Shoreface nourishment

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Definition of Shoreface nourishment:
Artificial sand supply in the subtidal active zone of the coastal profile with sand imported from a source outside the active coastal zone.
This is the common definition for Shoreface nourishment, other definitions can be discussed in the article


On barred coasts, as along much of the Dutch coast, shoreface nourishments are commonly placed as a shore-parallel berm against the seaward flank of the outer nearshore bar (Fig. 2). Shoreface nourishments are less costly than beach nourishment. They are primarily intended to stabilize an eroding coast and to halt shoreline retreat. Compared with beach nourishment, shoreface nourishment is less suitable when rapid widening of the dry beach is required, because its effect on the shoreline is indirect and delayed.


Why shoreface nourishment

If offshore sand borrow locations are abundant and not far away, the price per m3 of shoreface nourishment is substantially lower than for beach nourishment. Sand can be deposited directly by dredging vessels equipped with rainbowing lances or with a split hull. Intermediate barges and pipelines to supply the sand on the nourishment site are not necessary. The cost-effectiveness of shoreface nourishments is even greater if they are combined with maintenance dredging of nearby navigation channels. Along the Dutch coast, the morphological influence of shoreface nourishments generally persists substantially longer than the direct shoreline response to beach nourishment..

Shoreface nourishment programmes contribute to the long-term sand balance of the coastal zone, provided there are no substantial sand losses to deep water, i.e., beyond the seaward boundary of the active coastal zone where almost no wave-induced sand transport occurs. However, estimating long-term net sediment transport to deep water is problematic, as current measurement techniques do not allow for accurate estimates. Nearshore sandbars often tend to migrate offshore in cases where the adjusted Dean number [math]\Omega_{\beta}[/math] is large, see Shoreface profile#Downslope sediment transport. Even in these cases, however, the net sand transport from the upper shoreface to the lower shoreface is probably small. For example, model simulations for the Dutch coast suggest that the net sand transport across the seaward boundary of the active coastal zone (estimated depth of 20 m) is more likely directed landward than seaward[1]. This may also apply for other gently sloping coastal zones situated on wide continental shelves[2]. The long-term positive sand balance of shoreface nourishments therefore creates a sand buffer that is progressively spread over the active coastal zone by longshore and cross-shore transport processes. By adding sediment to the active coastal zone, repeated shoreface nourishments increase the sediment volume available for redistribution as the coastal profile adapts to sea level rise.

Processes

Shoreline accretion landward of a shoreface nourishment is related to two different mechanisms: the leeside effect and the feeder effect.

The leeside effect (also known as reef or breaker bar effect) is related to increased wave dissipation across the nourished sandbar. The smaller waves near the shoreline generate less stirring of the sediment bed and a decrease of the wave-induced undertow current and associated offshore-directed sand transport[3]. With oblique wave incidence, the wave attenuation behind the nourishment induces a gradient in the alongshore sediment transport, resulting in updrift sedimentation and downdrift erosion. The resulting accretion behind the nourishment need not consist of nourished sand; part can result from convergence of longshore sediment transport.

The feeder effect is the landward redistribution of nourished sand. Nonlinear waves can produce net onshore sediment transport because the stronger shoreward orbital velocities beneath wave crests transport more sediment than the weaker offshore velocities beneath the troughs. Wave breaking over a finite-length nourishment can additionally generate a horizontal circulation, with an onshore-directed current over the central part of the nourishment and offshore return currents around its lateral ends (see also Rip current). The latter circulation therefore transports part of the nourished sand offshore, an effect that is relatively stronger for short nourishments[4].

In summary, both processes, the leeside effect and the feeder effect, produce net accretion in the zone between the shoreline and the nourishment. The leeside effect is strongest for oblique wave incidence, whereas the feeder may dominate under normal wave incidence. Observations showed a slow decline of the nourished sandbar, a slow landward migration of the bar crest, a deepening of the trough landward of the bar and net accretion further nearshore[4]. This observed accretion-erosion pattern is qualitatively consistent with the processes described above.

Observations further showed that considerable cross-shore profile change takes place after a shoreface nourishment. Repeated shoreface nourishments at the nourishment site near Camperduin (Fig. 2, North Holland coast, with bi-modal wave climate) compensated for the natural structural shoreline retreat. Even a small net shoreline advance of around 2 m/yr was observed[5]. This contrast with the impact of repeated shoreface nourishments on the shoreline position at another site, the Wadden island Ameland, where the wave climate is unidirectional with stronger net littoral drift. Here, only the structural erosion of the shoreline was compensated.

Shoreline advance and sediment accretion in the subtidal zone may impact the sub-aerial beach and dunes through two mechanisms. Over longer periods, shoreface nourishment can increase the sediment volume of the beach and dunes, although the direct transfer of nourished sand from the shoreface to the dry beach is difficult to quantify. A wider beach provides greater fetch for aeolian sand transport to the dunes, while a fuller nearshore profile can also reduce storm erosion of the upper beach and dune foot. These combined effects can yield a seaward migration of the dunefoot. [3]


Experience from shoreface nourishments on the Dutch coast

In 1990, the Dutch government has adopted a coastline maintenance policy by means of sand nourishments. From that time, more than 300 nourishments have been carried out, of which 90 shoreface nourishments with a total volume of about 160 Mm3. From the experience of these nourishments some rules of thumb have been derived to optimize the effectiveness of shoreface nourishments. These rules pertain to the Dutch coastal conditions, shown in Fig. 1. Similar conditions are found at sandy coasts along many other shelf seas.


Fig. 1. Characteristics of the Dutch coast. The figure shows a bathymetric map [m] of the coastal zone and wind roses (significant wave height and peak period) at the Holland coast (indicated by the arrow), where the wave climate has a typical bi-modal character. Other characteristics of the Dutch coastal zone are: Tidal range between 1.5 and 3.5 m, medium dune grain size between 0.15 and 0.25 mm, slope of the beach-foreshore in the order of 1/50 and typical beach width of 100-300 m.
Fig. 2. Typical Dutch shoreface-nourishment configuration: sand is placed as a submerged berm against the seaward flank of the outer nearshore bar. Coastal profile near Camperduin, North Holland; elevations relative to mean sea level.


Fig. 3. Shoreface nourishment Camperduin (North Holland coast) in 2002[6]. The figure shows a time stack of coastal profiles at the same Camperduin location with a 5-year interval (observed intermediate annual profiles are left out). The vertical scale of 10 m is equal to the distance between the 5-year interval lines. The migration (on average directed offshore) of the crests of the nearshore bars is indicated by blue lines. The shoreface nourishment was positioned in 2002 just seaward from the fading outer bar. The nourishment crest location is indicated by the red line. The nourishment was stable (slightly moving offshore) for more than 10 years. After applying the nourishment, the position of the inner bar was also stable and even moved slightly onshore.


Along much of the Dutch coast, shoreline behavior is coupled to the multi-year offshore migration cycle of nearshore bars (see the article Nearshore sandbars). Shoreface nourishment placed seaward of the outer bar can interrupt this cycle for several years. Most shoreface nourishments are applied as an artificial bar just seaward of the outer bar (Fig. 2); in one case the nourishment was placed in the trough between the outer and inner bar. The lifespan of the artificial nourished bar is defined here as the time from nourishment until integration of the artificial bar into the original bar pattern and the resumption of the offshore migration cycle.


The evaluation of the nourishments brought forward the following observations about the effects and effectiveness[7][6][8][9], see also Fig. 3:

  • The nourishments were effective to stop the offshore bar cycle and shoreline retreat during the lifespan of the nourishment;
  • The lifespan of the nourishments increased with increasing nourishment concentration (the sand volume nourished per meter);
  • The lifespan of the nourishments increased with increasing grainsize;
  • The nourishment became less mobile and persisted longer when placed deeper relative to the local wave climate and depth of closure;
  • The lifespan of the nourishments increased with increasing bar cycle period (the time it takes for an offshore moving bar to reach the position of the preceding bar);
  • A more landward position of the nourishment reduced dune erosion during storms;
  • A small fraction (less than 10%) of the nourished sand was lost offshore;
  • The nourished sandbar developed a trough at the shoreward flank;
  • The bar shoreward of the nourished bar migrated onshore;
  • Part of the nourished sand was distributed alongshore to neighbouring coastal stretches;
  • For the analyzed Dutch nourishments, direct feeding of the submerged profile with nourished sand could be demonstrated;
  • Net accretion of the supratidal beach occurred in some cases, mainly due to the leeside effect of the nourishments – the local reduction of wave heights leading to convergence of longshore sediment transport at the nourishment site.

Placement of the nourishment against the outer nearshore bar appears to be more effective than placement in the trough landward of the bar. Nourished sediment placed in the trough of a breaker bar increases offshore transport because of strong local erosion in the nourished sediment[10].


Design rules for shoreface nourishments in the Netherlands[11]

Shoreface nourishments are applied where longshore sandbars are present on the shoreface. The preferred nourishment location is at the seaward side of the outer sandbar (Fig. 2). The average volume of shoreface nourishments is 400-500 m3/m, similar to the volume of the outer bar; the average length is 4 km. This not only stops seaward bar migration, but may even initiate a landward migration of the inner sandbars (Fig. 3). The result is an increase of the sand volume of the subtidal and intertidal beach and a corresponding seaward advance of the coastline (according to the Dutch definition of shoreline position, which is related to the supratidal, intertidal and subtidal beach volume, see Coastline). For the Dutch North Sea coast, a berm crest level around NAP −5 m has generally proved effective for feeding the subtidal beach (This value should not be transferred directly to other coasts: nourishment mobility depends primarily on placement depth relative to the local wave climate and active-profile depth). It takes a few years before a shoreface nourishment has an effect on the shoreline position. For Dutch shoreface nourishments, the average time that the shoreline resumes its pre-nourishment position is 4-10 years.


Appendix A: Diffusion of a shoreface nourishment seaward of the surf zone

Larson and Hanson[12] have shown, using a simplified model for cross-shore sand transport, that a shoreface nourishment placed outside the breaker zone decays and gradually spreads in the cross-shore direction. The derivation of this model is demonstrated below, starting from the Bailard model for sand transport. It is assumed that the nourishment is implemented in the form of a long, shore-parallel sandbar whose height in the cross-shore direction is given by [math]\Delta z_b(x,t)[/math] relative to the equilibrium profile [math]z_b^{eq}(x)[/math]. It is further assumed that, prior to the nourishment, the coastal cross-shore profile deviates only slightly from an equilibrium profile, and that the depth perturbation [math]\Delta z_b(x,t) \equiv z_b(x,t) – z_b^{eq}(x)[/math] by the nourishment is small compared to the water depth.


The following symbols are used: [math]\tan \beta \equiv \partial z_b / \partial x =[/math] cross-shore slope; [math]x =[/math] cross-shore coordinate (shoreward = positive direction); [math]t =[/math] time; [math]q(x,t) =[/math] instantaneous cross-shore sand transport; [math]q^{eq}(x,t) = [/math] cross-shore sand transport for the equilibrium shoreface; [math]q_w(x,t) =[/math] cross-shore sand transport by waves only (including wave asymmetry, undertow, etc., disregarding the effect of the shoreface slope on sand transport ); [math]u_w(x,t) =[/math] cross-shore wave orbital velocity near the seabed; [math]U_w[/math] amplitude of wave orbital velocity; [math]\phi \sim 30° =[/math] sand friction angle; [math]\gamma_b =[/math] coefficient; [math]n =[/math] is the porosity of the sand bar. The brackets [math] \langle … \rangle[/math] indicate average over the wave period and the superscript [math]^{eq}[/math] indicates the equilibrium value.


The Bailard formula for bedload sand transport consists of a wave-induced transport term and a down-slope transport term: [13]

[math]q = q_w \, \Big(1- \dfrac{\gamma_b}{\tan \phi} \dfrac{u_w}{U_w} \dfrac{\partial z_b}{\partial x} \Big) . \qquad (A1) [/math]

The wave-averaged sand transport vanishes for the equilibrium surface profile:

[math]\langle q^{eq} \rangle = \langle q_w^{eq} \rangle - \dfrac{\gamma_b}{U_w \, \tan \phi} \langle q_w^{eq} u_w^{eq} \rangle \dfrac{\partial z_b^{eq}}{\partial x} = 0 . \qquad (A2)[/math]

Here, equilibrium means that the wave-averaged cross-shore transport predicted by this idealized model vanishes; substantial instantaneous and alongshore sediment transport may still occur. Because it is assumed that, prior to the nourishment, the coastal cross-shore profile deviates only slightly from an equilibrium profile and that the depth perturbation [math]\Delta z_b(x,t)[/math] by the nourishment is small compared to the water depth, one may approximate

[math] \langle q_w u_w \rangle \approx \langle q_w^{eq} u_w^{eq} \rangle [/math] and [math]\langle q_w \rangle \approx \langle q_w^{eq} \rangle + \langle q' \rangle \, , [/math]

where [math]q'(x,t)[/math] is a small transport component representing the influence of the depth perturbation on the cross-shore sand transport.

Substitution in Eqs. (A1, A2) gives

[math]\langle q \rangle = \langle q' \rangle - \dfrac{\gamma_b}{U_w \, \tan \phi} \langle q_w^{eq} u_w^{eq} \rangle \, \dfrac{\partial \Delta z_b}{\partial x} \, . \qquad (A3)[/math]

Cross-shore sand transport modifies the seabed profile according to the sand balance equation (Exner equation)

[math]\dfrac{1}{1-n} \dfrac{\partial \langle q \rangle}{\partial x} + \dfrac{\partial \Delta z_b}{\partial t} = 0 \, . \qquad (A4) [/math]

Substitution of Eq. (A3) gives

[math]\dfrac{\partial \Delta z_b}{\partial t} = - \dfrac{1}{1-n} \dfrac{\partial \langle q' \rangle}{\partial x} + \epsilon_d \dfrac{\partial^2 \Delta z_b}{\partial x^2} \, , \quad \epsilon_d = \dfrac{\gamma_b \, \langle q_w^{eq} u_w^{eq} \rangle }{(1-n) \, U_w \, \tan \phi} \, . \qquad (A5)[/math]

If [math]q'[/math] can be neglected, then Eq. (A5) reduces to a diffusion equation, similar to the equation derived by Larson and Hanson (2015[12]). The nourishment [math]\Delta z_b(x,t)[/math] thus decays and spreads as

[math]\Delta z_b \propto 1 / \sqrt{t \, \epsilon _d} \sim U_w^{-3/2} \, t^{-1/2} \, , \qquad (A6)[/math]

where it has been assumed that the wave-induced sand transport scales with the third power of velocity, [math]q_w \sim U_w^3[/math]. This scaling illustrates the behavior of the simplified diffusion model; it should not be used as a general prediction of shoreface-nourishment lifetime.

Field data fitting suggest that the diffusion coefficient [math]\epsilon_d[/math] should be in the order of [math]\epsilon_d \sim 0.004 \, U_w^3/g[/math]. [12] The diffusion term represents the tendency of the nourishment relief to flatten. It does not by itself predict whether nourished sand moves preferentially landward or seaward; this requires the additional transport term [math]q'[/math].

The influence of [math]q'[/math] was studied by Chen and Dodd (2019[14]), who found that the term [math]\dfrac{1}{1-n} \dfrac{\partial \langle q' \rangle}{\partial x}[/math] favors onshore transport of the nourishment. This occurs, on the one hand, due to the shoaling effect and amplification of wave asymmetry effects if the nourishment is located far seaward from the breakpoint, and on the other hand, due to wave dissipation and a reduction of the undertow current if the nourishment is situated close to the breakpoint.


Related articles

Shore nourishment
Beach nourishment
Shoreface profile
Nearshore sandbars
Rip current
Dealing with coastal erosion
Ecological impacts of seabed sand mining
Coastal Hydrodynamics And Transport Processes
Experiences with beach nourishments in Portugal


References

  1. ↑ Grasmeijer, B., Huisman, B., Luijendijk, A., Schrijvershof, R., van der Werf, J., Zijl, F., de Looff, H. and de Vries, W. 2022. Modelling of annual sand transports at the Dutch lower shoreface. Ocean and Coastal Management 217, 105984
  2. ↑ Anthony, E.J. and Aagaard, T. 2020. The lower shoreface: Morphodynamics and sediment connectivity with the upper shoreface and beach. Earth-Science Reviews 210, 103334
  3. ↑ 3.0 3.1 van der Werf, J.J., Huisman, B.J.A., Price, T.D., Larsen, B.E., de Schipper, M.A., McFall, B.C., Krafft, D.R., Lodder, Q.J. and Ruessink, B.G. 2025. Shoreface nourishments: Research advances and future perspectives. Earth-Science Reviews 267, 105138
  4. ↑ 4.0 4.1 Huisman, B.J.A., Walstra, D.J.R., Radermacher, M., De Schipper, M.A. and Ruessink, B.G. 2019. Observations and modelling of shoreface nourishment behaviour. J. Mar. Sci. Eng. 7, 59
  5. ↑ Löhr, J.S., Ruessink, B.G. and Price, T.D. 2026. Satellite-based shoreline dynamics of repeatedly nourished Dutch beaches with contrasting directional wave climates. Coastal Engineering 208, 105015
  6. ↑ 6.0 6.1 Van der Spek, A.F.J. and Elias, E., 2013. The effects of nourishments on autonomous coastal behaviour. Procs. Coastal Dynamics Conf. 2013, pp. 1753-1762
  7. ↑ Ojeda E., Ruessink, B.G. and Guillen, J. 2008. Morphodynamic response of a two-barred beach to a shoreface nourishment. Coastal Engineering 55: 1185-1196
  8. ↑ Gijsman, R., Visscher, J. and Schlurmann, T. 2019. The lifetime of shoreface nourishments in fields with nearshore sandbar migration. Coastal Engineering 152, 103521
  9. ↑ Atkinson, A.L. and Baldock, T.E. 2020. Laboratory investigation of nourishment options to mitigate sea level rise induced erosion. Coastal Engineering 161, 103769
  10. ↑ Jacobsen, N.G. and Fredsøe, J. 2014. Cross-shore redistribution of nourished sand near a breaker bar. J. Waterw. Port Coast. Ocean Eng. 140: 125–134
  11. ↑ Brand, E., Ramaekers, G. and Lodder, Q. 2022. Dutch experience with sand nourishments for dynamic coastline conservation – An operational overview. Ocean and Coastal Management 217, 106008
  12. ↑ 12.0 12.1 12.2 Larson, M. and Hanson, H. 2015. Model of the evolution of mounds placed in the nearshore. J. Integr. Coast. Zone Manag. 15: 21–33
  13. ↑ Bailard, J.A. 1981. An energetic total load sediment transport model for a plane sloping beach. J. Geophysical Res. 86: 10938-10954
  14. ↑ Chen, W. and Dodd, N. 2019. An idealised study for the evolution of a shoreface nourishment. Cont. Shelf Res. 178: 15–26


The main author of this article is Job Dronkers
Please note that others may also have edited the contents of this article.
Citation: Job Dronkers (2026): Shoreface nourishment. Available from http://www.coastalwiki.org/wiki/Shoreface_nourishment [accessed on 21-08-2026]
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