, Volume 8, Issue 5, pp 353-366

A comparison of 28 natural and dredged material salt marshes in Texas with an emphasis on geomorphological variables

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Abstract

Fourteen dredged material marshes andfourteen natural marshes along the Texas, USA, coastare compared on the basis of 1) edge: area ratios, 2)relative exposure index values, 3) elevation profiles,4) elevation of Spartina alterniflora, 5) soilorganic carbon content, 6) soil silt-clay content, and7) belowground plant biomass. Although edge: areacomparisons cannot detect certain types of differencesin geomorphology, comparisons clearly show thatdredged material marshes, on average, have fewer pondsand flooded depressions than natural marshes. Comparisons of relative exposure index values suggestthat wave protection structures associated with somedredged material marshes may be overbuilt. Elevationprofiles illustrate the potential for structures suchas berms to lead to differences between dredgedmaterial marshes and natural marshes, but they alsoshow the high variability in elevation profiles thatexists among both dredged material and naturalmarshes. S. alternifloraelevations in dredgedmaterial marshes are not significantly different fromthose of natural marshes. Soil organic carbon andsilt-clay content of dredged material marshes are notsignificantly different from those of natural marshes. Although belowground biomass of dredged materialmarshes is significantly lower than that of naturalmarshes, regression analysis suggests that belowgroundbiomass will increase over time. Findings reportedhere suggest several points that should be consideredduring planning and design of dredged material marshesin Texas: 1) if an objective of marsh construction isto mimic natural marsh geomorphology, methods toincrease the amount of unconnected edge need to bedeveloped, 2) methods of effectively summarizinggeomorphic characteristics need further development,and 3) there is some evidence suggesting thatprotective structures may be over-built, and the needfor substantial structural protection should bebalanced against the costs of structures and risk ofsite failure during project design. Lastly, a methodfor increasing the amount of unconnected edge thatinvolves excavation of bay bottom before placement ofdredged material is suggested.