EXAMPLE PROBLEM
GIVEN: Lake Erie has a mean depth of d = 61
220 miles or 116,160 feet.
feet and its length
FIND: The fundamental period of oscillation TJ
if j = 1.
SOLUTION: From Equation 2-83 for an enclosed basin,
2^3 1
T1
j (gd)h
2(116,160)
1
1
[32.2(61)]*
Tj = 52,420 sec. or 14.56 hrs.
Considering the variability of the actual lake cross-section, this
resuit is surprisingly close to the actual observed period of 14.38
hours (Platzman and Rao, 1963). Such close agreement may not always
resuit.
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Note: Additional discussion of seiching is presented in Section 3.84.
2.54 WAVE REFLECTION FROM BEACHES
The amount of wave energy reflected from a beach dépends upon the
roughness, permeability and slope of the beach in addition to the steepness and angle of approach of incident waves. Miche (1951) assumed that
the reflection coefficient for a beach x, could be described as the
product of two factors by the expression,
X = XjX2 ,
(2-85)
where xx dépends on the roughness and permeability of the beach and is
independent of the slope, while
depends on the beach slope and the
wave steepness.
Based on measurements made by Schoemaker and Thijsse (1949), Miche
found that xx * 0-8 for smooth impervious beaches. A value of
2-117