The northern test area also reached the highest beach volume ever measured at the end of the test period (2011). Previously the nourished beach sand in the northern test area eroded rapidly. The northern test area was nourished 1.5 years before the test started. The total volume of sand in the 3 km test area was larger in 2011, at the end of the test, than measured In 2006, after the nourishment.
Yes, Deltares defined a 3 km long reference area south of the southern test are
The beach volume in the reference area also started to grow after installation of the Ecobeach system.
At the same time that the beach nourishment at Egmond was carried out, also a beach nourishment of 0.5 million m3 was carried out at Castricum, just south of the intended reference area. The intended reference area was wedged in between this beach nourishment and the southern test area Due to the northward littoral drift, the sand of this nourishment entered the intended reference area, but was held up by the reduced longshore drift in the southern test area
Along the Dutch coast there are no undisturbed areas. All parts of the coast are affected by human actions. Also adjacent coastal effects do influence each other. You cannot interfere in one part of the coast and then say that the adjacent part of the coast is not affected by your actions.
The vertical drains increase the vertical permeability of the beach. Therefore near the drains the beach dries faster after wave run-up and during falling water (ebb). The vertical drains also penetrate less permeable silt layers which have been deposited by the sea and connect more permeable shell deposits. As the beach dries faster, onshore wind transport of finer grains is enhanced. Therefore the net onshore sand transport is increased. Fine sand are blown towards the dune and the sand on the beach eventually becomes coarser. Coarser sand has been found in the southern test area at Egmond and at two test areas at Hvide Sande in Denmark. Coarser sand is more permeable and also more stable. At Egmond only the upper 2 m, the active zone, had become coarser. After removal of the Ecobeach system, the beach has gone back to the original grain diameter. Also outflow of fresh water has been observed through a drain near the low water line. Therefore also the interaction between salt and fresh water may play a role.
The change in grain size at the northern test area was not significant. The drains at the northern test area were placed in a beach nourishment. The nourished sand was rather uniform and came from the seabed. Therefore there was no coarse fraction available. Also regular beach combing has taken place at the northern test area during the test to clean the beach. At the southern test area, coarser sand was available between the sand bars.
After removal of the drains, the accreted sand in the southern test area (and intended reference area) was rapidly eroded by the sea. The grain size in the southern test area has gone back to normal. In the northern test area a new nourishment was carried out just after the end of the test, early 2011.
The beach nourishment was already planned a few years earlier. Also in Dutch law it is given that we have to nourish the Dutch beaches with 12 million m3/year.
We think the Ecobeach system acts as a trigger, which slightly changes natural processes, such as increasing net onshore sand transport and affecting ground water flow. However, when the drains are removed, the effect also stops. So the presence of the drains is essential. The natural processes such as tide, waves, wind and sediment transport and deposition change the beach back to its original situation.
Yes, we are presently doing research with the University of Amsterdam on grain sizes in the Ecobeach test areas in the Netherlands and Denmark. We want to reinstall the Ecobeach system shortly in another beach area in the Netherlands. We think the system has great potential and can also be applied to increase the lifetime of beach nourishments.