Beach – Dune Interactions in microtidal environments
Longshore and cross-shore foredune dynamics
Beach-dunes seasonal elevation changes, bathymetrical surveys, shoreline evolution investigations, sand transport measurements and estimates of vegetation density role on the sedimentary flux on the beach and in the foredunes were carried out on Sfantu Gheorghe beach, Danube delta coast, from 1997 to present, aiming to determine the characteristics of coastal dunes development on a microtidal, dry temperate climate. Medium-term trends (years) and seasonal cycles of sediment transport are estimated from analyses of the wind and topographical time series.
For this 8-year interval, we identified a uniform rate of sand accumulation in foredunes sector (4-5 m3/m/year) both for the stable coasts and for progradating shoreline sectors, while the sub aerial beach volume experiences dramatically biennial changes. The multiannual beach width-volume correlation is logarithmic in the case of stable shoreline sectors and a linear regression for the coasts with progradating behavior.
- Subaerial beach width – volume corellation for stable coasts with a strong seasonally-controled dynamics – R 48 benchmark (A), and for beaches dominated by depositional processes – NBuival benchmark (B).
Unlike the coastal dune evolution in a temperate humid climate, the pattern of sediment deposition in the study area (see figure below) exhibits a vast disposition of the accumulative sectors which frequently cover large parts of the foredune slopes with a regular increased deposition rate on the landward one; the dune toe rarely encounters deposition rates higher than the others. Indepedently of the wind direction, the dune crest encounters the highest foredune transport rates whereas the seaward slope experiments larger transport rates than the landward slope.
Beach-dune morphodynamics at NBuival benchmark: (A) sweep zone. The classification from A to D represents different degree of morphodynamics intensity from the most stable (A zone) to the most dynamic (D zone); (B) Volums of the subaerial beach, foredune and beach-dune relative to the shoreline oscillations; (C) cross-shore profiles showing the general evolutionary trend.
Largely, the pattern of erosion and deposition resulted from topographical surveys, are in good agreement with the sand transport measurements. They both show the presence of a vigourous sand flux which remains fairly high over the entire foredune even if at smaller rates (20-50%) than on the beach surface. In a dune landscape, the aeolian processes are gradually impeded by plant biomass production which here, on the Sf. Gheorghe beach (sensu stricto) or in temperate dry climate (sensu largo), remains very low. The vegetation and secondarily, the general foredune morphology which has a gently slope seaward, are responsible for the more uniform disposition of the sand flux over the foredunes, compared to its counterpart in the temperate humid climate.
(A) DEM of R48 polygon in April 2003; (B) Level changes resulted from the subtraction of the DEM’s surveyed in April 2003 and August 2004.
The computed beach volume, beach width and foredunes volume on R48 profile. Note the interconection between beach volume and beach width and the independent development of the foredune.
The morphologic impact of storms
The most part of the storms are induced by northern strong shore-parallel winds. The situation is different for normal or oblique oriented storms that spend a considerably larger quantity of energy on the beach-dune system for a similar intensity of the storm. Important insights in sediments dynamics and level changes in the beach-dune system were gained as we performed accurate topographical surveys and computed the modifications just before and after storms.
For onshore storms, the wave attack angle is directed inland resulting in important morphodynamic impacts even when the storm magnitude is moderate. Figure below displays the cross-shore volume changes induced by the oblique-onshore January 1998 great storm. The strong impact of the hydrodynamic erosional processes is comparable with the eolian reworking.
- Level changes imposed by 1998 storm on an equilibrium-state beach (OGA 53 benchmark)
Aeolian sand transport on temperate dry coasts
A key factor in aeolian landforms evolution is the blown sand flux. Aeolian sand fluxes, particle pathways, pattern of erosion and deposition are important issues for the understanding and prediction of the aeolian landforms evolution.
In the temperate dry climate, feedback mechanisms between the vegetation cover and the sedimentary flux result in small and medium size foredunes with gently sloping flanks.
This is the case of the aeolian landforms on the Danube delta coast and from Letea, Caraorman and Saraturile beach-ridge plains which are investigated in a project of Sfantu Gheorghe Marine and Fluvial Research Station.
Our research interests in this field are represented by:
- Comparative measurements of aeolian fluxes on the beach and in the dunes;
- Fetch effect on sand flux;
- Linkage between sedimentary rates and morphometrical characteristics of the landforms in the temperate dry climate.
Simultaneous display of the wind speed (a.), shear stress (b.) and sand transport rate (c.)
Sand transport measurements: A) Sand flux vs. beach fetch; average wind speed is 8 m/s and wind direction is normal to the shoreline (east); B) Measured sand transport rates compared with the potential transport; C) Sand fluxes measured simultaneously on the beach and foredune (solid line), compared with potential transport (Bagnold, 1941) expressed as coefficient of achievement (dotted line); Traps 9 and 10 are placed on the beach while T11 – 13 are placed on the dunes D) Measured sand transport rates compared to the potential transport.
Nearshore sandbars morphodynamics on microtidal coasts
Seasonal and multiannual dynamics of nearshore sandbars on Sulina – Sfantu Gheorghe beach (Danube delta coast)
