Final Bridge Hydraulic Studya123.g.akamai.net/7/123/11558/abc123/forestservic... · 2017-07-21 · The bridge routines in HEC-RAS allow the modeler to analyze a bridge with several

TOBIAS-FLYNN BRIDGE Within the City of Sedona, Arizona

FINAL BRIDGE HYDRAULIC STUDY

June 2013

Prepared for: Red Rock Ranger District Coconino National Forest

8375 State Route 179 Sedona, Arizona 86341

(928) 203-7500

Submitted by: Timothy Huskett, E.I.T., Design Engineer

Southwest Environmental Consultants, Inc. 20 Stutz Bearcat Drive #6

Sedona, Arizona 86336 (928) 282-7787

Tobias-Flynn Bridge Final Bridge Hydraulic Study

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Table of Contents SECTION 1: INTRODUCTION ................................................................................................. 3

1.1 Purpose of Study .......................................................................................................................................... 3

1.2 Location of Study ......................................................................................................................................... 3

1.3 Study Description ........................................................................................................................................ 3

SECTION 2: EXISTING HYDRAULIC ANALYSIS ................................................................. 6 2.1 Previous Hydraulic Models ..................................................................................................................... 6

2.2 Hydrology ....................................................................................................................................................... 7

2.3 Hydraulic Modeling Approach ............................................................................................................... 8

SECTION 3: PROPOSED HYDRAULIC ANALYSIS ........................................................... 10 3.1 Proposed Bridge ....................................................................................................................................... 10

3.2 Hydraulic Modeling Approach ............................................................................................................ 10

3.3 Hydraulic Performance of Proposed Conditions ......................................................................... 10

3.4 Comparison of Water Surface Profiles............................................................................................. 11

SECTION 4: STABILITY AND SCOUR ASSESSMENT ..................................................... 12 4.1 Scour Assessment ..................................................................................................................................... 12

4.2 Contraction Scour ..................................................................................................................................... 12

4.3 Abutment Scour ........................................................................................................................................ 12

4.4 Pier Scour..................................................................................................................................................... 13

SECTION 5: CONCLUSION .................................................................................................... 17


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Table of Contents (cont’d) LIST OF APPENDICES

Appendix A: HEC-RAS Duplicate Model Results Appendix B: HEC-RAS Corrected Model Results Appendix C: HEC-RAS Existing Conditions without Base Flow Channel Model Results Appendix D: HEC-RAS Existing Conditions with Base Flow Channel Model Results Appendix E: HEC-RAS Alternative B Model Results Appendix F: HEC-RAS Alternative C Model Results Appendix G: Photographs Appendix H: Cross Section Map Appendix I: FEMA Floodplain Maps

LIST OF FIGURES Figure 1-1: Study Area Figure 1-2: Left Overbank near Alternative B Figure 1-3: Right Overbank near Alternative B Figure 2-1: Gauge Station Historical Graph Figure 4-1: I-17 Bridge at Dry Creek Abutment Scour Figure 4-2: I-17 Bridge at Dry Creek Pier Scour Figure 4-3: Pier Scour at I-17 Bridge Figure 4-4: Scour Depth and Width Figure 5-1: Natural Exposed Bedrock Figure 5-2: Exposed Bedrock

LIST OF TABLES Table 2-1: Old vs New Model Table 2-2: Hydraulic Model Comparison Table 3-1: Summary of Results from HEC-RAS Model of Proposed Conditions at XS 3772 Table 3-2: Summary of Results from HEC-RAS Model of Proposed Conditions at XS 4650 Table 3-3: Comparison of Water Surface Profiles on Oak Creek Table 4-1: Scour Summary for Proposed Design


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SECTION 1: INTRODUCTION 1.1 Purpose of Study This report summarizes a proposed plan to construct a bridge across Oak Creek within Nation Forest System lands. The study involved the analysis of two alternatives. This report has been prepared for the United States Forest Service Red Rock Ranger District for the Coconino National Forest and is based on conceptual designs and should not be used for construction purposes. 1.2 Location of Study The reach of Oak Creek being evaluated for the improvements is located within the city limits of Sedona, Arizona within Yavapai County more specifically within the East portion of Section 24 of Township 17 North, Range 5 East of the Gila and Salt River Base and Meridian. The reach begins 250 feet west of the Yavapai/Coconino County Boundary and ends approximately 2,900 feet west of the Yavapai/Coconino County Boundary. Figure 1.1 shows the portion of Oak Creek being analyzed within this report. Figure 1-1 Study Area

1.3 Study Description The portion of Oak Creek analyzed within this report is a well contained perennial stream surrounded by steep cliff walls within a rural portion of Sedona, Arizona. The profile of the creek is overall mildly sloped but portions of the creek especially near the proposed bridge locations

PROJECT LOCATION


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have deep pools. Oak Creek is currently designated a FEMA Zone AE Special Flood Hazard Area as shown in the effective Flood Insurance Rate Map (FIRM), Map Number 04025C1435G dated September 3, 2010. Zone AE floodplain areas are subject to inundation by the 1-percent annual chance flood event determined by detailed methods. Zone AE areas also have determined and shown Base Flood Elevations (BFE). The base flow channel, which typically carries a discharge of 32 cubic feet per second, primarily consists of gravel and cobble material with some areas containing medium to large boulders. The toe of the bank consists of deep rooted perennial grasses which provide stability to the stream banks which include, but not limited to, deer grass. The floodplain overbanks are dominated by dense brush and mature riparian trees (Figure 1-3). In addition the overbanks have considerable medium sized bedrock and exposed rock outcrops near the main channel as shown in Figure 1-2. Figure 1-2 Left Overbank near Alternative B

Soils of the Oak Creek flood plain are developing in alluvium from mixed sources, and comprise deep bouldery and very bouldery sands, and riverwash, 0 to 5%. They are susceptible to wind erosion, have a low water holding capacity, and are subject to frequent flooding. Soils of the steep slopes adjacent to the flood plain are developing in place on hills, 15 to 40% slopes, from sandstone residuum, and are comprised of moderately deep extremely gravelly fine sandy loams, and shallow extremely gravelly fine sandy loams. Erosion hazards are only moderate because of the gravelly nature of the soils.


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Figure 1-3 Right Overbank near Alternative B


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SECTION 2: EXISTING HYDRAULIC ANALYSIS The hydraulic analysis of Oak Creek was performed using HEC-RAS version 4.1.0, a 1-dimentional hydraulic simulation program developed and maintained by the Hydrologic Engineering Center of the U.S. Army Corps of Engineers. A steady flow analysis with a mixed flow regime was used to evaluate the hydraulic performance of Oak Creek. Four hydraulic models were developed for this report:

• Duplicate Model • Corrected Model • Existing Conditions Model without Base Flow Channel • Existing Conditions Model with Base Flow Channel

The limitation of HEC-RAS is that it is a one-dimensional analysis therefore assumes all flow moves along a singular dimension. For a given cross section, all of the flow is assumed to move either downstream, or all of it moves upstream, along the singular dimension. The consequence of this is that there is only one water surface elevation (WSEL) and one total flow for a given time step at a given cross section. All the other variables for a given cross section within the profile output table are derived from the stage and flow values. HEC-RAS was chosen as the most excepted software for modeling the hydraulic conditions of Oak Creek. The Federal Emergency Management Agency (FEMA) accepts this software for developing Flood Insurance Studies (FIS) and developing Flood Insurance Rate Maps (FIRM). In addition the floodplain of Oak Creek was historically developed using HEC-1 which is the foundation for the currently HEC-RAS software. HEC-RAS contains a bridge routine which uses user-defined cross-sections in the computation of energy losses due to the bridge structure. The bridge routines in HEC-RAS allow the modeler to analyze a bridge with several different methods without changing the bridge geometry. The bridge routines have the ability to model low flow, low flow and weir flow, pressure flow, pressure and weir flow, and highly submerged flows. HEC-RAS was recommended by the Forest Service for this analysis. 2.1 Previous Hydraulic Models A hydraulic model of Oak Creek had been previously developed in accordance with FEMA guidelines in preparation of the FEMA Flood Insurance Rate Map for Oak Creek within Yavapai County. The original model was prepared using HEC-2. A duplicate model for Oak Creek was attempted using the HEC-2 data within HEC-RAS but the original water surface elevations could not be duplicated (See Appendix A). Table 2-1 summarizes the water surface elevations difference between the HEC-2 Model and the recently developed Duplicate HEC-RAS Model. The difference in water surface elevation can be associated with the development of the hydraulic modeling software from HEC-2 to HEC-RAS.


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Table 2-1 Old vs New Model

Cross Section

100-yr WSEL (ft-NGVD 29) FEMA HEC-2 Model

HEC-RAS Duplicate

Model Difference in

WSEL

FG-5486 4093.1 4095.5 2.4 FF-4915 4085.6 4089.8 4.2 FE-3791 4079.9 4080.4 0.5 FD-2568 4064.4 4065.2 0.8 FC-1250 4054.5 4054.5 0.0

Once a duplicate model that most closely matches the original FEMA hydraulic model was established the topography of the cross sections were updated to reflect the current conditions to create a Corrected Model (See Appendix B). The Corrected Model only used those cross sections from the Duplicate Model that could be located on the FEMA Flood Insurance Rate Map (FIRM) for Oak Creek, a total of 5 cross sections were included within the Corrected Model. The Corrected Model was then used to build an Existing Conditions Model without Base Flow Channel (See Appendix C) or Current Model which contained an additional 11 delineated cross sections using the most recent aerial topography. The Existing Conditions Model without Base Flow Channel contains delineated cross sections which does not include the topography of the base flow channel since the aerial topography captures the top surface of the water flowing within Oak Creek. Table 2-2 summarizes the determined water surface elevations from each model.

Table 2-2 Hydraulic Model Comparison

Cross Section

100-yr WSEL (ft-NAVD 88) Difference in WSEL (ft)

Duplicate Model

Corrected Model

Current Model

Current - Duplicate

Current - Corrected

FG-5486 4098.1 4100.1 4100.6 2.5 0.5 FF-4915 4092.4 4095.0 4095.0 2.6 0.0 FE-3791 4083.0 - - N/A N/A

3772 - 4080.6 4083.9 N/A 3.3 FD-2568 4067.8 4069.4 4068.8 1.0 -0.6 FC-1250 4057.1 4057.1 4057.1 0.0 0.0

The Duplicate Model, Corrected Model and Existing Conditions Model without Base Flow Channel were developed using a steady flow analysis with a subcritical flow regime. Cross Section FE was not used in the Corrected and Existing Conditions Model without Base Flow Channel since this cross section was at a location that overlayed the location of a proposed bridge. Cross Section 3772 was implemented in place of Cross Section 3791 which is located immediately upstream of the proposed bridge location approximately 20 feet downstream of Cross Section FE. The locations of Cross Sections FC through FG are in correlation with FEMA FIRM Map Number 04025C1460F dated June 6, 2001 to match the original Flood Insurance Study (FIS) performed on Oak Creek.


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2.2 Hydrology The hydraulic models used in this study were developed using the peak discharges for Oak Creek published in the Flood Insurance Study for Yavapai County, Arizona dated June 6, 2001. The 10-year, 50-year, 100-year and 500-year flows listed in the FEMA FIS at the Yavapai/Coconino County boundary are 9,450, 20,300, 26,900 and 45,650 cubic feet per second (cfs) respectively. The creek has an average base flow of 32 cubic feet per second determined using USGS Gauge Station 09504420 identified as Oak Creek Near Sedona, AZ (See Figure 2-1) which contains approved discharge data from 1982 to present. This station is located approximately 2 miles upstream of the project area and is managed by the USGS Arizona Water Science Center Flagstaff Field Office. It is desired to base the hydraulic calculations on the most defensible hydrologic discharges. Figure 2-1 Gauge Station Historical Graph

2.3 Hydraulic Modeling Approach A hydraulic model of Oak Creek, identified as Existing Conditions Model with Base Flow Channel (See Appendix D) was developed from approximately 1,100 feet downstream of proposed Alternative B bridge location to approximately 600 feet upstream of proposed Alternative C bridge location. This model differs from the previous Existing Conditions Model without Base Flow Channel since this model contains topography developed with a recent survey of the base flow channel integrated with the aerial topography. The upper and lower limit


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locations of the model were chosen to allow for the potential hydraulic impacts of a bridge crossing Oak Creek to be encompassed within the model. The downstream water surface elevation (WSEL) or boundary condition was set for each modeled discharge by HEC-RAS as a known water surface elevation, extracted from the most recent FEMA Flood Insurance Study for Oak Creek except the Base Flow starting water surface which was determined based on Normal Depth calculations. The geometry for the model cross sections was delineated in AutoCAD from topographic mapping data supplied by Yavapai County. The county topography was based on the vertical datum North American Vertical Datum of 1988 (NAVD 88). In addition to the county supplied topography, SEC Inc. performed addition survey to capture the topography of the low flow channel which had to be incorporated into the delineated cross sections from the county topography which did not define the low flow channel. Model cross sections were placed at locations to correspond to FEMA delineated cross sections and to effectively determine the hydraulic properties of Oak Creek with and without the bridges. Cross section locations are displayed in Appendix A. Channel and overbank roughness estimates were extracted from the HEC-2 model of Oak Creek previously developed within the Flood Insurance Study for Yavapai County Flood Control District. Manning’s roughness coefficient values for the main channel were set between 0.045 and 0.06 for a channel with a rock bed, while the overbanks were between 0.09 for moderately vegetated areas and 0.15 for areas with mature trees and large boulders. During high flows some vegetation such as small brush and grasses become negligible consistent with low Manning’s Roughness Coefficients. Hydraulic models for analyzing the existing condition and proposed condition for each bridge alternative were performed for the 10-year, 50-year, 100-year, and 500-year peak flow rates and for the average Base Flow.

SECTION 3: PROPOSED HYDRAULIC ANALYSIS 3.1 Proposed Bridge A hydraulic model was developed for two proposed bridge locations along Oak Creek. The conceptual alternatives are intended for planning level analysis, and are not considered to be sufficient for purposes of construction. Each recommendation will require further study and analysis to refine the solution for implementation and development of construction documents. 3.2 Hydraulic Modeling Approach The proposed condition was modeled in HEC-RAS to determine and compare the impacts on the water surface elevations for the various peak discharges, and to provide hydraulic design information for the structural design of the bridge and computing maximum scour depths. The hydraulic modeling approach for the proposed condition is essentially the same as discussed for the existing conditions except for inclusion of bridge location and layout. As shown in Appendix H Alternative B bridge is modeled between cross sections 3689 and 3772, Alternative C bridge is modeled between cross sections 4568 and 4650. 3.3 Hydraulic Performance of Proposed Conditions


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Appendix E and F contain cross section plots and a detailed table of results from the proposed condition HEC-RAS model for each alternative bridge location. Table 3-1 summarizes the results of the hydraulic analysis of Alternative B and Table 3-2 summarizes the results of the hydraulic analysis of Alternative C.

Table 3-1 Summary of Results from HEC-RAS Model of Proposed Conditions at XS 3772 Recurrence

Interval Proposed

Water Surface Elev.

Main Channel Ave. Velocity

Ave. Velocity at

Pier 1

Ave. Velocity at

Pier 2

Ave. Velocity at

Pier 3

Ave. Velocity at

Pier 4 (yrs) (ft, NAVD) (fps) (fps) (fps) (fps) (fps) 10 4077.8 9.81 2.27 0.97 2.42 2.58 50 4081.5 12.95 3.76 2.04 3.43 4.09 100 4083.3 14.29 4.43 2.52 3.88 4.77 500 4087.5 17.22 5.92 3.56 4.86 6.28

Table 3-2 Summary of Results from HEC-RAS Model of Proposed Conditions at XS 4650 Recurrence

Interval Proposed

Water Surface Elev.

Main Channel Ave. Velocity

Ave. Velocity at Pier 1



(yrs) (ft, NAVD) (fps) (fps) (fps) (fps) 10 4085.2 9.38 1.13 2.09 0.52 50 4089.7 13.97 2.21 3.44 1.86 100 4091.7 15.98 2.78 4.06 2.47 500 4096.5 20.19 4.41 5.41 3.82

3.4 Comparison of Water Surface Profiles Table 3-3 was developed to summarize the change in the 100-year Water Surface Elevation (WSEL) between the existing water surface elevation and the proposed installation of either Alternative B or Alternative C. The table shows the calculated Water Surface Elevations for the existing Oak Creek floodplain and the change in the water surface elevation with either Alternative B or Alternative C bridge installation. The final two columns of the table show the calculated difference in water surface elevation between the existing water surface elevation and the water surface elevation after either bridge alternative has been implemented.


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Table 3-3 Comparison of Water Surface Profiles on Oak Creek

Cross Section

100-yr WSEL (ft-NAVD) Difference in WSEL (ft)

Existing Bridge Alternative B

Bridge Alternative C

ALT B - No Bridge

ALT C - No Bridge

5200 4097.1 4097.1 4097.1 0.0 0.0 4915 4094.6 4094.6 4094.6 0.0 0.0 4650 4091.6 4091.6 4091.7 0.0 0.1

Bridge ALT C

4568 4090.6 4090.6 4090.6 0.0 0.0 4400 4089.0 4089.1 4089.0 0.1 0.0 4200 4087.3 4087.4 4087.3 0.1 0.0 3949 4084.8 4084.9 4084.8 0.1 0.0 3772 4083.0 4083.3 4083.0 0.3 0.0

Bridge ALT B

3689 4082.1 4082.1 4082.1 0.0 0.0 3500 4078.5 4078.5 4078.5 0.0 0.0 3160 4073.9 4073.9 4073.9 0.0 0.0 2568 4067.0 4067.0 4067.0 0.0 0.0

The results of the HEC-RAS hydraulic model for 100-year storm event indicates that the installation of a bridge at either Alternative B or Alternative C will result in a small increase in the 100-year Water Surface Elevation in comparison to the existing water surface elevation of Oak Creek. If Alternative B were implemented then there would be a maximum 0.3 foot rise in the 100-year water surface near the bridge. If Alternative C were implemented then there would be a maximum 0.1 foot rise in the 100-year water surface near the proposed location. There also would be no rise in the water surface elevation for the Base Flow since no piers or bridge structures would be located within the base flow channel.

SECTION 4: STABILITY AND SCOUR ASSESSMENT 4.1 Scour Assessment A scour assessment was conducted using HEC-RAS bridge scour analysis tool for each bridge alternative. Three types of scour were analyzed; contraction scour, pier scour and abutment scour. The scour calculations were performed in accordance with the Federal Highway Administration document HEC-18, “Evaluating Scour at Bridges” (Richardson et al 2001). During the 100-year storm event Bridge Alternative B has 4 piers subject to flooding while Bridge Alternative C has 3 piers subject to flooding. The following sections discuss the potential scour components and the approach to assessing the scour potential. For the scour assessment the 100-year design flow rates was used as this will demonstrate a worst case scenario. Scour will not be an issue during the typical Base Flow event considering the already established channel for conveying the base discharge and no piers are located within the Base Flow Channel.


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4.2 Contraction Scour Contraction scour is the general lowering of the streambed within the bridge opening waterway. It usually occurs over most or the entire bridge opening. Contraction scour is the result of constrictions in the floodplain flow area caused by the bridge structure and roadway embankments. In the case of the proposed design, minor flow area constriction is caused by the placement of a pier in the floodplain. The contraction scour calculations were performed in accordance with the Federal Highway Administration document HEC-18, “Evaluating Scour at Bridges” (Richardson et al. 2001). A 100-year discharge rate of 26,900 was used for the scour calculations. The 100-year flow velocities do not exceed the critical velocity for entraining sediment across the entire flow width in the project reach. As a result of the low velocities, clear-water contraction scour is expected across the entire bridge waterway. In clear-water contraction scour, sediment transport into the contracted section is essentially zero and maximum scour occurs when shear stress reduces to critical shear stress of the bed material. The computed contraction scour depth for both bridge alternatives is zero for the 100-year flood. 4.3 Abutment Scour Abutment scour is localized, deep erosion that occurs at bridge abutments. It is caused by the redirection of flow that is exerted by road embankments and the abutment itself. For Bridge Alternative B a scour depth of 0.55 feet was calculated for the right overbank abutment and no scour at the left overbank abutment since the abutment is out of the 100-year flood area. For Bridge Alternative C no abutment scour is expected because both right and left overbank abutments are not located within the 100-year flood area. Figure 4-1 I-17 Bridge at Dry Beaver Creek Abutment Scour


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4.4 Pier Scour Pier scour is localized, deep erosion that occurs at bridge piers. High velocity flow against a pier causes an intense, horseshoe-shaped, horizontal vortex at the upstream end of the pier and along both sides. A vertical vortex forms just downstream in the wake of the pier. The horseshoe vortex and wake vortex exert erosive power on the stream bottom at the base of the pier. The depth of pier scour is affected by:

• Velocity and depth of the flow • Width of the pier • Shape of the pier • Attack angle of the flow in relation to the axis of the pier • Length of the pier if the flow is not aligned with the pier • Competence of the streambed material to resist scour such as bedrock or course cobble

and boulder For this report the Base Flow, 10-year, and 100-year flood events were used to determine the potential scour at the proposed alternative bridge locations. The pier scour calculations were computed using a circular cylinder shaped pier with a width of 5 feet and a length of 32 feet. The flow was assumed to be aligned with the pier axis because the relatively straight and uniform planform of the channel and floodplain. Figure 4-2 I-17 Bridge at Dry Beaver Creek Pier Scour


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Figure 4-3 Pier Scour at I-17 Bridge

Table 4-1 summarizes the potential pier scour computed for each proposed bridge alternative during the Base Flow, 10-year, and 100-year storm events. This summary does not account for the effects of scour-resistant bedrock or scour countermeasure mitigations. The detailed scour calculations are included in Appendix E and Appendix F.

Table 4-1 Pier Scour for Proposed Alternative Design

Pier Base Flow Event 10-yr Event 100-yr Event (depth in feet) (depth in feet) (depth in feet)

Alternative B

1 0.00 5.29 7.77 2 0.00 3.37 5.88 3 0.00 5.90 7.68 4 0.00 5.65 8.06 Alternative C

1 0.00 0.00 5.85 2 0.00 5.53 7.89 3 0.00 2.30 5.76 4 0.00 0.00 0.00 5 0.00 0.00 0.00 6 0.00 0.00 0.00


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Using Figure 7.18 of the Federal Highway Administration HEC-18 Evaluating Scour at Bridges, Fifth Edition, dated April 2012, the top width of the scour hole at the piers can be estimated to be 2 times the depth of the scour. Figure 4-4 Scour Depth and Width

Based on scour calculations during a typical Base Flow discharge no pier scour, abutment or contraction scour will be caused by the proposed installation of Bridge Alternative B or Bridge Alternative C. During the 10-year storm discharge Bridge Alternative B will experience pier scour along all four piers. Pier scour depths for Alternative B is shown within Table 4-1. No abutment or contraction scour will occur at Alternative B for the 10-year event. During the 100-year peak discharge Bridge Alternative B will experience pier scour along all four piers at depths shown within Table 4-1. Along the right abutment a scour depth of 0.55 feet will occur and no contraction scour is predicted at this time. For Bridge Alternative C two out of the proposed six piers will experience scour at depths shown within Table 4-1. No abutment or contraction scour will occur at Alternative C for the 10-year event. During the 100-year peak discharge Bridge Alternative C will experience pier scour along three out of the six proposed piers at depths shown within Table 4-1. No contraction or abutment scour will occur at Alternative C for the 100-year event. In addition there will be no scour during the Base Flow event since no pier structures will be located within the Base Flow Channel.


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SECTION 5: CONCLUSION

It should be noted that no specific or final bridge design has been selected at the time of this report. The alternative bridge locations described within this report are best from an engineering approach on finding the most direct and efficient crossing of Oak Creek. The scour depths calculated within Section 4 of this report are the predicted maximum depths based on observed physical conditions. There may be some piers which are located within the natural bedrock of Oak Creek and therefore have no scour as calculated in the previous section. Figure 5-1 Natural Exposed Bedrock

Scour may be reduced by pinning the foundation of the piers to bedrock or armoring the surrounding areas of the piers with scour resistant material such as concrete or rip rap at widths greater than the predicted scour hole widths. For both bridge alternatives contraction scour will not have an impact therefore there will be no scour along the entire width of the floodplain which could lower the local streambed at the bridge which in turn changes the flow characteristics of the upstream and downstream channels. Any scour that occurs because of the installation of a bridge will be localized to the piers or abutment. Effective scour countermeasures would be incorporated in the final design. The installation of any structure within the floodplain of Oak Creek will have a small rise in the 100-year Water Surface Elevation and scour around the proposed piers but both bridge alternatives were laid out to reduce the amount of impact to the area while still serving the needs of the potential users. The scour at the bridge location will be limited to localized scour at the proposed piers or abutment. There will be no scour during the Base Flow event.


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During the 100-year storm event, Bridge Alternative B would have 4 piers subject to flooding while Bridge Alternative C would have 3 piers subject to flooding, and potentially experience pier scour. In addition, the Alternative C pier located on the left-overbank may be scour resistant considering the amount of exposed bedrock located within the area, therefore only two piers may experience localized scour. Figure 5-2 Exposed Bedrock

Documents

Final Bridge Hydraulic Studya123.g.akamai.net/7/123/11558/abc123/forestservic... · 2017-07-21 · The bridge routines in HEC-RAS allow the modeler to analyze a bridge with several