
Coastal Design Manual - Design Example

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Ohio Coastal Design Manual

Chapter 4.5 B
armor stone revetment
@ a moderate bluff

download Chapter 4.5 B

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.

Design example 4.5 B demonstrates the design process at a fictitious site of an armor stone revetment as erosion protection at along a stretch of shore with moderate (20 to 30-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 common the south coast of Lake Erie along the Central 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

Project Purpose
Site Description
Field Survey
Analysis
Design
Discussion
Final Survey Products
Design Drawings
Legal Description
Submerged Lands Lease

Project Purpose

The purpose of Example Project B is to protect the toe of the glacial till 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 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. At the east end of the property there is a small embayment suggesting increased erosion in this area.

The bluffs at the project site are 15 to 20 feet in height and have been partially regraded to an approximately 1.7 horizontal to 1 vertical slope. The bluff extends from 575.1 feet at the toe to a top elevation of 589.1 feet as referenced to the International Great Lakes Datum of 1985 (IGLD 1985).

A 15 to 20-foot wide sand and gravel beach is present at the project site.
The bluffs are composed primarily of till overlain with glaciolacustrine 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.

Closer to shore, in the beach zone, sand beaches are trapped by the area’s groin structures and range from 0.5 to 3-feet thick. The bottom slope from 100 to 1500 feet offshore is approximately 100 horizontal to 1 vertical.

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 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 rubble mound revetment. The structure is in poor condition due to undersized concrete rubble being fractured and displaced by wave action.

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Field Survey

The upland parcel is located within the Firelands portion of the Connecticut Western Reserve district of Ohio’s Public Lands Survey System more 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 to north right of way of the 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 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 “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 an attempt to recover the station was 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.” A closed level circuit was completed. 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 was found, and held as controlling stations included 5/8-inch iron pins at the southwest corner of Sub Lot 5 and 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.

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Analysis

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 in accordance with Ohio Administrative Code (OAC) Section 4733-37 was included and the Ohio registered professional surveyor’s signature and seal were affixed to the survey plat (see Existing Site Plan “A”).

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Design

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. Placing a 2H:1V sloped revetment over the existing 1.7H:1V slope also offers the advantage of not having to excavate the existing slope while minimizing the amount of fill required. The toe of the structure is entrenched 2.5 feet into shale bedrock at an elevation of 567.5 feet IGLD 1985.

The project site is located in the Huron to Vermilion reach of the “Revised Report on Great Lakes Open Coast Flooding,” (USACE 1988) and has a design water level of 575.5 feet IGLD 1985 for a 30-year return period.

A 5.5-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 4.3 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 8.0 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.

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 0.4 tons for the non-scour case. The minimum median armor stone size was 1.1 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 1.1 ton per unit value from Hudson’s Equation, the safety factor results in a lower limit for the armor stone of 2.2 tons per unit and a range of 1.6 to 2.7 tons per unit. The selected 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 5.7 feet to an elevation of 581.2 feet IGLD 1985 was calculated for the initial design case. If the toe of the structure is scoured, the wave run-up increases to 7.6 feet, to an elevation of 583.1 feet IGLD 1985. For an economical design, the crest of the revetment is set to 582.0 feet IGLD 1985 and a splash apron is specified to 585.0 feet IGLD 1985.

The splash apron is specified as a double layer of new ODOT 601 Type “B” stone. The upper bluff will be stabilized by re-grading a gentle slope from the top of the splash apron at 585.0 feet IGLD 1985 to the top of the bluff at 589.1 feet IGLD 1985. A thin layer of ODOT 601 Type 56 stone will be used as a base for the re-graded slope in the area of the 12 to 24-inch filter layer stone.

To prevent sliding failure along the slope of the revetment, 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 west end of revetment will be curved back into the bluff face with a radius approximately equal to the plan view width of the armor layer. The east end of the structure is extended to the property line to be continuous with the existing revetment on the eastern adjacent property. This should sufficiently reduce the risk to adjacent properties and prevent potential upland slope failure at the ends of structure.

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Discussion

In this example, the revetment has been designed to closely follow the shape of the shore. The revetment will extend a maximum of 29.2 feet from the toe of the existing bluff. This distance is determined by the required crest elevation and revetment slope and cannot be reduced without compromising the functionality or stability of the structure. Therefore this structure has been appropriately designed to minimize effects on lake processes and adjacent properties.

The revetment is intended to prevent erosion of the existing bluff and will 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 29 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 existing rubble mound revetment on the eastern adjacent property. It is unlikely that this structure will trap sediment. The structure may cause changes in wave energy that could adversely affect adjacent properties. This risk has been addressed 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.

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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 project site includes two separate parcels, and 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.”)

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