Ohio Coastal Design Manual
|Chapter 4 |
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Promote better projects along the coast that balance the use of Lake Erie as a shared natural resource along with the property owners’ need for lakefront erosion protection and the benefit of access to the lake.
Chapter 4 addresses the design of stone revetments and seawalls, the most common structures used to prevent erosion along the shore of Lake Erie. These structures are designed to protect against the erosion of the lower portion of the bluff due to wave action. Erosion of the upland caused by surface water runoff, groundwater seepage or the natural weathering of the bluff may require separate, additional measures. Since they are usually designed in conjunction, guidelines for upland erosion control measures are also included in this section.
The five design examples are intended to demonstrate the design process. The example sites are fictitious. The site conditions, parcel boundaries, addresses and parcel numbers were invented to illustrate the range of engineering and surveying methods involved in design. The example sites include typical coastal features and are intended to be applicable to a large portion of Lake Erie’s south shore.
Chapter 4. Erosion Control Structures
4.1 General Design Guidelines for Erosion Control Structures
This chapter addresses the design of stone revetments and seawalls, the most common structures used to prevent erosion along the shore of Lake Erie. These structures are designed to protect against the erosion of the lower portion of the bluff due to wave action. Erosion of the upland caused by surface water runoff, groundwater seepage or the natural weathering of the bluff may require separate, additional measures. Since they are usually designed in conjunction, guidelines for upland erosion control measures are also included in this section.
The five design examples are intended to demonstrate the design process. The example sites are fictitious. The site conditions, parcel boundaries, addresses and parcel numbers were invented to illustrate the range of engineering and surveying methods involved in design. The example sites include typical coastal features and are intended to be applicable to a large portion of Lake Erie’s south shore. (top)
Protection against wave-based erosion
The guidelines below address the elements of shore structure design common to nearly all erosion control structures subject to direct wave action and run-up.
1. Minimize the extent lakeward.
Erosion control structures should be designed with the smallest lakeward footprint possible. This minimizes the occupation of the lake bottom, limits habitat loss and usually results in a lower cost to construct the project.
In the case of stone revetments, the crest width should be only as wide as necessary for a stable structure. In general, the revetment should follow the cross-section of the bluff and be located as close to the bluff as possible.
For seawalls, the distance that the structure extends lakeward of the upland must be minimized. If the seawall height is appropriately designed to prevent the majority of overtopping, there is no engineering rationale based only on erosion control which justifies extending a seawall out into the lake.
2. Minimize the impacts to adjacent properties.
The design of the structure must consider the potential for damaging adjacent property.
Projects designed to extend lakeward of the shore will affect the movement of littoral material, reducing the overall beach forming process which in turn may cause accelerated erosion on adjacent or down-drift properties with less protective beaches.
Seawalls, (and to a lesser extent, stone revetments) change the direction (wave reflection) and intensity of wave energy along the shore. Wave reflection can cause an increase in the total energy at the seawall or revetment interface with the water, allowing sand and gravel to remain suspended in the water, which will usually prevent formation of a beach directly fronting the structure. This effect may impact the adjacent downdrift properties by either reducing beach formation (immediately adjacent) or potentially increasing beach formation (further downdrift). In extreme conditions wave reflection may allow littoral material to be transported off shore rather than along the shore, which would potentially remove that material from the littoral system and starve downdrift beaches.
3. Structural Stability.
The design must include the applicable calculations to demonstrate that the proposed structure will have long-term stability. These principles were introduced in Chapter 3.
For stone revetments, the stability of the structure depends on the unit weight of the armor stone, the slope and the design wave height. The most common calculation used is Hudson’s Equation, which relates the design wave height and design slope of the revetment to the weight (and size) of the stone needed to resist uplift (and displacement) from wave energy. This calculation is presented in the revetment design section and the examples that follow.
The stability of a seawall depends on its total weight in cross-section, location lakeward of the shoreline, cap elevation, underlying geology, and the degree to which it is used to retain the upland bluff. For the purposes of this manual, a seawall is a shore-parallel structure with a nominally vertical face. Typical seawall designs common along the Lake Erie shore include stacked pre-cast concrete block, cast-in-place concrete walls and stone-filled cribs.
The design should include details and specifications that show how blocks or cribs are to be connected and sufficient reinforcing detail that shows how cast-in-place concrete walls and caps will be connected. How the seawall is to be anchored into the underlying strata must also be detailed.
4. Materials of Construction.
The specifications for all materials to be used as part of the erosion control structure must be included in the design drawings. Particular attention should be paid to the specifications of fill materials that may be used under armor stone or behind seawalls. Demolition debris and common clean fill (dirt) are not acceptable materials for structures potentially exposed to the waters of Lake Erie (either during construction or post-construction).
Concrete rubble, if specified as fill, must include a size (weight) range and be clean and free of smaller material and exposed rebar. Concrete rubble should never be specified for any exposed portion of any structure.
5. End Effects / Flanking.
The design should avoid abrupt, shore-perpendicular ends at property boundaries. In general, both revetments and seawalls should be “rounded” off at the ends and/or meet the existing bluff slope contours. This will reduce the potential for erosion at the adjacent property working its way back behind the structure and causing upland slope failure and possible failure of the end of the revetment or seawall. If existing structures are present at adjacent properties, the proposed design should transition to these as smoothly as possible.
6. Design of Toe Protection.
Adequate toe protection should be included in the design to prevent sliding failures, scour and undermining at the base of a seawall. Both revetments and seawalls should also be adequately set into the underlying strata.
For armor stone revetments it is common practice to specify that stone at the upper end of the armor stone size range be placed at the toe, or toe stone 1 to 2 tons or greater than the design median armor stone size.
Many seawalls are used for recreational or watercraft access. The use of armor stone as toe protection in the design of a seawall may interfere with this function. Nevertheless, toe protection at the seawall base is recommended as a means of preventing the scouring and undermining of the structure and increasing its expected life. (top)
Protection against upland erosion
The height and composition of the bluffs along Lake Erie’s coast are highly variable. Addressing the erosion caused by groundwater seepage, surface water run-off and natural weathering is dependent on site conditions. The Lake Erie Shore Erosion Management Plan (LESEMP) addresses many of these issues on a regional and reach basis and should be consulted as a supplement to this Manual. LESEMP information is found online at: www.ohiodnr.com/tabid/20501/default.aspx.
The general guidelines presented here are intended to apply to the bluff and upland areas landward of a well-designed and constructed erosion control structure.
The design of the upland erosion control features at a site should complement and work in concert with the proposed shore structure. Options for stabilizing the upland include:
1. Re-grade the existing slope to at least 2 horizontal to 1 vertical.
This option applies where there is adequate distance between the shore structure top elevation and upland structures. Stabilization through re-grading and vegetating the bluff slope has been fairly successful along shores with bluffs composed of till and bluffs of medium elevation (less than about 40 feet).
2. Retain as much existing vegetation as possible.
Native trees, shrubs, and perennials are the best means of limiting erosion from surface water run-off and naturally reducing flows from groundwater seeps. This is especially important along areas with medium to high till bluffs (40-60 feet). Tree and shrub roots are also extremely effective at stabilizing existing upper bluff soils.
3. Reduce or re-direct surface water sheet flow
or collected surface water drainage.
A slight swale at the top of the bluff, coupled with a well designed trench drain can eliminate most of the sheet flow down the bluff. Collected surface water should be diverted landward if at all possible; if not, the conveyance pipe should be run down the bluff to as close to the lake elevation as possible. Outlet protection should be placed at the down-slope end of the pipe to prevent erosion at that location.
As a general consideration, the less surface water conveyed over the edge of the bluff, even as limited sheet flow, the better. For sites with unfavorable geology leading to perched water and seeps at the bluff face, the less upland surface water allowed to infiltrate into the groundwater the better.
If downspouts, other surface drainage or basement sumps are currently collected and conveyed over the bluff, the optimum means of discharge should be a pipe that extends the full distance to the toe of the bluff. Pipes suspended over the bluff allow the water to erode the bluff below it.
4. Terrace the upland.
Low-height terracing can be a cost-effective means of stabilizing the upland bluff and can be designed to provide access pathways to the lake. As a general consideration, multiple, 3 to 4-foot high terraces will be less prone to a large scale bluff failure, lower in initial cost, and easier to repair than fewer, higher retaining walls. Terracing can also be effective in intercepting groundwater seeps and diverting the water along the terraces.
4.2 Armor Stone Revetment Design
This section presents a simplified approach to the design of the most common type of revetment: rough, angular stone armoring. Examples A, B and C in section 4.5 illustrate revetment designs at three types of site settings.
The primary references for the design of armor stone revetments are the U.S. Army Corps of Engineers “Coastal Engineering Manual” (the CEM) and Engineering Manual 1110-2-1614, “Design of Coastal Revetments, Seawalls and Bulkheads.” (top)
Components of an armor stone revetment are:
The armor layer consists of sufficiently sized stone and a thickness designed to be stable under the design wave conditions and the design slope.
- The filter layer consists of smaller stone or rubble that supports the larger armor stone and prevents erosion of the underlying bluff material. This layer may also be called a bedding layer. If this material is intended to be impermeable, it may be referred to as a “core”. Many revetments include geotextile fabric under the filter layer to further reduce the potential for erosion of underlying fine-grained bluff material.
- The toe stone consists of heavier stone placed at the lakeward edge of the revetment, and serves to prevent slipping failure of the upper revetment. In many cases the toe stone will also be placed in an excavated trench into the underlying natural material.
- The crest is the upper elevation of armor stone.
When the crest is designed as a horizontal feature, it is nominally as wide as the armor stone layer thickness. The height of the crest above the design water level is determined by the calculated run-up elevation of the design wave.
In addition to its recreational and aesthetic features, the presence of a beach lakeward of an armor stone revetment will aid in erosion protection.
- The splash apron is located above the crest and usually consists of much smaller stone. It serves as a less costly means of dissipating the remaining wave run-up, splash and spray that can extend above the armor layer. (see illustration)
The majority of armor stone used along Lake Erie is quarried limestone. Sandstone is also available.
Allowances for the lighter mass density of sandstone (specific gravity of 2.2 to 2.5 for sandstone versus 2.6 for limestone) must be included in the design calculations. Sandstone is more resistant to cracking than limestone but it is also a softer material and more easily eroded. The use of concrete block or specialty concrete forms as armor material is addressed in the Corps of Engineers’ CEM. Concrete typically has a specific gravity of 2.4, but it can be much lighter.
Concrete rubble should never be used as armor material due to its tendency to crack and break apart easily, reducing the unit weight of the block. It is also difficult to obtain concrete rubble of a sufficient weight per piece that would be needed to resist wave forces. Further, it is also difficult to control the size and shape of rubble since most rubble tends to be from slabs that are limited in one dimension (the slab thickness). This shape limitation tends to result in both breaks and the creation of large voids, neither of which favor a stable structure. (top)
The maximum recommended slope of a random-placed armor stone revetment is 1.5 horizontal to 1 vertical. Slopes greater than this will tend to be unstable. A 1.5H to 1V slope results in the smallest stable footprint along the shore. Where possible, revetment slopes should be selected to match the existing bluff/bank slope’s stable angle of repose. In practice, revetment slopes range from 1.5 to 1 to 2.5 to 1. Slopes greater than 3 horizontal to 1 vertical are rarely specified along the Lake Erie shore, mostly due to the higher cost of armor stone needed to construct what would be a wider revetment than might be necessary. (top)
The basis of the design for sizing the necessary weight and size of the armor stone units is the relationship between the force of the design wave (design wave height) and the slope of the structure. This relationship is expressed as follows:
is the 50th
percentile (median) weight of the stone (lbs)
is the unit mass of the stone (lb/ft3
) Limestone typically is 160-165 lb/ft3
H is the design wave height (ft) at the toe of the structure
= 62.4 lb/ft3
is an empirical value based on physical testing.
For randomly placed, angular stone KD
is the design slope of the revetment. For a 2:1 slope, cot
Hudson’s equation addresses only the stability of armor stone with respect to wave forces at a given slope. The calculation relies on the risk assumed with a given design water level (the return period) and wave height, both of which may be exceeded during the life of the structure.
The other factors that can affect long term stability include the quality of the stone, the range of actual sizes supplied, the placement on the slope, fracturing of the stone over time and the effect of ice forces. These factors are independent of each other and can all add to the long-term risk of failure of the revetment.
Ice forces are very unpredictable and difficult to calculate for revetments. Ice may act laterally against the slope moving and displacing stone, large ice blocks may drag stone lakeward as the ice recedes and ice can exert an uplift force on the stone as it forms along the shore and is thrust landward by wave action.
Armor stone is subject to fracturing over time and during transportation and placement. The stone will fracture due to ice, freeze and thaw and wave forces, losing its unit size/weight and thus its stability.
OCM recommends a safety factor be applied to the calculated unit stone weight as a measure of risk reduction against fracturing, ice forces, and variability in stone size and placement. The engineer should consider how these factors apply to each design and assign an appropriate safety factor that also incorporates the level of risk the property owner is willing to accept in return for the cost difference between larger or smaller armor units.
It is common to specify a range of stone size, using the design weight from Hudson’s equation as the lower value in the range. A range of stone size may also be a factor in the available supply of stone from a quarry. If a range of armor sizes is used, the design should specify that the larger stones be placed on the exposed layer directly receiving wave forces. This results in a conservative design that helps counter damage and poor placement of the stone during construction. USACE (in EM 1110-2-1614, “Design of Coastal Revetments, Bulkheads and Seawalls”) recommends a range of armor stone between 0.75 x W50 and 1.25 x W50. USACE in the CEM notes that uniform sizing of armor units is more economical for design wave heights greater than 4.5 feet.
The thickness of the armor layer is determined by the dimensions of the stone size selected for stability. The most common, and perhaps most cost effective arrangement is to specify two layers of armor stone. The approximate diameters for armor stone weights and the calculated layer thickness for a two-layer armor design are included in the table on this page. The armor layer thickness will tend to be slightly less than those in the table if a larger range is specified due to closer packing of stones. The design armor layer thickness can be calculated using a formula from the CEM that requires one to assume the number of layers and the unit stone size. The rubble mound revetment design module in the ACES software also includes this calculation.
A single layer of armor stone cannot be expected to have long-term stability or effectively prevent erosion. A single displaced stone could allow wash-out and erosion of the filter layer, and potentially the bluff material, leading to failure of the revetment. (top)
The crest elevation for an armor stone revetment is based on the wave run-up expected given the revetment slope, the design wave height, wave period and water level. The equations used to calculate run-up were presented in Chapter 3. The empirical formula shown below will generally result in a conservative run-up value.
The calculated height of run-up is added to the DWL elevation to arrive at a conservative design elevation for the revetment crest.
Function of the filter layer
The filter layer consists of graded rock or riprap and in some cases a geotextile fabric. It acts as a transition between the underlying soil and the armor structure. It prevents the migration of fine soil particles through voids in the structure, distributes the weight of the armor material to provide more uniform settlement and permit relief of hydrostatic pressures within the soils. In the case of revetments which extend above the water level, filter layers also help prevent surface water from causing erosion beneath the armor material. (top)
Filter layer design
The long-term stability of the revetment armor layer rests, in part, on the design of the filter layer. The material(s) for the filter layer should meet the following conditions:
The above photo shows typical concrete rubble of greatly varying size. The larger slabs may not be suitable as filter layer material.
A revetment is shown in the above photo.
- The material should be resistant to erosion caused by run-up and water washing through the armor stone. Fine grained material or a mix of larger material with fines should not be specified.
- The material should be capable of supporting the weight of the armor stone layer without significant displacement or creation of significant voids. Random pieces of concrete rubble are problematic as filter material due to the potential for large voids and uneven settlement.
- The material should be capable of preventing erosion and loss of the underlying bluff material. Geotextile fabric placed between the filter layer material and the bluff material can prevent loss of the fine grained bluff material.
The filter layer should be designed to minimize the amount of fill needed. The slope of the filter layer will usually be the same as the slope of the armor layer. The thickness will be determined by the cross-section of the bluff and the type and size of material to be used. In general, the filter layer thickness is two to three times the average stone size used in the filter layer.
As a design guideline, the USACE recommends a filter layer stone size that is 10 percent of the size of the armor stone. The use of larger stone or rubble increases the potential for uneven settling and the creation of large voids. Smaller filter layer stone can be specified if it is underlain by impermeable bluff material and a geotextile fabric to reduce the loss of fine material from the bluff.
Neither the filter layer nor any underlying fill should ever be exposed to direct wave action or run-up. (top)
4.3 Seawall Design
Seawalls can be effective erosion control structures and have the added functionality of providing direct access to the lake. The negative aspects of using a seawall to control wave-based erosion include:
- The vertical or near-vertical wall generally will create higher wave run-up, splash and spray compared to a sloped stone revetment.
- The wave energy exerted on the vertical seawall is not dissipated as it is over the slope and irregular surface of a revetment. This results in greater forces on the structure and more potential for damage.
- The vertical wall will reflect a high proportion of the wave energy which increases the energy in the nearshore. This may preclude the formation of a beach directly lakeward of a seawall unless the wall is well landward of the water and a stable beach is already present.
- The toe of a seawall is subject to scour and undermining due to direct and reflected wave energy. This effect can be magnified as lake levels change seasonally and year to year. Long-term scouring at the seawall may eventually lead to the down-cutting of the lake bed, resulting in a lower lakebed elevation (and higher water level, thus higher waves) lakeward of the wall.
Seawalls along the shore of Lake Erie have been designed and constructed in many different configurations using steel sheet pile, cast-in-place concrete, pre-cast block and rock-filled cribs. There are locations along the shore where each of these types might be appropriate, cost-effective, and feasible.
In this section the focus is on the most common types of seawalls: pre-cast block and stone-filled cribs. These types of seawalls are built with modular unit construction (blocks or cribs) that can facilitate construction and result in lower cost. Both types can be considered gravity structures in that the weight of the structure is expected to resist the wave forces as well as any earth pressures from the fill landward of the seawall.
Seawall design components:
- Location of the seawall with respect to the shore. This is a critical design choice since it is directly related to the existing site bathymetry, the existing conditions present at the bluff, and materials along the shore.
- The height of the lake-facing wall with respect to the design water level.
- Total weight of the wall to the degree that its components act as a single mass.
- The structural connections that assure a stable, unified structure able to resist sliding and overturning forces. These may include design features such as reinforcing steel to connect the vertical wall to the cap, reinforcing or cabling to connect unit blocks together, and the means of connecting the members of the crib structure together.
- Fill material landward of the seawall face or placed within the crib structure.
- The seawall cap which prevents overtopped water from eroding the fill material.
- The provisions for drainage of run-up, splash, spray and groundwater.
- Provisions for toe protection or prevention of scour or undermining.
Wall & cap height
Unlike revetments, the height of a seawall is not often determined solely by calculating the run-up height and adding it to the DWL. There usually are functional concerns that come into play which result in a wall height less than what would be needed to prevent run-up and overtopping. The most common functional issue is access to the waters of the lake.
A seawall cap elevation 10 feet above the average summer lake level (about 571.5 feet IGLD 1985) would prevent run-up and overtopping a large percentage of the time, but it would also make access more difficult.
Seawalls designed to be higher than the upland elevation (protecting low-lying areas) are an example of when run-up and overtopping under severe flooding conditions are the most important design parameters.
Seawall height is an important aspect of the overall stability of the structure. Concrete blocks stacked more than three units high have a tendency to be much less stable unless significant interconnection and tie-backs are included in the design. Similarly, steel frame cribs become more susceptible to bending and overtopping stresses as the crib height increases.
It is common to design seawalls with relatively low wall heights (elevation 576 to 580 feet IGLD 1985) and include a retaining wall landward of the cap that serves to contain the run-up and overtopping that would be expected under high lake water level and severe storm conditions.
If a seawall height is determined for functional reasons it is appropriate that this basis of design be identified in the design information.
Run-up & Overtopping
Run-up height for seawalls can be estimated using the same equations presented in Chapter 3. The estimate using the empirical (FEMA) equation R = 0.7 x Hb will tend to underestimate the run-up, especially considering that the vertical face of most seawalls will force significant amounts of water into the air, which can then be carried by the wind over the crest of the wall. While the wind-borne wash may create overtopping volumes that need to be addressed for erosion or drainage control, this effect is not as significant from a structural standpoint.
If run-up and overtopping volumes will be significant, a straight-forward option is to include a second wall landward of the cap that would serve as a barrier to overtopping water reaching the bluff or bank. In many cases this would be a wall of lower height that would also function as a retaining wall. It is not recommended that the seawall cap width be designed as the means for attenuating overtopping effects. Wider seawall caps are in opposition to the design goal of minimizing the lakeward extent of an erosion control structure.
Sliding and Overturning
Each seawall design should be checked for both sliding and overturning. Every site and every design will have different wave conditions, materials of construction and upland geology. The engineer should carefully evaluate all potential forces acting on the seawall and the expected types of structural failure. The simplified illustrations on this page describe some of the conditions that may be present at a site and the two most common failure modes that need to be checked. The Design Examples for concrete block and steel frame crib seawalls that follow in section 4.5 more fully detail both sliding and overturning calculations for specific site conditions including wave forces.
Basic Sliding Safety Factor Equations
Overturning safety factor
The factor of safety for overturning is the sum of the resisting moments divided by the sum of the overturning moments. The diagram below and the two formulas show the basic relationships.
This case assumes that the wall is under static conditions and that the forces due to the height of water are equal on both sides. The moments due to wave forces will act in the opposite direction as the earth pressure forces, so the static condition, ignoring the wave forces (assuming that there are no waves) is considered the worst case.
Every design will have very specific conditions that must be analyzed on a case by case basis. The equations and assumptions above should be considered only as a very simple example.
Pre-cast concrete block seawall design
There are many pre-cast concrete block configurations and sizes. The most common use a transverse tongue and groove to resist sliding forces of the stacked blocks. The specific sizes used will depend on factors such as the equipment available for installation, the pre-cast forms used by a manufacturer, the engineer’s or contractor’s familiarity with a specific type of block and the overall dimensions needed for the seawall.
As the table shows, pre-cast concrete block unit weights are in the same range as typical armor stone. There are also block seawalls that use large hollow pre-cast units. These are usually connected with reinforcing bars and the open space then filled with grout or concrete.
Block seawall general arrangement
The following guidelines reflect OCM’s experience reviewing many designs and observing the performance of existing structures along the shore.
The layout of the seawall should match the plan of the shore. If the shore is curved, the seawall should be designed to match the shore plan.
A second row of block landward of the lower tier of block may provide additional stability and reduce the potential for sliding failure. The two blocks of the first tier should be structurally connected.
Designs that include a slight over-hang of the cap (with a chamfer) can help reduce overtopping by redirecting a portion of the wave energy lakeward.
Stepped block seawalls, with each tier slightly set back from the one below will generally result in a more stable structure with reduced run-up and overtopping.
Block seawall structural design
Structural design considerations include:
The first tier of block must be set on firm material, with sufficient bearing capacity to resist settling. Shale and hard glacial till are present below the nearshore beach material along much of the Lake Erie shore from Erie County east. The conditions at each site should be verified, as there are numerous anomalous buried stream beds and discontinuities along the shore.
One of the most common reasons for the failure of block seawalls is the eventual undercutting of the nearshore, causing scour of the foundation material under the block. This is due to changing lake levels and to the reflected wave energy from the seawall itself. To counter this common long-term threat to the structure the design can:
Include entrenchment into the underlying material;
Provide stone toe protection to reduce scour; or
Locate the seawall as far landward as possible, which reduces the amount of wave energy at the structure’s toe.
The importance of providing substantial interconnection of the blocks, cap, and any required tie-backs cannot be overstated. Although individual block units may have sufficient weight to resist wave forces, a unified structure is the best means of preventing significant failure of the seawall.
Steel frame crib design
The use of shore-fabricated steel frame cribs as an element of erosion control structures along the Lake Erie shore has been on the increase since the 1990s. Prior to this, timber frame cribs were more common. While this chapter addresses the use of cribs as seawalls for erosion protection, cribs are also commonly used as pier segments for watercraft access structures.
Steel frame cribs are essentially rigid baskets that are filled with appropriately sized stone or rubble to create a unified gravity structure that is capable of resisting the Design Wave forces. The total weight of the stone fill within the crib acts as a single mass. This is an advantage when compared with the required unit weight, size and cost of armor stone under the same design conditions.
Since the crib structure is partially open to the water on the face, and the rock is generally large in diameter, the crib must be considered a porous structure that allows the transmission of wave energy through it.
Key design issues for steel frame cribs include:
A steel frame crib structure is shown above from four different angels. The top picture is the lakeward most crib in the bottom picture. The top-middle picture is taken from standing atop the lakeward-most crib. Design Example E describes a typical steel frame crib.
- Sufficiently sized steel members connected in a rigid fashion to resist bending due to wave and ice forces and due to the lateral forces exerted by the stone/rock fill. The crib design should be evaluated using standard structural steel design methods.
- Steel frame cribs are particularly susceptible to damage by ice. Bent and twisted steel cribs caused by ice heaving have been noted by the OCM and property owners. Ice forces along the shore include both horizontal and vertical forces. Vertical forces can lift the crib, potentially causing displacement of the entire structure and certainly stressing steel frame members facing the ice. Horizontal ice forces from the thermal expansion of the ice built up along the shore as temperatures rise can result in bending stresses at the crib’s exposed members. Recommended design values for horizontal ice forces on cribs range from 5,000 to 20,000 lb/ft (from: “Ice Engineering Design for Marinas,” C. Allen Worley, World Marina ’91 Conference, American Society of Civil Engineers). For a detailed discussion of ice forces refer to Chapter 6 of the USACE “Ice Engineering” EM-1110-2-1612, Sept. 2006.
- A means of resisting sliding forces from both wave action and earth pressure. This may be in the form of driven or drilled piles to which the crib is attached. This design feature will be highly dependent on the underlying strata. Pile can be driven into till and to a lesser extent shale, but in some cases holes must be drilled into which the pile is set and then grouted.
- The spacing between cross-members needs to be small enough to retain the size of the stone or concrete rubble used as fill.
- Since the crib fill is typically smaller in diameter and less in unit weight than armor stone, wave forces transmitted through the crib will cause uplift and movement of the material within the crib. This will cause the fill material to fracture, wear and re-settle. This leads to a common type of failure of cribs, as over time they lose a portion of the fill which reduces the total mass and increases the potential for overturning and sliding. This is a good reason to specify small armor stone rather than concrete rubble because rubble will tend to break up faster and to a greater degree than stone.
- In most cases, the crib will include a cast-in-place reinforced concrete cap which is typically 1 foot or less in thickness.
The most common designs for steel frame cribs are modular units 10-feet wide by 15 to 20-feet long, with the height variable to meet the site bathymetry. Diagonal cross bracing is included in nearly all designs. In most designs, crib members are welded, with multiple cribs bolted together. A typical spacing between transverse members along the sides is 1 foot, allowing stone of 0.5 to 1.0 tons per unit to be used as fill.
4.4 Construction, Inspection & Maintenance
Erosion control structures must be constructed as shown on the approved design drawings. This ensures that the specified materials and the location of the project features as selected by the engineer are built in a manner that leads to a stable, long-lasting installation. It is also a condition of the permit authorization.
In many cases the construction process is under the full control of the contractor, who may or may not have had input into the design. This does not relieve the owner of the responsibility of ensuring that the work is fully consistent with the design and within the footprint shown on the approved design drawings.
A property owner can engage the engineer to oversee construction activities to ensure that a project is built according to plan.
It is helpful to plan a pre-construction meeting to be attended by the contractor, engineer and property owner to discuss the project schedule and logistics, and identify any potential changes that appear necessary to the existing design. If changes to the design are needed, the engineer of record can then submit the revised drawings to the regulatory authorities for review. Construction should not proceed until those changes are approved by all authorizing agencies.
Although it is not as common with smaller projects at single owner sites, the property owner can engage the engineer to oversee the construction activities to ensure that the project is constructed according to the plans.
While experienced contractors have developed significant cost savings and potentially effective changes to designed and permitted shore structures, there have been far more instances where contractor-initiated changes have resulted in poorer structures and in some cases serious failures. While some changes may appear to be minor, they can lead to catastrophic failure. For example, the substituting of smaller armor stone, would lead to a premature failure of the armoring. Effects of changes like this may not be seen for years, but eventually a high lake level and storm conditions similar to the design conditions will impact the site and the damage will be evident. This is why all proposed changes to approved designs must be approved by the engineer of record for the project and revised authorizations be obtained from all involved agencies. (top)
Inspection and monitoring
The development and implementation of a monitoring and inspection plan is a critical component for the long-term success of any coastal project.
For erosion control structures such as revetments and seawalls, the recommended monitoring and inspection can be as straight-forward as looking for and documenting any significant changes after construction has been completed and on a periodic basis thereafter. Typical post-construction problems that should be identified include:
Displacement down the revetment slope or movement of armor stone.
Cracked armor stone or concrete.
Uneven settling of a seawall section or crib.
Slumped upland bluff areas above the revetment or seawall.
Increased erosion at the ends or flanks of the construction.
Significant changes to the beach either at the site or along adjacent or nearby properties.
Inspection and monitoring should be performed on a routine basis, at least once a year. Documentation can include photos, record of the water level at the time of inspection, and notations about the condition of the various features of the structure.
It is appropriate to engage the services of the design engineer for inspection and monitoring, especially if the project is complex or difficult to access, but in most cases the property owner can carry out inspections and document the results without difficulty. (top)
Maintenance and repair
Through the monitoring and inspection of a project the required maintenance is likely to be discovered.
Minor repairs to authorized structures may not require additional regulatory authorization. It is advisable to contact the applicable regulatory agencies to determine if authorizations are required prior to completing any planned maintenance or repairs. (top)
4.5 Design Examples
The five design examples that follow include three armor stone revetment designs (A, B and C) and two seawall designs (D and E). Each example begins with a narrative describing the site and the engineering and surveying performed. This is followed by engineering calculation sheets, the engineering design drawings, the submerged lands lease metes and bounds descriptions and plat. Examples C and D share the same fictional site location, as do Examples B and E.