Ohio Coastal Design Manual
Chapter 4.5 C
armor stone revetmnet
@ a low bluff
download Chapter 4.5 C
Our Goal:
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. |
This design example demonstrates the design process at a fictitious site of an armor stone revetment as erosion protection at along a stretch of shore with low(0 to 15-foot) bluffs.
The site conditions, parcel boundaries, addresses and parcel numbers were invented to illustrate the range of engineering and surveying methods involved in design, but are similar to the coastal features along the south coast of Lake Erie's Western Basin.
Coastal Design Manual example sites include typical coastal features and are intended to be applicable to a large portion of Lake Erie’s south shore.
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Project Purpose
The purpose of Example Project C is to protect the toe of the silt and clay bluff from erosion due to wave action. An armor stone revetment is selected to best achieve the project purpose.
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Site Description
The project site is located along the shore of Lake Erie in Ottawa County, between Port Clinton and Catawba Island. The shore in this area is oriented from west to east, and is irregular in shape with small bays and headlands. The predominant direction of sediment transport in the littoral zone is from northeast to southwest.
The shore at the project site consists of a 30 to 40-foot wide sand and gravel beach that fronts a 6-foot high bluff (embankment). The bluff extends from a toe elevation of 572.7 feet to 579.0 feet at the crest as referenced to the International Great Lakes Datum of 1985 (IGLD 1985). A timber crib pier is present at the center of the site property and is trapping a small amount of sediment on its east edge. The crib pier is made up of two 16-foot long by 8-foot wide timber cribs with a crest elevation of 576.0 IGLD 1985.
The bluff is composed primarily of silt and clay with a thin layer of topsoil. A 2 to 4-foot thick layer of sand covers till in the nearshore zone and is distributed in a bar system. Limestone bedrock is present at an unknown depth. The nearshore slopes at approximately 4 degrees for the first 100 feet then levels to approximately 1 degree.
The site is exposed to storm waves from west-northwest to north directions but is partially protected by Catawba Island and the Bass Islands from northeast waves. A review of historic wave information results in a significant wave height of 1.6 feet at a period of 3.4 seconds. The most frequent wave direction was from 225.0 degrees (referenced to 0/360 degrees north). The largest wave recorded over the 32-year study was 6.9 feet with a 7.0 second period. The average direction of the largest waves was 321.0 degrees. Wave data was measured at WIS station E04 located approximately 3.5 miles north of the project site in 20-foot deep water.
The expected erosion rate at the project site is 0.0 feet over 30 years based on the 2010 Coastal Erosion Area maps. The site is not located in a designated Coastal Erosion Area. 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 beach width varies from 30 to 40 feet on both the eastern and western adjoining properties. There are no existing shore structures on either adjoining property.
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Field Survey
The upland parcel is located within the Congress Lands district north and east of the First Principal Meridian of the Public Lands Survey System more specifically part of Fractional Section 35, Town 7 North, Range 17 East. Being within Catawba Island Township, and outside of any incorporated municipal boundaries, the parcel boundary extends to the centerline of the county road with a 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 “Clintport AZ MK” (PID MC1546) which is approximately 1.5 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 1995. Therefore this station was used within the horizontal control network. A closed traverse was performed between station “Clintport AZ MK” and the inter-visible station “Clintport” (PID MC1541) with intermediate stations located close to the project site. A least squares adjustment was made to generate resultant coordinates based upon Ohio State Plane 3401(NAD 83).
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 “J 317” (PID MC0994) which is approximately three (3) kilometers southwest. 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 and 2009. The NGS stainless steel rod, established in 1980, has a reported dynamic height of 585.05 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 “J 317.” The resultant adjusted elevations provided by a closed level circuit were utilized for the project after confirming the elevation, relative to IGLD85, 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 #9063079 (Marblehead).
With the horizontal and vertical control network established, recovery of boundary evidence was performed. Monumentation was found, and held as controlling stations included 5/8-inch iron pins at the intersecting centerlines of 60-foot Sand Road and 50-foot Spring Valley Road, and along the centerline of Sand Road. 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 the timber crib pier along the shore and centered on the upland parcel, the natural shoreline appears to be unaltered by artificially placed fill material.
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Analysis
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 May 1956 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 1993 limited warranty deed for the subject parcel.
Parcel data provided by the Ottawa 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 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 “C”).
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Design
The customary minimum slope based on standard engineering design practice for 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 is selected for a conservative design. The toe of the structure will be entrenched to an elevation of 567.8 feet IGLD 1985 into the underlying till.
The project site is located in the Locust Point to Marblehead reach of the “Revised Report on Great Lakes Open Coast Flooding,” (USACE 1988) and has a design water level of 576.2 feet IGLD 1985.
Sand and gravel in the footprint of the revetment will be sidecast into the lake, however, beach sand immediately lakeward of the structure will be left in place as shown in Section B-B. The water depth for the initial design condition includes the small amount of beach cover over the toe of the revetment. An initial 4.2-foot structure depth was calculated from the beach profile elevation at the toe of the structure and the design water level. Based on the breaking wave equation a design wave height of 3.3 feet was calculated for the initial case. If the beach sand and till at the toe of the structure are scoured, the water depth at the toe of the structure would increase to 8.4 feet. In this case, the design wave height would increase to 6.6 feet.
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 0.2 tons for the non-scour case. The minimum median armor stone size was 1.3 tons 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 1.3 ton per unit value from Hudson’s Equation, the safety factor results in a lower limit for the armor stone of 2.6 tons per unit. 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 was specified.
Wave run-up on the structure is calculated using the empirical formula introduced in Chapters 3 and 4. Wave run-up of 4.1 feet to an elevation of 580.3 feet IGLD 1985 is calculated for the initial design case. If the toe of the structure is scoured the wave run-up increases to 6.9 feet to an elevation of 583.1 feet IGLD 1985. In this case, the crest of the revetment is set to 584.0 feet IGLD 1985. It should be noted that in this case the crest of the revetment will be well above the 579.0 feet IGLD 1985 elevation of the top of the bluff. The higher crest elevation in this location along the shore will help protect the upland during periods of open coast flooding associated with high lake water levels and northeast storms.
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 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.
On the landward site of the revetment, smaller ODOT 601 Type “B” stone is specified at a slope of 1.75 horizontal to 1 vertical. A smaller stone is acceptable in this application because it will not be subject to wave action. A 2 to 4-ton armor stone entrenched 2 feet into the top of the bluff is specified to prevent sliding failures on the landward slope.
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Discussion
Although the entire structure is located on the beach area above the water level at the time of the survey, an appropriate design still considers minimization of the overall project footprint. The revetment will extend lakeward a maximum of 23 feet from the toe of the existing bluff. 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. The revetment was also designed so that the armor layer is immediately adjacent to the existing bluff face.
This reduces the amount of fill added to the site as well as the lakeward extent of the structure. In this case it also allows for the largest possible width of beach to be preserved lakeward of the structure.
The revetment is intended to prevent erosion of the existing bluff and will therefore decrease the amount of material added to the littoral system. Any 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 23 feet toward the lake, it may also affect the littoral transport of material along the shore in high water conditions. The structure may cause changes in wave energy that could adversely affect the stability of the beach at this site and on adjacent properties. The interaction between the wave and structure will cause an increase in wave energy in the nearshore zone due to wave reflection. The structure’s effect on wave motions also increases the likelihood of scour of the beach fronting the structure.
These risks have been reduced with the use of rough angular limestone 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. Observing and measuring changes to the beach over time should be part of the routine inspection of the structure’s performance in the years following construction. A beach monitoring plan should be developed to quantify and mitigate long term effects of the structure.
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Final Survey Products
Based upon the design from the Ohio registered professional engineer, a plat that depicted the boundaries of the submerged lands lease has been prepared. The proposed design of the armour stone revetment locates its occupation landward of the natural shoreline and therefore is not included in any lease parcel. The existing occupation of the timber crib pier is bisected by the location of the natural shoreline and therefore the lease parcel only includes the area lakeward of said natural shoreline (see Submerged Lands Plat).
A metes and bounds description has been written for the area 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 Description).
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Design Drawings
Example C – Low Bluff
(Calculation sheet 1 of 3)
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Example C – Low Bluff
(Calculation sheet 2 of 3)
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Example C – Low Bluff
(Calculation sheet 3 of 3)
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Project Site – Vicinity Map
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Existing Site Plan (plan view)
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Existing Site Plan (cross section)
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Proposed Site Plan (plan view)
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Proposed Site Plan
(cross section 1 of 1)
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Legal Description & Submerged Lands Lease
Lake Erie Submerged Lands
Legal Description
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Submerged Lands Lease
Plat of Survey
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Chapter 4.5 C - Low Bluff
Armor Stone Revetment
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