J.06_Final_EA_Main_N_Floodway.pdf

Final Environmental Assessment Improvements to the North and Main Floodways Levee Systems

Lead Agency:

United States Section, International Boundary and Water Commission El Paso, Texas

Technical Support:

Parsons Austin, Texas

November 2007

Cooperating Agency:

Texas Parks and Wildlife Department Austin, Texas

COVER SHEET

FINAL ENVIRONMENTAL ASSESSMENT

AND

FINDING OF NO SIGNIFICANT IMPACT

IMPROVEMENTS TO THE

MAIN AND NORTH FLOODWAY LEVEE SYSTEMS

Lead Agency: United States Section, International Boundary and Water Commission

Cooperating Agency: Texas Parks and Wildlife Department

Proposed Action: Raising approximately 76 miles of the Main and North Floodway Levee Systems to meet current requirements for flood control.

Report Designation: Final Environmental Assessment

Abstract: The United States Section, International Boundary and Water Commission (USIBWC) is considering raising portions of the 29-mile Main Floodway Levee System and portions of the 46-mile North Floodway Levee System to meet current flood control requirements. The Proposed Action would increase the flood containment capacity of the Main and North Floodways Levee System by raising elevation of a number of levee segments for improved flood protection. Fill material would be added to the existing levee to bring height to its original design specifications, or to meet a 2-foot freeboard design criterion. Typical height increases in improvement areas would be less than 1 foot and would not require expansion of the existing levee footprint.

In some locations, up to 2 feet of fill material would be placed on top of the levee, extending levee footprint up to a maximum of 12 feet form the current toe of the levee. This expansion would take place along the approximately 20 feet service corridor currently utilized for levee maintenance, inside the maintained floodway, and entirely within the flood control project right-of-way. In some instances, adjustment in levee slope would be made to eliminate the need for levee footprint expansion, when required due to engineering considerations or for protection of biological or cultural resources. The need for excavation outside the levee structure is not anticipated.

The Environmental Assessment assesses potential environmental impacts of the Proposed Action and the No Action Alternative. Potential impacts on natural, cultural, and other resources were evaluated, and mitigation measures were incorporated, as applicable, into the Proposed Action. A Finding of No Significant Impact was issued for the Proposed Action based on a review of the facts and analyses contained in the Environmental Assessment.

FINAL ENVIRONMENTAL ASSESSMENT

IMPROVEMENTS TO THE MAIN AND NORTH FLOODWAY LEVEE

SYSTEMS

Lead Agency:

UNITED STATES SECTION,

INTERNATIONAL BOUNDARY AND WATER COMMISSION

UNITED STATES AND MEXICO

Cooperating Agency:

TEXAS PARKS AND

WILDLIFE DEPARTMENT

Technical Support:

Parsons

8000 Centre Park Dr., Suite 200 Austin, Texas 78754

USIBWC Contract IBM04D0002, Task Order IBM06T0020

NOVEMBER 2007

Main and North Floodway Levee Systems Environmental Assessment Contents i USIBWC

TABLE OF CONTENTS

SECTION 1 PURPOSE OF AND NEED FOR THE PROPOSED ACTION......................................... 1-1

1.1 Purpose of and Need for Action........................................................................................ 1-1

1.2 USIBWC Authority ............................................................................................................ 1-1

1.3 Scope of the Environmental Review................................................................................. 1-2

1.4 Environmental Coordination and Compliance Analysis.................................................... 1-3

1.5 Organization of the Environmental Assessment............................................................... 1-5

SECTION 2 DESCRIPTION OF PROPOSED ACTION ...................................................................... 2-1

2.1 Levee System Description ................................................................................................ 2-1

2.2 Proposed Action................................................................................................................ 2-2

2.3 Alternatives Considered and Eliminated from Detailed study........................................... 2-8

2.4 Other Actions With Potential Cumulative Impacts ............................................................ 2-8

2.5 Summary Comparison of Environmental Consequences of the Alternatives................... 2-9

2.5.1 No Action Alternative............................................................................................... 2-9

2.5.2 Proposed Action...................................................................................................... 2-9

SECTION 3 AFFECTED ENVIRONMENT ...................................................................................3-1

3.1 Biological Resources ........................................................................................................ 3-1

3.1.1 Vegetation ............................................................................................................... 3-1

3.1.2 Wildlife..................................................................................................................... 3-4

3.1.3 Threatened and Endangered Species .................................................................... 3-6

3.1.4 Wetlands and Aquatic Habitat................................................................................. 3-6

3.2 Cultural Resources ........................................................................................................... 3-8

3.2.1 Previous Cultural Resources Studies ..................................................................... 3-9

3.2.2 Historic and Prehistoric Archaeological Resources .............................................. 3-11

3.2.3 Historic-age Resources......................................................................................... 3-13

3.2.4 Cemeteries ............................................................................................................ 3-13

3.3 Water Resources ............................................................................................................ 3-16

3.3.1 Regional Flood Control ......................................................................................... 3-16

3.3.2 Water Flow ............................................................................................................ 3-17

3.4 Land Use......................................................................................................................... 3-19

3.4.1 Natural Resources Management Areas................................................................ 3-19

3.4.2 Agricultural Land ................................................................................................... 3-19

3.4.3 Urban Areas .......................................................................................................... 3-19

3.5 Community Resources.................................................................................................... 3-20

3.5.1 Socioeconomics .................................................................................................... 3-20

3.5.2 Environmental Justice ........................................................................................... 3-23

3.5.3 Transportation ....................................................................................................... 3-23

3.6 Environmental Health...................................................................................................... 3-24

3.6.1 Air Quality.............................................................................................................. 3-24 ii USIBWC

3.6.2 Noise ..................................................................................................................... 3-25

3.6.3 Hazardous and Toxic Waste ................................................................................. 3-26

SECTION 4 ENVIRONMENTAL CONSEQUENCES ....................................................................4-1

4.1 Biological Resources ........................................................................................................ 4-1

4.1.1 Vegetation ............................................................................................................... 4-1

4.1.2 Wildlife..................................................................................................................... 4-1

4.1.3 Threatened and Endangered Species .................................................................... 4-4

4.1.4 Jurisdictional Wetlands and Aquatic Habitat........................................................... 4-5

4.1.5 Cumulative Effects .................................................................................................. 4-5

4.2 Cultural Resources ........................................................................................................... 4-6

4.2.1 Historic and Prehistoric Archaeological Resources ................................................ 4-6

4.2.2 Historic-age Resources........................................................................................... 4-6

4.3 Water Resources .............................................................................................................. 4-8

4.3.1 Flood Control........................................................................................................... 4-8

4.3.2 Water Flow .............................................................................................................. 4-9

4.4 Land Use........................................................................................................................... 4-9

4.5 Community Resources.................................................................................................... 4-10

4.5.1 Socioeconomics .................................................................................................... 4-10

4.5.2 Environmental Justice ........................................................................................... 4-12

4.5.3 Transportation ....................................................................................................... 4-12

4.6 Environmental Health...................................................................................................... 4-13

4.6.1 Air Quality.............................................................................................................. 4-13

4.6.2 Noise ..................................................................................................................... 4-13

4.6.3 Hazardous and Toxic Waste ................................................................................. 4-14

4.7 Indirect and Cumulative Impacts..................................................................................... 4-14

SECTION 5 BEST MANAGEMENT PRACTICES............................................................................... 5-1

5.1 Engineering Measures ...................................................................................................... 5-1

5.2 Natural Resources ............................................................................................................ 5-1

5.3 Cultural Resources ........................................................................................................... 5-2

SECTION 6 ENVIRONMENTAL COMPLIANCE AND COORDINATION .......................................6-1

6.1 Consultation ...................................................................................................................... 6-1

6.1.1 Cooperating Agency Review................................................................................... 6-1

6.1.2 Public Review of Draft EA ....................................................................................... 6-1

6.2 List of Contributors............................................................................................................ 6-2

SECTION 7 REFERENCES................................................................................................................. 7-1 iii USIBWC

APPENDICES

Appendix A Detailed Maps of Levee Alignment, Right-of-Way and Potential Improvement Areas Appendix B Habitat of Threatened and Endangered Species Potentially Occurring Within

Counties Intersecting the Levee System Appendix C Detailed Maps of Cultural Resources Along the Main and North Floodway Levee

Systems Study Corridor Appendix D Comment Letters on the Draft EA Appendix E Cultural Resources Evaluation (Sundenmeyer et al. 2007).

Appendix F Technical Support Studies Report (Parsons 2007) [Appendices E and F are provided in the attached CD-ROM]

LIST OF TABLES

Table 1.1 Summary of Environmental Coordination and Compliance............................................. 1-3

Table 2.1 Summary of Environmental Impacts of Proposed Improvements to the Main and North Floodways.................................................................................................... 2-10

Table 3.1 Acreages of Plant Communities along the Levee Systems............................................. 3-5

Table 3.2 Wetlands within Main and North Floodway Levee ROW................................................. 3-7

Table 3.3 Known Archaeological Resources and HPAs Identified Within or Near the Study Corridor................................................................................................................ 3-12

Table 3.4 Historic-age Resources Identified Within the Study Corridor ........................................ 3-14

Table 3.5 State and Federal Conservation Lands Located Along the Main and North Floodway Levee Systems.............................................................................................. 3-20

Table 3.6 Population Growth in Cameron, Hidalgo, and Willacy Counties Adjacent to the Main and North Floodway Levee Systems.................................................................... 3-21

Table 3.7 Percentage of Minority Populations and Poverty Rates in the Main and North Floodway Levee Area.................................................................................................... 3-21

Table 3.8 Estimated Total Employment for Cameron, Hidalgo, and Willacy Counties ................. 3-22

Table 3.9 Summary Search Report for the Main and North Floodway Levee Systems, McAllen, Texas Vicinity................................................................................................. 3-27

Table 3.10 Summary Search Report for the Main and North Floodway Levee Systems, Santa Monica, Texas Vicinity ....................................................................................... 3-28

Table 4.1 Potential Impacts to Vegetation within Improvement Areas of the Main and North Floodway Levee Systems................................................................................................ 4-2

Table 4.2 Levee Improvement Areas along State and Federal Conservation Lands...................... 4-3

Table 4.3 Potential for impacts to Known Archaeological Resources and HPAs Identified Within the Study Corridor................................................................................................ 4-7 iv USIBWC

Table 4.4 Potential for impacts on Historic-age Resources Identified Within the Study Corridor ........................................................................................................................... 4-8

Table 4.5 Potential Economic Impacts Improvements to the Main and North Floodway Levee Systems for Hidalgo County .......................................................................................... 4-10

Table 4.6 Potential Economic Impacts Improvements to the Main and North Floodway Levee Systems for Cameron County ....................................................................................... 4-11

Table 4.7 Potential Economic Impacts Improvements to the Main and North Floodway Levee Systems for Willacy County........................................................................................... 4-11

Table 4.8 Air Emissions for Improvements to the Main and North Floodway Levee Systems...... 4-13

Table 4.9 Potential Cumulative Impacts of Ongoing and Planned TPWD Initiatives .................... 4-15

Table 6.1 Preparers of the Environmental Assessment and Technical Studies ............................. 6-2

Table 6.2 Technical Review of the Environmental Assessment...................................................... 6-3

LIST OF FIGURES

Figure 2.1 Main and North Floodway Levee Systems Location Map ............................................... 2-3

Figure 2.2 Levee Miles 0 – 20........................................................................................................... 2-4

Figure 2.3 Levee Miles 20 – 36......................................................................................................... 2-5

Figure 2.4 Levee Miles 36 – 57......................................................................................................... 2-6

Figure 2.5 Levee Miles 55 – 69......................................................................................................... 2-7

Figure 3.1 Index of Detailed Cultural Resources Maps along the Study Corridor Provided in Appendix C ............................................................................................................... 3-10

Environmental Assessment Acronyms and Abbreviations v USIBWC

ACRONYMS AND ABBREVIATIONS

AQCR air quality control region

CERCLIS Comprehensive Environmental Response, Compensation, and Liability Information System

CFR Code of Federal Regulations cfs cubic feet per second db decibel dbA A-weighted sound level in decibels DNL day-night average sound level

EA environmental assessment EIS environmental impact statement EO executive order

ERNS Emergency Response Notification System of Spills ESA Endangered Species Act FM farm-to-market

GENS generator of hazardous waste GIS geographic information system

GPS global positioning system HPA High probability areas

IBWC International Boundary and Water Commission LRGCC Lower Rio Grande Canal Company

LRGFCP Lower Rio Grande Flood Control Project LRGV Lower Rio Grande Valley m meter MxIBWC Mexican Section, International Boundary and Water Commission NAAQS National Ambient Air Quality Standards

NEPA National Environmental Policy Act NRHP National Register of Historic Places

NPL National Priority List NRCS Natural Resources Conservation Service NWR national wildlife refuge

RCRA Resource Conservation and Recovery Act ROW right-of-way T&E threatened and endangered

TCEQ Texas Commission on Environmental Quality THSA Texas Archeological Sites Atlas TPWD Texas Parks and Wildlife Department

TSD transport, storage, and disposal USACE U.S. Army Corps of Engineers USEPA U.S. Environmental Protection Agency USFWS U.S. Fish and Wildlife Service

USIBWC United States Section, International Boundary and Water Commission UST underground storage tank

WMA wildlife management area

Environmental Assessment Purpose of and Need for the Proposed Action

1-1 USIBWC

SECTION 1

PURPOSE OF AND NEED FOR THE PROPOSED ACTION

This section discusses the purpose of and need for the Proposed Action; the authority of the United States Section, International Boundary and Water Commission (USIBWC) to conduct the project as part of its mission; the scope of the environmental review; a summary of environmental compliance requirements; and the organization of this document.

1.1 PURPOSE OF AND NEED FOR ACTION

The USIBWC, in cooperation with the Texas Parks and Wildlife Department (TPWD), prepared this Environmental Assessment (EA) for the Proposed Action of raising sections of the Main and North Floodway Levee Systems located in Hidalgo, Cameron, and Willacy Counties, Texas. These two levee systems are part of the Lower Rio Grande Flood Control Project (LRGFCP) that extends approximately 180 miles from the Town of Peñitas in south Texas to the Gulf of Mexico. The Proposed Action would increase the flood containment capacity of the Main and North Floodways Levee System by raising elevation of a number of levee segments for improved flood protection. Fill material would be added to the existing levee to bring height to its original design specifications, or to meet a 2-foot freeboard design criterion. In some locations, where up to 2 feet of fill material would be placed on top of the levee, the current levee footprint could be extended up to a maximum of 12 feet. This expansion would take place along the approximate 20-foot service corridor currently utilized for levee maintenance, inside the maintained floodway, and entirely within the flood control project right-of-way (ROW). The need for excavation outside the levee structure is not anticipated.

1.2 USIBWC AUTHORITY

The International Boundary and Water Commission (IBWC), which before 1944 was known as the International Boundary Commission, was created by the Convention of 1889, and consists of a United States Section (USIBWC) and a Mexican Section (MxIBWC). The IBWC was established to apply the rights and obligations the Governments of the United States and Mexico assumed under the numerous boundary and water treaties and related agreements.

Application of the rights and obligations are accomplished in a way that benefits the social and economic welfare of the people on both sides of the boundary and improves relations between the two countries. The mission of the USIBWC is to assure:

• Regulation and conservation of waters of the Rio Grande for use by the United States and Mexico through joint construction, operation, and maintenance of international storage dams and reservoirs and plants for generating hydroelectric energy at the dams, and regulation of the Colorado River waters allocated to Mexico;

• Distribution of waters of the Rio Grande and the Colorado River between the two countries;

1-2 USIBWC

• Protection of land along the Rio Grande from floods through levee and floodway projects, and solution of border sanitation and other border water quality problems;

• Preservation of the Rio Grande and Colorado River as the international boundary; and

• Demarcation of the land boundary.

The mission of the USIBWC has five components, the third of which covers the proposed raising of the Main and North Floodway Levee Systems.

1.3 SCOPE OF THE ENVIRONMENTAL REVIEW

Federal agencies are required to take into consideration the environmental consequences of proposed and alternative actions in the decision-making process under the National Environmental Policy Act (NEPA) of 1969, as amended. The President’s Council on Environmental Quality issued regulations to implement NEPA that include provisions for both the content and procedural aspects of the required environmental analysis. In 1978, the Council on Environmental Quality issued regulations implementing the process (40 Code of Federal Regulations [CFR] 1500-1508).

The USIBWC regulations for implementing NEPA are specified in Operational Procedures for Implementing Section 102 of the National Environmental Policy Act of 1969, Other Laws Pertaining to Specifics Aspects of the Environment and Applicable Executive Orders (46 FR 44083, September 2, 1981; Appendix 501-A). These federal regulations establish both the administrative process and substantive scope of the environmental impact evaluation designed to ensure that deciding authorities have a proper understanding of the potential environmental consequences of a contemplated course of action. The Council on Environmental Quality regulations require that an EA:

• Briefly provide evidence and analysis to determine whether the Proposed Action might have significant effects that would require preparation of an environmental impact statement (EIS). If analysis determines that the environmental effects would not be significant, a Finding of No Significant Impact is prepared;

• Facilitate the preparation of an EIS, when required; or

• Aid an agency’s compliance with NEPA when no EIS is necessary.

This EA identifies and evaluates the potential environmental consequences that may result from implementation of the Proposed Action and No Action alternative. It also characterizes the affected environment and describes, when required, mitigation measures to prevent or minimize impacts to environmental resources. The following resource areas are analyzed for potential environmental consequences: biological resources; cultural resources; water resources; land use; and community resources (socioeconomics, environmental justice, and transportation). Environmental health issues are also evaluated (air quality, noise, and hazardous and toxic waste).

1-3 USIBWC

The analysis of environmental resources for the affected environment and environmental consequences was based on a potential impact corridor around the existing Main and North Floodway Levee Systems.

Analyses of environmental consequences also include potential indirect impacts adjacent to the levee corridor and the region depending on the resource and its relationship to the Proposed Action and alternatives. Reference values for air quality, cultural resources, socioeconomics, and environmental justice are evaluated on a regional basis (county level).

Results of studies conducted in support of the EA preparation were reported in the document Technical Support Studies for the Environmental Assessment of Flood Control Improvements to the Main and North Floodway Levee Systems (Parsons 2007). Findings of these studies were used to document baseline conditions for biological resources, cultural resources, wetlands, and waste storage and disposal. The report also documents potential performance of the levee system based on hydraulic model simulations, and an evaluation of environmental compliance requirements and coordination activities.

Recent published information is used for impact analyses that is based for the time period covered during construction and subsequent flood control improvement conditions. Potential environmental consequences of the Main and North Floodway Levee Systems for each resource area are discussed separately in Section 4 of this EA.

1.4 ENVIRONMENTAL COORDINATION AND COMPLIANCE ANALYSIS

Table 1.1 is a summary of regulatory and/or permitting requirements potentially applicable to improvements under consideration, potential compliance issues, and anticipated level of environmental coordination.

Table 1.1 Summary of Environmental Coordination and Compliance

Agency or Organization

Regulation or Issue Level of USIBWC Coordination

Biological Resources

U.S. Fish and Wildlife Service

(USFWS)

Endangered Species Act of 1973 (Public Law 93-205) and amendments of 1988 (Public Law 100-478) USFWS Coordination Act (916 USC 661, et seq.)

Section 7 of the Act requires formal consultation if significant adverse impacts to federally listed threatened and endangered species, and migratory birds, could occur.

Consultation with USFWS regarding impacts on various units of the LRGV National Wildlife Refuge adjacent to the levee systems.

Texas Parks and Wildlife Department

(TPWD)

Chapters 67 and 68 of the TPWD Code, and Section 65.171-65.184 of the Texas Administrative Code

Coordination with Wildlife Division concerning potential impacts of the levee-raising project to wildlife.

Coordination with State Parks Division concerning potential impacts on park tracts.

1-4 USIBWC

Agency or Organization

Regulation or Issue Level of USIBWC Coordination

Cultural Resources Texas Historic Commission

(THC)

National Historic Preservation Act of 1966, as amended (16 USC 470, et seq.)

Compliance with Section 106 requirements for potential impacts to archaeological and historic resources.

Water Resources

U.S. Army Corps of Engineers

(USACE)

Section 10 of the Rivers and Harbors Act of 1899 Section 404 of the Clean Water Act (33 USC 1344)

Permit application if waters of the United States are affected. Mitigation plan and permit application for potential impacts to wetlands.

Texas Commission on Environmental Quality

(TCEQ)

Section 401 of the Clean Water Act (33 USC 1344);

Section 26.040 of Texas Water Code

Section 401 Certification: conditions and mitigation measures may be stipulated for the 401 permit; coordination is typically a function of the USACE permitting process.

United States Environmental Protection Agency

(USEPA)

Section 402 of the Clean Water Act Section 404 of the Clean Water Act

Requirements for NPDES construction permit and Storm Water Pollution Prevention Plan preparation.

Section 404 Certification; coordination is typically a function of the USACE permitting process.

Other Issues

Texas Coastal Zone Management

Section 306 of the Federal Coastal Zone Management Act

Compliance with General Land Office (GLO) for project compatibility with coastal zone management for Cameron and Willacy Counties.

Natural Resources Conservation Service

(NRCS)

Farmland Protection Policy Act Determination that no unique or prime farmland would be affected by the federal project.

U.S. Customs and Border Patrol

Levee Road Usage Coordination during construction activities.

Irrigation Districts

Modifications to intake channel and construction along irrigation canals

McAllen, San Juan, Donna, and Progreso Irrigation Districts in Hidalgo County: levee construction along segments of the Main Floodway.

Mercedes and Santa Maria Irrigation Districts in Hidalgo County; La Feria, Adams Garden, and Harlingen Irrigation Districts in Cameron County: levee construction along the North Floodway.

1-5 USIBWC

1.5 ORGANIZATION OF THE ENVIRONMENTAL ASSESSMENT

This Environmental Assessment is composed of the following sections:

Section 1 identifies the purpose of and need for the Proposed Action, defines the scope of the environmental review, and provides an environmental coordination and compliance analysis.

Section 2 describes the Proposed Action and No Action Alternative, and summarizes potential environmental impacts.

Section 3 presents information on the affected environment, providing a basis for analyzing the impacts of the Proposed Action.

Section 4 analyzes the environmental consequences of the flood control improvements of the Main and North Floodway Levee Systems.

Section 5 describes best management practices for construction and potential mitigation actions.

Section 6 describes the consultation process and lists persons and agencies consulted, and contributors to the EA preparation.

Section 7 is a list of cited references and source documents relevant to preparation of the

EA.

Support documentation is provided in appendices, as follows:

• Appendix A: detailed maps of levee alignment, right-of-way, and potential improvement areas.

• Appendix B: habitat of Threatened and Endangered Species potentially occurring within counties intersecting the levee system.

• Appendix C: maps of cultural resources located near the levee system.

• Appendix D: review comments on the Draft EA

• Appendices E and F contain, respectively, the Cultural Resources Evaluation and Technical Support Studies Report prepared in support of the EA development.

Appendices E and F are provided in the attached CD-ROM.

Environmental Assessment Description of Proposed Action

2-1 USIBWC

SECTION 2

DESCRIPTION OF PROPOSED ACTION

This section presents a description of the Proposed Action for improvements of the Main and North Floodway Levee Systems. An overview of the Main and North Floodway Levee Systems is presented in Figures 2.1 to 2.5. Appendix A presents detailed maps of levee alignment, potential levee improvement areas, and land use in the levee system vicinity. A summary of potential environmental impacts, subsequently discussed in Section 4, is provided at the end of Section 2.

2.1 LEVEE SYSTEM DESCRIPTION

The LRGFCP was constructed to protect urban, suburban, and highly developed irrigated farmlands in the Rio Grande delta from floods, in both the United States and Mexico. The LRGFCP includes 102 miles of grass-covered earthen structures along the United States margin of the Rio Grande and Anzalduas Diversion Dam. The dam diverts floodwaters into a United States interior floodway system flanked by 168 miles of levees. A second dam, Retamal Diversion Dam, routes Rio Grande floodwaters into Mexico’s interior floodway. The distance between the United States and Mexican levees along the Rio Grande ranges from approximately 400 feet to 3 miles.

The LRGFCP interior floodway system is composed of the Main, North and Arroyo Colorado Floodways. Floodwater flows from the Rio Grande are diverted eastward from Anzalduas Dam into the Main Floodway that, at the City of Mercedes, divides into the North and Arroyo Colorado Floodways. The North Floodway extends northward from Mercedes, and turns eastward near La Villa to a terminal point north of Arroyo City, near the Laguna Madre.

Sections of the interior floodway system were identified by hydraulic modeling as priority areas to improve flood containment. The hydraulic evaluation indicated that an increase in levee height, up to 2 feet, would be needed in a number of sections of the Main and North Floodways to meet design criteria for flood protection (USIBWC 2003a).

The Main Floodway extends approximately 29 miles east from the Banker Weir near the Anzalduas Dam. Near the town of Mercedes, the Main Floodway diverges into the North Floodway and the Arroyo Colorado. The North Floodway is approximately 46 miles long. The levee ROW for both the Main and North Floodways runs primarily through agricultural areas.

The interior floodway systems have a total area of 27,013 acres between the levees in Hidalgo, Cameron, and Willacy Counties.

The levee systems run largely along agricultural areas and natural resources management lands. Those lands include the Estero Llano Grande State Park and units of the Las Palomas Wildlife Management areas operated by the TPWD, and two national wildlife refuges (NWRs) maintained by the U.S. Fish and Wildlife Service (USFWS), the Lower Rio Grande NWR and the Laguna Atascosa NWR. Urban development in the Main Floodway System vicinity is primarily limited to the portions of McAllen, Weslaco, and Mercedes. Along the North Floodway System, there are some residential areas located between Weslaco and Mercedes.

No residential developments are allowed within the floodways, largely used for agriculture.

Environmental Assessment Description of Proposed Action

2-2 USIBWC

2.2 PROPOSED ACTION

The existing levee is a raised trapezoidal compacted-earth structure with a crown width of 16 feet, a typical height ranging from 10 to 15 feet, and an approximate 3:1 side slope ratio (units of horizontal run in feet per foot of vertical rise). The levee crown is an unpaved service road with restricted public access. The existing levee footprint typically ranges from 70 to 100 feet, depending on location. A typical levee cross-section is shown in the diagram below.

The Proposed Action would increase flood containment capacity by raising elevation of a number of levee segments for improved flood control. Fill material would be added to the existing levee to bring height to its original design specifications, or to meet a 2-foot freeboard design criterion. Typical height increases in improvement areas would be less than 1 foot and would not require expansion of the existing levee footprint.

In some locations, up to 2 feet of fill material would be placed on top of the levee, potentially extending levee footprint. For a typical levee cross-section, shown in the diagram below, a 2-foot increase in levee height would result in a maximum 12-foot increase in the footprint. The need for excavation outside the levee structure is not anticipated.

Figures 2.2 to 2.5 present an overview of the Main and North levee systems. Along each levee system, potential material fill additions are indicated for the 0 to 1-foot range (green line), and 1 to 2-foot range (red line). Appendix A presents detailed information for 26 individual segments on levee improvement areas, state and federal natural resources management lands, and locations of wetlands identified during a field survey conducted in support of the EA preparation (Parsons 2007).

3:1 SLOPE

16 ft

88 ft

12 ft

16 ft

88 ft

13 ft 12 ft

16 ft

88 ft12 ft. Max.

Expansion

14 ft 12 ft

3:1 SLOPE

Mile 0

Mile 68

Mile 60

Mile 50Mile 40

Mile 30

Mile 20

Mile 10

Mile 0

Mile 69

Mile 60

Mile 50

Mile 40

Mile 30

Mile 20

Mile 10

TEXAS

MEXICO

! Brownsville McAllen

Rio Grande

¹

Fi gure 2.1 Main and North Floodways Levee System

Location Map Main and North F loodwa ys Lev e e Sys tem EA

International Boundary and Water Commission, Unit ed Sta te s S ectio n

0 5 10 15 202.5 Miles

0 10 20 30 405 Kilometers

Rio Grande

Current Levee Footprint USFWS National Wildlife Refuge Tracts TPWD Wildlife Management Areas and Park Units

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Mission McAllen

Pharr

San Juan

Alamo Donna

Weslaco

Progresso

Progresso Lakes

Mercedes

La Villa

Combes

¹ 0 2.5 5 7.5 101.25 Kilometers

0 1 2 3 40.5 Miles

Figure 2.2 Levee Miles 0 - 20

Main and North Floodways Levee System EA International Boundary and Water Commission, United States Section

Mile Marker with Label Potential Levee Height Increase up to 1 foot Potential Levee Height Increase up to 2 feet Levee Footprint USFWS National Wildlife Refuge Tracts TPWD Wildlife Management Areas and Park Units

!. 5

Estero Llano Grande State Park

¹ 0 2.5 5 7.5 101.25 Kilometers

0 1 2 3 40.5 Miles

Figure 2.3 Levee Miles 20 - 36

Main and North Floodways Levee System EA International Boundary and Water Commission, United States Section

Mile Marker with Label Potential Levee Height Increase up to 1 foot Potential Levee Height Increase up to 2 feet Levee Footprint USFWS National Wildlife Refuge Tracts TPWD Wildlife Management Areas and Park Units

!. 5

504948

494847

46454443

¹ 0 2.5 5 7.5 101.25 Kilometers

0 1 2 3 40.5 Miles

Figure 2.4 Levee Miles 36 - 57

Main and North Floodways Levee System EA International Boundary and Water Commission, United States Section

Mile Marker with Label Potential Levee Height Increase up to 1 foot Potential Levee Height Increase up to 2 feet Levee Footprint USFWS National Wildlife Refuge Tracts TPWD Wildlife Management Areas and Park Units

!. 5