Dan Bowman

Late-Pleistocene seismites from Lake Issyk-Kul, the Tien Shan range, Kyrghyzstan

Abstract The aim of the study is to record the occurrence of sediment deformation structures in one of the tectonically most active areas on the globe, the Tien Shan range in Central Asia and to examine the significance of the deformations as indicators of palaeoseismicity. Soft-sediment deformation structures in form of balls and pseudo-nodules are exposed in the Issyk-Kul basin, within interfingering beds of shallow lacustrine, beach and fluviatile origin. Additional deformation structures that were encountered are: a complex and chaotic folded structure, giant balls and a “pillar” structure which has not been previously reported, where marl intrudes down into coarse pebbley sand and forms pillar morphology. Liquefaction features and bedforms related to storm and breaking waves were not encountered. Neither was there evidence of turbidites. Seven field criteria for relating soft-sediment deformation to palaeoseismic triggering provide strong evidence for a seismic origin of the deformation structures. Empirical relationships between magnitude and the maximum distance from an epicenter to liquefaction sites make the active epicentral zone north of Lake Issyk-Kul, with its frequent high magnitude events, the most favorable source for the deformation structures. Luminescence dating of the sediments gives a time window of 26±2.1 to 10.5±0.7 ka BP, indicating latest Pleistocene seismic activity.

Stepped-bed morphology in arid gravelly channels

The stepping phenomenon indicates systematic channel-bed variations that are not identical to the well-known pools and riffles. Channel beds in the Dead Sea area demonstrate that the main elements in a stepped bed are the regular and the rapid segments, which constitute distinct populations. The bed material of the stepped channel is heterogeneous in size, but significant uniformity prevails within the segment types. The cyclic spacing of the segment types deviates clearly from that of pools and riffles in that it is at closer intervals. The coarser the sediment, the more pronounced the segmentation. Stepping may produce variations in flow velocities from subcritical to supercritical and causes the overall flow regime to vary. Widening and braiding of the channels downstream does not replace the steps. Megasteps form in the canyons but are not cyclic. Conditions favoring stepping indicate a coarse fluviatile environment.

Sequential stage development of crescentic bars; HaHoterim Beach, southeastern Mediterranean

ABSTRACT Sequential bar development involving, in order, multiple bars, ridge and runnels, transverse/ oblique bars, and meandering or crescentic shore-attached or detached bars of both outer and nested inner systems were delineated at HaHoterim beach, northern Israel. The concomitant wave data, including wave directions, energy spectrum, significant wave height, and length of the calm period showed strong correlation with the bar stages. There is a notable lack of correlation between bar stages and incident wave periods, which were quite variable, even on a daily basis. Two bar developmental sequences were delineated: one shore-normal and the other initially oblique but gradually rotating to shore-normal in the mature stage. Threshold conditions for the initial stage development of shore-normal crescentic systems were defined as H1/3 < 1.0 m, peak spectral density of < 1 m2/Hz, and swell directions within ± 7° from the normal. The outer crescentic systems formed within 1 to 2 weeks. The nested inner crescentic bar system formed under different wave conditions. Development of inner micro-cresentic bars required 3 to 7 weeks of calm waves (H1/3 < 0.3 m), except for a short initiating pulse of wave energy from waves of about i m in height. The double crescentic pattern indicates the optimal evolutiona y stage of crescentic systems. The residence time of bar types was determined by the frequency of crossing of these threshold conditions. The outer bars and the mega-rip channels composed the most stable inshore elements. Out-of-phase relationships between inner and outer bar systems resulted from the lag in response of the outer bars behind changes in wave direction. Among the inner crescentic bars and shore rhythms, phase-correlation was the was the rule. Regional aerial photography and wave data indicate that the bar/wave relationships at HaHoterim axe representative of the eastern flank of the Nile, including most of the Israeli coast. Although the bar types described from other coasts are similar to those mapped at HaHoterim and the threshold wave conditions for initiation of bar stage development appear to be similar, the frequency of occurrence of specific bar types appears to be different at HaHoterim.

Geomorphology of the shore terraces of the late pleistocene Lisan lake (Israel)

Abstract The retreat of the Lisan Lake, the predecessor of the Dead Sea, left behind a sequence of shore terraces. Three cross-profiles of these terraces were taken with the aid of a level. The terraces were found to be horizontal, undisturbed and merging from profile to profile. Accordingly, the three cross-sections were integrated into one generalized profile consisting of 28 terraces. The good degree of preservation of the terraces is related mainly to a gravel capping. For the purpose of analysis, the terraces were divided into two sections, a riser triangle and that of the tread. According to the gradient of the treads and the distribution of the superficial gravel and boulders, the treads formed the foreshore whilst the terraces were active shorelines. By analysing the geometrical relationships between the terrace units, a method was found for distinguishing between actual and pseudo lake terraces. The height of the tread is suggested as a means for evaluating the wave height at the different stages of the Lisan Lake.

Bar morphology of dissipative beaches: An empirical model☆

This study of 150 examples of nearshore bar morphology along the highly dissipative beaches (ϵ > 33) of the southeast Mediterranean shoreline employs 32 years of aerial photography and wave data, aided by detailed short-term field studies. Three major bar types were delineated: non-rhythmic parallel/meandering bars, inner single-crescentic bars and double-crescentic bars. Each of these bar families includes variations. n nThe bars were related to daily ship and shore-wave data, including wave spectra, from the day of observation back to 60 days prior to the observation. The increase in total bar occurrence during summer is related to crossing of a major wave-energy threshold in the spring, when significant wave heights ⩽1 m sharply increase to 70–85% in April–May. The bar morphology/wave comparisons further indicate that as the significant wave heights decrease and remain below 1 m, non-rhythmic bars form within 7–10 days, single-crescentic bars require 15 days, and double-crescentic bars require 20–30 days. This adjustment period of the bars to wave power causes a delayed response which accounts for lack of coincidence between wave energy and bar occurrence. The formation of the initial double-crescentic bar, and its transformation to the mature double-crescentic type, requires a short pulse of wave energy (0.5 ⩽ H13 ⩽ 1.5 m). n nSome bar families occur throughout the year. The aseasonal occurrence is best shown by the mature double-crescentic type, which apparently is the final stage in the crescenticbar development sequence. However, other bar families show a tendency for a seasonal distribution which reflects their sensitivity to wave energy. Inner single-crescentic and initial double-crescentic bars are largely restricted to the calmest wave months of May/April to October/November. There is an antiphase relationship between the frequencies of non-rhythmic and crescentic bars.

Flow characteristics along the rip current system under low-energy conditions

Abstract This paper aims to determine the flow signature of the rip current under low-energy conditions based on current meters at Herzliyya Beach, Israel, on the southeastern Mediterranean shore. Field experiments, conducted under relatively calm wave conditions ( T sig = 3−6 s, H sig = 0.3−1.1 m), included current and wave monitoring with morphological mapping. Current data were analysed for shore-normal and resultant components and the time series of instantaneous and smoothed velocities provided current and orbital wave velocities. Statistical and spectral processing yielded energy distributions, specific current discharge, and directional data. Velocity and velocity-time asymmetry increased from the feeders to the neck and subsequently slowed down towards the rip head. The directional range of the flow was narrow in channeled feeders and even better concentrated at narrow necks with higher velocities. The flows at the rip head were typically divergent. Systematic spatial change of energy spectra was observed along the rip system: the subharmonic frequency band dominated the feeders and the inner surf embayments and decreased gradually toward the rip neck with a simultaneous gradual increase in incident wave frequency. The infragravity band frequency was minor everywhere. Velocity spectra at oblique bars with sheltered areas differed from spectral changes at transverse bars with an exposed inshore. The velocity at the rip neck was above the threshold for grain movement whereas at the head, it was usually below the threshold. Textural parameters of the beach sand could not be systematically related to the current velocity, i.e., no environmental groupiness was evident along the rip system.

Flow characteristics at the rip current neck under low energy conditions

Abstract This paper presents the flow signature of the rip current neck as determined by fourteen near-bottom velocity records, with a comparison made to 63 recordings in the surf zone. The field experiments, including wave monitoring and morphological mapping, were conducted under relatively calm, dissipative wave conditions (Tsig = 3–6 s, Hsig = 0.3-1.1 m) at Herzliya beach, Israel, on the Mediterranean shore. The current data were analysed for shore-normal and resultant components. The processing of the time series in the form of instantaneous and smoothed velocities, provided spectral energy, orbital and current velocity, specific current discharge and directional distribution data. The unique characteristic of the neck environment was found to be its continuous unidirectional seaward flow. Due to the channeling effect, this steady offshore drift constituted the fastest flow in the surf zone with a typically narrow (35°–94°) directional range. Through spectral analysis of the unidirectional return flow the morphodynamic process signature at the rip neck was defined by the relative contribution from the incident waves (78%), the subharmonic edge waves (14%) and the infragravity oscillations (8%). The pulsatory, jet-like rip model with concentration of energy at low frequencies is not corroborated by this study. The studied environment of the rip and bar pattern represented an inherited beach morphology, at an initial accretional, although not yet reflective, stage.

Residual colluvio-aeolian aprons in the Negev highlands (Israel) as palaeo-climatic indicator

Abstract Slope features, treated previously mainly pedogenetically and described as alluvial terraces or as a catenary compound of stony loessial Sierozem, are shown to be remains of former colluvial-loessial aprons. The loess was deposited in the central Negev during the uppermost Pleistocene by dust-laden rainstorms which triggered debris flows under conditions twice as humid as today. During the Holocene the aprons were selectively eroded by slope-runoff, resulting in a wide exposure of the basal slope, truncation of the wadi fill and the final formation of the present-day patchy slope morphology. The dust-laden rainy regime fits the last, more humid period, i.e. 70,000–10,000 yr B.P.

The Differential Effects of Prior Knowledge on Learning: A Study of Two Consecutive Courses in Earth Sciences.

We studied the occurrence and nature of learningin a university first year Introduction toGeomorphology course, and its relations with priorknowledge taught in a prerequisite course, and withthe prior knowledge in the to be learned subjects. Tendimensions of knowledge were tapped before and afterthe course by conventional and cognitive structuremeasures that were derived by the concept mappingmethodology. The fine-grain analysis of learningoutcomes yielded the following results: (a) studentsacquired only a small portion of the content in thecourse Introduction to Geomorphology, (b) the priorgeological and geomorphological knowledge did notaffect the learning of the new geomorphologicalcontents, (c) the minor effects appeared within ratherthan across knowledge dimensions, and they affectedmainly the learning of smaller knowledge units, and(d) concept definition cannot be considereda valid probe of knowledge. The differential effects of prior knowledge question thecentral, global and undifferentiated role that schematheories ascribe to prior knowledge in futurelearning. They call for greater reference to theexposed dimensions of knowledge by suggestingadditional factors to be considered in the sequencingof courses, as well as to the acquisition of complexknowledge with partial meaning of the basic knowledgeunits, and the use of new cognitive structure probesof knowledge.

Psychometric and Edumetric Validity of Dimensions of Geomorphological Knowledge Which Are Tapped by Concept Mapping.

The validity of several dimensions of knowledge which were inferred from concept maps was assessed for its psychometric and edumetric aspects. Data were collected from 14 students who enrolled in the university first-year introductory geomorphology course and in its prerequisite introductory geology course. They took an objective geomorphology test, the tree construction task, and the Standardized Concept Structuring Analysis Technique (SConSAT) version of concept mapping. Comparisons among these dimensions of knowledge before and after the geomorphology course yielded convergent evidence. For the psychometric perspective, the SConSAT version of concept mapping and tree construction had similar knowledge structure representations, and the cognitive map correctness was moderately positively correlated with the objective test but not with the geomorphology course test. For the edumetric perspective, the majority of the dimensions of knowledge structures from the SConSAT showed large improvements following the geomorphology course. This evidence shows that the knowledge structure dimensions have moderate to good construct validity which warrant their widespread use for evaluating learning outcomes in both experimental and classroom settings.