NM ERFO FS SNTFE 2022-1(1)_Hydraulic Report_Sealed_flat.pdf

1526 Cole Boulevard, Suite 370 Lakewood, CO 80401

720.768.6385 djanda.com

EL PORVENIR

BRIDGE

REPLACEMENT

Final Hydraulics Recommendations

Report

San Miguel County, New Mexico

Prepared for:

Federal Highway Administration Central Federal Lands Highway

Div.

12300 W. Dakota Ave.

Suite 167

Prepared by:

DJ&A, P.C.

1526 Cole Boulevard, Suite 370 Lakewood, CO 80401

May 15, 2024

Kent Steinhaus

Digitally signed by Kent Steinhaus

DN: C=US,

E=kent.steinhaus@djanda.com, O=DJ&A, CN=Kent Steinhaus Date: 2024.08.09 08:50:10-06'00'

Final Hydraulic Recommendations Report 05-15-2024

Table of Contents Figures Tables Introduction

Project Description Project Location FEMA Compliance Local Floodplain Compliance Site Assessment Design Parameters and Bridge Design

Hydrologic Calculations Hydraulic Model

Surface Materials Mesh Development Boundary Conditions 3D Structures Monitoring Locations Observation Coverage Existing Conditions Proposed Conditions Bridge Obstruction Scour Scour Countermeasures Results

Conclusions References Appendix A – FEMA Floodplain Maps Appendix B – Floodplain Administrator’s Email Confirmation Appendix C – Hydrologic Calculations Appendix D – National Stream and Aquatic Ecology Center El Porvenir Bridge Replacement Recommendations Memo Appendix E – SMS Modeling Data

Observation Line Results

Figures Figure 1: Site Map Figure 2: Zone A Map Figure 3: Existing Conditions 50yr Event Figure 4: Existing Conditions 100yr Event Figure 5: 50-ft Bridge with 50-Year Event Figure 6: 50-ft Bridge with 100-Year Event Figure 7: 90-ft Bridge with 100-Year Event and 30% Blockage Figure 8: Scour Plot Figure 9: Material Coverage Map Figure 10: Mesh with 3D Bridge Figure 11: Monitoring Locations Figure 12: Observation Locations Figure 13: Stream Profile Line Figure 14: Bridge Obstructions Figure 15: Scour Summary Table Figure 16: Riprap Analysis at Left Abutment Figure 17: Riprap Analysis at Right Abutment Figure 18: Riprap Extents and “wrap around” (HEC 23, figure 14.7)

Tables Table 1: NSAEC Proposed Bridge Dimensions Table 2: Stream Flows Table 3: Manning's Roughness Coefficients Table 4: 100-year Analysis of Waters Surface Elevations

Introduction Project Description The El Porvenir Bridge Replacement Project proposes the construction of a new bridge across Beaver Creek and the rehabilitation of the stream in the vicinity of the bridge. This project emerged as a critical infrastructure endeavor following the devastating Hermit Peak/Calf Canyon Fire, which resulted in the destruction of the previous bridge in 2022. The El Porvenir Bridge had served as a vital link for the nearby El Porvenir Christian Camp, in addition to supporting National Forest maintenance, logging, and service of the nearby El Porvenir campground. The bridge’s destruction during the wildfire necessitated the planning and execution of a project to rebuild this essential transportation corridor.

Project Location Situated approximately 13 miles northwest of Las Vegas, New Mexico, the bridge crosses Beaver Creek near the end of U.S Forest Service Road 261. This road is the only access point for El Porvenir Christian Camp located to the north and the El Porvenir Campground to the east. The new Bridge will be maintained by the United States Forest Service (USFS).

Figure 1: Site Map

FEMA Compliance The project site is located within Federal Emergency Management Agency (FEMA) Zone A Special Flood Hazard Area (SFHA). This flood zone is located within the San Miguel County Unincorporated Areas 350132, effective December 12th, 2010. Zone A is defined as the area with 1 percent annual chance of flooding and is determined using approximate methods with no flood depths defined. FEMA regulations state that for Zone A floodplains, all cumulative impacts to the system from the time of the original study cannot result in a water surface elevation increase of more than one foot (ft). These regulations are based on 44 Code of Federal Regulations (CFR) 60.3 (b). If these regulations are exceeded, a Conditional Letter of Map Revision (CLOMR) is required. This analysis aims to address FEMA requirements under CFR 60.3 (b) by comparing existing and proposed water surface elevations over the floodplain for the 100-year flood discharge. It is anticipated at this time that a Conditional Letter of Map Revision (CLOMR) or a Letter of Map Revision (LOMR) will not be required for this phase of the project as the proposed bridge will provide sufficient conveyance to keep potential increases in the water surface elevation to less than one foot.

Figure 2 below, shows the extents of the Zone A boundary with the full Flood Insurance Rate Map (FIRM) available in Appendix A.

Local Floodplain Compliance On April 10, 2024, DJ&A reached out to the local floodplain administrator of San Miguel County. The administrator was informed that as per FEMA regulations (44 CFR 60.3 (b)), a hydraulic analysis of the area had been conducted. This analysis concluded that the implementation of the bridge would not result in a water surface elevation rise greater than 1-ft. The floodplain administrator informed

Figure 2: Zone A Map

DJ&A, that no greater specific regulations or permitting from the local agency would be required, as the project would be conducted on lands belonging to the USFS. A copy of this conversation is located below in Appendix B.

Site Assessment Members of DJ&A, Central Federal Lands Highway Division (CFLHD), United States Department of Agriculture (USDA), USFS, the National Stream and Aquatic Ecology Center (NSAEC), MGE (A/E), Yeh & Associates (Geotech), and El Porvenir Camp performed a site assessment on August 23rd, 2023. This site assessment was conducted to understand the topography and geology of the area and identify flood-prone areas, morphology of the river, and potential sources of sediment transport and erosion. Observations concluded that before the fire, the bridge crossing Beaver Creek at El Porvenir was approximately 32-ft long with 14-ft wide approach roads. The burned bridge will be replaced with a similar structure that meets current design standards.

Given the increased peak floods in the Beaver Creek watershed due to the recent fire, the hydraulic design of the bridge is being carefully considered. Although immediate impacts from the fire are expected and unknown at this time, the long-term flow conditions are anticipated to improve, returning to similar conditions as before the fire. Additionally, stream restoration both upstream and downstream of the bridge crossing is planned after construction of the bridge. This restoration is expected to further improve the flow conditions and mitigate long-term degradation and scouring of the bridge abutments.

During the immediate recovery of the site following the fire, USFS crews cleared the bridge and creek material that was built up around the crossing and replaced it with a low water crossing. This process unfortunately violated the Clean Water Act and the U.S. Army Corps of Engineers has required that the creek be restored to the conditions that existed before the clean-up effort.

Design Parameters and Bridge Design The bridge structure must be designed to accommodate the design flow rate, as well as applicable design criteria from the Federal Highway Association (FHWA), American Association of State Highway and Transportation Officials (AASHTO) and USFS. This ensures that the bridge can withstand and safely pass peak flow rates associated with extreme weather events, providing a robust infrastructure that minimizes flood risk. The proposed design of the bridge opening should ensure that the bridge's waterway opening is properly sized to prevent constriction and potential flood risk.

During the site visit, DJ&A collected information related to the existing bridge; the collected information is summarized below and will be critical for the hydraulic analysis of the conditions of the channel before the fire.

Previous bridge geometry: The previous bridge was located at MP 0.5 on NM HWY 65 at Lat.

35.71078 degrees N., Long. -105.414312 degrees W., over Beaver Creek. The bridge had an overall length of 32-ft, a clear width of 14.2-ft, and an out-to-out width of 15.3-ft. The bridge was on a tangent alignment with no skew and was originally built in 1950.

The design criteria for the bridge were still being determined during the development of the preliminary recommendation report. It was originally thought that AASHTO’s guidance of a 50-year event and 2-ft of freeboard would dictate the design. A memo provided by Bob Gubernick of the NSAEC, dated 9/4/2023 and hereafter referred to as the NSAEC Memo, stated the recommended design parameters of the proposed bridge crossing and proposed channel restoration design based on AASHTO criteria. These parameters are summarized in the table below, with a copy of the full memo available in Appendix D.

Table 1: NSAEC Proposed Bridge Dimensions

Feature Dimension

Bankfull Width 29 to 30-ft wide

Bed Width 23 to 24-ft wide

Bankfull Depth 2.8 to 3-ft deep

Width Between Vertical Bridge Abutments 50-ft minimum width

Low Flow Channel Depth 0.5-ft

Bank Height From Bed Width 2.3-ft

Minimum Clearance Above Channel Bottom 7.25-ft

Between the submittal of the preliminary recommendations and the creation of this report, USFS requested that the bridge be designed to convey the 100-year flood event, have a freeboard of 3-ft while considering a 30-percent blockage of the bridge opening and feature scour protections designed to the 100-year event. The NSAEC design was analyzed under these conditions using 2D modeling. Unfortunately, hydraulic modeling revealed that the 50-ft bridge geometry was no longer acceptable in providing the required freeboard under USFS design conditions. This conclusion required a larger bridge geometry to be considered.

Hydrologic Calculations The hydrologic study of the bridge site uses modeling software to quantify peak flow rates for flooding events at the bridge site. The hydrologic analysis results obtained from United States Geological Survey’s (USGS) Streamstats and ERAMS for the El Porvenir Bridge project yielded similar results for the pre-fire 100-year flood flow rate equaling 1,560 cubic feet per second (cfs). Through meticulous examination, both Streamstats and ERAMS consistently produced data that corroborates the previously reported peak flow rate in the NSAEC Memo. This synchronization in results instills confidence in the accuracy and reliability of the hydrologic models. Ultimately the Streamstats flows were selected for the hydraulic calculations. These flow rates summarized below in Table 2 will be instrumental in informing the design process, ensuring that the El Porvenir Bridge is robustly engineered to withstand extreme flood events and guarantee the safety and resilience of the infrastructure. These hydrologic calculations are included in Appendix C of this report.

Table 2: Stream Flows

Return Interval (yr) Flows (CFS) from USGS Regression Equations

(Streamstats)

2 193

10 604

25 916

50 1210

100 1560

500 2570

Hydraulic Model DJ&A used SRH-2D (Sediment and River Hydraulics – 2 Dimensional) software, a tool developed collaboratively by the U.S. Bureau of Reclamation and the Federal Highway Administration, to assess the existing and proposed conditions of the bridge crossing. The functionality of this program is supported by Aquaveo’s SMS (Surface-water Modeling System) software, which facilitated the development of the model.

Surface Bathymetric cross-sections were surveyed at approximately 50-ft intervals along the channel, extending 500-ft both upstream and downstream of the crossing. To create a comprehensive terrain model, these bathymetric cross-sections were combined with a survey that included the road and its adjacent areas, extending approximately 100-ft on both sides. Additionally, LiDAR data was employed to supplement this data and form a complete surface representation of the entire floodplain. The resultant digital terrain model (DTM) was then imported into SRH-2D software, where it served as the primary data source for elevations.

Additional DTMs were created to represent the proposed features involved in the construction of the bridge, such as abutments and tweaked roadway alignment. This final DTM incorporated the recommended cross section shown in Appendix E as well as an additional cross section representing a 90-ft bridge span.

Materials Within the SMS application, polygons were utilized to delineate areas exhibiting similar land use and coverage characteristics. The boundaries of these polygons were established based on available aerial imagery, supplemented by survey and elevation data. Each polygon was then assigned a Manning’s n value within the materials coverage section of the model. These Manning’s n values were derived from site photographs taken during field visits for accuracy. Refer to Table 3 for a detailed listing of the Manning’s n values that were incorporated into the model, as well as Figure 9 which provides a visual of the coverages used. Following the Manning’s n guide listed in Two-

Dimensional Hydraulic Modeling for Highways in the River Environment (FHWA, 2019), the below values were assigned to the appropriate locations within the model.

Table 3: Manning's Roughness Coefficients

Material Coverage Manning’s n Gravel Roadway .023

Floodplain/ Wooded .10 Channel .05

Improved Channel .035 Bank Riprap .04

Mesh Development In the SMS software, mesh development is facilitated through the use of polygons within a mesh generator coverage. These polygons were drawn to reflect existing features to help represent variable conditions such as grade breaks and channel banks. Each polygon is designated to generate either triangle or quadrilateral cells. Quadrilateral cells typically offer greater stability as they can represent the same section with one cell instead of two; they are favored under certain conditions and are specifically employed in areas expected to experience flow and where grade breaks or linear features (e.g. roadway embankments) exist. In contrast, triangle cell types are utilized in the remaining areas of the mesh. The size of the cells varies across the mesh, with smaller cells employed near the channel for detailed flow representation and larger cells used in more distant areas. Furthermore, both the size and shape of the elements are refined based on the hydraulic results obtained. For a visual representation of the mesh extents, refer to Figure 3 through Figure 7 below.

Boundary Conditions Boundary conditions were strategically incorporated into the hydraulic model to accurately simulate the inflow and outflow conditions. The inflow boundary was established approximately 500-ft upstream of the crossing, at the mesh’s most upstream limit and incorporates the generated peak flow values. Conversely, the downstream boundary condition was set just beyond the mesh’s downstream limit and is about 670-ft downstream of the crossing. This condition incorporates a specific tailwater elevation, tailored to individual flow events. These boundary conditions are deliberately positioned at a far enough distance away from the primary area of interest to ensure that the flows stabilize before and after reaching the crossing. This setup also facilitates the observation of any potential alterations in flow conditions resulting from the installation of the bridge.

3D Structures Using the new 3D structures tool available within a SMS, a 3D representation of the bridge was inserted into the model. This tool replaces the previously developed 3D bridge tool and pressure flow conditions within the program. An image showing the model with the proposed bridge is available in Appendix E.

Monitoring Locations Several monitoring arcs and points have been dropped into the SMS model. These features monitor parameters such as water surface elevation (WSE) and flow. These points are not critical to the model but help to ensure the model has achieved steady-state, help check continuity, and monitor overall stability. Monitoring locations are shown in Appendix E.

Observation Coverage Observation arcs have been used to create a 2D cross section of the stream bed and subsequent flow conditions at that location. These arcs have been drawn at key locations, such as the upstream and downstream ends of the bridge as well as the length of the stream’s reach. Results from this coverage are available in Appendix E.

Existing Conditions The hydraulic impact of the current conditions (post-fire) surrounding the crossing were analyzed through the SRH-2D model. This analysis was conducted for both the 50-year and 100-year flood events, which correspond to peak flows of 1210-cfs, and 1560-cfs, respectively. The results of these models, illustrating the hydraulic impact under these specified events, can be viewed in Figure 3 and Figure 4.

Figure 3: Existing Conditions 50yr Event

Figure 4: Existing Conditions 100yr Event

Proposed Conditions The bridge crossing originally recommended by the USFS was the 50-ft bridge shown in Figure 5 and Figure 6 respectively. The model showed that the 50-ft span would not meet the freeboard criteria without significantly raising the grade of the road. To combat this issue, the bridge was modeled in multiple span variations starting from the 50-ft section until design criteria was met with the 90-ft span. This analysis was conducted to observe the hydraulic impact of proposed improvements and to see if a wider span would benefit in meeting design criteria. Analysis under design conditions found the 90-ft bridge was the smallest span that allowed the 100-year event to pass with 30% blockage while maintaining 3-ft of freeboard. These analyses incorporated updated DTM’s, a refined version of the mesh featuring enhanced resolution, specifically at the crossing area, and the addition of the 3D bridge structure shown in Figure 10. The results from these simulations, detailing the hydraulic impacts under the proposed conditions, are presented in Figure 5 through Figure 7.

Figure 5: 50-ft Bridge with 50-Year Event

Figure 6: 50-ft Bridge with 100-Year Event

Figure 7: 90-ft Bridge with 100-Year Event and 30% Blockage

Bridge Obstruction As requested by the USFS, a blockage of 30% has been included within the hydraulic model. This blockage was conducted using the obstruction tool available within SMS. To start the modeling approach, the opening area of the bridge was calculated and was found to be 485-sqft with a subsequent blocked area of 145.5-sqft. This blockage area was then split between both bridge abutments to simulate two large trees stuck on either side of the bridge. The simulated blockages are both 40-ft long with a diameter of 1.82-ft and have a drag coefficient of 1.3 (circular cylinder).

Appendix E shows the locations of the obstructions in more detail.

Scour Bridge scour was analyzed using the bridge scour coverage available within SMS. This coverage uses an array of arcs to map features such as channel centerline, contracted and approach sections, channel banks, and abutments. Scour inputs were taken from simulation results at the drawn locations and then exported to FHWA’s Hydraulic Toolbox, for analysis. Figure 8 below shows the scour calculated along the contracted bridge section with a summary of results calculated within the Hydraulic Toolbox shown in Appendix E.

Figure 8: Scour Plot

As seen above, both sides of the bridge experience scour with a slight variance between the two.

The left abutment will see approximately 2.71-ft of scour while the right abutment will see approximately 0.88-ft during the 100-year flood event. These sections will need to be reinforced to prevent scour under design conditions.

Scour Countermeasures Due to increased river forces near the bridge, there is a recognized potential for scour damage to the abutments and contraction during significant storm events, such as the 100-year flow event. To mitigate degradation around the bridge's abutments and overbanks, DJ&A conducted an analysis of this 100-year flow event using the Hydraulic Toolbox. Although this analysis identified a higher risk of degradation around the Left abutment, the recommended approach is to implement scour countermeasures at both abutments and banks. This proactive strategy is designed to ensure comprehensive protection against potential scour damage, irrespective of the current analysis predictions.

The prescribed measure involves deploying Class One riprap with a D50 of 0.54-ft (16.5-cm) to a depth of 12-inches around both abutments. This riprap will extend two times the flow depth for a total of 7-ft from the abutment toe and have a "wrap-around" extension beyond the abutment radius along the approach embankment of 25’. These specifications aim to provide a robust defense against scour. The findings and recommendations are detailed in Figure 16 and Figure 17, with Figure 18 illustrating the general layout of the riprap placement as per HEC 23, Figure 14.7.

Results The modeling approach discussed above resulted in a slight decrease in water surface elevation compared to the existing conditions. These details of these results can be found in Table 4 below. It should be noted that the bridge deck elevations and thicknesses vary between the 50 and 90-ft versions leading to different freeboard values.

Table 4: 100-year Analysis of Waters Surface Elevations

Model WSE (ft) Depth (ft) Freeboard (ft) WSE Rise (ft)

Existing Conditions 50-yr Event 7515.06 3.96 - -

Existing Conditions 100-yr Event 7515.51 4.42 - -

50-ft Bridge 50-yr Event 7514.81 5.54 2.94 -0.25

50-ft Bridge 100-yr Event 7515.44 6.16 2.30 -0.07

90-ft Bridge 100-yr Event W/ 30% Blockage 7514.84 3.26 3.16 -0.67

* All measurements were observed at the middle of the bridge on the upstream side.

Conclusions The analysis of the NSAEC proposed bridge section did not meet the project goals of conveying the 100-year flow event while maintaining a 3-ft freeboard and 30% blockage. Analysis showed that with no blockage, the 50-ft section provided approximately 2.30-ft of freeboard. Increasing the height of the bridge deck and embankments or dropping the channel bed enough to achieve the amount of needed freeboard was not feasible with project constraints. To meet the project’s criteria, a new 90-ft section was analyzed and was found to provide adequate freeboard under design conditions. This section followed a similar design of the original 50-ft section, featuring a low flow center section and overflow banks. Implementation provided a drop in WSE across the bridge’s span and thus provide a greater amount of freeboard. The 90-ft design also eliminated the need for water to flow around the southern abutment during high runoff as was originally proposed in NSAEC Memo. It should be noted that meeting freeboard requirements is especially important as flow rates will be much higher due to the recent fire, and likely will be for the next 1-3 years at minimum. These peak flow rates will be expected to decrease ver time as the watershed continues to recover and revegetate.

References FHWA. (2009). HEC-23 Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Deisgn Guidance- Third Edition. Washington, D.C.: U.S. Department of Transportation.

FHWA. (2012). Evaluating Scour at Bridges. Washington D.C.: U.S. Department of Transportation.

FHWA. (2012). HDS7 - Hydraulic Design of Safe Bridges. Washington D.C.: U.S. Department of

Transportation.

FHWA. (2012). HEC 20 Stream Stability at Highway Structures. Washington D.C.: U.S. Department of Transportation.

FHWA. (2019). Two-Dimensional Hydraulic Modeling for Highways in the River Environment.

Washington D.C.: U.S. Deptartment of Transportation.

FHWA. (2023). HEC 16 - Highways in the River Environment. Washington D.C.: U.S. Department of

Transportation.

USDA Forest Service. (2014, November 24). Transportation structures handbook, chapter 60 - Hydraulics and watershed protection.

Appendix A – FEMA Floodplain Maps

Project Site

Appendix B – Floodplain Administrator’s Email Confirmation

Appendix C – Hydrologic Calculations

11/9/23, 3:01 PM StreamStats https://streamstats.usgs.gov/ss/ 1/4

StreamStats Report

Collapse All

Basin Characteristics

Parameter Code Parameter Description Value Unit

BSLDEM30ff Mean basin slope computed from 30 m DEM in feet per foot 0.4 foot per foot

DRNAREA Area that drains to a point on a stream 22.9 square miles

ELEV Mean Basin Elevation 9438 feet

HIGHREG HIGHREG 1096 dimensionless

I24H100YA2 Maximum 24-hour precipitation that occurs on average once in 100 years from NOAA Atlas 2

4.67 inches

PRECIP Mean Annual Precipitation 33.1 inches

Region ID: NM Workspace ID: NM20231109204431745000 Clicked Point (Latitude, Longitude): 35.71099, -105.41481 Time: 2023-11-09 13:44:52 -0700 https://streamstats.usgs.gov/ss/ 2/4

Peak-Flow Statistics

Peak-Flow Statistics Parameters [Peak 2008 5119 N Mountain Flood Region 5]

Parameter Code Parameter Name Value Units Min Limit Max Limit

DRNAREA Drainage Area 22.9 square miles 0.606 3120

ELEV Mean Basin Elevation 9438 feet 7570 11500

I24H100YA2 24 Hour 100 Year Precipitation Atlas2 4.67 inches 3.08 5.51

HIGHREG High flow region number 1096 dimensionless

Peak-Flow Statistics Flow Report [Peak 2008 5119 N Mountain Flood Region 5]

PIL: Lower 90% Prediction Interval, PIU: Upper 90% Prediction Interval, ASEp: Average Standard Error of Prediction, SE: Standard Error (other -- see report)

Statistic Value Unit SE ASEp

50-percent AEP flood 193 ft^3/s 80 80

20-percent AEP flood 409 ft^3/s 65 65

10-percent AEP flood 604 ft^3/s 60 60

4-percent AEP flood 916 ft^3/s 60 60

2-percent AEP flood 1210 ft^3/s 64 64

1-percent AEP flood 1560 ft^3/s 68 68

0.2-percent AEP flood 2570 ft^3/s 84 84

Peak-Flow Statistics Citations

Waltemeyer, S.D.,2008, Analysis of the Magnitude and Frequency of Peak Discharge and Maximum Observed Peak Discharge in New Mexico and Surrounding Areas: U.S. Geological Survey Scientific Investigations Report 2008-5119, 105 p. (http://pubs.usgs.gov/sir/2008/5119/)

Bankfull Statistics

Bankfull Statistics Parameters [Rocky Mountain System D Bieger 2015]

Parameter Code Parameter Name Value Units Min Limit Max Limit

DRNAREA Drainage Area 22.9 square miles 0.15444 9730.1061

Bankfull Statistics Parameters [Southern Rocky Mountains P Bieger 2015]

Parameter Code Parameter Name Value Units Min Limit Max Limit

DRNAREA Drainage Area 22.9 square miles 1.540539 9729.943938 https://streamstats.usgs.gov/ss/ 3/4

Bankfull Statistics Parameters [USA Bieger 2015]

Parameter Code Parameter Name Value Units Min Limit Max Limit

DRNAREA Drainage Area 22.9 square miles 0.07722 59927.7393

Bankfull Statistics Flow Report [Rocky Mountain System D Bieger 2015]

Statistic Value Unit

Bieger_D_channel_width 24 ft

Bieger_D_channel_depth 1.89 ft

Bieger_D_channel_cross_sectional_area 35.7 ft^2

Bankfull Statistics Flow Report [Southern Rocky Mountains P Bieger 2015]

Statistic Value Unit

Bieger_P_channel_width 24.3 ft

Bieger_P_channel_depth 1.41 ft

Bieger_P_channel_cross_sectional_area 30 ft^2

Bankfull Statistics Flow Report [USA Bieger 2015]

Statistic Value Unit

Bieger_USA_channel_width 37.3 ft

Bieger_USA_channel_depth 2.35 ft

Bieger_USA_channel_cross_sectional_area 92.7 ft^2

Bankfull Statistics Flow Report [Area-Averaged]

Statistic Value Unit

Bieger_D_channel_width 24 ft

Bieger_D_channel_depth 1.89 ft

Bieger_D_channel_cross_sectional_area 35.7 ft^2

Bieger_P_channel_width 24.3 ft

Bieger_P_channel_depth 1.41 ft

Bieger_P_channel_cross_sectional_area 30 ft^2

Bieger_USA_channel_width 37.3 ft

Bieger_USA_channel_depth 2.35 ft

Bieger_USA_channel_cross_sectional_area 92.7 ft^2

Bankfull Statistics Citations

Bieger, Katrin; Rathjens, Hendrik; Allen, Peter M.; and Arnold, Jeffrey G.,2015, Development and Evaluation of Bankfull Hydraulic Geometry Relationships for the Physiographic Regions of the United https://streamstats.usgs.gov/ss/ 4/4

States, Publications from USDA-ARS / UNL Faculty, 17p.

(https://digitalcommons.unl.edu/usdaarsfacpub/1515?

utm_source=digitalcommons.unl.edu%2Fusdaarsfacpub%2F1515&utm_medium=PDF&utm_campaign=PDFCo

USGS Data Disclaimer: Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty.

USGS Software Disclaimer: This software has been approved for release by the U.S. Geological Survey (USGS). Although the software has been subjected to rigorous review, the USGS reserves the right to update the software as needed pursuant to further analysis and review. No warranty, expressed or implied, is made by the USGS or the U.S. Government as to the functionality of the software and related material nor shall the fact of release constitute any such warranty. Furthermore, the software is released on condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from its authorized or unauthorized use.

USGS Product Names Disclaimer: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the

U.S. Government.

Application Version: 4.18.1

StreamStats Services Version: 1.2.22

NSS Services Version: 2.2.1

Appendix D – National Stream and Aquatic Ecology Center El Porvenir Bridge Replacement Recommendations Memo

USDA FOREST SERVICE

National Stream and Aquatic

Ecology Center

Memo To: Amina Sena, District Ranger, Sante Fe N.F.

From: Bob Gubernick R.G., Engineering Geologist, NSAEC cc: Julia Dietsch, FHWA

Date: 9/4/2023

Re: El Porvenir Bridge Replacement

Introduction:

On August 8th 2023 a USFS team of specialists composed of Engineers, Hydrologists, and Engineering Geologist performed a site assessment to assist in determining the configuration of the road stream crossing at the El Porvenir Camp ground.

The USDA Forest Service Specialist team consisted of Bob Gubernick R.G., Engineering Geologist Traci Sylte P.E., Civil Engineer & Hydrologist Andy Efta, P.H., Hydrologist Joel Murray, P.H., Hydrologist Kip Klein, Civil Engineer John McCann, Hydrologist This memo documents the observations and recommendations for a replacement crossing design for El Porvenir.

From April to August 2022, the Hermit Peak/Calf Canyon wildfire complex burned across a large extent of the Santa Fe National Forest. The bridge over Beaver Creek accessing El Porvenir Campground (tributary to the Gallinas River) (Figure 1) was burned and subsequent rain over the high severity burn area produced flood flows that ultimately destroyed the bridge. Following these events, the stream in the vicinity of the bridge was bull dozed by fire crews to help improve hydraulic efficiency at the crossing. Large berms were constructed along the left and right banks of the channel to “protect” the road. Although well-intentioned, this work caused aggradation (sediment deposition) upstream at the bridge crossing along with a 5ft head cut at the downstream end of the dozer-straightened channel. This configuration makes the crossing more susceptible to instability during normal high storm events and especially future flood flows, and natural channel re-adjustments have potential to compromise any new bridge’s structural integrity and hydraulic capacity at the crossing.

Figure 1 - El Porvenir Bridge Crossing

Site Conditions:

Beaver Creek is comprised of 3 distinct rock units comprised of Arroyo Penasco group (Limestone and sandstone), Mesoproterozoic and Paleoproterozoic rocks (Igneous), Mafic and Felsic meta volcanics (Metavolcanic, Metaclastic, Gneiss, amphibolite), and the Madera Group (Limestone, sandstone, and shale).

No bedrock offering stream gradient control was observed in the immediate vicinity of the crossing, although a bedrock exposure is present in the river right bank approximately 1,200 feet upstream of the bridge crossing (Figure 2). The bank composition in this area was old debris flow deposits. The bank materials were matrix supported, fine grained silts, sands, and clays particles supporting larger gravels, cobbles, and boulders (Figure 3). Steep valley walls with shallow soils are typical in the watershed and

Figure 2 - Bedrock exposure 1200 feet upstream of bridge crossing when burned produce hydrophobic soil conditions or complete soil removal to bedrock in severely burned area. The high gradient slopes can initiate debris flows when monsoonal rains are heavy in the vicinity.

The debris flows run out on the valley floor, infilling the channel, and causing stream avulsions. These repetitive event cycles form the valley bottom as we see it in its present form.

The fire and subsequent flood flow has activated at least 17 sediment sources (small debris flows) most of which has made it into Beaver Creek, but have not contributed large amounts of sediment and debris downstream to the bridge crossing (Figure 4). Most of the sediment sources have an incident angle to the stream of 70 to 90 degrees, which tends to stall the sediment at the tributary junction. The deposition may cross the channel causing a small impoundment to form before overtopping and cutting back thru the deposition.

Figure 4 - channelized and un-channelized sediment sources and debris flows

No large volumes of debris were observed upstream and downstream of the crossing. Sediment deposition has occurred, and the river is transporting this material at higher flows. The lack of woody debris will be a continual problem at the site as mortality and age cause treefall or a large flow event cause lateral erosion and local recruitment. Only tree fragments were observed

Figure 3 - Debris flow deposits, matrix supported clasts.

in the debris. Mature trees in the vicinity are much larger than existing bankfull (active channel) dimensions and are not anticipated to mobilize. However, smaller trees and fragments with lengths or transport orientation widths that are less than bankfull (~30ft) should be anticipated to be mobilized. Any bridge crossing with a span less than approximately 30 feet is very likely to trap incoming mobilized trees/tree parts at the crossing.

Beaver creek at the road crossing is a moderate gradient transitional channel. It’s form ranges from step pool morphology (Rosgen A to B channel) with intermixed relatively closely spaced short riffles and shallow pools due to sediment supply downstream of the bridge to closely spaced to riffle and pool riffle morphology (Rosgen B and C channel) upstream of the bridge with wider spacing between grade control structure or bends (Figure 5).

Figure 5 - El Porvenir - Beaver Creek channel types. Left photo is pool riffle morphology upstream of the crossing. Right photo is step pool morphology downstream of the crossing.

The riparian forest has experienced moderate to high burn severity upstream of the crossing.

Prior to the fire, the riparian tree stands were well developed and had good tree density with fully mature trees. The remaining stand density has a high probability of retaining trees from debris flows that may occur in the near future. However, as the stand deteriorates, they will contribute additional wood loading to the stream and will be transported through the system over time.

Relative to restoration treatments an and needs, the narrow channel width and readily available local tree sources may allow construction of engineered log jams upstream of the crossing that will help catch woody debris and stall sediment to assist with maintaining a fully functional bridge orifice. The construction of these structures, channel planform, and other features will be evaluated for incorporation with the necessary channel restoration. The channel in its current form is unsustainable and the reach warrants a restoration action to provide stable channel planiform (Figure 6).

Because the bridge crossing will be replaced before the post-wildfire watershed response attenuates and the restoration action will not be constructed before the bridge design, the bridge crossing design must consider the need for a higher debris clearance with the pre restoration channel, and likely a deeper scour with the post restoration channel. A longitudinal profile along centerline of the channel was surveyed along with cross sections for modeling purposes in the bridge crossing reach. Channel dimension metrics were taken in areas that were relatively stable and beyond the channel damaged reach. Appendix A contains the approximate locations of the beginning and end of the restoration and proposed channel gradient profiles. Table 1 shows recommended channel dimensions for the bridge cross section.

Table 1 - Recommended channel dimensions for bridge replacement. See appendix A for more details

Feature Dimension and range (FT) Bank full width 29 to 30 FT wide Bed width 23 to 24 FT wide Bankfull depth 2.8 to 3 FT deep Width between vertical bridge abutments

50 FT minimum width (maximum allowable encroachment)

Low flow channel depth 0.5 FT Bank height from bed width

2.3 FT

A normal depth computation was made with a recommended bridge crossing cross section configuration to understand hydraulic capacity and clearance. USFS software WinXSpro cross section analyzer was used to evaluate a range of flows and their corresponding stage in relation to the bridge and road section. While the normal depth computation is not recommended solely for bridge design, it provides a starting point to help with bridge layout (elevations and dimensions) for further analysis in HEC-RAS.

Figure 6 - Regrading the channel after the fire has created a 5ft tall headcut that is migrating upstream towards the bridge crossing. Photo shows a 3.5 ft high headcut. a smaller one (1.5ft high) is located immediately downstream.

For the analysis, the bridge was set on a grade with a low section adjacent to the existing parking area. Clearance was set at 7.25 ft above the channel on the low end of the bottom chord. A concrete box beam wit a 2.5ft thickness was used to evaluate clearance.

Results of the normal depth computations are provided in the attached excel spreadsheet file “el porvenir normal depth computation 2023_0829”. Hydrology development is provided in Appendix B. Good correlation was found between field measured hydraulic geometry and Q100/Q2—relations, regional curves, and flood potential methodology for flows up to Q100. Under normal (i.e. general steady state; pre-wildfire disturbance) watershed conditions the design flow for a Q100 event would be ~1,560 CFS. Post-fire flow estimates suggested that flows within a 1– 3-year post-fire time frame may double or triple for a given flood event (3000 to 6000 CFS).

While observed conditions in burn-affected drainages suggest that post-fire flood response is likely to continue attenuating, watershed characteristics and flow estimates suggest that this drainage is naturally susceptible to large flow events.

Recommendations:

A conservative approach is warrant due to the proximity to the year of the burn. The watershed has not gone through a large flow event as of yet. Based on the gage data it is upwards of a Q10 event. However, that gage is farther much downstream and there are other tributaries that come in diluting the peak of flow of that event.

1. Minimum span between abutments is 50 ft

2. Clearance should be as high as possible. The bottom chord should be at least 7.25 ft above the restoration alignment. If the restoration channel isn’t regraded as part of the bridge construction, clearance should be raised.

3. Maintain a low road approach that can pass flood flow to reduce the clearance height of the structure.

4. Construct the approach out of gravel and do not harden the surface or embankment slopes. This is a desired failure point if a large flood happens in the next 3 years. It is cheaper to replace road embankment rather than the bridge.

5. Treat lower chord of the superstructure to survive debris impacts. I have used angle iron or wood (2x6”) facia in previous projects.

6. It is recommended that a roughed in version of the restoration profile be used to gain additional clearance. If the channel is constructed space grade controls 20 to 40 ft apart and construct out of boulders

7. The channel banks should be defined in accordance with the provided channel template.

The bermed material can be used to help form banks. If left as is the area is depositional due to over-width and will lose clearance height in time. The rate of head cutting is unknown and although observed to be migrating, it is impossible to determine when it will pass through the bridge orifice.

8. Increase scour depth to accommodate higher fire flood hydrology.

9. Maintain channel dimensions as shown on the drawings.

Any question or clarity required for contents within this report should be directed to Bob Gubernick at the National Stream and Aquatic Ecology Center Robert.Gubernick@usda.gov 218-491-4476

APPENDIX A

Date Generated 08/29/2023 Latitude 35.710773 Longitude -105.414231

Value Percentile [Range]

15.1 63.1 [0 - 74.3]

0.45 27.3 [0 - 6.89]

1.9 84.3 [0 - 2.86]

1.23 72.1 [0 - 2.82]

0.588 82.6 [0 - 1.621]

Flood Potential Watershed Analysis: Results Overview

Indices -- Watershed Flooding Characteristics Flood Potential Index (Pf)

Watershed Scale Ratio (Rf) Flood Variability Index (Vf)

Beard Flash Flood Index (F) Richards-Baker Flashiness Index (R-B)

Trends

Unadjusted Trend-Adjusted 5,550 5,550

10,500 10,500

Return Interval (yr) Percent Chance of Occurance Index Flood USGS Equations

(StreamStats) 500 0.2 3,190 2,570 200 0.5 2,100 ---- 100 1 1,520 1,560 50 2 1,080 1,210 25 4 762 917 10 10 457 604 5 20 290 409 2 50 120 193

1.5 66.7 72 ----

: directly comparable

Flood Discharge Estimates (cfs) Expected Flood Potential (Qefp)

Maximum Likely Flood Potential (Qmlf)

So uthern Transit ion (4

Zone 4 is experiencing no trend in the magnitudes of largest (5%) of annual peak discharges.

Zone 4 is experiencing no trend in the magnitudes of largest quarter (Q4, >4 yr RI) of annual peak discharges.

Zone 4 is experiencing no trend in the annual frequency of floods.

Zone 4 is experiencing no trend in the event frequency of large floods.

Zone 4 is experiencing a significant decreasing trend in flood flashiness.

See Flood Potential Portal (erams.com) for reference material regarding the Flood Potential methodology.

APPENDIX B

Appendix E – SMS Modeling Data

Figure 9: Material Coverage Map

Figure 10: Mesh with 3D Bridge

Figure 11: Monitoring Locations

Figure 12: Observation Locations

Figure 13: Stream Profile Line

Figure 14: Bridge Obstructions

Figure 15: Scour Summary Table

Figure 16: Riprap Analysis at Left Abutment

Figure 17: Riprap Analysis at Right Abutment

Figure 18: Riprap Extents and “wrap around” (HEC 23, figure 14.7)

Observation Line Results

Bridge

Bridge