ERDC/CHL CHETN-IV-36
June 2001
Chronic Beach Erosion Adjacent to Inlets
and Remediation by Composite
(T-Head) Groins
by Hans Hanson and Nicholas C. Kraus
PURPOSE: Beaches located directly downdrift of inlets may become isolated from sediment
sources and experience chronic erosion. Because shore-protection actions taken on a beach
isolated from sediment sources may not significantly disturb the downdrift beach, highly
efficient sand-retention structures such as T-head and L-head groins may be considered as a
remediation measure. The Coastal and Hydraulics Engineering Technical Note (CHETN)
described herein gives an overview of the performance and functional design procedures for
T-head and similar composite groins.
INLET PROCESSES AND DOWNDRIFT EROSION: Beaches located directly downdrift of
inlets where there is a dominant direction of net longshore transport can experience persistent
erosion. Recession of the shoreline can occur at a high rate, and breaching becomes a possibility
at inlets on barrier islands. This situation is illustrated in Figure 1 for Shinnecock Inlet, Long
Island, NY, in October 1996. The south shore of Long Island is oriented approximately east-
west, and impoundment at the east jetty (right side of figure) and erosion at the west jetty
indicate a net transport directed strongly to the west. In addition, the pattern of wave breaking
over the ebb-tidal shoal is asymmetric, with strong skewness toward the west, also indicating net
longshore transport to the west. The coast of Long Island is formed of glacial deposits, and the
grain size of the predominantly sand sediment ranges between 0.2 and 0.6 mm.
Figure 2 shows inferred sediment paths at Shinnecock Inlet based upon knowledge of the
morphology, waves, and currents. Sand moving from east to west can follow Path 1a (move
around ebb shoal and bypassing bar to reach the attachment bar) or follow Path 1b (enter the
channel and deposition basin). Material reaching the attachment bar can continue to the west
(Path 2a), or be transported east (Path 2b), depending upon the direction of wave incidence and
tidal current. Material on the west beach can move west or east (Path 3), and if it moves east it
can enter the channel to be deposited there, or move to the flood shoal or to the ebb shoal
(Path 4). Because sand bypassing along Path 1a and growth of the large attachment bar, which
can act as a groin, the beach segment between the west jetty and downdrift attachment bar has
become relatively isolated from adjacent sand sources. Therefore, the west beach experiences
chronic erosion. Bruun (1995) discusses the near field behavior (between downdrift jetty and
attachment bar) and far field behavior (further down drift from attachment bar) of longshore
transport and shoreline evolution.
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