Analysis of Structures on Shoreline of Squaw Bay, Lake Monona

Tim Santiago

Coastal Engineering - CEE 514

 

About Squaw Bay
Objective
Hypotheses
Paunack Park
Dellwood Circle
Boat Wakes vs. Water Waves
Determining 100 Year Water Level
Suggestions for Structures
Sources and Special Thanks
 


 



 

About Squaw Bay
     During the summer of 2000, the Madison area lakes recorded the highest water levels ever.  As a result, the city implemented a no wake zone for all of the lakes.  During the summer of 2001, high water levels again interfered with boat traffic on the lakes, however, this time, the no wake zone was implemented only on a small portion of Lake Monona, in a small bay in the southeast portion of the lake known as Squaw Bay.


       Map of Squaw Bay


 

Graph of High Water Levels on Lake Monona




 

Objective
       In observing the structures on Squaw Bay the objective was to observe the effects of high water on the shoreline by analyzing two different structures on either side of the bay.  Also, another goal of this project was to gain a better understanding of the course material discussed over the semester, and combine these topics with the research and data compiled an observations made.



 



 

Hypotheses
        Prior to my visit out to the lake, I devised a number of hypotheses that I thought were possible reasons to the reasoning to the closing of the bay to boat traffic during August of 2001.  The hypotheses are as follows:

 

 

 

Paunack Park
    Paunack Park was located on the south shore of Squaw Bay.  The structure there was a bulkhead built of single row of boulders, approximately 2 feet in diameter.  The structure at the park had eroded considerably; dirt was eroded as much as 18 inches from the bulkhead itself.  Due to the structures location on the south end of the bay, it was exposed to the most fetch (or open water area) than any other part of the bay.


        Picture of Paunack Park

 


        Profile of Beach



 

Dellwood Circle
    The Dellwood Circle site was located on the northern side of the bay.  The structure there was a sloped revetment built of a wall of 2 foot boulders near the shore, which tapered to a gentler slope of smaller (~2 inch rocks) above that.  The structure at the Dellwood Circle site showed little signs of erosion. Due to it's northern location, Dellwood Circle was exposed to the least fetch in comparison to any other part of the bay.
        

Picture of Dellwood Circle


        

Profile of Beach




 

Boat Wakes vs. Wind Waves

    Wind Waves
        As stated previously, Paunack Park's shoreline was exposed to more fetch than any other part of the bay. A wind of 35 mph, blowing over the average maximum fetch of 10800 feet (2.04 miles) onto Paunack Park, generates a wave height of approximately 1.4 feet. high.  In comparison to the water level of the lake during the high water levels of 2000 and 2001, it is obvious that a wave height such as this will have a significant impact on the shoreline.
        Since the location of Dellwood circle is not subjected to much fetch, the wind wave are assumed to be insignificant in terms of potential damage to the structure.

Picture on maximum potential fetch on Paunack Park

    Boat Wakes
        In comparison to wind generated waves, the energy generated by boat wakes are minimal in comparison.  Wind blows constantly, whereas boats only run for a few hours everyday, and generate only a few waves with each pass.  However, boat waves are a fairly significant factor in areas where wind waves are not much of a threat.  Dellwood Circle is such an example.  Boat wake height is dependent on a number of factors:  velocity, distance from shore, length of boat, and the draft of the boat (the draft, or depth of the boat beneath water, is quite dependent on speed).  A typical wave train from a boat wake may consist of 10-15 waves, only 3-4 of them will be of a fairly significant wave height.  It is not unlikely that a boat traveling close to shore (which is unavoidable in Squaw Bay due to it's small size) will generate a wave that will reach shore at a height of 1.5 feet.  Cabin cruisers and ski boats are notorious for generating the highest waves among recreational watercraft.  Again, as with wind generated waves, the impact on shorelines is quite significant, especially in times of especially high water.




 

Determining 100 year water level
    The water levels during the summer of 2000 were higher than were ever seen before, and it was quite obvious be comparing these water levels to beach profiles that the coastal structures were not adequately prepared for water levels this high. As a result, it is important to figure out how often events like this will occur in order to prepare for events of this severity in the future.
    Data was acquired from U.S. Geological Survey publications over 40 years (1961-2000), and from this data, an estimate of 100 year water level was calculated to determine the recurrence interval of events of this magnitude.  The values used were the maximum monthly values, the average monthly values, and the maximum annual values over the course of 40 years.  These values were used in two separate methods (Gumbel and Weibull) to determine this 100 year water level value.  From these multiple analyses, the best method (Gumbel method, Mean Monthly data, see below) shows that the recurrence interval for the water level during the year 2000 (847.47 ft above sea level, or 2.51 feet above the 40 year average) is expected to occur once every 80 years, where as the water level for the peak of 2001 (846.55 above MSL, 1.59 feet above average) would be expected once every 10 years.  In comparison to the 40 years of data that was analyzed, this shows to be fairly accurate.

 
100 Year Water Level Calculations
Distribution Data Type 100-Year   Water Level
Weibull Mean Monthly 8.15
Max Monthly 8.57
Max Annual 8.18
Gumbel Mean Monthly 7.57
Max Monthly 8.07
Max Annual 9.05

 

    It is important to note, however, that the chances of a "100-year" water level occurring become greater as the watershed becomes more developed.  With more urbanization on the watershed, more runoff is created, which ultimately finds its way to the lake.  Thus, what was once considered t be a 100-year water level 40 years ago is not the case today, as this level will be much more easily attained as urbanization affects the rise of the lake.

 



 

Suggestions for Structures

    Paunack
        It was quite obvious from analysis of the structure at Paunack Park, that the structure is in much need of improvement.  First of all, the bulkhead is in need of more protection from wind waves during high water levels, and this can be obtained through a greater variety in the sizes of riprap, and by making the structure sloped such that wave energy from wind and boat waves will dissipate during wave run-up as opposed to crashing into an under-protected shore.

    Dellwood Circle
        At Dellwood, the structure seemed to be built quite well, however, it is possible that the structure was damaged, and since rebuilt after sustaining damage from high water levels during 2000, and built better to prevent damage from similar events in the future.  In terms of improvements for the structure at Dellwood Circle, it is necessary to replace the smaller stones at the top of the revetment with larger one such that washout of the structure does not occur.  The two inch stones are only capable of withstanding the force of a 6 inch wave, and during high water levels in comparison with high boat traffic, these wave heights can easily be exceeded - the 1.5 foot wave heights, generated by boat wake, require cobble 6 inches in diameter or greater.

 


Sources

 - N.G. Bhowmik, et al. 11/1992. Waves Generated by Recreational Traffic on The Upper Mississippi River System. US Fish & Wildlife Service Report 92-SOO3

 - U.S. Geological Survey. Water Resources Data Publications, 1961-2000.

 - Philip Keillor. 2/09/01. Wave Energy Exposure on Shore of Inland Lakes and Reservoirs – Draft.

 - N.G. Bhowmik, et al.  Aquatic Plant Communities of Upper Mississippi River

 - Paul Komar. 1983. CRC Handbook of Coastal Processes and Erosion. CRC Press.

 - WDNR. 1991. Boating on Lakes Mendota and Monona. Wisconsin DNR, Madison WI.

 - Clarkson Map Company.  Map of Lake Monona, Map #1443.  Kaukauna, WI.

 - http://www.boatwashington.org/watching_your_wake.htm

 - http://www.co.dane.wi.us/press_release/exec/20010806.htm

 - http://wi.waterdata.usgs.gov/nwis/dv/?site_no=05429000&agency_cd=USGS

Special Thanks

    Chin Wu, Philip Keillor, Keith Puro
 

Other Important Links

    University of Wisconsin 

    UW College of Engineering 

    Dept. of Civil Engineering

    Coastal Engineering (CEE 514)

Created by Tim Santiago, December 20, 2001.  Email at trsantiago@students.wisc.edu