165 



Estuarine marshes embrace a wide variety of vegetative species in 

 diverse geologic settings where salinities are less than approximately 

 30 ppt. These marshes, comprising integral components of major estuarine 

 systems such as the Chesapeake Bay, occur in areas of quiescent waters and 

 ample sediment supply. Accretionary budgets differ widely, as seen from 

 Table 10.1. 



The data of Table 10.1 are plotted in Fig. 10.5 in terms of mean marsh 

 accretion rate against relative sea level rise rate, both in mm/yr. It is 

 noteworthy that with the exception of the three data points from Louisiana 

 and one in Georgia (Savannah R.) the remaining data points are confined 

 within a relatively narrow domain bounded by a range of 0.9 to 3.9 mm/yr 

 for sea level rise and 2.5 to 6.1 mm/yr for marsh accretion. The 

 corresponding mean values are 2.3 mm/yr and 4.3 mm/yr, respectively. Thus 

 overall marshes seem to have accreted at almost twice the rate of sea level 

 rise. 



The apparent discrepancy between sea level rise rate and accretion 

 rate is most likely to be due to the effect of compaction. The rate of 

 accretion can be thought of as the initial rate of sediment deposition, 

 which depends on the ambient suspended sediment concentration and the 

 sediment settling velocity. In the absence of compaction, and noting that 

 usually in the long run marsh level can at most keep pace with sea level 

 but not rise faster, the maximum marsh accretion rate must equal sea level 

 rise rate. This is indicated by the 45° dashed line (no compaction) in 

 Fig. 10.5. In general, however, compaction cannot be ignored. Laboratory 

 tests on the deposition of relatively thin fine-grained sediment beds 

 (Dixit, 1982) show that the density of the initial deposit (dry sediment 

 mass per unit volume) tends to increase from about 0.05-0.1 g/cm to 

 between 0.2-0.3 g/cm-^ after a few days of relatively rapid consolidation. 

 Beyond this period, further increase in density is very slow. Although 

 there would be a significant difference between these test results and 

 marsh compaction in the field, it is worthwhile to examine the implications 

 of compaction based on the laboratory evidence, qualitatively. Thus, if we 

 assume a two-fold increase in density, the line shown in Fig. 10.5 

 (compaction) will result. Several data points tend to corroborate the 

 observed linear trend. It can thus be surmised that compaction effects are 

 significant in marshes. 



