5. Hydrology. 
a. Water Level Variations. Figure 21 summarizes the observed mean monthly 
lake levels since 1920. Records are available back to 1860. 
b. Currents. Currents in the Holland area are variable. They generally 
appear to take a northward set during the summer and a southward set during the 
winter in response to the prevailing wins although reversals are frequent. 
There also appears to be evidence of a rotational current system in Lake 
Michigan which is not very well understood. In the immediate nearshore area 
it is likely that the currents are wave induced and will propagate alongshore 
in the direction of the dominant waves at the time. Inshore of the first bar, 
rip currents are extremely well developed and can add a significant amount of 
offshore water movement to the general longshore current. Intensity of the rip 
currents is evident from a number of rip channels which are gouged 1 to 2 feet 
below the adjacent ground. Longshore currents approaching Holland Harbor are 
deflected lakeward along its breakwaters. Aerial photos indicate that the de- 
flected current, after bypassing the harbor entrance, will move a considerable 
distance lakeward before turning parallel to and toward the shore off the down- 
drift coast. 
c. Ice. Floating ice is common in the Holland Harbor area during the 
winter months. Ice accumulates on the beach by wind and wave action and re- 
portedly often extends in a solid mass from the shoreline to more than 1,000 
feet offshore. Wave heights are substantially decreased within an ice field 
and ice along the shore blocks wave action to some extent. However, the action 
of ice can accelerate erosion processes; e.g., ice being pushed upon the beach 
will loosen consolidated beach and bluff material and some of the sediment be- 
comes embedded in the ice at the shore and over the longshore bars to be carried 
elsewhere if the ice drifts away during the spring thaw. The average ice season 
at Holland Harbor extends from late December to late March. 
6. Littoral Drift Estimate from Wave Statistics. 
Littoral drift has been computed from the available wave statistics dis- 
cussed earlier. The governing relationship used was (U.S. Army, Corps of 
Engineers, Coastal Engineering Research Center, 1977) 
Qy= 01000133 RER 
where Q is the potential littoral drift in cubic yards per day, and E the 
longshore momentum flux in feet per pound per day per foot of beach. The re- 
lationship between of, and a, is determined from linear theory in assuming 
that the bottom contours are straight and parallel. The number of waves per 
day for each wave height period-direction class is given by 
_ BS Ie 
be Sanu Os Sa 
where p is the probability of occurrence of that wave class. Computations 
were made using the wave statistics from SSMO data, Saville (1953), and Cole 
and Hilfiker (1970). The duration of computation is for the 9-month ice-free 
period from late March to late December. 
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