94 



For engineering projects, assessments of short-term water level changes 

 range from simple plotting of the data to more sophisticated mathematical 

 analyses. In some cases, some of the components which drive water level 

 changes can be isolated. To assess longer (multi-year) trends, it is important 

 to dampen or separate the effects of yearly variability so that the nature of the 

 secular trends becomes more pronounced. Least-squares regression methods 

 are typically inadequate because the secular trends often show pronounced 

 nonlinearity (Hicks 1972). It may also be important to examine long-term 

 periodic effects in a long data record such as the 18.6-year nodal period, 

 which Wells and Coleman (1981) concluded was important for mud flat 

 stabilization in Surinam. 



West Coast of North America 



The west coast of North America experiences extreme and complicated 

 water level variations. Short-term fluctuations are related to oceanographic 

 conditions like the El Nino-Southern Oscillation. This phenomenon occurs 

 periodically when equatorial trade winds in the southern Pacific diminish, 

 causing a seiching effect, which travels eastward as a wave of warm water. 

 This raises water levels all along the U.S. west coast. Normally the effect is 

 only a few centimetres, but during the 1982-83 event, sea level was elevated 

 35 cm at Newport, OR (Komar 1992). Seasonal winter storms along the 

 Pacific Northwest can combine with elevated water levels to produce tides 

 over 3.6 m. During the 1983 winter storms, water levels were up to 60 cm 

 over the predicted level. Tectonic instability along the U.S. west coast affects 

 long-term water level changes. Parts of the coast are rising and falling at 

 different rates. Studies in Oregon have shown that the state's northern coast is 

 falling while the southern part is rising relative to sea level (Komar 1992). 

 Along Alaska, some areas of the coast are rising nearly 1 cm/year. 



Great Lakes of North America 



On the Great Lakes of North America (Lakes Superior, Huron, Michigan, 

 Erie, and Ontario), astronomic tides have relatively little influence on water 

 levels. Short-term level fluctuations are primarily caused by local atmospheric 

 pressure changes and by winds. This is demonstrated in Figure 34, where the 

 first three wave bursts are shifted vertically from each other. In addition, 

 even during the recording of each 1,024-point burst (17.07 min long), the 

 mean water level changed. 



Long-term changes of water levels in the Great Lakes are caused by 

 regional hydrographic conditions such as precipitation, runoff, temperature 

 and evapotranspiration, snowmelt, and ice cover (Great Lakes Commission 

 1986). These factors in turn are affected by global climate variations. Crustal 

 movements also influence levels. For example, the eastern end of Lake 

 Superior is rebounding at a rate about 10 in. /century faster than the western 

 end, resulting in higher water at the west end at Duluth. Aquatic plant life 



Chapter 5 Analysis and Interpretation of Coastal Data 



