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.