GenCade:Structures

Types of Structures

GenCade simulates the effects of common coastal structures on the shoreline position. Generic types of structures that can be represented are groins, jetties, harbor breakwaters (with respect to their functioning as a jetty or groin), detached breakwaters, seawalls, and the “soft structure” or beach fill. Considerable flexibility is allowed in combining these basic structures to produce more complex configurations, e.g., T-shaped groins, Y-shaped and half-Y groins, and jetties with spurs. Combinations of these types of structures are also possible.

In shoreline change modeling, structures exert two direct effects:

• Structures that extend into the surf zone block a portion or all of the sand moving alongshore on their updrift sides and reduce the sand supply on their downdrift sides. These structures, referred to as ‘non- diffracting’ groins or jetties, only affect the sediment transport through direct blocking and have no effect on the waves.
• Groins/jetties with seaward ends extending well beyond the surf zone, referred to as ‘diffracting groins/jetties’, and detached breakwaters, produce wave diffraction. This causes the local wave height and direction to change, altering the longshore sand transport rate. In addition to this, the diffracting groins and jetties also block the sediment transport directly.

In reality, all groins and jetties are ‘diffracting’ to some degree. As diffracting structures require substantial computational effort, the user has been given the option to classify short groins and jetties as being ‘non- diffracting’, to save computational time. It is up to the user to determine for which groins and jetties wave diffraction may be ignored. Preferably, comparative simulations should be run, where short groins and jetties are represented as ‘diffracting’ and ‘non-diffracting’, respectively. Comparisons of simulation results may indicate the groins and jetties for which diffraction may be ignored. Only groins and jetties may be regarded as ‘non-diffracting’. Detached breakwaters are always regarded as diffracting as are groins and jetties that are combined with detached breakwaters or each other to form composite structures such as Y-groins.

Placement of groins, jetties,and detached breakwaters

This section describes capabilities and limitations in placing structures in GenCade. Idealized examples of plan views of various configurations are given for reference. It is noted that structures are represented as infinitesimally thin objects in the model. For example, a groin or jetty is located at the wall of a single cell and cannot occupy the position of more than one wall. In the figures below, structures are drawn thick for visibility only.

Three basic rules governing placement of groins, jetties, and detached breakwaters are:

• The position of a structure is defined by the location of its tip(s), and

these positions are located at cell walls;

• There must be at least two cells between groins and jetties. As an

important special case, a groin cannot be placed in the cell next to a lateral boundary;

• The locations of the tips of diffracting structures can coincide (be

located at the same cell wall), but they cannot overlap.

Figures 1 and 2 give schematic examples of structure placements in GenCade. Structures that affect the waves (detached breakwaters and diffracting groins and jetties) are denoted as ‘diffracting structures’ and indicated by a ‘)))’ symbol in the figures.

Figure 1 gives examples of allowable Non- diffracting groins/jetties may be placed behind a diffracting breakwater (but diffracting groins/jetties cannot) (upper left sketch). The other three sketches in this figure show situations involving the tips of two structures sharing the same grid cell. The tip of a detached breakwater and a diffracting groin/jetty can be at the same longshore grid cell, as can the tips of two detached breakwaters. Two or three diffracting tips may be put together to form an angled structure such as a spur groin/jetty (lower right sketch), Y-groin, angled groin, etc. In such a case, the attaching points must be located in the same cell, at the same distance offshore, and having the same depth (see text below).

Figure 2 illustrates the major restrictions on placement of structures. Groins/jetties must be placed at least two grid cells apart (upper left sketch). This is not a serious limitation, since groins in the field are typically placed one to two groin lengths apart. A groin/jetty cannot be placed in the cell adjacent to a boundary cell, whether the boundary is gated by a groin/jetty or being an open beach (upper right sketch). Diffracting structures of any type cannot overlap (lower left and lower right) (except having their tips at the same cell wall; Figure 1).