CHAPTER 2 



WATER LEVELS, SEDIMENT DEPOSITION, AND THE FORM OF THE MARSH 



FLUCTUATIONS IN WATER LEVEL 



While the long-term secular rise 

 in sea level due to glacial melting 

 and land subsidence has played an 

 important role in salt-marsh develop- 

 ment, other processes influence 

 water levels along the coast. 

 Although people often think of sea 

 level as a fixed datum, continuous 

 water level records (such as those 

 obtained from tide gauges) have shown 

 that sea level varies on virtually 

 ewery time scale. There are wind- 

 generated waves which may have periods 

 of seconds or minutes, as well as 

 semidiurnal or diurnal tidal waves. 

 The passage of atmospheric fronts with 

 different barometric pressures and 

 wind fields influences coastal water 

 levels for hours or days. Seasonal 

 and yearly changes in temperature, 

 salinity, and barometric pressure 

 influence the density and volume of 

 coastal waters, making them rise and 

 fall relative to the land. And 

 changes in coastal geomorphology that 

 may take place relatively rapidly 

 (dredging, breachway opening) or over 

 a number of years (developm.ent of a 

 barrier spit), may influence water 

 levels and tidal ranges. The name 

 "tidal marsh" reflects the widely 

 recognized importance of this 

 component of water level changes, and 

 it will receive particular attention 

 in the next section. First, it is 

 worthwhile to consider some other 

 processes that influence water levels 

 in the marsh on time scales longer 

 than a tidal cycle but considerably 

 shorter than the melting of glaciers. 



9 



Short-term Changes in Mean Sea Level 



Sea level is usually calculated 

 as the arithmetic mean of hourly water 

 level measurements collected over the 

 period of interest. Usually, water 

 level data are taken from tide gauges 

 that are designed to filter out short 

 period changes due to waves. The 

 elevation of the gauge itself is 

 usually leveled to U.S. Geological 

 Survey bench marks on land which can, 

 in turn, be related to the zero 

 elevation of the National Geodetic 

 Vertical Datum (Hicks 1978). 



Examinations of water level data 

 reveal a bewildering array of 

 nontidal changes, some of which are 

 irregular while others appear to be 

 cyclical. It is well known that there 

 is a rise in sea level associated with 

 storms which may reach 3 to 5 m (10 to 

 16 ft) above normal tide in the 

 extreme case of a hurricane. More 

 commonly, the effect of winter storms 

 along the northeast coast will 

 increase water levels by less than a 

 meter. This increase is due to a 

 short-term "surge" of water moving 

 into the area because of the low baro- 

 metric pressure associated with the 

 passage of the storm front (the 

 "inverse barometer effect," Smith 

 1979) and to a longer-term 

 accumulation of water against the 

 coast due to wind stress. Miller 

 (1958) studied these two processes on 

 the New England coast and found that 

 there was a time lag of 1 to 14 hr 

 with an average of 5 to 6 hr, between 

 maximum wind and maximum "set-up" or 



