Shoreface nourishment

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

Why shoreface nourishment

Shoreface nourishment is mainly intended to halt shoreline retreat. It is less effective for beach widening. If offshore sand borrow locations are abundant and not far away, the price per m³ is substantially lower than for beach nourishment. Sand can be deposited by split barges or through rainbowing. Shoreface nourishment programmes contribute to the long-term sand balance of the coastal zone if the volume of nourished sand is on average much larger than sand losses to deep water, beyond the seaward boundary of the active coastal zone where almost no wave-induced sand transport occurs. Accurate estimates of the net sand transport to deep water are beyond the possibilities of present measurement techniques. However, model simulations provide evidence that net sand transport across the seaward boundary of the active coastal zone (estimated depth of 20 m on the Dutch coast) is very small and more likely to be landward than seaward directed. This probably also holds for other gently sloping coastal zones situated on wide continental shelves. The long-term positive sand balance of shoreface nourishments thus creates a sand buffer that is progressively spread over the active coastal zone by longshore and cross-shore transport processes and so contributes to raising the seafloor and mitigating the effect of sea level rise.

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 characteristics 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 location indicated by the arrow. 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. Coastal profile near Egmond (North-Holland) with sand nourishment at the seaside of the outer sandbar. Depth is measured from mean sea level.

Fig. 3. Shoreface nourishment Camperduin (North Holland coast) in 2002. 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 the Dutch coast, long-term net shoreline retreat is related to the long-term net offshore migration of nearshore sandbars (see the article Nearshore sandbars). Most shoreface nourishments are applied as an artificial bar just seaward of the outer bar (Fig. 2); some other nourishments have been placed in the trough between the outer and inner bar. The lifetime of the artificial nourished bar is 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, see also Fig. 3:

• The nourishments were effective to stop the offshore bar cycle during the lifetime of the nourishment;
• The lifetime of the nourishments increased with increasing nourishment concentration (the sand volume nourished per meter);
• The lifetime of the nourishments increased with increasing grainsize;
• The lifetime of the nourishments increased with increasing placement depth;
• The lifetime 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 nourishments developed a trough at the shoreward flank;
• The bar shoreward of the nourishments migrated onshore;
• Part of the nourished sand was distributed alongshore to neighbouring coastal stretches;
• The "feeder effect" of the nourishments (sand supply from the nourishment to the beach) did not reach the supratidal beach;
• Accretion of the supratidal beach occurred in some cases by the "lee effect" of the nourishments – the convergence of longshore sediment transport at the nourishment site due to local reduction of wave heights, with downdrift erosion as a consequence.

Design rules shoreface nourishments

Fig. 4. Definition sketch of the volume-based coastline position used in the Netherlands, which is stable against sediment redistribution within a cross-shore zone. This cross-shore zone ranges from the dunefoot to a seaward boundary defined by the intersection of the beach profile with a lower plane situated [math]\small 2H[/math] below the dunefoot, where [math]\small H[/math] is the dunefoot level above LW. The beach volume [math]\small V \; [m^3/m] [/math] is defined as the sand volume in this zone. The volume-based coastline is situated at a distance [math]\small 0.5 V/H[/math] from the dunefoot. The location of the dunefoot is determined at a fixed distance [math]\small C[/math] from a fixed reference line.

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 (the Dutch definition of shoreline position is related to the supratidal, intertidal and subtidal beach volume, see Fig. 4). Shoreface nourishments are generally most effective in terms of feeding the subtidal beach when their crest is at approximately NAP -5 m. It takes a few years before a shoreface nourishment has an effect on the shoreline position. Shoreface nourishments have an average lifespan of 4–10 years, which is the time that the shoreline resumes its pre-nourishment position.

Related articles

Shore nourishment

Beach nourishment

Experiences with beach nourishments in Portugal

Shoreface profile

Nearshore sandbars

Dealing with coastal erosion

References

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 (2022): Shoreface nourishment. Available from http://www.coastalwiki.org/wiki/Shoreface_nourishment [accessed on 21-11-2024] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J