Bernard O. Bauer

A general framework for modeling sediment supply to coastal dunes including wind angle, beach geometry, and fetch effects

A series of spatially explicit equations are derived that form the foundation of a modeling framework that provides insight into how the interaction of the fetch effect and angle of wind approach leads to tradeoffs that govern the magnitude of aeolian sediment transport across beaches of different geometry. The spatial distribution of sediment transport rate per unit width at any point on the beach is shown to vary predictably as a function of wind angle, critical fetch, and beach geometry; and this has evident implications for the total volume and distribution of sediment transport into the dunes behind the beach as well as the proportion of sediment lost from the beach-dune system at the downwind margin. As the wind field shifts from onshore (shore perpendicular) to oblique (shore parallel) approach angle, total sediment transport rate across a dune line segment will reflect a tradeoff between transport reduction because of the cosine effect and transport enhancement because of potentially longer fetch distances traversed by the wind prior to encountering the dune line. This tradeoff is most evident on long, narrow beaches when beach width is less than the critical fetch. For such beaches and with onshore winds, the total sediment transport rate across the dune line will be less than that predicted for a wide beach because of the constraint imposed by the fetch effect. However, as the angle of wind approach becomes oblique, the available fetch becomes progressively longer, transport limitations imposed by the fetch effect are negated, and transport enhancement across the dune line is to be expected. With very large angles of wind approach, the cosine effect dominates the interaction and transport reductions across the dune line occur until the wind is shore parallel and sediment supply to the dunes ceases altogether. This sequence of adjustments and tradeoffs were only partially understood prior to this study, and yet they form the foundation of coastal dune modeling. The framework proposed in this paper serves to place future studies of process-form interaction in beach-dune systems on a robust theoretical foundation. It also facilitates the testing of various alternative hypotheses regarding the uneven spatial and temporal distribution of dune height and growth rate in coastal environments.

Dynamics of beach-dune systems

Beaches and coastal dunes are dynamic geomorphic systems that respond to process forcing over a broad spectrum of spatial and temporal scales. At all scales, there are potential suites of interaction between system forms and processes, and the mechanisms of interaction are stressed in this article. At micro-scales, this interaction is formalized through the concept of morphody namics, and deterministic and probabilistic approaches are used to model sediment transport and landform development over time scales of hours to months and space scales of metres to kilometres. Meso-scale interactions are conceptualized using a sediment-budget approach. Nine characteristic environments comprise the beach and dune sediment-budget ensemble, representing a classifica tion scheme for reciprocating coastal systems.

Influence of averaging interval on shear velocity estimates for aeolian transport modeling

Abstract Recent investigations of aeolian transport have focused on increasingly short time scales because of growing recognition that wind unsteadiness is a major factor in the dynamics of sediment transport. However, the statistical reliability of shear velocity ( u * ) estimates becomes increasingly uncertain as averaging interval is decreased. This study provides an empirical assessment of the influence of averaging interval on the reliability of u * estimates. The data consist of 15-min wind-speed profiles (1 Hz sampling) collected at four coastal sites. Each profile was subdivided into progressively shorter fixed-length time intervals, and estimates of u * and the 95% confidence interval for u * were determined for each time-block using standard statistical techniques. The logarithmic model accurately represents the measured wind-speed profiles, even with relatively brief averaging intervals. Mean r 2 values remain robust down to block lengths as short as 10–20 s, typically retaining better than 98% of the r 2 value found for the full-length data sets. Fewer than 2% of the individual 10-s blocks had r 2 values less than 0.9. However, mean confidence intervals typically expanded by 70–80% of the full-record value as block length decreased from 900 to 10 s. For highly log-linear profiles, this amounted to an absolute increase from about ±8% to only ±14% of u * , so that the additional information gained through the use of shorter averaging intervals may outweigh the increase in statistical uncertainty. Nevertheless, given that rates of aeolian transport are generally modeled as a function of u * 3 , this increase in uncertainty may be significant for transport modeling. Thus, very short averaging intervals should be used with caution when predicting aeolian sediment flux. It is proposed that transport modeling should incorporate the shear velocity confidence interval as an indicator of the potential error associated with this source of uncertainty.

Modification of a linear bar-trough system by a standing edge wave

Abstract Detailed fluid motions and bathymetric changes in the lacustrine nearshore zone at Wymbolwood Beach, Georgian Bay, Canada were monitored during storms to test theoretical postulates about the importance of low-frequency wave motions to nearshore circulation of water and sediments. Eighteen continuous-resistance wave staffs and two electromagnetic current meters were deployed along one shore-normal and two shore-parallel instrument arrays over a period of two months. During one intense storm, a mode 3, standing edge wave at 0.035 Hz was the predominant low-frequency motion. Prior to the storm, the bathymetry was characterized by linear bar-trough systems, and during the storm, these features migrated onshore and became rhythmic in response to the length scales and relatively fixed locations of nodal and antinodal lines of the standing edge wave. However, a well-defined crescentic bar, as predicted by the model under the condition of zero net sediment flux divergence, did not evolve because the hydrodynamic forcing ceased before an equilibrium condition could be attained. Nevertheless, the location of scour and accretion features demonstrated the tendency towards such a crescentic form. The offshore and alongshore irregularities of the post-storm bathymetry are not readily explained by alternative hydrodynamic interactions.

Coastal geomorphology through the looking glass

Abstract Coastal geomorphology will gain future prominence as environmentally sound coastal zone management strategies, requiring scientific information, begin to supplant engineered shoreline stabilization schemes for amelioration of coastal hazards. We anticipate substantial change and progress over the next two decades, but we do not predict revolutionary advances in theoretical understanding of coastal geomorphic systems. Paradigm shifts will not occur; knowledge will advance incrementally. We offer predictions for specific coastal systems delineated according to scale. For the surf zone, we predict advances in wave shoaling theory, but not for wave breaking. We also predict greater understanding of turbulent processes, and substantive improvements in surf-zone circulation and radiation stress models. Very few of these improvements are expected to be incorporated in geomorphic models of coastal processes. We do not envision improvements in the theory of sediment transport, although some new and exciting empirical observations are probable. At the beach and nearshore scale, we predict the development of theoretically-based, two- and three-dimensional morphodynamical models that account for non-linear, time-dependent feedback processes using empirically calibrated modules. Most of the geomorphic research effort, however, will be concentrated at the scale of littoral cells. This scale is appropriate for coastal zone management because processes at this scale are manageable using traditional geomorphic techniques. At the largest scale, little advance will occur in our understanding of how coastlines evolve. Any empirical knowledge that is gained will accrue indirectly. Finally, we contend that anthropogenic influences, directly and indirectly, will be powerful forces in steering the future of Coastal Geomorphology. “If you should suddenly feel the need for a lesson in humility, try forecasting the future…” (Kleppner, 1991, p. 10).

Design and field test of a continuously weighing, tipping‐bucket assembly for aeolian sand traps

A new tipping-bucket assembly (T-BASS) for aeolian sand traps has been designed and field tested with encouraging results. It facilitates high-frequency monitoring of sediment flux over extensive time periods, and therefore offers improved performance over other continuously weighing mechanisms that are generally limited to either a small total-load capacity or poor resolving ability. The T-BASS is a modified version of the tipping-bucket meteorological rain gauge mounted on a cantilever and pulley system linked to an electronic load cell. As trapped sand accumulates in one of the buckets, the increasing mass exerts a downward force on the cantilever arm, which translates into a slight deflection of the thin-beam element of the load cell. The resulting voltage output is proportional to the load, and the analogue signal may be monitored by a data-acquisition system. Eventually the bucket fills to capacity and tips, the sediment load is emptied into a reservoir container, the other bucket of the bucket pair is positioned beneath the funnel, and the system is automatically reset to zero load for continued measurement. In this way, a high-frequency record of sediment accumulation is obtained. Field testing of five prototypes demonstrated that the T-BASS can be configured to yield: (i) linear calibrations (for conversion of voltage to gram weight) with R2 values exceeding 0·99; (ii) weight resolution of 0·99; (ii) weight resolution of 0·1 g or better depending on load-cell specifications and bucket capacity; and (iii) detailed temporal information (order of 1 s) on sediment flux allowing investigation of its relation to attributes of the wind field. Suggested modifications may produce improved performance in future versions. Copyright

Beach steps: an evolutionary perspective

Abstract Field observation of contrasting beach-step behavior at Canaveral National Seashore on two subsequent days when incident-wave conditions in the inner surf zone were similar prompted this re-examination of our conceptual and quantitative understanding of beach steps. These lower-foreshore features are more complex than previously assumed, evolving through erosional as well as accretional phases, and displaying equifinality in geometric form but not necessarily internal sedimentary structure. Past and recent evidence is reviewed that links beach steps to incident waves at the surging-plunging transition and to the action of a backwash vortex. Tides and low-frequency waves likely play no direct role in beach-step initiation, although their presence can have pronounced influences on modulating nearshore hydrodynamics, and thus, on beach-step maintenance and evolution. A generalized, conceptual model capturing these aspects of beach-step dynamics is presented. Beach-step initiation proceeds via step “carving”, “excavation”, or “building” depending on the erosional-accretional character of the beach-foreshore system. Subsequent evolution of the step form may take one of several alternative morphodynamic pathways including stepface “retreat”, step “drag down”, or step “infilling/elimination” depending on tidal stage/range or wave set-up/setdown. Additional data on equilibrium beach-step forms and associated morphodynamic and hydrodynamic conditions in the field are necessary before quantitative models of beach-step existence and evolution can be formulated with realistic results.

Old Methodological Sneakers: Fashion and Function in a Cross-Training Era

Chorley, R.J. 1978. Bases for Theory in Geomorphology. In Geomorphology: Present Problems and Future Prospects, ed. C. Embleton, D. Brunsden, and D.K.C. Jones. pp. 1–13. Oxford, U.K.: Oxford University Press. Marcus, M.G.; Olson, J.M.; and Abler, R.F. 1992. Humanism and Science in Geography. In Geography’s Inner Worlds: Pervasive Themes in Contemporary American Geography, ed. R.F. Abler, M.G. Marcus, and J.M. Olson. 1992. pp. 326–41. New Brunswick, NJ: Rutgers University Press. Pickles, J., and Watts, M.J. 1992. Paradigms for Inquiry? In Geography’s Inner Worlds: Pervasive Themes in Contemporary American Geography, ed. R.F. Abler, M.G. Marcus, and J.M. Olson, pp. 302–26. New Brunswick, NJ: Rutgers University Press. Rediscovering Geography Committee, National Research Council. 1997. Rediscovering Geography: New Relevance for Science and Society. Washington, DC: National Academy Press. Rhoads, B.L., and Thorn, C.E., eds. 1996. The Scientific Nature of Geomorphology. Chichester, U.K.: John Wiley & Sons. Old Methodological Sneakers 679

Aeolian particle flux profiles and transport unsteadiness

Vertical profiles of aeolian sediment flux are commonly modeled as an exponential decay of particle (mass) transport with height above the surface. Data from field and wind-tunnel studies provide empirical support for this parameterization, although a large degree of variation in the precise shape of the vertical flux profile has been reported. This paper explores the potential influence of wind unsteadiness and time-varying intensity of transport on the geometry (slope, curvature) of aeolian particle flux profiles. Field evidence from a complex foredune environment demonstrates that (i) the time series of wind and sediment particle flux are often extremely variable with periods of intense transport (referred to herein as sediment “flurries”) separated by periods of weak or no transport; (ii) sediment flurries contribute the majority of transport in a minority of the time; (iii) the structure of a flurry includes a “ramp-up” phase lasting a few seconds, a “core” phase lasting a few seconds to many tens of seconds, and a “ramp-down” phase lasting a few seconds during which the system relaxes to a background, low-intensity transport state; and (iv) conditional averaging of flux profiles for flurry and nonflurry periods reveals differences between the geometry of the mean profiles and hence the transport states that produce them. These results caution against the indiscriminate reliance on regression statistics derived from time-averaged sediment flux profiles, especially those with significant flurry and nonflurry periods, when calibrating or assessing the validity of steady state models of aeolian saltation.

Assessing the Impact of an Organic Restoration Structure on Boat Wake Energy

ABSTRACT Erosion of unprotected levee banks decreases their structural integrity and increases the likelihood of failure. Several types of restoration structures for levee protection and stabilization have been used in the Sacramento–San Joaquin River Delta, California, to reduce erosion. The purpose of this paper is to describe the results of a field experiment designed to measure the effectiveness of organic restoration structures (brush bundles) in altering the hydrodynamic regime in the vicinity of levees, with specific focus on changes in boat wake energy. Two simple hypotheses were tested: 1) restoration structures reduce boat wake energy, and 2) energy reduction is dependent on water depth. Field work was conducted August 29–31, 2000 on Georgiana Slough, which is a tidally influenced (spring tidal range of 2 meters) distributary of the Sacramento River. Pressure sensors were deployed offshore and landward of the restoration structures. Data collection occurred with the bundles in place and with them removed. Boat wakes were generated during rising and falling tides to capture the effects of fluctuating water levels. Wakes were characterized by index wave height, period and energy. Comparing sample means of normalized energy with the bundles removed and with the bundles in place revealed a 60% reduction of energy by the bundles. It was also determined that energy reduction was tidally, or depth dependent. The reduction of energy by the structures indicates that they are an effective method to protect against boat-wake induced, levee erosion.