terrace (seaward from the foreshore, if the low-tide terrace is absent)
are the longshore troughs and longshore bars.
4.514
Profile Accuracy. Beach and nearshore profiles are the major
source of data for engineering studies of beach changes; sometimes littoral transport can be estimated from these profiles. Usually, beach
and nearshore profiles are measured at about the same time, but different teclmiques are needed for their measurement. The nearshore profile
is usually measured from a boat or amphibious craft, using an écho sounder
or leadline. or from a sea sied. (Kolessar and Reynolds, 1965-66; and
Reimnitz and Ross, 1971.) Beach profiles are usually surveyed by standard
leveling and taping techniques.
The accuracy of profile data is affected by four types of ^ror^
sounding error, spacing error, closure error, an error
f
fluctuations in the sea bottom. These errors are more signi
nearshore profiles than for beach profiles.
Saville and Caldwell (1953) discuss sounding and spacing ^rors.
Sounding error is the différence between the measure
eP
error may
actual depth. Under idéal conditions, average soni
rror may be
béas little as 0.1 foot, and average leadline soundi &
1953j (This
about twice the sonie sounding error. (Savil e an
, élévation
suggests that sonie sounding error may actually e |
rience witfc succhanges caused by transient features like ripp
*
t errors in practice
cessive soundings in the nearshore zone ind usuaiiy random and tend to
W approach 0.5 foot.) Sounding errors are usually * systematic error
average out when used in volume computati
, invoiVed. Long-period
due to the écho sounder or tide correctio
chanoine the water
water level fluctuations affect sounding accuracy by ^anging^the
level during the survey. At Santa Cruz, a i
(Magoon, 1970.)
hydrographie surveys was ±1.5 feet due to
.
artnal volume of a segSpacing error is the différence
singie profile across that
ment of shore and the volume estimate
important than sounding error,
segment. Spacing error is potentia y J
^P^ spacings between nearsince survey costs of long reaches u
pYamole, if a 2-mile segment of
shore profiles of thousands of feet. Fo
F x O00-foot spacings, then
shore 4,000 feet wide is surveyed by P™£
foot
beach front per survey
the spacing error is about 9 cubic yar
(1953, Figure 5). Thls error
according to the data of Saville and Ca w
localities.
equals a major part of the littoral budget in many
_ that the outer ends of
Closure error arises from the assump
élévation between tv>o
nearshore profiles hâve experienced no
often made in practice, and
successive surveys. Such an assumption i
, closure error of 0.1
W resuit in significant error. An uncomp
a profile, implies a
f°ot, spread over 1,000 feet at the seawar
of beach front where
change of 3.7 cubic yards per time interv
4-57
are the longshore troughs and longshore bars.
4.514
Profile Accuracy. Beach and nearshore profiles are the major
source of data for engineering studies of beach changes; sometimes littoral transport can be estimated from these profiles. Usually, beach
and nearshore profiles are measured at about the same time, but different teclmiques are needed for their measurement. The nearshore profile
is usually measured from a boat or amphibious craft, using an écho sounder
or leadline. or from a sea sied. (Kolessar and Reynolds, 1965-66; and
Reimnitz and Ross, 1971.) Beach profiles are usually surveyed by standard
leveling and taping techniques.
The accuracy of profile data is affected by four types of ^ror^
sounding error, spacing error, closure error, an error
f
fluctuations in the sea bottom. These errors are more signi
nearshore profiles than for beach profiles.
Saville and Caldwell (1953) discuss sounding and spacing ^rors.
Sounding error is the différence between the measure
eP
error may
actual depth. Under idéal conditions, average soni
rror may be
béas little as 0.1 foot, and average leadline soundi &
1953j (This
about twice the sonie sounding error. (Savil e an
, élévation
suggests that sonie sounding error may actually e |
rience witfc succhanges caused by transient features like ripp
*
t errors in practice
cessive soundings in the nearshore zone ind usuaiiy random and tend to
W approach 0.5 foot.) Sounding errors are usually * systematic error
average out when used in volume computati
, invoiVed. Long-period
due to the écho sounder or tide correctio
chanoine the water
water level fluctuations affect sounding accuracy by ^anging^the
level during the survey. At Santa Cruz, a i
(Magoon, 1970.)
hydrographie surveys was ±1.5 feet due to
.
artnal volume of a segSpacing error is the différence
singie profile across that
ment of shore and the volume estimate
important than sounding error,
segment. Spacing error is potentia y J
^P^ spacings between nearsince survey costs of long reaches u
pYamole, if a 2-mile segment of
shore profiles of thousands of feet. Fo
F x O00-foot spacings, then
shore 4,000 feet wide is surveyed by P™£
foot
beach front per survey
the spacing error is about 9 cubic yar
(1953, Figure 5). Thls error
according to the data of Saville and Ca w
localities.
equals a major part of the littoral budget in many
_ that the outer ends of
Closure error arises from the assump
élévation between tv>o
nearshore profiles hâve experienced no
often made in practice, and
successive surveys. Such an assumption i
, closure error of 0.1
W resuit in significant error. An uncomp
a profile, implies a
f°ot, spread over 1,000 feet at the seawar
of beach front where
change of 3.7 cubic yards per time interv
4-57