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.