113,230 research outputs found
Overtopping a truncated planar beach
No full textRun-up on a truncated impermeable beach is analysed theoretically and experimentally to find the volume of fluid, associated with a single wave event, that flows over the end of the beach. The theoretical calculations investigate the motion using the shallow-water equations and the fluid is allowed to flow freely over the end of the beach. Two models of wave events are considered: dam-break initial conditions, in which fluid collapses from rest to run-up and overtop the beach, and a waveform that models swash associated with the collapse of a long solitary bore. The calculations are made using quasi-analytical techniques, following the hodograph transformation of the governing equations. They yield predictions for the volume of fluid per unit width that overtops the beach, primarily as a function of the dimensionless length of the beach. These predictions are often far in excess of previous theoretical calculations. New experimental results are also reported in which the overtopping volumes due to flows initiated from dam-break conditions are studied for a range of reservoir lengths and heights and for a range of lengths and inclinations of the beach. Without the need for any empirically fitted parameters, good agreement is found between the experimental measurements and the theoretical predictions in regimes for which the effects of drag are negligible
Beach development, sediment budget and coastal erosion at Holderness.
No full textComplex relationships exist among offshore conditions, beach
sediment transport and morphology, and till cliff erosion. Modelled
and measured sediment transport rates established for the Holderness
coast are similar to those on comparable coasts elsewhere. The
direction of sediment drift depends on wave approach, and determining
sediment transport rates, cliff composition and cliff retreat
rates allows a sediment budget to be prepared. The beach response
predicted by the sediment budget was confirmed by field observations,
with budget surpluses and deficits coinciding with full and depleted
beach profiles respectively. The area of deficit in the north of the
study area was associated with the reduced sheltering effect of
Flamborough Head on sediment drift.
At most profiles, especially those with a sediment deficit, high
energy waves may remove the sand veneer completely, leaving the
till platform exposed. These bare till patches which elsewhere have
been called ords and have been regarded as unique, were thought, in
the present study,to represent a normal beach response to limited
sediment supply and prevailing offshore conditions.
Beach evolution was also modelled formally, the range of beach
profiles exhibited on the Holderness coast being grouped into a
number of distinct types, and evolution among them described and
predicted by a first-order Markov model. This can be refined to
provide different models for "winter" and "summer". Different modal
types occur at different locations, and certain types of transitions
between classes can be associated with particular ranges of wave
conditions.
Beach Development, Sediment Budget and Coastal Erosion at Holderness
Susan J. Mason.
Till cliff retreat at Holderness is extremely variable, both
spatially and temporally, being influenced by beach level, energy
conditions, cliff moisture content and the actions of man.
The sediment transport rates, cliff retreat data, sediment
budget and beach behaviour model are all essential elements of a
research programme currently being undertaken to find a cheap
method of protecting this coast
Beach and cliff retreat induced by storm groups at Forte Novo,Algarve (Portugal)
No full textLittoral cell dynamics may change through time as a reaction to modifications of the coastal system. Along the Vilamoura-Quarteira coastline, the construction of a groin field limited the sediment supply to the already narrow Forte Novo beach, located downdrift, enhanced wave action on the cliff base and lead to long-term persistent cliff retreat. This research uses a set of surveys from November 2009 to March 2010 to determine short-term soft cliff erosion associated to storm groups at this sediment starved area. Following the initial LiDAR survey, three subsequent surveys were performed using a reflectorless total station for monitoring the cliff face, and RTK-DGPS to monitor the position of the cliff top, the cliff foot and the topography of the adjacent beach. Results indicate an important reduction in beach levels during successive storms (without significant beach recovery in between), allowing waves to further attack the cliff base and contributing to further enhance the structural and permanent cliff retreat. This work demonstrates how a combination of magnitude and frequency of extreme events coupled with development of the coastline, has led to increased cliff-beach recession and to the permanent dislocation of the cliff face
Modelling of wave climate and sediment transport patterns at a tideless embayed beach, Pirita Beach, Estonia
No full textNearshore sand transport patterns along the tideless, embayed Pirita beach, Tallinn, Estonia, have been investigated utilizing high-resolution modelling of wave processes combined with bathymetric surveys and sediment textural analyses of the nearshore sea floor. Textural analysis showed the mean grain size is about 0.12 mm. Fine sand (0.063–0.125 mm) accounts for about 77% of the sediments. Coarser-grained sand (0.28 mm) dominates along the waterline. Based upon the spatial distribution of the mean grain size and basic features of the local wave activity, properties of the Dean Equilibrium Beach Profile were determined.
Alongshore sediment transport was calculated based upon a long-term time series of wave properties along the beach, and the CERC formula applied to about 500 m long beach sectors. The time series of wave fields and the properties of the local wave climate were modelled using a triple nested WAM wave model with an extended spectral range for short waves. The model is forced by open sea wind data from Kalbådagrund for the years 1981–2002. Results indicate that typical closure depth at Pirita is 2.5 m. The width and mean slope of the equilibrium profile are 250 m and 1:100, respectively. Southward transport dominates in the northern sections of the beach whereas no prevailing transport direction exists in the southern sections. This pattern has several nontrivial implications for the planning of beach protection activities
Impacts of Hurricanes Frances and Jeanne on Two Nourished Beaches along the Southeast Florida Coast
No full textSite inspections and beacli profile surveys of nourislied beaclies in the city of Boca Raton, and Town of Palm Beach, Florida show that the nourished beaches protected the shore from hurricane impacts in 2004. Striking the southeast coast of Florida within 20 days of each other. Hurricane Frances (Sept. 5, 2004) and Hurricane Jeanne (Sept. 25, 2004) had hurricane-force winds extending more than 120 miles from the center. The eye of Frances made landfall as a Category 2 storm and Jeanne made landfall as a Category 3 storm on the Saffir-Simpson Hurricane Intensity Scale, Above-average waves and surge affected the entire Florida east coast. Although these beaches were on the return or weak side (southwest quadrant with winds from the southwest as the eye traversed the shore) of both hurricanes, hurricane-uiduced waves affected the coast at least three days prior to landfall. Field inspection of the study sites after the passage of both hunicanes showed significant beach erosion and loss of berm elevation. Damage to infrastructure landward of the nourished beaches was minimal while non-nourished beaches located a few miles to the north and south of the renourished beaches sustained some damage. Beach profile surveys indicated that, as a general trend, beach and inner surfzone erosion was accompanied by the formation of well-developed storm bars seaward of pre-storm bars. Beach morphological responses at the town of Palm Beach were a function of offshore geomorphology of the reef system and the presence of high relief rock outcrops located within the surf zone. Sand that eroded from the renourished beach was deposited seaward of rock outcrops in the surf zone but the rock outcrops had no measurable sediment build up. Causes of the magmtude and trends of beach performance are hypothesized in an effort to explain the observed beach behavior.Hydraulic EngineeringCivil Engineering and Geoscience
Observations of nonlinear run-up patterns on plane and rhythmic beach morphology
No full textApplication of non-linear forecasting and bispectral analysis to video observations of run-up over cuspate topography shows that these alongshore patterns in the morphology are accompanied by changes to the fundamental behaviour of the run-up timeseries. Nonlinear forecasting indicates that at beach cusp horns, the behaviour of swash flow is more predictable and global (meaning that characteristics of individual swash events are well represented by the behaviour of the timeseries as a whole). Conversely, at beach cusp bays, the behaviour of swash flow is less predictable and more local (meaning that the characteristics of individual swash events are best represented by the behaviour of a small fraction of the timeseries). Bispectral analysis indicates that there is a nonlinear transfer of energy from the incident wave frequency f to infragravity frequency ~f/2 which only occurs in the bay, suggesting that the local behaviour is caused by interactions between successive swash cycles which are magnified by channelling caused by the beach cusp geometry. The local behaviour and the bispectral signatures are not present in offshore measurements, and are not present in runup timeseries collected when the beach was planar. These results provide evidence that interactions between successive run-ups are a fundamental characteristic of beach cusp bays. Ultimately, these interactions could lead to the growth of an infragravity wave with an alongshore wavelength forced by the presence of beach cusps
"Valuing Beach Closures on the Padre Island National Seashore"
No full textIn this paper we estimate the economic loss of hypothetical beach closures on the Padre Island National Seashore on the Gulf Coast of Texas. We use a travel cost random utility maximization (RUM) model with data from a random phone survey of Texas residents completed in 2001. We simulate realistic closures that may occur in event of an oil spill or other disruption. For comparison we valued the loss of beach closures in the heavily populated Galveston area. The aggregate losses on Padre Island were highest on weekend days in July estimated at ). They were lowest on weekdays in September at 28. A similar closure of beaches near Galveston resulted in losses of 852,000 (weekend day) with a per trip loss of $30.random utility model, beach use, non-market valuation
An analytical model for bore-driven run-up
No full textWe use a hodograph transformation and a boundary integral method to derive a new analytical solution to the shallow-water equations describing bore-generated run-up on a plane beach. This analytical solution differs from the classical Shen-Meyer runup solution in giving significantly deeper and less asymmetric swash flows, and also by predicting the inception of a secondary bore in both the backwash and the uprush in long surf. We suggest that this solution provides a significantly improved model for flows including swash events and the run-up following breaking tsunamis
Valuing a Beach Day with a Repeated Nested Logit Model of Participation, Site Choice, and Stochastic Time Value
No full textBeach recreation values are often needed by policy-makers and resource managers to efficiently manage coastal resources, especially in popular coastal areas like Southern California. This article presents welfare values derived from random utility maximization-based recreation demand models that explain an individual’s decisions about whether or not to visit a beach and which beach to visit. The models utilize labor market decisions to reveal each individual’s opportunity cost of recreation time. The value of having access to the beach in San Diego County is estimated to be between 23 per day.Recreation demand, repeated nested logit, labor supply, opportunity cost of leisure, time, beach recreation., Demand and Price Analysis, Environmental Economics and Policy, Institutional and Behavioral Economics, Q26, J22, Q51.,
The use of video imagery to analyse groundwater and shoreline dynamics on a dissipative beach
No full textGroundwater seepage is known to influence beach erosion and accretion processes. However, field measurements of the variation of the groundwater seepage line (GWSL) and the vertical elevation difference between the GWSL and the shoreline are limited. We developed a methodology to extract the temporal variability of the shoreline and the wet-dry boundary using video imagery, with the overarching aim to examine elevation differences between the wet-dry boundary and the shoreline position in relation to rainfall and wave characteristics, during a tidal cycle. The wet-dry boundary was detected from 10-minute time-averaged images collected at Ngaranui Beach, Raglan, New Zealand. An algorithm discriminated between the dry and wet cells using a threshold related to the maximum of the red, green and blue intensities in Hue-Saturation-Value. Field measurements showed this corresponded to the location where the watertable was within 2 cm of the beachface surface. Timestacks, time series of pixels extracted from cross-shore transects in the video imagery, were used to determine the location of the shoreline by manually digitizing the maximum run-up and minimum run-down location for each swash cycle, and averaging the result. In our test data set of 14 days covering a range of wave and rainfall conditions, we found 6 days when the elevation difference between the wet-dry boundary and the shoreline remained approximately constant during the tidal cycle. For these days, the wet-dry boundary corresponded to the upper limit of the swash zone. On the other 8 days, the wet-dry boundary and the shoreline decoupled with falling tide, leading to elevation differences of up to 2.5 m at low tide. Elevation differences between the GWSL and the shoreline at low-tide were particularly large when the cumulative rainfall in the preceding month was greater than 200 mm. This research shows that the wet-dry boundary (such as often used in video shoreline-finding algorithms) is related to groundwater seepage on low-sloped, medium to fine sand beaches such as Ngaranui Beach (mean grain size~0.27 mm, beach slope ~1:70) and may not be a good indicator of the position of the shoreline
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