Coastal Flooding
Causes of Coastal Flooding, Vulnerability to Flooding and How to Estimate Your Risk

Great Lakes high water levels are somewhat related over a period of several years. High water levels on other waterbodies are not usually related over time, but occur as discrete, short-term events. 

Potter (1992) demonstrated how the probability of exceeding a "100-year flood level" over the next few years varies greatly, depending on the chosen initial water level. If the Great Lake water level today is very high, there is a high probability that the flood level will be exceeded in the next couple of years. Conversely, at a very low lake level today, there is a very low probability of such exceedance. The 100-year flood level is the basis for federal flood insurance. It's defined as a level that's expected to be exceeded just once in 100 years, on average, over a long time.

Flooding at Sites Sheltered from Storm Waves
Some coastal sites in harbors, bays and behind islands are not exposed to storm waves of any significant size.



Flooding at Sites Exposed to Storm Waves
Coastal sites on the open coast are exposed to storm waves.



Urban Vulnerability to Flooding

Great Lakes cities and towns are often built in low-lying estuaries where rivers offered natural harbors when settlement occurred in the 1800s. These communities were developed on land with elevations believed adequately safe from flooding, based on short experience.  Record high lake levels threaten such communities,
their water treatment plants, sewage treatment plants, and other services depended upon by hundreds of thousands of people. The urban vulnerabilities at flood elevations higher than the record levels of 1985-1986 remain largely unexamined. 


How to Estimate the Risk of Flooding
Information on extreme high and low lake levels, storm surges and 100-year flood elevations for Canadian and U.S. shores of the Great Lakes can be found in tables of Wisconsin Sea Grant’s Coastal Processes Manual. 2nd Edition. 1998. The manual
shows (with examples) how to compare land elevations at particular sites with selected flood elevations. See the publications page for information on how to order it. 

Reference: Kenneth W. Potter. 1992. Estimating the probability distribution of annual maximum levels on the Great Lakes. Journal of Great Lakes Research. International Association for Great Lakes Research. 18(1):229-235.






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