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Ohio Coastal Design Manual

Chapter 3. Design Fundamentals

Design wave height

Waves can exert large forces on shore structures. Fresh water weighs 62.4 pounds per cubic foot and a large wave may bring thousands of pounds of force against a structure. The structural requirements for the stability of a structure are directly related to the DWH and the forces exerted by the design wave. The higher the wave, the larger the forces, and therefore, the larger and heavier the needed structure.

Waves along the shore of Lake Erie are produced primarily by wind. Waves can also be produced by boat wakes, but these do not reach the height or intensity of wind-driven waves. Wind-driven waves can come from any lakeward direction. Most of the Lake Erie shore will be subject to the waves generated by both the most common southwest storms (summer thunderstorms) and the more intense, but less frequent northeast storms more common in late fall and spring.

As wind velocity increases, the height of waves will increase until the waves break, decreasing the height. As waves approach the shore, and the water depth shallows or shoals, waves will increase in height until they break. It is the wave height as it approaches the shore and the proposed structures that is critical to design. This is why bathymetric profiles or contours establishing the depth of water in the nearshore are important to the design of shore structures.

In most cases the depth of the water at the structure under the DWL condition is the controlling dimension in determining the DWH.

A very complete description of wave theory, meteorology, the methods of developing design wave parameters and the behavior of waves in the near shore is found in Chapter II of the USACE’s Coastal Engineering Manual (CEM). The analytical methods described in the CEM are usually needed only for complex projects or when alternative design parameters are used.

For the design cases associated with less complex shore structures such as revetments and seawalls, the wave conditions can usually be calculated using simplified methods if certain assumptions are verified. The first assumption is that the nearshore is considered to be “shallow.” With respect to waves, a shallow condition on Lake Erie usually means depths of 20 feet or less, which is generally true along the entire Lake Erie shore. The second assumption is conservative in that it assumes that the design wave will break at the structure. This results in selecting a design wave that would exert the greatest force on the structure.

Waves in the nearshore will tend to break when the wave height reaches about 80 percent of the depth. A simple calculation based on this concept can be used to select the design wave, which is designated as “Hb” (height of the breaking wave). There are numerous equally valid means of calculating design waves based on transformation of wave hindcast data, on wave spectral analysis and based on wind conditions. In most cases the wave period, (T) and the slope (m) of the nearshore are required for those analyses. Programs such as the U.S. Army Corps of Engineers “ACES” (Automated Coastal Engineering System) software’s linear wave theory module can also be used to derive design waves.

The table above is a summary of Lake Erie off-shore hindcast wave data generated by the U.S. Army Corps of Engineers. The data is for the 16 wave information stations directly off Ohio’s coast. These are shown as numbers on the Lake Erie Basin map. Data is available for the other WIS locations, but not included here.

The data provides an overview of the varying wave climates along the lake. Average wave conditions all along the lake are dominated by waves from the west through the southwest which reflect the dominant weather pattern along the Lake Erie shore. The highest waves in the western basin are from the east; from the north into the Cleveland area, and then from the west as the shore becomes oriented southwest to northeast in Lake and Ashtabula counties. Wave heights are also limited by depth and fetch distances, with the shallower western part of the lake having lower average and maximum wave heights and periods than the eastern Ohio portion.

Offshore wave data can be used to calculate the DWH as the off-shore (or deep water) wave transforms into the shore. There are a number of methods that can be used including those in the ACES wave transformation modules. The limiting conditions and applicability of the various methods of transforming deep water waves into shallow water waves are fully discussed in the CEM. There are also numerical models available for evaluating wave data and assessing nearshore wave climate conditions. The complexity of using these methods is beyond the scope of this manual.

For simple design conditions, the following formula will provide a reasonable and conservative design wave height, H_b for the breaking wave.Where d_s is the depth of water at the structure toe under the DWL condition.

This calculation is independent of the nearshore slope and wave period and assumes that the design wave will break at the structure toe. This equation is derived from Figure 2.2 of EM 1110-2-1614, “Design of Coastal Revetments, Seawalls and Bulkhead,” USACE 1995. It should be noted that the depth of water at the structure toe (d_s) can change over time if there is the potential for scour at the toe. OCM typically assumes that the ultimate d_s will be the bottom elevation of the toe, even though it may be initially entrenched in the underlying lake bottom material.

If these assumptions are not valid for the proposed design or the site conditions are complex, then development of the design wave using methods documented in the CEM or other suitable design references may be necessary.

Run-up and overtopping of structures

The wave run-up height is the additional height above the DWL that the design wave will wash upwards along the slope or over the proposed structure. The run-up height is used to set the elevation of the crest of erosion control measures and should be used to assess the impact of high water level and severe storm conditions for seawalls that have their cap elevations below expected run-up heights.

Water and wind-driven spray from run-up can wash-out and erode the upland, displace smaller sized stone and lead to severe damage to the upland and the shore structure.

Overtopping refers to the volume of water that runs up and over the structure. It is sometimes helpful to estimate the overtopping volume to design the drainage features of a project. Overtopping can be a safety concern on access structures and portions of erosion control structures that have access incorporated into the design as water on the structures’ surfaces may cause slipping or falling.

Although calculating the overtopping volume is rarely required for erosion control projects, one very serious exception is for projects in the low-lying areas in the Western Basin that have been historically subject to lake flooding. Consideration of the wave climate during extreme high water years should be included in the determination of the crest height needed to prevent the overtopping of erosion control structures along the shore in these areas. The methods for calculating the overtopping volume are fully covered in the CEM and can be performed using the ACES software.

For structures such as seawalls and piers, it is not always possible to eliminate all run-up and overtopping and still have the desired functionality which usually is related to access to the water.

The equations to calculate run-up height described below can be used if the following conditions apply:

• The structure has a single slope of the same material.

• The design wave breaks at the toe of the structure.

• The structure is the same in cross section throughout the site.

The first equation for calculating the Run-up. Height (R) is based on the breaking wave height H_b multiplied by an empirical coefficient ().

Where

This equation assumes that the run-up is 70 percent of the breaking wave height, which is based on the Federal Emergency Management Agency (FEMA) run-up models. This equation will tend to underestimate the run-up height.

The second equation that can be used to calculate run-up is an empirical formula that also requires the calculation of the surf similarity parameter also known as the Iribarren number (Aherns and Heimbaugh, cited in EM1110-2-1614 Design of Coastal Revetments Seawalls and Bulkheads, USACE 1995).

*[Note: the values provided above for coefficients a and b apply only to single slope structures with rough, porous armoring. Coefficients a and b were derived by regression analysis of empirical data.]

The surf similarity parameter,  

tan  = revetment slope (e.g. 2:1 slope = 0.5)
g = 32.2 ft/sec2
T = wave period in seconds

The surf similarity parameter expresses the relationship of wave height to wave length at a given slope and is also useful in characterizing the types of breaking waves (shown on page 30).

Again, the basic and conservative design assumption is that the worst case condition is a breaking wave at the toe of the erosion control structure. Under these conditions the breaker will be collapsing onto the structure.

Calculating the run-up onto a structure can also be performed using a number of other formulae, including the calculation embedded into the ACES rubble-mound revetment design module. The second empirical equation above will tend to calculate a higher run-up value than the ACES module.

Types of breaking waves

Changes to the littoral system

The sand and gravel on beaches and moving in the littoral system are a part of the dynamic lake system. If the movement of this material is changed or interrupted, or if the total amount in the lake nearshore or entering the lake within an area is changed, there may be erosion losses at downdrift beaches.

When selecting pre-fill, sand that is similar to or heavier than the onsite sand will have a longer retention time at the project site.

Photo: Sand along the coast varies as illustrated in these photos from four of Ohio's 169 Lake Erie public access sites: Port Clinton City Beach (top), Lorain’s Lakeside Landing (middle top) Willowick City Hall (middle bottom) and Headlands State Park (bottom).

This is due to the transitory nature of beaches and the normal overall flow of littoral material across the lakeshore. Stable beaches require near constant replenishment from the littoral system. If there is a lack of sand and gravel reaching the beach, it will erode.

Eroding lakeshore bluffs are a source of material entering the littoral system. The placement of structures that minimize bluff erosion results in a decrease in the amount of material added to the littoral system. Over the design life of the structure, this can have impacts on the availability of material to form and sustain beaches.

The ODNR Division of Geological Survey frequently calculates the expected volume of littoral material prevented from entering the lake as part of the Survey’s review of projects along the lake. The calculations are based on the dimensions of the project, the bluff recession rate due to erosion and the reported fractions of sand and gravel present in the bluff material. Typical losses of littoral material to the lake over 30 years from a small erosion control project can be on the order of 100 cubic yards. This impact can be offset by periodic nourishment of the area with sand.

Structures that extend onto the shore or lakeward from the shore will have an impact on the natural movement of sand and gravel in the littoral system. In general, the farther lakeward a structure extends, the greater the potential impact.

Shore-parallel structures such as seawalls will tend to reflect sufficient wave energy to suspend even gravel-sized material in the water column which severely reduces the possibility of a stable beach forming immediately lakeward of the structure. Revetments result in less reflected wave energy than seawalls, but will also tend to reduce the potential for beach formation unless they are located well upland.

Shore-perpendicular structures such as groins, jetties and piers will usually result in significant changes to the movement of littoral material. In most cases these structures will entrap sand and gravel permanently by interrupting the natural transport of these materials along the shore. These structures may prevent the natural replenishment of adjacent or nearby beaches that are downdrift in the direction of transport.

The design of groins usually includes a calculated volume of pre-fill sand that is placed up-drift of the structure immediately following construction. The concept of pre-fill is based on the fact that groins are expected to permanently remove a volume of sand from the littoral system and form or stabilize a beach updrift of the structure. The pre-fill volume is needed to balance the littoral system by “filling” the groin compartment, so that the littoral material passes downdrift.

Piers are generally shore perpendicular structures that are used to access the waters of the lake. Many piers consist of a solid or mostly solid design that acts like a groin. To allow the unrestricted flow of littoral material past a pier, the usual design solution is to include an open span near the shore.

Jetties are structures that protect and reduce shoaling in a harbor channel, usually on a river or creek outlet to the lake. With respect to the movement of littoral material, jetties act like groins. Jetty design would need to potentially include both pre-fill along the up-drift side and a plan for surveying and measuring the volume of any accumulated littoral material and a means for by-passing the material on a regular basis.

If the structure will intentionally impound littoral material after construction, the design normally would include the placement of additional sand from an upland source equivalent to the calculated volume that will be impounded by the new structure. USACE design guidance recommends that the design volume include a factor of safety of 1.5 to 2. This added sand pre-fill will offset the negative impact to downdrift shorelines by minimizing the amount of native material impounded by the proposed structures. The sand pre-fill should be very close in particle size distribution to the existing material along the shore. Typically, sand that is lighter (smaller diameter) than the onsite sand will be more easily transported by waves away from the project site. Sand that is similar to or heavier than the onsite sand will have a longer retention time at the project site.

The methods of calculating the expected volume of littoral sand required to bring the project to equilibrium under design water levels include straight-forward volumetric estimates assuming a depth of the fill over the existing lake bottom and the use of beach profiles using multiple cross-sections to calculate fill volumes.

The littoral material on beaches is not usually considered to be suitable material for stable foundations for shore structures. Sand and gravel can be readily scoured at the base of and then under a structure, leading to settlement and potential failure of the structure. If the footprint of the structure will cover existing beach material, this would result in a loss to the overall littoral drift available in the lake. In such cases, beach material must be removed from the footprint to the depth of the underlying strata before construction and side-cast along the shore.

Effects on adjacent or nearby properties

The two most important questions related to effects on adjacent or nearby properties that must be addressed in the design of a shore structure are:

The design objective for all shore structures is to minimize the changes to wave energy at adjacent properties and to retain the same flow of littoral material along the shore.

• Will the structure sufficiently change the direction or magnitude of wave energy at an adjacent or nearby property to adversely affect the shore or bluff?

• Will the project change the flow patterns, interrupt or entrap sufficient littoral material to create a deficit of beach material and increased erosion along the shore on nearby properties?

The design objective for all shore structures is to minimize the changes to wave energy at adjacent properties and to not change the flow of littoral material along the shore. If the proposed structure will result in significant changes to wave energy or the littoral system, the engineer should prepare an explanation of the expected magnitude of the potential effects, justification for the extent of potential harm and a plan to mitigate such effects.

Impact of design on habitat

As discussed in Chapter 1, structures that would occupy existing beaches or the shallow nearshore areas along Lake Erie have impacts on these unique and limited habitats. In the simplest terms, structures use space that would otherwise be available to the organisms that would normally be there.

Beaches are ephemeral over seasons and years but they can be sustained and augmented with appropriate care and design. Unfortunately, shore structures such as revetments and seawalls can result in the complete loss of the beach. Once the nearshore is filled, it is lost and cannot be replaced.

The impact of one small project may seem inconsequential, but the cumulative impact of the addition of thousands of small shore structures along the shore over many decades has significantly changed both the quality and the quantity of beach and near shore habitats.

The most straight-forward design approach to minimize the impact on beach and nearshore habitats is not to construct on the beach but instead locate structures up the bluff or bank face. This is not always possible, so the next level of habitat-impact design is to minimize the distance the structure extends from the toe of the bluff or bank.

Structures that extend lakeward beyond a minimum distance from the toe of the bluff or bank must be balanced between one person’s use and the good of all the people, fish, birds, invertebrates and micro-organisms to whom Lake Erie has been entrusted.

Other design considerations in the
general arrangement of shore structures

There are factors in addition to those discussed above that need to be considered in planning the general arrangement of shore structures.

First, lake access structures such as seawalls may not be necessary along the full length of a property’s shore. In some cases projects will provide better functionality if access structures and erosion control measures are combined.

Construction atop a vertical shale bluff such as that pictured can be challenging as the weight of heavy trucks and equipment at the edge of a bluff can cause damage and lead to erosion.

Second, a structure needs to be rounded and merged into the upland as it approaches the littoral property boundary to avoid both impacts to adjacent property and to minimize the potential for flanking around the ends of the structure. Straight, shore perpendicular ends of structures can lead to chaotic wave conditions that can result in increased wave-based erosion at such corners.

One of the most common issues associated with shore structures is the large size and weight of the material required. In many cases, this also means that a significant area of the upland must be used for staging, movement of materials and heavy construction equipment such as dozers, track-hoes and cranes. Access for trucks is also usually required. The use of heavy equipment on a small residential lot can have a serious impact on the property and may even result in damage to the bluff or to neighboring properties. Experienced contractors and engineers who specialize in building along the shore of Lake Erie have valuable insight into the planning and logistics needed to deal with these issues. This is also a very good reason for coordinating projects along multiple parcels involving a number of property owners.

Another challenge is construction along the vertical shale bluffs (pictured above) present in Cuyahoga and Lorain counties. At some sites the bluff can be more than 50 feet high. Dumping of material from the top of bluffs is not a good construction practice and can have negative consequences such as unintended breaking up of the material, making it susceptible to movement by wave action onto adjacent properties and also pollution of the lake by fines associated with the material.

The weight of heavy trucks and equipment at the edge of a bluff can cause damage to the bluff itself leading to loss of sections of the upper bluff. The best alternative in high, vertical bluff areas is to place material from a barge.

Design drawings, engineering methods and calculations, materials specifications and supporting information

The purpose of creating design drawings and specifications for the materials of construction is communication. The drawings, specifications and supporting information are the means by which the intentions of the owner and the engineer are communicated to the contractor who will build the structure and to agencies that will review the design and authorize the construction.

The drawings and specifications become a part of regulatory authorizations. The documents also become a permanent public record of the approved design including the exact dimensions of the project and the specific materials described by the drawings and specifications. OCM offers the following Suggested Standards as a step toward the goals of decreasing the time required by agencies to review design submittals and eliminating the need to revise designs during and after regulatory reviews. The drawings and engineering calculation sheets included in the design examples in Chapter 4 have been prepared using these standards.

Suggested standards for engineering and surveying drawings

1. All drawings must be identified with information in the title block. This must include the project name, address, sheet title, sheet number and engineer’s name.

   
   

2. Plan views, cross sections and any other drawings depicting features of the site or structures are to be at standard scales and shall include a bar scale. The scale must be noted in the drawing title block.
   

   
   

3. The drawings must accurately and adequately show the features of the proposed structures and the existing site information. Existing conditions and proposed work must be on separate drawings.
   

   
   

4. The drawings and text on the drawings must be composed in a manner so that they can be reproduced by photocopy and scanning so that all features of the site are presented in a clear and easily readable fashion.
   

   
   

5. The existing plan view must include the existing contours of the upland, all potentially affected upland structures, and the existing beach and shore structures present along the beach, shore, or nearshore. The existing plan view drawing must not include any proposed structure or modification to the existing site conditions.
   

   
   

6. The existing plan view drawing must also include the profiles or contours of the off shore bathymetry to a distance of at least 100 feet beyond the extent of the proposed structure. The number of bathymetric profiles required to define the nearshore will vary with the project. As a general rule:

   • Property width less than 75 feet - 2 profiles
   • Property width 75 to 100 feet - 3 profiles.
   • Property width greater than 100 feet- 1 profile for every 50 feet.
   
   
   

7. All elevations, both bathymetric and upland topographic, must be referenced to the International Great Lakes Datum of 1985.

   
   

8. Plan view drawings of the proposed structures must include all of the site features present on the existing plan view. Changes to the upland topography after construction must be included. All proposed shore structures must be fully dimensioned in the plan view, including:

   • The linear distance along the shore.
     
     
   • The distance the structure extends from the existing toe of the bluff or shore at all significant features of the structure.
   • Elevations of structure crests, caps and toes.
   • Slopes of structures.
   • Location, extent and volume of sand pre-fill.
   • Location of the area where excavated or dredged sand by-pass is to be placed.
   
   
   

9. Cross sectional drawings of the proposed structures must be consistent with the plan view and the location of the cross sectional views must be shown on the proposed plan view. Cross sectional views must be sufficient to detail all aspects of the structure. If there are multiple components or significant differences in dimension or the materials of construction, multiple cross sectional views will be necessary.

   
   

10. The geology of the bluff or bank and the nearshore must be shown on the cross sectional views. The elevations of changes in strata must be shown. The existing profile of the bluff or bank must be shown on the cross sectional drawings.

    
    

11. The elevations, dimensions and the arrangement of, and note of, the materials of construction of all significant features of the structure must be shown on the cross sectional view. These include the following:

    • Elevations of structure crests, caps and toes.
    • Elevations where materials of construction change.
    • Elevation of the lake bottom at the toe of the structure.
    • Slope(s) of the structures.
    • Structure dimensions such as armor stone and underlayer thickness, and toe trench depth.
    • Distance from the toe of the existing bluff to the lakeward extent of the structures.
    • Design Water and Wave Height elevations.
    • Profile of any proposed sand pre-fill.
    • Materials of construction.
    
    
    

12. The details pertaining to the structural stability and construction details of the structures should be included on the plan and cross sectional drawing to the extent possible. Supplemental drawings providing sufficient detail to allow evaluation of the stability and the structural connections (re-bar, tie-backs, grouting, cables, etc.) between structural elements must be provided if these features cannot be clearly represented on the plan and cross sectional views. Materials of construction, re-bar sizing and spacing and similar details can be included as notes on the drawings. All design detail and specification of construction materials must be included on the drawings.

    
    

13. The signature, date and the stamp or seal of the Ohio registered professional engineer or professional surveyor who prepared the drawings must be affixed to each drawing sheet or an appropriate, bound cover sheet. In accordance with Ohio Revised Code 4733.14, “...Plans, specifications, plats, reports, and all other engineering or surveying work products issued by a registrant shall be stamped with the seal or bear a computer-generated seal in accordance with this section, and be signed and dated by the registrant.”

Suggested standards for engineering methods and design calculations

Design calculations must be clearly presented to document how the selection of the structure’s dimensions and materials of construction will result in a stable structure under the design water level and design wave conditions. The specific equations or engineering methods used must be noted. The basis for using assumed values must be stated.

The basis for the selected design water level and design wave height must be documented.

The basis of design for each key element of a structure must be stated. This would include design features such as revetment crest height and width, seawall cap height and width, and the length of piers.

Any specific coastal engineering data or information relied on by the engineer or related to design conditions must be clearly stated. This would include the use of wave hindcast data, wind developed wave conditions, or the assessment of fetch-limited wave conditions.

Excepting armor stone revetments and other rubble mound structures, all other proposed structures along the shore must be analyzed for both sliding and overturning stability. Consideration must be given to wave forces as well as passive earth forces if a structure will be acting as a retaining wall and a seawall.

Calculations related to the volume of littoral material required to reach equilibrium under design water levels by a structure must be fully documented, including all assumptions.

The specific, referenced engineering method, and the input values known and assumed must be cited. For example, if calculations are done using the Automated Coastal Engineering System (ACES) software, the specific module used and the input parameters must be listed.

Suggested standards for material specifications

All materials to be used in the construction must be specified and noted on the drawings. This includes:

• Cast-in-place concrete, strength and re-bar size and configuration.

• Armor stone size and weight.

• Other stone size and weight.

• Pre-cast concrete strength, reinforcing and dimensions.

• Geotextile-fabric filters: material type or manufacturer specification.

• Steel used as bulkhead or cribbing: size, weight and connection detail.

Particular care must be given to specifying fill. Common “clean, hard fill” that may be appropriate for upland applications is highly problematic when used as part of shore structure construction. A fill that contains a high percentage of fines, debris or vegetation will not be suitable for use along the shore of Lake Erie.

If “concrete rubble” is specified as a fill material it must be free of exposed rebar, free of all fines and contain no debris. The specific size or range of sizes for the concrete rubble must be included in the specification.

Sand to be used as pre-fill or beach nourishment must be specified using standard sieve sizing and gradation and in most cases must be specified as originating from an upland source.

Suggested standards for supporting information

Supporting information refers to information relative to the design of the shore structures in addition to that which appears on the design drawings and/or is documented in the engineering calculations. In many cases, this information would be submitted as part of an application for a regulating agency authorization.

The purpose or function of each major element of the proposed work must be clearly stated.

Any assumptions regarding the influence of the geology of the site must be included in the supporting information. This should include the identification of the upland strata and the composition of the nearshore.

Any expected effects on the littoral system as a result of the proposed structures must be discussed and documented.

A plan for long term monitoring, sand by-pass or beach nourishment that is to be conducted following construction, if needed as part of the project, must be included in the supporting information. The details of the plan can also be included as notes on one or more drawings that indicate monitoring profiles and the location(s) where by-passed sand is to be placed.

Any information used to develop the design or layout of the proposed work must be included as supporting information. This may include photos, studies, geotechnical or soil boring data, sediment or beach particle size data and pertinent historical information.