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At geologic timescales, a significant proportion of the sediment eroded from continents is transferred from rivers to littoral cells, then through submarine canyons to deep ocean basins. Accumulation and occasional discharge of sediment from canyon headwall regions is a likely driver for the sediment gravity flows believed to be responsible for much of this mass transport, but the responsible mechanisms and event timescales in canyon heads are imprecisely defined, largely due to the drowning of canyon heads since the last glaciation and the resulting dearth of modern depositional environments to observe. We investigated a littoral cell adjacent to Mattole submarine canyon in Northern California to better understand the influence of bathymetry and coast shape on sediment trajectories and hypothesize that canyon heads promote littoral sediment accumulation due to bathymetric sheltering of the adjacent coast from waves and convergence of littoral cells exposed to varying wave direction. The uppermost gullies of Mattole Canyon extend to the littoral cell, and a train of sediment waves within the uppermost thalweg of Mattole Canyon suggests that gravity-driven flows of shore-derived sediment may have recently occurred. Mattole Canyon and nearby Mendocino Canyon flow into the deep Mendocino Channel, which has the highest observed Late Holocene sediment accumulation rates in Northern California. We documented the beach slope, grain size, and sediment provenance along 15 km of coast adjacent to Mattole Canyon and conducted numerical wave modeling to infer the dominant direction and magnitude of sediment transport, relative wave sheltering by bathymetry, and estimated sediment mobility in the vicinity of the canyon head over a multi-year period. South of the canyon headwall, in the vicinity of the Mattole River outlet, coarser sediment rich in metasandstone clasts and steeper beach slopes coincide with decadal-scale beach accretion. North of the canyon head, sediments are generally finer, beaches are flatter, clast lithologies include more quartz and mudstone grains, and there is decadal-scale net erosion. Wave modeling suggests the Mattole headwall region is subject to sustained sheltering from waves due to canyon bathymetry, littoral sediment convergence above the canyon head for much of a typical year, and occasional bed entrainment of sediment in the canyon’s uppermost gullies by large waves. Larger winter waves from the northwest likely result in net southerly drift north of the canyon head, with a high likelihood of preferentially transporting fine-grained, more quartz-rich sediment towards the Mattole headwall. Smaller summer swells from the west and south likely results in net northerly transport of metasandstone clast-rich Mattole River-derived sediment towards the canyon head. The imbalance in seasonality in wave conditions, coast shape, and sediment delivery by the Mattole River and other coastal creeks is broadly consistent with the spatial patterns in grain size, mineralogy, slope, and beach width that we observe in the vicinity of the canyon, and the net delivery of sediment to the Mattole Canyon head. An approximate mass balance of sediment flux from coastal streams feeding Mattole River littoral cell suggests that sediment volumes likely accumulate in the canyon headwall region that could supply several density flow events in a typical decade. These findings highlight the importance of the connectivity of the canyon to the nearshore region for the processes and products in canyons and fans. We hypothesize that feedbacks between canyon bathymetry, sediment accumulation, and mass evacuating flows promotes the long-term persistence of canyon systems.

Guisado, E., Malvárez, G. and Navas, F., 2013. Morphodynamic environments of the Costa del Sol, Spain. The morphodynamic environments of the Costa del Sol, in southern Spain, have never been investigated at a littoral cell scale. In the last half of the 20th century the tourism industry have transformed the coastal landscapes and, importantly, the natural processes inducing irreversible damages and changes. For the most part, a project-based view on engineering works has focussed on individual beach behaviour, but no assessment on morphodynamics has been reported at physiographic scale. Using an integrated modelling approach, which combines wave energy propagation and digital terrain integration (land and sea), in this paper a new characterisation of coastal morphodynamics for western Costa del Sol is presented. Methodologically, the challenge of integrating land and sea has been overcome using existing methods, which takes high-resolution interoperable bathymetric models of the continental shelf of this section of the Mediterranean, and couples it with the Official Digital Terrain Model. This allows for propagation of waves from deep water to realistic nearshore environments. Wave input is represented by wave spectra from the recording network of the Spanish Port Authority, and the wave propagation model SWAN is used to propagate wave parameters onshore. Given the variability of wave climate in the region, characterised by long periods of calms (over 77% per year) followed by high energy events (dominated by high frequency storm waves with periods of less than 7 seconds and maximum wave heights exceeding 5 metres), a wave power index is used to identify and provide input data for extreme wave conditions of great significance for beach morphodynamics in the region. Results show that at littoral cell scale, even in the presence of the steep nearshore (i) the adaptation of the system tends towards generating dissipative environments, (ii) the presence of abrupt changes in local bathymetry affects significantly wave orbital velocities even at depths beyond wave base potentially inducing nearshore sediment transport, and (iii) high energy events affect the coastal stretch in complex ways, where highly active short-crested/high-frequency wave conditions induce transitional morphodynamic states. [ABSTRACT FROM AUTHOR]

The climates of wave-related nearshore processes important to coastal erosion have been investigated based on analyses of wave data measured at six sites along the Mediterranean coast of Egypt, spanning 0.92 to 5.25 years. Wave climates along this 900-km coastline are nearly identical in their significant wave heights, periods, and directions. The evaluated nearshore processes that depend on combinations of deep-water wave heights and periods are found to respond to seasonal changes in wave conditions. These processes include the climates of breaker heights on beaches, beach face slopes, the morphodynamic classification of beaches, swash runup elevations induced by storm waves, the depth of closure of beach profiles, and calculated sediment transport rates. The seasonal changes observed in the wave-related nearshore processes are ultimately responsible for the episodic morphologic changes identified in the beach and seabed environments of the Nile Delta. The largest storm waves measured during the study period had deep-water significant wave heights of 7.7 and 6.8 m at the Dabba and Abu Qir coastlines, respectively. [ABSTRACT FROM AUTHOR]

Beach profiles surveyed over a period of 18 y (1982 to 2000) at 41 sites were analyzed to assess beach changes, seabed scour, and grain sorting pattern fronting the 5 km long seawall built in 1991 to protect the Rosetta promontory from beach erosion, which had been retreating at a rate of -106 m/y. Although the seawall has succeeded in halting the recession, the shoreline shows adverse erosion at the leeside of the seawall ends (-14.4 m/y) associated with depth scour at a maximum of -0.50 m/y. The erosion along the promontory tip progressively decreases with longshore distance both to the east and to the southwest along the promontory flanks, reverting to accretion on both sides within the promontory saddles and yielding two nodal areas that represents zones where the sediment regime changes from erosion to deposition. Moreover, the seawalls have slightly altered the mean grain sizes of the beach sediment and seabed slope of the surf zone. The overall pattern of beach erosion and seabed scour results from wave refraction- induced longshore sediment transport along the promontory tip. The geographic correspondence between patterns of shoreline and seabed depth changes serve to refine boundaries of littoral subcells of the Rosetta promontory, including sediment paths, sources, sinks, nodal points, and zones of sediment transport convergence and divergence. The morphologic changes along this promontory reflect a combination of factors, including sediment availability, transport pathways from source areas, as well as the impact of protective structures at the Rosetta estuary. [ABSTRACT FROM AUTHOR]

In SW Spain, numerous tourist beaches have been nourished over recent years to counteract coastal retreat. Nourishment projects have not produced great endurance, mainly because of the lack of basic information on coastal dynamics and longshore sediment transport paths. In order to cover these aspects in a sandy coastal sector south of the Guadalquivir River mouth, a beach monitoring program was carried out between 1996 and 1998, with a secondary program between 2000 and 2002. Four main beach types were identified, each one with different volumetric trends during the monitoring periods. Most beaches recorded accretion because of the prevalence of fair weather conditions, which emphasized differences among beaches. In a broad sense, beaches located on intertidal rock platforms recorded erosion or no change. Those located updrift of rock platforms or human-made constructions (groins) recorded accretion. These structures divide the coast into several littoral cells, which control longshore sediment transport between beaches. All this information is very useful for planning replenishment projects on eroding beaches. The distribution of ceils, the sediment transport paths associated with them, and the volumetric trends obtained for every beach type were used for evaluating nourishment procedures, sand volume needs, and expected life time of the resulting artificial beaches. [ABSTRACT FROM AUTHOR]

Analyses have been undertaken of 68 beach-nearshore profiles spanning the years 1971–1995 to interpret sub-littoral cells of the Nile Delta that developed during cross-shore and alongshore sediment transport. Bottom sediments of these profiles taken along the 240-km length of the delta coastline have been analyzed to determine seabed level changes, textural and heavy mineral concentration distributions. The spatial distribution of seabed depth changes linked with the heavy mineral concentration allows identification of three zones of erosion (source areas) associated with accretion (sink areas). Erosion zones occur off the Nile promontories at Rosetta (−20 to −50 cm/yr), Burullus and Damietta (0 to −20 cm/yr). Accretion ranging from 0 to 40 cm/yr occurs between erosion zones at Abu Quir Bay, the Rosetta saddle, Gamasa embayments and along the western side of the jetty at the Suez Canal entrance, Port Said. This pattern was generally induced from the temporal variability in the direction and intensity of the incoming waves, currents, orientation of coastline, seafloor morphology and local protective structures. The correspondence among patterns of seabed depth change, heavy mineral concentration, and to some extent the texture of seabed sediments refine boundaries of littoral sub-cells of the Nile including sediment paths, sources, sinks, nodal points and zones of sediment transport convergence and divergence. This correspondence is not strongly correlated, as might be expected in this deltaic system where sediment has been recycled between recent and relict seabed settings moving in the alongshore and cross-shore directions. Nevertheless, the present investigation shows that the relationship between seabed level changes, heavy mineral concentration and to some extent mean grain size of the littoral zone can be practically applied in other deltaic settings to interpret sediment transport pathways particularly where there are less relict sediments. [Copyright &y& Elsevier]