DETERMINING THE CRITICAL VELOCITY OF GRASS SODS FOR WAVE OVERTOPPING BY A GRASS PULLING DEVICE
Authors: Bijlard Roel , Steendam Gosse , Verhagen Henk , Van der Meer Jentsje 
Published: June 2017

DETERMINING THE CRITICAL VELOCITY OF GRASS SODS FOR
WAVE OVERTOPPING BY A GRASS PULLING DEVICE
Roel Bijlard, Delft University of Technology, roelbijlard@gmail.com
Gosse Jan Steendam, INFRAM International, gosse.jan.steendam@infram.nl
Henk Jan Verhagen, Delft University of Technology, h.j.verhagen@tudelft.nl
Jentsje van der Meer, Van der Meer Consulting bv, jm@vandermeerconsulting.nl

INTRODUCTION
There is a shift in the approach for designing coastal
structures in the Netherlands, such as dikes or levees.
In the past dikes were designed on the probability of
exceedance of the water level during specific incoming
(wave) storm conditions. In the near future the design
criterion will be the probability of flooding of the
hinterland. In order to determine this flood probability,
the strength of the dike has to be known at which
failure occurs. During extreme storm conditions waves
will overtop the crest which can lead to erosion of the
grass sod on the landward slope. This can finally result
in instability of the dike and flooding of the hinterland.
Past research focused on the erosion of the grass sod
during different wave overtopping conditions, see
Steendam 2014. The last few years many tests have
been performed with the Wave Overtopping Simulator.
During these tests the Cumulative Hydraulic Overload
Method has been developed, see Van der Meer 2010
and Steendam 2014. With this method an estimation
of the critical velocity of the grass sod has to be made.
The critical velocity is a strength parameter for a grass
sod on a dike during loads induced by overtopping
wave volumes.

SOD PULLING TESTS
For safety assessments it would be beneficial if there
is also an easier way to determine the critical velocity
of the grass sod. However, it is important to measure
the actual strength of the grass cover, so a visual
inspection cannot be satisfactory. The sod pulling
test is developed in order to investigate the
resistance of the grass cover. It lifts the grass sod
perpendicular to the slope out of the sod and
measures the force as a function of the deformation.
In order to lift the sod, a pull frame is anchored into
the top layer with pins. This frame then is lifted out of
the grass sod by a hydraulic cylinder.
In order to insert the pins into the sod, the soil has to
be excavated on two sides (condition 2 test) or on all
4 sides (condition 4 test). This has the disadvantage
that the strength of an intact sod cannot be
measured directly. So a methodology is developed to
estimate the strength of an intact grass sod from the
measured data. A further introduction on the sod
pulling tests is given in Steendam 2014. The goal is
to rewrite the measured forces from the sod pulling
test into a critical velocity so that the Cumulative
Hydraulic Overload Method can be used for
determining the flooding probability of a dike.
Some of the locations tested with the wave
overtopping simulator have also been tested for the
strength of the grass cover with the sod pulling tests.
The two methods use the same failure mechanism of
the grass, erosion of the grass sod.
The top layer of a dike consists of soil and roots
growing in multiple directions. The roots anchor the
grass into the soil and can deform centimeters
without tearing. Pressures acting on the grass cover
will first break the weakest roots, but the forces will
be redistributed to other roots. Only when a critical
amount of roots are broken, the redistribution stops
and the grass cover will fail.

CONCLUSION
It is possible to rewrite the measured forces with the
sod pulling tests into a critical grass normal stress
(Ïƒgrass.c), which is one of the input parameters for
determining the critical velocity of a grass sod, see
Hoffmans 2012.

The equation also uses the pore water pressure (pw),
the relative turbulence intensity (r0) and the density of
the water (Ï). When the critical velocity resulting from
this equation is compared with the determined critical
velocity during the wave overtopping simulations,
there is good correspondence between the values for
the five tested locations. So the sod
pulling test could provide results that are reliable
enough to determine the critical velocity of a dike
section. Further elaboration and scientific background
will follow in the paper after the conference.

REFERENCES
Hoffmans (2012): The influence of turbulence on soil
erosion. Eburon, Delft.
Steendam, van Hoven, van der Meer, Hoffmans
(2014): Wave Overtopping Simulator tests on
transitions and obstacles at grass covered slopes of
dikes, proc. ICCE 2014 Seoul.
Van der Meer, Hardeman, Steendam, SchÃ¼ttrumpf,
Verheij (2010): Flow depths and velocities at crest
and inner slope of a dike, in theory and with the Wave
Overtopping Simulator, Proc. ICCE 2010, Shanghai.