OS33D-01
Rapid evolution of a marsh tidal creek network in response to sea level rise.
In the Santee River Delta (SRD), South Carolina, tidal creeks are extending rapidly onto the marsh platform. A time-series of aerial photographs establishes that these channels were initiated in the 1950's and are headward eroding at a rate of 1.9 m /yr. Short-term trends in sea level show an average relative sea level rise (RSLR) of 4.6 mm/yr over a 20-year tide gauge record from nearby Winyah Bay and Charleston Harbor (1975-1995). Longer-term (85-year) records in Charleston suggest a rate of 3.2 mm/yr. RSLR in the SRD is likely even higher as sediment cores reveal that the marsh is predominantly composed of fine-grained sediment, making it highly susceptible to compaction and subsidence. Furthermore, loss in elevation will have been exacerbated by the decrease in sediment supply due to the damming of the Santee River in 1939. The rapid rate of headward erosion indicates that the marsh platform is in disequilibrium; unable to keep pace with RSLR through accretionary processes and responding to an increased volume and frequency of inundation through the extension of the drainage network. The observed tidal creeks show no sinuosity and a distinctive morphology associated with their young age and biological mediation during their evolution. Feedbacks between tidal flow, vegetation and infauna play a strong role in the morphological development of the creeks. The creek heads are characterized by a region denuded of vegetation, the edges of which are densely populated and burrowed by Uca Pugnax (fiddler crab). Crab burrowing destabilizes sediment, destroys rooting and impacts drainage. Measured infiltration rates are three orders of magnitude higher in the burrowed regions than in a control area (1000 ml/min and 0.6 ml/min respectively). Infiltration of oxygenated water enhances decomposition of organic matter and root biomass is reduced within the creek head (marsh=4.3 kg/m3, head=0.6 kg/m3). These processes lead to the removal and collapse of the soils, producing topographically depressed regions at the creek heads. The depression focuses the ebb tidal flow into the creeks leading to strong ebb dominance in the creek heads and a net loss of suspended sediment through them. Thus the headward incision of tidal creeks is initiated by biologically driven subsidence at the creek heads. The results of this study provide an alternative scenario to marsh submergence as a response to increasing SLR and clear evidence of the importance of biological feedback in the evolving morphology of marsh tidal creeks.
OS33D-02
Morphology and Expansion of a Tidal Flat and Mangrove Forest, Firth of Thames, New Zealand
A morphological and sedimentological study was undertaken on a tidal flat/mangrove forest complex in the Firth of Thames (North Island, New Zealand), to elucidate patterns and rates of tidal flat progradation and associated mangrove habitat expansion along this wave-impacted mesotidal muddy coastline. Recent studies of mangrove habitat in the area document accelerated forest expansion over the past five decades. To better understand processes controlling progradation of unvegetated mudflats fronting mangrove forest, sediment cores and field observations were collected on a transect extending one km seaward of the mangrove fringe. Cores were X-radiographed and analyzed for grain size, water content, and the radioisotopes Pb-210, Be-7, and Cs-137 to evaluate sediment accumulation rates, and sediment mixing rates and depths. X-radiographs and Be-7 profiles indicate intense and rapid mixing (by waves) of the uppermost 3-7 cm of sediment on unvegetated flats. Pb-210 accumulation rates of 2-3 cm/y characterize the uppermost 40-50 cm of unvegetated flat sediments, much slower accumulation than the 5-10 cm/y accumulation rates observed in the seaward edges of mangrove forest. Our observations suggest that the wave-swept unvegetated mudflats accrete relatively slowly until an elevation threshold is reached that allows mangrove recruitment. Sediment accretion in the mangrove fringe remains low until vegetation is sufficiently dense to reduce wave exposure, whereupon more rapid sediment accumulation ensues, as the young trees mature. A simple sediment budget based on Pb-210 sedimentation rates and estimated local river sediment supply indicates that present sediment accumulation on the unvegetated mudflat exceeds present fluvial sediment discharge by a significant margin, suggesting that the system is not in steady state, but is still adjusting to massive sediment flux delivered following a period of forest clearance in the late 19th/early 20th century.
OS33D-03 INVITED
Early Salt-Marsh Development, an Example of a Turing Instability?
In the past decades, regular spatial patterns have been described in a wide range of ecosystems, ranging from arid lands to boreal peat lands. Pattern formation mechanisms in many of these ecosystems are caused by scale-dependent interactions between organisms and geophysical processes, causing facilitation between organisms at small spatial scale, but inhibition at larger spatial scales. This conforms to the activation-inhibition principle laid out by Alan Turing in 1953. We present a combination of experimental and modeling studies on early salt-marsh development that indicate that similar scale-dependent interactions determine the establishment of salt-marsh vegetation and early geomorphological development of the marsh. Based on these studies, we argue that the early development of salt-marsh ecosystems is characterized by a Turing instability, placed into a complex landscape setting.
OS33D-04
Geomorphic Structure of Tidal Hydrodynamics in Salt Marsh Creeks
We have developed a geomorphological theory of tidal basin response (tidal instantaneous geomorphologic
elementary response, or TIGER) to describe specific characteristics of tidal channel hydrodynamics. On the
basis of the instantaneous unit hydrograph approach, this framework relates the hydrodynamics of tidal
watersheds to the geomorphic structure of salt marshes and, specifically, to the distance traveled by water
particles within the channel network and on the marsh surface. The possibility of determining the water fluxes
from observations of geomorphic features is an appealing approach to the study of tidally driven flow rates.
Our formulation paves the way to the application of recent results on the geomorphic structure of salt
marshes and tidal networks to the determination of marsh creek hydrology. A case study shows how the
asymmetry in the stage-velocity relation and the existence of velocity surges typical of the tidal hydrographs
can be explained as an effect of the delay in the propagation of the tidal signal within the marsh area.
http://people.bu.edu/sergio
OS33D-05
Tidal Hydraulics and Morphological Response to Wetland Loss in Barataria Bay, Louisiana
Relative sea level rise (RSLR) in Barataria Bay, Louisiana is on the order of 1 cm/yr due to high rates of subsidence coupled with eustatic sea-level rise. The most pronounced physical response to this RSLR is wetland loss. Barataria is a large, shallow, interdistributary bay separated from the Gulf of Mexico by a chain of barrier islands. Four principal tidal inlets control the flux of water and nutrients between Barataria Bay and the Gulf of Mexico (west to east): Caminada Pass, Barataria Pass, Pass Abel, and Quatre Bayou. Between 1935 and 2006 wetland loss increased bay area by 1125 km2 resulting in greater tidal exchange and increasing tidal prism. During this period, inlet cross sectional areas enlarged by a combined 13,000 m2 and ebb-tidal deltas doubled in volume. The tidal wave in Barataria Bay is largely progressive, with only a minor standing wave component, consistent with a shallow estuary that contains additional frictional elements due to extensive marshlands and islands. Tides in Barataria Bay are diurnal and gradually reduce in amplitude from 0.53 m (tropic tide conditions) at the entrance to Barataria Pass to 0.07 m at Lake Cataouatche, located 70 km up basin. Much of the reduction in the upper reaches of the basin is due to bottlenecks, where narrow tidal channels connect to shallow lakes. The tidal wave crest reaches Lake Cataouatche 13 hours after propagating through Barataria Pass. Historically, the expanding inlet dimensions have increased tidal conductivity leading to an increase in tidal range throughout the bay. Tide gage records at Grand Isle and at eight other locations within the basin show that during the past 10 to 30 years (depending on station) tidal ranges increased from 9.8 mm/yr inside Barataria Pass to 3.1 mm/yr in Lake Salvador (60 km up basin). The magnitude of this change is significant due to the microtidal conditions that exists within the bay (TRAve = 0.32 m). Thus, the increasing extent of open water and dimensions of the tidal inlets are modifying the tidal signal in the backbarrier, producing a positive feedback that is increasing tidal prism.
OS33D-06
Can salt marshes survive sea level rise ?
Stability of salt marshes is a very delicate issue depending on the subtle interplay among hydrodynamics, morphodynamics and ecology. In fact, the elevation of the marsh platform depends essentially on three effects: i) the production of soil associated with sediments resuspended by tidal currents and wind waves in the adjacent tidal flats, advected to the marsh and settling therein; ii) production of organic sediments by the salt marsh vegetation; iii) soil 'loss' driven by sea level rise and subsidence. In order to gain insight into the mechanics of the process, we consider a schematic configuration consisting of a salt marsh located at the landward end of a tidal channel connected at the upstream end with a tidal sea, under different scenarios of sea level rise. We extend the simple 1D model for the morphodynamic evolution of a tidal channel formulated by Lanzoni and Seminara (2002, Journal of Geophysical Research-Oceans, 107, C1) allowing for sediment resuspension in the channel and vegetation growth in the marsh using the depth dependent model of biomass productivity of Spartina proposed by Morris et al. (2002, Ecology, 83, pp. 2869 – 2877). We first focus on the case of a tide dominated salt marsh neglecting wind driven sediment resuspension in the shoal. Results show that the production of biomass plays a crucial role on salt marsh stability and, provided productivity is high enough, it may turn out to be sufficient to counteract the effects of sea level rise even in the absence of significant supply of mineral sediments. The additional effect of wind resuspension is then introduced. Note that the wind action is twofold: on one hand, it generates wind waves the amplitude of which is strongly dependent on shoal depth and wind fetch; on the other hand, it generates currents driven by the surface setup induced by the shear stress acting on the free surface. Here, each contribution is analysed separately. Results show that the values of bottom stress induced by wind setup are small compared with those associated with wind waves. However, the permanence of wind currents makes them as significant as the oscillating tidal currents in determining the direction and the intensity of the residual sediment flux. Marshes are typically characterised by a variety of vegetation species competing for habitat space within the intertidal zone: we analyze this feature by considering the case of two different species. Preliminary results show that the presence of a species characterised by a narrower habitat range, lower optimum elevation and biomass productivity, has a positive feedback on the growth of the other species. Moreover, the presence of an invader raises marsh elevation above the value reached in the presence of just one species. Finally, we investigate the effect of a reduction of the amount of sediments supplied from the sea.
OS33D-07
Effects of Tidal Creek Geometry on Sediment Accretion
Field observations and numerical studies have shown that elevation, distance from a channel, and vegetation density are the major controls on sediment accretion rates and their spatial variability across intertidal salt marsh landscapes. An important aspect missing from these analyses however, is the effect of tidal creeks. We contend that tidal creek 2D and 3D structure controls the delivery of sediment to the salt marsh and these effects have not been thoroughly addressed. Therefore, the purpose of this study is to evaluate the geometric properties of 22 small tidal creeks on a 0.5 km2 salt marsh island, and to contrast those properties with observed sediment accretion rates determined from 45 sediment traps. Initial observations reveal a significant difference in accretion rates within a narrow range of elevations but at geomorphically distinct locations. Moreover, sediment accretion rates are positively correlated with variations in creek hypsometry. Together these observations indicate that tidal creek structure has a comparable role in sediment accretion relative to elevation, creek distance, and vegetation. Insight gleaned from this study on the geomorphic effects of sediment accretion will help quantify the uncertainty associated with point measurements, and constrain interpretations based on surface elevation tables (SETs).
OS33D-08
LONG TERM MORPHOLOGICAL EVOLUTION OF THE VENICE LAGOON: EVIDENCE FROM THE PAST AND TREND FOR THE FUTURE
During the last century, the Venice lagoon has been experiencing a generalized degradation consisting of the deepening of tidal flats and the reduction of salt marsh areas. A conceptual model describing the long term evolution of such lagoons has been recently proposed. According to the model, the long term degradation consists of two steps: a initial salt marsh deterioration phase followed by a tidal flat erosion phase. In this work we test the long term evolution model through the analysis of four different bathymetries of the Venice lagoon during the last century (1901, 1932, 1970, and 2000 respectively). The result of the analysis confirms that the recent past morphological evolution of the Venice lagoon has actually followed the two steps and highlights a slower erosive trend characterizing the Northern part of the lagoon compared to that in the Central-Southern part. This result enables us to infer the likely future evolution of the Venice lagoon as long as the present forcing conditions maintain.