Difference between revisions of "Shore nourishment"

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Nourishment can be divided into [[backshore]] nourishment, beach nourishment and shoreface nourishment.
 
  
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==What is nourishment?==
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<div style="border:1px solid #000000; text-align:center; align=center; padding=5px ; margin-right:20px; margin-left:20px">
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Nourishment can be regarded as a very natural way of combating coastal erosion and shore erosion as it artificially replaces a deficit in the sediment budget over a certain stretch with a corresponding volume of sand. However, as the cause of the erosion is not eliminated, erosion will continue in the nourished sand. It is thus inherent in the nourishment concept that the nourished sand is gradually sacrificed. This means that nourishment as a stand-alone method normally requires a long-term maintenance effort. In general, nourishment is only suited for major sections of shoreline; otherwise the loss of sand to neighbouring sections will be too large. Regular nourishment requires a permanent well-functioning organisation, which makes nourishment as a stand-alone solution unsuitable for privately owned coastlines.
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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.
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</div>
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The success of a nourishment scheme depends very much on the grain size of the nourished sand, the so-called borrow material, relative to the grain size of the native sand. As described in [[Coastal Hydrodynamics And Transport Processes#Onshore and Offshore Transport and Equilibrium Coastal Profile|Onshore and Offshore Transport and Equilibrium Coastal Profile]], the characteristics of the sand determine the overall shape of the coastal profile expressed in the equilibrium profile concept. Furthermore, in nature the hydrodynamic processes tend to sort the sediments in the profile so that the grain size decreases with increasing water depth.
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{{Definition|title=Shore nourishment
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|definition=Shore nourishments are projects where the coastal system is fed with sediments (sand) from a source (borrow area) at a sufficient distance from the project area so that coastal hydrodynamics remain unaffected.
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}}
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Other examples of projects where the coastal system is fed, are large scale land reclamation projects and the construction of an artificial island in the open sea. In these cases often huge volumes of sand are required; typically several hundreds of millions m<sup>3</sup> per project. Shore nourishment projects involve moderate volumes, typically in the order of 1-5 million m<sup>3</sup> (500-2000 m<sup>3</sup>/m) and rarely more than 10 or 20 million m<sup>3</sup> ('sandmotor' nourishment, order 5000 m<sup>3</sup>/m).  
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==Nourishment concept==
  
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==Equilibrium conditions==
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Shore nourishments can be regarded as a 'natural' way of combating dune erosion and beach erosion as it artificially replaces a deficit in the sediment budget over a certain coastal stretch with a corresponding volume of sand. A structural deficit in the sediment budget is generally due to divergence of longshore sediment transport. Shore nourishment does not eliminate this cause of erosion; erosion will continue, and the nourished sand will be carried away. It is thus inherent in the nourishment concept that the nourished sand is gradually sacrificed. This means that nourishment as a stand-alone method normally requires an ongoing maintenance effort. In general, nourishment is only suited for shoreline sections of substantial length; otherwise, the loss of sand to neighbouring sections will be too large. Regular nourishment requires a permanent well-functioning organisation, which makes nourishment as a stand-alone solution unsuitable for privately owned coastlines.
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When borrow sand is placed in a coastal profile, neither the profile nor the grain size distribution will match the equilibrium conditions. Nature will attempt to re-establish a new equilibrium profile so changes will always occur in the nourished profile. There will also be changes caused by the continued long-term erosion trend and the profile response to individual events. This means that in practice it is neither possible to perform a short-term nor a long-term stable nourishment at an eroding coast. It is inherently unstable on eroding shorelines. These are the basic realities, which the public, the politicians and those who fund the projects, find it hard to accept. On the other hand, as environmental concerns and requirements for sustainability are gaining in importance, nourishment has gradually increased its share of shoreline management schemes over the last decades.
 
  
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===Grain size===
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[[Image:nourishment methods.gif|thumb|left|300px|<small>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. </small>]]
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[[Image:nourished beaches equilibrium.jpg|350px|right|thumb|Fig. 1. Equilibrium conditions for nourished beaches required to obtain an additional beach width of &delta;w with borrow sand, which is finer and coarser than the native sand (upper and lower, respectively).]]
 
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As mentioned above, the performance of a nourishment scheme very much depends on the grain size of the borrow material relative to the grain size of the native material; see the discussion on [[Coastal Hydrodynamics And Transport Processes#Onshore and Offshore Transport and Equilibrium Coastal Profile|equilibrium profiles]].[[Image:nourish grain size.jpg|300px|thumb|right|Fig. 2. Relation between Nourishment Efficiency and the Grain Size Ratio for Nourishment<ref name="Vestkysten"/>]]
 
  
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If the borrow sand is finer than the native sand, it will tend to form a flatter profile than the natural one. The equilibrium reshaping of the nourished sand will reach out to the closure depth. If the objective of the nourishment is to obtain a wider beach, this will require very large volumes of sand, as illustrated in the upper part of Fig. 1.
 
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It is evident that the volume of sand needed to obtain a certain beach width increases drastically with the decreasing grain size of the nourished sand. Most coastal authorities realise this and some of them have introduced special bonuses for their nourishment contractors when they provide coarse sand. 
 
  
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It is evident from this figure that if borrow sand with a larger grain size than that of the native sand is nourished into a coastal profile, it will tend to form a steeper profile than the natural profile. This means that a wider beach will tend to be formed, see the lower part Fig. 1.
 
  
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Furthermore, coarser sand will be more stable in terms of longshore loss. This nourishment efficiency of the nourished sand has been studied by the Danish Coastal Authority on basis of many years of nourishment along the Danish North Sea Coast<ref name="Vestkysten">Vestkysten 2000 (in Danish) (The West Coast or the Danish North Sea Coast 2000), The Danish Coastal Authority.</ref>. The nourishment efficiency is defined as the ratio between the erosion rate for the natural sand (theoretical) and that of the nourished sand. The nourishment efficiency has been analysed as function of the ratio between the mean grain size of the borrow sand and that of the native sand:
 
  
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:<math>GSR_{Nourishment}=d_{50,Borrow}/d_{50,Native}</math>
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The design of a nourishment scheme depends very much on the grain size of the nourished sand, the so-called borrow material, relative to the grain size of the native sand. As described in [[Shoreface profile]], the characteristics of the sand determine the overall shape of the coastal profile expressed in the equilibrium profile concept. Furthermore, natural hydrodynamic processes tend to sort the sediments in the profile so that the grain size decreases with increasing water depth. Borrow material is generally mined from the seabed sufficiently far offshore to minimize any influence on the hydrodynamics of the coastal zone. Placement in onshore nourishment sites makes use of pipes; for offshore sites, sand is dumped by split barges or by rainbowing (Fig. 1). For small nourishments, land sources are sometimes used. 
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<br clear=all>
  
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The analysis covers effects of cross shore as well as longshore effects. The results are expressed as a relation between the nourishment efficiency versus the grain size ratio <math>GSR_{Nourishment}</math>. It is evident from the relation shown in Fig. 2. that the Nourishment Efficiency increases considerably with increasing Grain Size Ratio for Nourishment.
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==Nourishment applications==
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In the following, a short introduction is given to various different nourishment methods.
  
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===Steepness of profile===
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====Inner dune nourishment====
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Areas, which for a long time have been protected by hard coastal protection structures, have often developed steepened coastal profiles. Such areas are very far from their cross-shore equilibrium form. If nourishment is introduced in such areas it will require huge volumes of sand to restore the profile to the equilibrium profile, which is required to release the pressure on the coastal structures. In such cases, it is very important to find borrow sand, which is coarser than the native sand.  
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[[File:DuneNourishment.jpg|thumb|right]]
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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. 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.
  
  
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==Methods, functional characteristics and applicability==
 
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The three different nourishment methods will be discussed briefly in the following.
 
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[[Image:backshore nourishment.jpg|250px|right|thumb|Fig. 3. Principles in backshore nourishment, beach nourishment and shoreface nourishment]]
 
 
====Backshore nourishment====  
 
====Backshore nourishment====  
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[[Backshore]] nourishment is the strengthening of the upper part of the beach by placing nourishment on the [[backshore]] or at the foot of the dunes.  
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[[File:BackshoreNourishment.jpg|thumb|right]]
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The main objective of [[backshore]] nourishment is to strengthen the [[backshore]]/dune against erosion and [[breaching]] during extreme events.  The material is stockpiled in front of the dunes and acts as a buffer, which is sacrificed during extreme events. This kind of nourishment works more by volume than by trying to restore the natural wide beach. The loss is normally large during extreme events, whereby steep scarps are formed. [[Backshore]] nourishment can be characterised as a kind of emergency measure against dune setback/breach; it cannot, therefore, be characterised as a sustainable way of performing nourishment and it does not normally look very natural.  
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[[Backshore]] nourishment is the strengthening of the upper part of the beach by placing the nourishment on the [[backshore]] or at the foot of the dunes. The material is stockpiled in front of the dunes and acts as a buffer, which is sacrificed during extreme events.
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The main objective of [[backshore]] nourishment is to strengthen the [[backshore]]/dune against erosion and [[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 can be characterised as a kind of emergency measure against dune setback/breach; it cannot, therefore, be characterised as a sustainable way of performing nourishment and it does not normally look very natural.  
  
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[[Backshore]] nourishment can be performed by hydraulic pumping sand through pipes discharging at the foot of the dunes and later adjusted using a bulldozer. 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.  
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[[Backshore]] nourishment can be performed by hydraulic pumping 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====  
 
====Beach nourishment====  
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Beach nourishment is the supply of sand to the shore to increase the recreational value and/or to secure the beach against shore erosion by feeding sand on the beach. It is not a coastal protection measure, as the beach will normally be flooded during extreme events, but it will support possible coastal protection measures. When performing beach nourishment, the borrow sand must be similar to the native sand to adjust smoothly to the natural profile. It may be an advantage to use slightly coarser sand than the natural beach sand, as this will enhance the stability of the resulting slightly steeper profile.  Finer sand will very quickly be transferred to deeper water and will thus not contribute directly to a wider beach. However, the fine sand will help building up the outer part of the profile. See also [[experiences with beach nourishments in Portugal]].
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[[File:BeachNourishment.jpg|thumb|right]]
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[[Beach nourishment]] is the supply of sand to the shore to increase the recreational value and/or to secure the beach against shore erosion by feeding sand on the beach. It is not a protection measure against flooding, as the beach is normally submerged during extreme events. However, it enhances the protection function of the dune belt or other backshore flood defences. The sand is usually supplied via a cross-profile pipeline and adjusted with bulldozers to the natural beach profile. It is therefore necessary that the borrow sand has grain size similar to the native sand. It may be an advantage to use slightly coarser sand than the natural beach sand, as this will enhance the stability of the resulting slightly steeper profile.  Finer sand will very quickly be transferred to deeper water and will thus not contribute directly to a wider beach. However, the fine sand will help building up the outer part of the profile. See also [[Beach nourishment]] and  [[Experiences with beach nourishments in Portugal]].
  
 
====Shoreface nourishment====  
 
====Shoreface nourishment====  
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[[Image:nourishment methods.gif|thumb|Fig. 4. Nourishment methods in practice by the Danish Coastal Authority. Beach nourishment by pipe discharge on the beach and over the bow pumping and shoreface nourishment by split barge.]]
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[[File:OuterBarNourishment.jpg|thumb|right]]
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Shoreface nourishment is the supply of sand to the outer part of the coastal profile, typically on the seaside of the bar. It will strengthen the coastal profile and add sediment to the littoral budget in general. This type of nourishment is used in areas where coastal protection measures have steepened the coastal profile or in areas with a long-term sediment deficit. Shoreface nourishment is sometimes used with beach nourishment in order to strengthen the entire coastal profile. It is recommended for obtaining a nourished profile close to the equilibrium profile. Stand-alone shoreface nourishment acts only indirectly as a shore protection measure through slightly decreased wave exposure and as a shore restoration measure with considerable delay and little efficiency.
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[[Shoreface nourishment]] (also called profile nourishment) is the supply of sand to the subtidal part of the coastal profile. It will strengthen the coastal profile and add sediment to the littoral budget in general. This type of nourishment is used in areas where coastal protection measures have steepened the coastal profile or in areas with a long-term sediment deficit. 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]]. See [[Shoreface nourishment]] for further details.
  
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Shoreface nourishment is often performed using split barges. The unloading is fast and the unit price therefore low. Shoreface nourishment can profitably be used in connection with large beach nourishment schemes, in which borrow material, which does not fulfil the requirements for beach nourishment, can be used in the outer part of the profile where it naturally belongs.
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Shoreface nourishment is often performed using split barges (Fig. 1). The unloading is fast and the unit price therefore low. Shoreface nourishment can profitably be used in connection with large beach nourishment schemes, in which borrow material, which does not fulfil the requirements for beach nourishment, can be used in the outer part of the profile where it naturally belongs.
  
 
====Beach Scraping====
 
====Beach Scraping====
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A [[beach berm]] consisting of coarse sand or gravel is sometimes formed during relatively mild summer wave conditions, which tend to transport seabed material towards the beach. This coarse material can be brought to the upper part of the beach to protect the dune foot or the foot of the cliff against erosion by storm surges. This practice is called beach scraping and is normally performed using front loaders.
  
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=====Method=====
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This method can be used for beaches which are mainly exposed to seasonal erosion, whereas it is probably not suitable for locations exposed to long-term erosion. For other issues, see the page [[Beach scraping]].
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A [[beach berm]] consisting of coarse sand or gravel is sometimes formed during relatively mild summer wave conditions, which tend to transport seabed material towards the beach. Beach scraping is normally performed using front loaders.  
 
  
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=====Functional characteristics=====
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====Creation of a new beach====
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The purpose of beach scraping is to strengthen the upper part of the beach profile and the foot of the cliff. The material is placed in a position that reduces the erosion occurring during storm surge conditions.
 
  
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=====Applicability=====
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[[Image:BeachSlopeGrainSizeData.jpg|thumb|350px|right|<small>Figure 2. Correlation between sediment grainsize and related beach slope from 78 field studies, adapted from Bujan et al. (2019)<ref name=Bu>Bujan, N., Cox, R. and Masselink, G. 2019. From fine sand to boulders: Examining the relationship between beach-face slope and sediment size. Marine Geology 417, 106012</ref>. The violet area indicates the scatter of the data points.</small>]]
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This method can be used for beaches, which are mainly exposed to seasonal erosion, whereas it is probably not feasible for locations, which are exposed to long-term erosion. One disadvantage of the method is that the material used for strengthening the upper part of the beach profile is taken from the lower part of the same profile, which means that the method only contributes insignificantly to the overall stability of the beach profile. Another issue is that equipment operated during late summer may disturb recreational activities.
 
  
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====Beach De-watering or Beach Drain====
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In many places (e.g. along the Mediterranean coast) the coast is rocky with few wide sandy beaches. A beach can be created by artificial sand nourishment if this is desired for recreational purposes. There are two main differences with maintenance nourishment of an existing beach: (1) an entire equilibrium beach profile has to be created, up to the [[closure depth]], to minimize sand loss to deep water; (2) the new beach is subject to strong gradients in longshore sand transport that redistribute the nourishment alongshore. Additional measures are necessary to keep the nourished sand in place, such as confining the artificial beach between groynes or breakwaters. However, such structures must be carefully designed to ensure proper functioning, as explained in the articles [[Groynes]] and [[Detached breakwaters]]. The sand volume needed to create a stable beach strongly depends on the grain size. The equilibrium profile of a coarse sandy beach is much steeper than the equilibrium profile of a fine sandy beach (Fig. 2). With coarse sand smaller nourishment volumes are needed and longshore sand losses will be smaller.
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For more information on beach drainage systems see also [[Beach drainage]].
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<br clear=all>
  
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'''Definition'''
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====Channel wall nourishment====
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A beach de-watering system or beach drain, is a shore protection system working on the basis of a drain in the beach. The drain runs parallel to the shoreline in the wave up-rush zone. The beach drain increases the level of the beach near the installation line, thus also increasing the width of the beach. The beach drain method is patented world wide by GEO, Denmark.
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[[File:ChannelWallNourishment.jpg|thumb|right]]
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At places where the coast is interrupted by tidal inlets, channels of the ebb tidal delta may run some distance along the adjacent shores. The landward migration of these tidal channels can cause erosion of the beaches along these shores. In this case channel wall nourishments can be applied to counteract this landward migration, even when large nourishment volumes are required. However, because lateral channel migration is usually a rather slow process, the lifetime of the nourishment is generally longer than the lifetime of beach or shoreface nourishments. Channel wall nourishments therefore can be a good option to prevent beach erosion by onshore migrating channels. Extensive experience with channel wall nourishments has been gained at tidal inlets along the Dutch coast <ref>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</ref>.
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<br clear=all>
  
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=====Method=====
 
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The drain consists of a permeable plastic pipe installed 1.0 to 2.0 m below the beach surface in the wave up-rush zone. If there is a significant tide, the drain must be installed close to the MHWS line, i.e. near the shoreline. The drain is connected to a pumping well from which the drain water is pumped, either into a lagoon or back into the sea. The only visible part of the drain installation is the pumping well and a small control house.
 
  
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[[Image:beach drain fn.jpg|380px|center|thumb|Fig. 5. Principle of beach drain function]]
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==Related articles==
 +
:[[Beach nourishment]]
 +
:[[Shoreface nourishment]]
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:[[Shoreface profile]]
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:[[Experiences with beach nourishments in Portugal]]
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:[[Dealing with coastal erosion]]
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:[[Nearshore sandbars]]
  
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=====Functional characteristics=====
 
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The conditions influencing the function of the drain are summarised in the following:
 
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*The site must have a sandy beach. The beach sediments must be sand, preferably with a mean grain diameter in the range of 0.1 mm < d<sub>50</sub>  < 1.0 mm and preferably sorted to well sorted (C<sub>u</sub> = d<sub>60</sub>/d<sub>10</sub> < 3.5). These conditions give the permeability that provides optimal functionality of the beach drain.
 
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*The beach drain works by locally lowering the groundwater table in the uprush zone, which decreases the strength of the down-rush as a higher fraction of the water percolates into the beach. Furthermore, the physical strength properties of the beach sand is increased remarkably by the lowering of the water table in the wave up-rush zone thereby making the beach more resistant against erosion. The groundwater table in the beach is a function of several factors, the most important of which are: a) the groundwater table conditions in the coast and the hinterland, b) the groundwater table caused by tide and storm surge, and c) the groundwater table caused by waves. 
 
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*A high groundwater table in the coast and the hinterland influences beach stability and beach formation. The hinterland-based groundwater table saturates a large portion of the beach, causing groundwater seepage through the foreshore. This seepage tends to destabilise (fluidise) the foreshore. The beach drain locally lowers the groundwater table to the level of the drain and counteracts the destabilisation.
 
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*The beach drain works well at locations with relatively high tide because the tide generates an elevated groundwater table in the beach, which can be lowered considerably by the drain. It can therefore be stated that the presence of high tide at a location enhances the functionality of the drain.
 
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*The presence of high storm surges will affect the functionality of the drain by moving the uprush zone landwards away from the drain. The function of the drain during high surge conditions will mainly be indirect; the previously accumulated sand will act as a buffer for the erosion during the storm. When the storm surge falls, the elevated groundwater-level in the beach will increase beach erosion if there is no beach drain to prevent it.
 
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*Waves on a beach increase the height of the local groundwater table in the beach, partly due to the wave run-up on the foreshore and partly due to the locally elevated water-level in the uprush zone called wave set-up. Once again, the beach drain counteracts this.
 
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*The beach drain requires some wave activity on the beach as the drain works by manipulating the downrush conditions on the foreshore. Too small and too high waves make the beach drain inefficient. It works best on moderately exposed coasts.
 
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*As the beach drain system functions only on the foreshore in the uprush zone, it does not directly protect the entire active profile against erosion. Consequently, it is best suited at locations with seasonal beach fluctuations or where the objective is a wider beach at an otherwise stable section of the shoreline. For locations that experience on-going recession of the entire active coastal profile, the beach drain is probably only suitable combined with other measures. The long-term capability of the beach drain under such circumstances remains to be tested.
 
  
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=====Applicability=====
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==Further reading==
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The beach drain is best suited for the management of beaches with the following characteristics:
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*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
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*Sandy beaches
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*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.
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*Moderately exposed to waves
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*CIRIA, 2010. Beach Management Manual, 2nd Edition, Publication no. RP787, London
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*Exposed to tide
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*Coastal Engineering Manual 2006. part V Ch. 4 Beach fill design.
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*Suffering from high groundwater table on the coast and on the beach
 
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*Exposed to seasonal fluctuations of the shoreline
 
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*Exposed to minor long-term beach erosion
 
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*Locations with a narrow beach, where a wider beach is desired
 
  
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The beach drain is, however, not recommended as a primary shore or coastal protection at locations with the following characteristics:
 
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*Severely exposed locations
 
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*Protected locations
 
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*Locations exposed to severe long-term shore erosion and coast erosion
 
  
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==See also==
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==References==
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[[Theme 5#Coastal protection techniques|Coastal protection techniques]]
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<references/>
  
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[[Experiences with beach nourishments in Portugal]]
 
  
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[[Artificial nourishment]]
 
  
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[[Beach nourishment]]
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{{4Authors
 +
|AuthorID1=13331
 +
|AuthorFullName1=Mangor, Karsten
 +
|AuthorName1=Karsten
 +
|AuthorID2=11540
 +
|AuthorName2= Jan van de Graaff
 +
|AuthorFullName2= Jan van de Graaff
 +
|AuthorID3=16611
 +
|AuthorName3= AnnaKroon
 +
|AuthorFullName3= Anna Kroon
 +
|AuthorID4=120
 +
|AuthorFullName4=Job Dronkers
 +
|AuthorName4=Dronkers J
 +
}}
  
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==References==
 
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<references/>
 
−
 
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==Further reading==
 
−
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.
 
  
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{{author
+
[[Category:Coastal protection]]
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|AuthorID=13331
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[[Category:Soft coastal interventions]]
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|AuthorFullName=Mangor, Karsten
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[[Category:Beaches]]
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|AuthorName=Karsten}}
 
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[[category:Theme 5]]
 
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[[category:Theme 8]]
 
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[[Category:Shoreline management]]
 
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[[Category:Protection of coastal and marine zones]]
 
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[[Category:Coastal defence]]
 
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[[Category:Coastal flooding]]
 
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[[Category:Coastal erosion]]
 
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[[Category:Coastal flooding management]]
 
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[[Category:Coastal erosion management]]
 
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[[category:Coastal structures]]
 

Latest revision as of 14:32, 23 July 2023

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) at a sufficient distance from the project area so that coastal hydrodynamics remain unaffected.
This is the common definition for Shore nourishment, other definitions can be discussed in the article

Other examples of projects where the coastal system is fed, are large scale land reclamation projects and the construction of an artificial island in the open sea. In these cases often huge volumes of sand are required; typically several hundreds of millions m3 per project. Shore nourishment projects involve moderate volumes, typically in the order of 1-5 million m3 (500-2000 m3/m) and rarely more than 10 or 20 million m3 ('sandmotor' nourishment, order 5000 m3/m).


Nourishment concept

Shore nourishments can be regarded as a 'natural' way of combating dune erosion and beach erosion as it artificially replaces a deficit in the sediment budget over a certain coastal stretch with a corresponding volume of sand. A structural deficit in the sediment budget is generally due to divergence of longshore sediment transport. Shore nourishment does not eliminate this cause of erosion; erosion will continue, and the nourished sand will be carried away. It is thus inherent in the nourishment concept that the nourished sand is gradually sacrificed. This means that nourishment as a stand-alone method normally requires an ongoing maintenance effort. In general, nourishment is only suited for shoreline sections of substantial length; otherwise, the loss of sand to neighbouring sections will be too large. Regular nourishment requires a permanent well-functioning organisation, which makes nourishment as a stand-alone solution unsuitable for privately owned coastlines.

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 very much on the grain size of the nourished sand, the so-called borrow material, relative to the grain size of the native sand. As described in Shoreface profile, the characteristics of the sand determine the overall shape of the coastal profile expressed in the equilibrium profile concept. Furthermore, natural hydrodynamic processes tend to sort the sediments in the profile so that the grain size decreases with increasing water depth. Borrow material is generally mined from the seabed sufficiently far offshore to minimize any influence on the hydrodynamics of the coastal zone. Placement in onshore nourishment sites makes use of pipes; for offshore sites, sand is dumped by split barges or by rainbowing (Fig. 1). For small nourishments, land sources are sometimes used.

Nourishment applications

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

Inner dune nourishment

DuneNourishment.jpg

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. 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

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Backshore nourishment is the strengthening of the upper part of the beach by placing the nourishment on the backshore or at the foot of the dunes. The material is stockpiled in front of the dunes and acts as a buffer, which is sacrificed during extreme events. The main objective of backshore nourishment is to strengthen the backshore/dune against erosion and 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 can be characterised as a kind of emergency measure against dune setback/breach; it cannot, therefore, be characterised as a sustainable way of performing nourishment and it does not normally look very natural.

Backshore nourishment can be performed by hydraulic pumping 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

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Beach nourishment is the supply of sand to the shore to increase the recreational value and/or to secure the beach against shore erosion by feeding sand on the beach. It is not a protection measure against flooding, as the beach is normally submerged during extreme events. However, it enhances the protection function of the dune belt or other backshore flood defences. The sand is usually supplied via a cross-profile pipeline and adjusted with bulldozers to the natural beach profile. It is therefore necessary that the borrow sand has grain size similar to the native sand. It may be an advantage to use slightly coarser sand than the natural beach sand, as this will enhance the stability of the resulting slightly steeper profile. Finer sand will very quickly be transferred to deeper water and will thus not contribute directly to a wider beach. However, the fine sand will help building up the outer part of the profile. See also Beach nourishment and Experiences with beach nourishments in Portugal.

Shoreface nourishment

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Shoreface nourishment (also called profile nourishment) is the supply of sand to the subtidal part of the coastal profile. It will strengthen the coastal profile and add sediment to the littoral budget in general. This type of nourishment is used in areas where coastal protection measures have steepened the coastal profile or in areas with a long-term sediment deficit. 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. See Shoreface nourishment for further details.

Shoreface nourishment is often performed using split barges (Fig. 1). The unloading is fast and the unit price therefore low. Shoreface nourishment can profitably be used in connection with large beach nourishment schemes, in which borrow material, which does not fulfil the requirements for beach nourishment, can be used in the outer part of the profile where it naturally belongs.

Beach Scraping

A beach berm consisting of coarse sand or gravel is sometimes formed during relatively mild summer wave conditions, which tend to transport seabed material towards the beach. This coarse material can be brought to the upper part of the beach to protect the dune foot or the foot of the cliff against erosion by storm surges. This practice is called beach scraping and is normally performed using front loaders.

This method can be used for beaches which are mainly exposed to seasonal erosion, whereas it is probably not suitable for locations exposed to long-term erosion. For other issues, see the page Beach scraping.

Creation of a new beach

Figure 2. Correlation between sediment grainsize and related beach slope from 78 field studies, adapted from Bujan et al. (2019)[1]. The violet area indicates the scatter of the data points.

In many places (e.g. along the Mediterranean coast) the coast is rocky with few wide sandy beaches. A beach can be created by artificial sand nourishment if this is desired for recreational purposes. There are two main differences with maintenance nourishment of an existing beach: (1) an entire equilibrium beach profile has to be created, up to the closure depth, to minimize sand loss to deep water; (2) the new beach is subject to strong gradients in longshore sand transport that redistribute the nourishment alongshore. Additional measures are necessary to keep the nourished sand in place, such as confining the artificial beach between groynes or breakwaters. However, such structures must be carefully designed to ensure proper functioning, as explained in the articles Groynes and Detached breakwaters. The sand volume needed to create a stable beach strongly depends on the grain size. The equilibrium profile of a coarse sandy beach is much steeper than the equilibrium profile of a fine sandy beach (Fig. 2). With coarse sand smaller nourishment volumes are needed and longshore sand losses will be smaller.

Channel wall nourishment

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At places where the coast is interrupted by tidal inlets, channels of the ebb tidal delta may run some distance along the adjacent shores. The landward migration of these tidal channels can cause erosion of the beaches along these shores. In this case channel wall nourishments can be applied to counteract this landward migration, even when large nourishment volumes are required. However, because lateral channel migration is usually a rather slow process, the lifetime of the nourishment is generally longer than the lifetime of beach or shoreface nourishments. Channel wall nourishments therefore can be a good option to prevent beach erosion by onshore migrating channels. Extensive experience with channel wall nourishments has been gained at tidal inlets along the Dutch coast [2].


Related articles

Beach nourishment
Shoreface nourishment
Shoreface profile
Experiences with beach nourishments in Portugal
Dealing with coastal erosion
Nearshore sandbars


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

  1. ↑ Bujan, N., Cox, R. and Masselink, G. 2019. From fine sand to boulders: Examining the relationship between beach-face slope and sediment size. Marine Geology 417, 106012
  2. ↑ 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


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.
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