Mean water levels

L - Depth of pile below mean water level, ft... 

Note All estimates exclude the activities of the outer slopes. However in the case of Canton and Fanning Islands, these do not contribute to the virtually closed systems. These two reefs also lack reef-flats subject to inflowing water because of the continuous islands along the reef crests. 1 = Mean water level 2 = low water springs. 

Above the mean water level, the palustrine environment begins it can include peats, calcareous marsh and ponds (Figure 9.3B, C), and hydromorphic (pseudogley) soils depending on the nature of the morphoclimatic setting. 

For coastal waters in open communication with the sea, the mean water level (MWL) can often effectively be taken as a site-specific constant, being related to the mean sea level (MSL) of the oceans. In some areas, for example the eastern Mediterranean, the mean sea level varies slightly with the time of year, in a predictable way. 

The reference water level in an enclosed body of water which is not subject to human control should be taken as the mean value of all data on the water level for a certain time period. Surge and seiche effects cause changes in the transient water level only and do not significantly change the mean water level. The reference water level upon which the computed probable maximum storm surge or probable maximum seiche is superimposed should be seleaed so that the probability of its being exceeded over the lifetime of the plant is sufficiently low. ... 

Standard terminology defines the water level in the absence of wave effects as still water level, whereas wave setup will cause a departure from the still water level and this water level including the effects of the waves is the mean water level. As implied, the mean water level is determined as the average of the fluctuating water level over a suitable time frame usually taken as a number of multiples of the short wave period, say the spectral peak. In considering wave setup, often the location of interest is that of the maximum wave setup at the shoreline. This raises the question of whether wave setup is defined at elevations above the maximum rundown, say on the beach face where the water is present over only a portion of the wave period. Since wave setup is defined as the mean water level, over what period should the water surface be averaged on the beach face which is wetted over only a portion of the wave period If the time average is over only the portion of the period that water is present, in the upper limit, the maximum setup will be the maximum runup. For purposes here, wave setup will usually be defined only for conditions where water is present over a full wave period. 

Nielsen points out that previous field investigations have typically measured the mean water level elevation on the beach as opposed to the average fluctuating mean water level in the vertical plane (i.e., the wave setup as usually deflned), and that the two measurements are often different in part due to the beach permeability, which in turn is related to beach material size. The issue of extracting wave setup from runup and rundown on the beach is illustrated in Fig. 1.10. 

Fig. 1.10. Illustration of differences between mean water level (MWL) shoreline and mean water line on beach.

Wave records were analyzed by the zero-downcrossing method, and calculated results of characteristic wave heights and periods are presented in tabular forms. Prom these results, the ratios of and H ms to the local depth (inclusive of mean water level change) are calculated and plotted against the relative water depth h/Lo, as shown in Fig. 4.2. 

Figure 5.7 shows the time series of bubble diameter distribution just beneath the mean water level near the breaking point under the same wave condition as in Fig. 5.5. The bubble diameter here means the averaged value of the major and minor diameter. The horizontal axis is the characteristic bubble diameter, the vertical axis is the temporal change, and the contour indicates number of bubbles per unit area. The plunging wave breaking not only entrains the bulk of air but also generates a large number of bubbles in comparison with the spilling breaker. As discussed earlier, the void fraction decreases exponentially in time. The time series...

A Monte Carlo simulation of the wave climate during the period of time that is object of study, V, measured in years (see Fig. 33.2), will provide a sample containing N possible outcomes of the forcing during V years. Each of them can be discretized as a series of sea states characterized by the values of the significant wave height, peak period, mean water level, etc. 

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