The purpose of Example Project A is to protect the toe of the glacial till bluffs from erosion due to wave action. An armor stone revetment was selected to best achieve the project purpose at this site.
The Design Example A site is located along the shore of Lake Erie in Saybrook Township, Ashtabula County, Ohio, approximately 6 miles west of the Ashtabula Port. The shore along this stretch is fairly uniform with small embayments and headlands. The project shore is oriented from southwest to the northeast. The predominant direction of sediment transport in the littoral zone is from west to east.
|These photos show the view from standing atop the bluff looking out at Lake Erie (bottom) and from standing on the beach (above) looking toward the bluff. This site has similar characteristics to Design Example A.
The shore at the project site consists of 50 to 55 foot glacial till bluffs reaching an elevation of 630 feet as referenced to the International Great Lakes Datum of 1985 (IGLD 1985). The toe of the bluff is located at approximately 575 feet IGLD 1985 and is covered with concrete rubble fill. A narrow, 10 to 15-foot wide, sand and gravel beach is perched above a wall of existing concrete block modules.
The geology of the area consists of a thin layer of top soil or fill over a thick (25 to 30-foot) layer of soft glacial till. Below is a thick (30 to 35-foot) layer of hard glacial till over shale bedrock at approximately 570 feet of elevation (referenced to IGLD 1985). Shale bedrock is exposed in the nearshore and slopes at 3 to 4 degrees for the first 100 feet then continues at a shallower (1 degree) slope farther offshore.
The site is exposed to storm waves from all angles from west-southwest to east-northeast. A review of historic wave information results in a significant wave height of 3.3 feet at a period of 4.3 seconds. The most frequent wave direction was from the southwest. The largest wave recorded over the 32 year study was 16.4 feet with a 9.0 second wave period, from the west. The average direction of the largest waves was 264.0 degrees. Wave data was measured at WIS station E14 located approximately 10 miles north of the project site in 72-foot deep water.
The project site is located in a designated Coastal Erosion Area based on the Final 2010 mapping with an expected erosion rate of 34.2 to 42.9 feet over 30 years. There are no surface drainage issues causing erosion at the project site.
The shoreline in this area is generally consistent; the eastern and western adjacent properties are similar to the project site. The bluff and upland topography are constant in this area. Both the eastern and western adjoining properties include a small sand and gravel beach held in place with large concrete blocks and vertical concrete sewer pipes. The concrete rubble at the toe of the bluff is continuous across the site and adjoining properties.
The upland parcel is located within Connecticut Western Reserve district of Ohio’s Public Lands Survey System more specifically part of Original Lot (O.L.) 55, Fractional Section 3, Town 13 North, Range 4 West. Being within Saybrook Township and outside of any incorporated municipal boundaries, the parcel boundary extends to the centerline of the county road with a sixty (60) foot right of way reservation for public ingress and egress 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 “Woodring” (PID MB2112) which is approximately one (1) kilometer east. Based upon the NGS datasheet, the horizontal accuracy of the station is Third Order with reports that attempt to recover the station failed in 1993 and 1996. Therefore this station was not used within the horizontal control network.
Global Positioning System (GPS) observations of approximately thirty (30) minute in length were performed on two (2) control stations along Lake Road West. The raw data files were uploaded to the NGS Online Positioning User Service (OPUS) for the rapid-static sessions. The resultant Ohio State Plane 3401(NAD 83) coordinates provided by the OPUS solution were utilized as the controlling stations for an adjusted closed field traverse.
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 “P 8” (PID MB1001) which is approximately 4 kilometers southeast. Based upon the NGS datasheet the vertical accuracy of the station is First Order Class II with reports that attempt to recover the station were successful in 1993 and 2009. The U.S. Coast and Geodetic Survey disk, established in 1934, has a reported dynamic height of 645.93 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 OPUS solution.
The resultant adjusted elevations provided by a closed level circuit were utilized for the project after confirming the elevation, relative to IGLD 1985, of the control stations 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 #9063053 (Fairport Harbor).
With the horizontal and vertical control network established, recovery of boundary evidence was performed. Monumentation found and held as controlling stations included a ¾-inch iron pin in a monument box at the southwest corner of O.L. 55 and a 2-inch splined axel shaft at the southeast corner of O. L. 55. Subsequent intermediate points were located along Lake Road West including P-K nails found at the southwest and southeast corners of the subject parcel and were used in the final determination of the upland parcel 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. Presence of concrete modules and rubble along the bluff and shore indicate that fill material has been placed artificially and has altered the location of the natural shoreline.
A technical assistance request was made to the ODNR Office of Coastal Management to help in identifying the location of the natural shoreline prior to the artificial placement of the concrete material. A drawing was provided to the consultant that depicted the location of the natural shoreline on the April 1973 aerial photograph. This location was transferred to the site and compared to the descriptions within the current and previous title deeds. The natural shoreline was slightly adjusted based upon the description within the 1971 general warranty deed for the subject parcel.
Parcel data provided by the Ashtabula 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 kilometer. 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 that conforms to 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”).
The maximum slope normally considered for the long-term stability of an armor stone revetment is 1.5 horizontal to 1 vertical. Based on the wave climate in the area of the project site a slope of 2 horizontal to 1 vertical was selected for a conservative design, which also matches the planned re-graded upland slope. The existing concrete modules are to be removed and re-used as part of the revetment core. This allows the toe of the revetment to be placed at the 569.8 foot IGLD 1985 elevation of the shale bedrock at the shore.
The project site is located in the Saybrook to Kingsville reach of the “Revised Report on Great Lakes Open Coast Flooding” (USACE 1988) and has a design water level of 575.0 feet IGLD 1985 for a 30-year return period.
A 5.2-foot structure depth was calculated based on the lake bottom elevation at the structure toe and the design water level. Using the breaking wave equation presented in Chapter 3, a design wave height of 4.1 feet was calculated for this case.
Since the toe of the structure was designed to be entrenched 2.5 feet into the shale bedrock, the depth of the structure at the base of the toe will be 7.7 feet. Future scouring at the toe of the structure due to the fractures and wear of the shale would result in an increase in water depth from 5.2 to 7.7 feet and a design wave height of 6.0 feet for this conservative case. The scour of shale bedrock may not always be a reasonable assumption, but for this example, it was assumed that the fractures caused during entrenchment would lead to scour, aided by the presence of a significant amount of cobble and gravel along the near shore that could abrade the shale.
Hudson’s Equation was used to calculate the median armor stone size to resist displacement due to wave action. Using the unit weight for the specified limestone, the minimum median armor stone size is 0.3 tons for the non-scour case. The minimum median armor stone size was 1.0 ton per unit if the toe of the structure is scoured.
A factor of safety of 2.0 was selected for the armor stone size to account for potential effects of ice forces, and long-term fracturing of the stone. Using the conservative 1.0 ton per unit value from Hudson’s Equation, the safety factor results in a lower limit for the armor stone of 2.0 tons per unit. The resulting design specification of a 2 to 4-ton range for the armor stone layer also provides additional mass that improves the long-term ability of the revetment to resist earth forces from the upland. A double layer of 2 to 4-ton limestone will be stacked in a 6-foot thick armor layer.
The filter layer was specified as stone or clean concrete rubble about 1/3 of the diameter of the armor stone. For economy of design, the existing concrete modules and concrete rubble at the toe of the bluff will be relocated to form the filter layer for the revetment. Due to the variability of the filter layer material and the fine-grained till composition of the bluff a geotextile filter fabric is specified.
Wave run-up on the structure was calculated using the empirical formula introduced in Chapters 3. Wave run-up of 5.4 feet to an elevation of 580.4 feet IGLD 1985 was calculated for the initial design case. If the toe of the structure is scoured the wave run-up increases to 7.4 feet to an elevation of 582.4 feet IGLD 1985. The crest of the revetment was placed at 583.0 feet IGLD 1985.
To stabilize the upper portion of the bluff face the existing bluff will be re-graded to a 2 horizontal to 1 vertical slope above the revetment. To protect the re-graded bluff face from erosion resulting from spray, a splash apron was included in the design. The splash apron was specified as new ODOT 601 Type “B” stone and will extend to an elevation of 586.0 feet IGLD 1985.
To prevent sliding failure along the slope of the revetment, larger stones are placed at the lakeward base for toe protection. In this case 4 to 5-ton armor stones are to be entrenched 2.5 feet into the shale bedrock. Toe stones are typically specified to be 1 to 2 tons heavier than stones used for the armor layer.
To reduce the risk of causing increased erosion on adjacent properties and to prevent potential failure of the ends of structure, it is essential to appropriately terminate the structure at the property boundaries. To mitigate end effects, the ends of revetment are curved back into the bluff face. In this case, the ends of the structure are rounded off with a radius approximately equal to the plan view width of the armor layer.
To reduce the overall project footprint and minimize effects on littoral processes and adjacent properties the revetment has been placed with the armor layer immediately adjacent to the existing bluff face. The revetment has also been designed to closely follow the shape of the shore. The revetment will extend a maximum of 36.2 feet from the existing bluff toe. This distance is determined by the required crest elevation and revetment slope and can not be reduced without compromising the functionality or stability of the structure. In this way it has been appropriately designed to minimize effects on lake processes and adjacent properties.
The revetment is intended to prevent wave-based erosion of the existing bluff and will therefore decrease the amount of material added to the littoral system. Sand or gravel in the footprint of the revetment must be excavated and sidecast into the lake prior to construction to prevent sediment from being permanently removed from the littoral system.
As the structure will extend approximately 36 feet lakeward of the bluff toe, it will affect the littoral transport of material along the shore. In this case, the impact is expected to be minimal due to the location of concrete modules and rubble on adjacent properties. The structure may also cause changes in wave energy that could adversely affect adjacent properties. This risk has been reduced with the use of rough, angular limestone placed at a slope of 2H:1V. Much of the wave energy will be absorbed and dissipated by the revetment, minimizing the wave energy reflected in the nearshore zone.
Final Survey Products
Based on the design from the Ohio registered professional engineer, a plat that depicted the boundaries of the submerged lands lease was prepared. The adjusted historic natural shoreline serves as the southern limit of the lease. Due to the use of the artificially placed fill material (concrete rubble) two separate lease parcels are depicted according to the definitions provided within OAC 1501-6-01. (see Submerged Lands Plat “A”)
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”).