Final_Hydraulics_Report.pdf

Final Hydraulic Report

Peralta Creek Bridge Replacement

PR 85, Pueblo de Cochiti, NM

Project NM IRR 85(1) ii

Prepared by:

Veronica Ghelardi

Bart Bergendahl

Technical Services Hydraulic Engineers Central Federal Lands Highway Division

Lakewood, Colorado

December, 2014 iii

Contents

Project Background

Project Site Description

Hydrology

Central Federal Lands Scope of Work

Hydraulic Analysis

Scour

Summary

Existing Conditions Update

References

Appendix A – HEC‐RAS Output Summaries

Appendix B – Scour Computation Summaries

Appendix C – Incipient Motion Analysis

Project Background

This project is on Peralta Creek in Pueblo de Cochiti, New Mexico. The Peralta Creek watershed is located partially within the boundaries of the Pueblo and partially in the adjacent Bureau of Land

Management and National Forest Boundaries. The project location is at the PR 85 crossing of Peralta

Creek near the Pueblo de Cochiti village. (Figure 1)

Figure 1. Peralta Creek – NM 85(1) Project Location map

The 45 square mile watershed is located upstream of the Pueblo. In the summer of 2011, the Las

Conchas Fire burned approximately 28 square miles in the upper watershed of Peralta Creek (ref. USACE

Technical Assistance Report, September 2012). The US Army Corps of Engineers (Corps) conducted an in‐depth analysis of the burn area and concluded that the severity of the burn caused much of the vegetation and organic material to be lost. (Figure 2) This has increased the near‐term risk of flooding due to higher runoff from the burn area.

The severity of the fire significantly reduced the ability of much of the soil to allow rainfall infiltration resulting in increased runoff and reduced lag time between the peak precipitation and the peak runoff.

Small depth and average frequency precipitation events now initiate large flow events with magnitudes formerly seen only after large rainfall events.

Figure 2. Peralta Creek Watershed and Severity of Burn (courtesy of USACE, TAR, September 2012)

Project Site Description

Peralta Creek is composed largely of cobbles, gravels, and sandy soils that are highly erodible. (A pebble count conducted in October, 2012, indicates D50 = 0.069’ and D85 = 0.2’.) At the time of the Corps’ study, the Peralta Creek crossing at PR 85 consisted of 2‐10 foot diameter corrugated metal pipes and a drop structure of wire enclosed riprap baskets. (Photo 1)

Photo 1. Peralta Creek at PR 85: Two 10‐foot diameter CMP’s and gabion baskets

Fifteen feet of channel degradation has been stopped by the culverts and gabions serving as a grade control structure (GCS). There is a sanitary sewer line under the streambed just downstream of the crossing that is threatened by further degradation of the streambed, and the Sile Canal siphon, located further downstream of the project site is also threatened by more downcutting of the streambed.

Before the fire, the estimated flows in the creek were approximately 25% of the expected discharges resulting from the fire damage in the upper watershed. The existing creek bed will not be able to contain the increased flows and, without intervention, would overtop the culverts, inundate the village, and overtop the road south of the crossing. The Corps developed a conceptual mitigation design for

Peralta Creek that includes three GCSs at and downstream of the PR 85 crossing. Also included in the design were berms to protect the village and channel work upstream of the PR 85 crossing to contain the flood flows within the creek. The Corps determined a channel equilibrium slope of 0.01151 ft/ft and used that slope for the conceptual design. The Corps intended to retain the 2‐10 ft CMPs and tie the existing gabions into a reinforced GCS immediately downstream of the crossing.

Since the fire, the Peralta Creek flows have increased dramatically for even normal rainfall events. The

Bureau of Land Management (BLM) has constructed temporary berms to keep flood flows out of the village; however, the berms are being undermined and eroded by the flows and need constant attention. The Pueblo was concerned about the ability of the culverts to handle the higher flows and backing water into the community. Therefore, they removed the two culverts in July of 2012 in an effort to increase the conveyance through the crossing. (Photo 2) In addition, the roadway embankment, which effectively serves as an integral part of the existing grade control structure, was armored with riprap. Finally, a temporary, at‐grade crossing of PR 85 was constructed across Peralta Creek.

Photo 2. Peralta Creek PR 85 after removal of 2‐10 foot CMP culverts

When the culverts were removed, the riprap enclosed wire gabion baskets were left as the only grade control. In the few months since the culverts were removed, the larger flows from normal precipitation events have started to unravel the gabion baskets. (Photo 3) If the gabions are not reinforced or a new

GCS is built, the GCS could fail, resulting in a headcut. The headcut has the potential to migrate upstream past the PR 85 crossing and compromise the existing BLM’s flood mitigation measures and the

Corps’ conceptual design for the stream.

Photo 3. Riprap downstream of gabion GCS & wire enclosed riprap gabions unraveling in background

Hydrology

The Corps’ hydrological analysis was based on NOAA Atlas 14 rainfall information. HEC‐HMS 3.5 was used to generate runoff values from the Peralta Creek watershed with infiltration parameters reduced to represent the burn area soils. The analysis produced peak discharge rates for precipitation amounts from 1.0 inch to 5.0 inches.

Table 1 below compares the pre‐fire and post‐fire runoff rates for the watershed. The pre‐fire discharges were calculated from 2008 USGS Regression Equations for Region 6, the Central Mountains of New Mexico. As can be seen, the potential discharge magnitudes for Peralta Creek have increased significantly for all recurrence intervals.

Recurrence Interval (year)

Pre‐Fire Discharge (USGS Regression Equations)

(cfs)

Post –Fire Discharge (Corps HEC‐HMS analysis)

(cfs)

2 675 2000

5 1450 4490

10 2160 9290

25 3290 14400

50 4320 ‐

100 5520 20020

Table 1. Estimated Flood Flows before and after the Las Conchas Fire

As the upper watershed heals and vegetation regenerates over time, the post‐fire flood magnitudes are expected to trend back toward pre‐fire levels. However, at the request of the Pueblo, the 25‐year post‐ fire discharge of 14,490 cfs was used as the bridge capacity design flood to complement the Corps’ proposed channel stabilization/flood mitigation design. This is considered appropriate given the high magnitude and frequency of recent runoff events, and the immediate threat that such runoff poses to the Pueblo community. Given the discharge magnitude and the expectation that flows will return to pre‐fire levels, the scour design and check floods for the new bridge were also selected as the post‐fire

25‐yr event.

Central Federal Lands Scope of Work

Central Federal Lands Highway Division (CFL) was asked to design a new bridge structure at the PR 85 crossing site in November 2012. After a scoping trip, it was determined that CFL would take the project to the 30% design milestone, the design would not include any channel work, and the existing grade control structure would not be disturbed. It was agreed that the Corps’ concept design for the proposed channel improvement/flood mitigation project would be accommodated by the new bridge design. It was also agreed that CFL would use the Corps’ extensive hydrological analysis of the post‐burn watershed and the resulting 25‐yr flood flow magnitude for the bridge design flood standard. Finally, the new bridge was to account for the possible failure of the current GCS. In August, 2014, CFL was asked to develop a 100% design and assemble a complete PS&E package.

Hydraulic Analysis

The Corps provided their Technical Assistance Report to CFL that included their hydrological analysis, conceptual designs, and 2‐D hydraulic modeling results. CFL also conducted an updated survey of the local crossing area from which hydraulic models of the Peralta Creek crossing were developed using

HEC‐RAS v. 4.1.

Without reinforcing and extending the existing grade control structure, it is estimated that a flow of

4000 cfs or greater would overtop and likely damage the gabions and eventually wash out the GCS, allowing a headcut to migrate upstream. Therefore, the bridge design needs to accommodate the potential for future degradation of the channel at the crossing location, as well as allow for the proposed channel improvements should funds become available.

The Corps’ conceptual design included upstream channel work to contain the post‐fire flood flows and berms to protect the Pueblo, and building three grade control structures between PR 85 and the Rio

Grande River to protect PR 85, the Sile siphon, and the sewer line. The upstream GCS was to tie into the existing gabion structure and protect the existing sewer line.

The downstream degradation and widening reflects the flow events that have been experienced in the past. With much higher flows possible due to the fire, the downstream conditions could deteriorate further. Without channel stabilization work, it is expected that the degradation and widening occurring downstream of the crossing will migrate through the crossing.

The upstream channel is also unstable and can migrate laterally as it approaches the crossing. Unless the new bridge abutments are pulled back, countermeasures will be needed to protect the approach embankments and possibly the new abutment foundations from this potential lateral migration.

Given the uncertainties associated with future channel work, magnitude of flood events, stability of the channel and existing grade control structure, and funding, the type, size, and location (TS&L) for the new bridge was based on the Corps’ conceptual flood mitigation design for the grade control structure immediately downstream of the crossing. The conceptual GCS design downstream of the crossing included a 140 foot wide spillway. This width reflects the existing main channel width downstream of the crossing.

The new bridge design must account for the fact that the current gabion GCS could very well wash out before the Corps’ flood mitigation project is actually constructed. Consequently, the bridge opening at the crossing could potentially widen significantly over time. With the gabion GCS gone, the channel invert elevation could be approximately 15 ft. lower than the existing invert due to a headcut migrating upstream. Based on this lower invert elevation, a 140 ft. bottom width, and 2:1 abutment slopes, a 250

ft. bridge is needed. This length will result in the abutment foundations being placed in‐line with the maximum extent of the upstream channel banks, which will accommodate shifts in the channel thalweg location over time. Four‐foot diameter, circular drilled shafts were assumed for the bridge substructure.

Given the upstream locations of the temporary at‐grade crossing of the wash and the existing GCS, it was preferable that the proposed bridge be located on the current roadway alignment. (Doing this would also likely avoid acquiring additional right of way.) Consequently, to accommodate all conditions described above, as well as avoid disturbing the existing roadway embankment, which is an integral part of the existing GCS, the bridge was located directly above the existing roadway embankment.

Several HEC‐RAS models were developed to represent potential hydraulic and scour conditions at the proposed bridge location. The models are described below:

Hydraulic model 1 – Assessment of the capacity of the existing crossing with the GCS (wire enclosed gabion baskets) in place (discharge of 4000 cfs)

Hydraulic model 2 – Determination of potential contraction and pier scour depths for existing topography and proposed bridge subjected to a 25‐yr flood (discharge of 14,490 cfs)

Hydraulic model 3 – Determination of the potential pier scour of a widened 140 foot channel with an invert elevation reflecting a 15‐foot headcut through the crossing subjected to a 25‐yr flood (discharge of 14,490 cfs)

All model limits extended roughly 800 feet downstream of the crossing and 350 feet upstream of the crossing. A total of 9 cross sections were extracted from the project topographic mapping to represent the channel geometry through the reach. Each model had the same cross section locations as shown in

Figure 3, but the models differ in channel invert elevations and widths through the reach. Manning’s n‐ values of 0.035 and 0.04 were selected for the main channel and overbank areas, respectively. Models

1 and 2 used ineffective flow areas at Cross Sections 996 and 1080 to represent the opening in the existing roadway embankment.

Figure 3. Plan view of HEC‐RAS model cross section locations

Hydraulic model 1 – This model represents existing conditions with the gabion GCS in place. The current opening is about 40 ft. wide through the gabion grade control structure. As shown in Figure 4, the existing condition constriction causes increases in flow depths just upstream of the crossing during a flow of 4000 cfs… a magnitude just less than the post‐fire, 5‐year flood event (4490 cfs). Due to the abrupt drop at the end of the gabion structure, the flow goes through critical depth near the brink of the

GCS. The maximum elevation of the gabions at the downstream face of the GCS was surveyed at 5268.3

ft. Consequently, flows of 4000 cfs and greater are expected to unravel and/or out‐flank the existing

GCS, eroding the existing embankment and increasing the flow area with time.

XS 1080.093

XS 996.138

Figure 4. Model 1, water surface profile, existing conditions, Q = 4000 cfs

Hydraulic model 2 – The second hydraulic model reflects the existing topography and the proposed 250‐ foot bridge subjected to the 25‐year design flow of 14490 cfs. This model represents a worst case scenario of all flow reaching the existing opening by assuming the Corps’ recommended berms needed to protect the village from inundation are in place and the upstream channel work has been constructed. The flow magnitude results in a combination of pressure and overtopping flow occurring at the proposed bridge. To accommodate the new bridge, the top of the existing roadway embankment is removed (lowered) to elevation 5268.3 ft. (Note: The fact that the existing grade control structure is expected to begin failing at 4000 cfs makes this model a severe test.) This scenario provides the hydraulic variables needed to compute the worst‐case pier and contraction scour depths for the proposed bridge and represents the immediate post‐construction geometry. (See Figure 5)

0 200 400 600 800 1000 1200

Peralta_Wash_proposed1 Plan: Model 1, Existing XS, Adj IFA 9/12/2014

Main Channel Distance (ft)

E le va tio n ft)

Legend

EG < Corps 5 yr

WS < Corps 5 yr

Crit < Corps 5 yr

Ground

.1

.9

.6

.9

.1

.0

.4

.3

001 ALI

Grade Control Structure Downstream Face

Cross Section Location/Stationing

Water surface elevation = 5269.19 ft

Figure 5. Model 2, water‐surface profile, post‐construction conditions, Q = 14490 cfs

Hydraulic model 3 – This model represents the hydraulic conditions that would exist for 14,490 cfs if the gabion structure failed (or was removed) and a headcut migrated through the crossing. To represent the effects of the headcut, the cross section channel widths have been widened to 140 ft with an approximate channel elevation calculated using the Corps’ equilibrium slope starting at the current channel elevation at the first GCS and carrying it up through the crossing. The cross sections upstream of the existing gabion GCS were lowered approximately 15 feet. The most upstream cross section, which is out of the influence of the bridge, was left at its existing elevation. This model was used to obtain variables to compute the pier scour elevation for the failed grade control condition. (See Figure

6)

0 200 400 600 800 1000 1200

Peralta_Wash_proposed1 Plan: Mod 2 with GCS Template + Bridge 9/15/2014

Main Channel Distance (ft)

E le va tio n ft)

Legend

EG Corps Q25

Crit Corps Q25

WS Corps Q25

Ground

.1

.9

.6

.9

.1

.0

.4

.3

001 ALI

Cross Section Location/Stationing

Grade Control Structure Downstream Face

Water Surface Elevation = 5278.73 ft

Figure 6. Model 3, water‐surface profile, headcut has moved through crossing, Q = 14490 cfs

The full HEC‐RAS output summaries for all 3 models are included as Appendix A to this report.

Freeboard – The top gabions in the existing GCS are at approximately elevation 5268.3 ft. As flows exceed this elevation, the existing GCS is expect to begin failing. Allowing a minimum of 2 feet of freeboard above this elevation for Model 2 requires the minimum low chord elevation for the new bridge be set at 5270.3 ft. This elevation will allow clearance for any initial overtopping that may occur as the total flow reaches and exceeds a magnitude of 4000 cfs, as well as room for bridge inspection.

This will require the removal of existing roadway embankment above elevation 5268.3 ft. This removal is illustrated for Cross Section 1080 in Figure 7 (Model 1, existing conditions) and Figure 8 (Model 2, post‐construction conditions) below. Elevation 5270.3 will provide a maximum 15’ of freeboard for the potential Model 3 condition.

0 200 400 600 800 1000 1200

Peral ta_Wash_proposed1 Plan: Model 3Headcut_thru_crossing-x-s_widened 9/10/2014

Main Channel Distance (ft)

E le va tio n ft)

Legend

EG Corps Q25

Crit Corps Q25

WS Corps Q25

Ground

.1

.9

.6

.9

.1

.0

.4

.3

001 ALI

Cross Section

Location/Stationing

Upstream side of bridge; headcut moved through crossing; new invert elevation established.

Figure 7. Cross Section 1080 in Model 1, Existing conditions

Figure 8. Cross Section 1080 in Model 2, Post‐construction conditions

-200 -100 0 100 200 300 400 500

Peralta_Wash_proposed1 Plan: Model 1, Existing XS, Adj IFA 9/12/2014

Station (ft)

E le va tio n ft)

Legend

EG < Corps 5 yr

WS < Corps 5 yr

5.5 ft/s

6.0 ft/s

6.5 ft/s

7.0 ft/s

7.5 ft/s

8.0 ft/s

Ground

Ineff

Bank Sta

.04 .035 .04

-200 -100 0 100 200 300 400 500

Peralta_Wash_proposed1 Plan: Model 2 with GCS Template 9/12/2014

Station (ft)

E le va tio n ft)

Legend

EG Corps Q25

WS Corps Q25

Crit Corps Q25

2 ft/s

4 ft/s

6 ft/s

8 ft/s

10 ft/s

12 ft/s

14 ft/s

16 ft/s

Ground

Ineff

Bank Sta

.04 .04 .04 .035 .04 .04 .04

Scour

Hydraulic models 2 and 3 provided the variables to determine the worst‐case scour potential. Scour equations recommended by HEC‐18 were used to calculate the potential pier and contraction scour for the two scenarios. The potential for debris increasing the potential scour is considered captured because of: 1) the extreme magnitude used for the design discharge (14,490 cfs), 2) the 4‐foot diameter piers (a 3‐foot diameter was specified in the preliminary bridge design), 3) the solid bridge rail assumed in Model 2, and 4) the fact that the actually opening size will be significantly larger than the existing when a magnitude of 14,490 cfs is reached.

Long‐term degradation is not a concern in Model 2 because the grade control is assumed to stay in‐ place. Long‐term degradation in Model 3 is represented by the failure of the GCS and a 15‐ft. headcut migrating through the bridge opening.

Table 2 below shows the scour potential if the existing embankment/gabion structure stays in‐place to provide in‐kind grade control (Model 2), and the scour potential if the existing embankment/gabion grade control fails, allowing the headcut to move up through the Peralta Creek crossing (Model 3).

Hydraulic Model No.

Reference Cross Section

Channel Invert

Elevation (ft)

Water‐ Surface Elevation

(ft)

Contraction Scour Depth (ft)

Pier Scour Depth

Total Scour Depth (ft)

Total Scour

Elevation

1080.093 5261.9 5278.7 9.6 10.5 20.1 5241.8

1080.093 5243.7 5255.2 0 7.9 7.9 5235.8

Table 2. Design Scour Depths and Elevations

The worst‐case scenario is if the existing embankment/gabion GCS fails and a 15‐foot‐deep headcut migrates upstream through the crossing during the design storm event (Model 3). Because it is assumed that the existing GCS/roadway embankments fail in Model 3, there is no physical feature present to induce a true contraction scour. Therefore, no contraction scour is included in the total scour elevation.

Also, since the abutments for the proposed bridge will be set back from the 140’ wide channel/250’ long bridge, no local abutment scour will occur in the Model 3 scenario. Finally, because of the extreme magnitude of the design discharge, the same discharge is used for the scour design and check floods.

The spreadsheet summaries for the above scour computations are included as Appendix B to this report.

Summary

Based on the results of the above hydraulic analyses, the following design recommendations apply:

A 250 ft. long bridge centered on the existing grade control opening is recommended.

Elevation 5235.8 ft. is the design and check scour elevation for designing the proposed bridge piers.

The bottom‐of‐headcut elevation of 5243.7 ft. represents the design and check scour elevation for designing the proposed bridge abutment foundations.

To preserve the integrity of the existing grade control structure (GCS), it is recommended that the new bridge be built on the existing alignment, directly above the existing roadway embankments, without disturbing any existing shotcrete, gabion, or riprap installations.

The recommended minimum low chord elevation for the proposed bridge is 5270.3 ft.

Remove roadway embankment, as required, to provide a minimum 3‐foot clearance below the low chord of the new bridge. Embankment should not be removed below elevation 5267.3 ft.

Existing Conditions Update After the completion of the analyses described in this report, the existing grade control structure was widened from an approximately 40‐foot bottom width to about 90 feet. The slopes of the widened structure were also protected by shotcrete. This work, shown in Photo 4 below, was undertaken by the

Tribe presumably to increase flow capacity and decrease potential blockage by debris at the crossing;

thus, further decreasing the flooding risk to their community located upstream.

Photo 4: South side of widened grade control structure

A September 2014 survey captured the widened geometry and revealed that the shotcrete protection extends to an elevation of about 5266’ (6.7‐foot depth) along the sides of the widened grade control structure. Survey and other personnel on site indicated that the shotcrete protection appears not ‘toed‐ down’ below the channel bottom within the grade control structure to prevent local undermining. An incipient motion analysis indicates that the natural bed material within the grade control can begin moving at about 500 cfs, which correlates to a flow depth of only 1.3 feet (see Appendix C). The 6.7‐ foot flow depth correlates to a discharge of approximately 8600 cfs. Both flows are significantly less than the bridge design discharge of 14,490 cfs. Consequently, the grade control could begin failing along the sides, during discharges significantly less than design, by contraction scour, local pier scour, or a combination of both undermining the shotcrete toe and eroding the supporting embankment.

In addition to the above, the September 2014 survey also indicated that the channel invert immediately downstream of the grade control was raised at least 10’ during a recent flow event in late summer when large amounts of sediment were transported by the stream. During a site visit in October 2014 Tribe members indicated that a flow event in late summer 2014 dropped large amounts of sediment ~ 13 ft deep immediately downstream of the gabion GCS. This material buried the downstream face of the grade control, making it impossible to know the depth of any ‘toe‐down’ constructed along the downstream face of the widened grade control structure. However, knowing that 15‐feet of degradation occurred below the brink of the grade control structure from past flow events, the estimated discharges of around 9000 cfs suggests that the widened grade control structure could also fail along the downstream face at discharges significantly less than design.

Given the above information, as well as the channel conditions represented by Hydraulic Model 3 (i.e.

failure of the grade control structure), it is concluded that the widened grade control structure will have no effect on the above hydraulic design recommendations for the proposed bridge.

References

Federal Highway Administration (2008) “Project Development and Design Manual” Federal Lands

Highway Division. US Department of Transportation, Chapter 7.

HEC‐18, Evaluating Scour at Bridges, Fifth Edition, April 2012, Chapters 6 & 7.

HEC‐RAS, v. 4.1.0, January 2010. U.S. Army Corps of Engineers Hydrologic Engineering Center.

Technical Assistance Report for Pueblo De Cochiti, Peralta Creek Flood Mitigation Measures, September

2012, U.S. Army Corps of Engineers, south Pacific Division, Albuquerque District.

APPENDICES

APPENDIX A

HEC‐RAS Output

Model 1

Model 2

Model 2 (con’t)

Model 3

Model 3 (con’t.)

APPENDIX B

SCOUR ANALYSES

Model 2 – Contraction Scour

Model 2 – Pier Scour

Model 3 – Pier Scour

APPENDIX C

INCIPIENT MOTION ANALYSES