The purpose of Example Project E is to provide access to the waters of Lake Erie while still providing adequate protection to the bluff from wave based erosion. The replacement of a portion of the armor stone revetment with a steel crib seawall is proposed.
The project site is located along the shore of Lake Erie in Vermilion, Erie County, approximately 3.5 miles west of the Vermilion River. The shore in this area is oriented from west to east. The predominant direction of sediment transport in the littoral zone is from east to west.
The shore at the project site is irregular in shape due to the installation of the shore perpendicular structures. The site property is oriented in a slight northwest to southeast direction. The project site spans two parcels and is approximately 200 feet wide. At the east end of the property there is a small embayment suggesting increased erosion in this area.
The bluffs in this area are 15 to 20 feet in height and have been partially regraded to an approximately 1.7 horizontal to 1 vertical slope. The bluffs are composed primarily of till overlain with gaciolacustrine silts and clays. In the nearshore zone, shale makes up the bottom. Sand and a nearshore bar system are located as far as 700 feet offshore near the site location. The bottom slope from 100 to 1500 feet offshore is approximately 100 horizontal to 1 vertical.
An armor stone revetment has been constructed on the site as erosion protection. The revetment extends from an elevation of 567.5 feet as referenced to the International Great Lakes Datum of 1985 (IGLD 1985) at the base of the toe to 582 ft IGLD 1985 at the crest.
The revetment is constructed with a double layer of 2 to 4 ton armor stone over stone filter layer consisting of 12 to 24-inch stone. 4 to 5 ton armor stone entrenched 2.5 feet into bedrock is specified as toe protection. An ODOT 601 Type “B” stone splash apron extends from the revetment crest to 585 feet IGLD 1985. Above the splash apron the bluff has been regraded to the top of the bluff at approximately 589 feet IGLD 1985.
The site is exposed to storm waves from west-southwest to east-northeast. A review of historic wave information results in a significant wave height of 2.3 feet at a period of 3.6 seconds. The most frequent wave direction was from 180 degrees (referenced to 0/360 degrees north). The largest wave recorded over the 32 year study was 11.8 feet with a 9.0 second period. The average direction of the largest waves was 11.0 degrees. Wave data was measured at WIS station E06 located approximately 4.5 miles north of the project site in 33-foot deep water.
The project site is not located in a designated Coastal Erosion Area based on the 2010 mapping, but has an expected erosion rate of 0.1 to 0.8 feet over 30 years. There are no existing drainage measures causing localized erosion at the project site.
The eastern and western adjoining properties are similar to the project site in bluff elevation and upland topography. The western adjoining property is undeveloped and includes no shore protection. A 15 to 20-foot wide sand and gravel beach is present at the toe of the bluff. The eastern adjoining property includes an existing structure for erosion protection in the form of a revetment. The structure is in poor condition due to undersized concrete rubble being fractured and displaced by wave action.
The upland parcel is located within the Firelands portion of the Connecticut Western Reserve district of Ohio’s Public Lands Survey System, specifically part of Original Lot (O.L.) 34, Town 13 North, Range 20 West. Being within the incorporated boundaries of the city of Vermilion, the parcel boundary extends north of the sixty (60) foot dedicated right of way centered on said centerline.
Horizontal control was established for this site by evaluating the location of published monumentation through the National Geodetic Survey (NGS) website: www.ngs.noaa.gov. The closest station to this site was determined to be “A 319” (PID MC0927) which is approximately 2.5 kilometers east. Based upon the NGS datasheet, the horizontal accuracy of the station is reported as a Cooperative Base Network Control Station with reports that attempts to recover the station were successful in 2003, 2004 and 2009. Therefore, this station was used within the horizontal control network. An open traverse was performed between “A 319” and a Third Order station “Ceylon” (PID MC1118) with intermediate stations located close to the project site. No adjustment was made to the resultant coordinates.
Vertical control was established for this site by evaluating the location of published monumentation through the NGS website. The closest station to this site was determined to be “Z 318” (PID MC0928) which is approximately 0.1 kilometers south. Based upon the NGS datasheet the vertical accuracy of the station is First Order Class II with reports that attempts to recover the station were successful in 2004. The NGS stainless steel rod, established in 1980, has a reported dynamic height of 597.99 feet at 45 degrees latitude. NGS Vertical Datum Transformation software (VDatum) was used to adjust for the hydraulic corrections for the project location based upon the latitude and longitude positions in the NGS datasheet for station “A 319”.
Confirmation of the elevation, relative to IGLD 1985, of the control stations was performed by benching into the water level on a calm day with minimal wave activity and comparing that value to the water level station data retrieved from NOAA’s Great Lakes Online website: www.glakesonline.nos.noaa.gov/monitor.html for station #9063063 (Cleveland).
With the horizontal and vertical control network established, recovery of boundary evidence was performed. Monumentation found and held as controlling stations included a 5/8- inch iron pin at the southwest corner of Sub Lot 5 and another at the southeast corner of Sub Lot 6. Subsequent points were located along the north right of way of West Lake Road within the Water’s Edge Subdivision, and proration of any surplus was calculated and applied to the subject parcels in the final determination of the boundary lines.
A topographic survey was performed that located the cultural (i.e. buildings, survey monuments, coastal structures) and natural (i.e. top and toe of bluff) features on the subject parcel and adjoiners. Notwithstanding the presence of random rubble along the shore on the east portion of the upland parcel, the natural shoreline appears to be unaltered by artificially placed fill material.
Parcel data provided by the Erie County Auditor’s Office was imported into the computer-aided design (CAD) drawing to establish a general orientation of the shoreline for a reach of approximately 1.5 kilometers. Methodology for partitioning the boundaries between the littoral adjoiners was examined including extending the upland parcel boundary lakeward without deflection and a radial projection from the general alignment of the 1.5 kilometer reach of shore from the intersection of the natural shoreline and the parcel sidelines. The radial projection method provided the most equitable distribution between the subject parcel and the east and west adjoiners.
A base map was provided to the engineering consultant that depicted the locations of the existing site improvements relative to the established parcel boundaries and littoral partitions. A general statement that the survey and plat were prepared t in conformity with Ohio Administrative Code (OAC.) Section 4733-37 was included and the Ohio registered professional surveyor’s signature and seal were affixed to the plat of survey (see Existing Site Plan “A”).
Design specifications and details are identified on the following design example drawings and supported by the included design calculations.
In order to improve lake access at the project site while still providing adequate shore protection a portion of the existing armor stone revetment is to be removed and replaced with a seawall constructed with stone filled cribbing. The existing revetment spans both parcels of the site property and is approximately 208 linear feet long. Four steel crib units are proposed. Each crib will be 16 feet long and 10 feet wide. The cribs are to be placed just west of the center of the project site with 3 cribs on the west parcel and 1 on the east parcel.
The proposed seawall is intended to maintain existing erosion protection while providing access to the waters of Lake Erie for swimming and small watercraft. Therefore the cap elevation of the seawall is to be set at 576 feet IGLD 1985 based on the structure’s recreational function rather than wave run up and overtopping. The structure will be placed on bedrock at an elevation of 569.8 feet IGLD 1985. The cribs are to be placed as far into the revetment as possible to minimize the overall footprint of the structure. In this case, the cribs are recessed into the revetment so that the seawall cap meets the armor layer. The armor stone removed from the revetment will be retained and used as fill for the cribs.
The steel cribs will replace the toe stone and must be large enough to prevent sliding failures in the armor layer of the revetment. The trench left from excavating the revetment toe stone will be filled with ODOT 601 Type “B” stone as scour protection for the seawall. The vertical piles of the crib are to be set 1.5 feet into bedrock and grouted in place. In the area of the toe stone, the pile will be set 1.5 feet below the toe trench.
The 30-year return period design water level for this site is 575.5 feet IGLD 1985 as listed in the “Revised Report on Great Lakes Open Coast Flooding” (USACE 1988). At the 30-year design water level the water depth at the base of the seawall will be 5.7 feet. Based on the breaking wave equation, a design wave height of 4.4 feet can be calculated.
With the design water level just 0.3 feet below the seawall cap elevation, it is clear that the seawall will be severely overtopped by 4.4-foot waves. The intended use of the seawall for lake access necessitates the low crest elevation of the structure. Overtopping during storm conditions is acceptable as the recreational intent of the structure limits it use during severe storms. The risk of overtopping is minimal as the existing 2 to 4 ton armor stone revetment continues behind the seawall to an elevation of 582 feet IGLD 1985. Additionally, the existing ODOT 601 Type “B” stone splash apron extends to an elevation of 585 feet IGLD 1985. A 10-inch thick reinforced concrete cap is specified for the seawall to withstand overtopping forces.
In order to confirm the external stability of the seawall it must be checked for both sliding and overturning. Due to the variable water levels and wave forces expected at the site, a minimum of two design cases must be considered for the steel crib. In this example, the stability is assessed both at low water with no waves and at the design water level with the design wave height. This ensures the design is acceptable as a retaining structure for the armor stone revetment and is capable of withstanding significant hydrodynamic loads.
It is assumed that the stone filled crib and revetment are porous structures and that water passes through. In this case hydrostatic forces are the same on all sides of the structure and the resultant hydrostatic force is limited to the buoyancy of the submerged portion of the structure.
Case 1 – Low Water
In this case, the following forces will act on the structure: gravity, earth forces, reactive forces, and friction.
The force of gravity is determined by estimating the total weight of the structure’s cross section. A total of 56.3 tons was calculated for each 16-foot long by 10-foot wide crib. A structure weight of 3.5 tons per foot will be used in the design. The assumed low water level of 569.2 feet IGLD 1985 is below the base of the structure. Hydrostatic and buoyant forces will not affect this design case.
In the absence of other vertical forces the normal reactive force is equal to the structure weight. If a minimum angle of internal friction of 35 degrees is assumed, friction forces can be estimated at 2.5 tons per linear foot of structure.
In most cases soil borings are suggested to determine actual physical properties at the test site. For this design example, the bedrock beneath the revetment and seawall is assumed to be capable of supporting the structures with minimal settling. A 110 lb/ft3 unit weight is assumed for the backfill. Based on the 26.6 degree revetment slope and 35 degree angle of internal friction, an active pressure coefficient of 0.43 is calculated. Earth forces are estimated at 0.4 tons per foot of structure.
The pile resistance to sheer load is estimated using the rigid analysis described in International Building Code 1805.72. This method results in a minimal estimate for pile resistive forces and provides a more conservative design than balancing active and passive earth forces on a steel pile fixed in bedrock. The rigid pile analysis conservatively estimates pile resistance at 13 lbs per linear foot of structure.
The earth force due to retaining the existing revetment is the only anti-stabilizing force. Friction and the resistance on the pile are stabilizing forces in this design case. The factor of safety for sliding stability is the ratio of stabilizing to anti-stabilizing forces. A factor of safety of 6.1 was calculated for the low water case.
To verify the seawall will be stable against overturning, moments are calculated about the structure toe. A 5-foot moment arm was assumed for the center of gravity and a 2.1-foot moment arm was assumed for the center of pressure for the earth forces. Friction and pile resistance forces were assumed to act at the base of the structure with zero moment arms. This results in a 17.6 ft-tons stabilizing moment per linear foot of structure and a 0.9 ft-tons per linear foot anti-stabilizing moment. A factor of safety of 20.6 was calculated for the low water case.
The ability of the steel crib to retain the revetment can also easily be checked by comparing the weight of the steel crib to the weight of the existing revetment toe stone. The steel crib weighs approximately 3.5 ton per linear foot. The 4 to 5 ton toe stone will be approximately 4 to 4.5 feet in diameter weighing only 1.25 tons per linear foot.
Case 2 – Design Water Level and Wave Height
In this case, the following forces will act on the structure: gravity, earth forces, normal reactive forces, friction, wave uplift, and horizontal wave forces.
The force of gravity is determined in the same method as the low water case (3.5 tons per linear foot of structure). However, in this case the normal reactive force will be reduced by buoyancy and the vertical wave uplift forces. Based on the 5.9-foot structure depth, 10-foot structure width and assuming 30 percent porosity, 1.2 tons per linear foot of buoyant force is estimated.
Several methods are commonly used to predict the forces due to waves. In this design example, a method described in the USACE Coastal Engineering Manual is used. Wave forces are calculated based on the Goda Formula for irregular waves modified to include impulsive forces from breaking waves. This method was adapted to the geometry of the proposed seawall. In particular the calculations have been simplified based on the exclusion of a rubble foundation in the design and the assumptions that Bm = 0 and hs = d = h’ (water depth at toe of structure is the same as water depth in front of structure).
It should be noted that this method assumes a vertical impermeable structure. It does not take into account the dissipation of wave energy within the matrix of the stone inside the crib or the transmission of wave energy through the crib to the existing revetment. This method will provide a considerably conservative design.
The Goda method predicts a free surface height 6.7 feet above the design water level at the wave crest. Wave pressures are calculated at 225 lb/ft2 at the base of the structure, 262 lb/ft2 at the design water level and 242 lb/ft2 at the crest of the structure. Wave uplift pressures are also estimated at 221 lb/ft2. Based on these pressures, the total horizontal wave force is estimated to be approximately 0.7 tons per linear foot of structure and the wave uplift force is estimated to be approximately 0.4 tons per linear foot of structure.
The 26.6 degree revetment slope and 35 degree angle of internal friction is used to calculate a passive earth pressure coefficient of 2.34 for the backfill. Earth forces are estimated at 1.7 tons per foot of structure. Pile resistive forces are estimated to be similar to the low water design case: 13 lbs per linear foot of structure.
The resultant normal force is calculated from the structure weight, buoyancy and wave uplift forces (1.8 tons/ft). Friction is estimated at 1.3 tons per linear foot assuming a 35 degree internal angle of friction.
A total of 3.0 tons per foot of stabilizing forces (friction + earth forces + pile resistance) and 0.7 tons per foot of anti-stabilizing (wave) forces were calculated. This results in a factor of safety of 4.5 against sliding.
To verify overturning, stability moments are calculated about the structure heel. Assuming a 5-foot moment arm for the center of gravity and a 2.1-foot moment arm for the center of pressure for the earth forces results in a total stabilizing moment of 21.3 ft-tons per linear foot of structure. Assuming a 3.8-foot moment arm for the center of pressure of the horizontal wave forces and a 6.7-foot moment arm for the center of pressure of the wave uplift force results in a total anti-stabilizing moment of 5.4 ft-tons per linear foot of structure. A factor of safety of 3.9 is calculated for overturning stability.
The steel crib should be designed to ensure it has sufficient flexural strength to resist the forces expected in its design life. The design should be checked using Load and Resistance Factor Design methods (AISC Manual of Steel Construction).
The proposed steel crib seawall will extend approximately 28 feet from the pre-revetment location of the bluff toe. The proposed seawall will extend nearly to the toe of the existing revetment but will result in a slight reduction of the overall structure footprint.
Generally, vertical structures increase the wave energy in the nearshore zone due to wave reflection. The use of stone filled cribbing will limit the reflection of energy as a significant portion of the wave energy will be dissipated within the crib or allowed to pass over the crib to the revetment. The placement of the crib completely within the footprint of the existing revetment effectively isolates the new construction and will limit its effect on adjacent properties.
Final Survey Products
Based upon the design from the Ohio registered professional engineer, a plat that depicted the boundaries of the submerged lands lease was prepared. The project site includes two separate parcels, but a lot consolidation has not been planned by the parcel owner. Therefore, two separate lease parcels are depicted using the location of the water’s edge on the date of the field survey as the natural shoreline (see Submerged Lands Plat “B”).
Two metes and bounds descriptions have been written for the areas depicted on the plat of survey with direct relationship to the upland parcel boundaries as required in Ohio Revised Code Section 1506.11(B) (see Submerged Lands Lease Descriptions for Parcel “A” and “B”).