Beach nourishment behaviour Modeling of beach nourishment planform evolution: a case study of the coast of North Zealand

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1 Institut for Geovidenskab og Naturforvaltning Beach nourishment behaviour Modeling of beach nourishment planform evolution: a case study of the coast of North Zealand Serena Pilunnguaq Sørensen & Sofie Kamille Astrup Geografi & Geoinformatik, Københavns Universitet IGN,

2 Project background and inspiration Modeling of coastline evolution Collaboration with DHI Beach Nourishment Schematized nourishments North Zealand

3 Project background and inspiration Pilot project: carried out in millioner m3 sand, approximatly 20 years of sedimentloss Protecting a long stretch of coastline Recreational value

4 Project description 1) Investigation of beach nourishment planform evolution and transport patterns in idealized and constant conditions The effect of nourishment design One or more nourishment sites The effect of wave incidence wave angle 2) How would a beach nourishment evolve at the coast of North Zealand? Establishing a local sediment budget Simulation of local wave climate Beach nourishments and preexisting coastal structures 3) Evaluation and migration of beach Nourishments Evaluation of LITTLINE results Downstream migration of beach nourishments Comparison with observations from the real world

5 Evolution of schematized nourisment scenarios All nourishment scenarios over time converge towards a bellshape. All nourishment scenarios spread out symmetrically independently of varying the incoming wave angle within a range of 20 degrees. Rectangular nourishment designs increase the nourishments spatial stability when compared to the bellshaped nourishments. The lifespan of a beach nourishment increases with initial longshore width. The decay in nourishment crossshore width is exponential. A1 S1

6 Gradients in transportcapacity A strong diffusional phase is identified within the first year of wave action The gradient in longshore sediment transport is higher when applying waves with incidence angles of 10 degrees, and this causes the most rapid diffusion of the nourishment

7 Gradients in transportcapacity The coastline generally suffers from sediment starvation The coast is subject to chronic erosion; average shoreline recession rate for the past 60 years was determined to be ~0.16 m y -1. The erosion rate increases gradually from zero at Hundested to ~ m 3 y -1 at Gilleleje. Coastal protection measures halters erosion in some areas but in turn new erosional hotspots are created downdrift. The storm event in 2013, Bodil, eroded large amounts of sediment from the cliffs and dunes, which led to beach widening.

8 Local erosionsrates Tisvildeleje Coastline retreat: 0.1 m/yr Erosions rate: 2000 m3/yr Raageleje Coastline retreat: 0.2 m/yr Erosion rate: 5200 m3/yr Udsholt Strand With the perturbation: Coastline advance: 0.02 m/yr Deposition: 2900 m3/yr Without the perturbation: Coastline retreat: 0.3 m/yr Erosion rate: 2600 m3/yr

9 Simulation of nearshore wave climate using MIKE21 SW Tisvildeleje Raageleje Udsholt Strand Offshore Nearshore The disequilibrium in shoreline orientation at Udsholt Strand produced high potential littoral drift rates. The model was highly sensitive to changes in local shoreline orientation. yearly longshore sediment transport rates with waves extracted at 9m depth for bølger was used for further analysis Tisvildeleje: m3/yr - Raageleje: m3/yr - Udsholt Strand: m3/yr

10 Simulation of coastline evolution Tisvildeleje m3, 32 years of sedimentloss m3, 64 års years of sedimentloss

11 Simulation of coastline evolution Rågeleje m3, 13 years of sedimentloss m3, 26 years of sedimentloss

12 Simulation of coastline evolution Udsholt strand m3, 22 years of sedimentloss m3, 44 years of sedimentloss

13 Migration of beach nourishments on a sediment starved coast?

14 Downdrift migration of the Sandengine