Section 4.531 describes four methods for estimating longshore transport rate, and Sections 4.532 through 4.535 describe in detail how to use
two of these four methods. (See Methods 3 and 4.)
One approach to estimating longshore transport rate is to adopt a
proven estimate from a nearby locality, after making allowances for local
conditions. (See Method 1.) It requires considérable engineering judgement to détermine whether the rate given for the nearby locality is a
reliable estimate, and, if reliable, how the rate needs to be adjusted
to meet the changed conditions at the new locality.
Method 2 is an analysis of historical data. Such data may be foupd
in charts, maps, aerial photography, dredging records, beach fill records,
and related information. Section 4.811 describes some of these sources.
To apply Method 2, it is necessary to know or assume the transport
rate across one end of the littoral zone being considered. The most successful applications of Method 2 hâve been where the littoral zone is
bounded on one end by a littoral barrier which is assumed to completely
block ail longshore transport. The existence of such a complété littoral
barrier implies that the longshore transport rate is zéro across the
barrier, and this satisfies the requirement that the rate be known across
the end of the littoral zone being considered. Examples of complété littoral barriers include large jetties immediately after construction, or
spits building into deep quiet water.
Data on shoreline changes permit estimâtes of rates of érosion and
accretion that may give limits to the longshore transport rate. Figure
4-50 is a shoreline change map which was used to obtain the rate of
transport at Sandy Hook, New Jersey. (Caldwell, 1966.)
Method 3 (the energy flux method) is described in Section 4.532
with a worked example in Section 4.533. Method 4 (the empirical prédiction of gross longshore transport rate) is described in Section 4.534
with a worked example in Section 4.535. The essential factor in Methods
3 and 4, and often in Method 1, is the availability of wave data. Wave
data applicable to studies of littoral processes are discussed in detail
in Section 4.3.
4.832 Onshore-Offshore Motion. Typical problems requiring knowledge of
°nshore-off shore sédiment transport are described in Section 4.511. Four
classes of problems are treated:
(1) The seaward limit of significant sédiment transport. ^vail
able field data and theory suggest that waves are able to move sand durlng some days of the year over most of the Continental Shelf.
owever,
field evidence from bathymetry and sédiment size distribution suggest
tbat the zone of significant sédiment transport is confmed close to
i°re where bathymétrie contours approximately parailel t e s ore
.
1116 depth to the deepest shore-parallel contour tends to merease with
ayerage wave height, and typically varies from 15 to 60 feet.
4-153
two of these four methods. (See Methods 3 and 4.)
One approach to estimating longshore transport rate is to adopt a
proven estimate from a nearby locality, after making allowances for local
conditions. (See Method 1.) It requires considérable engineering judgement to détermine whether the rate given for the nearby locality is a
reliable estimate, and, if reliable, how the rate needs to be adjusted
to meet the changed conditions at the new locality.
Method 2 is an analysis of historical data. Such data may be foupd
in charts, maps, aerial photography, dredging records, beach fill records,
and related information. Section 4.811 describes some of these sources.
To apply Method 2, it is necessary to know or assume the transport
rate across one end of the littoral zone being considered. The most successful applications of Method 2 hâve been where the littoral zone is
bounded on one end by a littoral barrier which is assumed to completely
block ail longshore transport. The existence of such a complété littoral
barrier implies that the longshore transport rate is zéro across the
barrier, and this satisfies the requirement that the rate be known across
the end of the littoral zone being considered. Examples of complété littoral barriers include large jetties immediately after construction, or
spits building into deep quiet water.
Data on shoreline changes permit estimâtes of rates of érosion and
accretion that may give limits to the longshore transport rate. Figure
4-50 is a shoreline change map which was used to obtain the rate of
transport at Sandy Hook, New Jersey. (Caldwell, 1966.)
Method 3 (the energy flux method) is described in Section 4.532
with a worked example in Section 4.533. Method 4 (the empirical prédiction of gross longshore transport rate) is described in Section 4.534
with a worked example in Section 4.535. The essential factor in Methods
3 and 4, and often in Method 1, is the availability of wave data. Wave
data applicable to studies of littoral processes are discussed in detail
in Section 4.3.
4.832 Onshore-Offshore Motion. Typical problems requiring knowledge of
°nshore-off shore sédiment transport are described in Section 4.511. Four
classes of problems are treated:
(1) The seaward limit of significant sédiment transport. ^vail
able field data and theory suggest that waves are able to move sand durlng some days of the year over most of the Continental Shelf.
owever,
field evidence from bathymetry and sédiment size distribution suggest
tbat the zone of significant sédiment transport is confmed close to
i°re where bathymétrie contours approximately parailel t e s ore
.
1116 depth to the deepest shore-parallel contour tends to merease with
ayerage wave height, and typically varies from 15 to 60 feet.
4-153