J.06_2015_10_20_Channel_Maint_Alternatives.pdf

Channel Maintenance Alternatives and Sediment-transport Studies for the Rio Grande Canalization Project:

Final Report

Submitted to: International Boundary and Water Commission, U.S. Section 4171 North Mesa, Suite C-100

El Paso, Texas 79902-1441

Submitted by:

3801 Automation Way, Suite 100 Fort Collins, Colorado 80525

October 20, 2015 Contract No. IBM09D0006

Order No. IBM14T0016

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Final Report i

EXECUTIVE SUMMARY

ES-1. Background

The United States Section of the International Boundary and Water Commission (USIBWC) Rio Grande Canalization Project (RGCP) is a narrow river corridor that extends 105.4 miles from Percha Dam at River Mile (RM) 105.4 in Sierra County, New Mexico, to American Dam at RM 0 in El Paso, Texas (Parsons, 2004). The RGCP was constructed between 1938 and 1943 to facilitate compliance with the 1906 convention between the United States and Mexico, and to properly regulate and control, to the fullest extent possible, the water supply for use of the two countries as provided by the treaty (Parsons, 2004). Based on the Act of Congress of June 4, 1936 (Public Law 648; 49 Stat. 1463), USIBWC’s statutory duties within the RGCP are to provide for efficient water flows and flood protection. As a result, major elements of the project included acquisition of Right-of-Way (ROW) for the river channel and adjoining floodways (8,332 acres;

Parsons, 2004), improvement of the alignment and efficiency of the river channel conveyance for water delivery (conveyance efficiency), and flood-control measures that extended through the Rincon and Mesilla Valleys of New Mexico and El Paso Valley in Texas. As part of the RGCP, a deeper main channel was dredged to facilitate water delivery for irrigation. Flood protection levees were placed along two-thirds of the length of the RGCP where the channel was not confined by hillslopes or canyon walls (e.g., Selden Canyon). In addition to a variety of dams constructed prior to the 1960s, a number of Natural Resource Conservation Service (NRCS) sediment/flood-control dams were built between 1969 and 1975 on tributary arroyos to control flooding and sediment delivery to the RGCP from about 300 square miles of drainage basin downstream of Percha Dam.

Of the many challenges that the USIBWC faces in operating the RGCP, ongoing sediment delivery from the tributary arroyos has historically been among the most significant. Sediment deposition on the alluvial fans can result in sediment plugs, island formation, and aggradation that prevents draining of irrigation return flow that could result in increased water-surface elevations and associated impacts to levee freeboard and flood conditions. The sedimentation may also be affecting the delivery of water to U.S. stakeholders and Mexico due to reductions in channel and drain return efficiencies. One of the primary requirements of the USIBWC from the 2009 Record of Decision (ROD) involved identification of methods to improve river management through an evaluation of adaptive management strategies aimed at channel maintenance activities and levee protection. As part of the adaptive management strategy approach, the USIBWC is evaluating channel maintenance alternatives to address the sediment related problems along the RGCP.

ES-2. Study Objectives

This channel maintenance alternatives and sediment-transport study is intended to build upon previously developed conceptual restoration plans (USACE, 2009) and river management plans (Parsons, 2004) to specifically address issues associated with sedimentation along the RGCP (sediment plugs, island formation, raising of the river bed, reduced irrigation drain efficiency, and increased threats to levee freeboard and flooding). These problems occur at the nine representative problem locations that are evaluated in this study (Table ES-1; Figure ES-1). The nine problem locations are:

1. Tierra Blanca Creek to Sibley Arroyo (includes vortex weir below Tierra Blanca)

2. Salem Bridge to Placitas Arroyo (includes Hatch Bridge, Thurman Arroyo, and numerous islands)

3. Rincon Siphon A Restoration Site to Rincon Siphon (includes Garcia Arroyo)

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Figure ES-1. Map of the Rio Grande Canalization Project reach showing the locations of the nine problem locations that are considered in the Channel Maintenance Alternatives and Sediment-transport Studies project. Also shown are the Geomorphic Subreaches that were developed as part of the USACE (2007) study.

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Table ES-1. Summary of the nine problem locations evaluated in this study.

Problem Location

Identification Representation Geomorphic Subreach1

D/S Station2

U/S Station2

RM Range3 Length (miles)

Comment

Tierra Blanca

Creek to Sibley Arroyo

Vortex Weir 1 5168+00 5288+50

97.8 - 100.1

2.3 Draft Channel Maintenance Plan Low

Priority Area; includes vortex weir below Tierra Blanca.

Salem Bridge to Placitas Arroyo

Arroyos and Islands

2 4459+10 4658+80 84.4 - 88.2 3.8

Draft Channel Maintenance Plan Low and High Priority Areas; includes

Hatch Bridge, Thurman Arroyo and numerous islands.

Rincon Siphon A Restoration Site to Rincon Siphon

Restoration Sites and Siphon

2 4329+20 4371+40 82 - 82.8 0.8 Draft Channel Maintenance Plan Low and High Priority Areas; includes Garcia Arroyo

Rincon Arroyo to

Bignell Arroyo Arroyos and

Islands 2 3986+60 4169+40 75.5 - 79 3.5

Draft Channel Maintenance Plan Low and High Priority Areas; includes

Reed Arroyo

Rock Canyon to

1.4 mi below

Rincon/Tonuco Drain

Confluence

Drain and Mouth of

Seldon Canyon 3 3643+50 3798+30 68.9 - 71.8 2.9

Draft Channel Maintenance Plan Medium Priority Area; Includes Horse

Canyon Creek

Picacho Drain to below Mesilla Dam

Drain, Canals and Dam

5 and 6 2042+00 2167+10 38.8 - 41.2 2.4 Draft Channel Maintenance Plan Medium Priority Area; includes

California Lateral

East Drain to below Vinton

Bridge

Drain and Arroyo

6 and 7 785+70 876+00 14.8 - 16.6 1.8 Not a Draft Channel Maintenance Plan Priority Area but issues with sedimentation and flooding.

Upstream of Country Club

Bridge to NeMexas Siphon

No Inputs, Bridge, Populated Area, Levee

Encroachments

7 378+10 456+10 7.1 - 8.6 1.5 Draft Channel Maintenance Plan High

Priority Area; Levee encroachment and freeboard concerns

Montoya Drain to American Dam

Drain 7 0 139+90 0 - 2.7 2.7 Draft Channel Maintenance Plan High

Priority Area; Below Anapra Bridge

1From USACE (2007) Study.

2Station refers to the base model station line prepared for the USACE (2007) Study.

3Miles upstream from American Dam.

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4. Rincon Arroyo to Bignell Arroyo (including Reed Arroyo)

5. Rock Creek to 1.5 miles Below Rincon/Tonuco Drain Confluence (including Horse Canyon Creek)

6. Picacho Drain to downstream of Mesilla Dam

7. East Drain to downstream of Vinton Bridge

8. Upstream of Country Club Bridge to NeMexas Siphon

9. Montoya Drain to American Dam

Results from the study provide a suite of alternatives to reduce or minimize the sediment issues at the 9 problem locations, and identify the most efficient, sustainable and environmentally beneficial methods. Once identified, the preferred alternatives can then be applied to other locations along the RGCP that have similar issues to the problem locations evaluated in this study.

ES-3. Study Approach

In general, this study evaluated five channel maintenance alternatives (CMAs) at each of the nine problem locations. At each of the problem locations except for the Mesilla Dam site, three of the CMAs are classified as “sediment-removal alternatives” and include “Channel Excavation Short”, “Channel Excavation Long” and “Localized Sediment Removal” scenarios that involve excavation of sediments over varying distances and widths. At the Mesilla Dam problem location, the sediment removal alternatives only included the “short” and “long” scenarios. The remaining alternatives are classified as “Non-sediment Removal Alternatives” and vary by problem location, as discussed in Section ES-6, below.

A number of tasks were carried out as part of this assessment of CMAs for the RGCP that included a field assessment, targeted cross-section surveys, steady-state modeling of the overall RGCP, localized steady-state hydraulic modeling of the problem locations, sediment-transport modeling of the problem locations under existing and with-CMA conditions, and preparation of a benefit-cost/consequence analysis that was used to rank the alternatives and identify the two best CMAs at each location. These tasks are summarized in the following sections.

ES-4. Field Reconnaissance and Targeted Cross-section Surveys

A field reconnaissance of the Problem Locations 1 through 5 and 9 was carried out in October 2014 and Problem Locations 6 through 8 in February 2015 to assess the existing hydraulic conditions and geomorphic setting of the project reaches and preparation of this field assessment report. Sediment sampling was also conducted during the field reconnaissance to characterize the size distribution of the bed material.

The arroyos at the upper five problem locations (upstream from the Mesilla Diversion Dam problem location) have a significant effect on the hydraulic and sediment-transport conditions in the vicinity of the arroyos. Most of the arroyo fans create significant backwater effects that extend upstream over relatively long distances, and many of the fans have resulted in erosion of the opposite bank. Tributary derived sediments have accumulated downstream from the confluences and in many cases have created vegetated mid-channel or bank-attached bars that reduce conveyance efficiency.

At the lower four problem locations, the bed material is sand and the sedimentation issues have resulted in a variety of concerns. Sedimentation upstream from Mesilla Dam is affecting dam operations and is resulting in excessive sediment delivery to the Eastside and Westside Main Canals, requiring frequent maintenance by the Elephant Butte Irrigation District (EBID). At the

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East Drain Problem Location, a number of west-side arroyos deliver coarse sediments to the mostly sand-bed reach, resulting in aggradation that is affecting flood conveyance as well as the efficiency of the East Drain. Aggradation at the Country Club Problem Location could result in increased water-surface elevations and associated levee freeboard encroachments that would disqualify the levee FEMA certification. The relatively low gradient at the Montoya Drain Problem Location results in low sediment-transport capacities that in turn have resulted in system-wide aggradation and the formation of numerous vegetated islands, many of which have formed or enlarged during the recent, ongoing drought period (2010–2015).

A total of 79 monumented cross sections were initially surveyed at Problem Locations 1 through 5 and 9 by the Tetra Tech team. The cross sections that were surveyed by Tetra Tech included cross sections that are in the base hydraulic model (discussed below) and were selected to be representative of the channel geometry and hydraulic controls through the study reach. Additional cross sections were surveyed at the mouths of the arroyos and drains, as well as at hydraulic controls that were identified during the field reconnaissance. As part of the surveys at Problem Locations 1 through 5 and 9, a total of 24 survey control points and 158 monumented end points were set and surveyed by Del Sur Surveying, LLC (a licensed surveyor in the State of New Mexico), and the topographic/bathymetric surveys included a total of 3,395 survey points.

USIBWC conducted the cross section surveys at Problem Locations 6 through 8, and provided the data to Tetra Tech for purposes of conducting the hydraulic analysis. These surveys included a total of 79 cross sections, 49 of which were surveyed along the main channel of the RGCP, 19 of which were surveyed in the Eastside Main Canal and 11 of which were surveyed in the Westside Main Canal.

ES-5. Base Steady-state Hydraulic Modeling

The one-dimensional HEC-RAS hydraulic model that was developed for the USACE (2007) study was adopted as the base model of the RGCP for this study. This model was developed using in-channel survey data that was collected between 2004 and 2007 and 2004 LiDAR mapping in the overbanks. An updated base model of the overall RGCP was then developed by incorporating the cross-section survey data that was collected for this study at the nine problem locations. For the problem location cross sections that were not surveyed as part of this study, the geometry of the main channel was estimated by interpolating between the bounding surveyed cross sections.

The overbank portions of the updated base model cross-sections that extended beyond the limits of the 2014 survey data were updated using the 2011 LiDAR mapping that was provided by USIBWC. This model was executed over a range of discharges from 500 cfs up to the 100-year peak discharge and included the discharges of interest to this study:

Average annual spring hydrograph discharge (2,350 cfs above Mesilla Diversion Dam and 1,400 cfs downstream from the dam).

Mid-range channel capacity (3,000 cfs)

Upper range of channel capacity (3,500 cfs)

100-year routed peak discharge (ranging from 2,350 to 15,150 cfs)

A comparison of the predicted water-surface profiles from the base model and updated base model indicates that, as expected, the aggradation that has occurred along the majority of the reaches results in an increase in water-surface elevation. The largest increase in water-surface elevation occurred at Problem Location 3, where an average increase of about 3.0 feet was indicated (2,350 cfs). The smallest increase in water-surface elevation occurred at Problem Location 6 with an average increase of 0.2 feet (2,350 cfs), although this average value includes predicted decreases in water-surface elevation downstream from Mesilla Dam. Considering only

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Final Report vi the reach upstream from the dam, the average increase in water-surface elevation ranges from about 0.4 feet (3,500 cfs) to 0.6 feet (2,350 cfs). The only location where an average decrease in water-surface elevation occurs is at Problem Location 5 at the 100-year peak discharge.

However, it should be noted that the original base model was developed for a range of relatively low flows (up to 6,000 cfs), whereas the updated base model was developed for a much larger range of flows up to the 100-year peak discharge (14,100 cfs at this location) and it was therefore necessary to extend some of the cross sections farther into the overbanks to contain the higher discharges. This is especially true at Problem Location 5, where the extended cross sections in the updated base model results in higher overbank flow conveyance and thus a reduction to the 100-year peak discharge water-surface elevation.

Localized base models of the problem locations were developed using the geometry for the updated base model of the overall RGCP. The downstream boundary condition for each of the models was obtained from the updated base model of the overall RGCP. These models were executed over the same steady-state discharges that were included in the updated base model of the RGCP and as such the results are identical to the updated base model results.

ES-6. Channel Maintenance Alternatives Development

The design for the sediment-removal alternatives was prepared using the existing bed profiles and information from the base and updated base hydraulic modeling. For the “Channel Excavation Long” alternatives, the up- and downstream limits of the excavation were located at the limits of the convex bed profile shape because this type of profile typically represents areas with the most significant aggradation. This generally places the upstream limit of excavation within a few hundred feet upstream of the arroyo mouth. The resulting excavation lengths under the “long” alternatives ranged from 3,900 to 11,800 feet. For the “Channel Excavation Short” alternatives, the up- and downstream limits of the excavation varied by problem location, but in general the upstream limit was set in the vicinity of the upstream limit under the “long” alternative, and a maximum target excavation length of 2,600 feet was used to set the downstream limit. The resulting excavation lengths for the “short” alternatives ranged from 900 to 2,700 feet, although an excavation length of about 4,700 feet was used at the Mesilla Dam site (Problem Location 6).

The excavated bed profile under both the “short” and “long” alternatives was typically set to match the existing bed elevation at the downstream limit of the excavation with a slope that resulted in reasonable excavation depths through the excavated reach. Average excavation depths under both of these alternatives ranged from about 2.5 to 5 feet. The excavated channel width was then set such that the excavated channel has a flow capacity that restores the overall channel capacity to ~2004 conditions based on information presented in the USACE (2007) baseline study as well as the results from the 2007 baseline model. At locations where the existing channel capacity exceeds 3,500 cfs, the geometry of the excavated channel was designed to have a capacity of between 750 and 1,000 cfs since this range of discharges represents the lower regime of Caballo releases during normal operating conditions.

For the “Localized Sediment Removal” alternatives (also referred to as “Localized Excavation” or “Excavation at Mouth” alternatives), it was assumed that the excavated channel would span the entire width of channel and that the excavation profile would need to have a down-gradient slope and tie into the downstream existing bed profile to avoid creation of a pool/sediment trap. Target excavation lengths of 200 feet were used, but because the resolution of the available bed profile is derived from the modeled cross sections, which in some cases have a spacing that exceeds 300 feet, it was necessary to increase the excavation lengths beyond the target length.

Excavation lengths for the “local” alternatives ranged from 80 to 690 feet.

The non-sediment removal alternatives also varied by site. At many of the sites where tributary sediment loading is the primary concern, construction of arroyo sediment traps upstream from the

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Final Report vii confluence with the RGCP could greatly reduce the coarse-grained sediment supply to the RGCP.

Although it would be necessary to periodically excavate material from the sediment traps, the excavation would not occur in the bed of the active channel and loading/hauling costs could be reduced so this alternative could be less expensive than the excavations from the RGCP as historically practiced. At many of the sites where coarse-grained tributary sediments are resulting in sedimentation issues, another non-sediment removal alternative that could be employed involves construction of low-elevation spur dikes or vanes. The spurs could be designed to increase sediment-transport rates beyond the nose of the spurs while promoting deposition of coarser material between the spurs. The spurs along the bank opposite the alluvial fan would also provide the added benefit of bank protection. In addition, a number of site-specific alternatives were identified in the Statement of Work or developed during the course of this study.

ES-7. Steady-state Hydraulic Modeling of the CMAs

The localized steady-state base models were adjusted to represent with-alternative conditions to evaluate the short-term effects of the alternatives on hydraulic conditions with a specific focus on the effects on water-surface elevation. In general, the localized models for the CMAs were developed by adjusting the existing (updated base model) channel geometry, and in some cases the hydraulic roughness, to reflect elements associated with the alternatives.

The water-surface profile comparisons indicate that each of the alternatives evaluated would have at least have a localized effect on water-surface elevation. The effects of like alternatives vary by problem location, and in general show the largest effect at the lowest discharges and the smallest effect at the 100-year discharge. Many of the alternatives would result in no increase to predicted water-surface elevation over the range of modeled discharges and almost all of the alternatives (except the low-elevation spur dike alternative) would result in some localized decrease to predicted water-surface elevation. The lengths over which the alternatives would affect predicted water-surface elevation are dependent on both the longitudinal extent of the treatment as well the degree to which the treatment affects conveyance.

Based on the model results of the excavation alternatives, each scenario would result in reduced predicted water-surface elevations. None of the excavation alternatives would result in increases to predicted water-surface elevation except the short excavation alternative at Problem Locations 4 and 9, where very small, localized increases occur near the upstream limit of the excavations.

As expected, the reduced conveyance area associated with the low-elevation spur dike alternative generally results in increased predicted water-surface elevations. The alternatives involving island/bar destabilization and vegetation typically result in reduced predicted water-surface elevations that are a result of the 6-inch lowering of the island and bar surfaces and the reduction to the hydraulic roughness. The comparative predicted water-surface elevations for the site-specific alternatives indicate the effects of these alternatives vary with the degree of modification.

Modifications to the Tierra Blanca Vortex Weir at Problem Location 1 and installation of riprap revetment at Problem Location 8 result in very little change to predicted water-surface elevation.

At Problem Location 3, replacement of the Rincon Siphon with an elevated flume and removal of the grade-control structure would have a much more significant impact, reducing the average predicted water-surface elevation at all of the modeled discharges.

ES-8. Sediment-transport Modeling of the Problem Locations

The sediment-transport modeling was performed with sediment-routing models of the problem location reaches that were developed using the mobile bed sediment-transport feature in HEC- RAS Version 4.1 (USACE 2010). At Problem Location 6, where flow splits upstream from Mesilla Dam deliver flow and sediment to the Eastside and Westside Canals, it was necessary to use the beta-test version of HEC-RAS 5.0 because Version 4.1 is not capable of modeling sediment splits

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Final Report viii at distributary junctions. The model geometry and basic structure of the models were taken from the localized hydraulic models discussed in the previous sections. In general, the mobile bed sediment-transport feature in HEC-RAS requires input to define the existing bed material, the upstream and lateral sediment supplies, the hydrologic sequence over which sediment transport is evaluated, as well as a variety of other model input that is necessary for the sediment-transport computations.

The bed material model input was developed using the gradations of the sediment samples that were collected during the field reconnaissance. Two separate hydrologic series were prepared, including a series that represents a best estimate of the hydrology during the Water Year 2005 (WY2005) to WY2014 period for input into the model validation runs, and a separate series that represents normal operating flows during a drought condition for input to the base and alternative sediment-transport simulations. The hydrologic series for the validation period was selected because cross section surveys were conducted in 2004 and in 2014/2015 for this study, and that information provided a basis for assessing the reasonableness of the sediment-transport modeling results. These hydrologic series were developed using measured discharges at the Caballo and El Paso gages, taking into account adjustments that were necessary to reflect the various inflows and outflows along the RGCP. For the hydrologic series that were used as input to the base and alternative model simulations, the measured flows at Caballo and El Paso during the 2013 irrigation season were used, after making similar adjustments to account for inflows and outflows and additional adjustments to represent “normal flow conditions” of 2,350 cfs upstream from Mesilla Dam and 1,400 cfs below the dam. These adjusted WY2013 hydrographs were then duplicated 10 times to represent a 10-year period of extreme drought conditions.

For both the base and validation model runs, the upstream bed material supply was estimated using the HEC-RAS “Equilibrium Load” option, which computes the sediment load (and gradation of the sediment load) that is in balance with the sediment-transport capacity, thereby creating an equilibrium condition at the upstream limit of the model. Bed material sediment supply from the tributaries was input assuming the mean annual bed load (USACE, 2007) is delivered during a single monsoon-season event, along with an appropriate water discharge.

Two different sediment-transport functions were used in the models, including the Meyer-Peter Müller (MPM) and Yang formulae. The MPM formula was used at Problem Locations 1 through 5, where tributaries deliver coarse sediments to the RGCP that have a significant impact on sediment-transport conditions in the river. The Yang formula was used at Problem Locations where the bed material is primarily sand. Numerous other model input that is required for the sediment-transport simulations was prepared based on an inspection of the results from initial model runs, previous experience with sediment-transport modeling of the Rio Grande, and engineering judgement. At Problem Location 6, the model input also included gate opening time series that represent operations of Mesilla Dam and the canal headworks.

Results from the validation model runs indicate that the predicted aggradation and degradation patterns match the observed (survey-based) patterns reasonably well. As such, the sediment-transport models should provide a reasonable tool for evaluating the effects of the alternatives.

The localized base sediment-transport models were adjusted to reflect alternative conditions, and each were executed over the 10-year simulation period with duplicated drought condition (adjusted WY2013) annual hydrographs. Results from the modeling were used to compare the spatial and temporal effects of the alternatives on mean bed elevation change, aggradation and degradation along the modeled reach and downstream sediment deliveries. Results from the modeling of the sediment removal alternatives were also used to estimate the time over which the excavation volumes would backfill with sediment.

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To evaluate the long-term effects of the alternatives on water-surface elevation, the predicted model geometry at the end of the simulation was incorporated into the localized steady-state hydraulic models. The end-of-simulation (EOS) models were then executed over the same steady-state discharges that were evaluated in the original localized hydraulic models. To provide a basis for comparison, the EOS model suite included a version of the base condition model with end-of-simulation geometry. Comparative water-surface profiles for normal operating flows of 2,350 and 1,400 cfs (above and below Mesilla Dam, respectively) and at the 100-year peak flow were then prepared to assess the long-term changes in water-surface elevation. The EOS-model water-surface profiles at the 100-year peak flow were also used to assess levee freeboard in areas where freeboard encroachments could be of concern in the long-term.

ES-9. Channel Maintenance Alternatives Evaluation

The expected benefits, costs and consequences associated with the alternatives were assessed in concert to identify the two alternatives that had the highest benefit relative to cost/consequence at each problem location in accordance with the statement of work for this study. The benefits considered in this assessment included: (1) reduction in water-surface elevation along the modeled reach, (2) reduced levee freeboard encroachments, (3) groundwater benefits, which include the benefit of increased groundwater levels in the vicinity of restoration sites as well as reduced groundwater levels elsewhere, (4) reduction in aggradation and downstream sediment loading, (5) improved irrigation drain return flows, (6) durability of the alternative, (7) restoration benefits, in addition to those benefits associated with increased groundwater levels, and (8) additional site-specific benefits. The costs and consequences considered in this assessment included: (1) annualized total cost of the alternative based on the up-front construction cost and projected O&M costs, (2) increases to water-surface elevation along the modeled reach, (3) levee freeboard encroachments, (4) groundwater consequences, which include the consequence of decreased groundwater levels in the vicinity of restoration sites as well as increased groundwater levels elsewhere, (5) increases to aggradation and downstream sediment loading, (6) increased bank erosion potential, (7) restoration consequences, in addition to those consequences associated with increased groundwater levels, and (8) additional site-specific consequences.

Construction cost estimates were developed for all alternatives within each problem location, and included the capital costs, along with annual operations and maintenance (O&M) costs for each alternative. The estimates were prepared with the best information available at this time and are considered pre-feasibility level estimates that are for comparison purposes and not for budgeting purposes. It is anticipated that many of the assumptions used within the estimates will be modified as more detailed information becomes available. To prepare the O&M cost estimates, a 50-year project life cycle was used and a maintenance period was prepared using either the results from the sediment-transport modeling and engineering judgement.

A scoring system was developed for each of the benefit and cost/consequence parameters using the results from the hydraulic and sediment-transport modeling. These systems were used to score each of the alternatives under the various scoring parameters and prepare overall benefit and cost/consequence scores. The two alternatives at each of the problem locations with the highest difference of benefit to cost/consequence were then identified and recommended for further consideration. The difference between the benefits and costs/consequences was computed by summing the individual benefit parameter scores and subtracting the summation of the cost and consequence scores. The recommended alternatives for each problem location and the difference between the net benefit and net cost/consequence scores are presented in Table

ES-2.

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Table ES-2. Summary of the two alternatives that received the highest ranking at each of the problem locations evaluated in this study, and the difference between the net benefit and net cost/consequence scores. Also shown are the estimated total annualized costs.

Problem Location

Alternative

Difference b/w Benefits and

Costs/ Consequences

Total Annualized

Cost Rank

Arroyo Sediment Traps 14.8 $285,000 1

Vortex Weir 9.9 $4,100 2

Arroyo Sediment Traps 27.5 $90,600 1

Island Destabilization/Vegetation Removal

19.2 $77,000 2

Arroyo Sediment Traps 12.4 $14,100 1

Rincon Siphon Modifications 12.2 $100,400 2

Long Excavation 14.2 $653,500 1

Island Destabilization/Vegetation Removal

7.2 $97,500 2

Arroyo Sediment Traps 14.2 $34,800 1

Long Excavation 6.5 $243,300 2

Gate Automation 13.8 $164,200 1

Sluiceway and Check Structures 11.1 $154,800 2

Arroyo Sediment Traps 26.6 $77,500 1

Long Excavation 9.3 $164,800 2

Riprap 6.1 $28,300 1

Spur Dikes 2.5 $34,200 2

Island Destabilization/Vegetation Removal

19.8 $32,300 1

Long Excavation 18.1 $534,700 2

ES-10. Recommendations

Based on the findings of this study, a number of recommendations were identified for further evaluation, for incorporation into adaptive management practices, or for improving channel maintenance along the RGCP. These recommendations include:

1. Any of the recommended alternatives that are ultimately selected for implementation should be monitored locally as part of the adaptive management approach. It is recommended that this monitoring involves the establishment of monumented cross sections that are surveyed prior to and immediately after implementation to establish a base condition. Repeat surveys through time would be beneficial for evaluating channel response. These monitoring sections should also be included in the arroyos when applicable.

2. Because the sediment trap alternatives provide the highest benefit relative to costs and consequences, this alternative should be considered at all problem locations, and elsewhere along the RGCP, where tributaries deliver coarse sediment loads to the river. It is

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Final Report xi recommended that, as part of the adaptive management approach, at least one sediment trap be constructed as laid out conceptually in this report for purposes of testing the trap efficiencies. During the testing of the sediment traps, if it is determined that it is desirable to also eliminate the finer fractions of the tributary bed material sediment supply, the trap screens could be re-designed to also trap the sand, silt, and clay classes. However, because the fine (sand, silt and clay) sediment loads delivered by the tributaries represents a relatively small portion of the overall fine sediment loads supplied and transported by the RGCP, this may not be worthwhile.

3. The tributary loading was based on a relatively simple approach of targeting a high, but realistic sediment concentration. Because tributary loads are highly variable it is recommended that arroyo sediment traps be monitored annually and enlarged if sediment removal is required too frequently.

4. Any opportunity to construct sediment traps that are larger than those evaluated in this study, which are limited in size due to the ROW constraint, should be considered and evaluated in detail. These opportunities should include the potential for construction of sedimentation basins in the upstream portions of the arroyo watershed.

5. Localized and short excavation scenarios do not appear to provide good value due to the high frequency of the excavations that would be necessary for maintenance purposes, therefore, it is recommended that any excavations be conducted over reaches that are as long as possible. Based on the long excavation alternatives evaluated in this study, a minimum length of 3,900 feet should be targeted for the excavations.

6. During the field reconnaissance of Problem Location 5, a beaver dam was identified at the mouth of the Rincon/Tonuco Drain. The dam appears to have an effect on drain efficiency and probably results in increased groundwater levels, at least during the non-irrigation season, so it is recommended that the dam be removed and the beavers be relocated. It is also recommended that beaver activity be monitored at other drains along the RGCP to ensure beaver dams are not affecting drain performance or local groundwater levels.

7. Because the sedimentation issues result in varying degrees of problems among the problem locations, it is recommended that the problem locations be prioritized during development of the implementation plan. The island/bar destabilization and vegetation removal alternative was evaluated for this study under the assumption that the full set of islands and bars that are selected for treatment be cleared and grubbed in concert. In practice, this work can be prioritized such that the largest islands and bars that have the most significant hydraulic effect receive the highest priority. Similar prioritization of the other alternatives (e.g. identification of the most problematic tributary sediment loadings for installation of the arroyo sediment traps), is also recommended prior to implementation.

8. The non-sediment removal alternatives in the vicinity of Mesilla Dam at Problem Location 6 would be affected by 2-D and 3-D flow and sediment patterns that are not taken into account in the 1-D HEC-RAS modeling conducted for this study. If it is desirable to provide more certainty to the scoring and ranking of the alternatives, the automated gate operator and check/sluiceway structure alternatives that were identified as having the highest benefit to cost/consequence difference should be evaluated further using a 2-D model platform or a physical model. The cost for conducting 2-D modeling of these alternatives is estimated to be less than 5 percent of the estimated cost of construction for each of the alternatives, while the cost for a physical model would probably be about 10 percent of the estimated cost of construction.

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9. The proposed check structure and sluiceway alternative at Problem Location 6 will require relocation of the heading of the Del Rio Lateral Canal. Locating the new heading upstream from the Eastside Main Canal first check structure could result in undesirable sediment loading to the lateral if the heading were located upstream from the sluiceway. As such, the lateral canal heading should be located downstream from the sluiceway and upstream from the first check structure.

10. Many of the individual alternatives could be combined to enhance the expected relative benefits. For example, implementation of the modified Tierra Blanca Vortex Weir in addition to the sediment trap alternative may result in significant benefits for very little additional cost.

Similarly, a sediment trap combined with localized excavation could produce immediate and long-term benefits. Combining of alternatives should be evaluated using similar methods to those used in this study, at a minimum.

11. The proposed modifications at the Rincon Siphon would include removal of the grade control structure below the current siphon. If this alternative were implemented, downcutting would probably occur along a significant portion of the upstream reach that could affect the foundations of the NM 154 and ATSF Railroad Bridges. It is therefore recommended that the as-built information for both bridges be reviewed to determine the depth to which the piers are buried. If it is determined that the piers are not sufficiently embedded below the channel invert elevation plus scour relative to the downstream limit of the grade control structure, the bridge foundations could be at risk to undermining. As such, it would be necessary to reconstruct the pier footings, which could result in a significant increase to the cost for this alternative that would reduce the difference between benefit and cost/consequence and potentially change the alternative ranking. All of this should be heavily considered in more detailed evaluations of this alternative.

12. Although the model validation simulations represent the no-action scenario under normal flow conditions (WY2005 to WY2014), the alternative evaluation presented in this study is based on extreme drought condition hydrology (WY2013). [Although the models were validated using the estimated actual hydrological conditions over the period from WY2005 to WY2014, the analysis and scoring of the alternatives was based on a 10-year simulation of the WY2013 irrigation release (repeated 10 times). As such, the analysis presented herein includes two separate no-action model runs, including: (1) a scenario under which the actual hydrologic conditions that occurred from WY2005 to WY2014, which was represented by the model validation simulations, and (2) a scenario under which extreme drought conditions occur over an extended (10-year) period, which were represented by the base model simulations that were used as the basis for the alternative evaluations.] It is recommended that the alternative evaluation also be conducted for normal hydrology to determine whether or not the scoring and ranking of the alternatives would change under a different flow regime. This exercise would be relatively simple because the tools that were developed for this study, primarily the sediment-transport models and alternative scoring systems, are in place.

13. The preliminary cost estimates prepared for this study may not reflect the actual construction and O&M costs, so a detailed cost analysis of the top-ranking alternatives should be carried out to better compare the two alternatives recommended at each problem location.

14. The levee freeboard analysis presented in this study uses the elevations of the levees as indicated by the 2011 LiDAR topography. It is recommended that in areas where activities are planned to improve levee freeboard conditions, the top of the levees be surveyed to ensure that the LiDAR-based levee elevations and the associated analysis presented herein are accurate. The surveys of the levees need not be extensive, but should rather include “spot” elevations at selected model cross sections for purposes of validating the 2011 LiDAR-based

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Final Report xiii levee top elevations that are reflected in the localized hydraulic modeling prepared for this study.

15. Although this study was not intended to include a detailed evaluation of the habitat benefits and consequences, the Tetra Tech team of engineers and geomorphologists understands the importance of these considerations, so the parameters were included in the scoring matrix.

The scoring of the habitat benefit and consequence parameters presented in this report is somewhat subjective, so it is recommended that these parameters be re-evaluated in a separate study by an entity with appropriate expertise in riparian and aquatic ecology.

16. Each of the above recommendations should be considered as a task under a 5-year adaptive management plan, except recommendation number 8 if deemed unnecessary. In addition, it is recommended that each type of the generalized top-ranked alternatives be implemented at one of the problem locations, at a minimum, for purposes of testing under this 5-year plan.

These generalized top-ranked alternatives include arroyo sediment traps, island/bar destabilization with vegetation removal and long excavation. It is also recommended that the two site specific alternatives that received the highest rank, including the installation of additional automated gate operators at Mesilla Dam (Problem Location 6, after the recommended further evaluation) and installation of riprap revetment below Country Club Bridge (Problem Location 8), be implemented as part of this 5-year plan because these alternatives appear to achieve the desired benefits with relatively low cost and consequence.

As discussed in recommendation number 7, the 5-year plan should also prioritize the problem locations, and the details associated with the final design of the alternatives should be prioritized as part of the implementation plan.

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TABLE OF CONTENTS

Executive Summary ..................................................................................................................... i

ES-1. Background ................................................................................................................... i ES-2. Study Objectives ............................................................................................................ i ES-3. Study Approach ........................................................................................................... iv ES-4. Field Reconnaissance and Targeted Cross-section Surveys ....................................... iv ES-5. Base Steady-state Hydraulic Modeling ......................................................................... v ES-6. Channel Maintenance Alternatives Development ......................................................... vi ES-7. Steady-state Hydraulic Modeling of the CMAs ............................................................ vii ES-8. Sediment-transport Modeling of the Problem Locations .............................................. vii ES-9. Channel Maintenance Alternatives Evaluation ............................................................. ix ES-10. Recommendations ...................................................................................................... x

1 INTRODUCTION .............................................................................................................. 1.1

1.1. Background................................................................................................................ 1.1

1.1.1 The Rio Grande Canalization Project .................................................................. 1.1

1.1.2 Sedimentation Issues and Study Reasoning ....................................................... 1.3

1.2. Study Objectives ........................................................................................................ 1.3

1.3. Authorizations ............................................................................................................ 1.5

1.4. Study Approach ......................................................................................................... 1.5

1.5. General Information about Report Content ................................................................ 1.8

2 Geomorphic Subreach Descriptions .................................................................................. 2.1

2.1 Geomorphic Subreach 1 (Problem Location 1) .......................................................... 2.1

2.2 Geomorphic Subreach 2 (Problem Locations 2, 3 and 4) ........................................... 2.2

2.3 Geomorphic Subreach 3 (Problem Location 5) .......................................................... 2.2

2.4 Geomorphic Subreach 5 (Problem Location 6) .......................................................... 2.4

2.5 Geomorphic Subreach 6 (Problem Locations 6 and 7) ............................................... 2.4

2.6 Geomorphic Subreach 7 (Problem Locations 7, 8 and 9) ........................................... 2.4

3 Field Reconnaissance and Site Surveys ........................................................................... 3.1

3.1 Field Reconnaissance ................................................................................................ 3.1

3.1.1 Problem Location 1 ............................................................................................. 3.1

3.1.2 Problem Location 2 ............................................................................................. 3.5

3.1.3 Problem Location 3 ............................................................................................. 3.7

3.1.4 Problem Location 4 ............................................................................................. 3.7

3.1.5 Problem Location 5 ........................................................................................... 3.10

3.1.6 Problem Location 6 ........................................................................................... 3.10

3.1.7 Problem Location 7 ........................................................................................... 3.13

3.1.8 Problem Location 8 ........................................................................................... 3.15

3.1.9 Problem Location 9 ........................................................................................... 3.15

3.2 Site Surveys at Problem Locations 1 through 5 and 9 .............................................. 3.18

3.3 Site Surveys at Problem Locations 6 through 8 ........................................................ 3.21

3.4 Survey Data Reduction and Cross-section Development ......................................... 3.21

3.5 Pre-Work and Post-Work Cross-section Comparisons ............................................. 3.21

3.6 Discharge Measurements ........................................................................................ 3.23

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4 Hydraulic Modeling and Alternative Development ............................................................. 4.1

4.1 Existing Base Model of the RGCP ............................................................................. 4.1

4.2 Updated Base Model of the RGCP ............................................................................ 4.1

4.3 Comparison of Base Model and Updated Base Model Results .................................. 4.4

4.4 Localized Base Modeling of Problem Locations ......................................................... 4.6

4.5 Alternative Development ............................................................................................ 4.6

4.5.1 Solutions Applied on Other Streams and Rivers Similar to the RGCP ................. 4.6

4.5.2 Sediment Removal CMAs ................................................................................... 4.8

4.5.3 Non-Sediment Removal CMAs ........................................................................... 4.9

4.6 Site-specific Alternative Design Variations ............................................................... 4.16

4.6.1 Sediment Removal CMAs ................................................................................. 4.16

4.6.2 Arroyo Sediment Traps ..................................................................................... 4.21

4.6.3 Low-elevation Spur Dikes ................................................................................. 4.27

4.6.4 Island and Bar Destabilization with Vegetation Removal ...................