Gebruiker:HJVerhagen/Basalton

Basic Coastline

The basic coastline in the Netherlands is a representation of the low water line of 1990. This line is used to identify coastal erosion and coastal growth and to take measures if necessary.

Coastal Memorandum 1990

In the coastal Memorandum of 1990, the Dutch Government decides to maintain the 1990 coastline by beach nourishment. The coastline in question is the low-water line. For practical application, the definition of this does not appear to be unambiguous, which is why the Memorandum also defines the momentary coastline (also called instantaneous coastline) (SKL) and basic Coastline (BKL). Each year, the shoreline to be tested ( TKL) is determined on the basis of the MKL, and if it threatens to come inland from the BKL, a sand supplementation is carried out.

The momentary coastline (MKL)

The problem with the low water line mentioned in the 1990 Coastal Memorandum is that the height of the average low tide is well defined, but the position in the horizontal direction is not. See the attached figure, here the beach profile crosses three times the low water line. In fact, it is also not important to maintain a line, but to maintain the amount of sand in the active beach profile. To determine this volume, two heights are used, the average low water level (glw)and the height of the dune foot (dv). The height of the dune foot is basically determined by finding the intersection of the steep slope of the dune front and of the dry beach. In general, this theoretical dune foot point will be slightly below the sand. It is very difficult to redefine the height of the dune foot every year. Some administrators define the dune foot line as a certain elevation line, on which the dune foot usually lies. In relatively unalterable coastal sections, this is an acceptable approach. The method of determining the MKL is such that it is not very sensitive to the precise choice of the value dv. The location of the dune foot is thus determined by the height above NAP (National Datum, approx. Mean Sea Level) and the distance from that elevation line to the administrative coastline (Xdv). This administrative line has no physical meaning, but I simply the basis for survey work.

The recipe for calculating the position of the MKL is:

• Determine the location of the dune foot
• The height of the average low water (glw) is determined
• The height h of the dune foot above average low water is calculated
• The sand volume A is calculated; A is the volume of sand seaward of the dune foot and above the level (glw-h)
• The position of the momentary coastline (SKL) is defined in relation to the national beach pile line as: (A/2h) - XDV

The background of this method is that the thickness of the sand disc to be taken must be a function of the measuring wave height; however, it is unknown. But because the elevation of the dune foot is also a function of the measuring wave height, the value h is a good representation of the effect of both tide and wave influences. For the determination of the beach profiles, the so-called JarKus profiles are measured along the coastline.^[2] These profiles are roughly 250 metres apart and are measured annually from around 800 meters in the sea to just behind the dunes. These measurements are available throughout the coast from 1965 onwards. From the period from about 1850 there are also profile soundings available in some places, but these are often slightly shifted compared to the jarkus rowing and are therefore more difficult to analyse. In the case of groynes, the sounding is carried out exactly in the middle between the groynes.

The Basic Coastline (BKL)

The Basic Coastline is by definition the coastline of 1 January 1990. But of course there are no measurements made on exactly that date, moreover, there are always variations in the measurements. The BKL is therefore determined by taking the beach measurements of the approximately 10 years prior to 1990 and by determining the MKL for each of those years. These values are placed in a graph, a regression line is determined. Where this regression line cuts the date 1-1-1990 lies the basic coastline BKL. In principle, the location of the BKL is immutable. In very special cases, where the coast is substantially altered by a work, it can be decided to shift the BKL. This is not based on a technical or morphological calculation, but actually a political decision. An example of this is the Hondsbossche Zeewering where the BKL was actually on the toe of the dike. Due to the construction of the Hondsbossche Duinen, a piece of dune was added, of which the intention is to preserve it. So there’s the BKL shifted seaward.

The coastline to be tested (TKL)

The use of the coastline to be tested (TKL)

Within the framework of the coastal policy is determined annually whether nourishment is required in a given coastal sector. This is done by determining the coastline (TKL) to be tested before the reference date. This is determined in the same way as the BKL, namely by a regression analysis of the MKL values of the previous years. See the attached graph. In this example, a supplementation was carried out in 1990, causing the MKL to shift far seawards. The number of years over which the regression analysis can be carried out is therefore somewhat limited. If there are too few years available, a regression line is usually adopted parallel to the previous regression line (so it is assumed that the erosion before and after supplementation is approximately the same). By the way, the first year after supplementation is often more than average due to adjustment effects.

In this case, it appears that the TKL is still just satisfactory for 1995 and is no longer satisfactory for 1996. In principle, a supplement at this location would be required in the course of 1995. Now the decision to supplement does not depend on a single BKL exceedance, but only if multiple profiles are threatened to become negative. In order to assess this, coastal maps are issued annually by Rijkswaterstaat. ^[3] These maps indicate whether the coast is growing or eroding with a dark green or light green block. A red block indicates that in that place the TKL has exceeded the BKL, and that something has to happen there.

General reference work

■ Verhagen, H.J., Definition of flood defence and coastline: “Basic coastline” (http://resolver.tudelft.nl/uuid:e f1bffdf-34df-456a-b9c5-824ee7cc49ef). Rijkswatersstaat, Department for Road and Water Engineering, note WBA-N-S9125, Delft (1990). Consulted on 25-2-2019.

References

1.  Maij-Weggen, J.R.M., Rijkswaterstaat,  RIKZ, Coastal defence after 1990: Policy choice for coastal care (http://resolver.tudelft.nl/uuid:a4c7d579-d528-4f26-9364-1346817230ba). Ministerie van Verkeer & Waterstaat, 's-Gravenhage (1990), pp. 66. Consulted on 25-2-2019.

2.     The name comes from annual KUStmetingen

3.  coastline map (https://geoservices.rijkswaterstaat.nl/apps/pdokkaart/applicaties/kustlijnkaart). Rijkswaterstaat. Consulted on 25/03/2019.

Design of beach nourishment in the Netherlands

A beach nourishment to broaden the beach and maintain the coastline can be designed using mathematical calculation models or on the basis of beach measurements. In the Netherlands, Belgium and Germany,a nourishment design is mainly based on measurement, while mathematical models are mainly used elsewhere. A nourishment design for coastal maintenance and beach widening can be made much more reliable  based on measurement data, provided that they are present. If there are no good, long-term series of measurements of the beach profile, one must make the design using calculation models. In the Netherlands, the coast has been measured annually for years (JarKus measurements) and therefore the very reliable method based on measurements is used in the Netherlands for the design of supplements to prevent erosion.

Use of measurements

To compensate for coastal erosion, the design of a supplementation is actually very simple, every year the same amount of sand has to be applied as erosion disappears annually. The assumption is that there is no significant change in the wave climate and the orientation of the coastline. With most nourishments, this is a correct assumption. In case of substantial changes in the coastal orientation, this method is therefore not always usable (e.g. in the design of the sand engine). In practice, the length of the nourishment must be 20-40 times the width in order to apply this method.

In short, the method consists of the following steps:

1.   Make sure there are enough measured profiles (at least 10 years).

2.    Use these profiles to calculate the annual sand loss (in m³/year) for a coastal section.

3.   Multiply this amount by an appropriate lifetime (e.g. 5 years).

4.   Add a loss factor (order 40 %).

5.   Place this amount of sand somewhere on the beach between the low water line and the dune foot.

To determine the amount of sand in the profile, the same method can be used as used for the basic Coastline. Given the fact that the instantaneous coastline has been measured for the necessary years and thus the decline of this coastline, determining the loss of sand is quite simple. Suppose the decline of the MKL is 5 m/year, then the annual sand loss is 5*(2h) m³ per year per linear meter of coastline. Here is 2h the height of the active beach profile. Along the Dutch coast, h is near Hoek van Holland in the order of 4 m, so in the above example the erosion would be 40 m³ per year per linear meter of coast. For a nourishment with a length of 4 km and a lifespan of 5 years is therefore 40*4000*5 = 80 000 m^3. Because there is extra sand loss immediately after construction, a good amount is 1.4 *80000 = 112 000 m^3. This is a seaward shift of 1.4*5*5= 35 m.

In the practice of beach nourishments (from 1990 onwards), this method appears to work very well. Analyses of nourishments in northern Germany also show that this is a reliable method.

The starting point is that the grain size of the nourishment sand is equal to the original beach sand. If this is not the case, it must be corrected. In case of finer sand in the win area, the volume of the nourishment will need to increase.^[1]

Use of mathematical models

Single line model For relatively wide and short nourishments (such as the sand motor), a single-line model can be used. In this model, the coast is represented by a single line (e.g. the instantaneous coastline) and a constant profile along the entire coastline. For each profile, the orientation of the coast is given, and in each profile the sand transport is calculated by the surf induced current. If in a profile 1 the sand transport is larger than in a profile 2, there will be between profile 1 and 2 sedimentation, for details about the model, see ^[2]. As there is sedimentation, the coastal orientation will change, and thus also the transport of sand. This makes it possible to calculate the coastline change.

A classic example is the calculation of a relatively short and wide supplementation with straight waves. The single-line model can very well predict how such supplementation can develop over time.

The Unibest calculation model of Deltares is an example of a single-line model.^[3]

Field models

In highly two-dimensional situations, e.g. at a tidal inlet or the mouth of an estuary, or if the nourishment itself has a strong two-dimensional character (as with the Sand Engine), an approach with profile measurements is not possible. A single-line model is often inappropriate. In these cases, a two-dimensional sand transport model is made (usually with models such as Delft3D from Deltares from Delft or Mike21 of DHI ^[4] in Denmark). In such a model, the bed of the area is introduced as a depth map. Then there is a  tidal flow calculation and a wave penetration calculation. After that, the sand transport is calculated at each mesh-point and from the difference in sand transport between the different mesh-points, the sedimentation and erosion is calculated in all boxes. It can then be assessed whether a nourishment behaves as intended.

The problem with this type of model is that (apart from the fairly long computation times for the computer) the results are rather sensitive to inaccuracies in the input. For example, at the edge of the model, the water levels and flow rates must be properly entered, and the wave climate must be well known. Also variations in the sand composition (grain size) have a great influence.

Foreshore Nourishments

Instead of directly suppling the beach, it is also possible to supple the foreshore (underwater bank). The advantage of this is that the implementation of the nourishment is cheaper, and there is no direct effect of the work on the use of the beach. The sand is then transported over time by the waves from deeper water to the coast. A foreshore nourishment is calculated just like a beach nourishment, but the use of measurement data with beach profiles is then less easy, as a foreshore nourishment does not give a new beach line. Therefore, in those cases, a single-line model or a field model is usually used.^[5]

General reference work

■ VERHAGEN , H.J. (1992). Method for artificial beach nourishment (pdf). Proc. International Conference on Coastal Engineering 23rd. ICCE, Venice, Italy, 1992: 12 (American Society of Civil Engineers).

References

1.  Pilarczyk, K.W.,  VAN OVEREEM, J., B AKKER, W.T., (1986).  Design of a beach nourishment scheme (pdf). Proc. International Conference on Coastal Engineering 20rd. ICCE, Taipei, Taiwan, 19862: 1456-1470 (American Society of Civil Engineers).

2.  Bakker, W.T. (1971). The dynamics of coasts. Contribution to the PAO course Coastal Dynamics and Coastal Defence Study Report W.W.K. 71-222: 108pp (PAO).

3.  VAN DER SALM, G.L.S., Coastline modelling with Unibest: Areas close to structures (http://resolve r.tudelft.nl/uuid:f77f32d5-b9ca-47d8-9b43-decbe23c8080). TU Delft, MSc Thesis (28-2-2013).

4.  DHI (company) (https://en.wikipedia.org/wiki/DHI_(company)). wikipedia. Consulted on 5-4­2019.

5.  VAN DUIN, M.J.P.,W IERSMA,N.R., W ALSTRA,D.J.R., VAN R IJN, L.C., Stive, M.J.F., (2004). Nourishing the Shoreface: Observations and hindcasting of the Egmond case, The Netherlands (pdf). Coastal Engineering 51 (2004): 12 (Elsevier). DOI: doi:10.1016/j.coastaleng.2004.07.01. Archived from original on 1 July 2019. Consulted on 5 April 2019.