NederlandsVlaams Onderzoekscentrum
voor Kustconstructies 

Research lines
Stability
of concrete elements
In the present stability formulae for concrete elements the stability is calculated using a method based on armour stone. The additional friction because concrete elements hook into each other is added as a surcharge on the weight. This is fundamentally incorrect. For gentle shorelines the weight is determining, but for steep slopes the friction resistance. However, the friction also depends on the weight of the blocks above the block considered. So a stability formula is needed where apart from the weight also the friction and the force due to block on top are included. This force is a function of the block weight, the slope angle, the number of blocks and the friction between blocks and the filter layer.
For cubetype of elements (normal cubes or Antifer) pattern placing leads to much higher stability. Research by Robin van Buchem has shown that the constant in the Van der Meer stability formula is indeed a function of the packing density.
In order to understand this process one needs to have a good value of the friction. This work has been done by Marco de Lange.
Phases of the research:
Relevante Nevlock studies:
Zwanenburg 
2012 
The
influence of the wave height distribution on the stability of
single layer concrete armour units 
Van de Koppel  2012  Static and dynamic loads on the first row of interlocking, single layer armour units 
Van der Linde  2009  Stability of single layer armour units on low crestedbreakwaters 
De Lange, M  2010  Extraction Force Xbloc: Model Tests 
Van Buchem, R.V.  2009  Stability of a single top layer of cubes 
Van Zwicht, B.N.M.  2009  Effect of the concrete density on the stability of Xbloc armour units 
De Rover, R.A.  2007  Breakwater stability with damaged single layer armour units 
Toe structures
The stability of toe structures with various types
of foreshores is still a point of discussion. Worldwide there is no
agreement on the way of computing the stability, especially in case of
a "nonstandard" foreshore. in the past basic research on this matter
has been done by Gerding and by Docters van Leeuwen. The data of
Doctors van Leeuwen did not fit well into the whole scheme, and are
therefore disregarded by many authors. However, a good design method
should be as such that also her data fit into the scheme. Later Van
der Meer adapted the Gerding formula and extended it for shallow
water. This extension has been confirmed by the tests of Ebbens. Also
Baart and Dijkstra did work on toe stability. However, they all used a
"blackbox" approach, i.e. the stability of a given type of toe
depends on wave height and wave period in front of the structure.
However Baart showed that a much better approach is to determine the
hydraulic boundary conditions at the toe (i.e.using an appropriate
numerical model) and then determine the stone stability using an
appropriate stone stability formula (i.e. an Izbash type formula, but
including effects of turbulence and pressure gradients). Hofland and
Hoan have given some ideas for the development of such formulas,
however their work focussed on permanent flow, while under aa wave we
have an oscillating flow.
The measurements of NammuniKrohn show that the linear wave theory is not too bad for predicting the flow velocities. However some correction is needed. The following steps are foreseen to develop a new toe formula:
Relevant Nevlock studies:
NammuniKrohn, J.  2009  Flow velocity at rubble mound breakwaters 
Ebbens, R.E.  2009  Toe structures of rubble mound breakwater: stability in depth limited conditions 
Dijkstra, O.P.J.  2008  Armour layer stability on a bermed slope breakwater 
Baart, S.A.  2008  Toe structure for rubble mound breakwaters: analysis of toe bund design tools and a method for toe rock stability description 
Hoan, N.T.  2008  Stone stability under nonuniform flow 
Hofland, B,  2005  Rock and roll : turbulenceinduced damage to granular bed protections 
Van den Berk, M.  1999  Stability of toe and slope structures of rubble mound breakwaters 
Docters van Leeuwen, L.  1996  Toe stability of rubblemound breakwaters 
Gerding, E.  1993  Toe structure stability of rubble mound breakwaters 
Relevant Nevlock studies:
Hovestad, M.  2005  Breakwaters on steep foreshores 
Relevant Nevlock studies:
Dijkstra,
O.P.J
2008
Armour
layer stability on a bermed slope breakwater
Relevant other studies:
Vermeer,
A.C.M.
1986
Stability
of rubble mound berms and toe constructions.
Report on literature survey and model investigation
[in Dutch]. Report no M2006, WLDelft Hydraulics,
Delft.
Relevant Nevlock studies:
Martinez Pez  2013  Validation of Swash for wave overtopping 
Oosterloo  2013  Influence of very oblique waves on wave overtopping 
Suzuki et al.  2012  A numerical study on the effect of beach nourishment on wave overtopping in shallow foreshores 
Krom  2012  Wave overtopping at rubble mound breakwaters with a nonreshaping berm 
Lioutas  2012  Spatial distribution of overtopping 
Kester, D van  2009  Spatial distribution of wave overtopping 
Dijk, B. van  2001  The rear slope stability of rubble mound breakwaters 
Bosman, G.  2009  Velocity and flow depth variations during wave overtopping 
Relevant other studies:
Van
Gent, M.
2003
Wave
overtopping events at dikes. Proc 28th int
conf coastal engg, Cardiff, 7–12 Jul 2002. World
Scientific, Singapore
Van
Gent, M. and Puzueta, B.
2005
Rearside
stability of rubble mound structures. Proc int
conf coastal engg, Lisbon, 19–24 Sep 2004. World
Scientific, Singapore
Papadopoulos  2012  Scour below the toe of breakwaters 
Van de Sande  2012  Stability of open filter structures 
Caus  1998  Imperfecte
filterconstructies onder golfbrekers 
Booij  1998  Erosie onder een geometrisch open filter 
Verheij et al  2012  Interface stability of granular material under currents 
CUR 233 (Verheij & Hoffmans)  2011  Interface stability of granular filters 
Relevante Nevlock studies:
Jumelet, D  2010  The influence of core permeability on armour layer stability 
Vilaplana Domingo, A.  2010  Evaluation of the volumeexchange model with Van der Meer laboratory tests results 
Van Broekhoven, P.J.M.  2011  The influence of armour layer and core permeability on the wave runup 
Kik, R.  2011  The notional permeability of breakwaters: experimental research on the permeability factor P 
Kik et.al.  2012  Notional Permeability 
Kluwen, J.  2012  Physical model tests of the notional permeability on breakwaters 
Mellink, B.A.  2013  Numerical and experimental research of wave interaction with a porous breakwater 
Fases in het onderzoek:
Relevante Nevlock studies:
Van Veldhoven, V.J.C.G.L.  2002  Vooronderzoek schroefstraal Op een talud met breuksteen: stroomsnelheden en steenstabiliteit 
Schokking, L.A.  2002  Bowthrusterinduced Damage 
Nielsen, B.C.  2005  BowthrusterInduced Damage: A physical model study on bowthrusterinduced flow 
Van Blaaderen, E.A.  2006  Modelling bowthruster induced flow near a quaywall 
Van der Laan, T  2005  Het ontwikkelen van een model voor boegschroefstralen bij verticale kademuren 
Van Doorn, R.  2012  Bow Thruster Currents at Open Quay Constructions on Piles 
Van den Brink. A.J.W.  2014  Modelling scour depth at quay walls due to thrusters 
de Jong, J.  2014  Modeling thrusters with openFoam 
Roelse, F.  2014  Stability of slope material affected by bow thrusters at open quay structures 
Transport ot stones by currents and waves
The formulas of Shields and Izbash give an indication for the stability of stone in normal current. Jongeling, Hofland and Hoan have extended these formulas for application in case of an arbitrary turbulence. Also a change has been made to describe the stability instead of number of stones displaced by the entrainment rate. This allows to describe the transport of stone quantities also with this parmeter, so effectively an improvement of the Paintal formula
The mentioned equations are valid
for (turbulent) flow. The effect of waves has not been
included. For nonbreaking waves the formula of Sleath is
used, but also this formula has limitations. Some research
ahas been done by Terrile and Tromp, but this has not yet lead
to a practical design formula. new research on near bed
structures van Van de Heuvel has improved our knowledge, but
there are still questions related to scaling factors for the
physical model tests applied.
Also for the stability
of stones in high turbulent, high velocity currents (e.g. propeller jets) this is a promising approach, but no
practical solution exists yet.
Phases of the research:
Relevante Nevlock studies:
The DutchFlemish Centre
for Coastal Structures Research is a cooperation
between Delft University of Technology, Ghent
University and the contractors Van Oord, Royal
Boskalis Westiminster, BAM and Deme. The goal is to
initiate joint research in the field of construction
for coastal protection, like breakwaters, bed
protection, dike revetments, etc. For information:
mail@nevlock.nl.
Postal address: TUDelftSectie Waterbouwkunde  Postbus 5048  NL2600GA Delft  Netherlands 