MS NP GUIS 15(4)_Culvert Sizing and Scour Protection Design Evaluation and Recommendations Report_FINAL_adv.pdf

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Table of Contents

Table of Contents ........................................................................................................... i

Section 1 Introduction ................................................................................................ 1-1

1.1 Background .......................................................................................................................................................... 1-1

1.2 Study Objectives ................................................................................................................................................. 1-2

1.3 Site Investigation ............................................................................................................................................... 1-3

1.3.1 Geotechnical Investigation ............................................................................................................... 1-4

1.4 Assumptions ........................................................................................................................................................ 1-4

1.5 Report Organization ......................................................................................................................................... 1-5

Section 2 Methodology ............................................................................................... 2-1

2.1 Environmental Considerations .................................................................................................................... 2-1

2.2 Hydraulic Analysis ............................................................................................................................................. 2-2

2.2.1 Evaluation Criteria............................................................................................................................... 2-3

2.2.1.1 Environmental Compliance .............................................................................................. 2-3

2.2.1.2 Scour Countermeasure Performance ........................................................................... 2-3

2.2.2 Alternatives Analysis .......................................................................................................................... 2-4

2.2.2.1 Scenario 1- Existing 24-inch-diameter Culvert ....................................................... 2-4

2.2.2.2 Scenario 2- 20-foot-wide Open-bottom Bridge Culvert ....................................... 2-4

2.2.2.3 Other Evaluation Scenarios .............................................................................................. 2-5

2.3 Geotechnical Analysis ...................................................................................................................................... 2-6

2.3.1 Culvert Scour Countermeasures .................................................................................................... 2-7

2.3.1.1 Riprap ......................................................................................................................................... 2-7

2.3.1.2 Articulated Concrete Block ............................................................................................... 2-8

2.3.2 Embankment Scour Countermeasures ....................................................................................... 2-9

2.3.2.1 Riprap Armoring Seaward Embankment ................................................................... 2-9

2.3.2.2 Alternative Countermeasures Seaward Embankment ..................................... 2-10

2.3.2.3 Countermeasures at Landward Embankment ...................................................... 2-11

Section 3 Results ........................................................................................................ 3-1

3.1 Hydraulic Analysis ............................................................................................................................................. 3-1

3.2 Geotechnical Analysis ...................................................................................................................................... 3-3

3.2.1 General ...................................................................................................................................................... 3-3

3.2.2 Culvert Scour Countermeasures .................................................................................................... 3-3

3.2.3 Embankment Scour Countermeasures ....................................................................................... 3-3

3.2.4 Recommended Scour Countermeasures .................................................................................... 3-4

3.3 Environmental Analysis .................................................................................................................................. 3-4

3.3.1 Culvert Sizing and EFH Enhancement ......................................................................................... 3-5

3.3.2 Scour Protection ................................................................................................................................... 3-6

3.4 Additional Considerations .......................................................................................................................... 3-10

3.4.1 Adaptive Management Approach ............................................................................................... 3-10

3.4.2 Minimize Slopes on the Road Embankment .......................................................................... 3-10

3.4.3 Countersunk Box Culverts ............................................................................................................. 3-10

3.4.4 Pedestrian and Bike Lane Boardwalk Scour Protection................................................... 3-11

Section 4 Planning Level Cost Estimate........................................................................ 4-1

Section 5 Summary and Recommendations................................................................. 5-1

5.1 Results Matrix ..................................................................................................................................................... 5-1

Section 1• Introduction ii

5.2 Final Recommendation ................................................................................................................................... 5-2

List of Figures

Figure 1. Typical Class IV Riprap Scour Protection ......................................................................................... 2-4

Figure 2. Park Road Crossing Existing Conditions ........................................................................................... 2-4

Figure 3. 20-foot Bridge Culvert on Robert McGhee Road ........................................................................... 2-5

Figure 4. 10' x 2' Box Culvert with Angled Wing Walls .................................................................................. 2-6

Figure 5a. Schematic Plan- Typical Open-bottom Culvert Riprap Installation .................................... 2-7

Figure 5b. Schematic Section- Typical Open-bottom Culvert Riprap & Subgrade Stabilization .. 2-8

Figure 6. ACB Schematic Section ............................................................................................................................. 2-9

Figure 7. ACB Installation Sequence .................................................................................................................... 2-10

Figure 8a. ACB Construction Detail Section ...................................................................................................... 2-11

Figure 8b. ACB Construction Detail Plan ............................................................................................................ 2-11

Figure 8c. ACB Installation at Indian River Drive ........................................................................................... 2-11

Figure 9. Typical Salt Marsh Vegetated Slope .................................................................................................. 2-12

Figure 10. MDOT Common Native Roadside Wildflowers of Mississippi .............................................. 3-8

Figure 11. Two-lane Highway with Maintained Vegetated Side Slopes and Tidal Marsh Toe ...... 3-9

Figure 12. Vegetated ACB at Reedy Creek MP 736.55 in Sharon, MS ...................................................... 3-9

List of Tables

Table 1. Comparison of Model Outputs for Several Culvert Design Scenarios .................................... 3-2

Table 2. Comparison of Cost Items ......................................................................................................................... 4-1

Table 3. Matrix Summarizing the Results of Culvert Sizing and Scour Protection Design

Evaluations of Six Alternatives ................................................................................................................................ 5-2

Table 4. Matrix Summarizing the Costs of Embankment Scour Protection Alternatives ............... 5-2

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Section 1

Introduction

1.1 Background

As part of the larger Gulf Islands National Seashore NP-GUIS 15(3) Bike and Pedestrian Use

Enhancements at Park Road project, CDM Smith is tasked with reviewing and evaluating alternative solutions for the replacement of an existing culvert and associated scour protection related to the widening of Park Road at East Stark Bayou to accommodate a new bicycle and pedestrian lane. Park Road serves as the public entrance to Gulf Islands National

Seashore in Jackson County, Mississippi, and significant bicycle and pedestrian traffic share the existing two-lane road with vehicular traffic.

At present, land use in the vicinity of this road segment includes estuarine emergent wetlands

(i.e., tidal marsh), forested wetlands, and open waters. An existing 24-inch-diameter culvert passes under the road, allowing a tidal channel to flow in an east-west direction. Marsh floodplains abut the channel on either side of the embankment. The tidal channel is, on average, 50-feet wide on either side of the crossing. The current culvert serves as a bottleneck, limiting the flow of water, nutrients, sediment, and the movement of fish and other aquatic organisms between the area east of Park Road and the rest of Stark Bayou. Over time, sediment and debris have partially clogged the culvert, further restricting the flow of water, impeding fish passage, and leading to differences in the prevalent vegetation on either side of the culvert.

FHWA has overseen the design of the road widening project and have engaged internal and consultant resources to advance the design to the 90%-complete stage. CDM Smith was previously engaged to assist FHWA with an environmental assessment (e.g., wetland delineation), compliance (e.g., Section 401/404 permitting), and mitigation for minor impacts of the Park Road project on the adjacent marsh and tidal channel as a result of an increased footprint for the crossing. Mitigation initially included a combination of marsh creation, for which CDM Smith completed the design, controlled burns to enhance forested wetlands, and improved hydrologic connectivity between marsh areas on either side of the Park Road crossing to improve designated Essential Fish Habitat (EFH).

Increasing the size of the culvert will improve hydrologic connectivity and allow for a more natural tidal hydrologic regime, while also facilitating the use of the enhanced wetland by invertebrates and vertebrates. As water flow improves, it is anticipated that plant species composition on each side of the culvert would become more similar, with an increase in smooth cordgrass on the eastern side of Park Road. Restoring the free movement of fish, wildlife, and aquatic organisms by replacing the 24-inch-diameter pipe with a much larger structure would improve EFH and wetland habitats east of Park Road by allowing a much greater and more natural interaction of fauna with the physical and floral components of that habitat. Improvement of the hydrologic regime of the channel and associated wetlands and improved connectivity for fish passage would be seen immediately after the culvert has been replaced. Noticeable improvements to the habitats on either side of the Park Road crossing would likely occur gradually over the following 2-5 years, as the system reaches a new equilibrium.

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To improve the degree and rate of water exchange in this area which will benefit EFH, a 20-foot-wide, open-bottom box culvert was proposed by the NPS in the Environmental

Assessment/Finding of No Significant Impact and carried forward by FHWA in the design plans, specifications, and estimate. The size of the proposed culvert is based on the dimensions of the bridge culvert on Robert McGhee Road, approximately 0.35 miles northwest of the Park Road crossing. A hydraulic analysis was not conducted to size the proposed culvert. This reference culvert on Robert McGhee Road spans a much larger tidal channel with a larger contributing watershed than the one crossed by Park Road. The recommended scour protection for the new culvert incudes heavy (Class 4) riprap to protect the immediate vicinity of the structure, and recommendations for embankment stabilization that include partially grouted riprap (PGR) to protect the seaward (i.e., west side) embankment and bioengineering to protect the landward (i.e., east side) embankment.

FHWA requested CDM Smith provide an independent, third-party review of the proposed 20-foot structure and riprap scour protection to determine if other viable options are available to reduce the project cost, improve constructability, and improve aesthetics. The third-party review included an analysis to consider smaller culvert sizes that would adequately improve flow, water quality, and EFH. Additionally, scour countermeasures were evaluated to achieve safety and stabilization goals while minimizing impacts to the surrounding marsh community and providing a desired natural aesthetic and habitat value.

In addition to the 90% design plans, the CDM Smith reviewed the Hydraulic Evaluation for the

Gulf Islands National Seashore Park Road Improvement Project – Final Report (“WSP Hydraulic

Report”) (WSP 2019) completed in July 2019 to provide additional information on the basis for design. FHWA also provided geotechnical information in the form of boring logs and a laboratory testing report, 90% design cost estimates, and feedback from National Park Service

(NPS) on the design and history of the Park Road crossing. These project materials were used to inform the evaluation of the existing design and proposed alternatives.

1.2 Study Objectives

In support of the goals of this project – primarily the expansion of the roadway for bicycle and pedestrian use and improved tidal exchange to enhance EFH – the assessment of the Park

Road culvert and scour protections was completed consisted the following objectives:

Evaluate the current culvert sizing and design to evaluate if a smaller culvert(s) would meet the purpose and need for the project, including design objectives and EFH improvements.

Evaluate whether several smaller, open-bottom box culverts could provide the same benefits as one large, open-bottom box culvert.

Evaluate scour protection to determine whether the size and/or layout of the countermeasures can be reduced while still providing adequate protection of the road and culvert.

Prepare a design analysis and recommendations hydrology and hydraulics report summarizing the culvert hydraulic analyses and potential alternatives to reduce costs and provide for a more natural aesthetic design.

Section 1 • Introduction

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1.3 Site Investigation

For the purpose of this review and evaluation, an on-site investigation was not performed to inform the alternatives analysis. Instead, CDM Smith relied upon the site visit discussed in the previously performed hydraulic analysis (WSP Hydraulic Report), along with notes from a field visit conducted by Ben Medina, a project engineer with the FHWA Eastern Federal Lands

Highway Division (EFLHD), and personal communication with Mr. Gregg Gollot, a resident who lives adjacent to the marsh on the east side of Park Road.

Relevant observations from these sources are summarized below.

• Erosion Conditions (Mr. Medina, FHWA):

o The vegetation on the sides of the embankment was healthy with no bare soil or other signs of erosion.

• Existing Scouring (WSP Hydraulic Report):

o The base of most of the pilings [supporting the boardwalk] showed minimal scour. One piling, however, showed moderate scour up to 12” deep and 24” wide.

o The structure [on Robert McGhee Road] showed some signs of scour, notably the displacement of Class III riprap and eroded soil at the abutments.

o Overall, site conditions suggest that scour around the existing culvert [on Park

Road] has been neither negligible nor excessive.

• Past Flooding (Mr. Gollot, Nearby Resident):

o There is no recollection of Park Road overtopping since Hurricane Katrina

(2005) until Tropical Storm Cristobal (June 2020), when cars were observed driving across Park Road through several inches of water.

o An estimate of the water surface elevation of overtopping for Park Road during Katrina is around 15 feet (NAVD88), based on observations from property owners at nearby residences. While anecdotal, this elevation is significantly less than the 22 feet estimated in the previous hydraulic analysis.

o Pavement repairs were required along Park Road after Katrina, but there is no recollection of damage to the roadway embankment.

o There were no reported damages or specific erosion problems reported as a result of overtopping during Cristobal.

o Flood conditions are not affected by the upstream drainage area. Rather, they are solely a function of flooding from storm surge.

These observations appear to validate that the Park Road crossing has not historically experienced damaging erosion or flooding events and upland runoff is insignificant.

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1.3.1 Geotechnical Investigation

A previous subsurface investigation was conducted to observe the subsurface conditions at the project site on September 2017. The drilling program consisted of four (4) test borings

(17BH-1 through 17BH-4) advanced to depths of 20 to 52 feet-below ground surface (ft-bgs) by using 3 ¾" hollow-stem auger drilling technique. Split spoon samples were typically collected continuously to 20 ft-bgs and at five-foot intervals thereafter to borehole termination depths in accordance with ASTM D1586. Undisturbed (Shelby tube) samples were collected from Borings 17BH-1, 17BH-2, and 17BH–3, at depths 18, 20, and 15 ft-bgs, respectively, in accordance with ASTM D1587. Geotechnical laboratory testing was conducted by S&ME on two Shelby tube samples collected in the test borings to assist in estimating engineering properties. The laboratory test program consisted the following:

• Materials finer than No. 200 sieve in mineral aggregates by washing (AASHTO T 11- 05)

• Sieve analysis of fine and coarse aggregates (AASHTO T 27-14)

• Determining the liquid limit of soils (AASHTO T 89-13)

• Determining the plastic limit and plasticity index of soils (AASHTO T 90-16)

• Laboratory determination of moisture content of soils (AASHTO T 265-15,)

• Consolidated, undrained triaxial compression test on cohesive soils (AASHTO T 297- 94)

• One-dimensional consolidation properties of soils (AASHTO T 216-07)

The following subsurface conditions were interpreted from the test borings and geotechnical laboratory test results, and our understanding of the regional geology.

• The test borings typically encountered embankment fill, underlain by the coastal deposits.

• The embankment fill was encountered from roadway surface to 7.5 ft-bgs at the north side of the existing culvert and 12.5 ft-bgs at the south side of the existing culvert. The fill typically consisted of very loose to medium dense silty sand to fine sand with gravel.

• The coastal deposits consisted of very soft to soft clay, clay with peat, silty clay, fat clay with SPT N values ranging from 0 to 4 and very loose to medium dense fine sand, coarse sand, clayey sand, silty sand with gravel, poorly graded sand with silt with SPT N-values ranging from 0 to 14.

1.4 Assumptions

After review of the 90% design and other materials supplied by FHWA supporting the design, the following assumptions for the CDM Smith evaluation were noted:

• The evaluation of the site is based on existing conditions only and is focused on the culvert crossing and approximately 275 feet of adjacent embankment.

• All elevations are based on NAVD88.

• Future sea level rise (SLR) conditions are not analyzed as part of the evaluation.

• Relevant results of previously performed field surveys related to the Park Road project (geotechnical analysis, hydrologic and hydraulic information, and environmental criteria required to meet EFH mitigation and permitting requirements) will be sufficient to perform an adequate evaluation and inform recommendations.

• A hydraulic analysis was not previously completed to determine the size of the culvert in the 90% design.

Section 1 • Introduction

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• The team will reference the design standards in the FHWA Federal Lands Project Development and Design Manual (PDDM), particularly Chapter 6 – Geotechnical and Chapter 7 – Hydrology & Hydraulics.

• The culvert structure must be open-bottomed.

• Only alternatives that will reduce the size of the culvert and the footprint of scour protection, while also meeting design and mitigation objectives, will be considered.

• Up to four of the most suitable culvert design alternatives will be provided in a matrix that describes the pros/cons and costs to allow for easy comparison of feasible alternatives.

• For scour protection, alternatives will include a minimum of one riprap alternative and one additional design alternative (e.g., PGR, articulated concrete block, A-jacks, bioengineering, etc.).

• Cost estimates for the 90% design will be used for guidance and comparison.

1.5 Report Organization

The material presented in this report is organized into five sections. After this introductory section, Section 2 offers a description of the methodology used to evaluate environmental constraints, hydraulic analysis, and geotechnical analysis of the culvert sizing and scour protection design and defines the scenarios that were considered for this report. Section 3 provides the results of the environmental, hydraulic, and geotechnical analyses, as well as comparison tables of culvert and scour protection designs. Section 4 provides planning-level cost estimates, including culvert structure costs and relevant countermeasure costs of each recommended alternative design. Section 5 provides a summary of findings in a results matrix and final recommendations for culvert and scour protections designs.

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Section 2

Methodology

The task methodology included evaluation of existing and proposed culvert configurations from environmental, hydraulic capacity, and geotechnical performance perspectives.

2.1 Environmental Considerations

Prior to the construction of Park Road, the project area was an estuarine emergent wetland with a tidal creek system, referred to as East Stark Bayou. Historically, East Stark Bayou would have had unrestricted hydrologic connectivity to Halstead Bayou and Davis Bayou. The project area continues to contain estuarine emergent marsh; however, construction of Park Road and the resulting reduced hydrologic connectivity has altered the conditions of the marsh, particularly the composition of species present.

As part of the overall environmental compliance associated with the project, in email correspondence dated September 25, 2019, NOAA Fisheries determined that the proposed EFH mitigation of enhancing 4.95 acres of emergent marsh through the installation of an open-bottom culvert would mitigate for project EFH impacts. According to the NOAA EFH Mapper and the Gulf of Mexico Fishery Management Council GIS database, EFH has been designated for the following species and guilds in the East Stark Bayou area: red drum, shrimp, reef fish, and coastal migratory pelagics. Given the habitats present in the vicinity of the Park Road crossing, the area would most likely be used by red drum and white and brown shrimp. According to NOAA

Fisheries, the project would need to provide access to the 4.95-acre area east of Park Road to red drum and white and brown shrimp by naturally flushing with the ebb and flow of the tides and positively affecting water quality parameters such as salinity, dissolved oxygen, temperature, and turbidity. The agreed-upon ratio of enhancement to impact for EFH in the estuarine emergent wetlands is 23:1 (enhancement : impact).

As currently designed, the existing 24-inch-diameter culvert under Park Road conveying the tidal channel of East Stark Bayou will be replaced with an open-bottom 20-foot wide box culvert. A larger hydraulic opening associated with the box culvert will enhance EFH marsh areas to the east of Park Road by improving hydrologic connectivity, improving water quality, and facilitating the movement of fish, wildlife, and aquatic organisms.

Restoring the free movement of fish, wildlife, and aquatic organisms would improve wetland habitat east of Park Road by allowing a much greater and more natural interaction of fauna with the physical and floral components of that habitat. Improvements to the hydrologic regime of the wetland and fish passage would be seen immediately after the culvert has been replaced.

Noticeable improvements to channel and marsh habitats would likely occur gradually over the following 2-5 years.

Because one of the primary drivers for the project is improving EFH on the east side of Park Road, the existing design and all proposed alternatives to culvert sizing and scour protection are evaluated through an environmental lens to confirm EFH enhancements will be realized.

Section 2• Methodology

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While NOAA Fisheries did not identify specific performance or success metrics for EFH improvements, requirements for improved EFH would include the following:

• Sustaining suitable flows and water depths within the culvert.

• Providing suitable native substrates to facilitate aquatic organism movements and passage.

• Improving the exchange of water, nutrients, and sediments on either side of the Park

Road crossing, thereby improving water quality and habitat suitability.

• Reducing the potential for impacts to the surrounding marsh and tidal channel.

The USFWS Habitat Suitability Index (HSI) models for larval and juvenile red drum and northern

Gulf of Mexico brown and white shrimp provide additional specific habitat requirements for these target species with designated EFH:

• Salinity is an important factor in red drum hatching success. Eggs float at salinities > 25 ppt and sink at lower salinities. Reduced nearshore salinity drastically reduces hatching success.

• Larval and juvenile red drum require shallow (< 3 m), quiet bays with little current, muddy bottoms, and either submerged vegetation or emergent marsh.

• Brown and white shrimp spawn offshore, but post-larval shrimp move into estuaries and stay in these nurseries along the marsh-water interface until they transform into juveniles.

• Both shrimp species prefer muddy substrates and salinities < 30 ppt.

• Total yields of adult shrimp are directly proportional to the area of estuarine vegetation.

When marshes are separated from estuaries with levees or bulkheads, food resources are unavailable to shrimp and densities of post-larval shrimp and juveniles in the estuary are lower.

Increasing the tidal exchange on each side of the Park Road crossing through a larger structure should allow for an increase in red drum and shrimp habitat suitability on the east side of the road.

2.2 Hydraulic Analysis

In order to evaluate the culvert sizing for Park Road, a previously developed 2D hydraulic model was obtained from FHWA and modified to simulate each of the culvert scenarios. The hydraulic model used was SRH-2D, a two-dimensional sediment and river hydraulics software utilized within SMS (Surface-water Modeling System) as the pre and post processor. The basic parameters of the hydraulic model (n-values, surge hydrograph, etc.) were not changed, but the embankment dimensions of Park Road were modified to reflect the dimensions indicated in the

90% design plans, and the mesh was edited to account for the different culvert configurations.

Section 2 • Methodology

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The previous hydraulic evaluation showed that the maximum flow velocities occurred when the water surface elevation was at the top of the road (i.e. water surface elevation of approximately

5’), rather than when the water surface elevation was significantly higher than the roadway. The top of the road was also determined to be at an elevation less than the 8-year surge elevation.

The Federal Lands Highway Project Development and Design Manual (PDDM) classifies structures spanning 20 feet or greater as a bridge. Per the PDDM, the design criteria for a bridged waterway is the 50-year event (whereas the design criteria for a structure spanning less than 20 feet is the 25-year event). Therefore, the initial hydraulic analyses were based on the 50-year surge hydrograph as this encompassed all design criteria relevant to sizing the culvert opening. It is understood that a design variance will be required for the 25- or 50-year design criteria based the fact that there are no plans to raise the road elevation and that the road currently overtops at less than an 8-year surge event, based on the previous hydraulic study.

2.2.1 Evaluation Criteria

The analyses were based on two factors to evaluate the suitability of each alternative:

environmental compliance and scour countermeasure performance.

2.2.1.1 Environmental Compliance

The environmental compliance of an analysis alternative was evaluated based on meeting the mitigation requirements for EFH, due to the expansion of Park Road, as described in Section 2.1.

For adequate EFH mitigation along the tidally influenced creek, a soft bottom habitat must be maintained and the emergent marsh on the east side of Park Road must be functionally accessible for fish habitat. Consequently, the criteria chosen to demonstrate environmental compliance of an alternative include:

• facilitation of aquatic organism passage, as discussed in Section 7.5.1.3 of the PDDM, and

• minimal head loss between the western and eastern portions of the marsh on either side of the proposed culvert.

An equilibrium in the water surface elevation on both sides of Park Road shows that there is adequate exchange between the two areas of marsh. Adequate tidal exchange of water, nutrients, and sediments will improve access to and increase the suitability of EFH east of the Park Road crossing.

2.2.1.2 Scour Countermeasure Performance

For the analyses completed as part of this study, the scour countermeasures recommended for the 90% design plans based on the previous study represented the largest extent of scour protection.

The performance of scour countermeasures, designed to reduce the likelihood of local scour for proposed alternatives, was evaluated to confirm that the proposed scenarios would not result in additional scour countermeasures for the channel and marsh in the vicinity of the culvert greater than those recommended for the 90% design plans (i.e., Class 4 riprap in the vicinity of the culvert, three feet deep within 15 feet of the culvert).

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Figure 1. Typical Class IV riprap scour protection for open bottom structure (Conewago Enterprises)

2.2.2 Alternatives Analysis

Hydraulic evaluations completed for this study included analysis of the existing 24-inch-diameter culvert system, the 20-foot wide bridge structure represented in the 90% design plans, and other additional conveyance alternatives, along with their performances based on the evaluation criteria previously discussed.

2.2.2.1 Scenario 1 – Existing 24-inch-diameter Culvert

Scenario 1 evaluated the existing 24-inch-diameter culvert that is proposed to be replaced in conjunction with the widening of Park Road. This evaluation was performed to provide an understanding of how much better the alternative configurations would perform in meeting the environmental criteria.

Figure 2. Park Road crossing existing conditions: 24”-diameter culvert with wing walls (Google Earth)

2.2.2.2 Scenario 2 – 20-foot-wide Open-bottom Bridge Culvert

Scenario 2 evaluated the 20-foot-wide, open-bottom bridge culvert with proposed scour countermeasures as shown in the 90% design plans. Since the effects on EFH for the Park Road

2-5 expansion project were determined based on this structure, it provided the basis for comparison of the other scenarios. Additional scenarios were expected to substantially meet or exceed the performance of this baseline structure in order to be considered valid alternatives.

Figure 3. 20-foot bridge culvert on Robert McGhee Road (WSP 2019)

The baseline 20-foot-wide structure modelled in Scenario 2 is similar to the bridge culvert on

Robert McGhee Road northwest of Park Road on a larger tidal creek tributary of Stark Bayou

(Figure 3).

2.2.2.3 Other Evaluation Scenarios

Scenario 3 consists of a 10-foot wide culvert with a flat headwall. A 10-foot span was used as a reasonable starting point for hydraulic comparison because it represented a mid-point, with the understanding that shorter (or longer) spans would be evaluated depending on the results of the evaluation. The geotechnical evaluation determined that this span was the minimum size culvert that could be used without increasing the riprap class (see Section 2.3.1.1), so no smaller spans were evaluated. Spans between 10-foot and 20-foot would be valid options to meet the goals and objectives of this project, but would be expected to costs more and would present additional issues for construction related to site access and the desire to maintain access along Park Road during construction.

Scenario 4 evaluated the construction of two, 5-foot-wide culverts with flat headwalls to provide greater constructability options.

Scenario 5 evaluated the 10-foot-wide culvert with 45-degree wingwalls. Wing walls were evaluated to determine if they significantly improved the water exchange between the west and east marsh areas, or if they affected the scour velocities in the vicinity of the culvert. While 45-degree wingwalls were used in the model, the actual angle of these walls should be determined based on the sideslopes and other site constraints during the design process (Figure 4). Any variation in the design angle would be expected to make only a nominal difference in the performance of the structure.

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Figure 4. 10’ x 2’ box culvert with angled wing walls

An additional scenario was considered, which consisted of a 10-foot-wide culvert with flanking relief culverts consisting of four, 36-inch-diameter reinforced concrete pipes (RCPs) on each side of the culvert. However, this option was discounted due to negligible hydraulic or environmental benefits.

A final validation of the recommended option was to run the model using an average tide hydrograph to ensure that the tidal exchange between the west and east marsh was adequate.

2.3 Geotechnical Analysis

In order to evaluate culvert and embankment scour countermeasures, the following sources were used:

• results from the hydraulic analyses during this study,

• bridge and wave scour analyses conducted by WSP (WSP Hydraulic Report),

• findings of WSP site visit (November 2018),

• subsurface geotechnical investigation results, and

• FHWA HEC-23 guidelines.

In addition, riprap sizing for embankment scour countermeasures were verified by following

Design Standard 13 – Embankment Dams by U.S. Department of Interior, Bureau of Reclamation.

The WSP Hydraulic Report (2019) recommended Class 4 riprap for the culvert scour countermeasure and either Class V riprap or partially grouted riprap (PGR) for seaward embankment wave scour countermeasures. During this study, riprap as potential culvert and embankment scour countermeasures was re-evaluated based on new hydraulic scenarios.

Additionally, an alternative countermeasure for each potential scour location was evaluated.

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2.3.1 Culvert Scour Countermeasures

2.3.1.1 Riprap

Riprap is a very common scour countermeasure for culvert foundations. It is a proven method and has limited constructability concerns or environmental impacts. Due to the relatively low flow velocity at the Park Road crossing, evidence of Class 3 riprap displacement at the nearby

Robert McGhee Road structure and considering tolerable damage to the riprap from the worst wave conditions, Class 4 riprap is suitable to protect the culvert, apron, and wingwalls. Selection of Class 4 riprap is based on the culvert velocity of 3.63 f/s and upstream head of 4.7 feet. An increase in flow velocity would result in an increase of riprap size to the next higher class, (i.e., Class 5 riprap). Note that the average median particle weight (D50) of Class 4 and Class 5 riprap is

300 pounds and 500 pounds, respectively. The larger the riprap, the greater the subgrade bearing capacity required. As per FHWA HEC-23 guidance, the riprap should extend the entire length of the culvert wall and wingwalls (upstream and downstream) and cover the embankment slopes to

20 feet from the abutment walls. The riprap should be flushed below the thalweg elevation and be extended to the scour depth but not less than 2.25 times the actual D50 or 1.50 times the actual

D100 riprap size, whichever results in the greater thickness.

Due to weak coastal deposits, a minimum of 12 inches of crushed filter stone (AASHTO #57) bedding on top of the subgrade is required for a competent riprap foundation. The crushed filter stone and riprap can potentially settle down in weak subgrade conditions. To reduce settlement and improve the bearing capacity, a nonwoven geotextile supplemented with a heavy- duty triaxial geogrid (Tensar TX150 or equal) is required at the interface between the subgrade and crushed stone bed. For Class 5 riprap, such ground improvement methods might not be strong enough to resist the tensile failure and excessive creep settlement caused by larger-sized riprap.

Also, a layer of nonwoven geotextile is required at the interface of the riprap and crushed stone to protect the crushed stone in position. Figures 5a and 5b demonstrate the schematic plan and section of the proposed riprap described above.

Figure 5a. Schematic Plan – Typical Open-bottom Culvert Riprap Installation (modified from HEC 23, FHWA)

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Figure 5b. Schematic Section – Typical Open-bottom Culvert Riprap and Subgrade Stabilization (Modified from

INDOT Standard Drawing)

2.3.1.2 Articulated Concrete Block (ACB)

ACB systems have been used for streambank revetment, full-channel armoring, and countermeasures for bridge pier scour as an alternative to riprap. For the open-bottom culvert scour countermeasures, ACB with non-interlocking block geometries (cabled system) can be used by following FHWA HEC-23 guidance. A minimum of 9-inch-thick blocks are required. The block system must be sloped away from the culvert inlet and outlet such that the depth of the system is greater than the maximum scour.

A minimum 9-inch-thick granular stone filter layer (AASHTO #57) is required that extends fully beneath the ACB system to protect the subgrade scour. As stated above for riprap, to reduce settlement and provide competent base for the ACB system, a heavy-duty triaxial geogrid (Tensar TX150 or equal) can be applied in the interface of the subgrade and filter layer. A geotextile filter layer should be secured properly to the bottom of the ACB system (i.e., on filter stones). The seal where the ACB meets the culvert wall, the geotextile should be secured to the wall to reduce the potential for leaching of the bed materials from the joint. Figure 6 shows the schematic section of the proposed ACB system.

Although the ACB system has better constructability than riprap, it is prone to uplift pressured and displacement during extreme events. Also, there is limited performance data available for ACB associated with open-bottom culverts. For these reasons, ACB was discounted as a viable channel scour countermeasure option.

Crushed Stone Bedding (Min. 9 inches)

Class IV Riprap

Pile Foundation Nonwoven Geotextile below the

Geogrid

2-9

Figure 6. ACB Schematic Section (Modified from INDOT Standard Drawing)

2.3.2 Embankment Scour Countermeasures

2.3.2.1 Riprap Armoring Seaward Embankment (West Side Park Road)

In accordance with U.S. Bureau of Reclamation Design Standard No 13-Embankment Dams and ASCE Hazard Tools, riprap Class 4 with an application thickness of twice the D50 is sufficient to protect the seaward embankment against wave scour. Note that smaller stone than D50 Class 4 riprap is not recommended for embankment revetments because of tendency to move in response to the wave action. Alternatively, larger Class 5 riprap can also be used.

Although the rock is glued together, the PGR does not sacrifice the structural flexibility that is essential for the weak ground condition. The flexible nature of PGR system demonstrates “self-healing” properties to accommodate small settlement and shifting of underlying materials. Also, the PGR allows for use of smaller/lighter rocks than common ripraps while maintaining the scour resistance. Despite these benefits of the PGR, the 90% design recommends a 3-feet thick PGR that could cause the embankment instability due to weak subsurface conditions. Also, the PGR is expensive and a difficult to install than traditional riprap. The PGR consists of specifically sized rock with the grout filling only at 1/3 to 1/2 of the total void space (HEC-23). The topsoil for vegetation on the PGR could compromise the resiliency of structural integrity between the topsoil and the PGR. Because the interconnected void in the PGR facilitates to pass water freely and the PGR does not choke the topsoil. In the tidal setup, the topsoil trapped into the PGR voids would be constantly pulled out and the topsoil blanket could be pushed up from the position.

As discussed above, the weak subgrade conditions may not have the capacity to support the thick riprap revetment and comparatively thick PGR. Adding a thicker riprap section on the top of the embankment could potentially damage the embankment. Also, the rock armoring does not meet the aesthetic preferences of the NPS.

Pile Foundation

Geotextile Geogrid

Filter

2-10

2.3.2.2 Alternative Countermeasures Seaward Embankment (West Side Park Road)

Open-cell vegetated ACB (Contech ArmorFlex or equal) is an alternative to riprap for protection of the seaward embankment. Open-cell ACB is an interlocking matrix of concrete blocks of uniform size, shape, and weight connected by a series of cables which pass longitudinally through pre-formed ducts in each block. The open cell units have an open area of 18-23%, as measured from the base of the mat. Parallel strands of cable extend through two cable ducts in each block, allowing for longitudinal binding of the units within the mat and subsequent ability of the blocks to move freely along the cable. Each row of units should be laterally offset by one-half block width from the adjacent row, so that any given block is cabled to four other blocks.

The ACB system should be placed on a minimum of 6-inch-thick crushed stone filter foundation (AASHTO 57). A geotextile layer is secured at the interface of ACB system and crushed stone filter for additional stability. A heavy-duty geogrid (Tensar TriAx 150 or equal) should also be placed on the compacted subgrade before placing the filter stone. Finally, another layer of geotextile is required at the interface of the filter layer and subgrade soils.

An embankment slope of 3H:1V or flatter is desired for the vegetated ACB system. The gentle embankment slope helps keep topsoil in position. The top opening of ACB partially acts as a key to hold topsoil in place. The topsoil can be seeded and/or planted with native vegetation to add roughness, create habitat, and incorporate a more natural aesthetic. Figure 7 shows installation of open-cell vegetated ACB.

Figure 7. ACB installation sequence: (a) Sub-grade preparation for ACB installation; (b) ACB installation with geotextile/filter fabric; (c) Post-construction fully-vegetated ACB Installation (Contech Engineered Solutions)

The ACB is a promising functional alternative to the riprap as a durable erosion protection system and a proven aesthetic structure. Also, it is easy to install and has very similar or cheaper project costs than large riprap. The ACB system also adds approximately one-sixth the weight of the proposed riprap and has the tendency to distribute loads. It should be noted that the installation of the ACB system should avoid the submerged condition. Figure 8 (a) and (b) show the construction details of an ACB slope stabilization project in North Carolina. Figure 8(c) shows the ACB installation at tidal environment.

(a)

(b)

(c)

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Figure 8a. ACB Construction Detail Section (Glenville Lake Dam Rehabilitation, Fayetteville, NC)

Figure 8b. ACB Construction

Detail Plan (Glenville Lake Dam

Rehabilitation, Fayetteville, NC)

Figure 8c. ACB Installation at Indian River Dirve, St.

Lucie, FL (Contech Engineered Solutions)

2.3.2.3 Countermeasures at Landward Embankment (East Side Park Road)

In contrast to the recommendations of the WSP Hydraulic Report but in agreement with the 90% design, the evaluation determined that structural stabilization is not required for the landward embankment on the eastern side of the crossing. This embankment is protected and only experiences low-energy forces associated with free flow during storm events in which the road is overtopped. To provide habitat value and promote a natural aesthetic, the landward embankment should incorporate a native roadside stabilization seed mix in over turf and grasses typical of roadside stabilization designs. This native grass slope can incorporate a pollinator wildflower mix for additional habitat and aesthetic benefits and can be mowed annually to maintain appropriate site lines and disperse seeds of annual species. The seed mix may be supplemented by native

2-12 plantings, particularly native marsh plantings at the toe of slope. Figure 9 provides a section diagram of how a typical estuarine salt marsh slope would be vegetated. The NPS seed mix now specified in the SCRs, which include a mix of grasses and native wildflowers suitable for upland slopes, would be appropriate for the upper landward embankment slopes upgradient of the influence of estuarine hydrology. The lower slopes and toe of slope should be seeded and/or planted with native species more typical of the adjacent estuarine marsh (e.g., saltmeadow cordgrass, smooth cordgrass, black needlerush, saltgrass, etc.). Actual species composition for the lower Park Road side slopes would be specified during the design phase.

Figure 9. Typical salt marsh vegetated slope (Patrick Lynch)

3-1

Section 3

Results

Below is a summary of the results from the environmental review, hydraulic analyses, and geotechnical evaluations for the proposed culvert and scour protection improvements on the

East Stark Bayou crossing segment of Park Road.

3.1 Hydraulic Analysis

As discussed in Section 2, the existing conditions for the Park Road embankment with a 24-inch-diameter culvert pipe (Scenario 1), and the 20-foot-wide open-bottom box culvert

(Scenario 2) were modeled to establish the baseline conditions and design constraints for comparisons with proposed alternatives.

For each alternative scenario, velocities and depths from the model output were evaluated on both the east and west sides of the culvert at:

1. the face of the culvert (i.e., inlet and outlet locations),

2. 15 feet from the face of the culvert, which is the limit of scour countermeasures shown in the previously performed hydraulic analysis, and

3. 30 feet from the face of the culvert, which was determined to be far enough away from the effects of culvert hydraulics to reasonably reflect the balance of water surface elevations associated with the exchange of water between the two marsh areas.

Each scenario was evaluated using the 50-year surge hydrograph to evaluate the maximum velocities throughout the surge event and the exchange of water between the marsh areas prior to the overtopping of Park Road. As expected, the hydraulic model indicated that the velocities around the culvert decrease substantially by replacing the 24-inch-diameter pipe culvert with a 20-foot-wide span, taking the maximum velocity from 8.34 ft/s down to 2.12 ft/s. Similarly, the water surface elevations balanced more efficiently with the larger opening.

In the existing conditions (Scenario 1), the WSE in the marsh east of Park Road was shown to be 0.67 feet lower than in the marsh west of Park Road, whereas the difference in Scenario 2 was negligible at 0.01 feet.

Scenario 3, a 10-foot-wide open-bottom box culvert, was selected because it represented a mid-point between the existing 24-inch-diameter pipe culvert and the 20-foot-wide open-bottom box bridge culvert. Reducing the size of the span from 20 feet to 10 feet increased the maximum velocities and increased the difference in water surface elevations on each side of

Park Road. However, it did so within an acceptable variation of the values for the 20-foot culvert span. The maximum velocities increased from 2.12 ft/s for the 20-foot-wide span to

3.63 ft/s for the 10-foot-wide span. While this difference is acceptable for the proposed riprap channel countermeasures, it was determined through the geotechnical evaluation that the

Class 4 riprap proposed for the 20-foot-wide span would be sufficient for Scenario 3, as well.

Therefore, any structure with a wider span than 10-foot could be used, and the velocities would range between 2.12 ft/s and 3.63 ft/s, generally fitting a linear relationship between the two widths.