# 10.5.3: Groynes


A jetty only prevents accumulation of material in a small area or stimulates accumulation in another rather restricted area. Its influence is mostly local. A field of groynes, on the other hand, is a series of smaller jetties extending into the surf zone and spaced at relatively short intervals along a beach (see Fig. 10.13). They can be very effective in reducing the existing longshore sediment transport rate2 along a coast. By keeping the coastal sand trapped between two adjacent groynes, they tend to stabilise the entire coast along which they are built. As such, they can be used to defend an eroding coast (for instance over a length of 5 km), to widen a beach or to extend the lifetime of beach fills.

Two main types of groynes can be distinguished:

Impermeable, high-crested structures with crest levels above MSL+1 m;thesetypes of groynes are used to keep the sand within the compartment between adjacent groynes. The shoreline will be oriented perpendicular to the dominant wave direction within each compartment (saw-tooth appearance of overall shoreline).

Permeable, low-crested structures with crest levels between the MLW and MHW lines, such that structure-induced eddy generation is reduced, at least at high tide (cf. Fig. 5.45). These types of groynes are generally used on beaches with a small sediment deficit; the function of the groynes is to slightly reduce the littoral drift in the inner surf zone and to create a more regular shoreline (without saw-tooth effect). They are for instance made from sheet piles: a row of vertical (wooden or steel) piles purposely spaced so as to make a porous barrier thus reducing but not totally blocking the longshore transport (Fig. 10.14).

If properly designed, groynes can be used to achieve sediment transport curve $$b$$ in Fig. 10.7, which is the ideal situation in that case. Proper design implies that the right choices for groyne length, height and permeability to sand are made. The fine-tuning problem (length and length/mutual spacing ratio) is, however, a difficult problem to resolve. The physics of groyne systems are not completely understood, making the successful design of a groyne system more an art than a science. No generally applicable design rules are available although there are some design guidelines (see below). A spacing equal to a few times the length of the groynes is common.

Since for solution $$b$$ only a small portion of the sand transport is to be stopped, the groynes should be short in this case, viz. shorter than the width of the breaker zone. Long groynes extending through the breaker zone tend to reduce the sediment transport curve to (nearly) zero in stretch A-B, which will unnecessarily maximise the lee- side erosion. A partial reduction can also be achieved with impermeable groynes.

In the example case of Fig. 10.7, in the cross-section through point B, the existing sediment transport had to be reduced by a factor of approximately 0.5 in order to fulfil the requirements (i.e. achieving line b in Fig. 10.7; $$S_{B, new} \approx 0.5 S_{B, old}$$.). Note that for arbitrary cross-sections between A and B different, namely larger ratios are required. Let point C be half way between A and B. Then, according to Fig. 10.7, $$S_{C, new} \approx 0.7 S_{C, old}$$.

Another point of practical concern is the absolute magnitude of the net yearly long-shore sediment transport involved. Although the transport gradient over section A-B (viz. the difference $$V$$ between $$S_B$$ and $$S_A$$ over the considered length of coast) can be measured quite accurately (via measuring the volume loss out of the A-B), the absolute values of the net yearly sediment transport of either $$S_B$$ or $$S_A$$ are in fact not automatically known. Since it is difficult to calculate these sediment transport rates, errors are easily made in the proper quantification of $$S_B$$ and $$S_A$$. If in Fig. 10.7 both $$S_A$$ and $$S_B$$ are increased by $$\Delta S$$, the difference $$V$$ remains the same. But to achieve a constant sediment transport in section A-B, now quite different reduction factors from those mentioned previously are necessary.

Consider for example the following two situations:

1. Stretch of coast 5 km: $$S_{in} = 100000\ m^3/yr$$; $$S_{out} = 200000\ m^3/yr$$;
2. Stretch of coast 5 km: $$S_{in} = 200000\ m^3/yr$$; $$S_{out} = 300000\ m^3/yr$$.

In both cases, the annual loss is $$100000\ m^3/yr$$, which amounts to a rate of structural erosion of $$20\ m^3/myr$$, a quite usual annual structural loss. However, an adequate groyne system for case 1 should be quite different from an adequate groyne system for case 2. In case 1, the net sediment transport near the downdrift side of the stretch of coast must be reduced by 50 % in order to achieve that $$S_{out}$$ equals $$S_{in}$$. In case 2 the reduction should be only 30%.

The rate of reduction of the sediment transport depends among others of the length of the groynes. For an impermeable groyne, a rough estimate of the required length can be made by taking the cross-shore distribution of the undisturbed wave induced longshore sediment transport as a starting point (see Fig. 10.15).

The transport reduction can now be estimated by simply assuming that the groyne completely blocks the transport and that seaward of the groyne the sediment transport is unaffected by the groyne. Less crude estimates can be made by applying coastline models, such as Unibest-CL+ or Genesis (see Sect. 8.3.2). Such models take the impact of the shoreline development inside the groyne bays on the longshore sediment transport into account. Structure-induced eddies can also significantly impact the sediment transport patterns. These effects can only be taken into account in area models (2DH or 3D).

In the framework of Conscience, a European Union funded research program, the efficiency of groyne systems was investigated and a number of design guidelines were presented (Van Rijn, 2010). It was concluded, amongst others, that:

• Nowadays, the design of groyne fields is generally combined with beach nourishment inside the groyne compartments to widen the beaches for recreation and to reduce the downdrift impacts;
• Long, curved groynes can be used to protect a major beachfill at both ends creating a wide beach for recreational purposes (pocket beach) at locations where lee-side erosion is acceptable or manageable;
• The trapping efficiency can be enhanced by using, for instance, L-head or T-head groynes (see also Fig. 10.8) instead of straight groynes;
• Groynes preferably only extend over the inner surf zone (up to the landward flank of the inner bar trough), crest levels are relatively low and spacings are in the range of 1.5 to 3 times the length. This ensures sufficient sediment bypassing such that lee-side erosion is prevented as much as possible.
• By reducing lengths at the downdrift end of the groyne field downdrift erosion can be controlled and reduced (groyne tapering);
• By constructing groynes from downcoast to upcoast initial erosion of the area to be protected is avoided.

2. Not only wave-induced longshore transport rates will be reduced but tide-induced longshore transport rates as well since groynes push the tidal current away from the coast.

This page titled 10.5.3: Groynes is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Judith Bosboom & Marcel J.F. Stive (TU Delft Open) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.