create strong local current and rip currents which add to the offshore movement of beach material during

storms.

(4) Functional design.

(a) Insight from numerical models.

Some numerical models of shoreline change include groin field effects (e.g., GENESIS, Hanson and

Kraus 1989, see Part III-2-4 for details) Boundary conditions for groins in these models give insight

into how they must function (Gravens and Kraus 1989). They are as follows:

- As the groin length increases, its impact on the shoreline regarding time evolution and equilibrium

planform must increase.

- Increasing groin permeability should decrease the impact of the structure on the shoreline.

Different groin permeabilities must produce different equilibrium planforms.

- A permeability of 100 percent should give longshore transport rates and shoreline evolution

identical to that modeled with no structures.

Groin bypassing around the seaward end is calculated at each time step in the GENESIS model. Key

variables are the water depth at the tip and the breaking wave height. In GENESIS, the depth of

active longshore sediment transport is taken at 1.6 times the significant breaking wave height (from

Hallermeier 1981). Groin length relative to surf zone width could also be employed to calculate the

bypassing factor.

A permeability factor representing groin elevation, groin porosity and tidal range must also be

estimated in the model. These three variables represent over-passing, through-passing and shore-

passing respectively as sketched in Figure V-3-29. The permeability factor is assigned and must

approach unity to satisfy the third criterion previously described.

A third key factor is the ratio of net transport rate, Qn to the gross rate, Qg (Bodge 1992). When the

Qn/ Qg ratio is zero, a perfectly balanced transport (no net) exists to produce symmetrical fillets on

both sides of a single groin. The opposite extreme is Qn/Qg =1 meaning unidirectional transport. A

single fillet on one side results.

These key factors controlling groin functioning are summarized in Table V-3-9 (Kraus and

Bocamazo 2000) with symbols shown in Figure V-3-29.

These three process factors incorporate many of the 20 or more fundamental variables. The geometry

ratio of spacing, Xg to length Yg is also the controlling factor for groin systems, as found in the

literature. Note from Figure V-3-29 that Yg represents a mean groin length measured from the

average, nourished beach shoreline. Using Ygu (updrift) gives a larger ratio or using Ygd (downdrift)

produces a smaller ratio that may account for some variability. The *Shore Protection Manual *(1984)

says Xg/ Yg=2-3 for the proper functioning of shore-normal groins.

V-3-70

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