Graduation of Tymen Olthoff
14 January 2019 10:00 - Location: Lecture Hall F, Faculty Civil Engineering and Geosciences - By: Webredactie
"Longshore transport of coarse-grained material: An assessment of the longshore transport behavior of the dynamic rock slope at Maasvlakte 2." | Professor of graduation: Prof. dr. ir. S. G. J. Aarninkhof, supervisors: Dr. ir. B. Hofland (TU Delft), Ir. G. M. Smith (Van Oord / TU Delft), Ir. D. van Kester (Van Oord).
The consortium Projectorganisatie UitbreidingMaasvlakte (PUMA), consisting out of Van Oord and Boskalis, constructed a hard sea defence for the protection ofMaasvlakte 2, which is the expansion of the Port of Rotterdam.
This hard sea defence consists of a Dynamic Rock Slope (DRS) with a Block Dam (BD) situated in front of it. The BD is build up out of large concrete blocks which were recycled from Maasvlakte 1. The Dynamic Rock Slope behind it protects the hinterland through an approximate three to four meter thick layer of quarry rock. When waves travel obliquely through this Block Dam and interact with the Dynamic Rock Slope, stones are transported in alongshore direction. This process is called Longshore Transport (LT) and will have erosion and accretion of stones to effect along the DRS. In order to maintain the layer thickness of this stones PUMA made a prognosis on Longshore Transport for the coming 50 years after construction. From this prognosis a nourishment campaign was set-up, consisting of volumes of stone PUMA expected to nourish every 2.5 years.
Goal of this research
After a maintenance period of five years it was concluded by PUMA that the nourishment volumes were lower than expected. Therefore Van Oord asked to assess where differences in the prognosis of PUMA on Longshore Transport and the observed Longshore Transport rates over the period 2013 to 2018 come from. Within their prognosis, PUMA made use of a dimensional Longshore Transport relation. As this specifically calibrated Longshore Transport relation is only applicable for the computation of LT rates at a structure containing the characteristics of the DRS at Maasvlakte 2, this research looked into the possibilities of using a non-dimensional Longshore Transport relation for the computation of Longshore Transport rates at the Dynamic Rock Slope ofMaasvlakte 2. Would such an equation be applicable to the case ofMaasvlakte, it would subsequently be usable when designing a new Dynamic Rock Slope, containing different characteristics than those of theMaasvlakte 2.
Two Longshore Transport Relations
In order to calibrate the dimensional Longshore Transport relation of PUMA, HR Wallingford executed scale model tests on Longshore Transport, in assignment ofPUMA(HRWallingford, 2007a)(HRWallingford, 2007b) (HRWallingford, 2009c)(HRWallingford, 2009d). By means of these scale model tests it was shown in this research the non-dimensional General Longshore Transport (GLT) relation of Tomasicchio et al. (2016) is able to predict Longshore Transport rates following from these tests. As the GLT relation showed promising results regarding the prediction of Longshore Transport at a Dynamic Rock Slope for these scale model tests, this relation and the PUMA relation, were both taken into the assessment of the Longshore Transport behavior of the Dynamic Rock Slope atMaasvlakte 2.
Computing Longshore Transport rates that followfrom the two Longshore Transport relations over the period 2013 to 2018 required a calculation model. This calculationmodel made use of the wave data following from a SWAN run. The input wave data was taken at the Europlatform, situated approximately 50 kilometers offshore of Maasvlakte 2. The output of the SWAN run consisted of a representative wave climate for every 100 meters along the DRS, situated at the toe of the DRS. Consecutively, the calculation model altered the three main forcing parameters, wave height, wave direction and peak period, as a result of the limiting water depth and interaction with the Block Dam. In this way the transport rates following from the wave climate over the period 2013 to 2018 were computed. All the corresponding characteristics regarding the specific case of the Dynamic Rock Slope atMaasvlakte 2 were taken into account.
Next to the sole computation of wave data over the period 2013 to 2018, survey data over this period was analysed.
This survey data shows the actual occurred changes in volumes of stone along the interest area of the Dynamic Rock Slope atMaasvlakte. These displaced volumes over four different survey periods represent the amounts of erosion or accretion a section of 100 meters along the interest area of the DRS experienced over the corresponding survey periods. By comparison of the displaced volumes to the computation LongshoreTransport following from both LT relations, the predictive abilities of both relations was assessed. In other words, a validation was carried out on both the PUMA and the GLT relation.
Results PUMA relation
Following from computations using the wave climate of 2013 to 2018, it was seen this wave climate produces 32% less Longshore Transport over the interest area along the Dynamic Rock Slope at Maasvlakte 2, with respect
to the prognosis of PUMA. For this prognosis PUMA used the wave climate of 1979 to 2005. The cause for differences in wave climate, being the first reason for the lower computed transport rates, are twofold. Firstly it was seen the wave climate over the period 2013 to 2018 solely consisted of 1:5 year storm events, making this period a relatively mild wave climate, compared with the design storm on which the DRS was designed. Next to the mild wave climate a difference in wave climate is present resulting from a difference in refraction due to the morphodynamically active foreshore of Maasvlakte 2.
The second reason for the lower computed transport rates over the period 2013 to 2018 - and which are therefore
also a reason for the lower observed transport rates - is the higher placement of the Block Dam. Computed transport rates showed Longshore Transport would approximately be 2 times as high, compared to the transport rates which are currently present.
Following from the validation of the PUMA relation, it was concluded the PUMA relation was able to compute the areas over which erosion or accretion will occur. With respect to the accuracy of the PUMA relation an approximate accuracy in the order of §2 orders of magnitude was obtained. Meaning a computed volume of displaced stones over a 100 meters section can be twice as high or two times as small as the occurred stone displacement. The validation again showed the differences in prognosis and currently observed Longshore Transport at the DRS are circumstantial and are not due to the predictive abilities of the PUMA relation.
Results General Longshore Transport (GLT) relation
The General Longshore Transport (GLT) relation of Tomasicchio et al. (2016) appeared to compute approximately
similar amount of transport rates over the period 2013 to 2018, compared to the PUMA relation. On average the GLT relation computed 9% less transport over this period. Thereby it was concluded the nondimensional GLT relation is applicable for the case of the Dynamic Rock Slope atMaasvlakte 2. As the GLT relation computes approximately similar amounts of Longshore Transport with respect to the PUMA relation, the accuracy in predicting amounts of erosion or accretion over the different survey periods appeared to be similar as to accuracy of the PUMA relation. The accuracy over the period 2013 to 2018 therefore again amounted to §2 orders of magnitude. Thereby the GLT relation and the PUMA relation were equally validated within this research, keeping in mind this validation holds for the specific case of the Dynamic Rock Slope atMaasvlakte 2 and the period of 2013 to 2018.
Resulting from this research, multiple recommendations regarding the Longshore Transport behavior of the Dynamic Rock Slope at Maasvlakte 2 were given. Firstly it is recommended to acquire more field data on actual Longshore Transport processes behind the Block Dam. Such field data would give more insight into the processes leading to Longshore Transport and could consequently be used in an update of the maintenance plan of the DRS. Secondly, this more specific knowledge on the Longshore Transport process at the DRS, following fromfield data, could contribute in quantifying the effect of the changing foreshore on Longshore Transport. Lastly, it is recommended that with regard to the current maintenance plan, a pragmatic approach should be maintained. This research showed stormevents have a severe influence on the displacement of rock following fromsuch an event. One must be aware intensive maintenance will be required, would a severe stormoccur.
For further research on the topic of Longshore Transport of coarse-grained material it is recommended more field and scale model test data is required for the calibration of the General Longshore Transport relation of Tomasicchio et al. (2016), for the specific case of a Dynamic Rock Slope. It is important to investigate the performance of the GLT relation - and other Longshore Transport relations - when applying it on a case of a DRS which does not include a Block Dam and which preferably holds different characteristics, e.g. slope, orientation and stone size.