BEDROCK SHORELINES EROSION ASSESSMENT - … · are those with rock strengths that greatly exceed...

BEDROCK SHORELINES EROSION ASSESSMENT

Prepared for: International Joint Commission as part of the International Upper Great Lakes Study

June 2010 10076.450

Geomorphic & Environmental Sciences Hazard Land Assessment Creek Rehabilitation Water Resources Management

T • 416 • 213 • 7121 F • 905 • 890 • 8499 www.geomorphicsolutions.ca

GEOMORPHIC SOLUTIONS

141 Brunel Road Mississauga, ON L4Z 1X3

__________________________________________________________________________________________________________________________ Geomorphic Solutions 10076 Bedrock Shorelines Erosion Assessment A Member of The Sernas Group, Inc. June 2010

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INTRODUCTION This report has been completed in support of the International Joint Commission’s (IJC) International Upper Great Lakes Study (IUGLS). One of the mandates of the IUGLS is to determine the potential impacts of various water level scenarios as a result of regulating the outflow of Lake Superior on the rate of shoreline recession. Different shoreline types will have different responses to changes in the water level. For example, highly resistant shorelines will respond to changes in erosion forces over much longer time scales than less resistant shorelines. Bedrock shorelines can be classified as hard rock or soft rock depending on their resistance to wave attack. The bedrock shorelines of the Great Lakes have been classified in the 1993 Levels Reference Study as either resistant (hard rock) or non-resistant (soft rock). The resistant hard rock shorelines respond to water level fluctuations on a geologic time scale (centuries to millenia) rather than the much shorter time scale of concern for shoreline management (decades). Non- or less- resistant soft rock shorelines respond much more rapidly to changes in water level and may respond differently to the proposed water level scenarios over a management time scale. The processes governing evolution of hard and soft rock shorelines are discussed along with their distribution throughout the upper Great Lakes. This is followed by a justification for excluding hard rock shorelines from further erosion assessment in the IUGLS. This report follows on from the Erosion Theme Report (Baird, 2010) commissioned by the IJC which provides a brief review of the proportion of bedrock shorelines within the upper Great Lakes and a review of processes governing evolution of non-resistant bedrock shorelines. HARD AND SOFT ROCK SHORELINES The following is a brief discussion of hard and soft rock shorelines and the dominant processes controlling the development of these shorelines. Further information on bedrock shorelines can be found in texts such as Trenhaile (1987) and Sunamura (1992). Along rocky shorelines the rate of erosion and the sensitivity of the shoreline to changes in water level and erosive forces is a function of the strength of the materials. The weaker the materials the more prone the shoreline is to respond to adjustments in water level and wave forcing. Bedrock shorelines can be composed of sedimentary rocks such as shale and sandstone or igneous and metamorphic rocks such as granite or gneiss. Weakly cemented shale and sandstone can erode fairly quickly where-as unweathered crystalline rocks often show little response to, or modification by, wave action. The most resistant rock type and structure will only experience minor erosion along joints, faults and bedding planes which take the form of small caves or inlets. Less resistant rock will experience more dramatic modification in the form of large caves, coves, stacks, arches and shore platforms. As the resistance to erosion decreases, the shape of the profile is less a function of the planes of weakness within the geological structure and more a function of the wave forces. Weaker rock such as shale behaves similarly to cohesive clay shorelines where erosion and slope processes create reproducible shoreline profiles. Bedrock shorelines are often classified as either hard rock or soft rock to distinguish the extremes in shoreline response to erosive forces and slope processes. Hard rock shorelines are those with rock strengths that greatly exceed the mechanical erosional force of individual waves. They erode at only millimetres to up to a few metres per century (Kirk, 1977; Sunamura 1992). Erosion of hard rock shorelines occurs due to air compression and caviation along joints resulting in the infrequent removal of blocks rather than the gradual


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process of wearing down the surface. The form of hard rock shorelines is often a direct result of the pre-existing topography, since water levels in the Great Lakes have only been at their present position for a few thousand years. For example, the morphology of the igneous shoreline in Lake Superior is largely a function of past glacial processes rather than wave processes (Figure 1). Soft rock shorelines include weakly cemented or highly weathered rock such as shale and sandstone, as well as cohesive material with high clay content such as glacial till. Examples of shale and cohesive shorelines are shown in Figures 2 and 3 respectively. These shorelines often have recession rates of more than 10 m per century (Kirk, 1977; Sunamura 1992). Although influenced by antecedent form, these shorelines are more often products of recent water levels and wave climate. Erosion on these shorelines occurs more due to gradual wearing down of the surface than block removal (quarrying).

Figure 1: Example of an igneous (hard rock) shoreline in Lake Superior. Note the

absence of wave cut features though there is evidence of abrasion on the lower slopes (Photo: A. Trenhaile).


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Figure 2: Example of a highly weathered and eroding shale (soft rock) shoreline on

Lake Ontario.

Figure 3: Example of a cohesive clay shoreline near Port Alma, Lake Erie. Note the

undercut at the toe of the bluff due to wave attack (Photo: A. Trenhaile). The cohesive strength of both hard rock and soft rock shorelines generally leads to the formation of a cliffed shoreline morphology. The main components of a cliffed shoreline are


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shown in Figure 4 to aid in the discussion of the dominant wave processes. Table 1 summarizes the importance of the different wave erosional processes along the cross-shore profile of hard and soft rock shorelines. Mechanical wave erosion occurs due to direct wave impact and results in loosening and removal of block joints and rock fragments. The main processes include water hammer and air compression in joints and irregularities on the surface. Wave-generated bottom shear stresses are due to the onshore and offshore movement of the water over the lake bed under waves. Abrasion occurs when sand and fine gravel is dragged by the waves across the lake bed. The main difference in the erosional processes on hard and soft rock shorelines is that on soft rock shorelines wave-generated shear stresses are more significant relative to the strength of the materials and lead to substantial nearshore lowering.

toe(may be covered by

a sand or cobble beach)

cliff or bluff face

tableland

nearshore

platform(not present on

all soft rock shorelines)

Figure 4: Components of a typical cliffed shoreline. Table 1: Erosional wave processes and their cross-shore dominance on hard and soft

rock shorelines. Erosional Agent Hard Rock

Soft Rock

Mechanical wave erosion Cliff face, toe and platform Cliff/Bluff face and toe Wave-generated bottom shear stresses

Negligible Nearshore

Abrasion Platform and shallow nearshore

Nearshore

There is a wide spectrum of recession rates for hard and soft rock shorelines depending on their lithology. The difference in recession rates for hard and soft rock shorelines is put in perspective by the following generalizations from Sunamura (1992):

• <0.001 m per year for granitic rocks • 0.001 - 0.01 m per year for limestone • 0.01 - 0.1 m per year for shale • 0.1 – 1 m per year for chalk and Tertiary sedimentary rocks • 1 – 10 m per year for Quaternary deposits • > 10 m per year for volcanic ejecta.

The division between hard and soft rock recession rates can be approximated as between 0.01 - 0.1 m per year, however there is no exact definition. It should be noted that these are generalizations and different rock types could vary greatly due to other factors such as the degree of weathering, structural characteristics or wave attack. For example, the recession


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rate for the Queenston Formation (predominately shale) was found to be approximately 0.30 m per year on the north-west shore of Lake Ontario (Geomorphic Solutions, 2009; Davidson-Arnott and Mathew, 2006) which is greater than the range presented by Sunamura (1992) for shale. Table 2 lists examples of measured recession rates within the Great Lakes and elsewhere. It can be seen that the erosion rates for soft rock shorelines can be several orders of magnitude greater than those for hard rock shorelines. Table 2: Examples of measured erosion rates within the Great Lakes and elsewhere

for hard rock and soft rock shorelines. Material Location Recession rate or

platform erosion rate (m per year)

Reference

Hard Rock Bedrock (basalt and gabbro)

Lake Superior 0 – 0.20 (av = 0.08) Johnson and Johnston (1995)

Intertidal limestone and mudstone

Kaikoura Peninsula, New Zealand

0.00153 Kirk (1977)

Intertidal limestone and mudstone

Kaikoura Peninsula, New Zealand

0.00148 Stephenson and Kirk (1996)

Supratidal green schist

Start-Prawle Peninsula, Devon, UK

0.000625 Mottershead (1989)

Soft Rock and non-cohesive material

Clay Lake Superior 0 - 2.26 (av=0.78) Johnson and Johnston (1995)

Clay Lake Superior 0 - 0.31 (av=0.14) Johnson and Johnston (1995)

Water-laid sand and gravel

Lake Superior 0.76 - 6.77 (av=1.89) Johnson and Johnston (1995)

Till Lake Michigan 0.10 – 0.75 Jibson and Odum (1994)

Clay, sandy clay, clayey sand , sand, sandstone

Lake Superior 0.07 – 0.57 Swenson et al. (2006)

Queenston Shale Lake Ontario av=0.30 Geomorphic Solutions (2009); Davidson-Arnott and Mathew (2006)


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BEDROCK SHORELINES OF THE GREAT LAKES Shoreline classifications for the upper Great Lakes were compiled from:

• Great Lakes Levels Reference Study (International Joint Commission, 1993);

• Lake Michigan Potential Damages Study - LMPDS (Stewart, 1998); and

• Lower Great Lakes Erosion Study - LGLES (Stewart, 1999). Each study classified the shoreline into geomorphic categories. The LMPDS and LGLES used a revised version of the classification used in the Level Reference Study. The Levels Reference Study covered the entire Great Lakes. The LMPDS updated the Levels Reference Study classifications for Lake Michigan and the LGLES updated the Levels Reference Study classifications for the United States shoreline of Lake Erie. The most recent classification was used where data for more than one study overlapped. As described above, there is a wide range of bedrock types, each with varying degrees of resistance to erosion. Many of these types are present in the upper Great Lakes where they have been grouped as either resistant or non-resistant. Classifications used to summarize the proportions of hard and soft rock shorelines within the upper Great Lakes are defined in Table 3. Length and proportion of the total shoreline for hard and soft rock shorelines are shown in Figures 5 to 7. Maps of hard and soft rock shorelines for each lake are shown in Figures 8 to 11. The northern shoreline of Lake Superior is dominated by resistant igneous or metamorphic rocks of the Canadian Shield. Resistant igneous and metamorphic bedrock is also found along the northern and eastern side of Georgian Bay. Non-resistant (soft rock) shorelines such as weak limestone, siltstone, sandstone or shale occur in sections of Lake Michigan, Huron, and Erie. In total there are approximately 6950 km of resistant bedrock shoreline and 590 km of non-resistant bedrock shoreline within the upper Great Lakes. Resistant bedrock comprises 58 % of the shoreline of Lake Superior and 57 % of the shoreline of Lake Huron. Table 3: Categories used to define hard and soft rock shorelines from the classification

studies.

Levels Reference Study LMPDS LGLES Hard Rock (10) Bedrock Resistant (7) Bedrock

(Resistant) (7) Bedrock (Resistant)

Soft Rock (11) Bedrock (Non-Resistant)

(8) Bedrock (Non-Resistant)

(8) Bedrock (Non-Resistant)

More detail of the bedrock geology on the Canadian shoreline of the Great Lakes is available on maps provided by the Ontario Geological Survey (Ontario Geological Survey, 1991a; 1991b). Geology on the U.S. shoreline of the Great Lakes is available on a number of geological maps (Bownocker, 1920; Berg et al., 1980; Robert et al., 1987; Nicholson et al., 2004; Dicken et al., 2005).


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Figure 5: Proportion of resistant bedrock shoreline by lake.

Figure 6: Proportion of non-resistant bedrock shoreline by lake.


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Figure 7: Proportion of all bedrock shoreline by lake.

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KEY MAPN.T.S.

Base mapping: ESRI, 2006, Geomorphic Solutions, 2010;Shoreline Classification: International Joint Commission: 1993; 1998; 1999.

Resistant andNon-Resistant

BedrockLake Erie

DATE: JUNE 2010DRAWN BY: J.D., L.W.PROJECT: 10076.450 FIGURE 8

0 25 50Kilometres ³

1:2,000,000

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London

Toronto

Cleveland

Major HighwayFreeway / ExpresswayWaterbody

IJC Shoreline ClassificationLegend

Resistant BedrockNon-Resistant Bedrock

Lake Ontario

Lake Erie

Lake Huron

Buffalo

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Resistant BedrockNon-Resistant BedrockGeorgian Bay

Lake Huron

LakeOntario

Lake Superior

Lake Nipissing

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KEY MAPN.T.S.



BedrockLake Michigan



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Lake Huron

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BedrockLake Superior



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JUSTIFICATION FOR EXCLUSION FROM THE REGULATION PLAN EVALUATION One aim of the IUGLS is to determine the potential impacts of different water level scenarios on the rate of shoreline recession. The proposed water level scenarios do not present large changes in mean water levels or even long term trends in water level, but rather changes in the frequency and magnitude of water level fluctuation. These changes could have significant impacts on the rate of recession of shorelines currently with a high rate of recession which respond relatively quickly to changes in the frequency, intensity and elevation of wave attack. The proposed differences in water level scenarios are unlikely to have a perceivable impact on shorelines which respond very slowly to erosive processes. Hard rock shorelines are those which have erosion rates generally much less than 0.1 m per year. This translates to a maximum of 10 m over a 100 yr planning horizon. The most resistant hard rock shorelines will erode on the order of a millimetre a year which is as little as 0.1 m over 100 years. These shorelines have experienced little modification in shape over the past several thousand years and are unlikely to respond to the small changes in water levels proposed over a management time frame. Future climate change scenarios may lead to water levels outside the historic range. It is unlikely that the hard rock shorelines would even respond to changes outside historic levels within the time frame considered. It is recommended that these shorelines do not require further consideration in the IUGLS. Soft rock or non-resistant bedrock shorelines respond more rapidly to changes in the erosive forces than hard rock shorelines. Soft rock shorelines respond to erosive forces in much the same way as cohesive shorelines. The rate of shoreline adjustment is high enough to respond to short term fluctuations in water level. Further investigation of the response of these shorelines to the proposed water level scenarios will be included in the modelling of the erosion of cohesive shorelines. SUMMARY Future water level scenarios for the upper Great Lakes will be affected by the regulation of outflow from Lake Superior. The different potential water level scenarios will have different impacts on the various shoreline types present around the upper Great Lakes. The effect on bedrock shorelines will depend on whether the shoreline is hard rock (resistant) or soft rock (non-resistant). The morphological response time of hard rock shorelines is well outside the time scale of typical shoreline management. Further investigation of the potential impact to these shorelines through modelling is not warranted. Soft rock shorelines respond relatively quickly to changes in the erosive forces. The potential response of these shorelines will be investigated within the framework of the cohesive shoreline erosion assessment.


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REFERENCES Baird, 2010. Erosion Theme Report. Prepared for the International Joint Commission. Berg, T.M., Edmunds, W.E., Geyer, A.R., Glover, A.D., Hoskins, D.M., MacLachlan, D.B., Root, S.I., Sevon, W.D., and Socolow, A.A. 1980. Geologic map of Pennsylvania: Pennsylvania Geological Survey, Map 1, scale 1:250000. Bownocker, J.A. 1920. Geologic map of Ohio [Revised 1947]: Ohio Division of Geological Survey, Map 1, scale 1:500000. Davidson-Arnott, R.D. and Mathew, S. 2006. Shoreline Recession Rates in the vicinity of Burloak Park. Prepared for Halton Region Conservation Authority. 7pp. Dicken, C.L., Nicholson, S.W., Horton, J.D., Kinney, S.A., Gunther, Gregory, Foose, M.P., and Mueller, J.A.L. 2005. Preliminary integrated geologic map databases for the United States: Delaware, Maryland, New York, Pennsylvania, and Virginia: U.S. Geological Survey, Open-File Report OF-2005-1325 scale 1:250000. Geomorphic Solutions. 2009. Recession Study for the Lake Ontario shoreline at 2092 Old Lakeshore Road, Burlington, Ontario. Prepared for Halton Region Conservation Authority. 14pp. International Joint Commission. 1993. Methods of Alleviating the Adverse Consequences of Fluctuating Water Levels in the Great Lakes – St. Lawrence River Basin. A Report to the Governments of Canada and the United States, 53pp. Jibson, R.W., Odum, J.K. and Staude, J.M. 1994. Rates and processes of bluff recession along the Lake Michigan shoreline in Illinois. Journal of Great Lakes Research 20(1): 135 – 152. Johnson, B.L. and Johnston, C.A. 1995. Relationship of lithology and geomorphology to erosion of the western Lake Superior coast. Journal of Great Lakes Research 21(1):3-16. Kirk, R.M. 1977. Rate and forms of erosion on intertidal platforms at Kaikoura Peninsula, South Island, New Zealand. New Zealand Journal of Geology and Geophysics 20, 571-613. Mottershead, D.N. 1989. Rates and patterns of bedrock denudation by coastal salt spray weathering: a seven year record. Earth Surface Processes and Landforms 14, 383-398. Nicholson, S.W., Dicken, C.L., Foose, M.P., and Mueller, J. 2004. Integrated geologic map databases for the United States, the upper midwest states: Minnesota, Wisconsin,Michigan, Illinois, and Indiana: U.S. Geological Survey, Open-File Report OF-2004-1355, scale 1:500000. Ontario Geological Survey. 1991a. Bedrock geology of Ontario, explanatory notes and legend: Ontario Geological Survey, Map 2544. Ontario Geological Survey. 1991b. Bedrock geology of Ontario, explanatory notes and legend: Ontario Geological Survey, Map 2545.


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Robert C. and Daniels, J. 1987. Bedrock Geology of Northern Michigan: Michigan Department of Natural Resources, Geological Publication BG-01, scale 1:500000. Stephenson, W.J. and Kirk, R.M. 1996. Measuring erosion rates using the micro-erosion meter: 20 years of data from shore platforms, Kaikoura Peninsula, South Island, New Zealand. Marine Geology 131, 209-218. Steward, C.J. 1998. A revised geomorphic, shore protection and nearshore classification of the Lake Michigan shoreline – Lake Michigan Potential Damages Study. Consulting Report Prepared for USACE Buffalo District Stewart, C.J. 1999. A revised geomorphic, shore protection and nearshore classification of the Lake Erie, Lake Ontario, Niagara River and St. Lawrence River shorelines – Lower Great Lakes Erosion Study. Consulting Report Prepared for USACE Buffalo District. Sunamura, T. 1992. Geomorphology of Rocky Coasts. Wiley, New York. Swenson, M.J., Wu, C.H., Edil, T.B., and Mickelson, D.M. 2006. Bluff recession rates and wave impact along the Wisconsin coast of Lake Superior. Journal of Great Lakes Research 32, 512-530. Trenhaile, A.S. 1987. The Geomorphology of Rocky Coasts. Oxford University Press, Oxford.

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