Shore nourishment

The application of coastal nourishments requires insight into the processes that shape the coastal profile. For an introduction to these processes, the reader is referred to the article Shoreface profile and other articles mentioned herein.

Definition of Shore nourishment: Shore nourishments are projects where the coastal system is fed with sediments (sand) from a source (borrow area) outside the active coastal zone. This is the common definition for Shore nourishment, other definitions can be discussed in the article

Shore nourishment projects are usually aimed at restoring sedimentary coasts suffering from erosion. Other sand nourishment projects include large-scale land reclamation and the construction of artificial islands in the open sea. Such projects can require several hundred million m³ of sand. Shore nourishment projects generally involve much smaller volumes, typically of the order of 1–5 million m³ and rarely more than 10–20 million m³.

Nourishment concept

Shore nourishments can be regarded as a 'natural' way of combating dune and beach erosion as it aims at restoring an antecedent situation without structurally affecting hydrodynamic conditions. A persistent deficit in the sediment budget is generally due to divergence of littoral drift (longshore sediment transport). Nourishment does not eliminate this cause of erosion: erosion will continue and the added sand will gradually be carried away. A nourishment strategy for combating persistent coastal erosion therefore requires ongoing maintenance and corresponding long-term financial and institutional arrangements.

When shore erosion is mainly due to gradients in the littoral drift, most of the nourished sand remains in the active coastal sand system and is transported to adjacent coastal sections (see Littoral drift and shoreline modelling). A shore nourishment strategy should thus consider not just the nourishment site, but a wider coastal stretch. This may be a 'coastal sediment cell', a coastal sand unit morphodynamically connected by littoral drift.

Fig. 1. Generally practiced nourishment methods. Pipe discharge on the beach for beach nourishment, over the bow pumping (rainbowing) for nearshore nourishment and split barge for nourishment in the outer part of the upper shoreface.

The design of a nourishment scheme depends strongly on the grain size of the borrow material relative to that of the native sand. As described in Shoreface profile, sediment characteristics influence the overall shape of the coastal profile, as expressed by the equilibrium-profile concept. Hydrodynamic processes also sort sediment across the profile, with grain size generally decreasing with increasing water depth. Borrow material is usually mined sufficiently far offshore to avoid significant effects of the borrow pit on coastal hydrodynamics. Suitable nourishment sand can often be found on the inner continental shelf in sand ridges and in ancient deposits where shorefaces, river channels and ebb-tidal shoals were situated when sea levels were lower. Borrow sand is commonly placed onshore through pipelines; offshore nourishment is carried out by split barges or rainbowing (Fig. 1). Land-based sand sources are sometimes used for small nourishments.

Nourishment applications

In the following, a short introduction is given to various different nourishment strategies.

Inner dune nourishment

Fig. 2. Nourishment of the inner dune flank.

Inner dune nourishment is the strengthening of the dune belt to prevent breaching by an extreme storm surge. It is not intended to prevent shoreline retreat and usually not termed 'shore nourishment'. This measure can be applied when a single dune row that protects low-lying hinterland from flooding is not strong enough to withstand extreme storm surges. Sand is placed at the landward side of the front dune to increase the dune volume and/or to raise the dune crest. Beach morphodynamic processes are not affected by this measure. The amount of sand needed to achieve the desired protection level can be estimated with the methods described in Dune erosion. Inner dune nourishment is particularly effective if the dune foot location is stable i.e., no structural retreat due to ongoing erosion. In this case, a one-off intervention is sufficient, so that the additional costs for applying the sand on the land side are also a one-off investment. Recovery of dune vegetation can be promoted by depositing the top layer of the area to be nourished and later applying it as a cover layer over the nourishment.

Backshore nourishment

Fig. 3. Nourishment of the backshore.

Backshore nourishment is the strengthening of the upper part of the beach by creating a berm on the backshore or by placing the nourishment against the foot of the dunes. The main objective of backshore nourishment is to create a sand buffer that protects the backshore/dune against erosion and dune breaching during extreme events. This kind of nourishment works more by volume than by trying to restore a natural wide beach. The loss is normally large during extreme events, whereby steep scarps are formed. Backshore nourishment primarily provides a sacrificial sand buffer against severe erosion and dune attack. It does not by itself restore the complete active profile and therefore may require repeated replenishment where structural erosion continues. It is sometimes carried out as an emergency measure after a severe dune erosion event to reduce the risk of dune breach.

Backshore nourishment can be performed by hydraulic pumping of sand through pipes discharging at the foot of the dunes and later adjusted using a bulldozer (Fig. 1). The sand source can be either an offshore supply via a cross-profile pipeline, floating or buried, or it can be supplied along the shore from, for example, a sand bypassing plant. The sand can also be supplied via land transport by dumpers.

Beach nourishment

Fig. 4. Beach nourishment.

Beach nourishment involves adding sand to restore or widen the beach, enhance recreational value and neutralize shoreline erosion. It is also an indirect flood-protection measure, as a wide and high beach reduces wave attack on the dune face during storm surges and prevents fast removal of eroded dune material. Sand is placed via cross-shore pipelines and shaped with bulldozers to match the natural beach profile, using borrow material with a grain size similar to the native sand. Slightly coarser sand is often preferred because it forms a more stable, steeper profile. Finer sand is rapidly transported to deeper water and does not directly widen the beach, though it can help build the outer profile. Beach nourishments commonly experience rapid sand loss during the first major storm, after which beach volume gradually stabilizes as the shoreface profile adjusts toward equilibrium. Where structural erosion continues, the nourishment subsequently continues to lose sand and must eventually be repeated. Restoring an equilibrium beach profile normally requires a much larger subtidal than supratidal sand volume. It is therefore often advantageous to combine beach nourishment with less expensive shoreface nourishment. See also Beach nourishment and Experiences with beach nourishments in Portugal.

Shoreface nourishment

Fig. 5. Nourishment of the outer shoreface bar.

Shoreface nourishment (also called profile nourishment) is the supply of sand to the subtidal part of the coastal profile. It aims to restore a subtidal equilibrium profile and to replenish the littoral sand budget in general. This type of nourishment is often used where beach erosion is due to an over-steepened cross-shore profile or in areas with structural erosion due to a gradient in the littoral drift. Shoreface nourishment is sometimes used in combination with beach nourishment, thus creating a nourished profile close to the equilibrium over the entire active coastal zone.

On barred coasts, shoreface nourishment is often placed at the outer breaker bar. Field studies in the Dutch coastal zone and flume experiments show that such placement can have a favorable effect on shoreline stability; see Shoreface nourishment for further details. Shoreface nourishment alone, however, contributes little to widening the subaerial beach.

Shoreface nourishment is commonly carried out by rainbowing or split barges (Fig. 1). Because unloading is rapid, placement costs per unit volume are relatively low. In large nourishment schemes, the outer part of the profile may also accommodate borrow material that is unsuitable for direct placement on the beach but compatible with the native sediment at greater water depth.

Creation of a new beach

Figure 6. Correlation between sediment grainsize and related beach slope from 78 field studies, adapted from Bujan et al. (2019). The violet area indicates the scatter of the data points.

In many regions, for example along parts of the Mediterranean coast, predominantly rocky shorelines lack wide sandy beaches. Where recreational beaches are desired, an artificial beach can be created by sand nourishment. To limit redistribution toward deeper water, sufficient sand must be supplied to establish an equilibrium profile extending to the closure depth. Where divergent longshore sediment transport would cause substantial alongshore losses, groynes or breakwaters are sometimes used to retain the nourishment. However, studies of such hybrid solutions along the Portuguese coast show that adding a breakwater does not necessarily improve the overall sediment balance, even when the structure is carefully designed. See the articles Groynes and Detached breakwaters for more details.

The sand volume required to establish a stable beach depends strongly on sediment grain size. Coarse-grained beaches generally have steeper profiles than fine-grained beaches (see Shoreface profile; the corresponding relation for beach-face slope is illustrated in Fig. 6), resulting in smaller nourishment volumes and reduced longshore sediment losses. Extraction of coarse seabed sediment may, however, cause long-lasting ecological impacts, as discussed in Ecological impacts of seabed sand mining.

Channel wall nourishment

Fig. 7. Channel wall nourishment.

Where a coast is interrupted by tidal inlets, channels of the ebb-tidal delta may run for some distance along the adjacent shores. Landward migration of these channels can cause beach erosion. Channel-wall nourishment can then be used to counteract this migration, although substantial sand volumes may be required. Because lateral channel migration is generally slow, such nourishments often have a longer lifetime than beach or shoreface nourishments. Extensive experience with channel-wall nourishment has been gained at tidal inlets along the Dutch coast.

Related articles

Beach nourishment

Shoreface nourishment

Shoreface profile

Experiences with beach nourishments in Portugal

Dealing with coastal erosion

Nearshore sandbars

Ecological impacts of seabed sand mining

Further reading

• Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
• Reeve, D. E., Chadwick, A. C. and Fleming, C.A. 2018. Coastal Engineering: Processes, Theory and Design Practice. 3rd edition. Boca Raton, Florida, USA: CRC Press (Taylor & Francis Group), 512p.
• CIRIA, 2010. Beach Management Manual, 2nd Edition, Publication no. RP787, London
• Coastal Engineering Manual 2006. part V Ch. 4 Beach fill design.

References

The main authors of this article are Mangor, Karsten, Jan van de Graaff, Anna Kroon and Job Dronkers Please note that others may also have edited the contents of this article. For other articles by this author see Category:Articles by Mangor, Karsten For other articles by this author see Category:Articles by Jan van de Graaff For other articles by this author see Category:Articles by Anna Kroon For other articles by this author see Category:Articles by Job Dronkers