the time interval is the time between successive surveys. Such a volume
change may be an important quantity in the sédiment budget of the littoral zone.
A fourth source of error cornes from assuming that the measured beach
profiles (which are only an instantaneous picture), represent a long-term
condition. Actually, beach and nearshore profiles change rapidly in response to changing wave conditions, so that différences between successive surveys of a profile may merely reflect temporary différences in
bottom élévation caused by storms and seasonal changes in wave climate.
Such fluctuations obliterate long-term trends during the relatively short
time available to most engineering studies. This fact is illustrated for
nearshore profiles by the work of Taney (1961a, Appendix B) who identified
and tabulated 128 profile lines on the south shore of Long Island that had
been surveyed more than once from 192 7 to 1956. Of these, 47 are on
straight shorelines away from apparent influence by inlets, and extend
from Mean Low Water (MLW) to about -30 feet MLW. Most of these 47 profiles were surveyed three or more times, so that 86 separate volume
changes are available. These data lead to the following conclusions:
(a) The net volume change appears to be independent of the time
between surveys, even though the interval ranged from 2 months to 16
years. (See Figure 4-19.)
(b) Gross volume changes (the absolute sums of the 86 volume changes)
are far greater than net volume changes (the algebraic sums of the 86 volume changes). The gross volume change for ail 86 measured changes is 8,113
cubic yards per foot; the net change is -559 cubic yards per foot (loss in
volume).
(c) The mean net change between surveys, averaged over ail pairs of
surveys, is -559/86 or -6.5 cubic yards per foot of beach. The médian
time between surveys is 7 years, giving a nominal rate of volume change
ot about -1 cubic yard per year per foot.
These results point out that temporary changes in successive surveys
ri
S °rT Pr°fües are usually much larger than net changes, even when
n
between surveys is several years. These data show that care
enPineeZfnin.raSUring^ea?h°re Profiles
results are to be used in
Dretinv diffp-ro 16S’
djta also sug8est the need for caution in interg
rences obtained m two surveys of the same profiles.
recov^dPdurinonth°f1b!aCh?r0fileS.1111151 be marked so that they can be
be tied in bv en
° r1 T
tbe Pr°ject* The profile monuments should
term use for datait rk loca^.Perman®nt references. If there is a longerenced by survpv t
e Pro^^le Positions, the monuments should be refso that the exact nosîtStat%C°?rdinate system or other reference System,
Even if there is no
° j 10 profile maX be recovered in the future.
Coastal région are
îong“term need, future studies in any
veyed, retrievable bench^ârk^. W1U benefit greatly from accurately sur4-58
change may be an important quantity in the sédiment budget of the littoral zone.
A fourth source of error cornes from assuming that the measured beach
profiles (which are only an instantaneous picture), represent a long-term
condition. Actually, beach and nearshore profiles change rapidly in response to changing wave conditions, so that différences between successive surveys of a profile may merely reflect temporary différences in
bottom élévation caused by storms and seasonal changes in wave climate.
Such fluctuations obliterate long-term trends during the relatively short
time available to most engineering studies. This fact is illustrated for
nearshore profiles by the work of Taney (1961a, Appendix B) who identified
and tabulated 128 profile lines on the south shore of Long Island that had
been surveyed more than once from 192 7 to 1956. Of these, 47 are on
straight shorelines away from apparent influence by inlets, and extend
from Mean Low Water (MLW) to about -30 feet MLW. Most of these 47 profiles were surveyed three or more times, so that 86 separate volume
changes are available. These data lead to the following conclusions:
(a) The net volume change appears to be independent of the time
between surveys, even though the interval ranged from 2 months to 16
years. (See Figure 4-19.)
(b) Gross volume changes (the absolute sums of the 86 volume changes)
are far greater than net volume changes (the algebraic sums of the 86 volume changes). The gross volume change for ail 86 measured changes is 8,113
cubic yards per foot; the net change is -559 cubic yards per foot (loss in
volume).
(c) The mean net change between surveys, averaged over ail pairs of
surveys, is -559/86 or -6.5 cubic yards per foot of beach. The médian
time between surveys is 7 years, giving a nominal rate of volume change
ot about -1 cubic yard per year per foot.
These results point out that temporary changes in successive surveys
ri
S °rT Pr°fües are usually much larger than net changes, even when
n
between surveys is several years. These data show that care
enPineeZfnin.raSUring^ea?h°re Profiles
results are to be used in
Dretinv diffp-ro 16S’
djta also sug8est the need for caution in interg
rences obtained m two surveys of the same profiles.
recov^dPdurinonth°f1b!aCh?r0fileS.1111151 be marked so that they can be
be tied in bv en
° r1 T
tbe Pr°ject* The profile monuments should
term use for datait rk loca^.Perman®nt references. If there is a longerenced by survpv t
e Pro^^le Positions, the monuments should be refso that the exact nosîtStat%C°?rdinate system or other reference System,
Even if there is no
° j 10 profile maX be recovered in the future.
Coastal région are
îong“term need, future studies in any
veyed, retrievable bench^ârk^. W1U benefit greatly from accurately sur4-58