Beach Cycle Research Articles

RECOVERY OF SANDY BEACH AFTER TYPHOON WAVES - CASE STUDY ON CHIGASAKI COAST

Beach topography quickly changes in response to the action of storm waves, resulting in erosion of the foreshore with accretion under a calm wave condition after a storm. These seasonal beach changes may occur on beaches with protective measures or artificial beaches produced by beach nourishment. On these beaches, the shore protection function of a sandy beach is reduced when a trough is formed immediately offshore of the shoreline and the foreshore slope increases, indicating the importance of the study on topographic changes. Moreover, the time required for a beach recovery in response to wave conditions has not been sufficiently studied, along with the 3-D topographic changes associated with beach cycles. In this study, we aim to investigate these issues using the Narrow Multi-Beam survey data, wave data, and seabed materials data, taking the Chigasaki coast as an example. It was found that a seabed shallower than 2 and 3 m depths was eroded by rapid offshore sand transport during a storm event with the deposition of sand in a zone between 3 and 5 m depths, and then the beach recovered within 1-2 years after the storm. It was also confirmed that a bar and trough disappeared in 1-2 months under the conditions of HE = 0.5 m, TE = 8 s, and H/L = 0.005 when the crown depth of the bar was smaller than approximately 2 m. Thus, the topography after the storm waves recovers within several months or 1-2 years depending on wave conditions and the crown depth of the bar.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/W_P_3p_xd8U

Sandy coast erosion under the conditions of a storm surge combined with a spring tide

By comparing the changes of beaches and features in the southeastern coast of Xiamen Island before and after Typhoon No. 9914, the beach cycle and coast erosion processes under the condition of superimposed the spring tide of storm surge were discussed. The study showed that during Typhoon No. 9914, the coastal water level rose to 4.04m above the main sea level (MSL). The storm surge elevation was 1.43m. The stormy waves with 22m/s onshore winds brought overwash onto Huandao Road, up to the main sea level of more than 8.5m, depositing about 8-10cm of beach sands on the road. It caused the beach berm to disappear completely. The sand prism lying in the high tidal zone was scoured. Part of the sand of the prism was washed upward away from the beach, part of which was brought to the low tide zone. The bare shoreface without sand prism was scoured directly by waves and currents. Although the sandy sediment which was brought to the deeper zone by the storm can come back to the shoreface, but the overwash sand could not be returned after the storm, and the prime failed to return to its original condition for a long time, eventually leading to beach erosion and coast retreat.

Storm-driven cyclic beach morphodynamics of a mixed sand and gravel beach along the Mid-Atlantic Coast, USA

The morphodynamics of a mixed sand and gravel beach in Delaware were investigated based on 18 almost monthly beach-profile surveys at 46 locations from 2009 to 2011, 60 sediment cores, and 550 surface sediment samples collected at various alongshore and cross-shore transects. Three different atmospheric disturbances occurred within a 3-month window during the study period: 1) a distal hurricane, 2) an energetic winter storm, and 3) “Nor'Ida”, a long-lasting and extremely energetic event resulting from the collision of a hurricane and winter storm. The storm-induced beach changes and post-storm recovery following each of the three storms are evaluated. A distinctive beach cycle was identified consisting of a built-up berm profile and depleted nearly-planar storm profile. The time-scale of the beach cycle relates to the frequency and intensity of storm impact and duration of inter-storm recovery instead of simple seasonality. The initiation of post-storm recovery occurs during the subsiding phase of the storm, attributable to the reduction in wave height and steepness transitioning to accretionary swells. The sediment characteristics of the storm deposit associated with Nor'Ida demonstrated substantial cross-shore variation ranging from sandy-gravel and gravelly-sand within the storm swash zone (near the pre-storm dune edge) to well-sorted medium to coarse sand seaward of the storm swash zone. Storm deposits along mixed sand and gravel beaches demonstrate a variety of sedimentological characteristics. In addition, the studied beaches lacked a sandbar under all wave conditions. A new beach cycle model is proposed for the non-barred mixed sand and gravel beach. • Sedimentological characteristics of the Nor'Ida storm deposit showed cross-shore variation. • Storm deposits of mixed sand and gravel beaches cannot be based solely on a layer of poorly-sorted coarse sediment. • A sandbar did not form under all wave conditions encountered during this study. • A modified beach cycle model is developed for the mixed sand and gravel Delaware coast. • The model consists of a storm-driven time-scale and planar non-barred post-storm beach.

The quantitative description of beach cycles

A quantitative method is developed to describe sequential changes in beach profile morphology. Dominant cycles in temporal eigenfunctions (computed by empirical eigenfunction analysis and rotary component analysis) are defined and used to construct a sequence of profiles representing the energetic portions of beach cycles. Five sets of beach profile data from Torrey Pines Beach in southern California were analyzed with these techniques. Because the first and second eigenfunctions account for between 65 and 90% of the variability of the data, the beach profiles are accurately described with only these two variables and the mean profile. Using rotary component analysis, beach changes represented by the two dominant eigenfunctions can be separated into many elliptical components, each representing a particular frequency associated with cycles in beach state. Profiles are reconstructed using points from the perimeter (latera recta) of these beach state ellipses. A frequency of 1 cpy is the most energetic frequency in the first and second eigenfunctions, corresponding to the cyclic onshore—offshore sediment movement associated with “bar” and “berm” profiles. Profiles constructed from points on the elliptical path associated with annual cycles graphically depict the sequential development of bar and berm during the beach cycle. Beach cycles corresponding to a frequency of 2 cpy are also significant, primarily due to the energy of the second eigenfunction. No other frequencies are consistently found to be as significant as the annual and semi-annual cycles at this beach. In particular, monthly beach cycles have little energy associated with them. The method described provides a uniform way to objectively discriminate energetic beach cycles, and yields a concise representation for beach modeling and prediction. It should be a valuable tool for uniform, quantitative intercomparison of beaches and beach cycles.

ARTIFICIAL MANIPULATION OF BEACH PROFILES

A field study was conducted at Myrtle Beach, South Carolina (U.S.A.), to determine the response of natural beaches to artificial manipulation by sand scraping. Between March 1981 and May 1982, a total of 100,000 m3 of sand was shifted from the lower beach to the backbeach on three occasions over a R-km length of shoreline. Fiftyfour profile stations were surveyed to the -1.0 meter contour as many as ten times during the study to determine the effect of scraping and fill along a stable-to-slightly erosional beach. The purpose of the scraping and fill was to provide temporary erosion relief, protect existing dunes and structures, and provide a wider recreational beach at high tide. It was found that scraping and fill had little adverse effect on the beach cycle in the northern zone of the project area, which is fronted by a natural dune system. Fill placed at a gentle slope along the seaward margin of the dunes remained in place for up to ten months before eroding to the prefill surface. In contrast, similar quantities of fill along shore protection structures in the southern zone eroded in several weeks to four months. The study found significant differences in the response of armored versus unarmored shorelines with higher erosion rates and slower recovery of the beach at armored stations.

Simulation model analysis of seasonal beach cycles

In this paper, a simulation model is presented which describes the formation and destruction of longshore bars. The distribution of sediment in the nearshore zone is modeled using a sediment continuity equation. Mass fluxes of sediment are computed using semi-empirical equations outside the breaker zone. Inside the breaker line, transport is made proportional to the velocity inside a solitary wave, and the direction of transport is made dependent on the wave type, so that seaward transport in the surf zone is characteristic of plunging breakers. The model is successful in simulating the geometrical properties of longshore bars in two dimensions as well as the sequence of events in bar formation. Comparison with field data suggests that berm/bar transition takes place during plunging breaker conditions, therefore the wave type is probably of greater importance than wave height or steepness in predicting the profile type. Three modes of profile recovery following storms are modeled: bar stabilization, bar destruction, and bar migration. The type of response depends on the sequence of wave heights and periods during the post-storm time span.

MORPHODYNAMIC VARtABILITY OF HIGH-ENERGY BEACHES

Field observations of beach and inshore morphology and of surf and inshore current spectra using an array of pressure transducers and low-inertia bidirectional flow meters interfaced with an in-field mini-computer/logging system have been replicated on several beaches in southeastern Australia under a range of energy conditions. Two broad extremes of beach conditions are distinguished spatially and temporally: (1) reflective systems In which much of the incident wave energy Is reflected from the beach face; and (2) dissipative systems with wide surf zones and high turbulent viscosity. Reflectivity increases as the ratio of wave steepness to beach (or bed) steepness decreases. Compared to steep, unbarred reflective beaches which are common in deeply indented or partially protected compart-, ments, the topography of exposed dissipative systems is more complex and varied: six time-and-environment-dependent morphologic types with different bar patterns and bar-beach relationships are recognized. The greatest total dissipation prevails In regions of most abundant inshore sediments or during and immediately after severe storms. Between the reflective and dissipative extremes there is a hierarchy of observed resonant frequencies with the highest frequency resonance occurring in the most reflective cases. Results indicate that near the beach face, motions associated with resonance at periods greater than incident wave periods exhibit strong net seaward resultants and are probably important sources of beach erosion. Reflective beaches are sensitive to resonant excitations over a wider range of frequencies and under lower energy conditions than are dissipative beaches. Hence, although reflective beaches represent the accretive end point of a "beach cycle" they are also more delicately poised with a higher potential for erosion.

Temporal variations in beach and nearshore dynamics

ABSTRACT Analysis of time-series data from two adjacent barrier beaches on the Magdalen Islands in the Gulf of St. Lawrence shows that the exposed west coast has a distinct seasonal wave climate and that the sheltered east coast is a storm-wave environment. Application of the general concept that all coasts in the Gulf of St. Lawrence are characterized by a single wave climate region (locally-generated storm waves) does not apply at the local level due to variations in aspect. The two barriers have distinctive morphologies that reflect both the different levels of wave energy and temporal variations in energy levels on the two coasts. Wave energy levels are higher on the exposed coast and seasonal variations in energy levels result in a summer-winter beach cycle. On the sheltered east roast ave energy levels are variable throughout the year and beach cycles result from erosion and accretion during storm and post-storm recovery periods. The differences between these exposed and sheltered barrier beaches are similar to those reported for other west- and east-facing beaches in northern hemisphere mid-latitudes which are dominated by westerly winds.

Three-Dimensional Beach Changes

Beach forms, such as crescentic bars and cuspate shoreline, manifest a three-dimensional structure of rhythmic topography characterized by periodic longshore undulations of both the surf zone bed and the subaerial surface. The three-dimensional structure and the tendency for migration of rhythmic topography cause distinctive modes of beach change. Since crescentic outer bars tend to migrate parallel to shore, profile changes involving an outer-bar system generally reflect two unrelated processes, one arising from net transformation of individual profiles and the other from longshore displacement of a whole rhythmic system in response to currents. Crescentic inner bars exhibit a tendency for predominant shoreward migration during the time of swells. Under this condition, a subaerial beach will initially assume a ridge-and-runnel-type structure and eventually a uniform two-dimensional profile when full accretion is reached. Beach changes involving crescentic inner bars can produce a beach cycle in response to a succession of random storms. A model predicts the most likely cycle length, which agrees with the field observation. Crescentic bars forming between headlands appear to be generated by a standing edge wave continuously excited by an incident seiche of beat frequency, whereas rhythmic topographies on an unbounded coast appear to be caused by sand-wave trains which result from instability of the surf zone bed perturbed by longshore currents and waves.

Bimodal Composition and Cyclic Characteristics of Beach Sediment in Continuously Changing Profiles

ABSTRACT Sediment deposited on a continuously changing subaerial beach on the Outer Banks, North Carolina, exhibits variations in size distribution ranging from unimodal coarse through bimodal intermediate to unimodal fine. A shift from one end of this range to tile other coincides with the progress of a beach cycle in which a beach, after being eroded by a storm, restores itself to a fully accretive state over a period of approximately two months. In the early phase of beach recovery, coarse material with unimodal distribution appears at the beach front in the form of a swash bar. The swash bar, because of its porous texture, tends to arrest fine materials subsequently washing up on the beach, adding a second mode to the size distribution curve. Accretion is aided by the loss of backwash into runnels behind the swash bar, as well as into the swash bar itself, which leads to landward migration of the swash bar and the consequent spreading of fine materials on the beach. Thus, the second mode, representing fine fractions, gains in prominence relative to the first mode, representing coarse fractions, and finally becomes the sole mode in the size distribution curve by the time when the swash bar reaches the upper limit of run-up. As this beach is subsequently eroded by a storm, the swash bar tends to disintegrate and, correspondingly, the zone of coarse materials diminishes seaward, disappearing eventually into the sea bed when the beach assumes a maximum state of erosion. Under this condition, beach deposits continue to be unimodal fine sediment until a swash bar emerges at the horeline to initiate the process of beach recovery.

Systematic Beach Changes on the Outer Banks, North Carolina

A beach-profile transition model is derived from analysis of 291 semidiurnal beach profiles measured on the Outer Banks, North Carolina, which allows prediction of successive beach profiles in terms of beach width, sediment storage, and surface configuration. The prediction requires knowledge of the initial profile and the stage in the development of a wave field. The model explains beach cycles which are purely associated with wave activity.

Beach Cycle of the Ninomiya Coast, Kanagawa Prefecture, Kantô District

A series of coastal and near-shore survey was carried out in 1962 by members of the Japanse Hydrographic Office on the beach of Ninomiya, Kanagwa Prefecture. The author, leader of the survey, made an investigation into the seasonal change of the beach and the sea bottom near shore from the results of the survey conducted several times within the one-year period. Situated at the northwest corner of Sagami Bay, the Ninomiya coast has a narrow beach of 60 to 70 meters in width, running east-west on the south side of the Oiso Hills. The sea bottom near the shore of Ninomiya coast is flattened by both wave abrasion and sedimentation down to 20 meters in depth and continues to a steep slope of deeper than 20 meters which several gullies incise. Some beach developments by small waves were observed during summer season and erosion during late summer and late spring by stormy waves was also recognized. While, the sea bottom near shore was eroded during summer season by long. period waves and deposited during late summer by stormy typhoon waves. Major records of the beach cycle observed are as follows: The berm was cut back and extinguished by stormy waves during late spring and the beach fore-shore became more gently sloping. In the calm season of small waves, sands started piling up on the beach to form a new berm, which later showed a growth. At the typhoon season of late summer, again the berm was cut back and the beach fore-shore became gently sloping. Beach cusps occurred in accordance with the formation of berm during summer season, but they were sharpened upward with berm development. Both berms and beach cusps disappeared because of the typhoon. From measurement of the inner structure of beach sediments, it may be said that the beach was rapidly deposited with sands of about 1 meter in thickness after it was eroded by stormy waves. Then, gravels were deposited on the seaward edge of the berm as cusp materials no sooner than the formation of berms and cusps started. Distribution of gravels extended on the beach with the devel opment of berms and cusps during calm season. The sea bottom near shore showed a gentle profile with minor steps in front of large cusps in late spring. During summer season it was eroded and the profile was further smoothed away because of distinguishable erosion of the edge of the steps. Eroded areas were formed in front of the large cusps and elongated in north-south direction. Sea bottom in front of the cusps was filled with finer sediments, and was slightly elevated against other areas. The author thinks that the large cusps were on the route of the near-shore circulation of sediments, since the cusps adjacent to the eroded areas of the sea bottom received much sedimentation during summer season. At the typhoon season of late summer, the sea bottom near shore received considerable sedimentation in contrast to the beach where erosion took place simultaneously. Deposited areas were found on the seaward extention of the large cusps, coincident with the eroded areas formed during calm season. In such areas, the sea bottom again showed a profile with minor steps due primarily to the sheet flow of sediments by rip curreat. At the same time, offshore bars occurred temporarily by stormy waves, but within a month after the storm those offshore bars disappeared.

Beach Cycles Related to Tide and Local Wind Wave Regime

Along portions of the northwestern coast of the Gulf of California the beaches exhibit a fortnightly cycle of erosion and deposition which is related to the combined effects of the tides and the waves generated by the daily "sea breeze." In this area the higher waters of spring tide occur during the early afternoon when the "sea-breeze" regime is strongest. This coincidence of maximum wave intensity during the times of highest water causes the position of the beach berm to follow the elevation of the envelope of the higher high waters in a fortnightly cycle. A record of the beach cycle is preserved within the beach face by the bands of heavy minerals that are concentrated by wave action at the level of each high water.