HEC 25 - Tidal Hydrology, Hydraulics, and Scour at Bridges

Chapter 7 Other Coastal Engineering Considerations

7.1 COASTAL ZONES AND BEACH DYNAMICS

7.1.1 Introduction

This section provides an introduction to special considerations for highway structures in the coastal zone and dynamic processes of beaches in the near-shore zone. This material is extracted, primarily, from the AASHTO Model Drainage Manual (AASHTO 2004), AASHTO Highway Drainage Guidelines (AASHTO 2004), and the U.S. Army Corps of Engineers Coastal Engineering Manual (USACE 2002).

Coastal engineering is a specialized branch of the engineering profession which includes the many physical science and engineering disciplines having application in the coastal area. Coastal engineering addresses both the natural and man-induced changes in the coastal zone, the structural and non-structural protection against these changes, and the desirable and adverse impacts of possible solutions to problem areas on the coast. The coastal engineer's work can be divided into three phases: understanding the nearshore physical system and the shoreline's response to it; designing coastal works to meet project objectives within the bounds of acceptable coastal impact; and overseeing the construction of coastal works and monitoring their performance to ensure that projects function as planned (USACE 2002).

7.1.2 Coastal Zones

The boundary between the land and water is commonly called the coastline or shoreline. The strip of land of indefinite width that extends inland to the first major change in terrain is commonly referred to as the coast or coastal zone. The coastal zone may be several miles wide. Highways that are located near coastlines and shorelines of oceans, tidal basins, bays, estuaries, and the lower reaches of many major river systems present challenging design conditions for roadway, structural, and hydraulic engineers.

The forces in coastal zones are more diverse than in typical riverine conditions and the data requirements are more extensive. There are several distinct types of hydraulic problems that may be encountered:

• Wave surge and tidal action along a coastline
• Seasonal shifts of the shore
• Along shore and offshore transport of beach sands
• Floods resulting from upland runoff in combination with tides and waves
  
  

These problems are common to the Atlantic, Pacific, and Gulf coasts. Many coastlines are significantly affected by winter storms that bring large waves and storm surges. Generally the shift is a recession, increasing the exposure of beach locations to the hazard of damage by wave action.

Consequently, stabilization of the shoreline is one of the most important considerations when highway facilities are located in the coastal zone. The design of foundations and protective works must be predicted on knowledge of local soils and geology. It is also necessary to determine the dominant geomorphic processes of the shore line. This type of information may be obtained from borings, soil surveys, analysis of aerial photographs, and field reconnaissance. Because shorelines may have significant temporal variability, it is necessary to obtain sufficient historical data to identify either this variability or long-term trends. The hydraulic engineer needs to recognize that these changes may be seasonal, annual, or even longer at some locations.

7.1.3 Pacific Coastlines

The coastline of the states bordering on the Pacific are periodically faced with El Niño. The El Niño is the common name for what most scientists refer to as the ENSO (El Niño - Southern Oscillation) phenomenon related to the interactions between the ocean and the atmospheric circulation patterns with an inter-decadal scale variability. Typically, the storms affecting the west coast of the United States are generated in the North Pacific and the waves travel southerly. El Niño events cause waves to travel in a northerly direction along the coast. The waves associated with the El Niño are frequently as large or larger than the storm waves from the North Pacific. Usually, the northeastern seaboard of the United States can credit El Niño with milder-than-normal winters and relatively benign hurricane seasons.

Tsunamis are another coastal hazard for Pacific coastlines. A tsunami is a wave, or series of waves, generated in a body of water by an impulsive disturbance that displaces the water column in a vertical or horizontal direction. Earthquakes, landslides, volcanic eruptions, explosions and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life.

As a tsunami approaches shore, it begins to slow and grow in height. As the tsunami reaches the shoreline, part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and breaking or turbulence. Tsunamis have a large amount of energy and very long wavelengths. As it approaches, the shoreline the wavelength becomes shorter causing a very large increase in wave height. This large wave height has great potential for erosion and destruction. Frequently, this results in stripping beach material and depositing it landward as well as undermining trees and destroying large structures. Tsunamis have had a maximum vertical runup of as much as 100 feet.

7.1.4 Dynamic Beach Processes

The beach and near-shore zone of a coast is the region where the forces of the sea react against the land. The physical system within this region is composed primarily of the motion of the sea, which supplies energy to the system, and the shore, which absorbs this energy. Because the shoreline is the intersection of the air, land and water, the physical interactions that occur in this region are unique, very complex, and difficult to fully understand. While there have been significant advances in understanding beach processes in recent years, the ability to predict changes is still limited.

On coasts where the shoreline is unconsolidated sediment such as a clay, sand or silt, the energy from the waves, wind and tide can cause rapid change in the shape and dimensions of the shoreline. Waves are the most significant factor to cause shoreline change. As waves move from offshore to the beach they will often break, reform and break again. The process of breaking results in a portion of the wave energy being dissipated. Additional energy is dissipated on the beach with the resultant transport of the beach sediment.

Figures 7.1 and 7.2 illustrate the principal features of the beach and nearshore wave environment, or the littoral zone. The offshore region lies beyond the zone of wave breaking. On many sandy coasts the landward end of this region is characterized by the presence of a longshore bar. The inshore region extends from the bar (or bars) across the surf zone to position of the tidal low water line. The foreshore extends from the low water line to the upper limit of swash and the beginning of the beach backshore. On beaches where dunes are present, the seaward toe of the dune marks the end of the backshore. If dunes are not present on the beach, the landward limit of the beach backshore is generally considered to be the upper limit of storm wave impacts. Other important features illustrated in these figures include the berm and the trough (just inshore of the alongshore bar).

Figure 7.1. Beach profile terminology (USACE 1992)

Figure 7.2. Nearshore wave processes terminology (USACE 1992).

The widths of the breaker and surf zones shown in Figure 7.2 change with wave conditions. During storms, when the waves are relatively large, these zones extend further offshore as the waves break in deeper water. Similarly, the swash zone will also be larger (and penetrate further landward) during storm conditions. A complete discussion of the nature of waves and sediment transport can be found in the U.S. Army Corps of Engineers Engineer Manual EM1110-2-1502: "Coastal Littoral Transport" (USACE 1992).

Normal Conditions. As a wave moves toward the shore it will break when the wave height is equal to about three-quarters of the water depth. The actual depth at breaking is a function of the beach slope and the wave length and period. Breakers are classified as four types: plunging, spilling, surging and collapsing. Plunging breakers have distinct curls, spilling breakers break more gradually, and have characteristic white water and surging breakers begin to form a plunging face, but reach the beach before this face is formed. Collapsing breakers are a transition category between plunging and surging.

The form of breakers is controlled by wave steepness and nearshore bottom slope. Breaking results in a dissipation of wave energy by the generation of turbulence in the water and by the suspension and transport of sediment. The broken wave forms a bore that moves across the surf zone to the beach where it forms the wave uprush and backwash in the swash zone. The formation of the bar is directly related to the sediment transport characteristics of the breaking waves. The dimensions and locations of the various wave zones are functions of the wave characteristics and the stage of the tide. Regions with relatively large tide ranges will have wider limits to the positions of these zones.

Storm Conditions. The high winds associated with storms generate large waves. In open water, the actual size and period of the waves are a result of a combination of the size of the storm (fetch), the length of time the storm winds have been blowing across the fetch (duration), and of course the magnitude of the wind itself (see Section 2.4). In enclosed bodies of water, such as bays and estuaries, the shape of the shoreline as well as the depth of the water also affect the wave conditions. As the storm waves move to the coast, they are modified by the presence of the shallow water, and when they reach their limiting depth, they break. These breakers, and the associated energy dissipated are greater than during normal conditions, and therefore there is more energy available to erode the shoreline. These changes often include the movement of the bar offshore, the recession of the beach, and in extreme storms, the erosion of the dune. Since the storm conditions may also include the presence of a storm surge, the portion of the beach profile exposed to wave attack is greater than during normal non-storm conditions.

Figure 7.3 illustrates the changes that are likely to occur on a beach as a result of a storm. As the waves and surge increase, sediment is moved offshore as the bar migrates to deeper water. The bar may in fact grow large enough to cause the storm waves to break further offshore thereby reducing the wave energy in the breaker zone. This process of bar migration offshore can be thought of as a process by which the shoreline is protecting itself from further erosion by the storm waves. Figure 7.3, Profile B, illustrates this mechanism.

The beach berm is naturally built by the waves during periods of relatively low wave energy and sediment accretion. The berm elevation approximates the highest elevation reached by normal waves. When storm waves erode the berm and transport the sediment offshore, the protective value of the berm is reduced and large waves can penetrate further landward across the beach backshore. The width of the berm at the time of a storm is thus an important factor in the amount of dune and upland damage a storm can inflict.

Figure 7.3. Schematic Diagram of Storm Wave Attack on Beach and Dune (from U.S. Army Corps of Engineers 1984).

During severe storms, such as hurricanes (or large northeasters), the higher water levels resulting from storm surge may lead to dune erosion. It is not unusual for 20 to 30 m wide dunes to disappear in a few hours. This dune erosion will be greater when the period of maximum storm surge coincides with a high astronomic tide (Figure 7.3, Profile C).

After the storm has passed and the waves return to normal size and period, the beach goes through a period of recovery. Material is transported from the bar and nearshore profile back to the beach above mean water level. The berm builds out, and when the sediment dries, is transported by the wind to rebuild the dune. This mechanism of beach rebuilding is illustrated in Figure 7.3, Profile D.

During very large storms the combination of the surge and large waves may succeed in completely overtopping the dunes causing extensive coastal flooding. When this occurs, the water transports beach and dune sediments landward in a process referred to as overwash. In some cases, on barrier islands, the overwash may transport sediment completely across the island and deposit the material in the estuary (sound or bay). This transport of material out of the littoral zone represents a net loss of material from the beach and nearshore. In rare cases, the overwash and storm flooding (from both the ocean and the estuary) may erode enough sediment to cut an inlet across the island. Such an inlet may close within a matter of weeks or months, or in extreme cases become a new feature of the barrier island.

Beach and Dune Recovery. Following a storm there is a return to more normal conditions that are characterized by low wave heights and longer periods than during storms. These waves that are not generated by the local winds along the coast are termed swell. As noted above, these waves tend to transport material back to the shoreline, moving the bar shoreward and rebuilding the berm. Often the rebuilding of the beach is incomplete, as there is a net loss of material from the system as material is transported far offshore, or along the beach. This latter transport is referred to as longshore transport.

On some shorelines there is a characteristic seasonal change in the shape of the beach. During the winter months, with relatively frequent storms, the beach is cut back so it appears to be relatively narrow and flat. During the summer months the beach rebuilds, the berm widens and the foreshore returns to the characteristic summer profile.

7.1.5 Longshore Current and Sediment Transport

Longshore current is the flow of water parallel to the coastline that is driven by the longshore component of the wave-induced thrust as it enters the surf zone. The longshore currents are responsible for the downdrift movement of sediments in the nearshore zone. Longshore sediment transport is influenced by the wave height, period, direction of approach, and the beach slope (AASHTO 2004).

The sediment that is carried with the longshore current is called longshore or littoral transport. Onshore-offshore transport is determined primarily by wave steepness, sediment size and beach slope. In general, high steep waves move material offshore and low waves of long period (low steepness waves) move material onshore. Longshore transport results from the stirring up of sediment by the breaking wave and the movement of this sediment by the longshore current generated by the breaking waves.

The direction of longshore transport is directly related to the direction of wave approach and the angle of the wave crest to the shore. Thus, due to the variability of wave approach, longshore transport direction can vary from season to season, day to day, or hour to hour. The rate of longshore transport is dependent on the angle of wave approach, duration and wave height. Thus, high storm waves will generally move more material per unit time than that moved by low waves. Because reversals in transport direction occur and because different types of waves transport material at different rates, two components of the longshore transport rate become important. The first is the net rate, the net amount of material passing a particular point in the predominant direction during an average year. The second component is gross rate, the total of all material moving past a given point in a year regardless of direction. Most shores consistently have a net annual longshore transport in one direction. Determining the direction and average net and gross annual amount of longshore transport is important in developing shore or highway protection plans.

Inlets can have significant effects on adjacent shores by interrupting the longshore transport and trapping onshore-offshore moving sand. During ebb tide, sand transported to the inlet by waves is carried seaward a short distance and deposited on an ebb tide bar. When this bar becomes large enough, the waves begin to break on it, moving the sand over the bar back toward the beach. During flood tide, when water flows through the inlet into the bay, sand in the inlet is carried a short distance into the lagoon and deposited. This process creates mounds of sediment, known as shoals, over which the depth of flow is comparatively shallow. Shoals in the landward end of the inlet are known as flood tide shoals or inner bars. Later, ebb flows may return some of the material in these shoals to the ocean, but some is almost always lost from the littoral system and the down drift beaches. In this way, tidal inlets store sand and reduce the supply of sand to adjacent shores. Since bridges cross many inlets they are of particular concern to a transportation engineer. A highway crossing at Indian River Inlet, Delaware, is shown in Figure 7.4. Some inlets are considered to be unstable in that their position changes with time. In some cases the inlet will migrate along the coast in a persistent direction while in others the inlet will tend to shift with no net change in general location. Some of these inlets have been stabilized by the construction of jetties. Jetties are generally designed to improve navigation through inlets, but they will also fix the position of the inlet and can interrupt longshore transport.

Figure 7.4. Indian River Inlet, Delaware.

7.2 Wave Analysis

7.2.1 Wind Waves

This section provides an introduction to short period waves in tidal waterways and is primarily focused on defining the variables and characteristics that are pertinent to predicting wind-induced wave heights in the vicinity of bridges. The primary variables (Figure 7.5) in describing waves are length (L, the horizontal distance between wave crests), height (H, the vertical difference between the wave crest and adjacent trough) and period (T, the time between successive crests). The wave speed, or celerity, is the wave length divided by the period (C=L/T). Another factor that affects wave height is the still-water depth, which is the depth of water if there were no waves.

Waves are classified as deep, transitional and shallow water waves. For deep water waves, the wave height is virtually unaffected by the depth and the wave celerity is unaffected by the bottom. For transitional water waves the bottom has some affect on the wave height and celerity. For shallow water waves the celerity is only a function of depth. If the water depth is greater than 0.5 times the wave length, it is considered a deep water wave. If the water depth is less than 0.04 times the wave length, it is a shallow water wave. Transitional water waves are in the range between 0.04 and 0.5 times the water depth.

Figure 7.5. Wave characteristics.

Waves that are produced by wind are affected by the wind speed, wind duration and fetch. Fetch is the distance that an unobstructed and constant wind, both in terms of speed and direction, acts over a body of water. Land is an absolute limit to fetch but changes in water depth and wind direction can also limit fetch. For very large bodies of water, the change in wind directions due to the circular wind field of a hurricane can limit fetch.

It is possible to predict wave heights for specific wind and waterway conditions. The primary factors are water depth, wind speed and fetch. If the wind duration is not sufficient to produce the computed wave height, then the waves are duration limited rather than fetch limited. If the waves are duration limited, the fetch distance used for computations should be reduced until the required duration equals the actual wind duration.

Wave heights and lengths also have a random nature in that each wave, even in a constant wind field, does not have the same height or arrive at a consistent period. The predictive equations are developed to predict the significant wave height, H_s, which is defined as the average of the one-third highest waves. The heights of less frequent waves can be estimated based on the significant wave height. For example, H_0.10, H_0.01, and H_0.001, the average of the ten, one and one-tenth percent highest waves, are approximately 1.27, 1.67, and 2.0 times the significant wave height. The frequency of these waves can be estimated by using the wave period (T) divided by the percentage represented as a fraction.

7.2.2 Wave Height Computations

It is useful to determine both the surge and wave height when establishing the low chord of tidal bridge decks. South Carolina, for example, uses the 10-year surge plus wave height plus two feet freeboard as the minimum elevation for the bottom of the deck. NOAA (1975) provides surge elevations (10-, 50-, 100-, and 500-year) for the South Carolina coast. It is reasonable to use a relatively low hurricane wind (Category 1) to compute wave heights for a 10-year surge in South Carolina. Wind speeds for a Category 1 hurricane, which has maximum wind speeds between 74 and 95 mph, should be used for this relatively frequent event. The maximum winds occur along the right side of the hurricane eye. For other areas of the coastline, refer to Appendix C to assess the frequency of various hurricane categories. From these figures it appears that Category 1 hurricane wind speeds should be used to compute 10-year wave heights for the entire coastline except for the southern Florida coast between Key Largo and Port Salerno, and the area between Cape Hatteras and Oregon Inlet, North Carolina. For these areas, Category 2 wind speeds appear to be more appropriate.

The methodology recommended to compute wave heights is from the Coastal Engineering Manual (USACE 2002). The data required to compute wave heights are wind speed, fetch length, channel flow depth, floodplain flow depth and 10-year surge elevation. Separate wave height computations should be conducted for the channel and floodplain. The computed wave height is the significant wave height, which is defined as the average height of the one-third highest waves. This wave height should be converted into a one percent wave, H_0.01, the average of the one percent highest waves. The waves may be limited by fetch or by the duration of the wind. If the waves are limited by wind duration, then the fetch should be reduced until the computed duration equals the actual wind duration.

It is generally assumed that 70 percent of the computed wave height is added to the surge elevation (still water level). The period is time between successive waves, which is the wave length divided by the wave speed. Waves are classified as deep water, where the wave height is virtually unaffected by the bottom and the celerity is unaffected by the water depth, transitional waves, where the bottom affects the wave height and depth affects the wave celerity, and shallow water waves, where celerity is a function of depth only and waves are more likely to break. The maximum wave height is approximately 0.78 times the flow depth. This limiting wave height is unlikely in the deep channel area but a reasonable estimate for waves in shallow floodplain areas. Therefore, for shallow areas the maximum water surface height (still water plus wave) is 1.55 times the depth (0.7 x 0.78 +1).

Figure 7.6. Wave height plus surge.

For the purposes of computing wave heights within a bridge opening, the definition of fetch requires the greatest judgment. Fetch is the distance of unobstructed wind with fairly uniform speed and direction. Figure 7.7 shows a road embankment and bridge crossing a floodplain and channel. The floodplain is assumed to have some relatively shallow depth of flooding during the surge. The wind is assumed to be oriented in the worst case direction with respect to the channel, but within a range of directions that can be reasonably produced near the peak of a the storm surge. The range of directions should be limited to within 45 degrees of the storm track. Land is an absolute limit to the fetch. Because waves tend to break in shallow water, the length of deeper channel could limit the fetch. It is reasonable, however, to extend the fetch somewhat upwind of the deep channel area, perhaps by 1,000 to 2,000 feet. For small tidal waterways in heavily wooded floodplains, it is reasonable to assume that wind waves will be minimal during a storm surge.

7.3 Shore Protection Countermeasures

The dynamic shoreline environment frequently necessitates that some type of protective device be installed to ensure the stability of highway and bridge infrastructure. This section provides a summary of many such devices and is extracted from AASHTO Highway Drainage Guidelines (2004). The reader is directed to other references for more detailed information. (For example see U.S. Army Corps of Engineers (1984)). Hydraulic Engineering Circular (HEC-23) "Bridge Scour and Stream Instability Countermeasures" also provides experience, selection, and design guidance for a wide range of countermeasures that would be applicable to the channel, estuary, or inlet portions of the tidal environment. Shore protection devices may be classified according to the materials used for construction, the general shape of the device, or their function or application. The typical classification used in design and that provided in the following paragraphs is by function or application.

Figure 7.7. Definition sketch for wave calculations.

7.3.1 Seawalls

Seawalls are essentially vertical structures, constructed parallel to the shoreline, that separate land and water areas and are primarily designed to prevent erosion and other damage due to wave action. These can be used to protect shorelines during storm events such as hurricanes. Design considerations are given by the U.S. Army Corps of Engineers (1995). Seawalls have been used to protect vital infrastructure such as the Great Ocean Highway in San Francisco, as shown in Figure 7.8, and the City of Galveston, Texas, as shown in Figure 7.9. The seawall in Galveston was constructed shortly after the hurricane of September 1900 that drowned approximately 6,000 people and it has continued to protect the City against flooding from subsequent hurricanes. These are typically massive structures built from reinforced concrete with pile foundations.

Figure 7.8. Seawall at San Francisco's Great Ocean Highway.

Figure 7.9. Seawall at Galveston, Texas.

7.3.2 Revetments

Revetments are shoreline structures constructed parallel to the shoreline and generally sloped in such a way as to mimic the natural slope of the shoreline profile and dissipate wave energy as the wave is directed up the slope. Revetments are the more common type of protective device because the protection is in direct contact with the embankment. Moreover, there are a wide range of economical materials available and designs adaptable to specific sites.

Controlling shoreline retreat through construction of a seawall or revetment does nothing to retain a beach or maintain a nearshore profile. As retreat continues, the beach is eroded and wave energy is expended directly on the base of the structure. This removes additional sediment, increases the water depth at the base and makes the unit vulnerable to being undermined. Therefore steps must be taken to protect the toe of the structure using features such as the stepped foundation of the seawall shown in Figure 7.8.

Rigid Revetment. Rigid revetments can be used in low energy environments and where the structure can be protected from settlement and flanking. This type of structure provides protection from moderate waves and currents but usually cannot withstand a severe environment. Failure often occurs if portions of the semi-monolithic structure are cracked, washed away or undermined. Below are two special types of revetment:

• Soil and cement (soil cement), mixed to form a solid, somewhat rigid embankment

• Rigid type armoring, such as Portland Cement Concrete (PCC), grouted rock and sacked concrete structures

The latter must be well founded to avoid undermining or flanking and should be placed so that it will not be overtopped for any but extreme events. Rigid revetments are usually most appropriate for areas of "quiet water" such as inlets, coves and backwater zones.

Flexible Revetment. In light wave conditions, flexible type armor protection, such as Articulating Concrete Block Systems (ACBs) blocks, gabions, articulated revetments and even oyster shells, have been successfully used for shore protection. In more moderately exposed locations stone is more frequently used as shown in Figure 7.10. All of these types are able to adjust with settlement of the foundation without creating a catastrophic failure.

When designing shoreline protection for large sea states, heavy rock slope protection is frequently used. When adequate stone size is not available, precast concrete armor sections such as tetrapods, core-loc, dolos and other special shapes and designed for specific purposes are used. Rock protection is usually the most economical when stones of sufficient size, quality and quantity are available. Rock shore protection has several other advantages and is the most commonly used embankment protection for ocean or lakeshore exposure. The following determinations must be made in the design of rock slope protection:

• Size of stone

• Foundation depth (below scour depth or to solid rock)

• Height of riprap placement (at an elevation above wave run-up or deep water wave height for protection from splash and spray)

• Thickness (sufficient to accommodate the largest stones; additional thickness on the slope will not compensate for undersized stones)

• Filter blanket (uniformly graded stone filter, geotextile filter fabric or both to prevent embankment material from being exfiltrated through the voids of the face stone)

• Slope of face (usually determined by the angle of repose of the embankment material but smaller stones could be used on flatter slopes)

Figure 7.10. Stone revetment.

7.3.3 Riprap Shore Protection

When used for shore protection, riprap reduces wave runup as compared to smooth types of protection. Other types of armor can be used to revet the slope, but stone is frequently the least expensive and more readily available at least for projects for which the waves are not greater than 6 feet. Equally important in the success of the protection is the placement of the stone and the underlying filter materials. The typical section schematic in Figure 7.11 identifies the relationship of the section to the bank of the existing shoreline (AASHTO 2004). The section also identifies the toe trench that is typical with all revetments. The toe trench is used to prevent the scour from occurring and undermining the revetment. Sometimes a sheetpile wall at the toe of the revetment fills this function.

The proper stone or armor unit size to use on a reveted slope will be a function of the wave height, slope of the revetment, type of armor placement, and the specific gravity of the armor. This presumes that adequate filter layer is present as a part of the design. The purpose of the design is to design the size of the armor unit just large enough to resist the forces of the wave uprush and downrush that would cause the armor to move. Based upon numerous laboratory experiments and field verification, the "Hudson Equation" (U.S. Army Corps of Engineers 2002) was developed to provide that relationship between the stone size and these parameters. The Hudson equation is:

(7.1)

in which

W = design weight of armor unit
γ_s = unit weight of armor
H = wave height of typically H ₁₀
K_D = dimensionless coefficient
S = specific gravity of armor
Θ = angle of revement with the horizontal

Figure 7.11. Riprap rock shore protection - typical design configuration.

Typically, W is in pounds force, H is in feet and g_s is in pounds force per cubic foot. The dimensionless coefficient, K_D, is based on laboratory experiments with different types of armor. Values for typical armor types are given in Table 7.1. Nomographs are provided in (Racin, Hoover, and Crosset-Avila 2000)that can be used in place of the Hudson Equation, Equation 7.1.

The example given in Figure 7.11 clearly shows a toe trench that holds the revetment against scouring. The toe stone is typically the same size as the armor stone and is placed upon a filter fabric. The height of the revetment should extend to the limits of the runup calculated and then provide freeboard for protection against the effects of wind driving the runup even higher. Consideration should also be given to protecting the bank above the rock slope protection from splash and spray. Sometimes a filter material is used instead of a geotextile filter. If a geotextile filter is used it should be tucked under the outer stones in the toe trench to prevent the fabric from unraveling from the stone.

In placing the toe, stone should be founded in a toe trench dug to hard rock or keyed into soft rock. If bedrock is not within reach, the toe should be carried below the depth of the scour. If the scour depth is questionable, extra thickness of rock may be placed at the toe that will adjust and provide deeper support. In determining the elevation of the scoured beach line, the designer should observe conditions during the winter season, consult records, or ask persons who have knowledge of past conditions.Thickness of the protection must be sufficient to accommodate the largest stones. Except for toes on questionable foundation, as explained above, additional thickness will not compensate for undersized stones. When properly constructed, the largest stones will be on the outside, and if the wave forces displace these, additional thickness will only add slightly to the time of complete failure. As the lower portion of the slope protection is subjected to the greater forces, it will usually be economical to specify larger stones in this portion and smaller stones in the upper portion. The important factor in this economy is that a thinner section may be used for the smaller stones. If the section is tapered from bottom to top, the larger stones can be selected from a single graded supply.

7.3.4 Jetties

A jetty is an elongated artificial obstruction projecting into the sea, reservoir or lake from the shore to direct and confine the stream or tidal flow to a selected channel. Many jetties are constructed at the mouth of a river or entrance to a bay to help deepen and stabilize a channel and thus facilitate navigation, as shown in Figure 7.12. Jetties interrupt the littoral drift causing deposition on the updrift side of the entrance and prevent accretion and shoaling in the channel area. This interruption of the littoral drift can result in accelerated shoreline erosion on the downdrift side of the jetty. Jetties are typically constructed from the materials listed below using landbased equipment. In some instances larger projects will require construction also from floating equipment.

Stone. As with revetment for slope protection, the most economical material for the construction of jetties is rock where stones of sufficient size, quality and quantity are available. It is important to note that broken concrete is sometimes used as a substitute for stone but it is never an adequate substitute. Broken concrete is usually platy with unsatisfactory relative dimensions and low specific gravity.

Figure 7.12. Jetty entrance for navigation channel. (Note the updrift sediment and starved downdrift shoreline.)

Precast Concrete Armor Units. Where stones of sufficient size and quantity are not available, precast solid or hollow concrete armor units may be substituted for rock in jetty construction. Details of this type of construction can be obtained from U.S. Army Corps of Engineers (1995). Solid, reinforced and nonreinforced prismatic concrete blocks have long been a substitute for rock. These blocky shapes do not easily interlock or provide enough voids between blocks to dissipate the energy of large waves.

Various shapes of concrete blocks have been devised that combine interlocking properties with improved voids between blocks for the dissipation of energy from large waves. Examples of this kind of jetty armor are:

• Quadripod
• Dolos
• Core-loc
  
  

An example of a Core-loc structure is given in Figure 7.13. The construction shows the placement of the Core-loc over the previous dolos armor layer.

Figure 7.13. Core-loc Armor Units on Manasquan Jetties being placed over dolos. (The darker units are dolos.)

7.3.5 Groins

A groin is a relatively slender permeable or impermeable barrier structure aligned and constructed to trap littoral drift or retard erosion of the shore. Basically, it is a spur structure extending outward from the backshore. A schematic example is shown in Figure 7.14. Factors pertinent to design are:

• Material
• Alignment
• Grade
• Permeability
• Length
• Spacing
• End configuration
  
  

Materials. Materials used in groin construction are similar to that used in jetties. However, the groin is typically exposed to less severe wave conditions since it only extends through the surf zone. Examples of construction materials are given below.

Stone. Stone is the most common material used in the construction of groins. Other materials used for groin construction are concrete, steel and timber. Stone groins are built like a jetty or breakwater structure, usually massive with a core of smaller, graded stone, having a trapezoidal section and dependent on weight for stability.

Figure 7.14. Schematic showing effect of a single groin on the Shoreline and the effect of a continuous groin field.

Concrete. Concrete groins are usually precast heavy solid blocks, fillable cells or interlocking shapes like tetrapods. Like those constructed with stone, they typically have a smaller, graded stone core. An example of a groin constructed with precast concrete units is shown in Figure 7.15.

Steel. Steel groins may be as simple as a line of sheet piling, or combination of H-piling, waling and sheeting. Corrosion of steel in salt water is a major factor in comparing costs of alternative materials for groin construction. Steel must be specially coated with an epoxy finish when used in seawater.

Timber. Timber piling in single or multiple rows may be used for groin construction. An example of a timber groin using sheet piling inserted between king piles is shown in Figure 7.16.

Figure 7.15. Concrete groin constructed with precast units. (Note the stone protection around the "T" at the seaward end.)

Figure 7.16. Timber pile groin.

Alignment. The conventional alignment of groins is normal to the shore. The obvious factors that might influence alignment are:

Effectiveness in retaining or detaining littoral drift
Protection of the groin itself from damage by wave action

The seaward end of groins have been finished in various configurations. For stone, there is typically an enlarged head to reduce the slope. Timber, concrete and steel have been frequently constructed with an "T," "L" or angle at the end of the section.

Permeability. Permeability of a groin may be a desirable characteristic but has not generally been considered to be a necessary element in the design of groins. Currently, more consideration is being given to groin permeability to allow downdrift movement of sediment (4).

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