Dune growth on natural and nourished beaches: A new perspective

Transcription

1 Dune growth on natural and nourished beaches: A new perspective Themadag duinaangroei Thijs Damsma 28 May 2009

2 Supervising Committee Prof. dr. ir. M.J.F. Stive dr. ir. S.G.J. Aarninkhof dr. ir. M. Van Koningsveld ir. D.J.R. Walstra ir. S. de Vries drs. N. Geleynse TU Delft Boskalis Van Oord & TU Delft Deltares & TU Delft TU Delft TU Delft 2

3 Presentation Introduction & Objectives State of the art Literature Modelling capabilities Data analysis Several techniques Model Conclusion & Recommendations 3

4 Hoek van Holland Den Haag 4

5 Big impact, both for nature and people 5

6 Sand engine One large, concentrated nourishment (time and space) Let natural processes redistribute the sediment: Building with Nature Expected dune growth is one of the design criteria 6

7 Objective Determine if and how a major (subaqueous) coastal intervention will influence the dune system. How will the coastal dunes develop in the ten years after the sand engine plan is executed? 7

8 State of the art 8

9 Study of dunes Coastal engineers Sedimentologists Geologists Ecologists (Safety) (Individual sand grains) (Very large scales) (Natural value) The continuous cycle of erosion & recovery in the coastal dunes is largely unstudied 9

10 What do we know? During fair weather dunes accrete (aeolian processes) 10

11 What do we know? During storms dunes erode (hydrodynamic processes) 11

12 What do we know? Dune evolution is dictated by the balance between the erosive and accretive processes Wide beach: Increase of aeolian deposition (+ dunes) Reduction of hydrodynamic erosion (+ dunes) Narrow beach: Reduction of aeolian deposition (- dunes) Increase of hydrodynamic erosion (- dunes) At a certain beach width an equilibrium is reached 12

13 Roelvink relation Direct link between beachwidth and dunefoot migration. Equilibrium beach width Dunefoot movement (m /year) Beach width (m) Derived by averaging long term data (150 yr) Derived for whole Holland coast (118 km) 13

14 Alongshore Length scale Limits of predictability 100 km 10 km Qualitative aeolian modelling Sand engine Roelvink relation 1 km 100 m Quantitative hydraulic modelling Qualitative hydraulic modelling Profile Instantaneous Storm Month Year Decade Century Millennium Time Scale 14

15 Relative importance wind and water Yearly measurements indicate that at wider beaches, more sand is deposited in the dunes. Conclusion in literature: the relation between beach width and dune growth is caused primarily by the aeolian processes 15

16 Implications for dune predictions We must understand aeolian processes to predict dune evolution (but we don t) Roelvink s empirical relation is the only applicable relation we have (but it is derived for a very long timescale) 16

17 Data research 17

18 Data research Objectives Test Roelvink s relation for smaller timescales Also look at erosion component 18

19 Data availability Dunefoot Mean high waterline Mean low waterline Dutch Beach Lines (1843 present) Only positions of dunefoot, mean high water and mean low water positions One measurement per year, per km JARKUS (1963 present) Complete profile measurement One measurement per year, per 250m 19

20 Data visualization Numbers are just numbers... Until: GoogleEarth 20

21 Linear trends Coastline Beach width Dunefoot 21

22 Linear trends 165 years 55 years 15 years 5 years 22

23 Linear trends 165 years 55 years 15 years 5 years 23

24 Spatial and temporal patterns 2007 Relative beach width 1965 wide beach average beach narrow beach 2007 Relative dune erosion/accretion 1965 dune growth average dune development dune erosion 24

25 Erosion calculation Dune erosion Outcome depends on storm parameters and on shape initial profile 25

26 Susceptibility to erosion Dune erosion calculated with model DUROSTA For every cross-shore transect, every year Same storm parameters, different initial profile Relative outcome is a measure for the susceptibility to erosion 26

27 Spatial and temporal patterns 2007 Relative beach width 1965 wide beach average beach narrow beach Relative dune erosion/accretion Relative susceptibility to erosion 1965 dune growth average dune dune erosion 1965 development less susceptible average more susceptible 27

28 Spatial and temporal patterns Apparently it is not variability in rates of dune accretion, but variability in rates of dune erosion that determine dune evolution. A storm forces a profile into shape 28

29 Empirical evidence Monthly dataset of Noordwijk (Quartel, 2008) - Constant growth, irregular erosion Dunefoot position Wave height Water level 29

30 Conclusions data research Variability in dunefoot migration rates is determined by variability in the erosive (hydrodynamic) processes, not by variability in the accretive (aeolian) processes. Yearly variability in storms explains why Roelvink s relation does not hold for short terms, only for long terms. Variations in rates of aeolian deposition in literature can be also be explained by variations in erosion because of yearly measurements. 30

31 Model 31

32 Model setup Main assumption: Aeolian transport is constant 2D Profile Assumption of alongshore uniformity Not good enough for sand engine, but provides starting point Modular setup 32

33 Modules Is there a storm? yes Dune erosion no Dune accretion Underwater profile Beach Time step = Time step+1 Dune erosion (DUROSTA) Dune accretion (Constant process) (a) (b) (c) (d) Foreshore evolution (Fixed shape) Beach evolution (Rubber band) 33

34 Demonstration 1

35 Demonstration 1 Just before storm

36 Demonstration 1 After storm

37 Demonstration 1 Dune grows

38 Demonstration 1 Dune grows more

39 Demonstration 1 Storm; very steep profile

40 Demonstration 1 Profile is smoothed

41 Demonstration 2 Two profiles

42 Demonstration 2 One profile is nourished Foreshore moves seaward Wider beach

43 Demonstration 2 No storms, identical dune evolution

44 Demonstration 2 After a storm, there is a difference

45 Demonstration 2 After a few storms Difference in beach width smaller

46 Conclusie Variabiliteit in duinaangroei wordt gedomineerd door de (hydrodynamische) afslag, en niet door de (eolische) aangroei 46

47 Conclusion 47

48 Conclusion Variability in dunefoot migration rates is determined by variability in the erosive (hydrodynamic) processes, not by variability in the accretive (aeolian) processes. Own data study Quartel dataset Explanation for Roelvink relation Not conflicting with literature Successful modelling possible 48

49 Recommendations For the sand engine: Compare design by erosion reduction Future measurements: Measure right before and after storms Model: Validate model with real data Refine assumptions Include alongshore transport 49

50 Dune growth on natural and nourished beaches: A new perspective M.Sc. Thesis presentation Thijs Damsma 19 May 2009