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Condition Assessment of U.S. International Boundary and Water Commission, Lower Rio Grande Valley Levees, South Texas Volume I, Main Text and Appendices

Joseph B. Dunbar, Robert F. Ballard, William L.

Murphy, Thomas E. McGill, Jose L. Llopis, Linda D.

Peyman-Dove, and Michael J. Bishop

October 2003

E ng in ee r R es ea rc h an d

D ev el op m en t C en te r

Distribution authorized to U.S. Government agencies only; administrative or operational use; October 2003. Other requests for this document shall be referred to Geotechnical and Structures Laboratory, ATTN: CEERD-GS-G, U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199.

ERDC TR-03-4

October 2003

Condition Assessment of U.S. International Boundary and Water Commission, Lower Rio Grande Valley Levees, South Texas Volume I, Main Text and Appendices

Joseph B. Dunbar, Robert F. Ballard, William L. Murphy, Thomas E. McGill, Jose L. Llopis

Geotechnical and Structures Laboratory U.S. Army Engineer Research and Development Center 3909 Halls Ferry Road Vicksburg, MS 39180-6199

Linda D. Peyman-Dove, Michael J. Bishop

Environmental Laboratory U.S. Army Engineer Research and Development Center 3909 Halls Ferry Road Vicksburg, MS 39180-6199

Final report

Distribution authorized to U.S. Government agencies only;

administrative or operational use; October 2003. Other requests for this document shall be referred to Geotechnical and Structures Laboratory, ATTN: CEERD-GS-G, U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199.

Prepared for U.S. Army Corps of Engineers Washington, DC 20314-1000

ABSTRACT: The U.S. Army Engineer Research and Development Center (ERDC) performed a condition assess-ment of the U.S. section of International Boundary and Water Commission (IBWC) levees along the Lower Rio Grande River Valley (LRGV) and its floodways in south Texas. ERDC and the IBWC conducted the condition as-sessment using airborne and surface geophysics, geologic studies, groundtruth investigations, and an Enterprise Geographic Information System (eGIS). The assessment encompassed 270 miles of levee and more than 600 associated structures within the Lower Rio Grande Flood Control Project (LRGFCP) in Hidalgo, Cameron, and Willacy Counties of Texas. The goal of the assessment was to detect, identify, and map potentially unstable levee reaches that require repair or replacement. In addition, the condition assessment will enable managers to prioritize tools to identify preventive and remedial measures for embankments based on best engineering practice.

Airborne, or overhead, geophysical surveying was the primary method used to assess the condition of IBWC levees in south Texas. Airborne methods included multifrequency electromagnetic (EM), digital video, magnetic (magnetometer), and high-resolution topography using LiDAR (an acronym for Light Detection and Ranging) tech-nology. Results of the study indicated the soil conductivities measured by the EM system and their spatial signa-tures accurately identified levee composition and predicted foundation geology. The 270 miles of levees were di-vided into 164 segments based on their EM signatures. A levee condition rating system based on 10 factors was used to numerically score and rank the levee segments. Of the 164 levee segments scored, 11 segments, or 7 percent, rated “Good,” 119 segments, or 72 percent, rated “Acceptable,” and 34 segments, or 21 percent, rated “Marginal.” No levee segment was rated “Poor.” A comprehensive eGIS was developed for the LRGV levee c dition assessment study area. GIS technology was used to analyze the levee condition assessment factors and func tioned as one of the project’s primary data management tools.

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Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products.

All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.

DESTRUCTION NOTICE: For classified documents, follow the procedures in DOD 5200.22-M, Industrial Security Manual, Section II-19, or DOD 5200.1-R, Information Security Program Regulation, Chapter IX. For unclassified, limited documents, destroy by any method that will prevent disclosure of contents or reconstruction of the document.

Contents

Conversion Factors, Non-SI to SI Units of Measurement.................................... vi Preface ................................................................................................................. vii Executive Summary ............................................................................................. ix Acronyms and Abbreviations.............................................................................. xii 1—Introduction

Purpose Scope Background and Study Area Approach

2—Development of Condition Assessment Criteria Levee Condition Assessment Design Considerations

Levee segments Workshop

Ten-Point Criteria Performance history under flood stage Construction history Visual inspection Material type Topographic irregularity Slope stability Man-made intrusions Geology Proximity to borrow area Anomalies

Condition Assessment Results

3—Geographic Information System Purpose and Scope Physical Requirements and Standards GIS Database Development

GIS database scope GIS data content requirements GIS data compilation GIS data organization iii

GIS database processing Baseline data EM data LiDAR data Engineering geology data Levee assessment

Technology Transfer Data availability ArcIMS server

4—Airborne Reconnaissance and Surveying Introduction LiDAR

Description Video Fli-Map software

Electromagnetic and Magnetic Flight path Data acquisition and processing

Airborne Survey Results

5—Geologic Investigation Introduction Regional and Local Geologic Settings Geologic Mapping of Levee Areas Construction of Geologic Cross Sections

6—Surface Geophysical Investigation Purpose and Scope Data Acquisition

Ground-based survey locations Electrical methods Magnetic methods

Results

7—Ground Truth Procedures Purpose Data Acquisition Procedures

Selection of field sites Modified cone penetrometer (SCAPS) Soil sampling and analysis

Results of Field Investigation SCAPS probes Laboratory soils tests Observations and analyses of selected sites

8—Conclusions and Recommendations Conclusions Recommendations iv v

References Appendix A-1: Scores for Performance History

Appendix A-2: Scores for Construction History

Appendix A-3: Scores for Visual Inspection

Appendix A-4: Scores for Material Type

Appendix A-5: Scores for Topographic Irregularity

Appendix A-6: Scores for Slope Stability

Appendix A-7: Scores for Man-made Intrusions

Appendix A-8: Scores for Geology

Appendix A-9: Scores for Borrow Areas

Appendix A-10: Scores for Anomalies

Appendix A-11: Average Overall Scores

Appendix B: GIS Data Dictionary

Appendix C: Fugro Airborne Surveys, Inc. Report

Appendix D: SCAPS Logs and Field and Laboratory Sample Descriptions

SF 298

Volume II, Oversize Plates, was reproduced separately.

Conversion Factors, Non-SI to SI Units of Measurement

Non-SI units of measurement used in this report can be converted to SI units as follows:

Multiply By To Obtain cubic feet 0.02832 cubic meters cubic feet per day 28.32 liters per day degrees 0.01745 radians degrees Fahrenheit (F-32) 5/9 degrees Centigrade feet 0.3048 meters feet per day 0.0003528 centimeters per second gallon 3.785 liter gallons per day 3.785 liters per day gallons per day per foot 12.42 liters per day per meter gallons per minute per foot 12.42 liters per minute per meter cubic feet per minute 28.32 liters per minute feet per minute 0.508 centimeters/second inches 2.54 centimeters inches 25.4 mm inches 25,400 microns mile 1.609 kilometer parts per billion 1.0 micrograms per liter parts per million 1.0 milligrams per liter square miles 2.590 square kilometers square feet 0.0929 square meters square feet per day 0.0929 square meters per day pounds force per square inch 6894.757 pascals pounds (force) 4.448222 newtons vi

Preface

This report documents Phases I and II innovative scientific investigations by the U.S. Army Engineer Research and Development Center (ERDC) of the physical condition of flood-control levees maintained and operated by the United States section of the International Boundary and Water Commission (USIBWC) in South Texas. The report is presented in two volumes. Volume I contains the text, figures, and tables. Volume II contains oversized colored plates of maps and cross sections. A compilation of digital data on CD format accompanies the report. The work took place between June 2001 and February 2003. Funding for the project was authorized in Memorandum of Agreement, Contract No. IBM- 01-12, signed December 5, 2000, from USIBWC to ERDC, and in subsequent Individual Work Orders.

This investigation and report was a joint effort of two ERDC, Vicksburg, MS, laboratories and private industry. The Geotechnical and Structures Laboratory (GSL) provided technical oversight of airborne geophysical surveying and performed technical and geological analysis of data. GSL also performed ground-truth field investigations to corroborate the airborne survey results. The Environmental Laboratory (EL) developed and documented a Geographical Information System (GIS) to store and present the data. The report was written by Messrs. Joseph B. Dunbar, William L. Murphy, Thomas E.

McGill, Jose L. Llopis, and Robert F. Ballard, GSL; Ms. Linda D. Peyman-Dove and Mr. Michael J. Bishop, EL. Messrs. James Stefanov and James Robinson, USIBWC, El Paso, TX, monitored the work.

Site Characterization and Penetrometer System field operations and data acquisition were performed by Mr. Karl F. Konecny under the direct supervision of Mr. John H. Ballard, EL, Mr. Jeff F. Powell, Instrumentation Engineering Branch, EL, and Mr. Don S. Harris, Public Works. Airborne geophysical surveying of the levees was performed by Fugro Airborne Surveys, Mississauga, Ontario, Canada. Complementary airborne LiDAR surveys were performed by John Chance Land Surveys, Inc., Lafayette, LA, a Fugro subsidiary.

The authors gratefully acknowledge the support and assistance of IBWC personnel in several offices, particularly Messrs. Chris Anzalduas and Gary Jones, Mercedes, TX, field office, and Messrs. Stefanov, and Robinson.

Work was performed under the direct supervision of Dr. Lillian Wakeley, Chief, Engineering Geology and Geophysics Branch, Dr. Robert Hall, Chief, Geosciences and Structures Division, GSL, and Mr. Harold W. West, Chief, and vii

Dr. M. Rose Kress, Environmental Systems Branch, EL. Dr. David W. Pittman was Acting Director, GSL, and Dr. Elizabeth C. Fleming was Acting Director, EL, during the period of this investigation.

Dr. James R. Houston was Director of ERDC, and COL James R. Rowan, EN, was Commander and Executive Director.

viii

Executive Summary

The U.S. Army Engineer Research and Development Center (ERDC) per-formed a condition assessment of the U.S. section of International Boundary and Water Commission (IBWC) levees along the Lower Rio Grande River Valley (LRGV) and its floodways in South Texas. ERDC and the IBWC conducted the condition assessment using airborne and surface geophysics, geologic studies, ground truth investigations, and an Enterprise Geographic Information System (eGIS). The assessment encompassed 270 miles of levee and more than 600 associated structures within the Lower Rio Grande Flood Control Project (LRGFCP) in Hidalgo, Cameron, and Willacy Counties of Texas. The goal of the assessment was to detect, identify, and map potentially unstable levee reaches that require repair or replacement. In addition, the condition assessment will enable managers to prioritize tools to identify preventive and remedial measures for embankments based on best engineering practice.

A basic understanding of the processes of levee failure along the Mississippi River exists from historical experience and documented study. ERDC investiga-tors recognized, however, that geology, construction techniques, and material properties unique to Texas levees may cause the levees to exhibit failure mecha-nisms different from those of the Mississippi system. It was necessary, therefore, to establish an understanding of Texas levee and foundation characteristics before appropriate geophysical techniques could be effectively applied for condition assessment. Levee failures are governed in large part by the soils that form the embankments and their foundations. Water seepage through the levee embankment can produce internal erosion of the levee soils. Foundation problems in levees usually are caused by under-seepage and are related to geologic conditions at the site. Because levees are underlain by floodplain deposits, knowledge of the geology of local floodplain deposits is an important consideration in flood control management. Levee systems are often built atop highly permeable, relatively coarse-grained river deposits representing point bars, old channels, and courses of the river system abandoned during its history of meandering over the floodplain. Sandy sediments associated with the deposits represent the most likely locations for under-seepage in levee systems. Similar conditions exist on the Rio Grande system. It was imperative that geologic features be identified in the earliest stages of a levee condition assessment so that other exploratory methods could be used to map their distribution. Knowledge of fluvial processes and the ability to recognize depositional environments in the geologic record are the key to identifying locations along modern levees where under-seepage has the greatest potential to occur. Development of a ix comprehensive GIS to effectively manage long-term levee condition assessment and flood-control data was also essential.

ERDC held a 1-day workshop to develop procedures for performing numerical levee condition assessments using quantitative and qualitative descrip-tors. As a result of the workshop, ERDC produced a levee condition tabulation form with 10 criteria. Important criteria discussed in the workshop included construction history, past performance, material type, floodplain geology, anoma-lous conductivity zones, and river migration rates. In this rating system, a higher rating number or score always represents a better condition. Of the 164 levee segments scored, 11 segments, or 7 percent, rated “Good,” 119 segments, or 72 percent, rated “Acceptable,” and 34 segments, or 21 percent, rated “Marginal.” No levee segment was “Poor” when rated using the mean of all 10 criteria.

A comprehensive GIS database was developed for the LRGV levee condition assessment study area. GIS technology was used to develop and analyze the database to address levee condition assessment factors. The GIS also functioned as one of the project’s primary data management tools. The GIS data management tool consisted of a functional, integrated geospatial information base containing available baseline data, engineering geology data, and geophysical data acquired from existing sources or collected specifically for this project. It was designed to provide support for geo-information integration, storage, access, retrieval, and sharing to all study partners. The GIS can be used to fully integrate, analyze, and display data in such a way that up-to-date levee conditions are readily available. In the future, preventive and remediation efforts can be reliably planned and prioritized based upon the best information available.

Airborne, or overhead, geophysical surveying was the primary method used to assess the condition of IBWC levees in South Texas. Airborne methods included multi-frequency electromagnetic (EM), digital video, magnetic (magnetometer), and high-resolution topography using LiDAR (an acronym for Light Detection and Ranging) technology. Phases I and II surveys were flown in June and July 2001. This survey of U.S. IBWC levees is the first known documented application of overhead EM and LiDAR in an integrated fashion to perform levee condition assessments. Fugro Airborne Surveys conducted an airborne, multi-frequency electromagnetic survey using a helicopter-borne Digital Helicopter Electromagnetic (DIGHEM) system to collect resistivity and conductivity measurements for the levee surveys in five discreet frequencies.

Three survey lines were flown along the levees to adequately describe the levee electrical and magnetic characteristics. One line was flown down the levee centerline and one each along the toes of the levee about 160 ft from the center line. The total width of survey was about 320 ft. The towed bird sensor operated at a nominal survey altitude of 100 ft with an average aircraft speed of about 50 knots. Sample rate was equivalent to one sample every 10 ft at a survey speed of 60 knots. GPS data were collected simultaneously.

Airborne EM results for the entire condition assessment study area are per-haps best discussed with reference to the 25 kHz frequency data. Effective depth of investigation at this frequency is approximately to and immediately below the base of the levees, roughly 15 to 20 ft in depth. Conductivities reflect material x type and physical condition of soils primarily within the levees and in the upper few feet of foundation. Assuming that low conductivity identifies sand and/or more porous levee soils and high conductivity identifies clayey soils, one can predict or estimate composition of LRGFCP levees and foundations.

The main river levee along and parallel to the Rio Grande from La Joya to Brownsville is a mix of high- and low-conductivity soils. This picture is consis-tent with the varied depositional facies that typify the floodplain environment of point bars, clay plugs, and fine-grained top stratum. From Progreso quadrangle east, the main river levee consists predominantly of low-conductivity soils, either sands or clay with air voids (desiccation cracks?). Airborne LiDAR data showed that the crests of the levees east (downstream) of the Retamal dike area are lower by up to several inches. The LiDAR data may imply that the levees have perhaps subsided or that levee soils have settled into internal voids.

Airborne and ground-based geophysical investigations provided remotely sensed data relating to the electrical and magnetic properties of the ground surface and the subsurface soils. Because there is often an inherent ambiguity in the interpretation of remotely sensed data, ERDC performed on-site physical confirmation of material properties (called ground-truthing) identified in the remote sensing surveys. ERDC conducted Site Characterization and Penetrometer System (SCAPS) and ground-based geophysical surveys on IBWC levees to provide ground truth. The SCAPS probed the levees and the levee foundations (to a depth of at least twice the levee height) and produced logs of soil physical properties and soil samples for laboratory testing. Ground-based surveying used electromagnetic sensors and a magnetometer deployed near the ground surface at selected levee sites.

xi

Acronyms and Abbreviations

ArcIMS – Arc Internet Mapping Service ASCII – American Standard Code for Information Interchange BIL – Band Interleaved by Line (a data format) CD – compact disc CD-ROM – compact disc, read-only memory cm – centimeter(s) CPT – Cone penetrometer technology DEM – Digital Elevation Model DIGHEM – Digital Helicopter Electromagnetic DOQQ – digital orthophoto quarter quadrangle DRG – Digital Raster Graphics EM – electromagnetic EMI – electromagnetic induction ERDC – Engineer Research and Development Center ESRI – Environmental Systems Research Institute FDEM – Frequency-domain electromagnetic FGDC – Federal Geographic Data Committee Fli-Map –software and hardware in support of airborne surveying system ft – feet FY – fiscal year GIS – geographic information system GPS – global positioning system Hz – Hertz (unit of electromagnetic frequency) IBWC – International Boundary and Water Commission JCLS – John Chance Land Surveys K – hydraulic conductivity Klab – Laboratory measured hydraulic conductivity LIDAR – light detection and ranging LRGFCP – Lower Rio Grande Flood Control Project LRGV – Lower Rio Grande Valley M - million MrSID – Multi-resolution Seamless Image database NAD – North American Datum NED – National Elevation Dataset NIM – Nuclear Research Instrument NLCD – National Land Cover Dataset NRCS – Natural Resources Conservation Service OD or O.D. – outside diameter xii

Pg – geologic map symbol for Pliocene Goliad formation PTO – power take-off QA – quality assurance Qam – geologic map symbol for Quaternary alluvial muds and silts Qas – geologic map symbol for Quaternary alluvial sands Qb – geologic map symbol for Quaternary Beaumont formation Qds – geologic map symbol for Quaternary eolian dune sands Ql – geologic map symbol for Quaternary Lissie formation RTCM – Radio Technical Commission for Maritime services SARIS – Scintrix automated electrical resistivity system SCAPS – Site Characterization and Analysis Penetrometer SDSFIE – Spatial Data Standard for Facilities, Infrastructure and Environment sec – second(s) SSURGO – Soil Survey Geographic Database S-VHS – Super Video Home System TDEM – Time-domain electromagnetic TIFF – Tagged Image File Format TOWNS – Technologies and Innovations for Urban Watershed Networks USCS – Unified Soil Classification System USGS – United States Geological Survey UTC – Universal Coordinated Time XYZ - A three-coordinate data point or file µm – micron (one millionth of a meter) xiii

1 Introduction

Purpose This report documents a condition assessment of the U.S. section of Interna-tional Boundary and Water Commission (IBWC) levees along the Lower Rio Grande River Valley (LRGV) and its floodways in south Texas. The U.S. Army Engineer Research and Development Center (ERDC) and the IBWC carried out the integrated condition assessment using airborne and surface geophysics, geo-logic studies, ground truth investigations, and an Enterprise Geographic Informa-tion System (eGIS). The assessment encompassed 270 miles of levee and more than 600 associated structures within the Lower Rio Grande Flood Control Project (LRGFCP) in Hidalgo, Cameron, and Willacy Counties of Texas (Figure 1-1). The goal of the assessment was to detect, identify, and map potentially unstable levee reaches that require repair or replacement. In addition, the products of the condition assessment will enable managers to prioritize tools to identify preventive and remedial measures for embankments based on best engineering practice.

Scope Phases I and II, the subjects of this report, were accomplished in 2001 through 2003. Phase I activities included development of GIS data management tools and procedures, preliminary geologic data input, and airborne mapping of 270 miles of IBWC levees. Phase II consisted of implementing surface geophysical and other ground truth investigations, completion of geologic interpretation and description, and development of a GIS database. Phases I and II are the first of four phases planned for investigation of levees in Texas and New Mexico.

The methodology described in this report is applicable at a research and development level nationally and globally for other areas that might require rapid assessment of levee condition.

Background and Study Area Flood control in the LRGV began in 1925 with the construction of flood con-trol works in selected areas by county governments. These early efforts were not

Chapter 1 Introduction 1 effective, and the federal government was asked to develop a plan for a system of protective works. The federal system included a continuous river levee and an interior floodway to efficiently discharge floodwaters to the Gulf of Mexico (Fig-ure 1-1). The interior floodway incorporated existing drainage and portions of older Rio Grande courses. Congress first authorized construction of the LRGFCP in 1933. The initial phase of construction occurred from 1933 to 1940.

Many of the pre-1933 levees constructed by counties and individual farmers were integrated into the project.

Subsequent phases of construction took place during 1958-1961, 1968-1969, 1973, and 1977. Large dams Falcon and Amistad, constructed upstream of the project area in the 1950’s and 1960’s, control the release of flood waters from major storm events within the headwaters and tributaries of middle Rio Grande River system.

The LRGFCP is operated and maintained by the U.S. section of the IBWC.

The Project’s principle features are Anzalduas and Retamal Dams, the 270 miles of levees, the interior floodway system, which includes Banker Floodway, Main Floodway, North floodway, and arroyo Colorado Floodway1.

The flood control project was redesigned and improved after the Hurricane Beulah flood of 220,000 cubic feet per second (cfs) in 1967. Figure 1-2 is a schematic map of the flood control system. The design flood of 250,000 cfs reduces to 235,000 cfs at Anzalduas Dam. Distribution of the design flood flow is as follows1:

Anzalduas Dam diverts 105,000 cfs into the Banker floodway and thence through the Main floodways to Mercedes, where 84,000 cfs is passed to the North Floodway and 21,000 cfs to the Arroyo Colorado floodway. One hundred thirty thousand cubic feet per second moves through Anzalduas Dam and down the Rio Grande, along which it reduces to 125,000 cfs at Retamal Dam. Retamal Dam diverts 105,000 cfs into Mexico’s interior floodway.

The levee system contains both new (early 1970’s construction following Hurricane Beulah) and old levees. Older levees have been upgraded or raised to meet the demands of rapid urban growth in the 1980’s and 1990’s and local flood control requirements. Many of the older levees within the IBWC system contain the original core of the early 1930’s levees. Typically, IBWC levees have a 1:3 (vertical:horizontal) side slope and are 10 to 15 ft higher than the surrounding ground surface. Most are 10 to 15 ft wide at the crest and 60 to 75 ft wide at the base.

IBWC program engineers recognized a potential need for assessment of LRGFCP levees in the late 1990’s. Engineers foresaw potential levee instability from movement of river water through or under the levees, removing soil in the foundation or levee by piping. Eventually, prolonged seepage and piping might lead to partial or complete failure of the levee, and result in breaching by floodwaters. Visual inspection alone would not adequately detect warning signs of an impending levee failure.

1 Joe Tucker, personal communication.

2 Chapter 1 Introduction

Representatives from the IBWC’s El Paso office approached the U.S. Army Corps of Engineers’ Galveston District for assistance. Galveston District person-nel referred IBWC to ERDC, where a proposal for condition assessment was pre-sented to IBWC in 2000. The ERDC is a world leader in flood control. Under authority granted him by the Flood Control Act of 1928, the Chief of Engineers, U.S. Army, authorized construction of the research center in 1929 after cata-strophic flooding occurred in the Lower Mississippi River Valley in 1927. That facility, originally named the Waterways Experiment Station, evolved into ERDC during the 1990’s. The ERDC has become the Corps’ premier research facility and a worldwide leader in the areas of soil and levee stability, embankment failure, expedient techniques for mitigating levee deterioration, engineering geology, and engineering geophysics.

IBWC subsequently funded ERDC for a 2-Phase condition assessment of LRGV levees. Phase I and II areas incorporate 25 U.S.G.S. 7-1/2 minute topo-graphic quadrangles, which comprise the study area and form the basis for the condition assessment evaluation (Figure 1-1). Phase I work included the upper river quadrangles La Joya, Mission, Hidalgo, Las Milpas, San Juan SE, and Progeso. Phase II work addressed the remaining 19 quadrangles of Figure 1-1.

Phase I proceeded with FY01 funding and Phase II with FY02 funding. Suc-cess of the work accomplished in Phases I and II provided the basis for continu-ation of the condition assessment approach to other levees. IBWC funded ERDC for technical oversight on additional levee assessments with Phase III (FY02) and Phase IV (FY03). Phases III and IV will include geophysical and geological investigations to assess the structural integrity of levees in the Presidio Flood Control and Rio Grande Rectification (Texas), and Rio Grande Canalization (New Mexico) projects. Phase IV work will include additional field investigation and sampling of levees in the Phase II area.

Approach A basic understanding of the processes of levee failure along the Mississippi River exists from historical experience and documented study (Wolf 2001;

USACE 1956, 1 and 2). ERDC investigators recognized, however, that geology, construction techniques, and material properties unique to Texas levees may cause the levees to exhibit failure mechanisms different from those of the Mississippi system. It was necessary, therefore, to establish an understanding of Texas levee and foundation characteristics before appropriate geophysical techniques could be effectively applied for condition assessment. Levee failures are governed in large part by the soils that form the embankments and their foundations. Failure of the levee occurs as a result of the river scouring the toe of the levee, resulting in failure of the embankment into the channel, by overtopping of the embankment, and by seepage and piping. Water seepage through the levee embankment (through-seepage) can produce internal erosion of the levee soils. Foundation problems in levees usually are caused by under-seepage and are related to geologic conditions at the site.

Chapter 1 Introduction 3

Through-seepage is not normally a major concern for levees constructed of clay soils unless flood stage is long enough that the embankment becomes satu-rated, or when defects in the levee, such as animal burrows or desiccation cracks, produce concentrated flows. Seepage through levees consisting of silt and sand can produce boils and piping of levee materials, erosion at the land-side slope, and can lead to breaching of the levee if the seepage remains uncontrolled.

Seepage of water beneath a levee constructed on a highly erodible foundation of silt or sand is more critical than through-seepage. Identifying erodible founda-tion soils is an important step in preventing bank failures. Because levees are underlain by floodplain deposits, knowledge of the geology of local floodplain deposits is an important consideration in flood control management. Topstratum thickness, desiccation potential, and bulk permeabilities of the fine-grained mate-rials are important factors in determining whether uncontrolled seepage is possi-ble. A greater topstratum thickness reduces the potential for uncontrolled seepage. Levee systems are often built atop highly permeable, relatively coarse-grained river deposits representing point bars, old channels, and courses of the river system abandoned during its history of meandering over the floodplain.

Sandy sediments associated with the deposits represent the most likely locations for under-seepage in levee systems (USACE 1956 and 1958, Wolf 2001).

Similar conditions exist on the Rio Grande system (Brown et al. 1980). It was imperative that geologic features be identified in the earliest stages of a levee condition assessment so that other exploratory methods could be used to confirm their existence and map their distribution. Knowledge of fluvial processes and the ability to recognize depositional environments in the geologic record are the key to identifying locations along modern levees where underseepage has the greatest potential to occur. Development of a comprehensive GIS to effectively manage long-term levee condition assessment and flood-control data was also essential.

ERDC and IBWC identified four task areas with specific sub-tasks required to accomplish the levee condition assessment:

a. Task 1 – Data Management. Develop a comprehensive eGIS data man-agement tool:

(1) Define functional and content requirements.

(2) Define physical requirements and standards.

(3) Locate and compile existing base, geologic engineering, and historic data.

(4) Provide guidelines for new data products.

(5) Establish protocol for data access.

(6) Maintain, update, and provide quality assurance (QA).

(7) Provide study partner support.

4 Chapter 1 Introduction

(8) Document, archive.

(9) Develop plan for technology transfer to IBWC site and long-term maintenance.

(10) Store GIS data on an ERDC server accessible to the IBWC (with an ultimate goal of transferring all data to IBWC servers).

b. Task 2 – Conduct geologic studies of the area to prioritize problem zones and river reaches:

(1) Locate and compile existing geologic and related data.

(2) Describe the geology underlying the levees.

(3) Identify specific floodplain depositional environments.

(4) Map on topographic bases the depositional environments and, spe-cifically, the distribution of abandoned river channels and courses.

(5) Produce representative geologic cross sections from boring and other data.

c. Task 3 – Conduct airborne geophysical/photographic surveys using the following methods/technologies:

(1) Multi-frequency electromagnetic (EM) induction.

(2) Magnetometry.

(3) Aerial photography.

(4) Digital flight line video.

(5) High-resolution topographic mapping using Light Detection and Ranging (LiDAR).

(6) On-board differential GPS for position referencing of airborne data.

(7) Process data and provide interpretation.

d. Task 4 – Perform ground truth investigations using ground surface geo-physical surveys and cone-penetrometer technology. The ground-based geophysics investigation was funded under the U.S. Army Corps of Engineers’ Technologies and Operational Innovations for Urban Watershed Networks (TOWNS) program.

(1) Electromagnetic induction.

(2) Total-field magnetometry.

Chapter 1 Introduction 5

(3) Electrical resistivity.

(4) Cone-penetrometer and in-place resistivity/conductivity using the Site Characterization and Analysis Penetrometer System (SCAPS).

(5) Soil sampling with SCAPS and field logging of soils.

(6) Selected laboratory testing of soil samples.

Ground-truth operations discussed in this report apply only to the Phase I area (the western half of the LRV) in Progreso, San Juan SE, Donna, Pharr, Las Milpas, Hidalgo, Mission, and La Joya 7-1/2 minute U.S.G.S. topographic quadrangles. Ground-truth for the Phase II (eastern half) is scheduled for 2003, and results will be documented in a separate report.

6 Chapter 1 Introduction

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8 Chapter 1 Introduction

2 Development of Condition Assessment Criteria

Levee Condition Assessment Design Considerations Levee segments

An immediate objective of the levee condition assessment was to identify and quantify, if possible, those factors that influence the stability and performance of IBWC levees. Using the factors as criteria for assessing levee condition, a numeric score could then be applied to levee segments. A numeric score would allow IBWC managers to more effectively monitor those levee segments that are distressed or have the potential to become unstable. The first step in developing a rating system was to create a workable catalog of levee reaches, or segments. Early concepts of segmenting levee reaches considered using geographic locations or uniform lengths. However, after reviewing the results of the airborne geophysical surveys and the color-coded EM conductivity2 maps that the surveys produced, it became apparent that segmenting levee reaches by EM conductivity would be a logical approach.

ERDC divided the south Texas LRGV levees into segments that were based on uniform EM airborne conductivity and assigned a number to each segment sequentially on topographic quadrangle base maps (see Plate 1 of Volume II for locations of levee segments). ERDC assigned and rated approximately 250 levee segments on 25 topographic quadrangles mapped in Phase I and II areas. Fig-ure 2-1 is a portion of the Progreso quadrangle showing numbered levee segments for that quadrangle.

Workshop

ERDC held a one-day workshop on 17 July, 2001 to develop procedures for performing numerical levee condition assessments using quantitative and qualita-tive descriptors. Attendees included 12 engineers and scientists from ERDC and the U.S. Army Engineers Lower Mississippi River Valley Division. As a result

2 Unless otherwise stated, the term “conductivity” used throughout this report refers to electrical conductivity.

Chapter 2 Developments of Condition Assessment Criteria 9 of the workshop and feedback from its attendees, ERDC produced a levee condition tabulation form with ten criteria, or factors. Table 2-1 lists the ten condition factors. Other factors discussed in the workshop but not selected for the analysis included river migration rate, levee design criteria, potential loss in event of levee failure, embankment erosion, and channel scour. Figure 2-2 is an example of a completed rating form for levee segment 3 of San Juan SE quadrangle.

In this rating system, a higher rating number or score always represents a bet-ter condition. The workshop established the following definitions for use in the ten-point criteria condition assessment of levees:

a. Descriptor – term describing factors related to a levee segment’s past and probable future performance.

b. Weight – number relating the degree of importance of the descriptor to levee performance, assigned to a descriptor on a scale of 0 to 3, where 0 = not important, 1 = somewhat important, 2 = important, 3 = very important.

c. Value – number relating how well a levee segment performs relative to the descriptor without regard to the weight assigned the descriptor (i.e., the score before adjusting for weight). Values range from 0 (worst per-formance) to 4 (best performance).

d. Score – a number representing the levee segment’s condition relative to that descriptor, or criterion, taken as the product of the “weight” and the “value”, and ranging from 0 to 12.

e. Condition – numerical expression of levee condition based on the average (mean) scores of all 10 descriptors (fewer than 10 in some cases), where 0-3.99 = Poor, 4-6.99 = Marginal, 7-9.99 = Acceptable, and 10-12 = Good.

It was recognized that even if a levee segment scored “Acceptable” or “Good” on condition, a low score in any one of the ten descriptors (criteria) could result in poor performance and possibly levee failure under flood conditions. Thus, each descriptor was scored individually. ERDC presented segment rankings to IBWC as a series of maps depicting levee segments and representing scores for all ten individual descriptors and a cumulative ranking of overall levee condition (Volume II, Plates 8-18). Figure 2-3 is a portion of the condition map prepared for the “Geology” criterion for the San Juan SE quadrangle. The map shows color-coded segments of levee and the values assigned for geology, from 0 to 4. Note that the lower segment of levee near the center of the map (this is the Retamal Dam levee segment) scored a zero for geology, but scored in the acceptable range (around 7) for the overall condition.

Plate 18 of Volume II shows the overall levee condition score as the mean of scores for all 10 criteria for all levee segments (numbers carried to 2 decimal places).

10 Chapter 2 Development of Condition Assessment Criteria

Adverse geological conditions beneath Retamal dike and apparent levee des-iccation make this segment a candidate for stability problems. The two arms of an abandoned meander channel beneath this levee are potential paths for seepage and attendant piping under high water conditions. There is also the possibility that expansive clays were used in the construction of Retamal dike and could lead to internal cracking. This reach of levee was built in 1971 and has never been subjected to flooding. Drought conditions in the LRGV during the past 10 years may have caused drying and possible internal cracking.

Ten-Point Criteria Ten condition criteria developed during the July, 2001 workshop enabled an evaluation of levee condition for each segment based on design, performance during past floods, material properties, and other factors. IBWC personnel assigned ratings for the first three criteria—performance history, construction history, and visual inspection of apparent condition. IBWC provided a rating for each quadrangle of the study area with, generally, all levee segments contained in a given quadrangle receiving the same rating for each of the three criteria.

Exceptions were the Mission, San Juan SE, Progreso, and Santa Maria quadrangles, where both river and main floodway levees were present. In those three quadrangles, IBWC assigned separate ratings to the river and main floodway levees.

ERDC assigned ratings for the remaining seven criteria. Criteria ratings ranged from 0 for worst condition to 4. They were applied qualitatively for some criteria and quantitatively for others. The following sections explain the philos-ophy and approach used in developing the ratings. Ratings were assigned to each of the ten criteria and entered into the condition-rating chart (Figure 2-2) as a “Value.” Data from the airborne EM survey were the primary tool for segmenting the levees and were used by ERDC analysts to derive ranking values for six of seven criteria. The last criterion was based on the LiDAR survey and ranked segment elevations based on deviations in levee height.

Volume II of this report presents large-format colored maps of the levees showing conductivity for each frequency, magnetometer data, and weighted scores for each of the ten criteria for all levee segments. Plate 18 shows the mean scores for all criteria (the condition assessment) for all levee segments. The reader can make a rapid assessment of the locations and distribution of potential trouble spots by studying this collection of maps.

Performance history under flood stage

IBWC subjectively rated each quadrangle’s levees by how they performed under past flood events. Levees that “had never been tested” by flood received no rating. All of the main floodway levees, except a short segment in Mission quadrangle, received a rating of 4, implying that they performed very well. River levees east of Progreso quadrangle received a 1.5 for all segments because the levees there are generally older and are poorly maintained. The levees are below

Chapter 2 Developments of Condition Assessment Criteria 11

Retamal Dam, are lower in height, and are not subjected to major flooding because of the dam. Table 2-2 is a summary of IBWC ratings for performance history, construction history, and visual inspection.

Construction history

IBWC rated LRGFCP levees based on their construction history, which is as much a factor of age (when they were built) as how they were built. It was assumed that levee construction practices were less rigorous during early construction periods. River levees east of Progreso quadrangle, some built in the 1930’s, are older than other levees, and were downgraded accordingly (to a 2).

See Table 2-2 for a summary of ratings for construction history.

Visual inspection

IBWC engineers and technicians toured the levees in 2002 and made visual inspections, from which they assigned condition values for the levee condition assessment. Typical conditions observed and noted in the visual inspection were rills and cracks on levee slopes, depressed levee crests, vehicle ruts on levee slopes and crest roadway, and seeps or “soft spots” below water supply reservoirs. Table 2-2 summarizes the IBWC ratings for visual inspection.

Material type

The material type criterion assessed the relative suitability of soils used in levee construction. The assessment assumed that materials in the top stratum soils adjacent to the levees were used as borrow (based on historic construction photos and locations of borrow pits). The rating scale (the “Value”) for this criterion was 0 (worst condition, thick sandy soils) to 4 (best condition, low-shrink clay soils). The scale was as follows:

Value Description Best , 4 mainly clay, low to moderate shrink/swell potential Good, 3 or 3.5 mainly clay, some mixed soils, high shrink/swell1

Intermediate, 2 or 2.5 mixed soils (sand, silt, little clay) Marginal, 1or 1.5 primarily sand and/or thin top-stratum Worst, 0 or 0.5 sand and/or no top-stratum, thick sandy substratum 1 Assuming the shrinkage is within tolerable limits. Note that a high shrink/swell clay downgrades the levee condition relative to a clay with low to moderate shrink/swell.

Material type was derived from soil maps. Study of aerial photographs enabled interpretation of geological environments of deposition and location of borrow areas. Analysis of soil maps and aerial photographs was the primary method used to infer materials of levee construction. Documentation of newer levees provided data on locations of borrow pits and associated engineering data.

12 Chapter 2 Development of Condition Assessment Criteria

Topographic irregularity

LiDAR data produced high-resolution maps of the elevations (local heights) of the levee system. ERDC investigators used centerline elevation and departures from an arbitrary “norm” datum to characterize topographic irregularities. Large departures were considered the worst case. The scale for topographic irregularity was as follows:

Value Description Best, 4 < 6 in. deviation from local slope Good, 3 or 3.5 6-11.99 in. deviation from local slope Intermediate, 2 or 2.5 12-17.99 in. departure Marginal, 1 or 1.5 18-23.99 in. deviation from local slope Worst, 0 or 0.5 24 in. or greater deviation from local slope

There were 64 of 165 levee segments (39 percent, or more than a third of the levee segments) with a score for topographic irregularity of 2 or less, indicating they showed 12 in. or more of departure.

Slope stability

The term “slope stability” refers to the ability of a levee to maintain its integ-rity with few signs of distress, which would be manifested in open surface cracks, seepage, slumping of the sides of the levee, detached vegetation, potholes, or areas of subsidence. Additionally, the slope stability criterion considered the proximity of the levee toe to the Rio Grande, Arroyo Colorado, and/or deep borrow areas. Levees distant from streams and deep borrow areas were considered potentially more stable than those near. The primary considerations in ranking levees for slope stability were proximity to the rivers or deep borrow areas and levee soil type, with sandy levees considered most troublesome. A secondary consideration was relative electrical conductivity of the soils in the levee and in the foundation, with low electrical conductivity soils considered a worse case condition. High conductivity, clay-rich soils were considered to have high cohesion and low potential for erosion and piping. Clay soils with a capacity to shrink or swell with changing moisture were downgraded.

The scale for slope stability ranking was as follows:

VALUE DESCRIPTION

Best, 4 >500 ft from river, clay levee, high conductivity) Good, 3 or 3.5 300-500 ft from river, clay levee with high shrink/swell potential, high conductivity, shallow borrow pit at toe Intermediate, 2 or 2.5 200-299 ft from river, clay levee, low conductivity foundation, shallow borrow pit at toe Marginal, 1 or 1.5 100-199 ft from river, sandy levee or foundation, low conductivity Worst, 0 or 0.5 <100 ft from river, sandy levee, low conductivity

Chapter 2 Developments of Condition Assessment Criteria 13

Man-made intrusions

This criterion considered the presence of man-made structures, such as pump stations within the levee and utility crossings and bridges passing over, through, or coincident with a levee. Built-in structures like pump stations may create a zone of poor compaction where the concrete or wood structure contacts the levee soil. Contacts are potential sites for seepage and attendant piping and erosion, and were considered to downgrade levee condition. Levee segments with no man-made structures received a value of 4 (best condition). Ranking for this criterion was subjective. In general, levee segments with greater numbers of intrusions received the lowest ranking, but consideration was also given the size, depth, geometry and type of intruding structure. IBWC provided maps showing locations and types of man-made structures. The locations were digitized and overlaid on the EM data for comparison and evaluation.

The scale for effects of man-made intrusions was as follows:

VALUE DESCRIPTION

Best, 4 no intrusions Good, 3 or 3.5 1 to 2 with small diameter or EM signature Intermediate, 2 or 2.5 3 to 4 with small diameter or EM signature Marginal, 1 or 1.5 1 to 2 with large diameter or EM signature Worst, 0 or 0.5 3 or more with large diameter or EM signature

Geology

The criterion for geology considered the stratigraphy, structure, and physical properties of soils underlying the levee in its foundation and of adjacent soils from which levee materials were borrowed. The geologic setting of the Rio Grande Valley in south Texas is predominantly a result of Recent (less than 10,000 years) to Pleistocene (10,000 to 2 million years) alluvial and deltaic sedimentation. Most of the levees are within the meander belt of the recent Rio Grande and its tributaries. Geomorphic features in this area were created and controlled by the movement, deposition, and erosion of those streams.