GenCade:Representation of Inlets

GenCade employs the Inlet Reservoir Model (IRM) as first presented in Kraus (2000) and further developed by Larson et al. (2003, 2006). Each inlet is represented by six morphological elements (shoals and bars) plus the inlet channel (Figure 1). Each morphological element is, in turn, represented by an actual sand volume Vx and an equilibrium volume $V_{xq}$, where the subscript x is a placeholder for subscripts a (attachment bars), b (bypass bars), e (ebb shoal), or f (flood shoal). Each morphological element is assumed to have a certain equilibrium volume for fixed hydrodynamic and sediment conditions. The flux of sediment out of each morphological element is given by:

$Q_{ox}=Q_{ix}{\frac {V_{x}}{V_{xq}}}$

where $Q_{ox}$ represents the flux out of the element x and $Q_{ox}$ is the flux into the element. The attachment and bypass bars also have a third index, l or b, in front of the other two, where index l stands for the left side of the inlet and index r stands for the right side when looking seaward from land. When the sediment transport goes from left to right, the attachment and bypass bars on the left-hand side of the inlet are not active, as the sediment is assumed to be transferred from the beach on the left-hand side onto the ebb shoal without passing through the attachment and bypass bars on that side. The corresponding situation occurs as sediment is moving from right to left.

In Figure 1, the transport goes from left to right. A transport rate $Q_{lst}$ is moving alongshore towards the inlet, which may or may not be stabilized by a jetty. If there is a jetty, a portion of this sediment $Q_{j}$ will be trapped by the jetty (thus, when no jetty, $Q_{j}=0$) whereas the remaining part $Q_{in}$ will enter into the inlet system. A part of this rate may go to the ebb shoal, $Q_{ie}$ , depending on how full the ebb and flood shoals are with respect to the equilibrium volume, while the other portion, $Q_{ic}$ , will go into the inlet channel. This will, in turn, feed the ebb and flood shoals in proportion to their relative volumes.

Unless the system is completely full at equilibrium, only a portion $Q_{out}$ of the incoming rate $Q_{in}$ will leave the inlet system and be transported further along the beach. Initial and equilibrium volumes of the respective morphological elements are specified as input values to the model as are the respective locations of the attachment bars.

Figure 1. Interactions between the morphological elements of an inlet

Walton and Adams (1976) derived empirical equations for the equilibrium ebb shoal volume based on field data from 43 United States inlets. Walton and Adams’ definition of the ebb shoal approximately corresponds to the sum of the IRM’s volumes of the ebb shoal and bypass bars. To employ these equations for computing Vxq of the different morphologic units, some assumptions must be made concerning the size relationship among them. Historical aerial photographs or bathymetric surveys can be employed to determine the relative proportions of the morphological elements.