN

6.1 Overview

URS did not perform a geotechnical field investigation under this scope of work. In completing the geotechnical design of this project, URS used existing geotechnical information prepared by others and furnished by USIBWC, and has relied on this information as accurate and complete.

URS applied engineering judgment in the review of the data based on our experience in the region and with similar geologic materials, but URS does not warrant the accuracy or completeness of information provided by others. The following sections summarize the geotechnical design approach and conclusions for this project.

6.2 Summary of Existing Information

Previous Geotechnical Studies 6.2.1

Existing geotechnical information used in the design of this project included the following previous geotechnical studies:

S&B Infrastructure, Ltd. in association with Raba-Kistner Consultants, Inc. (RKCI 2008), Final Technical Memorandum, Geotechnical Explorations of Levee System within the Rio Grande Canalization Project, Hatch-Tonuco Levee Segment. Volumes I-IIE, August 4, 2008.

Kleinfelder West, Inc. (Kleinfelder 2010), Final Geotechnical Report, NMDOT D1310, Hatch Pavement Remediation, Hatch, New Mexico, April 23, 2010.

A total of 18 test borings from the two previous studies were examined as potentially relevant to this project. Borings are summarized in Table 6-1. The approximate boring locations are depicted on Figure 6-1. Additional discussion on the relevancy of the geotechnical borings is provided in subsequent sections.

Groundwater Information 6.2.2

Available groundwater data are primarily historical information, including: (1) water levels encountered during drilling by RKCI 2008 and Kleinfelder 2010; (2) piezometers installed by RKCI 2008; (3) USIBWC well data; and (4) U.S. Geological Survey (USGS) well data.

Note that the majority of existing wells are located at a great distance from the site, particularly the USIBWC wells located along the Rio Grande approximately 8,800 to 9,900 feet downstream (east) and approximately 21,800 to 40,800 feet upstream (west) of the site. Due to the great distance from the site, groundwater elevations at the USIBWC wells are not expected to reflect actual groundwater elevations at the site. However, URS expects the trends in groundwater levels exhibited at each well to be indicative of the groundwater trends at the site and along the Rio Grande as a whole. Additional discussion on the relevancy of the well data is provided in subsequent sections. Approximate locations of USIBWC and USGS groundwater wells are shown in Figure 6-2.

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For the purposes of this report, maximum ground water elevation at both Thurman I and II Arroyos was assumed to be at El. 4058.0.

Groundwater Conditions 6.2.3

6.2.3.1 River Stage Data

In the Rio Grande Valley, the flow regimes of the Rio Grande are generally differentiated by irrigation season and non-irrigation season. During the irrigation season, which typically runs from March 1 through October 31 (but may vary during dry years with a decrease in water supply), the U.S. Bureau of Reclamation releases flows from the upstream flood control structures to provide irrigation water for crops downstream. This ultimately results in higher river flows and river stage elevations during the irrigation versus the non-irrigation seasons. The Contractor should be fully aware of the irrigation season schedule prior to beginning any construction activity.

Due to the proximity of the site to the Rio Grande, river stage data were examined as part of the groundwater evaluation to develop an understanding of river fluctuation and degree of communication between river levels and groundwater levels. The nearest upstream river gage is below Caballo Dam ("CAAN5") and the nearest downstream gage is at Hayners Bridge near Rincon ("RHB5'). Gages CAAN5 and RHB5 were located approximately 20.5 and 11.5 miles from the project site, respectively. Approximate locations of the river gages are shown in Figure 6-3. River stage data from the nearest gaging stations are summarized in Table 6-2. Plots of the data are presented in Figures 6-4 and 6-5.

URS examined U.S. Army Corps of Engineers (USACE) 1996 data (Fixed-Bend Cross Sections HEC-2, Rio Grande Canalization Improvement Project, Percha Diversion Dam, New Mexico, to American Diversion Dam, Texas, July 1996) and Tetra Tech 2015 data to estimate channel geometry at the river gage locations. This information included minimum riverbed elevation (thalweg), riverbank elevation, and both low flow and 100-yr water surface elevations.

Similarly, these sources were used to estimate Rio Grande channel geometry at Thurman I and II Arroyos. This information was used to provide a reference point for the river gage data, and to permit interpolation of river data at the project site. Interpretations were limited by the fact that the USACE 1996 and Tetra Tech 2015 data did not extend fully to the upstream river gage (located approximately 6,000 ft north). Another limitation was lack of definitive river gage datum; several online resources had differing datum elevation or no datum elevation. URS made a best estimate from available data in selecting datum elevation. The graphical data are presented in Appendix B.

In general, river water surface elevations appear to hold relatively constant throughout the non-irrigation season, with "static" readings of approximately Elev. 4142.5 and 4009.5 feet for gages CAAN5 and RHB5, respectively. While elevations vary considerably during the irrigation season, a recurring "static" peak can be observed at approximately Elev. 4149 and 4012 feet for CAAN5 and RHB5, respectively. Corresponding irrigation season levels are approximately 6.5 and 3.5 feet higher than the non-irrigation season levels at each gage, respectively.

June 2018 6-3

6.2.3.2 Short-Term Data from Borings

Complete details of groundwater conditions encountered during drilling of borings associated with previous studies are provided in Table 6-3. In summary, groundwater was observed in all RKCI 2008 borings at the time of drilling. The depth varied from approximately 6.5 feet to 13.5 feet below ground surface (bgs). Maximum and minimum groundwater elevation encountered in these borings was Elev. 4054.7 and 4051.6 feet above mean sea level (ft MSL), respectively.

These borings were performed in March 2008, which is during the irrigation season when the Rio Grande levels – and by association groundwater levels – would be expected to be higher than yearly average.

Groundwater was not observed in Kleinfelder 2010 borings. This is likely due to both the distance of these borings from the Rio Grande, and termination elevations of the borings being approximately 10 feet higher than where groundwater was encountered in the RKCI 2008 borings.

Note these observations represent groundwater conditions at the time of the field exploration and may not be indicative of other times or at other locations. Groundwater conditions can change with varying seasonal and weather conditions and other factors.

6.2.3.3 Long-Term Data from Monitoring Wells

Monitoring well data dating back to as early as 1994 and 2013 for USGS and USIBWC wells, respectively, were examined and are plotted over time in Figures 6-4 and 6-5. Data plots of wells considered to be most relevant to the project are shown as solid lines, while other wells examined are shown as dotted lines. Ground surface elevation and initial reading (or reading at time of drilling) for each boring location and well are shown as point values. River stage data are also presented. Note that water level during drilling was lower than initial well reading in some cases for USIBWC wells; URS believes this may be attributed to rapid rise of river levels (and hence groundwater levels) shortly after installation, and/or delayed water entry associated with lower-permeability soils.

USGS wells considered to be most relevant to the project include wells "USGS-H-13" and "USGS-.434" (abbreviated to the last digits of the USGS identification number). USGS-H-13 is located approximately 6,600 ft upstream of Thurman II Arroyo, while USGS-.434 is located approximately 3,100 ft downstream of Thurman I Arroyo. Both are located within 200 feet of the Rio Grande banks. The USIBWC wells considered to be most relevant to the project include wells RS-MW-6 and RS-MW-7, which are both located downstream of Thurman I Arroyo by approximately 8,800 and 9,900 feet, respectively, and within about 400 feet of the Rio Grande banks.

Examination of the long-term data indicates that, overall, the amplitude of groundwater level fluctuations generally does not exceed about 10 feet. The amplitude of fluctuation is closer to about 5 feet for USGS wells. Nearly all of the wells exhibited cyclic water levels that correlate very strongly with timing of spikes in river stage data, confirming that groundwater is strongly influenced by water levels in the Rio Grande. The delay in communication with river and groundwater was difficult to assess due to infrequency of readings in some piezometers, but URS expects the response to be fairly rapid based on expected high permeability of subsurface soils, June 2018 6-4 and could range from as little as days to weeks based on the data. One exception was USGS- .434, which did not exhibit a strong cyclic response, and may be due to the presence of more clayey (lower permeability) subsurface soils at this location.

6.3 Site Surface Conditions

The Thurman I and II Arroyos sites are located south of the foothills of Redhouse Mountain, south of U.S. Hwy 85 and north of the Rio Grande. The area is generally undeveloped floodplain area and farmland. A USIBWC levee access road, part of the Hatch-Tonuco levee reach, crosses the arroyo alignments at a nearly perpendicular orientation, and is located approximately 400 to 500 feet north of the Rio Grande.

The topography of the site is relatively flat and gently slopes towards the Rio Grande. Ground surface elevations range from about Elev. 4060 to 4065 feet on the upstream end of Thurman I and II Arroyos at the northern limit of the USIBWC right-of-way (ROW). At the confluence of the arroyos and the Rio Grande, located approximately 500 to 600 feet south of the ROW, ground elevations range from approximately Elev. 4060 to 4062 feet.

The ground surface in Thurman I and II Arroyos is sparsely vegetated and mostly barren. The ground surface is relatively rocky in the arroyos, particularly at the confluence of the Rio Grande where sediment has accumulated. Visible particles generally consist of rounded coarse gravels and cobbles carried downslope from the mountains and foothills. Particle sizes generally grade larger at the confluence of the Rio Grande, where a relatively high occurrence of boulder-size particles was observed by URS.

6.4 Subsurface Characterization

Geology 6.4.1

According to published geologic mapping by the New Mexico Bureau of Geology and Mineral Resources (2003), the site is primarily underlain by the Quaternary-aged alluvium (Qa).

Alluvium consists of floodplain deposits of the Rio Grande and contains varying proportions of sand, silt, clay, and gravel.

The specific project setting is located along the levee of the Rio Grande which was channelized in the 1930's as part of a valley-wide flood control and irrigation project. According to RKCI 2008 (after Dena, 2000 and Doser et al., 2001), the Rio Grande Floodplain deposits represent the most recent incision of the Rio Grande into the Camp Rice formation during the late Pleistocene through the Holocene Epochs. The Rio Grande Floodplain deposits are reported to be up to 200 feet thick and are comprised of distant and local source sediments ranging from gravel to silt size. Crevasse splays are the typical deposition pattern encountered on the Rio Grande floodplains, which are comprised of thin fans of sand and silt spread across the floodplain during a break in a natural or artificial levee caused by a flood event. The splay grows uniformly progressively coarser and sandier until channels are cut into the top of the splay. These channels are then filled with coarse sediment that becomes progressively finer-grained upward, opposite the majority of the splay deposition. These channels are discontinuous, often become commingled, and are difficult to correlate across short distances.

June 2018 6-5

Near-Surface Soil Mapping 6.4.2

Near-surface soil maps published by NRCS Web Soil Survey indicate several soil units are present at each site. "Near-surface" refers to soils in the upper 80 inches (6.7 feet) bgs. Mapping was originally performed in the 1950's to 1970's with periodic updates by NRCS (formerly SCS), but changes to near-surface conditions resulting from recent human activity or on-going environmental/geologic processes may not always be captured by these maps. The mapped soil units along the arroyo alignment for each site are listed below. Note that other soil units are mapped in the project area, but are not discussed herein.

Thurman I Arroyo:

o Brazito loamy fine sand, 0 to 1% slopes MLRA 42.2 (Br).

o Riverwash (RE).

Thurman II Arroyo:

o Belen loam (Be).

o Brazito very fine sandy loam, thick surface (Bs).

o Riverwash (RE).

Published properties of these soil units are provided in Tables 6-4 and 6-5. The intent of presenting this information is to identify potential variations between the relatively widely spaced existing borings.

Properties of Arroyo Sediments 6.4.3

Tetra Tech 2015 collected a bulk sample to represent the sediment gradation delivered by Thurman I Arroyo to the Rio Grande confluence. The sample was designated as Pebble Count PC3, and was taken from the surface of the Thurman I Arroyo fan. The sample included cobble-and gravel-size particles, with the approximate grain size distribution presented in Table 6-6.

Photographs of the sediment surface taken by URS staff during a site visit on October 20, 2016 provide confirmation of the coarse-grained nature of accumulated sediments in both Thurman I and II Arroyos. However, the URS photographs indicate even larger particles are present in the fans of both arroyos, in some cases appearing to be upwards of 12 to 18 inches in diameter or larger (boulder-size particles). These areas are presented in Photos 6-1 and 6-2.

Subsurface Stratigraphy 6.4.4

Generalized subsurface stratigraphy is based on the nearest borings to each specific arroyo, which included only RKCI 2008 borings. Conditions in the Kleinfelder 2010 borings were found to be appreciably different and are not believed to be representative of conditions in the portions of arroyos located within the USIBWC ROW. In the event that this evaluation needs to consider conditions in the arroyos outside USIBWC ROW and nearer to U.S. Hwy 85, the Kleinfelder 2010 data should be revisited.

June 2018 6-6

Photo 6-1. Example of deposited sediment in Thurman I Arroyo.

Photo 6-2. Example of deposited sediment in Thurman I Arroyo.

The generalized subsurface stratigraphy and measured soil properties within each stratum area are shown in Tables 6-7 through 6-10. Borings on the opposite side of the Rio Grande from the arroyos (HT-162, HT-163, and HT-164) were assumed to be representative of conditions on the downstream end of the arroyos near the Rio Grande, while the other nearby borings (HT-74, HT-75, HT-76, and HT-77) were assumed to be representative of conditions on the upstream end of the arroyos. In general, similar soil layering was observed at all locations. However, it is unknown whether theses borings are representative of the actual subsurface conditions at the arroyos due to the distance from actual arroyo alignments (i.e., 200- to 600-foot offsets). Also, June 2018 6-7 the "upstream" and "downstream" borings are spaced approximately 800 to 900 feet apart, and it is unknown whether subsurface conditions near the middle portions of the arroyo alignments are similar. Consequently, URS suggests that additional pre-construction geotechnical data (borings, field testing, etc.) be conducted to validate the assumptions contained herein.

The subsurface stratigraphy presented below does not account for the arroyo sediments, which includes many large cobble- and boulder-size particles. These materials were not sampled in any of the borings, and the depth/thickness of these materials are not known. Therefore, the generalized subsurface stratigraphy should be considered as representative of conditions "outside" the arroyo channels. For the purposes of this report, URS has assumed the upper 5 feet of soil "inside" the arroyo channels (bottom and slopes) consists of arroyo sediments, believed to be a mixture of gravel- to boulder-sized particles in a silty sand matrix. Further discussion and recommendations related to stratigraphy are presented in subsequent sections.

6.5 Seismic Design Considerations

Site Seismic Classification 6.5.1

Site soils can be classified as Site Class E ("Soft Soil") according to the 2012 International Building Code (IBC) - Section 1613.3.3 and ASCE 7-10, Chapter 20, Table 20.3-1. Site Class E is for soft/loose soil profiles with an average SPT N-value less than 15 averaged over the upper 100 feet bgs. While the maximum exploration depth of the considered borings was only 20 feet, URS conservatively assumed conditions below 20-foot depth were similar. Additional geotechnical explorations could potentially justify use of improved Site Class designation.

Site Coefficients 6.5.2

In accordance with Section 1613.3 of the IBC, the spectral response accelerations for the "Risk- Targeted Maximum Considered Earthquake" (MCER) were obtained from the USGS website.

These design parameters are summarized in Table 6-11.

Based on the mapped acceleration parameters, the site is classified under the Seismic Design Category C, which indicates a Low to Moderate Seismic Risk Level per Section 11.4 of ASCE 7-10. The mapped acceleration parameters mentioned above are for information purposes only and will need to be confirmed by the Project Structural Engineer-of-Record for final design.

Liquefaction Potential 6.5.3

As required per ASCE 7-10, liquefaction potential was evaluated for the 2% 50-year event (2,475-year mean return period). A de-aggregation of seismic hazard was performed using the USGS' online 2008 Interactive De-aggregation tool. The analysis provides the moment Magnitude (M) and Peak Horizontal Ground Acceleration (PGA) for the NEHRP "BC-rock" boundary, which corresponds to a site underlain by rock.