The document summarizes research on testing transitions in grass-covered slopes of flood defenses using a wave impact generator. It describes: (1) The development and validation of the wave impact generator to simulate wave impacts; (2) Tests on 14 sections of 4 dikes, including 5 reference sections and 9 sections with transitions or objects; (3) Preliminary results showing significantly more erosion around transitions than references, likely due to secondary effects like maintenance difficulties and changes to soil composition rather than primary hydraulic effects.

1. IAHR 28 June – 3 July 2015, The Hague, the Netherlands
Transition structures in grass covered slopes
of primary flood defences tested with the wave
impact generator
Paul van Steeg (Deltares)
Astrid Labrujere (Rijkswaterstaat)
Roy Mom (Infram)
Guido Wolters (Deltares, presentation)

2. • Introduction
• Transitions in grass covers
• The wave impact generator
• Test set up and methodology
• Results and discussion
• Conclusions
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

3. Introduction
• Research programme WTI-2017 regarding safety of dikes
• Focus on river dikes
• This presentation: transitions in wave impact zone
• (2. Outer slope) Wave impact zone: this presentation
• (3. Outer slope) Wave run-up / run down zone
• (4. Crest) Overtopping zone
• (5. Inner slope) Overtopping zone
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

4. Introduction: transition structures
- Many types of transitions in grass slopes
- Geometric (e.g. sharp bend at toe or berm)
- Change between type of revetment (e.g. grass-asphalt)
- Objects (e.g. poles, trees, stairs, NWO’s)
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

5. Introduction
To study grass and transitions under wave impact loads a wave
impact generator is developed
Project subdivided in four parts:
- Phase 1: Development of wave impact generator
- Phase 2: Validation of wave impact generator in Delta Flume
- Phase 3: Testing with wave impact generator in the field
- Including transition structures
Focus of this presentation
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

6. Introduction: the wave impact generator
Based on the ‘wave overtopping simulator’ a Wave Impact Generator
was developed.
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

7. Introduction: the wave impact generator
• Testing five alternative wave impact generators on slope equipped
with pressure sensors
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

8. Introduction: the wave impact generator
Wave impact generator gives hydraulic loads that represents wave
impacts of irregular waves with Hs ≈ 0.6 – 0.7 m (based on pressure
sensors)
Van Steeg, P., Klein Breteler, M., Labrujere, A., ‘Design of wave impact generator to test stability of grass slopes
under wave attack’, ComCoast, Varna, Bulgaria, 2014
Validation of erosion rate: comparison of wave impact generator with
full scale tests in the Deltares Delta Flume
Van Steeg, P., Klein Breteler, M., Labrujere, A., Wave impact generator and wave flume tests to determine strength
of grass on dikes under wave loads, ICCE, Seoul, South Korea, 2014
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

9. Test set-up and methodology
Wave impact generator is applied on
- 4 dikes (Harculo, Oosterbierum, Olst, Berkum)
- 14 test sections
- 5 reference test sections (no transition)
- 9 tests with transitions or objects
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

10. Test set-up and methodology
Oo.3 Ol.2Oo.7
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

11. Test set up and methodology
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

12. Test set up and methodology
- Up to 5800 impacts are released per test
- (equivalent to storm of approx. 14 hours)
- Erosion was measured during the tests
- Erosion is compared to reference test (no transition but same
grass/ clay conditions).
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

13. Example of results
transition
No transition
(= ref. test)
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

14. Results and discussion
• In general the erosion of the grass around the transition was
significantly more than the erosion at the reference test (without
transition)
• Reason are in almost all tested cases due to so-called secondary
effects which are explained next.
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

15. Results and discussion
Primary effects are:
- Lower stability due to interruption of grass sod
- Higher hydraulic loads due to local acceleration, jets etc.
Secundary effects are indirect effects due to:
- Maintenance (mowing is more difficult)
- Construction (sand instead of clay)
- Other effects
- Small animals around transitions
- Shadow (leading to lower grass quality)
- Large animals (sheep) walking along transition
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

16. Conclusions
• Transitions are potentially weak parts of a grass cover as
illustrated with the wave impact generator.
• In almost all cases secondary effects of the transitions caused the
lower stability of the grass cover.
• Therefore secondary effects should be minimized in the design of
transitions of grass dikes and should be compensated with
mitigating measures.
• It is recommended to include transitions in the design,
constructions, assessment and maintenance of grass dikes.
IAHR 28 June – 3 July 2015, The Hague, the Netherlands

17. Acknowledgements
IAHR 28 June – 3 July 2015, The Hague, the Netherlands