J.12_Ysleta_to_Fabens_Design_Doc_Report_Supplement_20160715.pdf

Rehabilitation of Wasteways 1 and 2 Within the Ysleta to Fabens Levee Reach El Paso County, Texas

USIBWC IBM15B0003

Supplement 1 to the Design Documentation Report

July 15, 2016

Prepared by:

URS GROUP, INC.

Texas Registered Firm 3162 9400 Amberglen Blvd Austin, TX 78729 Tel: 512-454-4797 Fax: 512-454-8807

Supplement 1 to the Design Documentation Report Wasteways 1 and 2

URS Group, Inc. ii USIBWC, El Paso, Texas

STATEMENT OF LIMITATIONS

This Document is for USIBWC use only. The scope of services performed within may not be appropriate to satisfy the needs of other users. Any use or re-use of this document or of the findings, conclusions, or recommendations presented herein is the sole risk of the user.

URS Group, Inc. iii USIBWC, El Paso, Texas

TABLE OF CONTENTS

1.0 PROJECT INFORMATION ........................................................................................ 1-1

1.1 General ................................................................................................................. 1-1

1.2 Authorization ....................................................................................................... 1-1

1.3 Purpose of Design Documentation Report .......................................................... 1-1

1.4 Project Description ............................................................................................... 1-1

1.4.1 Project Location ....................................................................................... 1-1

1.4.2 Existing Features ...................................................................................... 1-2

1.4.3 Project Objectives .................................................................................... 1-2

1.4.4 Project Requirements ............................................................................... 1-2

1.4.5 Description of Alterations ........................................................................ 1-4

1.5 Project Deliverables ............................................................................................. 1-4

2.0 PERTINENT DATA ...................................................................................................... 2-1

2.1 Project Purpose .................................................................................................... 2-1

2.2 Opinion of Probable Construction Cost ............................................................... 2-1

2.3 Existing Utilities .................................................................................................. 2-1

2.4 Design Data .......................................................................................................... 2-1

2.4.1 Controlling Elevations ............................................................................. 2-1

2.4.2 Hydraulics Data ....................................................................................... 2-1

2.4.3 Survey Data .............................................................................................. 2-1

2.4.4 Soil Borings ............................................................................................. 2-2

3.0 DESIGN INFORMATION AND REFERENCES ...................................................... 3-1

3.1 Design Criteria ..................................................................................................... 3-1

3.2 Other Codes and Standards Used in Design ........................................................ 3-2

3.3 General References .............................................................................................. 3-3

3.4 Approval Criteria Waivers ................................................................................... 3-4

3.5 Computer Programs Used .................................................................................... 3-4

3.6 Government-Provided Information ...................................................................... 3-4

4.0 ENGINEERING DESIGN ............................................................................................ 4-1

4.1 Hydrology and Hydraulics ................................................................................... 4-1

4.1.1 Background .............................................................................................. 4-1

4.1.2 Hydraulic Features ................................................................................... 4-1

4.2 Geotechnical ........................................................................................................ 4-1

4.2.1 Objectives ................................................................................................ 4-1

4.2.2 Background .............................................................................................. 4-2

4.2.3 Nearby Structures..................................................................................... 4-3

4.3 Project Setting ...................................................................................................... 4-3

4.3.1 Regional Geology .................................................................................... 4-3

4.3.2 Site Seismicity ......................................................................................... 4-4

4.3.3 Geomorphology ....................................................................................... 4-4

URS Group, Inc. iv USIBWC, El Paso, Texas

4.4 Summary of Analyses Performed ........................................................................ 4-4

4.5 Analysis Results ................................................................................................... 4-4

4.5.1 Seepage and Slope Stability ..................................................................... 4-4

4.5.2 Liquefaction ............................................................................................. 4-6

4.5.3 Erosion Control ........................................................................................ 4-6

4.6 Geotechnical Design Features .............................................................................. 4-6

4.6.1 Seepage Control Measures ....................................................................... 4-6

4.7 Environmental Considerations ............................................................................. 4-7

4.7.1 Overview .................................................................................................. 4-7

4.7.2 Biological Resources ............................................................................... 4-7

4.7.3 Cultural Resources ................................................................................... 4-7

4.7.4 Construction Approaches ......................................................................... 4-8

4.8 Structural Design Features ................................................................................... 4-8

4.8.1 Overview .................................................................................................. 4-8

4.8.2 Load Conditions ....................................................................................... 4-8

4.8.3 Structural Design of Drainage Structures ................................................ 4-9

4.9 Loads .................................................................................................................... 4-9

4.9.1 General Loading....................................................................................... 4-9

4.10 Concrete Design Criteria .................................................................................... 4-11

4.10.1 Strength Reduction and Load Factors .................................................... 4-11

4.10.2 Reinforcing Steel ................................................................................... 4-12

4.11 Steel Design Criteria .......................................................................................... 4-12

4.12 Civil Design Features ......................................................................................... 4-12

4.12.1 Excavation of Existing Levee ................................................................ 4-12

4.12.2 Typical New Levee Section ................................................................... 4-12

4.12.3 Surface Drainage .................................................................................... 4-13

4.12.4 Interim Flood Protection ........................................................................ 4-13

4.12.5 Roadways ............................................................................................... 4-13

4.12.6 Utility Relocation ................................................................................... 4-13

4.12.7 Traffic Plan ............................................................................................ 4-13

4.13 Construction Plan ............................................................................................... 4-13

4.13.1 Sequence of Construction ...................................................................... 4-13

4.14 Real Estate Information ..................................................................................... 4-14

4.14.1 New Levee Right-of-Way ...................................................................... 4-14

4.14.2 Construction Laydown/Staging Area ..................................................... 4-14

4.15 Government-Furnished Borrow Material Location ........................................... 4-14

4.16 Haul Route ......................................................................................................... 4-15

4.17 Access Roads ..................................................................................................... 4-15

4.18 Utility Removal .................................................................................................. 4-15

4.19 Dewatering Plan ................................................................................................. 4-15

5.0 CONSTRUCTION SCHEDULE .................................................................................. 5-1

5.1 Overview .............................................................................................................. 5-1

5.2 Schedule Constraints ............................................................................................ 5-1

5.3 Assumptions for the Construction Schedule ........................................................ 5-1

5.3.1 Time Extensions for Unusually Severe Weather ..................................... 5-3

URS Group, Inc. v USIBWC, El Paso, Texas

5.4 Description of Critical Path Items........................................................................ 5-3

5.5 Potential Delays for Construction ........................................................................ 5-3

5.5.1 Environmental Issues ............................................................................... 5-3

6.0 ADDITIONAL INFORMATION REQUIRED .......................................................... 6-1

LIST OF TABLES

1-1 Summary of Wasteways 1 and 2 Structures 1-2 Milestones 4-1 Drainage Structures 4-2 Summary of Computed Factors of Safety for Subreach 1 4-3 Summary of Exit Seepage Gradients for Subreach 1 4-4 Summary of Computed Factors of Safety for Subreach 2 4-5 Summary of Exit Seepage Gradients for Subreach 2 4-6 Load Conditions for Drainage Structure Design 4-7 Unit Weights 5-1 Estimated Procurement and Construction Schedule 5-2 Monthly Anticipated Adverse Weather Calendar Days

LIST OF FIGURES

1 Project Location and Vicinity Map 2 Geologic Map of Project Area

URS Group, Inc. vi USIBWC, El Paso, Texas

Acronyms and Abbreviations

AASHTO American Association of State Highway and Transportation Officials

ACI American Concrete Institute

APE Area of Potential Effect

ASTM American Society for Testing Materials

DDR Design Documentation Report

EA Environmental Assessment

EPCWID #1 El Paso County Water Improvement District #1

FEMA Federal Emergency Management Administration fps feet per second

HEC-RAS Hydrologic Engineering Center River Analysis System

LF linear feet

NOAA National Oceanic and Atmospheric Administration

RGRP Rio Grande Rectification Project

RKCI Raba-Kistner Consultants, Inc.

ROW right-of-way

SOW Scope of Work

THC Texas Historical Commission

TOLE Tope of Levee Elevation

TPWD Texas Parks and Wildlife Department

TRS Temporary Restraining Structure

TxDOT Texas Department of Transportation

URS URS Group, Inc.

USACE U.S. Army Corps of Engineers

USIBWC United States International Boundary and Water Commission

WSE Water Surface Elevation

URS Group, Inc. 1-1 USIBWC, El Paso, Texas

1.0 PROJECT INFORMATION

1.1 General

Supplement 1 to "Design of Rehabilitation Levee Improvements of the Rectification Project from Ysleta to Fabens, Texas El Paso County, Texas," dated June 2012, addresses revisions to the design of levee improvements to reflect changes submitted by the U.S. International Boundary and Water Commission (USIBWC). The current review involves a design of drainage structure and reconstruction of adjacent levee sections amounting to a total of 1,250 linear feet (LF) and the reconstruction of four (4) ramps as described in "RFP IBM14-04_Design SOW Wasteways 1 and 2."

The portion of the Rio Grande Rectification Project (RGRP) located in El Paso and Hudspeth Counties, Texas from Ysleta to Fabens was constructed from 1934 to 1938. The purpose of the project was to stabilize the international river boundary and provide flood protection for both counties in urban, suburban, and agricultural areas. The RGRP also provides irrigation and drainage benefits to agricultural areas. USIBWC investigated the structural condition of the levee from Ysleta to Fabens based on a 100-year flood event. This project represents approximately 1,250 LF and two drainage structures known as Wasteway 1 and 2, which are all part of the 91 levee miles in the RGRP on the U.S. side. The work required under this Scope of Work (SOW) will be limited to these 1,250 LF of levee and structural work.

1.2 Authorization

USIBWC retained the services of URS Group, Inc. (URS) under Task Order IBM14T0021 of Contract No. IBM09D0008 to provide services for the design and preparation of the plans and specifications for the improvements project. The SOW entitled "RFP IBM14-04_Design SOW Wasteways 1 and 2 Final_2," dated September 2014, was provided to URS by USIBWC.

1.3 Purpose of Design Documentation Report

This Design Documentation Report (DDR) presents the basis for design and the essential data, assumptions, criteria, and computations for developing the design, plans, specifications, and costs for improvements to the Ysleta to Fabens Reach. The purpose of the DDR is to provide sufficient design detail and analyses to support the preparation and review of the deliverables for this project.

1.4 Project Description

1.4.1 Project Location

Wasteways 1 and 2 within the Ysleta to Fabens Reach of the USIBWC levee are located within El Paso County in Texas, as shown on the project location map in Figure 1. The SOW for this levee reach covers approximately 1,250 LF.

URS Group, Inc. 1-2 USIBWC, El Paso, Texas

1.4.2 Existing Features

The existing Wasteways 1 and 2 structures within the footprint of the design scope within the Ysleta to Fabens Reach consists of an earthen embankment constructed of silts, sands, and clays with an approximate crown width of 16 feet, an average height of 8.7 feet, and a series of 6-ft x 5-ft box culverts for each. These structures are owned by the El Paso County Water Improvement District #1 (EPCWID #1).

The "Final Environmental Assessment Improvements to the Rio Grande Rectification Project," March 2009, identified three structures located within the boundaries of the Wasteways 1 and 2 project limits.

1.4.3 Project Objectives

The objectives for the design of the Wasteways 1 and 2 project included:

Rehabilitating wasteway structures. The design must allow for closure of structures during flood events and must encompass necessary structural repair or replacement.

Certifying the levee to withstand the 100-year flood with +3 feet of freeboard.

1.4.4 Project Requirements

The following were requirements for the design of Wasteways 1 and 2, per the SOW:

Follow U.S. Army Corps of Engineers (USACE) and Federal Emergency Management Administration (FEMA) criteria for flood risk reduction design (see Section 3).

Design a minimum 16-foot-wide maintenance road at the crest of the levee consisting of a 6-inch layer of flex-base material directly placed on the finished embankment material.

Design sideslopes to be no steeper than 3H:1V on the river side and no steeper than 3H:1V on the land side. However, if existing right-of-way (ROW) is insufficient, then landside sideslopes cannot be steeper than 2.5H:1V unless the design does not require mowing.

Design of handrails per Safety and Health Requirements Manual, EM 385-1-1 (2008 with all consolidated amendments through August 2011).

Design of guardrails per American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide, 4th Edition, October 2011, including signage per the MUTDC.

Conduct a geotechnical investigation of levee embankment materials and foundation soils and perform seepage and stability analyses to assess levee performance during the design flood event and other critical hydraulic cases.

Minimum Top of Levee Elevation (TOLE).

URS Group, Inc. 1-3 USIBWC, El Paso, Texas

Table 1-1. Summary of Wasteways 1 and 2 Structures

Structure Station

Structure Elevation (ft msl)

Utility Flow Line

Coordinates Structure

Name Structure Size Structure

Usage Ownership Notes X (Easting) Y (Northing) 933+30 Top of

Levee:

3663.29*

3649.8 438302.34 10606233.55 Wasteway No. 1

Five sluice gates with five 6-ft x 5-ft box culverts

Drainage/ Irrigation

EPCWID

#1

1319+12.00 Top of Levee:

3633.05*

3625.02 455147.81 10574171.16 Wasteway No. 2

Three sluice gates with three 6-ft x 5-ft box culverts

Drainage/ Irrigation

EPCWID

#1

* Estimated elevations from as-built plans.

Note, there are no known utilities within the limits of construction for this project.

URS Group, Inc. 1-4 USIBWC, El Paso, Texas

1.4.5 Description of Alterations

The alterations for the Wasteway improvements include:

Structures:

o Wasteway 1 box culvert replacement – Replace five existing 6-ft x 5-ft box culverts with five new 6-ft x 5-ft box culverts.

o Wasteway 2 box culvert replacement – Replace three existing 6-ft x 5-ft box culverts with three new 6-ft x 5-ft box culverts and new slide gates.

Ramps:

o Two ramps on the river side of Wasteway 1 at STA 932+50 and STA 934+00.

o Two ramps on the river side of Wasteway 2 at STA 1318+50 and STA 1319+50.

Handrails:

o Design of handrails for health and safety at both Wasteways 1 and 2.

1.5 Project Deliverables

Table 1-2. Milestones

Deliverable Due Date for Submittal 90% Construction Drawings, Specifications, Opinion of Probable Cost, Design Documentation Report 26 January 2015

Final Construction Drawings 23 February 2015

URS Group, Inc. 2-1 USIBWC, El Paso, Texas

2.0 PERTINENT DATA

2.1 Project Purpose

The purpose of this project is to perform rehabilitation needs for the existing flood protection to contain the Design Protection Elevation + 3 feet of freeboard. The project will include the evaluation and replacement of deteriorated structures.

2.2 Opinion of Probable Construction Cost

The Opinion of Probable Cost is attached under separate cover.

2.3 Existing Utilities

The existing utilities adjacent to the work included in the construction of Wasteways 1 and 2 are only the Wasteways themselves. Extensive research of all utilities in the area has been done using field verification and historical documentation and by contacting known utilities in the area to verify the locations of their utilities. A complete list of all known utilities and their respective locations within the project limits and elevations of crossing is provided in Table 1-1.

2.4 Design Data

2.4.1 Controlling Elevations

The controlling elevation is the minimum TOLE, which is equal to the 100-year water surface elevation (WSE) plus 3 feet of freeboard.

2.4.2 Hydraulics Data

The following hydraulics data were provided by USIBWC under the previous design contract for the Riverside-Fabens Rectification project:

Excel® spreadsheet with 100-year WSE and minimum TOLE for the Riverside Dam to Fabens Reach; and

Hydrologic Engineering Center River Analysis System (HEC-RAS) Model for the Riverside Dams to Fabens Reach.

2.4.3 Survey Data

Existing topographic data shown on the plans were produced from an on-the-ground survey performed by Frank X. Spencer & Associates, Inc., El Paso, Texas during October/November 2010. An additional topographic survey was conducted for the canal and submitted in June 2011. The horizontal coordinates shown on the plans are referenced to the Texas State Plan Coordinate System, Central Zone, NAD83, U.S. Survey feet. Coordinates shown are surface coordinates, using a combined scale factor of 1.0002414337. To convert to grid coordinates, multiply by 0.99758625 for this project. Surface elevations shown on the plans are referenced to

NAVD88.

URS Group, Inc. 2-2 USIBWC, El Paso, Texas

2.4.4 Soil Borings

Soil borings were provided by Raba-Kistner Consultants, Inc. (RKCI) as part of the "Geotechnical Analysis and Engineering Evaluation Report," dated November 2010 (Geotechnical Report). Additional soil borings were provided by RKCI as part of the "Supplemental Geotechnical Analysis for Final Design," dated June 2012 (Geotechnical Supplement). RKCI drilled all borings for the project.

The original 2010 subsurface investigation included a total of 86 borings. The borings were generally drilled to a depth of 50 feet. Borings were advanced using a modified hollow-stem auger procedure. Hollow-stem augers were used from the ground surface until caving soils below the groundwater table were encountered. Mud rotary methods were then employed until reaching the boring termination depth. Upon completion of drilling, bentonite-cement grout was used to backfill the boreholes using tremie methods in accordance with USACE guidelines.

For the supplemental subsurface investigation, two borings each were drilled at Wasteway 1 (Sta. 933+13) and Wasteway 2 (Sta. 1319+23), consisting of one boring on the existing levee crest and one boring on the landside bank of the canal. The wasteway borings were drilled to a depth of 50 feet below existing grade.

URS Group, Inc. 3-1 USIBWC, El Paso, Texas

3.0 DESIGN INFORMATION AND REFERENCES

3.1 Design Criteria

The following standard is a requirement from USIBWC for the design of the project:

A/E/C CADD Standard, Release 4.0, July 2009.

The following design criteria are part of USACE requirements for the design of the project:

EC 1110-2-1806, "Earthquake Design and Evaluation for Civil Works." July 31, 1995.

EC 1110-2-1913, "Design and Construction of Levees." April 30, 2000.

EC 1110-2-6067, "Process for the National Flood Insurance Program (NFIP) Levee System Evaluation." August 31, 2010.

EM 1110-1-1002, "Survey Markers and Monumentation," 14 September 1990.

EM 1110-1-1005, "Control and Topographic Mapping," 31 August 1994.

EM 1110-2-301, "Guidelines for Landscaping Planting at Floodwalls, Levees, and Embankment Dams," 1 January 2000.

ER 1110-2-1200, "Plans and Specifications for Civil Works Projects," 12 June 1972.

EM 1110-2-1205, "Environmental Engineering and Local Flood Control Channels," 15 November 1989.

EM 1110-2-1601, "Hydraulic Design of Flood Control Channels," 1 July 1991.

EM 1110-2-1902, "Slope Stability," 31 October 2003.

EM 1110-2-1908, "Instrumentation of Embankment Dams and Levees," 30 June 1995.

EM 1110-2-2000, "Standard Practice for Concrete for Civil Works Structures," 1 February 1994.

EM 1110-2-2007, "Structural Design of Concrete Lined Flood Control Channels," 30 April 1995.

EM 1110-2-2104, "Strength Design for Reinforced Concrete Hydraulic Structures," 30 June 1992.

EM 1110-2-2105, "Design of Hydraulic Steel Structures," 31 March 1993.

EM 1110-2-2902, "Conduits, Culverts, and Pipes," 31 October 1997.

URS Group, Inc. 3-2 USIBWC, El Paso, Texas

ER 1110-1-261, "Quality Assurance of Laboratory Testing Procedures," 28 April 1999.

ER 1110-1-8100, "Laboratory Investigations and Testing," 31 December 1997.

ER 1110-1-8155, "Specifications," 10 October 2003.

ER 1110-2-401, "Operation, Maintenance, Repair, Replacement, and Rehabilitation Manual for Projects and Separable Elements Managed by Project Sponsors," 30 September 1994.

ER 1110-2-1150, "Engineering and Design for Civil Works Projects," 31 August 1999.

ER 1110-2-8152, "Planning and Design of Temporary Cofferdams & Braced Excavations," 31 August 1994.

ETL 1110-2-256, "Sliding Stability for Concrete Structures," 24 June 1981.

ETL 1110-2-307, "Floatation Stability Criteria for Concrete Hydraulic Structures," 20 August 1987.

ETL 1110-2-569, "Design Guidance for Levee Underseepage," 1 May 2005.

ETL 1110-2-571, "Guidelines for Landscape Planting and Vegetation Management at Levees, Floodwalls, Embankment Dams, and Appurtenant Structures," 10 April 2009.

ETL 1110-2-573, "Construction Cost Estimating Guide for Civil Works," 30 September 2008.

SWD-AEIM Architectural and Engineering Instructions Manual.

SWFP 1150-2-1, "Criteria for Construction within the Limits of the Existing Federal Flood Protection Projects," 31 October 2003.

TM 5-818-8, "Engineering Use of Geotextiles," 20 July 1995.

USACE, "Engineering Manual File Format Specification Version: EM06," 7 May 2009.

3.2 Other Codes and Standards Used in Design

The following codes and standards were utilized in completing the design of the Project:

AASHTO, Standard Specifications for Highway Bridges.

American Concrete Institute (ACI), Building Code Requirements for Structural Concrete, ACI 318-99.

ACI 350-06, "Code Requirements for Environmental Engineering Concrete Structures."

URS Group, Inc. 3-3 USIBWC, El Paso, Texas

American Society of Civil Engineers/Structural Engineering Institute, Standard 7-05.

American Society for Testing Materials (ASTM) International Standards.

Bureau of Economic Geology at the University of Texas at Austin, "Geologic Atlas of Texas," Van-Horn/El Paso Sheet, 1968.

Texas Department of Transportation (TxDOT), Roadway Design Manual, 2004.

TxDOT Specifications and Standards.

Web Soil Survey of El Paso County, U.S. Department of Agriculture, Natural Resources of Conservation Service, http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx.

3.3 General References

The following documents were reviewed during the design of the Project:

Mapping Alliance Partnership (MAPVI). "Flood Frequency Determination, El Paso County and Incorporated Communities, Texas – Disaster Response Flood Recovery Data." Submitted to U.S. Dept. of Homeland Security, FEMA. March 26, 2007.

RKCI. "DRAFT – Feasibility Study and Conceptual Design for Paisano Reach (Phase II) Upper Rio Grande Flood Control System Rectification Project, El Paso, El Paso County, Texas. Draft Geotechnical Memorandum." Prepared for URS, February 12, 2011.

RKCI. "Geotechnical Analysis and Engineering Evaluation of Rectification Project Levee from Riverside Dam to Fabens, El Paso County, TX." Prepared for URS, November 2010.

RKCI. " Supplemental Geotechnical Analysis for Final Design Rectification Project Levee From Riverside Dam to Fabens, El Paso County, TX." Prepared for URS, December 2011.

RKCI. "Geotechnical Memorandum for Courchesne Reach, El Paso, El Paso County, Texas; Nemexas Reach, Sunland Park, Dona Ana County, New Mexico; Upper Rio Grande Flood Control System Rectification Project." March 2, 2011.

Terracon. "Geotechnical Engineering Report – IBWC Upper Rio Grande Levee Evaluation: Fabens, Texas." December 21, 2009.

URS. "DRAFT – Guidance Document for Geotechnical Analyses." Prepared for Sacramento, CA Department of Water Resources, Urban Levee Geotechnical Evaluations Program Contract. Revision 9, January, 2011.

URS. "Design of Rehabilitation Levee Improvements of the Rectification Project from Ysleta to Fabens, Texas El Paso County, Texas," June 2012.

URS Group, Inc. 3-4 USIBWC, El Paso, Texas

USACE. ERDC TR-03-4 Report 2: "Condition Assessment of U.S. International Boundary and Water Commission, Texas and New Mexico Levees. Volume III:

Rectification Levees," March 2004.

RKCI. "Supplemental Geotechnical Analysis for Final Design Rectification Project Levee from Riverside Dam to Fabens," June 22, 2012.

3.4 Approval Criteria Waivers

No waivers on criteria have been submitted for approval.

3.5 Computer Programs Used

The following are the computer programs used in development of the Project:

Adobe Portable Document Format Writer and Reader; Adobe Professional (Versions 9.0 and 10.0); Adobe Systems Inc.

AutoCAD Civil 3D (2009): Computer-aided drafting and design (CADD) software, AutoDesk, Inc.

Microsoft Excel (2010): A Software for Storing, Organizing and Manipulating Data;

Microsoft Corporation

Microsoft Word (2010): Word Processing Software; Microsoft Corporation

SAP2000: Structural Analysis and Design Software; CSI, Computer & Structures, Inc.

Specifications-Kept-Intact (SpecsIntact, Version 4.4.2): An Automated System for Preparing Standardized Facility Construction Specifications; National Aeronautics and Space Administration - Information Management Communications Services, Kennedy Space Center.

3.6 Government-Provided Information

USIBWC provided the following project documents to URS for preparation of this DDR and the plans and specifications for the Wasteways 1 and 2 improvements project.

As-Builts:

o EPWCID #1 drawing set showing details for the wasteway structures, "Riverside WW1 Impvts."

URS Group, Inc. 4-1 USIBWC, El Paso, Texas

4.0 ENGINEERING DESIGN

4.1 Hydrology and Hydraulics

4.1.1 Background

In December 2007, USIBWC completed a hydraulic model of the RGRP to analyze existing conditions along the Rio Grande. The detailed technical analysis was performed using USACE's HEC-RAS Version 3.12. The study determined the capacity of the existing RGRP structure to convey the design flood flows without encroaching on the freeboard. At the time that the analysis was completed, the proposed freeboard for the Rio Grande levee was 3 feet.

The HEC-RAS model provided by USIBWC shows that flows are conservatively assumed throughout the watershed at 11,000 cubic feet per second (fps). Velocities in the Rio Grande channel range from 5.45 to 15.75 fps along the alignment; however, the velocities are much lower along the left bank of the floodway and on the side slopes of the levee. These velocities range from 0.48 to 1.55 fps.

4.1.2 Hydraulic Features

The hydraulic features that will convey the flow of water along the Ysleta to Fabens Reach are Wasteway No. 1 and Wasteway No. 2. Their specific locations are listed in Table 4-1.

Table 4-1. Drainage Structures

Structure STA / Location WSE Existing Size Proposed Size

Wasteway No. 1 933+29.42 3650.75 5 barrels; 5 ft x 6 ft Same as Existing

Wasteway No. 2 1219+22.98 3630.69 3 barrels; 5 ft x 6 ft Same as Existing

4.2 Geotechnical

4.2.1 Objectives

The geotechnical investigation was performed for the 15.8-mile levee segment from Riverside Dam in the Ysleta region of El Paso to Fabens, Texas with specific focus on the Wasteways 1 and 2 structures. Borings were drilled to determine subsurface conditions of soils and to obtain soil samples for laboratory testing. The subsurface information collected was supplemented by the USACE report "Condition Assessment of the USIBWC, Texas and New Mexico Levees," dated June 2004. The investigation and analysis were performed by RKCI under contract with

URS.

For more information, refer to the following reports:

Geotechnical Analysis and Engineering Evaluation ("Geotechnical Report"), submitted by RKCI in November 2010; and

URS Group, Inc. 4-2 USIBWC, El Paso, Texas

Supplemental Geotechnical Analysis for Final Design ("Geotechnical Supplement"), submitted by RKCI in December 2011 and revised in June 2012.

4.2.2 Background

Levee System Components

This project, like others surrounding the Rio Grande in this area, is historically linked to irrigation of local farmlands and sharing irrigation water between the U.S. and Mexico. This objective is accomplished through a network of irrigation canals that parallel the levee for portions of this project. The primary water-delivery component is the Riverside Canal that receives water via the Riverside Diversion Dam and supplies the irrigation network extending throughout the farmlands in Upper Rio Grande Valley. A second component is a smaller unlined canal that is topographically lower than the primary canal and facilitates drainage of the farmlands by means of percolation and is generally referred to by the Irrigation District as the "drain." This drain runs parallel to the levee alignment. The drain is located in some areas between the canal and the levee, and in other areas, the canal is located between the drain and the levee. Additionally, there are small irrigation ditches on the landside of the levee system that provide local irrigation water. Water in these ditches is controlled by slide gates between the canal, drain, and ditches.

Previous Flood Events and Past Performance

In 2006, El Paso County experienced a storm event during the last few days of July and the first few days of August. Typically, storms in this area are short-duration, high-intensity storms.

However, this event was atypical as it was a lower intensity storm with a longer duration, on the order of 10 days. A previous study (MAPVI, 2007) reported that for some watersheds in El Paso County, the 24-hour storm had return periods ranging from the 50-year storm to the 400-year storm. According to the same report, this 2006 storm had the highest daily rainfall on record for the El Paso Airport rain gage (one of several rain gages in the area), reporting 2.84 inches of rainfall. However, neither the return period for the rainfall event nor the return period for the flood event is known for the exact area along the Ysleta to Fabens Reach, as there are not enough gage data to accurately determine these. USIBWC reports that despite the levee overtopping in places during this event, overall the levee performed well, and the integrity was not compromised. There have also been reports of flooding in 1986; however, very little information is readily available about this storm event.

During a site visit on March 9, 2011 that included URS, USIBWC, and Irrigation District staff, localized areas of superficial erosion, sloughing, and rutting were observed. In a few localized areas, possible evidence of minor through-seepage (on the drain slope) and sand boils (on the concrete-lined canal slope) were observed. These occurrences were most likely the result of the 2006 flood event, which was indicated to be in excess of the 100-year-storm design flood.

USIBWC's maintenance crews regularly inspect the levee slopes and perform maintenance repairs as needed. However, some of these irregularities had not been repaired because the levee slopes had not been affected, and the affected area was small and difficult to access. These areas are under the jurisdiction of the Irrigation District.

URS Group, Inc. 4-3 USIBWC, El Paso, Texas

An additional challenge associated with maintaining the levee is the occurrence of animal burrows (such as those created by gophers and foxes) that are frequently encountered within the levee embankment. According to USIBWC staff, these burrows are typically small and can usually be remediated by prompt excavation and backfilling. However, one occurrence of these burrows resulted in a large area of subsidence during the 2006 flood, which involved about a 1-foot loss of freeboard in the levee over an area approximately 25 feet by 20 feet. USIBWC staff remediated the affected area by excavating and recompacting levee soils. No additional remediation approaches have been implemented at this time to address animal burrows.

4.2.3 Nearby Structures

Border Security Fence

The border security fence runs parallel to the Wasteways 1 and 2 project. In general, the fence is located on the landside slope of the levee near the crest. The foundation of the border fence consists of reinforced concrete strip footing ("trench" footing) that extends 6 feet below the ground surface. There were several gaps in the border fence and foundation along the levee reach, including gaps at Wasteways No. 1 and 2, that were closed in 2011, since the project was started.

4.3 Project Setting

4.3.1 Regional Geology

The Geologic Atlas of Texas – Van Horn to El Paso Sheet (Bureau of Economic Geology, 1968) was used to summarize the regional geology in the vicinity of the project site. Geologic units visible on the Atlas within a few miles of the levee alignment are described in order from youngest to oldest. The geologic map with the superimposed project alignment is provided on Figure 2.

The youngest geologic units found near the project site are Quaternary units from the Holocene period. These include Windblown sands (Qws) that may exhibit areas of large dunes (Qwsd).

Also present from the Holocene period are Young Quaternary deposits, which consist of alluvium along the Rio Grande (Qalr), alluvium and low terrace deposits along other streams (Qal), and lacustrine and fluviatile deposits of clay, silt, sand, and gypsum in bolsons (Qb).

Quaternary units from the Pleistocene period include Old Quaternary deposits, which include alluvium, colluvium, caliche, and gypsite on surface dissected by modern drainage, and Terraces along the Rio Grande (Qao).

Also present in the vicinity of the project alignment are Tertiary units of the Oligocene period, which include Bolson deposits (QTb) consisting of clay, sand, and gravel, and are in part gypsiferous with some caliche.

A number of faults are indicated on the Geologic Atlas, the closest of which is approximately 8 miles east of the project alignment. Faults appear to be randomly oriented for the most part, with a series of faults trending nearly north-south and others trending nearly east-west.

URS Group, Inc. 4-4 USIBWC, El Paso, Texas

4.3.2 Site Seismicity

Seismic coefficients were determined by RKCI using Section 1613 – Earthquake Loads of the 2006 International Building Code. Because the project alignment covers a substantial geographic range, the seismic coefficients vary along the project. The following parameters are presented as a range of values that represent borings B-1 through B-86.

The Spectral Response Acceleration of the Maximum Mapped Earthquake Ground Motion for a 0.2-second period is Ss = 0.325 to 0.334 g.

The Spectral Response Acceleration of the Maximum Mapped for Earthquake Ground Motion for a 1-second period is S1 = 0.104 to 0.105 g.

4.3.3 Geomorphology

The primary geomorphologic feature of the site included the meandering nature of the historic Rio Grande River. Engineering controls implemented in the late 1930s and early 1940s effectively straightened the river alignment. Geomorphologic data available for review included aerial photographs from 1936 and 1937, and 1936 treaty parcel maps (developed under the Convention of 1933) that illustrated the Historic Rio Grande Alignment of 1933. This information was used to construct the historic river alignment with respect to the current river alignment to facilitate an evaluation of the geomorphologic effects on the subsurface soils. Both the historic Rio Grande alignment and another unknown stream were discovered in the photos. It was determined that these historic river channel meanders have had a significant impact over the entire project area, effectively normalizing any localized impacts.

4.4 Summary of Analyses Performed

A seepage and sliding stability analysis was performed and discussed in the "Supplemental Geotechnical Analysis for Final Design Rectification Project Levee from Riverside Dam to Fabens," dated June 2012.

4.5 Analysis Results

4.5.1 Seepage and Slope Stability

A summary of the analysis results is provided below. Complete seepage analysis and slope stability results are provided in the Geotechnical Supplement. Note that the critical analysis condition for levee sections was considered to be when the 100-year flood WSE was developed on the riverside, whereas the critical condition for wasteway sections was considered to be when the canal (located on the landside of the levee) reached its full flow depth.

Wasteway No. 1

Seepage analyses performed at Wasteway No. 1 (Sta. 933+30) indicated acceptable results per USACE criteria, but under-seepage entry gradients are sufficiently high that piping is a concern.

Consequently, seepage control measures, consisting of a turned-down upstream foundation or hanging sheet pile at least 3 feet below finished grade, are recommended. Slope stability

URS Group, Inc. 4-5 USIBWC, El Paso, Texas analyses indicated that factors of safety met USACE minimum criteria. The computed factors of safety are summarized in Table 4-2, and the exit seepage gradients are in Table 4-3.

Table 4-2. Summary of Computed Factors of Safety for Subreach 1

Stability Condition Required Factor of Safety Range of Computed

Factor of Safety Rapid Drawdown 1.0* NA

Long-Term Steady Seepage 1.4 2.3 – 3.1

* A minimum Factor of Safety of 1.0 applies to water levels prior to sudden drawdown for conditions where these water levels are unlikely to persist for lengthy periods preceding drawdown.

Table 4-3. Summary of Exit Seepage Gradients for Subreach 1

Stability Condition Required Maximum

Exit Gradient Computed Maximum

Exit Gradient* Rapid Drawdown 0.5 <0.5

Long-Term Steady Seepage 0.5 <0.5

* Exit gradients were computed with drain/canal filled where applicable.

Wasteway No. 2

Seepage analyses performed at Wasteway No. 2 (Sta. 1319+12) demonstrated excessive under-seepage gradients and uplift on the wasteway culverts. Additional analyses indicated that an 8-ft-deep seepage cutoff can effectively reduce gradients and uplift to meet USACE criteria.

Slope stability analyses indicated that factors of safety met USACE minimum criteria. The computed factors of safety are summarized in Table 4-4, and the exit seepage gradients are in Table 4-5.

Table 4-4. Summary of Computed Factors of Safety for Subreach 2

Stability Condition Required Factor of Safety Range of Computed

Factor of Safety Rapid Drawdown 1.0* NA

Long-Term Steady Seepage 1.4 1.9 – 3.5

* A minimum Factor of Safety of 1.0 applies to water levels prior to sudden drawdown for conditions where these water levels are unlikely to persist for lengthy periods preceding drawdown.

Table 4-5. Summary of Exit Seepage Gradients for Subreach 2

Stability Condition Required Maximum

Exit Gradient Computed Maximum

Exit Gradient* Rapid Drawdown 0.5 <0.5

Long-Term Steady Seepage 0.5 <0.5

* Exit gradients were computed with drain/canal filled where applicable.

URS Group, Inc. 4-6 USIBWC, El Paso, Texas

4.5.2 Liquefaction

Soil liquefaction results from loss of strength during cyclic loading, such as that imposed by earthquakes. Soils most susceptible to liquefaction are clean, loose, saturated, uniformly graded, and fine-grained sands. Empirical evidence indicates that loose silty sands are also potentially liquefiable.

While design for liquefaction is not a requirement of EM 1110-2-1913 for earthen levees, an evaluation of liquefaction potential was performed due to the presence of saturated, very loose to loose, cohesionless soils near the ground surface. The analysis found that when groundwater is present, the wasteway structures have a significant potential for liquefaction during earthquake events, particularly when the earthquake magnitude exceeds 6.5. Following a discussion with USIBWC staff regarding the risk of damage to the two wasteway structures as a result of liquefaction and the measures that would be required to limit the damage, USIBWC indicated that mitigation measures would likely be more expensive than structure replacement. As a result, this report does not contain recommendations for measures to limit liquefaction-induced damage to the levees or the two wasteway structures.

4.5.3 Erosion Control

Based on past performance during flooding in which only minor erosion was observed on the riverside slopes, erosion from the design storm is not anticipated to affect the integrity of the levee. Consistent with the rehabilitation of other reaches of the Upper Rio Grande Levee, segments of the levee that are not undergoing rehabilitation are not receiving any erosion protection.

Rock riprap will be placed behind each wasteway to prevent erosion of soils behind the wasteway headwalls and wingwalls as has previously occurred. This riprap as well as riprap in the wasteway exit channels has been designed with consideration for the maximum velocity of the river during flood events as well as rainfall runoff.

4.6 Geotechnical Design Features

4.6.1 Seepage Control Measures

Wasteways No. 1 and 2

Analyses indicate that seepage cut-off barriers, such as sheet pile walls, are required at the wasteway structures to reduce under-seepage gradients. The sheet piles should be cast into the concrete for the wasteway structures and extend 3 feet and 8 feet below the base of Wasteways No. 1 and 2, respectively. Along the sides of the Wasteways, the sheet pile should extend from the top of the wasteway structures to the appropriate depth below the base. Sheet piles should also extend 100 feet to either side of the Wasteways.

Since both Wasteways are to be reconstructed, these control measures should be implemented during the reconstruction. Due to the possibility of high water on both the river side and in the canal during construction, temporary excavation support and dewatering will likely be required.

The contractor will be responsible for excavation support and dewatering.

URS Group, Inc. 4-7 USIBWC, El Paso, Texas

4.7 Environmental Considerations

4.7.1 Overview

In March 2009, USIBWC, in cooperation with the Texas Parks and Wildlife Department (TPWD), completed an Environmental Assessment (EA) for the proposed improvements to flood control infrastructure along sections of the RGRP entitled "Final Environmental Assessment for Improvements to the Rio Grande Rectification Project," which is available at:

ftp://63.96.218.8/Rectification_FinalEA_Mar2009.pdf. The Ysleta to Fabens Reach is located within the project area studied in the EA.

The conclusions of the EA are based on conceptual-level design of the levee system and the assumption that embankment material would be added to the existing levee to bring height to its original design specifications or to meet a 3-foot freeboard design criterion. The EA also assumed that raising the levee would require an expansion of the footprint in various locations, which would remain entirely within the existing ROW. The Area of Potential Effect (APE) for the RGRP improvements consists of USIBWC ROW, including the levee. This ROW is sufficient to include any staging areas needed for construction activities associated with the proposed action.

4.7.2 Biological Resources

The EA concluded that all expansion would take place within the current levee service corridor with no significant effects on wildlife habitat in the vicinity of the levee system. In areas requiring levee footprint expansion, impact on vegetation would be limited to non-native, managed salt cedar habitats and managed old-field habitats along that levee that are of very limited value as wildlife habitat. This grass cover is expected to be rapidly re-established after project completion. In areas requiring levee footprint expansion, it was anticipated that no woodland communities would be impacted. The EA concluded that the levee expansion may remove some habitat for the Species of Concern, the Burrowing Owl, but levee expansion would occur outside the breeding season of the owls to reduce the impacts. To protect the burrowing owl, coordination between TPWD and USIBWC would move the nests or replace them with artificial nests as required (EA Section 5.2). Impacts expected in expansion areas are not located within natural resource conservation areas. The placement of embankment material would be required within these areas but without levee footprint expansion.

4.7.3 Cultural Resources

Historic Resources

The EA concluded that improvements to the levee system were not expected to adversely impact historical resources. This finding was based on the assumption that only irrigation canals and minor drainage structures, such as weir gates and standpipes, are located within or near stretches of the levee that would require footprint expansion and that nearly all drainage structures would be retained in their current condition.

URS Group, Inc. 4-8 USIBWC, El Paso, Texas

However, it has been determined that some of the irrigation structures will need to be replaced to meet current USACE design criteria. The removal of these structures will require coordination with the Texas Historical Commission (THC).

Archaeology

No archaeological resources have been previously identified within the APE. No intensive archaeological surveys have been performed in the project area. There are three existing structures that will be impacted by construction activities. Wasteway No. 1, Wasteway No. 2, and the wastewater effluent discharge. The historic nature of these structures is unknown. Prior to any construction, these structures should be verified by USIBWC through the THC.

4.7.4 Construction Approaches

The best management practices and mitigation measures indicated in Section 5 of the EA report will be implemented as part of the Proposed Action to minimize the potential for impacts to natural resources. A work plan for the Phase I survey will be developed and submitted to the Texas State Historical Preservation Office (SPHO) for review and approval. Best management practices during construction could include use of sediment barriers and soil wetting to minimize erosion and dust, revegetation, and coordination with natural resources agencies. Please refer to "Final Environmental Assessment for Improvements to the Rio Grande Rectification Project," Chapter 5 for a complete list of best management practices and mitigation measures.

4.8 Structural Design Features

4.8.1 Overview

Two drainage structures – Wasteway No. 1 and Wasteway No.