The "Zand Rivier" as essential phenomenon in foreshore dynamics

The “Zand Rivier” or in English, the “Sand River” is a phenomenon used by Dutch’ Rijkswaterstaat in the run-up to their integral “Coastal Policy 1990”. With the aim to determine how much sand was buffered and moving in the active part of the Dutch coast, how far it extends, what the predominant sediment transport mechanisms are, their volumes, direction, etc.

In analogy with an ordinary inland river, but as a more or less autonomous entity following the coastline over its full length. The physics and related processes are definitely comparable and therefore a suitable instrument to determine the required base line as well as for cases such as the consequences of possible human intervention. An approach that has worked very well for aforementioned??Coastal Policy 1990” and as laid down in the so-called “Kustnota 1990”, the document that has been issued at that time to support this policy. An approach and experience I would like to share with you here, this time focused on the current situation as well as the future.

The overall setting

For the Dutch, the coastal “Zand Rivier” begins on the border with Belgium. From there, under the influence of tides, wind and waves, seawater, sand and silt it flows along their North Sea coast towards the North, deflecting towards the East at the Wadden Sea and leaves again towards their Eastern neighbours at the level of the Eems-Dollard into Germany.?

In terms of quantities, on average about as much sand enters the Netherlands along this route as it leaves again. Which amounts to about a half to one million cubic meters per year. Thanks to this virtually constant input and output, there is a high degree of consistency and continuity in terms of net sand transport through the coastal system.

During which all kind of obstacles are encountered and to overcome, such as the passing in tidal inlets and channels, fore-deltas, estuaries, waterways and structures such as piers and breakwaters. Still, all in all, these seem to have little or no influence on the total net sand transport. Nor are the frequent nourishments that are carried out annually along the Dutch coast and outer deltas. In total, on average about 10 to 15 million cubic meters per year. Extra sand, which also “disappears” again along the way and does not reach the finish line.

The “Zand Rivier” as river

The flow pattern of the “Zand Rivier”, just like an ordinary river, consists of a spiral-shaped, forward moving circular motion, with in this case the coastline as its right bank and the open water of the North Sea as its left bank.??Due to the obvious and typical difference in flow resistance between its two bank types, the “Zand Rivier” also meanders just like an ordinary river.

As far as this meandering is concerned, past studies point to a cycle of between 10 and 20 years in which the sand river, and with it its coastline, makes a harmonic undulating movement of successive seaward and landward movements.?

In the past, so-called groynes and other forms of shoreline structures were built along various coastal stretches in order to contain this process and thus limit unwanted landward movements. Comparable to groynes in a river.

An additional interesting fact is that these studies also show that people actually proceeded with building these groynes at the moment that the natural decline had actually reached its maximum and shortly afterwards it would have moved forward again in the same natural way, seaward.

As a result, such an intervention actually did (and does) more harm than good, because such a hard structure in a sandy coast is disastrous for the natural dynamics and therefore usually fatal for its ability to recover.

This is why, at the time of the 'Coastal Policy 1990', it was advised to opt for beach nourishment as the preferred solution as a maintenance measure for coastline enforcement as much as possible, unless hard local interventions are the only solution for other reasons.

Applying beach nourishment means that a sand buffer is prepared on the beach at the location to be maintained to be collected by the sea when necessary and thus replenishes the position, shape and volume of that coastal section.

The determining processes

With regard to the water and sediment movement (transport) through the “Zand Rivier” as mentioned, it consists of a spiral-shaped, on average, daily (resulting) clockwise northward moving water and sand mass with (1) as its right bank the sandy coastline with its long-term harmonic periodic forward and receding foreshore and sandbanks moves and (2) as left bank the open water of the North Sea.

Partly under the influence of the (almost) always present waves with their associated elliptical orbital movement, what results is a clockwise rotating flow pattern. The same applies to the sediment carried in it (sand and silt)

Which means that the timeframe the sand can settle on the higher parts of the foreshore, bank-system and along the waterline is theoretically at most about 25 percent of the total cycle time. With the additional condition that a flow-delay is required for permanent deposition in the form of, for example, a depression (pit) or shadow effect (waves and/or current) relative to a structure, sand ripples, etc.

For the remaining three-quarters of the cycle (time), the movement is in the opposite direction, downwards and away from the coast. A process in which gravity contributes to deposition and counteracts the movement of sand along the slope to higher parts.

All in all, this means that the net sand transport and deposition phase (time window) is reduced to an average of about 10 to 15 percent of the entire cycle time (tide plus wave and wind influence). Thus directly proportionally reducing the ability for natural coastal recovery by sand that is supposed to be supplied naturally from deeper water and deposited on the higher parts of the slope and the (wet) beach.

The closure depth of the Sand River is determined by the prevailing combination of tide, waves (short and long) and sediment properties. This results in a partly flat (sloping) bottom with a series of sand waves or banks up to the (high) water line. It is the combination of each of these factors and their relative share in the total dynamics or duration and frequency of occurrence that determines the end result. For example, a coastline exposed to long waves looks substantially different from one that is mainly exposed to shorter waves.

As a result, both behave differently, a phenomenon that can also occur sequentially, depending on the order, frequency and duration of both phenomena.?

This is why, in this situation, the analysis of dynamic coastal profiles should not be based on a generic wave climate in which all observations are lumped together. Yet this error is often signalled.

Ergo, analysis of the right bank of the Sand River, consisting of the wet beach, waterline(s), the bank system and the deeper foreshore, requires thorough knowledge of and experience with the entirety of hydro-morphological processes, respective time scales and interactions. Crucial when considering intervention.

In addition to this, it may be interesting to touch on the following point, although there is (still) little to say about it: that of an accelerated or steady, non-accelerated sea level rise. In relation to which, it is often stipulated that in the event of such a rise, the seabed must also rise at the same rate. People like to refer to “the rule of Per Bruun”, who stated in 1962 that such a correlation does exist.?

It cannot be ruled out that such a relationship may exist for estuaries and a relatively similar sheltered sandy coast. For a dynamic coastal environment, this however is still very much up for debate. In the years following his initial definition, in this respect Per Bruun himself has also added a number of comments to his rule.

Certainly in view of the present structurally observed shift of short(er) wave energy in the direction of long wave energy, and the inextricably linked influence on coastal and dune erosion, it is obvious now that such a linear relationship for the North Sea and Wadden coasts is out of the question. Rather the opposite is true. Resulting in an equal or deeper equilibrium profile of the foreshore instead of a proportionally rising profile according to Bruun’s initial assumption. Partly in view of the (extensive) time scales inextricably linked to this phenomenon, we will leave it at this for now and note it as “work in progress”

Imperfections

Just like the flow along normal rivers, the “Zand Rivier”??can also encounter obstructions or imperfections, which can disrupt its course temporarily or for a longer period of time.

Probably the best-known example of this is the formation of sand deposits on the foreshore during the storm season as a result of dune erosion and/or high(er) water levels as a result of atmospheric conditions elsewhere.

Sand from the higher part of the coast (including dune front and higher) then washes away and usually deposits on the lower parts of the foreshore. As a result, among other things, the bed of the sand river no longer matches the average conditions and current-profile that will return later.

Along the path of least resistance, also the “Zand Rivier” will want to restore its old course and bed and redistribute the sand mass lying in the way.

In practice, this means that as a result of the clockwise rotating current, most of this sand is discharged to deep(er) water, where it settles in thin, untraceable deposits where it remains (outside the active zone). As a consequence in fact it is lost for the natural coastline dynamics and can therefore no longer make a positive contribution to the “Zand Rivier”.

While at the same time the higher part of the cross shore profile contains relatively too little sand due to the erosion after the storm, a deficit the “Zand Rivier” will (want to) restore naturally, assisted by the tidal effect (high and low water) and wind. We have already seen that, in terms of hydraulics, the time span in which this can happen is only 10 to 15 percent of the entire cycle.

If, partly as a result of this, the time scale on which this natural recovery takes place (or should take place) does not correspond (takes too long) with the preferences of the (coastal) manager, then the latter can artificially (try to) compensate for the shortage by means of nourishment.?

As already mentioned, extensive research has shown that the most effective method of doing this is through the already mentioned and proven “beach nourishment”.

A second shortcoming may arise as a result of constructive (hard) intervention or changing circumstances, otherwise the coastline erodes and systematically moves landward.

To be able to quantify this behaviour (erosion) more or less unambiguously, use is made of the so-called Basis Kustlijn (BKL), which is a measure of a theoretical amount of sand in the coastal profile and associated coordinates.

When this BKL moves too far inland according to the manager's wishes, it can be decided to intervene, depending on the interests at stake. In line with the 1990 Coastal Policy, with a preference for nourishment, unless it appears that, for example, a rock dumping offers a better, more effective and more sustainable solution.

Control and management

When is chosen for nourishment, first it must be assessed whether the required volume of sand can best be placed on the beach or on the foreshore. Where, as we have already seen, sand that is dumped directly on the foreshore immediately forms an obstacle for free flow and therefore by definition has a (much) shorter lifespan than when it is stored on the beach, just above the active zone.

Judging by the performance of the so-called foreshore and/or trench wall replenishments, as they have been generously added to the system to date, it can only be concluded that they are (or have been) hardly effective, if at all. In addition to the direct obstacle effect, there are a number of other reasons for this.

Such as incorrect timing in relation to the natural (harmonic) meandering behaviour of the Dutch coastline, comparable to the crucial errors that were made in the past in the timing of the construction of groynes and other fixed structures as mentioned earlier.?

In addition, in order to be somewhat successful at all, it is necessary that the sand mass to be replenished “fits” in all respects within the periodic hydro-morphological conditions on site, autonomous as well as human-induced (captive).

Assessment of historic nourishments on this subject shows us that this requirement has not been met in almost all cases. The nourishments carried out were too small in terms of quantities or far too large and/or literally misplaced in terms of choice of location in terms of depth (lines) and/or in terms of location against or between the banks and/or position along the coastline or channel (wall).

It has also been found that the actual purpose of many of these nourishments was often not or insufficiently clearly defined, making it impossible to determine the desired effectiveness. Many of these types of “projects” have been carried out under the heading of “research” or “pilot”, whereby the often lack of clear planning and design conditions further complicates matters.

Synthesis

The “Zand Rivier” phenomenon as outlined here is a real element in the functioning and understanding of the behaviour of the sandy coast and a crucial factor when it comes to planning and implementing any necessary measures.

Given its multi-dimensional appearance, thorough knowledge of the relevant processes and their interaction is an absolute necessity.

For which phenomenological research based on factual measurements must be leading. In some areas, the use of one or more parameter-specific models can sometimes contribute to contribute to spatial insight and spread of the parameter in question. It is an illusion to assume that there are models that encompass the whole.

It cannot be emphasized enough that factual knowledge of and experience with the all of the (sub) processes is crucial and therefore leading in the analysis of these systems and their desired as well as disruptive behaviour or appearance.

Evaluation of the effect of the many nourishment practices imposed so far does show that a great deal of work still needs to be done on this point in order to (be able to) be effective, sustainable and financially justifiable. It is therefore recommended to 1) evaluate the current nourishment program as a whole, 2) adjust those parts that clearly need improvement and (last but certainly not least) 3) suspend the program as a whole until these conditions are met.