EVALUATION OF DEPTH OF CLOSURE USING DATA FROM DUCK, NC, USA

Citation
Rj. Nicholls et al., EVALUATION OF DEPTH OF CLOSURE USING DATA FROM DUCK, NC, USA, Marine geology, 148(3-4), 1998, pp. 179-201
Citations number
38
Categorie Soggetti
Oceanografhy,"Geosciences, Interdisciplinary
Journal title
ISSN journal
00253227
Volume
148
Issue
3-4
Year of publication
1998
Pages
179 - 201
Database
ISI
SICI code
0025-3227(1998)148:3-4<179:EODOCU>2.0.ZU;2-I
Abstract
Using 12 years of frequent high precision profiles collected on a wave -dominated sandy oceanic beach to 8-m depth, the characteristics and i nterpretation of depth of closure - or the seaward limit of significan t profile change - is critically examined. This includes evaluation of the predictive capability of d(1) - the seaward boundary of the litto ral zone - Its originally defined by Hallermeier (1981) [Hallermeier, R.J., 1981. A profile zonation for seasonal sand beaches from wave cli mate. Coastal Eng. 4, 253-277.]. Depth of closure during major erosion al events is usually produced by the seaward limit of offshore bar mov ement. Following the original recommendation of Hallermeier (1981), d( 1) based on 12 h exceeded wave height and a reference depth of mean lo w water provides a robust limit to the observations using a 6-cm chang e criterion. Empirically, the observed depth of closure is 69% of d(1) although the scatter is large. This scatter is partly controlled by p re-event profile shape, most particularly bar configuration. Depth of closure under accretional conditions can also be measured, but it is t ime-scale-dependent as accretion is a slow, steady process, d(1) under predicts closure for accretional situations. Time-interval (e.g., annu al) depth of closure represents the integral effect of erosional and a ccretional events. An important observation of the data is that depth of closure increases with time scale. However, the full population of time-interval observations is not resolved at Duck due to the measurem ent limit (8-m depth). As time interval increases from 1 year to 8 yea rs, less cases close. Most non-dosing time-interval cases coincide wit h the periods influenced by the most energetic wave events. However, t ime-interval closures are generally deeper than the biggest event clos ure in the period, showing that it represents more than the largest ev ent. The frequency distribution of the data suggests that most, if not all, of these missing data simply represent closures deeper than 8-m depth. Up to a 4-year time interval, d(1,t) appears to provide a reaso nable limit to the quantified observations. It is an interesting resul t that d(1) (an event-based approach) might provide a limit to closure over periods up to 4 years as these time-interval closures are genera lly larger than those produced by single storms. Therefore, the genera l applicability of this result for more complete data sets, or on rapi dly evolving (i.e., eroding or accreting) coasts is uncertain. The pre sent data also suggest that on swell-dominated coastlines where accret ional processes are dominant, d(1) may underpredict closure, suggestin g an important limitation to this approach. But as sediment would be m oving onshore, this may not be a practical problem. Therefore, within the limits of the data set, Hallermeier's (1981) approach is found to define robust estimates of depth of closure, particularly for individu al erosional events. This useful result is expected to find widespread application in coastal geology and engineering. (C) 1998 Elsevier Sci ence B.V. All rights reserved.