| Popis: |
Previous results show that overwash provides an important sediment source to back-barrier marshes, sustaining a narrow marsh state under conditions in which marsh drowning would otherwise occur. We expand the coupled barrier island-marsh evolution model GEOMBEST+ to explore the effects of wind waves on back-barrier marshes. We find that the addition of marsh edge erosion leads to wider, more resilient marshes and that horizontal erosion of the marsh edge is a more efficient sediment source than vertical erosion of the marsh surface as it drowns. Where marshes and bays are vertically keeping up with sea level, and the net rate of sediment imported to (or exported from) the basin is known, the rate of marsh edge erosion or progradation can be predicted knowing only the present basin geometry, sea level rise rate, and the net rate of sediment input (without considering the erosion or progradation mechanisms). If the rate of sediment input/export is known, this relationship applies whether sediment exchange with the open ocean is negligible (as in basins dominated by riverine sediment input) or is significant (including the loss of sediment remobilized by waves in the bay). Analysis of these results reveals that geometry and stratigraphy can exert a first-order control on back-barrier marsh evolution and on the marsh-barrier island system as a whole and provides new insights into the resilience of back-barrier marshes and on the interconnectedness of the barrier-marsh system. Plain Language Summary Sand washed across barrier islands during storms (called overwash) provides sediment for salt marshes behind those islands, and can allow a marsh which otherwise would drown to grow vertically fast enough to keep up with sea level. We use a barrier island-marsh evolution model (GEOMBEST+) to see what effect marsh edge erosion by waves has on overwash-supported marshes. Consistent with previous research, we find that wave erosion can make marshes more resilient by freeing sediment that can be used elsewhere on the marsh surface. We add that horizontal erosion of the marsh edge provides more sediment per volume eroded than vertical erosion of the marsh surface. This is because the bottom layers of the marsh contain more sediment (that can stay on marsh surfaces), while the surface layers include plant material (that drifts away or decomposes). We also find that when the marsh and bay are keeping up with sea level, expanding or eroding the marsh is the only way to change the volume of the bay, so how fast the marsh is expanding or eroding can be predicted using geometry, knowing only the size of the basin, sea-level-rise rate, and the net rate of sediment import or export. |
