DISCUSSION 



Effects of Alternate Assumptions 



Geodynamic changes in elevation of land relative to "global" sea level are a function of 

 glacial isostatic rebound affecting large portions of continents, regional adjustments to plate 

 tectonics, subregional isostatic adjustments to sedimentary loading, and local subsidence due to 

 withdrawal of groundwater and oil and compaction of sediments. Because relative sea level at any 

 particular tidal gauge is also affected by barometric pressure, wind direction, and coastal 

 currents, at least 35 years of data are needed to separate the various components of local sea 

 level to detect a 1 mm/yr trend with 95 percent confidence (IAPSO 1985). The average rate of 

 glacial isostatic submergence for the East Coast is 0.6 mm/yr (IAPSO 1985), which would mean 

 that the simulation would be advanced by approximately three years over a hundred-year period 

 compared with a 0.0 value for subsidence. If a value of 1.2 mm/yr is used, based on Hicks et al. 

 (1983), the simulation is advanced by six years over a hundred-year period. 



Simulation of sea level response at Bombay Hook, Delaware, shows how subsidence assump- 

 tions affect wetland response. If subsidence is considered as negligible (held to 0.0 in computer 

 runs), only a slightly different outcome results by the year 2100 than if subsidence is considered 

 to be 2.9 mm/yr (Table 4-7). Under the low scenario, higher subsidence results in a slightly larger 

 wetland area because conversion of lowland occurs. Marsh area expands at the expense of unde- 

 veloped lowland by virtue of its 5 mm/yr accretion rate beginning around the turn of the century. 

 However, subsidence assumptions make no difference through 2050. 



TABLE 4-7 



PERCENT MARSH FOR DIFFERENT MODEL CONDITIONS; DELAWARE BAY: 

 TOTAL AREA = 30.800 ha 



Low High 



A = 5 A = Variable A = 5 A = Variable 



Year S = S = 2.9 S = S = 2.9 S = S = 2.9 S = 9 S = 2.9 



2050 29.2 29.2 29.2 29.2 29.2 29.2 29.2 29.2 

 2100 34.0 34.7 34.0 34.7 30.5 22.7 39.2 30.2 



A = Accretion Rate in mm/yr; S = Subsidence Rate in mm/yr 



In the Gulf Coast, average subsidence ranges from 0.0 and 1.5 mm/yr (Holdahl and Morrison 

 1974). Subsidence is essentially zero for most of the Gulf Coast areas simulated, except for the 

 northern Texas Coast, where a subsidence value of 3.5 mm/yr was used, and for the Mississippi 

 Delta, where values of 3.5 to 11 mm/year were used. Because the tidal range is 0.3 m along the 

 Texas Coast, a 3.5 mm/yr subsidence doubles the rate of change in coastal features compared to 

 the default of 0.0. The results of these alternative values are shown in Table 4-8. 



As expected, holding accretion rate constant, rather than allowing it to increase as marshes 

 expand, has an impact similar to that of introducing a small subsidence rate (Table 4-7). The net 

 effect is loss of most wetlands that would have been gained under the higher accretion rate by the 

 year 2100. However, the total wetland area was nearly equal under the two conditions. Differences 

 are more striking under the high scenario. Here the increase in accretion to 10 mm/yr, which 

 began in 2075, enables salt marsh expansion. In contrast, if the accretion rate is held constant, 

 marsh never accumulates beyond its original area in 2075, and fewer areas are suitable for marsh 

 expansion. Therefore, by the year 2100 the marsh area was reduced to 30.5 percent. In contrast, 

 the total area of wetlands with rising accretion but no subsidence equalled about 39 percent. 



120 



