18-_Design_Report.pdf

International Boundary and Water Commission United States Section

Donna Pump to Brownsville Levee Rehabilitation

Progreso Levee (Sub-Reach #1)

Hidalgo and Cameron Counties, Texas

Contract No. IBM09D0006 Order No. IBM11T0002

Design Report Final Design Submittal

December 2013

TETRA TECH, INC.

17885 Von Karman Ave, Suite 500 Irvine, California 92614

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Donna Pump to Brownsville Levee Rehabilitation

Progreso Levee (Sub-Reach 1)

Hidalgo and Cameron Counties, Texas

Design Report Final Design Submittal

Prepared for:

International Boundary and Water Commission

United States Section 4171 North Mesa, Suite C100

El Paso, Texas 79902

Prepared by:

Tetra Tech, Inc.

17885 Von Karman Avenue, Suite 500 Irvine, California 92614

December 2013

Donna Pump to Brownsville Levee- Progreso Levee Contract No. IBM09D0006 Hidalgo and Cameron Counties, Texas Order No. IBM11T0002 iii Final Design Report

TABLE OF CONTENTS

1. Purpose of Project

2. Review of Previous Reports and Studies

2.1 Environmental Assessment

2.2 Surveying and Mapping

2.3 Survey Control Lines

2.4 Right-of-Way

3. Field Investigation

4. Hydrology and Hydraulics

4.1 Hydrology

4.2 Hydraulic Model

5. Compliance with Code of Federal Regulations, Title 44, Section 65.10

5.1 Freeboard

5.2 Closures

5.3 Embankment Protection

5.4 Embankment and Foundation Stability

5.5 Settlement

5.6 Interior Drainage

5.7 Other Design Criteria – Sediment Transport

5.8 Operation Plans

5.9 Maintenance Plans

5.10 Ongoing FEMA Study Efforts

6. Geotechnical Recommendations

6.1 Settlement

6.2 Seepage

6.3 Slope Stability

6.4 Evaluation of Recent Fill Placement

6.5 Levee Embankment and Toe Drain Material Specification

6.7 Traffic Surface

7. Civil Design

7.1 Existing Facilities

7.2 Gatewell Structures and Closures

7.3 Top of Levee Raising and Widening

7.4 Levee Improvements Adjacent to Canals

7.5 Top of Levee Access Road

7.6 Roadway Intersections

7.7 Maintenance Access Ramp

7.8 Maintenance Recommendations

7.9 Vegetation Removal Zone

8. Engineer’s Opinion of Probable Cost

9. Conclusion

10. References iv Final Design Report

APPENDICES

Appendix A – Field Investigation Forms and Photos Appendix B – HEC-RAS Model Outputs Appendix C – Survey Control Data Appendix D – Engineer’s Estimate of Probable Cost Backup v Final Design Report

LIST OF TABLES

Table 2.1 – Survey Control Line Summary Table 2.2 – Rehabilitation outside Existing Levee Right-of-Way Table 2.3 – Existing Landside Toe Locations with Less Than 15 Feet Table 3.1 – Summary of Existing Gatewells from Field Investigation of Progreso Levee Table 4.1 – Hydraulic Summary along Donna Pump to Brownsville Progreso Levee (100-

Year Flood Discharge) Table 4.2 – Top of Levee Increase along Donna Pump to Brownsville Progreso Levee Table 5.1 – Minimum Factors of Safety Table 6.1 – Soil Density and Strength Parameters Table 6.2 – Computed Factors of Safety Table 6.3 – Toe Drain Gap Length Summary Table 7.2 – Maintenance and Replacement Summary for Existing Structures Table 7.3 – Summary of Study Segment Table 8.1 – Engineer’s Opinion of Probable Cost for Progreso Levee Rehabilitation

LIST OF FIGURES

Figure 1.1 – Location Map (Donna Pump to Brownsville Levee System) Figure 1.2 – Location Map Donna Pump to Brownsville Levee Rehabilitation- Progreso

Levee Figure 3.1 – Existing Gatewells Identified during Field Investigation of Progreso Levee, Gatewells 231 to 220 Figure 3.2 – Existing Gatewells Identified during Field Investigation of Progreso Levee, Gatewells 219 to 208 Figure 3.3 – Existing Gatewells Identified during Field Investigation of Progreso Levee, Gatewells 207 to 200 Figure 4.1 – Locations of HEC-RAS Cross Sections Figure 5.1 – Maximum Average Daily Flow by Water Year Recorded at USIBWC Gage

No. 04-4692.00 below Anzalduas Dam Figure 7.1 – Typical Section Showing Levee Improvements – Levee Raising and Widening

(Landside) Figure 7.2 – Typical Section Showing Levee Improvements – Levee Raising and Widening

(Riverside) Figure 7.3 – Typical Section Showing Levee Improvements Adjacent to Utility Power

Poles Figure 7.4 – Typical Maintenance Access Ramp Figure 7.5 – FEMA Minimum Vegetation-Free Zone for Basic Levee vi Final Design Report

1 Final Design Report

1. Purpose of Project

The United States Section of the International Boundary and Water Commission (USIBWC) operates and maintains the approximately 65-mile Donna Pump to Brownsville levee, which is part of the Lower Rio Grande Flood Control Project (LRGFCP) levee system, located in Hidalgo and Cameron Counties, Texas. The existing levee system was designed to protect the existing farmlands and commercial/residential buildings against the 100-year flood event. The 2006 Federal Emergency Management Agency’s (FEMA) decertification of the levee system has resulted in public and local government pressure to upgrade the levees to meet the certification requirements of National Flood Insurance Program (NFIP) regulations (44 CFR 65.10). The USIBWC contracted with Tetra Tech to provide engineering services to prepare levee rehabilitation improvement design and construction documents for the Donna Pump to Brownsville levee project.

As part of the effort to design rehabilitation improvements for the levee to meet the FEMA levee certification requirements for the Donna Pump to Brownsville levee system, Tetra Tech completed the following:

Review of a detailed field inspection (performed by USIBWC operation and maintenance [O&M] crews)

Review of all previous reports and studies Visual inspection of the existing gatewell structures (approximately 10) Geotechnical investigations and analyses Field surveys of the levee centerline and right-of-way (ROW) mapping

The minimum freeboard elevations were provided by USIBWC, and minor hydraulic analysis was performed. According to the minimum top of levee spreadsheet, the water surface elevation is 3 feet below the minimum freeboard elevations. The levee rehabilitation will raise the existing levee to a minimum elevation determined by USIBWC. The side slopes of the levee will be cleared of all vegetation. All levee side slopes that are disturbed during vegetation removal or affected by grading will be reseeded to establish ground cover. On the riverside, the maximum levee side slope will be 3H:1V (horizontal to vertical); on the landside, it will vary between 3H:1V and 2.5H:1V. A maximum landside slope of 2.5H:1V or steeper may be used as long as special measures are taken on the slope where mowing will not be required (as stated in the SOW). The top of the levee will be reconstructed to provide a minimum width of 16 feet, with 6 inches of flexible material (caliche based) on the roadway surface. USIBWC requested that Tetra Tech identify areas where the landside horizontal width is less than 15 feet wide. In these identified areas, USIBWC has determined that the side slopes may be graded at 2.5:1 to maximize the horizontal width.

The design work was utilized in the preparation of plans, specifications, and an engineer’s estimate of probable cost to rehabilitate the levee to address design related criteria of the FEMA levee certification requirements (44 CFR 65.10).

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This project covers the approximately 65-mile Donna Pump to Brownsville reach of the LRGFCP levee system, extending 16 miles and 49 miles within Hidalgo County and Cameron County, respectively (Figure 1.1). For planning purposes, the reach has been divided into five sub-reaches. The proposed levee rehabilitation investigation and design described in this report relate to the Donna Pump to Brownsville Levee Rehabilitation- Progreso Levee (Sub-Reach #1), which extends approximately 6 miles within Hidalgo County and a couple of feet within Cameron County (Figure 1.2). A majority of the levee within the Progreso Levee has been improved by Department of Homeland Security (DHS) as part of the construction of the Border Fence. The segments not improved are between Stations 199+70.94 to 307+66.36 and Stations 637+40.37 to 850+53.40 in Hidalgo County and from Stations 0+00 to 0+43.03 in Cameron County. These segments are part of contract to improve the levee.

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Figure 1.1 – Location Map (Donna Pump to Brownsville Levee System)

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Figure 1.2 – Location Map Donna Pump to Brownsville Levee Rehabilitation- Progreso Levee

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2. Review of Previous Reports and Studies

2.1 Environmental Assessment

USIBWC completed an environmental assessment (EA) in September 2007 (USIBWC 2007).

The proposed project evaluated in the EA would increase the flood containment capacity of the Donna Pump to Brownsville levee system by raising the elevation of a number of levee segments to meet a 3-foot freeboard design criterion for flood protection. Height increases of up to 2 feet were proposed to satisfy the design freeboard requirement. The increase in levee height would result in an expansion of the levee footprint due to lateral extension of the structure. Structural improvements may be required for some levee segments where seepage is a potential problem.

A memorandum submitted to USIBWC documents Tetra Tech’s understanding of and assumptions about the 2007 EA (Tetra Tech 2011b). The following assumptions have been incorporated into the design of the Donna Pump to Brownsville levee system:

The levee is centered in the 160-foot-wide survey corridor.

The levee footprint can be expanded on one side by 24 feet with an additional 15 feet allowed for clearing and grading of the service area. This expansion can occur on either the landside or riverside of the levee.

Any proposed expansion of the levee footprint would be located on the landside, except where expansion on the landside is constrained by permanent fencing, canals, and/or ROW, in which case expansion on the riverside would be required.

The proposed subsurface seepage improvement to include toe drains on the landside of the levee is currently being refined and incorporated in the design plans.

Per FEMA and U.S. Army Corp of Engineers (USACE) requirements for levee certification, the design of the levee system will include a minimum vegetation-free zone of 15 feet from the levee toe or to ROW on the landside and riverside.

The proposed vegetation removal within the levee expansion corridor has been evaluated and accepted by USIBWC.

Ongoing Section 106 consultation with the Texas Historical Commission will be performed by USIBWC.

All levee improvements are anticipated to be within the 160-foot-wide survey corridor as indicated in the EA. The Texas Historical Commission has requested that preliminary plans be submitted for its review as part of the Section 106 consultation.

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2.2 Surveying and Mapping

Melden & Hunt, Inc., conducted field surveys along the project levee in January 2011. Existing drainage features, ROW markers, and centerline survey markers were surveyed. Existing ROW markers were verified.

A survey control plan is included in the design plans which includes levee alignment, levee stationing, ROW information, and survey control points.

The horizontal control for the surveying and mapping was established based on the Texas Coordinate System, South Zone NAD 83 in feet.

2.3 Survey Control Lines

For the levee remediation design, the existing levee centerline and stationing were obtained from USIBWC survey field books and field survey verification. Design features and existing facilities that are part of this project will be located in the field during construction, based on their positioning relative to the levee centerline.

The existing centerline alignment indicated in the USIBWC survey field books differs from the alignment indicated by the field centerline markers. As a result of research and investigation, an alignment was compiled using the survey field book data and the field survey to correct the field book centerline alignment. The corrected centerline alignment is within the top of the levee. The revised centerline alignment and the stationing are different from those shown in the original survey field books. There is no direct correlation between the USIBWC field book centerline stationing and the as-built plans. There are currently two separate stationings listed in the USIBWC survey field books. From information obtained in response to request for information (RFI) No. 2, it was determined that because of the discrepancies in the USIBWC survey field books, a new survey control line stationing will be used. The first survey control stationing is from Donna Pump to the Hidalgo and Cameron County line, and the second is from the county line to the end of the levee. The following is a summary of the survey control line stationing from Donna Pump to Brownsville.

Table 2.1 – Survey Control Line Summary

Design Sub-Reach Location Begin Station End Station

Hidalgo County 0+00.00 850+53.40 Cameron County 0+00.00 0+43.03

2 Cameron County 0+43.03 635+50.41 3 Cameron County 635+50.41 1270+72.00 4 Cameron County 1270+72.00 1904+85.12 5 Cameron County 1904+85.12 2566+63.04

2.4 Right-of-Way

The ROW documents, which were provided by USIBWC, were used to prepare a map for use in the design. The existing levee was verified to be mostly within the existing ROW. Rehabilitation

7 Final Design Report of the levee will be conducted within the existing levee ROW except for a location indicated in Table 2.2. Landside levee improvements between Stations 280+00 to 281+40 will need a permanent ROW/Easement; the levee improvements extend beyond the existing ROW by 5-feet.

Table 2.2 – Rehabilitation outside Existing Levee Right-of-Way

Begin Station End Station Private Property Owner 280+00 281+40 Unknown

Tetra Tech has identified locations along the Progreso Levee where the distance from the landside toe to the ROW, existing property fence, or DHS border wall is less than 15 feet (Table 2.3). In such areas, the landside side slope will be excavated to a 2.5H: 1V side slope to provide the maximum horizontal width possible.

Table 2.3 – Existing Landside Toe Locations with Less Than 15 Feet

Begin Station End Station Comments 237+00 248+00 ROW, levee side slopes have been graded to 2.5:1 to provide the maximum horizontal width 278+00 287+00 ROW, levee side slopes have been graded to 2.5:1 to provide the maximum horizontal width ROW = right-of-way

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9 Final Design Report

3. Field Investigation

The Tetra Tech team conducted a field investigation from January 3 to 7, 2011. The investigation was conducted by driving and walking the entire length of the Progreso Levee system while visually assessing the existing flood protection elements. Notable findings and the existing conditions of the levee during the walk were documented with photographs, and georeferenced locations were recorded with a GPS unit. During the field investigation, the team used a summary maintenance field log prepared by USIBWC. The field log indicates gatewells that need repair as part of the operation and maintenance and sluice gates that need to be replaced.

Heavy vegetated areas on either the landside or riverside of the levee prevented the team from verifying several inlet and outlet structures.

A construction note within the design plans instructs the contractor to clean out or clear the area to allow USIBWC to inspect the gatewells and determine the necessary action if needed. The trees and bushes within the levee’s vegetation-free zone need to be completely removed for completion of the investigation and to satisfy USACE/FEMA requirements. The existing gatewells are shown on Figures 3.1, 3.2 and 3.3. Photographs of the existing levee features are included in Appendix A.

Several gatewells need to be cleaned out because the pipe is silted up and the existing ground is higher than the invert of the pipe. The findings from the field investigation are summarized in Table 3.1.

The field investigation did not include an inspection of the operation of the sluice gate within the gatewell. It was determined that the USIBWC Mercedes O&M crew is currently performing this activity and that the maintenance summary sheet in the field log provided by USIBWC was sufficient for Tetra Tech’s structural verifications (Duran 2011). USIBWC will modify the Tetra Tech scope of work to reflect this revision. The USIBWC Mercedes O&M crew’s field investigation from January to February 2011, findings are included in Table 3.1.

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Figure 3.1 – Existing Gatewells Identified during Field Investigation of Progreso Levee, Gatewells 231 to 220

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Figure 3.2 – Existing Gatewells Identified during Field Investigation of Progreso Levee, Gatewells 219 to 208

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Figure 3.3 – Existing Gatewells Identified during Field Investigation of Progreso Levee, Gatewells 207 to 200

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Table 3.1 – Summary of Existing Gatewells from Field Investigation of Progreso Levee

Gatewell Structure

USIBWC Notes

223 Abandoned and plugged gatewell structure by USIBWC as noted in the Patrol Book and the summary maintenance field log. Tetra Tech unable to locate and survey this abandoned structure during the field investigations. If gatewell structure is found in the field during construction, USIBWC may use the abandon and plug the gatewell structure as an optional item.

222 Abandoned Structure 221 Abandoned Structure 220 Protect in place, good working condition 219 Protect in place, good working condition 218 Protect in place, good working condition

203 Abandoned and plugged gatewell structure by USIBWC as noted in the locate and survey this abandoned structure during the field investigations. If gatewell structure is found in the field during construction, USIBWC may use the abandon and plug the gatewell structure as an optional item.

202 Abandoned and plugged gatewell structure by USIBWC as noted in the locate and survey this abandoned structure during the field investigations. If gatewell structure is found in the field during construction, USIBWC may use the abandon and plug the gatewell structure as an optional item.

201 Protect in place, good working condition

200 Tetra Tech located a T-Post, but did not locate the gatewell structure.

USIBWC noted in the field notes that this gatewell structure has been abandoned and plugged. If gatewell structure is found in the field during construction, USIBWC may use the abandoned and plugged the gatewell structure as an optional item.

Gatewell structures 223, 203, and 202 are noted in the USIBWC Patrol Book and in the summary maintenance field log as abandoned/plugged. Tetra Tech was unable to locate these structures during the field investigation. The approximate locations of these structures are shown on the plans for the Contractor to locate and notify USIBWC if found. USIBWC may abandon the gatewell structure and inlet/outlet pipes per the details shown on the plans as optional items. All previous abandoned gatewell structures and inlet/outlet pipes need to be properly abandoned and plugged as shown on the plans to prevent seepage and flood flows from passing through the levee.

Pipe penetrations that are not shown on the USIBWC summary maintenance field log or Patrol Book are assumed illegal and unlicensed penetrations through the levee. If these pipe penetrations are found during construction the Contractor needs to alert USIBWC for corrective actions.

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4. Hydrology and Hydraulics

4.1 Hydrology

No hydrologic analysis was performed by Tetra Tech for this project. Based on a study completed by USIBWC in 2003, the design flood for the LRGFCP is based on a peak flow of 250,000 cubic feet per second (cfs) at Rio Grande City, which attenuates to 235,000 cfs at Peñitas. During the design flood, Anzalduas Diversion Dam and Retamal Diversion Dam would each divert 105,000 cfs into the United States and Mexico. Flow diversion during the design flood would limit flood flows through the Brownsville-Matamoros area to 20,000 cfs (USIBWC 2003).

4.2 Hydraulic Model

The hydraulic analysis of the project reach along the Rio Grande was provided by USIBWC (2003). The hydraulic model was created using the Hydrologic Engineering Center-River Analysis System (HEC-RAS) program, version 3.0 (USACE 2001). The 2003 model was based on the geometrical cross-sectional data used in a 1992 HEC-2 model for the channel of the Rio Grande and the floodways in the United States. The Rio Grande data were modified for the established Restricted Use Zone (floodplains on both sides of the Rio Grande) and additional bridges and to include data for 24 additional new cross sections that were surveyed.

The 2003 HEC-RAS model contains 153 cross sections between Anzalduas Diversion Dam and the downstream extent of the U.S. levee embankment near Brownsville, and it extends beyond the limits of this study at the upstream end. A total of 113 HEC-RAS cross sections are located within the project limits for the Donna Pump to Brownsville levee rehabilitation; 28 HEC-RAS cross sections are located within the Progreso Levee. The locations of HEC-RAS cross sections within the Progreso Levee are shown on Figure 4.1.

The computed water surface elevations for the design discharge of 20,000 cfs from the hydraulic analysis for the levee rehabilitation are presented in Table 4.1. The elevations in the model are referenced to the NAVD88. The outputs from the HEC-RAS model are included in Appendix B.

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Figure 4.1 – Locations of HEC-RAS Cross Sections

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Table 4.1 – Hydraulic Summary along Donna Pump to Brownsville Progreso Levee (100-Year Flood Discharge)

HEC-RAS

River Station

Invert Elevation

(feet)

Water Surface

Elevation (feet)

Flow Velocity (feet/second)

Top Width (feet)

Froude Number

129.216 55.64 90.46 6.86 162 0.21

129.214 55.64 83.75 8.68 82 0.29

129.213 55.64 82.44 12.04 62 0.41

129.212 55.64 82.41 12.05 62 0.41

129.211 55.64 83.46 3.05 299.18 0.11

129.21 55.64 83.47 2.73 327.24 0.1

129.2 55.04 83.47 2.67 776.19 0.11

129.03 53.84 83.26 2.54 1400 0.11

127.7 49.94 82.3 1.84 6874.96 0.08

126.7 50.94 81.69 2.59 9460.68 0.11 125 50.24 80.5 2.86 5309.03 0.1

123.8 49.74 79.53 2.65 2550.72 0.1

122.5 51.94 78.7 3.05 577.12 0.11

121.46 45.44 78.3 3.38 1061.64 0.12

120.7 50.84 77.88 2.67 843.83 0.11

120.62 46.54 77.79 2.91 614.93 0.11

120.59 46.54 77.77 2.91 614.83 0.11

120.5 49.84 77.71 2.61 1286.35 0.1

120.38 51.64 77.38 2.33 3647.8 0.09

117.87 49.34 76.24 2.87 5098.22 0.11

117.2 46.84 75.73 3.09 3841.67 0.12

116.5 47.54 75.37 3.42 1576.25 0.13

116.1 45.84 75.13 2.97 478.47 0.12

115.7 48.14 74.88 3.23 1346.1 0.12 115 36.14 74.36 3.24 1476.76 0.12

114.8 46.94 74.11 3.34 1577.64 0.13

114.65 42.54 74.07 2.53 2286.92 0.1

114.1 42.04 73.48 2.29 9144.73 0.1

Note: All elevations are based on NAVD88.

The minimum amount that the top of the levee must be raised was determined by USIBWC and provided as part of the design. The necessary increases in top of levee elevation are indicated in Table 4.2. The total length of the existing levee within the Progreso Levee is approximately 12 miles, per USIBWC limits. Approximately 6 miles of the levee was improved by DHS as part of the border fence construction, this portion has been excluded as part of our contract.

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Table 4.2 – Top of Levee Increase along Donna Pump to Brownsville Progreso Levee

HEC-RAS

River Station

Existing Top of Levee

Elevation1 (feet)

Water Surface

Elevation (feet)

Existing Freeboard

(feet)

Minimum Top of Levee

Elevation2 (feet)

Increased top of Levee3

(feet)

129.211 88.87 83.46 5.41 86.46 0

129.03 88.80 83.26 5.54 86.26 0

127.7 87.68 82.30 5.38 85.30 0

126.7 86.87 81.69 5.18 84.69 0 125 86.32 80.50 5.82 83.50 0

123.8 86.31 79.53 6.78 82.53 0

122.5 86.65 78.70 7.95 81.70 0

121.46 82.51 78.30 4.21 71.30 0

120.7 80.99 77.88 3.11 80.88 0

120.5 81.09 77.71 3.38 80.71 0

120.38 82.04 77.38 4.66 80.38 0

117.87 80.34 76.24 4.1 79.24 0

117.2 79.94 75.73 4.21 78.73 0

116.5 83.17 75.37 7.8 78.37 0

116.1 80.05 75.13 4.92 78.13 0

115.7 80.14 74.88 5.26 77.88 0 115 77.30 74.36 2.94 77.36 0.06

114.65 75.73 74.07 1.66 77.07 1.34

114.1 76.63 73.48 3.15 76.48 0

Note: All elevations are based on NAVD88.

1 Top of levee elevations were provided by USIBWC.

2 USIBWC assume 3 feet above the water surface elevation.

3 Increase in top of levee elevation was done by USIBWC in 2007, not part of this project.

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5. Compliance with Code of Federal Regulations, Title 44, Section 65.10

The Code of Federal Regulations (44 CFR 65.10) includes a description of the types of information FEMA needs in order to recognize (or “accredit”), on NFIP maps, that a levee system provides protection from the base flood. The following information addresses the design criteria identified in 44 CFR 65.10 and is intended to provide additional information for use by USIBWC in developing the FEMA levee certification package. Tetra Tech’s scope of work does not include developing the information/analyses to support the FEMA levee certification package; however, the following information is provided to aid USIBWC in development of that information and analyses.

Specific design criteria are listed, as well as other criteria as specified on the MT-2 Forms. MT-2 Form 3 (Riverine Structures) includes a description of required sediment transport information.

5.1 Freeboard

According to 44 CFR 65.10, the freeboard requirements are as follows:

A minimum of 3 feet above base flood (100-year) water surface level.

An additional 1 foot above minimum within 100 feet of structures/bridges/constrictions.

An additional 0.5 foot at the upstream end of the levee, tapering to not less than the minimum at the downstream end of the levee.

A comparison of the 100-year water surface elevations from the USIBWC HEC-RAS model of the Rio Grande along the Progreso Levee and the proposed top of levee elevations shown on the drawings of the proposed design (Tetra Tech 2013a).

5.2 Closures

The Progreso Levee includes 10 gatewell structure side drains within our contract area. Of these gatewells, 4 are operational and 6 have been abandoned/plugged per the USIBWC Patrol Book and in the summary maintenance field log. Gatewell structure 223, 203, and 202 are noted as abandoned/plugged but were not located during the field investigation. The approximate locations of these missing structures are shown on the plans for the Contractor to locate and notify USIBWC if found during construction. USIBWC may abandon and plug the structures as optional items to meet FEMA’s requirements.

The remaining abandoned gatewells are to be excavated, and brick and mortar sealed to provide permanent closure. The soil within the existing gatewell structure will be excavated and the steel plates to be removed. The outlet pipe, inlet pipe and cavity of the gatewell structures will be filled with flowable fill and the top of the gatewell structure is to be removed to 5 feet below the existing levee side slope. The four operational gatewells include working existing sluice gates, which meet FEMA’s requirements for a closure device.

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USACE requires that all discharge pipes be inspected using a television camera or other visual inspection at least every 5 years. This is not included specifically as part of FEMA’s requirements for levee certification.

The Mercedes Canal crossing located between Stations 686+19.66 to 688+23.95 does not have a closure structure and the system will not meet FEMA certification without the construction of a closure structure. USIBWC has been informed of this opening and a closure structure is to be designed in the future.

5.3 Embankment Protection

According to 44 CFR 65.10, engineering analyses must be submitted that demonstrate that no appreciable erosion of the levee embankment can be expected during the base flood and that anticipated erosion will not result in failure of the levee embankment.

To demonstrate that no appreciable erosion of the levee embankment can be expected during the base flood, flow velocities were extracted from the HEC-RAS model of the Rio Grande along the Progreso Levee provided by the USIBWC. No appreciable erosion of the levee embankment is expected during the base flood based on velocities identified in the HEC-RAS model along the left/north bank (maximum of 1.3 feet per second [fps]) or the maximum velocity expected through the bends based on the equation on Plate B-33 in Engineer Manual (EM) 1110-2-1601, a maximum of 1.7 fps (USACE 1994). Flood waters become erosive at velocities greater than 5 fps for grass-lined earthen channels per EM 1110-2-1601.

5.4 Embankment and Foundation Stability

According to 44 CFR 65.10, engineering analyses must be submitted that demonstrate that seepage into or through the levee foundation and embankment will not jeopardize embankment or levee stability. The geotechnical evaluation of this reach performed by Raba-Kistner Consultants, Inc. (R-K) indicated that construction of internal drainage was required to mitigate potential seepage conditions during flood stage (R-K 2013a). It was recommended that a toe drain with a minimum base width of 3 feet extending at least 18 inches below the landside toe of the levee be included in the proposed rehabilitation design. Seepage analysis indicated that implementation of the toe drain would control seepage pressures within the levee and foundation soils and would reduce hydraulic exit gradients to less than 0.5. The toe drain detail has been incorporated into the design drawings and specifications. With this condition, seepage associated with the base flood is not expected to jeopardize the embankment or foundation stability.

Slope stability analyses were performed on pertinent cross sections of the proposed rehabilitated levee. The results of the analyses relative to FEMA criteria are provided in Table 5.1.

Table 5.1 – Minimum Factors of Safety

Condition Calculated Minimum Factor of

Safety USACE Required Factor of

Safety End of construction 1.3 1.3 Long-term (steady seepage) 1.5 1.4 Rapid drawdown 1.4 1.0 to 1.2

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The results of the analyses indicate that the proposed levee design meets the FEMA levee criteria for embankment and foundation stability.

5.5 Settlement

According to 44 CFR 65.10, engineering analyses must be submitted that assess the potential and magnitude of future losses of freeboard as a result of levee settlement and demonstrate that freeboard will be maintained within the minimum freeboard standards. The geotechnical evaluation for this project indicated that placement of 3 feet of additional fill height could induce total settlement (compression and long-term consolidation) of approximately 3 to 3-1/2 inches.

The proposed design would not significantly raise the current levee height but rather it would reconstruct portions of the existing levee with engineered fill. As a conservative measure to maintain the required freeboard, the proposed design height of the levee has been increased by additional 4 inches to mitigate potential settlement.

5.6 Interior Drainage

According to 44 CFR 65.10, joint probability analysis of the sources of flooding landward of the levee and the river itself must be completed and the extent of the flooded area provided. It is expected that this will be a significant effort because of the number of side drains (approximately 10 side drains over the Progreso Levee) and the ownership of these systems outside of USIBWC right-of-way. USIBWC should note that this analysis will be required as part of a levee certification submittal to FEMA. Tetra Tech’s current SOW does not include providing any support with regard to the interior drainage analysis.

5.7 Other Design Criteria – Sediment Transport

A sediment transport analysis is required if there is any indication from historical records that scour and deposition can affect the base flood elevation (BFE), or if based on stream morphology, vegetative cover, development of the watershed, and bank conditions, there is a potential for debris and sediment transport to affect the BFE or a structure. If sediment transport will not affect the BFE or any structures, an explanation is required as to why a sediment transport analysis was not performed.

Related to the U.S. (north) levee embankment structure along the Rio Grande between Retamal Dam (south of Donna, Texas) and Brownsville, Texas, the following initial investigations were conducted to identify whether there is potential for sediment transport to affect the BFE or the levee embankment. These investigations focused on the Rio Grande along the Progreso Levee but extended upstream as required by the available information.

5.7.1 Analysis of Long-Term Aggradation/Degradation

A trend of long-term aggradation could reduce channel capacity and impact the freeboard of the levee; a trend of degradation could impact channel stability, which could in turn influence the stability of the levee. A review of hydrologic conditions was conducted to characterize the influence of floods on the potential to mobilize, transport, and deposit sediment within the Rio Grande along the Progreso Levee.

22 Final Design Report

USIBWC operates a flow gage on the Rio Grande upstream of the north levee embankment (Gage No. 08-4692.00 below Anzalduas Dam near Reynosa, Tamaulipas, Mexico). Construction of Anzalduas Dam was completed in April 1960 so that floodwaters could be diverted from the Rio Grande into the interior U.S. floodway. Construction of upgrades to Retamal Dam, located approximately 39 miles downstream of Anzalduas Dam, were completed in May 1975 to divert floodwaters to the interior Mexican floodway and to limit floods at Brownsville-Matamoros to the safe capacity of the Rio Grande. According to USIBWC Minute No. 238, the design flood flow below Anzalduas Dam is 20,000 cfs (USIBWC 2003). A review of the average daily discharges recorded between 1960 (when Anzalduas Dam was constructed) and 2010 indicates that floods in excess of the design flow occurred in 9 years. Other than the flooding associated with Hurricane Beulah in September 1967 (maximum average daily discharge of 121,000 cfs), the maximum average daily flow rate was 41,300 cfs in water year 1972 (Figure 5.1). However, considering only the period after 1975 when the upgrades to Retamal Dam were completed, four floods with maximum average daily discharges exceeding 20,000 cfs occurred; the maximum average daily flow of 27,300 cfs occurred in water year 1976. Further investigation of the magnitude of the design flood is warranted to confirm that 20,000 cfs is consistent with the peak flow during the 100-year flood (i.e., the base flood). The current effective FEMA Flood Insurance Study (FIS) indicates that a discharge of 20,000 cfs has a 500-year recurrence level.

Figure 5.1 – Maximum Average Daily Flow by Water Year Recorded at USIBWC Gage No. 04-

4692.00 below Anzalduas Dam

On a long-term basis, most bed material sediment is mobilized, transported, and deposited during flood flows. Since the design flood has been exceeded four times since the upgrades to Retamal Dam were completed in 1975, the system has been tested by major flooding. Assuming the flood

10,000

20,000

30,000

40,000

50,000

60,000

M ax im u m A ve ra g e

D ai ly F lo w cf s)

121,000

Design Flood Downstream of Anzalduas Dam = 20,000 cfs

1960 - Anzalduas Dam Construction Completed 1975 - Retamal Dam Upgrades Completed

23 Final Design Report regime since 1975 is representative of the future flood regime (due to the regulation of floods by Anzalduas Dam and Retamal Dam), it is expected that future opportunities for floods to transport sediment will be similar. Therefore, any existing trends in long-term aggradation or degradation that are in response to flood hydrology may continue into the future.

Little information was available to evaluate whether long-term aggradation/degradation has the potential to affect the BFE or the Progreso Levee. Due to the set-back of the levee embankment from the Rio Grande channel, the risk of degradation of the channel and corresponding bank failures appears low; however, this expectation should be confirmed by USIBWC maintenance staff. Confirmation would include a statement that USIBWC does not need to conduct regular dredging of excessive sediment deposits that would indicate the potential for aggradation. The sediment trapping of the impoundments upstream of Anzalduas Dam and Retamal Dam likely limit aggradation potential, but this should be confirmed through USIBWC maintenance records.

5.7.2 Analysis of Bend Scour

In meandering alluvial channels, such as the Rio Grande along the Progreso Levee, the bed profile and the planform of the river are related. Pools form due to scour in the bends, and riffles or crossings form between bends. The increase in depth through a pool corresponds to an increase in the height of the bank, which increases the potential for the bend scour to lead to bank instabilities. The planform of the Rio Grande channel was reviewed to identify bends that were located within 500 feet of the levee embankment. An equation developed by Thorne (1988) was used to calculate maximum bend scour as a function of bend geometry. An assumed failure plane with a 2H:1V slope was extended up from the calculated maximum scour depth to evaluate whether bank failure could impact the levee embankment.

5.7.3 Conclusion

The initial review of available information related to sediment transport in the Rio Grande along the Progreso Levee indicates limited potential for increases in the BFE or threats to the integrity and performance of the levee embankment due to local scour or general aggradation/degradation processes. The combined influence of Anzalduas Dam and Retamal Dam have limited the frequency of major floods capable of transporting substantial amounts of bed material sediment between Retamal Dam and Brownsville. Where existing bends are within 500 feet of the levee embankment, calculations of expected maximum bend scour indicate that except as previously noted, expected bank failure will not likely threaten the levee embankment or foundation.

The potential for long-term aggradation/degradation to affect the BFE or the levee embankment is expected to be minimal due to (1) the controlling influence of the diversion dams on the flood regime, (2) the capacity of the dams to trap sediment that could otherwise aggrade in the channel along the Progreso Levee, and (3) the setback between the Rio Grande channel and the levee embankment, allowing for bank failures without threatening the levee embankment or foundation, but further analyses of operation and maintenance records should be made to confirm this expectation.

It should be noted that the analyses to determine the impact of sediment transport on the water surface elevation and freeboard is not part of Tetra Tech’s scope of work. However, this initial

24 Final Design Report information has been developed to aid USIBWC in the development of the necessary information to support a FEMA levee certification package. Details of additional investigations that could be undertaken by USIBWC to further develop the information related to sediment transport and levee certification are identified in the following section.

5.7.4 Additional Investigations

The following additional investigations could be performed to confirm that sediment transport is not expected to increase the BFE or threaten riverine structures:

Specific Gage – Using long-term stage and flow measurements collected at the USIBWC gauging station downstream of Retamal Dam (or the gage near Brownsville, Texas, and Matamoros, Tamaulipas, Mexico, Gage No. 08-4750.00) changes in gage height over time for a specified flow rate can be determined. The specific gage analysis is useful in indicating whether there is a trend of aggradation or degradation. Assuming the USIBWC gage is installed in an alluvial reach of the Rio Grande channel, the history of shifts in the rating curve or a specific gage analysis would indicate changes in channel morphology over time. While this gage is located upstream of the Progreso Levee, the analysis would still provide useful information related to the Progreso Levee.

As-Built Plans –The as-built plans for crossings of the Rio Grande (i.e., bridges, diversion structures, dams, and utilities) should show the local elevation of the Rio Grande channel, possibly even the geometry of the channel. The channel geometry on the plans can be compared to the channel geometry represented in the HEC-RAS model to identify an average rate of change over the period of comparison. Any historical surveys that include the channel geometry could be used for these comparisons.

Comparison of Historical Aerial Photographs – The planform of the Rio Grande can be compared at different points in time (e.g., preceding and following flood events) to identify whether sediment transport is sufficient to alter the alignment of the channel. Initially a comparison could be made using recent imagery and imagery closely following the 1975 completion of the upgrades to Retamal Dam. Comparisons to earlier imagery could be made to determine whether there was an ongoing trend before the construction/upgrades to the diversion dams, but comparisons before and after 1975 need to consider the influence of the dam operations on the hydrologic regime. If substantial changes in planform are apparent between 1975 and 2010, intermediate dates could be compared, preferably close in time to major flooding.

Of specific interest in the comparison of historical aerial photographs would be the location of bends in the Rio Grande channel. It is clear in recent aerial imagery that bends form and cut off over time. If bends have historically migrated across the floodplain and encroached upon the levee embankment, USIBWC operation and maintenance procedures may show how USIBWC has historically protected the levee embankment.

Review of Operation and Maintenance Records – The records maintained by USIBWC could be reviewed to identify whether natural responses of unbalanced sediment transport have

25 Final Design Report historically resulted in the need for intervention to maintain channel conveyance. Intervention may include the following:

Construction of grade control structures (e.g., diversion structures, drop structures, weirs, or sills) to prevent down cutting of the bed

Installation of revetments to arrest bank erosion

Excavation/dredging of deposited sediment

Repairs such as unclogging or reinforcing outlets of drainage structures that enter the channel

5.8 Operation Plans

According to 44 CFR 65.10(c), an officially adopted operation manual will need to be provided.

The current USIBWC manual may need to be updated to meet FEMA requirements. Because the system includes closures (such as sluice gates at side drains), and locations within the Mercedes Concrete Canal (Station 738+00 to Station 750+00) requires USIBWC to provide a one foot full canal by boarding out the canal and pumping it full of water during a flood event, the following information must be included in the manual:

Documentation of the flood warning system that will be used to trigger emergency operation activities and demonstration that sufficient flood warning time exists to permit activation of the mechanized portions of the drainage system

Specific actions and assignments of responsibility by individual name or title

Provisions for periodic operation of the closure at not less than 1-year intervals

FEMA will not accept a general operation plan that is not specific to the levee system, i.e., one that covers an entire county or state.

5.9 Maintenance Plans

According to 44 CFR 65.10(d), an officially adopted maintenance manual will need to be provided. Upon submittal of a levee certification request, FEMA may review all available inspection information to ensure that it is in compliance with the maintenance manual. In order for FEMA to recognize the certification and accredit the levee on the NFIP maps, any issues of noncompliance identified during inspections will require documentation of the correction necessary to meet the standards in the maintenance plan.

5.10 Ongoing FEMA Study Efforts

FEMA is currently working with the local communities to update the flood information for the Rio Grande in the project area. USIBWC should consider coordinating with that effort to ensure that all base information (such as the 100-year discharge) is consistent and that no conflicts arise (such as designation of a floodway in an area where levee improvements are planned).

26 Final Design Report

27 Final Design Report

6. Geotechnical Recommendations

In July 2009, Raba Kistner (R-K) completed the subsurface exploration and engineering analyses for the proposed rehabilitation improvements to the Donna Pump to Brownsville levee reach (R- K 2009). The geotechnical analyses of the levee were performed in general accordance with EM 1110-2-1913 (USACE 2000) and FEMA Requirements for Mapping Levees (44 CFR 65.10).

In June 2011, R-K evaluated recently constructed levee fill that was placed along portions of the Donna Pump to Brownsville levee reach (R-K 2011a). Portions of the Progreso Levee were included in that study.

In August 2012, R-K completed the subsurface exploration and engineering analyses for additional geotechnical borings along the Progreso Levee to complete the levee rehabilitation.

The testing results and recommendations for the additional geotechnical borings can be found in the Geotechnical Addendum- Sub-Reach 1 (R-K 2013a).

6.1 Settlement

On the basis of the review of the conditions encountered along the Progreso Levee, maximum estimated settlements of 3 to 3-1/2 inches were calculated (R-K 2013a). Based on the settlement evaluation, the proposed design of the Progreso Levee includes a vertical overbuild of 4 inches to mitigate potential settlement after construction.

6.2 Seepage

Potential seepage though and under the Progreso Levee was evaluated by R-K using the computer software SLIDE developed by Rocscience. The analysis indicated that some form of internal drainage was necessary in order to effectively control seepage pressures and hydraulic exit gradients. It was recommended that a toe drain with a minimum base width of 3 feet extending at least 1.5 feet below the landside toe of the levee be included in the proposed levee design. The seepage analysis indicated that implementation of the toe drain would control seepage pressures within the levee and foundation soils and would reduce hydraulic exit gradients to less than 0.5. The toe drain detail has been incorporated into the proposed levee design drawings and specifications.

The seepage analyses results require that the Mercedes Concrete Canal between Stations 738+00 to 750+00 is to have a full canal of one foot deep during a flood event. USIBWC will board the canal at this segment and pump water into the canal to maintain a full canal during a flood event to satisfy the seepage requirements.

6.3 Slope Stability

The analyses of slope stability were conducted by R-K using SLIDE. Soil density and strength parameters were based on the data collected as part of the 2009 study (R-K 2009). The assumed parameters consisted of various subsurface conditions shown in (Table 6.1).

28 Final Design Report

Table 6.1 – Soil Density and Strength Parameters

Material (Classification under USCS)

Unit Weight (pounds/cubic foot)

Short-Term and Sudden Drawdown (Undrained)

Long-Term Drawdown (Drained)

Cohesion (pounds/square foot)

Friction Angles

(degrees)

Cohesion (pounds/square foot)

Friction Angles

(degrees) Poorly Graded Sands with Silts (SP-SM)

117 0 33 0 33

Poorly Graded Silty Sand (SM)

117 0 32 0 32

Clays (CL and CH) 120 200 24 400-425 0 Gravel (Drainage Blanket/ Toe Drain)

125 0 35 0 35

Clay Fill (CL and

CH)

120 200 24 400-425 0

USCS = Unified Soil Classification System

The analyses of slope stability included seepage conditions estimated on the basis of a landside toe drain as described in the Section 6.2. The results of the analyses are summarized in Table 6.2.

Table 6.2 – Computed Factors of Safety

End of Construction-

Riverside

End of Construction-

Landside

Steady State at Flood Stage –

Riverside

Steady State at Flood Stage –

Landside

Sudden Drawdown –

Riverside Station 302+10

1.8 2.1 2.2 2.1 1.8 Station 748+80 1

1.9 1.6 3.3 2.3 1.9 Station 837+70 2

1.3 1.5 2.8 2.4 1.4 1 Requires one feet of water in the Mercedes Concrete Canal 2 Requires embedded toe drain

Based on the results of the analyses, the proposed levee embankment and slope stability for this project meet the current USACE requirements.

6.4 Evaluation of Recent Fill Placement

An evaluation of the recently constructed fills placed on the top of portions of the levee within the Progreso Levee was performed by R-K (R-K 2011a). The evaluation indicated that significant portions of the recent fills were not compacted to the specified density criteria and that some of these fills do not meet the gradation and material type criteria. Therefore, for areas where recent fills have been placed, it was recommended that material be removed to a depth of 18 inches below the top of existing levee before the placement of fill to reach final design grade.

In areas where recent fills have not been placed, the removal requirement is 12 inches below the top of existing levee.

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6.5 Levee Embankment and Toe Drain Material Specification

On the basis of the field testing, the excavated materials are expected to be reusable as fill materials for the levee embankment. However, the existing caliche-based surface material should be segregated and disposed of offsite. Only material classified as CL, CL-ML, and SC soils under the Unified Soil Classification System may be reused for the levee embankment. Material acceptable for reuse should have a maximum liquid limit of 45, and a plasticity index between 20 and 30.

The approved reusable materials or import soils used as levee embankment fill should be placed in loose lifts not exceeding 8 inches in thickness and compacted to at least 95 percent of maximum dry density as determined by ASTM D1557.