Scour is addressed elsewhere in this Manual for cases involving hydraulics for bridges (Chapter 9), reservoirs (Chapter 12), and storm water management (Chapter 13). Scour occurring in a coastal environment is fundamentally different than riverine scour. In the coastal environment, the primary driver of scour is from head differences across a structure driven by storm tide, which are propagating inland. Riverine scour is typically opposite in direction and is driven primarily by storm runoff, resulting in different mechanisms and directions for scour considerations. Many coastal structures require analyzing for both riverine and coastal scour, and some scenarios require evaluation of combined effects. classifies three levels of approach for coastal scour which will be discussed later. A Level 3 analysis should be evaluated by a precertified TxDOT engineer with experience in coastal processes.

This section focuses on scour due to coastal exposure including storm surge, waves, and tidal currents. Scour should be assessed or evaluated for most projects, especially in locations that may have significant water velocities at the large scale or localized levels. The potential for scour is influenced by a combination of factors including water velocity, structure configuration, bathymetry, bay/inlet configuration, sediment characteristics, and wave climate. Of particular interest for TxDOT projects is the potential for scour at bridges and roadway embankments. Scour can occur along roadway embankments from several scour mechanisms including wave action, weir-flow, and shoreline change. These mechanisms will be discussed in the next section.

All bridges need to be evaluated for scour (23 CFR 650.313(e) and FHWA, HEC-18, Evaluating Scour at Bridges). Especially susceptible locations are areas with abrupt changes in bathymetry, or tidal inlets that can cause flow constrictions or focus wave energy, and areas of active wave breaking. Other resources available to provide in-depth information, equations, and procedures to perform coastal scour include the First and Second volumes of , Highways in the Coastal Environment, and (Sections 9.7 and 9.8).

Scour results from the erosive action of flowing water, which may remove and carry away material from the bed, banks of waterway, and from around coastal structures, such as piers and abutments. Different materials scour at different rates. Loose granular soils are rapidly eroded by flowing water, while cohesive or cemented soils are generally more scour-resistant.

Determining the magnitude of scour risk at structures in coastal waters is complex, as they are at the confluence of concurrent hydraulic forces (e.g., currents and breaking waves) that arise due to the interface of the land and water. Reviewing site conditions can lead to a better anticipation of the types of scour that may be present and should be evaluated. Once the types of existing structures or planned structures are known, the list of typical scour mechanisms can be reviewed for applicability. Table 15-10 lists common scour mechanisms. Examples of many of these mechanisms are also shown in Figure 15-21.

Table 15-10: Common Types of Scour Mechanisms

General Scour Mechanisms	Additional Coastal Scour Mechanisms
Scour around piers and abutments	Roadway damage by wave attack
Erosion along toe of highway embankment
Erosion of embankment due to overtopping flow
Long term vertical degradation of stream bed	Roadway and railway damage by coastal “weir-flow”
Horizontal migration of stream banks
Debris impact on structure	Roadway damage by bluff erosion and shoreline recession
Clogging due to debris causing redirection of flow

Hydraulic analysis must consider the magnitude of design storms, characteristics and geometry of the tidal inlet, estuary, or bay, and the long-term effects due to placement of the bridge or other transportation asset. Structures located in coastal environments must also consider scour resulting from flow in two directions due to tidal fluctuations. In addition, the analysis must consider the long-term effects of normal tidal cycles. This can be achieved by modeling and reviewing the time-dependent tidal flows, velocities, and depth on the following processes:

Geotechnical investigations are commonly used to evaluate scour mechanisms. Understanding the local soil and rock properties is important, as it provides a basis for describing common engineering properties of geomaterials and how different materials may behave under various conditions. Geotechnical investigation can include surface samples, borings, and SEDflume testing. Surface samples and boring can be evaluated via Unified Soil Classification System (USCS) classifications, gradation, plasticity index, and hydrometer testing.

Due to the dynamic nature of sediment transport in the coastal zone, most technical guidance is based on empirical equations, experience, and field observations. The type of sediment located throughout the study area and suspended in the water column should be considered and equations based on available data used as appropriate. One area of focus that can vary the level of analysis needed or types of equations used is the type of soil to be evaluated, such as cohesive, non-cohesive, coarse, or erodible rock. For non-cohesive material scour equations, it is important to know the D

50

and D

90

sediment diameters. For cohesive materials, when possible, it is important to know the erosion rate and critical velocity of sediment particles. These values can also be derived empirically if the D

50

and classification are known.

Long-term sediment budgets are also important for understanding areas prone to scouring. The long-term sediment budget can be generally characterized by typical sediment movement within the littoral zone. The littoral zone is the zone from the shoreline to just beyond the breaking wave zone. Littoral drift is the movement of beach material in the littoral zone by waves and currents. An aggrading or stable waterway may exist if the sediment supply to the project area from the littoral drift is large. Such a situation may minimize the effect of contraction scour, and possibly local scour. Conversely, long-term degradation, contraction, and local scour can be exacerbated if the sediment from littoral drift is reduced. Important information for evaluating the effect of littoral drift includes historical information, future (dredging, installation of jetties, etc.), and sources of sediment. The

Shoreline Change

subsection provides a more in-depth discussion for long-term shoreline change and sediment budgets.

Once the project location is determined to be coastal (as described and shown in the maps in Section 1), the asset type to be designed or evaluated is chosen (such as a bridge), and the site’s geotechnical background is developed, then the scour mechanism(s) that apply can be evaluated. Besides the list in Table 15-10, the USACE provides examples of locations that have a high potential for coastal scour (Figure 15-22). Other methods of determining the observed or potential scour mechanism include reviewing maintenance reports, reports from community engagement, and photos after storms.

Guidance on methods for evaluating scour analysis in tidal waterways are outlined in , , , , and USACE .

Understanding sediment budgets and geomorphic existing conditions provides a valuable tool for evaluating the potential for scour in tidal waterways. In most cases, this principle is not easy to quantify without direct measurements and hydraulic modeling. For the analysis of roadway structures located in tidal waterways, a three-level analysis approach, similar to the approach outlined in , is recommended. Determining the appropriate level of analysis is dependent on-site conditions, level of approach, available data, and potential applicable scour mechanisms.

Figure 15-23 summarizes the guidance on level of approach as it pertains to scour estimation methods.

There are several methods of infrastructure adaptation that can be incorporated during planning, designing, construction, operating, or maintaining infrastructure to mitigate for coastal scour. recommends a five-part approach: manage and maintain, increase redundancy, protect, accommodate, and relocate. Table 15-11 summarizes each of the five steps in the recommended approach.

Table 15-11: Summary of the HEC-25 Recommended Five-Part Approach for Scour Mitigation

Manage and Maintain Increase and target maintenance activities (e.g., culvert cleaning) Relocate movable assets prior to forecast storm events (e.g., moving maintenance vehicles and equipment to less vulnerable areas) Enhance and practice emergency procedures Stockpile and strategically place fuel, temporary bridges, and road construction materials for quick deployment	Increase Redundancy Identify and enhance, as appropriate, alternative roads, routes, or modes to serve transportation needs during times of compromised service (e.g., enhanced ferry service) Consider constructing or enhancing closely spaced roads perpendicular to the coastline versus one roadway parallel to coastline
Relocate Relocating infrastructure further inland away from the coastline Repurposing or reclassifying paved road to all-terrain vehicle road Reconditioning a damaged vehicular bridge to serve as a pedestrian bridge or fishing pier	Accommodate Increasing bridge deck elevations and strengthen bridge structures, Lowering roadway profiles to allow overwash without pavement damage during extreme events Raising tunnel portal walls to reduce likelihood of flooding
Protection New or enlarged seawalls, bulkheads, revetments, etc. (hard structures to provide barriers or resistance to damaging forces) Retaining walls, mechanically stabilized earth (MSE), etc. Beach nourishment (soft strategy to move water away from infrastructure) Dune restoration and vegetation (soft strategies to provide barriers and resistance to damaging forces) Living shorelines along sheltered coasts (combination of hard and soft structures/strategies to increase resistance to damaging forces)

If an existing transportation asset is identified as at risk to erosion, then protection might be the first option to immediately mitigate risk. All options can be reviewed and incorporated for proposed transportation assets. For general scour conditions, such as local scour, contraction scour, stream instability, and overtopping flow, refer to Table 2.1 (Stream Instability and Bridge Scour Countermeasure Matrix) in for guidance on reviewing the hydraulic countermeasures, structural countermeasures, biotechnical countermeasures, and monitoring as part of a scour evaluation. Since those measures are applicable for the above conditions and more well-known in riverine analysis, the remainder of this section will focus on examples of coastal protection and countermeasures.

Many different natural processes and forces impact roads near the coast. This section will focus on the most common erosion issues (e.g., overwashing, coastal weir-flow damage, and wave action) for roadway infrastructure.

Overwashing

:

For roadways in coastal environments, highway overwashing is a very common occurrence due to nearshore locations and low elevations. There are several mechanisms that damage pavements subject to overwash including:

Coastal weir-flow and wave action will be discussed as they relate to impacts from these mechanisms later in this section.

Strategies for minimizing damage during the overwashing condition include:

Figure 15-24, below, shows the first three strategies in a roadway cross section view.

One scour mechanism that damages pavements during overwashing is the coastal weir-flow damage mechanism. During this mechanism, water overtops the roadway embankment. The embankment acts as a broad-crested weir to the incoming storm surge. The water that flows over the embankment can become supercritical as it reaches the edge of pavement and have a free fall or a high tailwater condition. It is likely that waves will exacerbate the weir-flow damage mechanism and increase the scour risk. This happens when waves moving across the pavement with the storm surge increase the instantaneous flow velocities on the downstream shoulder. In addition to incoming storm impacts, the embankment can act as a broad-crested weir and experience similar damage mechanisms as storm surge recedes. There is also a concern of uplift on overtopped pavements due to a potential increase in underlying pore pressure in the sandy base materials.

Mitigation measures for weir-flow are similar to general overwashing. Lowering the roadway elevation to an elevation at or below adjacent ground elevations can prevent weir-flow from occurring, since the crest of the pavement is no longer the highest portion of the grade. In addition, considerations can be incorporated into the pavement design to mitigate pore pressure increases. Armoring the shoulder to withstand weir-flow mechanisms is recommended where lowering the roadway is not possible.

Waves, as discussed in Section 3, cause some of the primary hydraulic forces on coastal roadway embankments. Some key variables that are often considered in revetment design to protect roadways include design wave height, potential for wave runup, and potential for overtopping splash. Where wave action and wave runup are a concern, revetment, seawalls, and bulkheads may be considered for shoreline or roadway embankment protection, as previously discussed. Relocation further inland can also reduce impacts of wave action.

Revetments, seawalls, and bulkheads are all coastal protection features designed to protect roadways and other coastal assets from dynamic coastal processes as shown in the examples in Figure 15-25. The preferred use of each option is dependent on the local conditions. Revetments are layers of protection on top of a sloped surface to protect the underlying, elevated soil. Seawalls are free-standing structures that can be stable at greater heights than revetment and are designed to protect against large wave forces. Bulkheads are designed primarily to retain the soil behind a vertical wall and are primarily used in areas with limited wave action. The fetch is a length of ocean or lake over which the wind blows in a constant direction. Given the relationship between wave height and fetch, the flowing discussion distinguishes between the three types of coastal protection and their applicability:

Although these three structural alternatives are the traditional approaches to coastal armoring, combinations of them with softer methods, as later discussed, are becoming more viable and can often provide more dynamic long-term protection. It is important to note that the more robust the structural alternative, such as a seawall, the more likely for residual shoreline or structural erosion adjacent to the project site. Residual impacts should be considered when evaluating project solutions.

A well-designed and constructed revetment can protect roadway embankments from waves. The rubble is efficient at absorbing wave energy and any damage that the revetment receives is often inexpensive to repair. and provide detailed procedures for the design of stone revetments and recommend Hudson’s equation to estimate stone size for the outer layer of rock revetments subject to wave action. Hudson’s equation accounts for the most important variables, including design wave height, structure slopes, and different stone densities and angularities. HEC-25 provides a more detailed description of calculations for the design wave heights for revetment design. HEC-25 also provides examples for the applicability of design alternatives depending on specific project site conditions and requirements.

Some revetment design failure mechanisms include: inadequate armor layer design for wave action, inadequate under layer, flanking, toe scour, and overtopping splash (Figure 15-26).

A - Toe Scour causing concrete median barriers to topple along the shoreline adjacent to SH87 near High Island, Texas

B - Inadequate Armor Layer - A revetment with rocks too small to withstand wave attack along SH316 in Calhoun County, Texas

To prevent failure mechanisms, precaution should be taken when applying design equations, like from HEC-23 or HEC-25, to ensure values used represent the intent of the equations. Stone should be sized to ensure wave action is appropriately addressed. Toe design should have significant depth of stones to prevent undermining. The protection should tie back into natural structures or adjacent resistant structures to prevent flanking and should extend high enough to prevent overtopping splash damage. The biggest risk is exposure of the underlying soil. Concrete block and panel revetment have not had a good performance record in severe coastal environments; neither approach matches the performance and flexibility of stone revetment (FHWA, 2008).

New technologies are emerging for scour protection; some examples include scour control mats and geo-fabric stone bags. As case studies develop and results are reviewed, these technologies might serve as an additional option for shoreline and structure protection (Figure 15-27).

Other hard coastal structures that are successful in shoreline protection include groins, breakwaters, and hybrid structures (Figure 15-28). Groins are narrow, roughly shore-normal structures built to reduce longshore currents, and/or to trap and retain littoral material. Breakwaters are structures protecting a shore area, harbor, anchorage, or basin from waves. Hybrid structures are some functional combination of groins, breakwaters, or structural measures functioning in a complementary fashion. Further studies need to be conducted on all coastal protection measures, but redundancy in protection methods should be considered for adaptive designs in coastal environments.

Living shorelines, discussed more in Section 6, are techniques that involve combining ecologically-based, “soft” mitigation techniques, such as coastal vegetation, with hardened solutions, such as breakwaters. This is also a more adaptable system that can potentially provide improved long-term mitigation due to its more dynamic response to future coastal change (e.g., vegetated features can migrate with sea level rise). One example of a soft coastal protection method along roadways is beach nourishment, particularly on Gulf shorelines. Placing sand to widen a beach along with hard coastal structures can lead to a great balance between structural needs and environmental conditions to mitigate erosion.

A - Wetlands and submerged oyster reef barriers provide living shoreline protection for Broadway Street in Aransas County, Texas

B - Groin fields shelter local roadways in Calhoun County, Texas

Engineering coastal bridges can be complex and requires consideration of forces and processes unique to the coastal environment, including tidal bridge scour and hydrodynamic loads from waves and tidal currents. The types of scour that occur at bridges in the coastal environment include the same general categories as found at riverine bridges. However, in the case of coastal bridges, scour can potentially be caused by more varied directions and magnitudes, thus increasing the complexity of analysis. Additionally, coastal bridges can experience scour as a result of wave action (wave scour) and localized areas of high velocity flows.

Methods of design equations to consider in coastal scour evaluation can be found in , , , and the USACE . Scour as defined for coastal bridges is broken into three approaches per HEC-25. For a level 3 approach as shown previously in Figure 15-23 where advanced modeling is needed because of the variety of nearshore processes, a precertified TxDOT coastal engineer should be involved. Each design storm or nearshore process scenario that is simulated should be considered for impacts on scour. Combinations of different physical processes that could increase the scour rate should be considered. For instance, if a bridge experiences tidal, wave, and storm surge conditions as part of the design storm scenarios, the hydrodynamic impacts of each condition above should be reviewed for variables like water elevation, velocity, depth, and other factors needed in scour evaluations.

Guidance documents for countermeasures included in apply for coastal areas as they do for riverine, and include:

Design Guideline 11 - Rock Riprap at Bridge Piers

and

Design Guideline 17 - Riprap Design for Wave Attack

. In Texas, riprap often refers to concrete; in FHWA guidance, riprap is typically equivalent to stone revetment. For the purpose of this chapter, riprap directly referencing a design guideline title means stone revetment.

Stone revetment is a common revetment countermeasure at bridge piers and abutments. Stone revetment should follow HEC-23

Design Guideline 4 - Riprap Revetment

for guidance on sizing, gradation, placement methods, and failure mechanisms. Some failure mechanisms of stone revetment include: particle size being too small, channel changes like migration or channelization, improper gradation, improper placement, and lack of or inadequate filter. To avoid the failure mechanisms, attempts should be made to evaluate variables used for appropriateness when applying equations to size stone revetment, and any channel changes as evaluated per HEC-20 should be considered in the application for the design life of the bridge. Similar to riverine applications, HEC-23 for countermeasure design can be utilized for coastal applications methods, but all available coastal inputs should be considered and applied in the design (such as tides, wave, and storm surge).