iahr2015 - transition structures in grass covered slopes of primary flood defences, steeg, deltares,...
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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)
• 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
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
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
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
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
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
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
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
Test set-up and methodology
Oo.3 Ol.2 Oo.7 IAHR 28 June – 3 July 2015, The Hague, the Netherlands
Test set up and methodology
IAHR 28 June – 3 July 2015, The Hague, the Netherlands
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
Example of results
transition
No transition (= ref. test)
IAHR 28 June – 3 July 2015, The Hague, the Netherlands
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
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
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
Acknowledgements
IAHR 28 June – 3 July 2015, The Hague, the Netherlands