Hydraulic Report - 10.3 WV ERFO FS MNGAH921 2018-1(1) Appendix C.pdf

APPENDIX C

SMS Trail Bridges Analysis

TR 487 – MP 1.2

DSR 2018-2.2

Hydraulics Report TR 487 – MP 1.2

WV ERFO FS MNGAH921 2018-1(1) DSR 2018-2.2

TABLE OF CONTENTS

1. Purpose

2. Footbridge Structure

2.1. Location

2.2. Existing Geometry

2.3. Proposed Geometry

3. Hydrology

4. Structure Selection

5. Hydraulic Analysis

5.1. Modeling Method and Program Used

5.2. Topographic and Bathymetric Data

5.3. Model Geometry

5.4. Roughness Assignments

5.5. Boundary Conditions

5.6. Modeling of Proposed Structure

5.7. Model Run Control Parameters

6. Results

7. Scour Calculations

7.1. Methodology

7.2. General Scour (Long-Term Degradation)

7.3. Contraction Scour

7.4. Local Scour at Piers

7.5. Local Scour at Abutments

7.6. Scour Countermeasure Design

8. Culvert Background / Assessment

8.1. Culvert Hydrology

8.2. Hydrology Characteristics

8.3. Hydraulic Modeling

8.4. Existing Conditions

8.5. Proposed Conditions

LIST OF FIGURES

Figure 1: Project Location Map Figure 2: 3D-Rotated View of SMS Generated Mesh Displaying 0.5-Foot Contours Figure 3: Mesh Elements Figure 4: ARR Mesh Quality Plot Figure 5: Boundary Conditions Figure 6: 3D UGrid Proposed Bridge Figure 7: Critical Velocity Equation (HEC-18 Eqn. 6.1) Figure 8: Culvert Outlet. Photos taken December 2021 Figure 9: FSR 86 DSR 2018-2.2 Culvert Drainage Area Map Figure 10: Rating Curve for Proposed Condition ....................................... Error! Bookmark not defined.

LIST OF TABLES

Table 1: Williams Trail footbridge survey geometry Table 2: Williams Trail footbridge proposed geometry Table 3: StreamStats Peak Discharge Data Table 4: Hydraulic Design, Scour Design, and Scour Design Check Flood Frequencies Table 5: Peak Discharge Results Table 6: Summary of Hydrologic Analysis Table 7: Summary of Culvert Design and Analysis Parameters

1. Purpose The purpose of this report is to assess the proposed hydraulic conditions at DSR 2018-2.2 FSR 487 which conveys flows of an unnamed tributary (Stream) through Williams Trail (Trail) pedestrian bridge (Bridge) for Williams River (River). Additionally, this report will also assess the proposed hydraulic conditions of a culvert that is located 35 feet to the west of 2018-2.2 bridge replacement (Culvert).

2. Footbridge Structure

2.1. Location

Figure 1: Project Location Map

2.2. Existing Geometry

Pedestrian trail bridge of the following parameters:

Table 1: Williams Trail footbridge survey geometry

Material Low Chord (ft)

High Chord

(ft) Length (ft) Width (ft)

Wood 3014.95 3016.04 14.35 4.28

Project Location:

Williams River Trail MP 1.20

LAT: 38.32903

LONG: -80.2169300

2.3. Proposed Geometry

The wooden footbridge for Williams Trail over the unnamed tributary is to be removed and replaced with proposed bridge and abutments in a new location. The proposed footbridge has the following parameters.

Table 2: Williams Trail footbridge proposed geometry

Material Bottom Chord Elevation

Top Chord Elevation

Length (ft)

Width (ft)

Wood 3015.04 3016.85 35.0 5.0

3. Hydrology The unnamed tributary through Williams Trail pedestrian bridge to Williams River is an unregulated and ungaged stream. Williams River is a FEMA regulated stream. The stream is classified as Zone A flood hazard area, or areas with a 1% annual chance of flooding. The Bridge is located outside of the Williams River Zone A flood hazard area. Peak discharges were estimated using USGS StreamStats. The drainage area delineated using StreamStats was found to be 0.0492 mi2. The resulting peak discharge data produced using StreamStats is presented below:

Table 3: StreamStats Peak Discharge Data

Frequency Flow Rate (ft3/s)

2-YR 5.01

10-YR 9.10

25-YR 17.60

50-YR 22.00

100-YR 26.70

200-YR 31.80

500-YR 39.50

Three flood frequencies are used to design and evaluate the proposed bridge and bridge foundations. These are the hydraulic design flood frequency, the scour design flood frequency and the scour design check flood frequency as presented in Table 4.

Table 4: Hydraulic Design, Scour Design, and Scour Design Check Flood Frequencies

Hydraulic Design Flood Frequency

Scour Design Flood Frequency

Scour Design Check Flood Frequency

Q25 Q50 Q100

4. Structure Selection The existing pedestrian trail bridge and abutments are to be removed and replaced with a new pedestrian bridge and abutments with wingwalls, located approximately 20 feet downstream from the existing. The aggregate surface for the approach trail, approximately 1,000-ft to trail head, is to be repaired. Additionally, an existing 12-inch trail cross-pipe about 35 feet to the west of the proposed Williams Trail pedestrian bridge is to be removed and replaced.

5. Hydraulic Analysis

5.1. Modeling Method and Program Used

WSP developed a two-dimensional model of the 2018-2.2 crossing using the Sedimentation and River Hydraulics model (SRH-2D) within the Surface-water Modeling System (SMS Version 13.3.11) graphical user interface. SRH-2D was developed by the US Bureau of Reclamation (USBR), with cooperating technical development from the Federal Highway Administration (FHWA). SRH-2D is the preferred river modeling approach by FHWA for river crossing structures.

SRH-2D is a depth-averaged two-dimensional hydraulic model that uses a flexible mesh based finite element analysis of hydraulic systems. The model provides robust analysis of 2D dominated flow fields such as flows with in-stream structures, through bends, perched rivers, side channel and agricultural returns, and braided channel systems. SMS allows for the use of HY-8, the FHWA culvert analysis program, to create culverts within SRH-2D models. This means that culverts are represented in SRH-2D models as 1D structures. Bridges are represented as 3D structures in SRH-2D models.

The design objective of the 2-D hydraulic modeling in this study was to develop depth and velocity inputs for analysis of scour potential at the proposed bridge crossing. Modeling details and results are summarized in the following sections.

5.2. Topographic and Bathymetric Data

Field topographic and bathymetric survey data was collected by JMT in 2021. This survey data extended approximately 90 feet upstream of the crossing to approximately 80 feet downstream. Due to the area required for the floodplain model, WSP utilized secondary data from the USGS Original Project Resolution WV_FEMA_R3_East_Lidar_2016_D16 EB206, for areas outside the surveyed area, including topography surrounding the channel for the unnamed tributary and the floodplain for William River. The vertical datum for all sources is NAVD 88, the horizontal datum for all sources is NAD 83. Projection was set to State Plane Coordinate System, Zone: West Virginia South (FIPS 4702).

A vertical difference of approximately 3 feet was found between the survey data and the USGS elevation data. To provide a smooth transition with the survey surface and a positive slope for the flow path, the USGS data was raised to correspond to the surveyed elevations using ArcMap 10.8.2. A surface was created for the proposed bridge, wingwall abutments, and trail approaches, based on design data, in AutoDesk Civil3D 2022, then merged with the survey surface to create a proposed conditions surface. The proposed conditions surface and the secondary data were merged to create a complete terrain using HEC-RAS 6.4.1 RAS Mapper software.

Figure 2: 3D-Rotated View of SMS Generated Mesh Displaying 0.5-Foot Contours

5.3. Model Geometry

The modeling mesh is a collection of vertices, edges, and faces defining the three-dimensional spatial information of the ground surface. SRH-2D uses a finite element mesh to define the geometric surface and set the modeling limits. The SRH-2D modeling site extends from approximately 80 feet upstream of the proposed crossing to approximately 125 feet downstream. The model domain was sized to use available survey and USGS data. Further than 80 feet upstream of the bridge, the channel is not sufficiently well-defined in the available USGS data to model accurately. The downstream model limit is the confluence of the tributary with the river. No available elevation data for the riverbed was found, so river flow could not be accurately modeled, and the tributary model could not extend past the confluence and the limit of elevation data.

Both the patch and paving mesh type were utilized to create the model mesh. Triangular paving was primarily used in the overbank regions while rectangular patching was used throughout the channel and the crossing. The mesh contains roughly 6,215 elements.

The mesh extents and total number of elements was developed to best capture geometric features impacting conveyance while also minimizing model inefficiencies and increased computational time. Upstream and downstream model extents were set to be far enough upstream and downstream to fully depict any impacts caused by the project. Initially, a coarse mesh was developed using larger elements and used in a simulation.

The model results informed the insertion of additional resolution throughout the domain. This iterative refinement process continued until changes in simulation solutions were minimal.

Figure 3: Mesh Elements

Figure 4: ARR Mesh Quality Plot

5.4. Roughness Assignments

A Manning’s roughness of 0.035 was used for the channel, 0.02 for the trail, 0.05 for the riprap, 0.013 for the concrete abutments and wingwalls, and 0.10 was used for overbank areas. These values were estimated based upon available site photos.

5.5. Boundary Conditions

The peak discharges, determined using the USGS StreamStats, were used as Inlet Q boundary condition constant discharges in the SRH-2D model runs. These discharges were presented in Table 3. The SMS “Channel Calculator” tool was used to compute and assign a normal depth for the outflow boundary condition for all simulations.

Figure 5: Boundary Conditions

5.6. Modeling of Proposed Structure

The SMS 3D Structure coverage was used to generate a 3D UGrid to visualize the bridge in the graphics window and generate a ceiling elevation dataset to represent the bridge pressure zones. The terrain file used in the proposed conditions SMS simulations was modified to incorporate proposed grading and riprap layout through the bridge crossing.

Figure 6: 3D UGrid Proposed Bridge

5.7. Model Run Control Parameters

All SRH-2D model controls were set to a 0.50-second timestep. A time step less than 1-second was found to be necessary to maintain hydraulic computational stability during the model run for all mesh options. All simulations were set to a dry initial condition, and the total period that the simulation run analyzed was set to 4-hours. This was a long enough period to allow the model to reach a steady state solution for all simulation runs. Output frequency was set to 15-minute intervals. The default parabolic turbulence parameter option was selected and set to the default value of 0.7. Each simulation required less than 15 minutes of total run time.

6. Results

Table 5: Proposed Conditions Freeboard Results (Design Flood Event – 25-Year)

Low Chord

Elevation (ft) Average Water Surface Elevation

(ft) Freeboard (ft) Proposed Bridge 3015.04 3011.81 3.23

Average Water Surface Elevation values extracted from an SMS arc along upstream face of proposed bridge

Table 6: Summary of Hydraulic Performance

Discharge

Q (ft3/s) Contracted Section Main

Channel Average Velocity (ft/s) Contracted Section Main

Average Water Surface Elevation (ft) Q2 5.0 3.2 3011.44

Q25 17.6 4.2 3011.75 Q50 22.0 4.7 3011.77 Q100 26.7 5.2 3011.81

Contracted Section Main Channel Average Velocity from SMS Scour Tool Output Average Water Surface Elevation values extracted from an SMS arc along upstream face of proposed bridge

7. Scour Calculations

7.1. Methodology

The scour analysis performed for this site was based on the evaluation of several empirical equations outlined in the Federal Highway Administration’s Hydraulic Engineering Circular No. 18 (HEC-18)

(FHWA 2012).

While the empirical equations from HEC-18 are considered the best available quantitative methods for the design and analysis of scour at bridge piers, scour estimates should be weighed against historical observations of actual scour in the field and professional judgment of scour potential given the material makeup and conditions of the specific channel. For this analysis, the equations used to produce the scour results are considered conservative.

Four scour processes are considered as part of this analysis:

1. General Scour

2. Contraction Scour

3. Pier Scour (local scour)

4. Abutment Scour

The SMS Bridge Scour tool was used to export model output parameters for analyzing the bridge crossing.

This tool uses arcs to define the approach and contracted sections, bank stations, and abutments and then extracts both main channel and overbank averaged hydraulic parameters for bridge scour analysis.

7.2. General Scour (Long-Term Degradation)

Over time, changes to the channel bed elevations can occur due to the natural trend of the stream or changes within the stream or contributing watershed. Long-term degradation does not include the cutting of the streambed at the bridge due to a significant storm event, that is accounted for following the contraction and local scour analysis. An estimated 1-foot of general scour is applied at this site.

7.3. Contraction Scour

Live Bed Determination (FHWA 2012)

One of the first steps in a scour analysis is a determination of whether live-bed or clear-water contraction scour conditions prevail at the project location. Generally, live-bed contraction scour occurs when bed sediment transport is actively occurring. Clear-water contraction scour occurs when there is no such bed sediment transport. These conditions are determined through comparing the average velocity of the flow in the main channel upstream of the bridge opening against the critical velocity for initiating motion of the D50 size of the channel bed material.

Sediment samples were taken at the proposed crossing location. These samples resulted in an average particle size D50 of 3.1 mm and D95 of 22.1 mm. These particle sizes were used throughout the scour analysis. The equation for critical velocity is shown in Figure 7.

𝑉𝑉𝑐𝑐 = 𝐾𝐾𝑢𝑢𝑦𝑦

6𝐷𝐷

Figure 7: Critical Velocity Equation (HEC-18 Eqn. 6.1)

Where:

Vc = Critical velocity above which bed material of size D and smaller will be transported, ft/s y = Average depth of flow upstream of the bridge, ft D = Particle size for Vc, ft D50 = Particle size in a mixture of which 50 percent are smaller, ft Ku = 11.17 English units

A summary of model outputs and computations at are presented below:

Table 7: Upstream Velocity Comparisons

Storm Event Approach Section Critical Velocity (ft/s)

Approach Section Average Velocity (ft/s)

Design (50-Year) 2.2 5.6 Check (100-Year) 2.2 5.8

These calculations result in a critical velocity lower than the average velocity of flow in the main channel upstream of the bridge opening. Therefore, there is a live-bed scour condition at this site. An excel workbook was utilized to carry out the live-bed contraction scour equations which are described in detail within HEC-18 Section 6.

Table 8: Contraction Scour Depth Summary Storm Event Live Bed Computed Scour Depth (ft)

Design (50-Year) 0.0 Check (100-Year) 0.0

7.4. Local Scour at Piers

The trail pedestrian bridge does not have piers; therefore, this scour process was not considered.

7.5. Local Scour at Abutments

HEC-18 NCHRP 24-20 Abutment Scour Approach (FHWA 2012) NCHRP 24-20 methods provide a single combined local and contraction scour depth value based on velocity, depth, and area for the flow obstructed by the abutments. Overbank hydraulic parameters were extracted from the SRH-2D simulations and used to compute abutment scour using an excel workbook.

Considering the proximity of each abutment to the main channel, scour condition location type a (main channel) was selected.

Table 9: Summary of Calculated Scour

Flood Frequency Abutment General Scour (ft)

Contraction Scour (ft)

Abutment Scour (ft)

Total Scour Elevation (ft)

Design (50-Year) Left 1.0 0.0 1.9 3009.7 Right 1.0 0.0 1.9 3009.8

Check (100-Year) Left 1.0 0.0 2.0 3009.5 Right 1.0 0.0 1.9 3009.8

Note: Abutment Scour includes the contraction scour and long-term scour depths

7.6. Scour Countermeasure Design

The proposed bridge riprap extents are designed to withstand the effects of scour up to and including the 100-year Scour Countermeasure Design Flood Frequency. Riprap along the abutments and pedestrian trail embankments are recommended as scour countermeasures. Riprap protection will be toed down to the 100-year abutment scour elevation.

Methodologies outlined in the FHWA Hydraulic Engineering Circular No. 23 Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance (HEC-23) were used for sizing riprap at abutments.

According to FHWA’s HEC-23 Volume 2 guideline 14, the recommended design equations for sizing rock riprap for spill-through and vertical wall abutments is as presented below.

For Froude numbers ≤ 0.8:

𝐷𝐷50

𝑦𝑦

𝐾𝐾

𝑆𝑆 − 1

𝑉𝑉2

𝑔𝑔𝑔𝑔

Where:

D50 = median stone diameter (ft);

V = Characteristic average velocity in the contracted section (ft/s);

S = Specific gravity of the riprap (usually taken as 2.65);

g = Acceleration due to gravity, 32.2 ft/s2.

y = depth of flow in the contracted bridge opening (ft);

K = 0.89 for spill-through abutment, 1.02 for a vertical wall abutment.

For Froude numbers > 0.8:

𝐷𝐷50

𝑦𝑦

𝐾𝐾

𝑆𝑆 − 1

𝑉𝑉2

𝑔𝑔𝑔𝑔

0.14

K = 0.61 for spill-through abutment, 0.69 for a vertical wall abutment

Inputs to these equations was taken directly from the SMS 100-year simulation results at the proposed bridge.

Table 10: Abutment Riprap Analysis Inputs Froude Number Near Abutments

(SMS 100-yr Output) 1.53

Average Velocity in Contracted Section (SMS 100-yr Output) 5.2

Abutment Type Spill-Through Contracted Section Flow Depth 0.74 Specific Gravity of Rock Riprap 2.65

Gravitational Acceleration 32.2 K 0.61

D50 (ft) 0.3 D50 (mm) 84.9

A Class IV riprap gradation is recommended. The riprap thickness should be at minimum 2.0 times the D50 value.

8. Culvert Background / Assessment The culvert site is located 35 feet to the west of 2018-2.2 bridge replacement. The culvert is located on Williams River Trail. A visual assessment was performed of the culvert in December 2021 by performed by the U.S. Forest Service personnel (Forest Service). The culvert outlets into a small tributary before discharging to Williams River. The existing culvert that crosses Williams River Trails is a 12-inch CMP.

Figure 8 shows the conditions of the culvert. The repairs for this culvert include remove and replace the existing trail cross pipe. To support the proposed design, a hydraulic HY-8 model was run to confirm the proposed size of the replacement culvert.

Figure 8: Culvert Outlet. Photos taken December 2021.

8.1. Culvert Hydrology

The FSR 86 DSR 2018-2.2 is located within Pocahontas County and has a drainage area of 0.07 acres. Peak discharges at the project location were calculated using the Rational method. The Rational method is the selected method for estimating peak discharges of drainage areas less than 200 acres. Calculation is based on a simple formula: 𝑄𝑄 = 𝐶𝐶𝐶𝐶𝐶𝐶, where Q (cfs) is the peak discharges, C is the runoff coefficient, i (in/hr) is the design rainfall intensity, and A (acres) is the drainage area.

Runoff coefficients for various types of land use, hydrologic soil groups and land slopes are provided by West Virginia DOT Drainage Manual. Time of concentration and the drainage area were manually delineated in ESRI GIS using County LIDAR contours. Rainfall intensity was read from the NOAA Atlas 14 point precipitation frequency estimates data center with an intensity duration determined based upon the time of concentration. Peak discharges results are shown in Table 1.

Table 5: Peak Discharge Results

Return Period (year) 10 25 Rainfall Intensity (in/hr) 4.95 5.71 Peak Discharges (cfs) 0.11 0.13

The culvert discharges into a smaller tributary before directly into a larger stream system, Williams River, which controls the backwater conditions through the culvert.

8.2. Hydrology Characteristics

Forest is the only land uses identified within this watershed. The soil type for this watershed is approximately 25% type A soil and 75% type C soil. Figure 9 shows the culvert site location, the stream channel, and the drainage area extents.

Figure 9: FSR 86 DSR 2018-2.2 Culvert Drainage Area Map

Hydrologic parameters are shown in Table 6. The runoff coefficient was based on the forested and wetland land use and the soil types. When determining the runoff coefficient (C), factors such as land use, soil type, and slope were taken into consideration.

Table 6: Summary of Hydrologic Analysis

Drainage Area (Ac) Runoff Coefficient (C)

Tc (hours)

0.07 0.35 0.12

8.3. Hydraulic Modeling

The Federal Highway Administrations (FHWA) HY-8 program was used for hydraulic modeling and analysis of the culvert replacement. The roadway functional classification is low-standard which correlates to a 25-year design storm for the culvert following PDDM standards.

The HY-8 tailwater conditions were analyzed by using outlet elevation at downstream of the culvert outlet.

8.4. Existing Conditions

The existing culvert consists of a 12-inch circular corrugated metal pipe. Survey file extents did not include assumed location of 12-inch CMP trail cross-pipe. The culvert length and invert in/out information were based assumptions made from LIDAR. The existing 12-inch CMP has sufficient capacity to convey the 25-year storm.

8.5. Proposed Conditions

The HY-8 analysis resulted in a recommendation of a 12-inch HDPE pipe. The proposed culvert is to extend beyond the slope, following US Forest Service culvert design standards. Per project correspondence, the USFS does not prefer the use of headwalls or fabricated end sections for culvert pipes under 48 inches in diameter. Headwalls are not required per Eastern Federal Lands Federal Highway Administration (EFLHD) Project Development and Design Manual (PDDM) for culverts with an equivalent opening of 48 inches or less. In proposed conditions, the 12-inch HDPE pipe conveys the 25-yr storm. The 12” size is proposed to minimize the need for excavation and additional site disturbance. Twelve inches is below the PDDM recommended minimum culvert diameter, however due to the sufficient capacity shown in the model and the fact that the crossing is on a trail rather than roadway, this size was considered appropriate. A formal design exception was requested and is attached to this report.

The parameters for a culvert designed to convey the 25-year design storm peak discharge are shown in Table 7.

Table 7: Summary of Culvert Design and Analysis Parameters

Type Single Barrel - HDPE Q25 0.13 cfs Diameter* 12 inches Length* 7.0 feet Upstream Invert Elev.* 3011.25 feet Downstream Invert Elev.* 3011.0 feet Pipe Slope* 3.57% Manning’s n Value (Pipe) 0.012 (smooth HDPE) Roadway Crest Elevation* 3012.5 feet Outlet Velocity 3.40 ft/s

*Survey information was unavailable. These values were based on assumptions.

Per HY-8 analysis, the culvert is outlet controlled at the design storm event. For the design flow of 0.13 cfs, the headwater elevation is 3011.44 ft and does not overtop the trail. The existing trail elevation is assumed to be 3012.50 ft at the location of the proposed culvert.

Attachment

Trail Bridge Scour Calculations

Design By: MTC

Modeled Scenario Ku y (ft)

D50

(mm) D50

(ft) Vc

(ft/s)

Approach section main channel average velocity

V (ft/s) V/Vc < 1?

Contraction Scour Method

Q50 11.17 0.54 3.08 0.01 2.2 5.6 No Live-Bed Q100 11.17 0.61 3.08 0.01 2.2 5.8 No Live-Bed

Summary of Results

Compute Contraction Scour (Check for live-bed or clear-water)

Critical Velocity Determination

[HEC-18 p. 6.2, Eqn. 6.1]

Clear-Water: Approach Section Velocity < Vc

Live-Bed: Approach Section Velocity > Vc

Modeled Scenario S1

Approach section main channel average depth y1

(ft) V*

(ft/s) T

(ft/s) V*/T k1

Q50 0.033990 0.54 0.77 0.83 0.93 0.64 Q100 0.038126 0.61 0.87 0.83 1.04 0.64

Modeled Scenario

Contracted section main channel average depth yo

(ft)

Approach Section Main Channel Flow

Q1

(cfs)

Contracted section main channel flow

Q2

(cfs)

Approach section main channel width W1

(ft)

Contracted section main channel adjusted width W2

(ft) y2

(ft) ys

(ft) * Q50 0.69 23.5 21.1 7.74 6.35 0.56 -0.13

Q100 0.74 27.4 24.9 7.74 6.33 0.64 -0.10

Compute Contraction Scour Live-Bed Contraction Scour Calculations

Inputs

Live-Bed Contraction Scour - Summary of Results

*Note: If calculated ys returns a negative answer, the contraction scour depth equals zero.

[HEC-18 p. 6.10, Eqn. 6.2]

[HEC-18 p. 6.10, Eqn. 6.3]

Simulation Abutment Scour Condition Abutment Type

Unit Discharge, Upstream in Main Chnanel, cfs/ft q1

Unit Discharge in Constricted Area, cfs/ft q2 q2 / q1

Upstream Flow Depth (y1)

Amplification Factor

Left Abutment A Spill-through 3.03 3.32 1.10 0.54 1.60 Right Abutment A Spill-through 3.53 3.96 1.10 0.54 1.60 Left Abutment A Spill-through 3.53 3.94 1.12 0.61 1.62

Right Abutment A Spill-through 5.14 5.59 1.12 0.61 1.62

Simulation Abutment

Flow depth including contraction scour, ft yc

Scour Depth from Long Term Degradation (ft)

Maximum Flow Depth including Abutment Scour, ft ymax

Depth at Abutment Toe Prior to Scour, ft yo

Abutment Scour Hole Depth ys

(ft)

Left Abutment 0.58 1.0 1.93 0.07 1.9 Right Abutment 0.58 1.0 1.93 0.03 1.9 Left Abutment 0.67 1.0 2.09 0.07 2.0

Right Abutment 0.67 1.0 2.09 0.11 2.0

Simulation Abutment Reference Location

Elevation (ft) Total Scour Elevation at

Abutment (ft) Left Abutment 3011.56 3009.7

Right Abutment 3011.74 3009.8 Left Abutment 3011.56 3009.5

Right Abutment 3011.74 3009.8

Compute Abutment Scour Scour Condition A - Live Bed

Inputs

Q50

Q100

Summary of Results

Q50

Q100

Q50

Q100

*Note: If calculated ys returns a negative answer, the contraction scour depth equals zero.

12” Culvert Waiver And

HY-8 Output

HY-8 Culvert Analysis Report Crossing Discharge Data Discharge Selection Method: Specify Minimum, Design, and Maximum Flow Minimum Flow: 0.00 cfs Design Flow: 0.13 cfs Maximum Flow: 20.00 cfs

Table 1 - Summary of Culvert Flows at Crossing: 2018-2.2-Existing Headwater Elevation (ft)

Total Discharge (cfs)

Culvert 1 Discharge (cfs)

Roadway Discharge (cfs)

Iterations

3011.47 0.13 0.13 0.00 1

3012.21 2.00 2.00 0.00 1

3012.58 4.00 2.91 1.08 14

3012.65 6.00 3.06 2.93 6

3012.70 8.00 3.17 4.82 5

3012.75 10.00 3.26 6.74 5

3012.80 12.00 3.34 8.65 4

3012.84 14.00 3.42 10.58 4

3012.88 16.00 3.49 12.51 4

3012.92 18.00 3.55 14.45 4

3012.95 20.00 3.61 16.39 3

3012.50 2.75 2.75 0.00 Overtopping

Rating Curve Plot for Crossing: 2018-2.2-Existing

Culvert Data: Culvert 1

Table 1 - Culvert Summary Table: Culvert 1 Total Discha rge (cfs)

Culvert Discha rge (cfs)

Headwa ter Elevatio n (ft)

Inlet Contr ol Dept h (ft)

Outle t Contr ol Dept h (ft)

Flo w Ty pe

Norm al Dept h (ft)

Critic al Dept h (ft)

Outl et Dep th (ft)

Tailwa ter Depth (ft)

Outle t Veloci ty (ft/s)

Tailwa ter Velocit y (ft/s)

0.13 cfs 0.13 cfs 3011.47 0.22 0.0* 1- S2n

0.13 0.15 0.13 0.08 2.19 0.73

2.00 cfs 2.00 cfs 3012.21 0.96 0.599 1- S2n

0.53 0.60 0.53 0.37 4.72 1.75

4.00 cfs 2.91 cfs 3012.58 1.33 1.003 5- S2n

0.67 0.73 0.68 0.53 5.14 2.12

6.00 cfs 3.06 cfs 3012.65 1.40 1.074 5- S2n

0.70 0.75 0.70 0.64 5.19 2.37

8.00 cfs 3.17 cfs 3012.70 1.45 1.248 5- S2n

0.72 0.76 0.72 0.74 5.22 2.56

10.00 cfs

3.26 cfs 3012.75 1.50 1.227 5- S2n

0.74 0.77 0.74 0.82 5.24 2.71

12.00 cfs

3.34 cfs 3012.80 1.55 1.335 5- S2n

0.75 0.78 0.75 0.90 5.28 2.85

14.00 cfs

3.42 cfs 3012.84 1.59 1.459 5- S1t

0.77 0.79 0.97 0.97 4.40 2.96

16.00 cfs

3.49 cfs 3012.88 1.63 1.524 4- FFf

0.78 0.80 1.00 1.03 4.44 3.07

18.00 cfs

3.55 cfs 3012.92 1.67 1.608 4- FFf

0.79 0.80 1.00 1.08 4.52 3.16

20.00 cfs

3.61 cfs 3012.95 1.70 1.688 4- FFf

0.81 0.81 1.00 1.14 4.59 3.25

* Full Flow Headwater elevation is below inlet invert.

Culvert Barrel Data Culvert Barrel Type Straight Culvert Inlet Elevation (invert): 3011.25 ft, Outlet Elevation (invert): 3011.00 ft Culvert Length: 7.00 ft, Culvert Slope: 0.0357

Culvert Performance Curve Plot: Culvert 1

Water Surface Profile Plot for Culvert: Culvert 1

Site Data - Culvert 1 Site Data Option: Culvert Invert Data Inlet Station: 0.00 ft Inlet Elevation: 3011.25 ft Outlet Station: 7.00 ft Outlet Elevation: 3011.00 ft Number of Barrels: 1

Culvert Data Summary - Culvert 1 Barrel Shape: Circular Barrel Diameter: 1.00 ft Barrel Material: Corrugated Steel Embedment: 0.00 in Barrel Manning's n: 0.0240 Culvert Type: Straight Inlet Configuration: Mitered to Conform to Slope Inlet Depression: None

Tailwater Data for Crossing: 2018-2.2-Existing

Table 2 - Downstream Channel Rating Curve (Crossing: 2018-2.2-Existing) Flow (cfs) Water Surface

Elev (ft) Velocity (ft/s) Depth (ft) Shear (psf) Froude

Number

0.13 3011.08 0.08 0.73 0.05 0.48

2.00 3011.37 0.37 1.75 0.23 0.59

4.00 3011.53 0.53 2.12 0.33 0.62

6.00 3011.64 0.64 2.37 0.40 0.64

8.00 3011.74 0.74 2.56 0.46 0.65

10.00 3011.82 0.82 2.71 0.51 0.66

12.00 3011.90 0.90 2.85 0.56 0.66

14.00 3011.97 0.97 2.96 0.60 0.67

16.00 3012.03 1.03 3.07 0.64 0.68

18.00 3012.08 1.08 3.16 0.68 0.68

20.00 3012.14 1.14 3.25 0.71 0.69

Tailwater Channel Data - 2018-2.2-Existing Tailwater Channel Option: Trapezoidal Channel Bottom Width: 2.00 ft Side Slope (H:V): 3.00 (_:1) Channel Slope: 0.0100 Channel Manning's n: 0.0350 Channel Invert Elevation: 3011.00 ft

Roadway Data for Crossing: 2018-2.2-Existing Roadway Profile Shape: Constant Roadway Elevation Crest Length: 20.00 ft Crest Elevation: 3012.50 ft Roadway Surface: Gravel Roadway Top Width: 5.00 ft

Crossing Discharge Data Discharge Selection Method: Specify Minimum, Design, and Maximum Flow Minimum Flow: 0.00 cfs Design Flow: 0.13 cfs Maximum Flow: 20.00 cfs

Table 3 - Summary of Culvert Flows at Crossing: 2018-2.2-Proposed Headwater Elevation (ft)

Total Discharge (cfs)

Culvert 1 Discharge (cfs)

Roadway Discharge (cfs)

Iterations

3011.47 0.13 0.13 0.00 1

3012.21 2.00 2.00 0.00 1

3012.58 4.00 2.91 1.08 14

3012.65 6.00 3.06 2.93 6

3012.70 8.00 3.17 4.82 5

3012.75 10.00 3.26 6.74 5

3012.80 12.00 3.34 8.65 4

3012.84 14.00 3.42 10.58 4

3012.88 16.00 3.49 12.51 4

3012.92 18.00 3.55 14.45 4

3012.95 20.00 3.61 16.39 3

3012.50 2.75 2.75 0.00 Overtopping

Rating Curve Plot for Crossing: 2018-2.2-Proposed

Culvert Data: Culvert 1

Table 2 - Culvert Summary Table: Culvert 1 Total Discha rge (cfs)

Culvert Discha rge (cfs)

Headwa ter Elevatio n (ft)

Inlet Contr ol Dept h (ft)

Outle t Contr ol Dept h (ft)

Flo w Ty pe

Norm al Dept h (ft)

Critic al Dept h (ft)

Outl et Dep th (ft)

Tailwa ter Depth (ft)

Outle t Veloci ty (ft/s)

Tailwa ter Velocit y (ft/s)

0.13 cfs 0.13 cfs 3011.47 0.22 0.0* 1- S2n

0.09 0.15 0.10 0.08 3.40 0.73

2.00 cfs 2.00 cfs 3012.21 0.96 0.543 1- S2n

0.36 0.60 0.43 0.37 6.11 1.75

4.00 cfs 2.91 cfs 3012.58 1.33 0.884 5- S2n

0.44 0.73 0.54 0.53 6.67 2.12

6.00 cfs 3.06 cfs 3012.65 1.40 0.943 5- S2n

0.45 0.75 0.56 0.64 6.75 2.37

8.00 cfs 3.17 cfs 3012.70 1.45 1.107 5- S2n

0.46 0.76 0.57 0.74 6.82 2.56

10.00 cfs

3.26 cfs 3012.75 1.50 1.078 5- S2n

0.47 0.77 0.58 0.82 6.87 2.71

12.00 cfs

3.34 cfs 3012.80 1.55 1.178 5- S2n

0.47 0.78 0.59 0.90 6.92 2.85

14.00 cfs

3.42 cfs 3012.84 1.59 1.269 5- S2n

0.48 0.79 0.60 0.97 6.96 2.96

16.00 cfs

3.49 cfs 3012.88 1.63 1.353 5- S2n

0.49 0.80 0.61 1.03 7.00 3.07

18.00 cfs

3.55 cfs 3012.92 1.67 1.432 5- S2n

0.49 0.80 0.61 1.08 7.04 3.16

20.00 cfs

3.61 cfs 3012.95 1.70 1.522 5- S1f

0.50 0.81 1.00 1.14 4.59 3.25

* Full Flow Headwater elevation is below inlet invert.

Culvert Barrel Data Culvert Barrel Type Straight Culvert Inlet Elevation (invert): 3011.25 ft, Outlet Elevation (invert): 3011.00 ft Culvert Length: 7.00 ft, Culvert Slope: 0.0357

Culvert Performance Curve Plot: Culvert 1

Water Surface Profile Plot for Culvert: Culvert 1

Site Data - Culvert 1 Site Data Option: Culvert Invert Data Inlet Station: 0.00 ft Inlet Elevation: 3011.25 ft Outlet Station: 7.00 ft Outlet Elevation: 3011.00 ft Number of Barrels: 1

Culvert Data Summary - Culvert 1 Barrel Shape: Circular Barrel Diameter: 1.00 ft Barrel Material: Concrete Embedment: 0.00 in Barrel Manning's n: 0.0120 Culvert Type: Straight Inlet Configuration: Mitered to Conform to Slope Inlet Depression: None

Tailwater Data for Crossing: 2018-2.2-Proposed

Table 4 - Downstream Channel Rating Curve (Crossing: 2018-2.2-Proposed) Flow (cfs) Water Surface

Elev (ft) Velocity (ft/s) Depth (ft) Shear (psf) Froude

Number

0.13 3011.08 0.08 0.73 0.05 0.48

2.00 3011.37 0.37 1.75 0.23 0.59

4.00 3011.53 0.53 2.12 0.33 0.62

6.00 3011.64 0.64 2.37 0.40 0.64

8.00 3011.74 0.74 2.56 0.46 0.65

10.00 3011.82 0.82 2.71 0.51 0.66

12.00 3011.90 0.90 2.85 0.56 0.66

14.00 3011.97 0.97 2.96 0.60 0.67

16.00 3012.03 1.03 3.07 0.64 0.68

18.00 3012.08 1.08 3.16 0.68 0.68

20.00 3012.14 1.14 3.25 0.71 0.69

Tailwater Channel Data - 2018-2.2-Proposed Tailwater Channel Option: Trapezoidal Channel Bottom Width: 2.00 ft Side Slope (H:V): 3.00 (_:1) Channel Slope: 0.0100 Channel Manning's n: 0.0350 Channel Invert Elevation: 3011.00 ft

Roadway Data for Crossing: 2018-2.2-Proposed Roadway Profile Shape: Constant Roadway Elevation Crest Length: 20.00 ft Crest Elevation: 3012.50 ft Roadway Surface: Gravel Roadway Top Width: 5.00 ft

TR 111 – MP 0.1

DSR 2018-2.6

Hydraulics Report TR 111 – MP 0.1

WV ERFO FS MNGAH921 2018-1(1) DSR 2018-2.6

TABLE OF CONTENTS

1. Purpose

2. Footbridge Structure

2.1. Location

2.2. Existing Geometry

2.3. Proposed Geometry

3. Hydrology

4. Structure Selection

5. Hydraulic Analysis

5.1. Modeling Method and Program Used

5.2. Topographic and Bathymetric Data

5.3. Model Geometry

5.4. Roughness Assignments

5.5. Boundary Conditions

5.6. Modeling of Proposed Structure

5.7. Model Run Control Parameters

6. Results

7. Scour Calculations

7.1. Methodology

7.2. General Scour (Long-Term Degradation)

7.3. Contraction Scour

7.4. Local Scour at Piers

7.5. Local Scour at Abutments

8. Scour Countermeasure Design

LIST OF FIGURES

Figure 1: Project Location Map Figure 2: 3D-Rotated View of SMS Generated Mesh Displaying 1 Foot Contours Figure 3: Mesh Elements Figure 4: Local Mesh Elements Near Footbridge Figure 5: ARR Mesh Quality Plot Figure 6: Boundary Conditions Figure 7: 3D UGrid Proposed Bridge Figure 8: Critical Velocity Equation (HEC-18 Eqn. 6.1) Figure 9: HEC-23 Equation 14.1 Figure 10: HEC-23 Equation 14.2

Table 1: Two Camp Trail footbridge survey geometry Table 2: Two Camp Trail Footbridge Proposed Geometry Table 3: StreamStats Peak Discharge data for Horseshoe Run Table 4: Hydraulic Design, Scour Design, and Scour Design Check Flood Frequencies Table 5: Proposed Conditions Freeboard Results (Design Flood Event – 25-Year) Table 6: Summary of Hydraulic Performance Table 7: Upstream Velocity Comparisons Table 8: Contraction Scour Depth Summary Table 9: Summary of Calculated Scour Table 10: Abutment Riprap Analysis Inputs

1. Purpose The purpose of this report is to assess the proposed hydraulic conditions at DSR 2018-2.6 –Trail 111 (Two Camp Trail), which crosses an unnamed tributary (Tributary) of Horseshoe Run.

2. Footbridge Structure

2.2. Existing Geometry

Trail 111-Two Camp Trail (Trail) crosses over the unnamed tributary to Horseshoe Run with a wooden footbridge. The existing footbridge has the following parameters.

Table 1: Two Camp Trail footbridge survey geometry

Material Bottom Chord

Elevation Top Chord Elevation

Length (ft)

Width (ft)

Wood 1694.15 1695.19 50.0 4.5

Project Location:

LAT: 39.177506

LONG: -79.603284

2.3. Proposed Geometry

The existing timber bridge and abutments will be removed and replaced with a new proposed bridge and footing in a new location. The proposed footbridge has the following parameters.

Table 2: Two Camp Trail Footbridge Proposed Geometry

Material Bottom Chord

Elevation Top Chord Elevation

Length (ft)

Width (ft)

Wood 1697.42 1699.27 42.5 6

3. Hydrology The Tributary is an unregulated and un-gaged stream. The drainage area to the tributary was delineated to be 2.16 acres (ac), with a peak discharge of 1.95 cubic feet per second (cfs) during the 100-year storm event.

However, the Tributary channel and the crossing with the Trail are located within the floodplain of Horseshoe Run. The flood flows from Horseshoe Run are of greater concern for the stability of the footbridge than the run-off from the drainage area to the Tributary, so hydrologic data for Horseshoe Run is used as the basis for the hydraulic model and scour calculations for both existing and proposed conditions.

The hydrology of this site is the same from existing to proposed conditions.

Though Horseshoe Run is within a FEMA Zone A, no hydrologic data or results are provided close the project area. Therefore, the drainage area to Horseshoe Run was delineated using USGS StreamStats and was determined to be 41.9 square miles. The peak flows were estimated with StreamStats. The results for peak flow data are presented below:

Table 3: StreamStats Peak Discharge data for Horseshoe Run

Frequency Flow Rate (ft3/s)

2-YR 2720

10-YR 3410

25-YR 4340

50-YR 5090

100-YR 5900

200-YR 6710

500-YR 7880

Three flood frequencies are used to design and evaluate the proposed bridge and bridge foundations. These are the hydraulic design flood frequency, the scour design flood frequency and the scour design check flood frequency as presented in Table 4.

Table 4: Hydraulic Design, Scour Design, and Scour Design Check Flood Frequencies

Hydraulic Design Flood Frequency

Scour Design Flood Frequency

Scour Design Check Flood Frequency

Q25 Q50 Q100

4. Structure Selection As outlined in the design scoping report, the existing footbridge for the Trail crossing over the Tributary will be removed and replaced with a new crossing structure in a new location.

5. Hydraulic Analysis

5.1. Modeling Method and Program Used

WSP developed a two-dimensional model of the 2019-1.11 crossing using the Sedimentation and River Hydraulics model (SRH-2D) within the Surface-water Modeling System (SMS Version 13.3.7) graphical user interface. SRH-2D was developed by the US Bureau of Reclamation (USBR), with cooperating technical development from the Federal Highway Administration (FHWA). SRH-2D is the preferred river modeling approach by FHWA for river crossing structures.

SRH-2D is a depth-averaged two-dimensional hydraulic model that uses a flexible mesh based finite element analysis of hydraulic systems. The model provides robust analysis of 2D dominated flow fields such as flows with in-stream structures, through bends, perched rivers, side channel and agricultural returns, and braided channel systems.

The design objective of the 2-D hydraulic modeling in this study was to develop depth and velocity inputs for analysis of scour potential at the proposed bridge crossing. Modeling details and results are summarized in the following sections.

5.2. Topographic and Bathymetric Data

Field topographic and bathymetric survey data was collected by JMT in 2022. This survey data extended approximately 180 feet upstream and 150 feet downstream of the footbridge at the trail crossing. Due to the area required for the floodplain model, WSP utilized secondary data from the Randolph County FEMA 2018-2019 dataset, available through West Virginia University, for areas outside the surveyed area, including topography for the channel for Horseshoe Run and the floodplain. These datasets were merged to create a complete terrain within the SMS software. The vertical datum for all sources is NAVD 88, the horizontal datum for all sources is NAD 83. Projection was set to State Plane Coordinate System, Zone:

West Virginia North (FIPS 4701).

Figure 2: 3D-Rotated View of SMS Generated Mesh Displaying 1 Foot Contours

5.3. Model Geometry

The modeling mesh is a collection of vertices, edges, and faces defining the three-dimensional spatial information of the ground surface. SRH-2D uses a finite element mesh to define the geometric surface and set the modeling limits. The SRH-2D modeling for the floodplain of Horseshoe Run extends from approximately 1200 feet upstream of the crossing to approximately 1200 feet downstream. The model domain width was sized to extend a conservative distance to capture the anticipated flood boundaries.

Both the patch and paving mesh type were utilized to create the model mesh. Triangular paving was primarily used in the overbank regions while rectangular patching was used throughout the channel and the crossing. The mesh contains roughly 26,270 elements.

The mesh extents and total number of elements was developed to best capture geometric features impacting conveyance while also minimizing model inefficiencies and increased computational time. Upstream and downstream model extents were set to be far enough upstream and downstream to fully depict any impacts caused by the project. Initially, a coarse mesh was developed using larger elements and used in a simulation.

The model results informed the insertion of additional resolution throughout the domain. This iterative

Figure 3: Mesh Elements

Figure 4: Local Mesh Elements Near Footbridge

Figure 5: ARR Mesh Quality Plot

5.4. Roughness Assignments

A Manning’s roughness of 0.035 was used for the channels area of the Tributary and Horseshoe Run, 0.025 was used for the trail, 0.08 was used for overbank and floodplain areas, and 0.05 was used for the riprap countermeasure extents.

5.5. Boundary Conditions

Peak discharges, determined using StreamStats, were used as Inlet Q boundary conditions in the SRH-2D simulations. The SMS “Channel Calculator” tool was used to compute and assign a normal depth for the outflow boundary condition for all simulations.

Figure 6: Boundary Conditions

5.6. Modeling of Proposed Structure

The SMS 3D Structure coverage was used to generate a 3D UGrid to visualize the bridge in the graphics window and generate a ceiling elevation dataset to represent the bridge pressure zones. The terrain file used

Figure 7: 3D UGrid Proposed Bridge

5.7. Model Run Control Parameters

All SRH-2D model controls were set to a 0.75-second timestep. A time step less than 1-second was found to be necessary to maintain hydraulic computational stability during the model run for all mesh options. All simulations were set to a dry initial condition, and the total period that the simulation run analyzed was set to 5-hours. This was a long enough period to allow the model to reach a steady state solution for all simulation runs. Output frequency was set to 15-minute intervals. The default parabolic turbulence parameter option was selected and set to the default value of 0.7. Each simulation required less than 15 minutes of total run time.

Table 5: Proposed Conditions Freeboard Results (Design Flood Event – 25-Year)

Average Water Surface Elevation

(ft) Freeboard (ft) Proposed Bridge 1697.42 1693.62 3.80

Average Water Surface Elevation values extracted from an SMS arc along upstream face of proposed bridge

Table 6: Summary of Hydraulic Performance

Discharge Q (ft3/s)

Contracted Section Main Channel Average Velocity (ft/s) Average Water Surface Elevation (ft)

Q2 2720 0.3 1691.93 Q25 4340 7.6 1693.62 Q50 5090 8.8 1694.41

Q100 5900 8.9 1694.88 Contracted Section Main Channel Average Velocity from SMS Scour Tool Output Average Water Surface Elevation values extracted from an SMS arc along upstream face of proposed bridge

7. Scour Calculations

7.1. Methodology

The scour analysis performed for proposed conditions for this site was based on the evaluation of several empirical equations outlined in the Federal Highway Administration’s Hydraulic Engineering Circular No.

18 (HEC-18) (FHWA 2012).

While the empirical equations from HEC-18 are considered the best available quantitative methods for the design and analysis of scour at bridge piers, scour estimates should be weighed against historical observations of actual scour in the field and professional judgment of scour potential given the material makeup and conditions of the specific channel. For this analysis, the equations used to produce the scour results are considered conservative.

Four scour processes are considered as part of this analysis:

1. General Scour

2. Contraction Scour

3. Pier Scour (local scour)

4. Abutment Scour

The SMS Bridge Scour tool was used to export model output parameters for analyzing the bridge crossing.

This tool uses arcs to define the approach and contracted sections, bank stations, and abutments and then extracts both main channel and overbank averaged hydraulic parameters for bridge scour analysis.

7.2. General Scour (Long-Term Degradation)

Over time, changes to the channel bed elevations can occur due to the natural trend of the stream or changes within the stream or contributing watershed. Long-term degradation does not include the cutting of the streambed at the bridge due to a significant storm event, that is accounted for following the contraction and local scour analysis. An estimated 1-foot of general scour is applied at this site.

7.3. Contraction Scour

Live Bed Determination (FHWA 2012)

One of the first steps in a scour analysis is a determination of whether live-bed or clear-water contraction scour conditions prevail at the project location. Generally, live-bed contraction scour occurs when bed sediment transport is actively occurring. Clear-water contraction scour occurs when there is no such bed sediment transport. These conditions are determined through comparing the average velocity of the flow in the main channel upstream of the bridge opening against the critical velocity for initiating motion of the D50 size of the channel bed material.

Sediment samples were taken at the proposed crossing location. These samples resulted in an average particle size D50 of 4.2 mm and D95 of 24.4 mm. These particle sizes were used throughout the scour analysis.

The equation for critical velocity is shown in Figure 8.

𝑉𝑉𝑐𝑐 = 𝐾𝐾𝑢𝑢𝑦𝑦

6𝐷𝐷

Figure 8: Critical Velocity Equation (HEC-18 Eqn. 6.1)

Where:

Vc = Critical velocity above which bed material of size D and smaller will be transported, ft/s y = Average depth of flow upstream of the bridge, ft D = Particle size for Vc, ft D50 = Particle size in a mixture of which 50 percent are smaller, ft Ku = 11.17 English units

A summary of model outputs and computations at are presented below:

Table 7: Upstream Velocity Comparisons

Storm Event Approach Section Critical Velocity (ft/s)

Approach Section Average Velocity (ft/s)

Design (50-Year) 0.8 4.3 Check (100-Year) 0.8 4.7

These calculations result in a critical velocity lower than the average velocity of flow in the main channel upstream of the bridge opening. Therefore, there is a livebed scour condition at this site. Spreadsheet based scour calculations wre utilized to carry out the live-bed contraction scour equations which are described in detail within HEC-18 Section 6.

Table 8: Contraction Scour Depth Summary

Storm Event Live Bed Computed Scour Depth (ft) Design (50-Year) 2.3 Check (100-Year) 2.1

7.4. Local Scour at Piers

The proposed footbridge does not have piers; therefore, this scour process was not considered.

7.5. Local Scour at Abutments

HEC-18 NCHRP 24-20 Abutment Scour Approach (FHWA 2012) NCHRP 24-20 methods provide a single combined local and contraction scour depth value based on velocity, depth, and area for the flow obstructed by the abutments. Overbank hydraulic parameters were extracted from the SHR-2D simulations and used to compute abutment scour using the FHWA Hydraulic Toolbox. Considering the proximity of each abutment to the main channel, scour condition location type a (main channel) was selected.

Table 9: Summary of Calculated Scour

Flood Frequency Abutment General Scour (ft)

Contraction Scour (ft)

Abutment Scour (ft)

Total Scour Elevation (ft)

Design (50-Year) Left 1 2.3 6.4 1686.2 Right 1 2.3 6.2 1686.3

Check (100-Year) Left 1 2.1 6.5 1686.1 Right 1 2.1 6.2 1686.4

Note: Abutment Scour includes the contraction scour and long-term scour depths

8. Scour Countermeasure Design The proposed bridge riprap extents are designed to withstand the effects of scour up to and including the 100-year Scour Countermeasure Design Flood Frequency. Riprap along the abutments and pedestrian trail embankments are recommended as scour countermeasures. Riprap protection will be toed down to the 100-year abutment scour elevation.

Methodologies outlined in the FHWA Hydraulic Engineering Circular No. 23 Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance (HEC-23) were used for sizing riprap at abutments.

According to FHWA’s HEC-23 Volume 2 guideline 14, the recommended design equations for sizing rock riprap for spill-through and vertical wall abutments is as presented below.

For Froude numbers ≤ 0.8:

𝐷𝐷50

𝑦𝑦

𝐾𝐾

𝑆𝑆 − 1

𝑉𝑉2

𝑔𝑔𝑔𝑔

Figure 9: HEC-23 Equation 14.1

Where:

D50 = median stone diameter (ft);

V = Characteristic average velocity in the contracted section (ft/s);

S = Specific gravity of the riprap (usually taken as 2.65);

g = Acceleration due to gravity, 32.2 ft/s2.

y = depth of flow in the contracted bridge opening (ft);

K = 0.89 for spill-through abutment, 1.02 for a vertical wall abutment.

For Froude numbers > 0.8:

D50 y

K S − 1

V2 gy

Figure 10: HEC-23 Equation 14.2

K = 0.61 for spill-through abutment, 0.69 for a vertical wall abutment

Inputs to these equations was taken directly from the SMS 100-year simulation results at the proposed

Table 10: Abutment Riprap Analysis Inputs

Froude Number Near Abutments (SMS 100-yr Output) > 0.80

Average Velocity in Contracted Section (SMS 100-yr Output) 8.9

Abutment Type Spill-Through Contracted Section Flow Depth 3.8

Specific Gravity of Rock Riprap 2.65 Gravitational Acceleration 32.2

K 0.61 D50 (ft) 1.3

D50 (mm) 402.9

A Class IV riprap gradation is recommended. The riprap thickness should be at minimum 2.0 times the D50 value.

y (ft)

D50

(mm) D50

(ft) Vc

(ft/s)

Approach section main channel average velocity

V (ft/s) V/Vc < 1?

Contraction Scour Method

Q50 11.17 2.87 0.07 0.00 0.8 4.3 No Live-Bed Q100 11.17 3.23 0.07 0.00 0.8 4.7 No Live-Bed

Summary of Results

Compute Contraction Scour (Check for live-bed or clear-water)

Critical Velocity Determination

[HEC-18 p. 6.2, Eqn. 6.1]

Approach section main channel average depth y1

(ft) V*

(ft/s) T

(ft/s) V*/T k1

Q50 0.016110 2.87 1.22 0.01 122.02 0.69 Q100 0.015066 3.23 1.25 0.01 125.18 0.69

Modeled Scenario

Contracted section main channel average depth yo

(ft)

Approach Section Main Channel Flow

Q1

(cfs)

Contracted section main channel flow

Q2

(cfs)

Approach section main channel width W1

(ft)

Contracted section main channel adjusted width W2

(ft) y2

(ft) ys

(ft) * Q50 3.26 382.6 565.5 31.34 19.70 5.53 2.27

Q100 3.78…