Swash overtopping and sediment overwash on a truncated beach
86
Citation
49
Reference
10
Related Paper
Citation Trend
Keywords:
Swash
Overwash
Berm
Berm
Swash
Progradation
Beach morphodynamics
Plage
Surf zone
Cite
Citations (91)
New field observations of sediment overwash on a natural beach are presented and compared to predictions from a process based hydrodynamic and swash sediment transport model. The observations were made at an intermittently closed and open lake or lagoon (ICOLL) and show rapid vertical growth of an initially low beach berm as a result of swash overtopping and sediment overwash. Total sediment overwash volumes passing the crest of the beach for a series of high tide overtopping events were obtained from survey data over a period of 12 days either side of spring tides. A process based model for monochromatic swash is extended to random wave conditions through robust parameterisations of wave runup, wave runup distributions and sediment transport and is applied to predict the total overwash transport. This approach avoids the requirement for a surf zone transformation model to obtain nearshore bore heights. The sediment transport calculations use independently determined friction factors from both field and laboratory studies. Predicted sediment transport overwash rates are well correlated with the measurements (r2=0.93), and agree with observed tidal and cumulative overwash rates to within about 20%. The model has applications over a wide range of beach and wave conditions and represents a first step in modelling the growth and recovery of natural beach berms.
Overwash
Swash
Berm
Beach morphodynamics
Cite
Citations (0)
This paper presents new experimental data on swash overtopping and sediment overwash on a truncated laboratory scale beach. The experiments were designed to allow direct measurement of total uprush sediment transport rate past a fixed section of beach. The overtopping data are compared to an existing analytical solution and predictions based on a recently developed numerical model, and show good agreement with the latter. The improvement is a result of more realistic predictions of the flow depth in the swash zone by the numerical model. Subsequently, the model is extended to enable predictions of sediment overwash using well accepted sediment transport equations. Model predictions using standard coefficients are in good agreement with the data. The model performance suggests it may be applicable for modelling the accretion of berms on natural beaches, a process which requires overtopping and sediment transport above the existing berm crest.
Berm
Swash
Overwash
Cite
Citations (3)
Many ultrasonic wave gages were placed with a small spacing across the swash zone to monitor either sand level or water level. Continuous monitoring conducted for a few years enabled the collection of data on the change in wave properties as well as swash-zone profiles. Data sets including two cases of large-scale berm erosion were analyzed. The results showed that 1) shoreline erosion started when high waves with significant power in long-period (1 to 2 min.) waves reached the top of a well-developed berm with the help of rising tide; 2) the beach in the swash zone was eroded with higher elevation being more depressed, while the bottom elevation just outside the swash zone remained almost unchanged; and 3) erosion stopped in a few hours after the berm was completely eroded or the swash-zone slope became uniformly mild. These findings strongly suggest that long waves play a dominant role in the swash-zone dynamics associated with these erosional events.
Swash
Berm
Elevation (ballistics)
Cite
Citations (2)
ABSTRACT Weekly topographic profile measurements across a southward migrating recurved‐spit complex throughout a summer period have revealed three different mechanisms of berm development, each reflected by a distinctive sedimentary sequence. Each mechanism dominates berm widening along certain sections of the active spit with transition zones separating each one. Along the straight beach sections where a net longshore transport is well developed, sand accumulates at the distal high‐tide swash mark during neap tide. These sandy accumulations are neap berms which are later redistributed over the main berm by swash occurring at spring high water. The main berm grows vertically and horizontally as a result. To the south, along the middle portion of the recurved spit, swash bars or ridge‐and‐runnel systems actively develop, migrate, and weld onto the established berms. This is the second method of berm widening and results from an excess of sand carried into this portion of the spit due to the steadily decreasing transport of the longshore current system. Berm‐ridges develop along the southernmost portion of the active recurved spit and represent the third and most rapid form of beach progradation. Wide, broad swash bars build nearly up to the spring high tide level. At neap high tide, the swash cannot extend over this feature. Wave energy is expended on the seaward margin of the swash bar initially developing a low‐angle beach face. Rapidly, this beach face steepens and a new berm (beach face and berm top) is developed on top of the swash bar. This berm structure still retains much of its swash bar or ridge appearance, hence the term‘berm‐ridge'. Numerous trenches dug into the beach provide data to model the distribution of primary sedimentary structures in recurved spits. Berm‐ridges are the most important features along rapidly accreting spits, and structures associated with these features are volumetrically the most significant. Berm‐ridges also develop arcuate, vegetated ridges separated by low lying, marsh‐infilled swales. These features are commonly seen within barrier islands and designate former inlets.
Berm
Swash
Plage
Longshore drift
Progradation
Beach morphodynamics
Cite
Citations (169)
Abstract Low‐lying barrier islands are ubiquitous features of the world's coastlines, and the processes responsible for their formation, maintenance, and destruction are related to the evolution of smaller, superimposed features including sand dunes, beach berms, and sandbars. The barrier island and its superimposed features interact with oceanographic forces (e.g., overwash) and exchange sediment with each other and other parts of the barrier island system. These interactions are modulated by changes in storminess. An opportunity to study these interactions resulted from the placement and subsequent evolution of a 2 m high sand berm constructed along the northern Chandeleur Islands, LA. We show that observed berm length evolution is well predicted by a model that was fit to the observations by estimating two parameters describing the rate of berm length change. The model evaluates the probability and duration of berm overwash to predict episodic berm erosion. A constant berm length change rate is also predicted that persists even when there is no overwash. The analysis is extended to a 16 year time series that includes both intraannual and interannual variability of overwash events. This analysis predicts that as many as 10 or as few as 1 day of overwash conditions would be expected each year. And an increase in berm elevation from 2 m to 3.5 m above mean sea level would reduce the expected frequency of overwash events from 4 to just 0.5 event‐days per year. This approach can be applied to understanding barrier island and berm evolution at other locations using past and future storm climatologies.
Berm
Overwash
Elevation (ballistics)
Storm Surge
Cite
Citations (44)
Swash
Overwash
Surf zone
Wave tank
Wave height
Cite
Citations (45)
Swash
Overwash
Berm
Cite
Citations (86)
Shore protection projects require the prediction of coastal storm damage and economic loss but the damage processes are not well understood. An exploratory experiment consisting of 11 tests was conducted in a wave flume with a sand beach to examine the movement of 10 wooden blocks (floatable objects) placed on the foreshore and berm as well as on short and long pilings. The still water level was varied to create accretional and erosional profile changes. The cross-shore wave transformation on the beach and the wave overtopping and overwash of the berm were measured in 101 runs of irregular waves where each run lasted 400 s. The initial block elevation above the sand surface had little effect on the hydrodynamics, sediment transport, and profile evolution in this experiment with widely- spaced blocks. The block floating and sliding on the sand surface and the block falling from the pilings depended on the swash hydrodynamics and block clearance above the foreshore and berm whose profile varied during each test. A simple probabilistic model is developed to estimate the immersion, sliding, and floating probabilities for the blocks in the swash zone. The predicted probabilities are compared with the observed cross-shore variation of the block response on or above the accretional and erosional beach profiles. The accurate prediction of the block response is shown to require the accurate prediction of the beach profile change.
Swash
Berm
Overwash
Beach morphodynamics
Waterline
Flume
Cite
Citations (1)
Understanding of shoreline erosion and accretion has been inhibited by the lack of data on bed-level changes at the fundamental time scale of individual waves. A new ultrasonic bed-level technique was used to measure shoreline erosion and accretion on a sand (Truc Vert, Bordeaux, France) and gravel (Slapton Sands, Devon, UK) beach in March and May 2008, respectively. The ultrasonic sensors were deployed in a linear array across the high-tide shoreline and measured bed-level changes on a swash-by-swash basis with an accuracy of 0.001 m (1 mm). A characteristic accretionary (berm-building) tide was selected from each dataset. Both the sand and gravel berms built up close to the high-tide swash limit, accreting around 0.07 m, with the sand berm extending 10 m cross-shore and the gravel berm being half as wide. The net morphological change integrated across the swash zone reached over 900 kg of accretion per metre width on the sand beach and 150 kg m-1 on the gravel beach. Swash-by-swash bed-level changes at the location of berm deposition were < 0.005 m for the sand beach and < 0.01 m for the gravel beach. At both sites during this period of berm building there were almost as many swash-by-swash erosion events as accretion events suggesting that a form of equilibrium acts to limit net bed-level changes. The swash-by-swash accretion events were slightly larger and slightly more numerous than the erosive events, resulting in overall accretion and berm formation.
Swash
Berm
Plage
Cite
Citations (13)