The objective of this task is to determine the interdependence between coastal processes and the extent, type and quality of structural shore protection put in place along the Lake Michigan shoreline. Many coastal processes influence the effectiveness of shore protection structures over their design life. Alternatively, structural shore protection has a direct and measurable effect upon alongshore sediment transport interfering with natural processes of beach accretion and erosion.
There are three key activities associated with this task:
Analysis of Shore Protection Impacts
The objective of this task is to develop an assessment of the potential damage to typical shoreline structures due to changing lake levels. The objectives of the assessment described in here are as follows:
- Select typical shore protection structures
- Develop typical configurations for each selected structure
- Select potential high and low water levels for each structure
- Evaluate the effects of high and low water levels on these structures
The study area for this report includes structures along the shoreline of five selected counties in Michigan and Wisconsin. These counties include Ottawa, Allegan, Ozaukee, Sheboygan and Manitowoc. An inventory of shore protection types for these counties has been prepared by Orca Technologies International, Inc. (Stewart, 1999). A review of this inventory indicated that there are three main types of structures along the shorelines of the selected counties: 1) revetments, 2) seawalls/bulkheads; and 3) groins. Therefore, these three structure types were selected for evaluation. In particular, only "typical" structures within each of the three categories were selected. For revetments, this included "rip-rap" revetments; for seawalls and bulkheads, this included vertical walls constructed using steel or timber sheetpiles; and for groins, this included grout-filled geotextile groins, which are more and more common along these shorelines.
In conducting the evaluation, there were several coastal design parameters that had to be considered:
Lake Level - Lake Michigan water levels fluctuate naturally between high and low levels that have been recorded over a period of almost a century. During that period, record high and low levels of approximately 177.74 m (583 feet IGLD 1985) and 175.91m (577 feet IGLD 1985) have been recorded. The location of the lake level relative to the shore protection structure affects its stability. In this evaluation, several lake level elevations were selected, relative to the shore protection structure, to evaluate this effect.
Lake Bed Slope - Lake bed, or nearshore slope affects the potential for scour and the wave breaking height and has a minor effect on global slope stability (stability of the structure/slope usually associated with movements including rotational slope failure, sliding and overturning). Based on discussions with W. F. Baird and Associates, typical bed slopes in the study area range from 1 vertical to 10 horizontal and 1 vertical to 20 horizontal. These slopes were used in the analyses.
Wave Characteristics - For this evaluation, for each structure, we evaluated the effect of a critical wave on the stability of the structures. The wave characteristics were provided by W.F. Baird and Associates. The height and period of the selected wave are 5.0 m and 9 seconds, respectively.
Soil Stratigraphy and Sheer Strength - Soil stratigraphy has a significant effect on the stability of shore protection structures. The stresses sustained by these structures and their global stability are dependent on the type of foundation and retained soils and their shear strengths. In general, cohesive soils usually apply higher loads than cohesionless soils and require special drainage design features. To represent a range of potential soil stratigraphy, we selected two typical soil profiles; a cohesive and a cohesionless soil profiles. For the cohesive soils, we selected the following typical effective shear strength parameters: a total density of 19.2 kN/m3 (122 pcf), a cohesion and internal friction angle of 0.0 Pa and 28°, respectively, and a friction angle between the soil and seawall/bulkheads of 10°. For the cohesionless soils, we selected the following typical shear strength parameters: a total density of 19.2 kN/m3 (122 pcf), an internal friction angle of 30° and a friction angle between the soil and seawalls/bulkheads of 14°.
Ice Forces - Ice can affect marine structures in a number of ways. Moving surface ice can cause significant crushing and bending forces as well as large impact loadings. Vertical forces can be caused by the weight of ice on structures during falling water levels and by buoyant uplift caused by ice masses frozen to structural elements during rising water levels.
Sand Supply - Continued land development along both inland rivers and coastal areas has been accompanied by erosion control methods which have deprived the coastal areas of sediment formerly available through the natural erosion process. These methods reduce the amount of sand transported along the coast. Because the natural sand supply has been reduced, the erosion of the shore may become gradually more severe, which may affect the stability of shore protection structures, and/or shorten their life. However, the future effects of sand supply reduction on the stability of shore protection structures is not included in this study.
Summary of Results
Changing lake levels are expected to have some effects on the evaluated structures. Rising and falling water levels have different effects on the stability of these structures; however, cycles of rising and falling water levels may result in a complex combined effect. For example, low levels may increase toe scour, reducing the toe resistance for sheetpile wall bulkheads, and high water levels increase the likelihood of overtopping which may increase stresses on the structure.
We note that the effects described here are associated with the evaluated structures under the selected wave characteristics and changes in the lake levels. More severe wave characteristics and lake level changes may have even more severe effects. In this section, we summarize the potential effects evaluated in this study, and present our conclusions.
Based on the evaluation completed in this study and engineering judgment, we developed an estimate of the percentage loss of the values of the shore protection structures over a 50-year period. This estimate was prepared to aid in the development of a cost estimate for the potential damage due to potential changes in Lake Michigan levels and should be used for this purpose only. The estimates are for privately-owned structures only. It is assumed for this study that publicly-owned structures were adequately designed, constructed, and maintained.
Seawalls/Bulkheads
It is clear that a reduction in lake level affects global stability of these structures. Low levels are associated with low toe resistance and high scour rates. The results of this study indicate that low lake levels may result in a reduction of approximately 20% in the factor of safety against sliding. If low lake levels persist, toe scour will further reduce the factor of safety.
Drawdown/seiche of approximately 1.07m (3.5 ft) may be associated with global instabilities along the shorelines and will not be limited to areas with protection structures. The results of this study indicate that, based on the assumptions in our analyses, a reduction in the factor of safety due to drawdown/seiche may exceed 30%.
Low lake levels reduce the potential for overtopping. However, if overtopping occurs, when the lake level is low, due to high waves, the additional stresses on the structure combined with the reduction in toe resistance associated with the low levels may result in instability.
Rising lake levels have a mixed effects on the stability of seawalls and bulkheads. A 1.06 m (3.48 ft) rise in lake level is expected to increase the factor of safety against global stability by approximately 20%, due to an increase in the toe resistance, and decrease the factor of safety by approximately 18% to 33%, due to an increase in the stresses on the structure assuming overtopping occurs with insufficient drainage features in place. Again, mitigation of overtopping effects will minimize the effects of rising lake levels on these structures.
Flanking continues to be a major failure mode. It is essential that measures be implemented to minimize the potential for flanking. All isolated structures that lack wingwalls will fail due to flanking in a 50-year period.
In summary, we conclude that, while we have no influence on the lake levels, it appears that mitigating overtopping effects by installing drainage features and providing toe scour protection and edge flanking protection may minimize the potential for failure and increase the useful life of the structures. Structures without these features have a high probability of failure.
The estimated loss of structure values for the seawalls/bulkheads in a 50-year period ranges from 35-100% for a extreme low lake level and from 50-100% for an extreme high lake level.
Revetments
Falling lake levels are expected to adversely affect the global stability of revetments. However, the factors of safety of the structures evaluated in this study are satisfactory for the range of lake levels assumed. In addition, the potential for adverse effects due to toe scour is less at low lake levels because riprap revetments usually include toe protection. However, armor stability is expected to be affected by rising lake levels. Rising lake levels from one-third to two-thirds the height of the structure will double the breaker height, which may damage the armor units.
Again, rapid drawdown/seiche may cause instability if drainage features are not in-place. Flanking is an important failure mode and rising lake levels are expected to accelerate flanking.
Rising lake levels may also result in overtopping. The effect of overtopping on revetments is expected to be small since riprap revetments usually allow for rapid drainage. This is assuming that the construction of these revetments included an adequate filter layer.
The estimated loss of structure values for the seawalls/bulkheads in a 50-year period ranges from 10-75% for a extreme low lake level and from 75-90% for an extreme high lake level.
Groins
Groins evaluated in this study will be subjected to breaking waves. These groins are expected to fail under breaking wave conditions for the design wave. Rising lake levels will reduce the dynamic force against the groin due to a decrease in the above-water portion of the groin. On the other hand, falling lake levels are expected to increase the dynamic breaking wave force against the groin due to the increase in the above-water portion of the groin. We estimated that the groin will lose 100% of its structure value during a 50-year period.
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