By Gerlyn Hinds, Reed Vetovitz, and Katie Buckler
Sitting directly along the southern shore of Lake Erie and bordering the city of Cleveland to the west, Lake County is home to more than 230,000 residents in northeastern Ohio. The Lake County Raw Water Pump Station draws water from the lake to supply approximately 40,000 people with clean water. However, strong ice and wave action along the southern Lake Erie shoreline was eroding the bluff on which the pump station was located by about 1.8-ft/year.
In 2018, a Project Partnership Agreement had been signed between the Buffalo District of the U.S. Army Corps of Engineers (USACE) and Lake County Utilities to protect the services of safe drinking water to the citizens of the county.
To keep the pump station from needing to be relocated, a $4 million project to construct a 600-ft revetment was completed during the summer of 2020. Several factors were considered as part of the design, including the bluff recession rate, water elevation, wave height, geotechnical stability, and environmental features.
ANALYZING COASTAL FACTORS
To design the revetment, a coastal analysis was first conducted. For projects on Lake Erie and Lake Ontario, best practices consider the 10-year wave and 20-year water level for determining the design storm conditions. The design storm wave heights are used for the determination of the crest elevation and stone sizing.
Bluff Recession Rate. Every 10 years, the Ohio Department of Natural Resources Division of Geological Survey identifies and maps coastal erosion areas along the state’s 262-mi Lake Erie shore. The objective of the Coastal Erosion Area Program is to minimize economic losses by ensuring that new development along the shore is adequately protected from coastal erosion. The recession rate for Lake County was estimated to be 1.8-ft/year.
Water Elevations. Water elevations are recorded at gauges along the shoreline. The datum used in this project follows the North American Vertical Datum (NAVD) 1988, which is based on an orthometric height and based upon an elevation at Point Rimouski/Father’s Point, Quebec. International Great Lakes Datum (IGLD) 1985 is expressed as a dynamic height; for Lake Erie it is based on an elevation of 569.2-ft about Point Rimouski/ Father’s Point, Quebec. The Great Lakes are referenced to the IGLD 1985 dynamic heights.
The difference between NAVD 1988 and IGLD 1985 dynamic heights is a hydraulic corrector and conversion tool. For the Lake County Raw Water Pump Station, NAVD 1988 are expressed 0.2-ft higher than IGLD 1985. Low Water Datum (LWD) is a fixed reference plane selected by Canada and the United States, which is 569.2 IGLD 1985 for Lake Erie. This allowed the majority of the elevations during navigation season to be above that plane. The water levels observed the 10-year and 20-year occurrence for the purpose of design.
The design depth is the combination of the water elevation and the depth of water below LWD at the proposed structural toe. Depth of water below LWD was acquired from a 2016 USACE underwater single beam survey. Each line was 1,200-ft long and measured the lake bottom elevations from about LWD out to -18.0 ft LWD. It was approximated that the design depths for 10-year and 20-year were 10.4-ft and 10.6-ft, respectively.
Wave Height. Determining the design storm wave conditions used the Wave Information Studies, a USACE-sponsored project that generates consistent, hourly, long-term wave climatologies along all U.S. coastlines, including the Great Lakes. These waves are sorted into class angles and frequency of occurrence, 10-year and 20-year waves. Wave Station 92057 provided 55 years of hindcast wind and wave information, resulting in 944 storms with wave heights greater than 6.6-ft.
Because waves are irregular in height, period, and direction, it is necessary to incorporate substantial modeling data to arrive at satisfactory conclusions. Standard design practice within the Great Lakes has been to use the greater of the combination of the 20-year wave and 10-year water level as the design condition. The design storm wave heights calculated at the toe of the structure ranged from 7.1-ft to 7.9-ft for each occurrence and class angle. Wave crest elevation is determined by following the procedures outlined in Coastal Engineering Manual 1110-2-1100 for breaking wave conditions and wave runup. Based on the maximum incident waves, the runup computations resulted in a crest elevation of +15-ft LWD.
DESIGNING THE REVETMENT
Following the coastal analysis, the results were implemented into the design.
Stone Sizing. The range of required armor stone size was determined to be 0.9W to 2.0W (where W is the individual stone weight). This resulted in 2,350-lb to 5,230-lb stone. However, based on consultation with the Ohio Department of Natural Resources, similar stone sizes used at a privately constructed project in the vicinity were required. This increased the size to 2-T to 5-T. The standard armor layer is a minimum of two armor stone deep to allow for a stable interlocking slope.
The range of required underlayer size was determined to be 0.06W to 0.2W. This resulted in a range of 270-lb to 890-lb. Like the armor stone, underlayer stone also has a layer thickness on two stones deep. The range of required bedding stone size was determined to be 0.00015W to 0.01W, or a range of 0.67-lb to 44-lb.
Developing the Cross Section. Based on the stone weight and sizes, a typical cross section was developed. This revetment consists of two layers of 2-T to 5-T armor stones at a 1 vertical: 2 horizontal slope, with a crest elevation of +15-ft LWD. The largest armor stone in the gradation is used at the base as a toe stone. The armor stone rests on top of two layers of Ohio Department of Transportation Type B or on another layer of Ohio Department of Transportation Type D material. Excavation of the revetment slope base/toe occurred down to -5-ft LWD. New precast concrete blocks were used at the crest for the creation of a maintenance road.
Geotechnical Investigation. A geotechnical investigation was conducted to determine if special measures were needed to prevent foundation bearing failure or account for significant settlement. Borings drilled along the top of the existing bluff encountered relatively thin strata of stiff clayey fill and stiff glacial lacustrine clay. These strata were underlain by glacial till that was encountered at depths between 5-ft and 8.5-ft below the ground surface. The glacial till consisted of fissured, stiff to hard, lean/silty clay with sand and a trace of gravel. Groundwater at elevations above the revetment crest is primarily limited to that within fissures. As a result, a geotextile filter was used beneath the revetment to maintain separation between the stone fill from the foundation soils under the anticipated dynamic hydraulic conditions.
At the base of the bluff where the revetment was constructed, the foundation soils primarily consist of stiff to hard glacial till. These foundation soils are relatively strong and incompressible. No special measures were needed.
BOLSTERING THE ENVIRONMENT
To stabilize the bluff above +15-ft LWD, wattles, native plantings, and live stakes were incorporated. Coir wattles, which are made from all-natural materials, were placed along the slope to break it up, slow down sheet flow, and encourage water to permeate into the soil rather than flow off the surface.
A planting plan comprised of native grass seed and live stakes was developed to create a robust native plant community on the upper slope. The extensive root systems of native vegetation will help hold the soils in place and also provide stopover habitat for numerous migratory bird species. The best time to plant at this site is early spring, before plants have broken their dormancy. Seeding was best done while freezing temperatures prevailed, as freezing and thawing helped break seed dormancy. Soil heaving caused by the freeze-thaw process also helped to improve the seed-to-soil contact, which was key to successful germination.
In addition to seeding, live stakes were used as a cost-effective method of generating a large amount of plant biomass. The stakes were driven into the soil such that at least 75 percent of the length of the stake was in contact with moist soil. If handled and installed correctly, the stake produced roots from nodes on the stem, and leaves appeared immediately upon breaking dormancy.
Through a collaborative partnership, the project is now completed. The Lake County Raw Water Pump Station is protected from further shoreline erosion and will continue to provide clean drinking water to residents in northeastern Ohio.
Gerlyn Hinds is Civil Engineer, Reed Vetovitz is Geotechnical Engineer, and Katie Buckler is Biologist, USACE Buffalo District. They can be reached at gerlyn.j.hinds@usace. army.mil; firstname.lastname@example.org; and email@example.com.
Joseph Ruszala, Tom Mullenhour, Kevin Lesika, Michael Mohr, William Butler, Shanon Chader, and Frank Lewandowski contributed to this article.
[This article first published in the July-August 2021 issue of The Military Engineer.]