Restoration of a boulder reef in Kattegat a numerical study of the design parameters and impact on sediment transport

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1 Restoration of a boulder reef in Kattegat a numerical study of the design parameters and impact on sediment transport S. Niemann, K. Bundgaard, et. al. DHI Water Environment Health, Denmark Boulder reef restoration project. Recreation of natural habitat for fish, lobsters and other crustaceans. Long term stability of the reef ensured through numerical modelling of design parameters. Impacts on the nearby and local morphology of the seabed determined through numerical modelling of sediment transport. Sustainable solution reached. Dredging site in Kattegat, Denmark

2 Background Reefs in shallow water is a rare type of reef in Denmark only 0.01 % is left. This reef was subject of stone fishing for harbours. Especially large stones was removed. Remaining reef subject to erosion. Flora and fauna degenerating Restauration project launched supported by EU and Danish authorities to restore reef

3 Facts on the restoration 7 acres of reef to be restored Extensive field investigations prior to actual restoration. Extensive numerical investigations on waves, hydrodynamics and sediment transport m 3 of stones mainly from norway and sweeden to be used. Project to be carried out slowly in while closely monoring the migration of flora and fauna into the reef. All progress and further information can be viewed at

4 DHI investigations DHI was commisioned to do hydrographic and sediment transport modelling. The purpose of the hydrophic modelling was to establish knowledge of the hydronamics in the area including waves and currents for normal conditions and extreme conditions. This includes design data Modelling of sediment transport was for the purpose of determining a layout for the reef where a minimum of erosion and accretion whould occur.

5 Hydrographic conditions The current in the Kattegat estuary is often 3 dimensional and the effect of meteorology, salinity, temperature and global circulations has to be taken into account. This includes the flow in and out of the Baltic sea. The reef itself is very local and needs special care when modelling. Waves will break over it and wave driven currents will be present. Modelling the reef will require good bathymetric and sediment data and results from a global hydrodynamic 3D model.

6 Global hydrodynamic model

7 Local hydrodynamic model

8 Wave model

9 Physical conditions for new layout Bathymetry of Læsø Trindel. The restoration area is shown (right) with and without the new structures.

10 Physical conditions for new layout Water depths are generally between 8m and 12m in the main part of the plateau. Locally the water depth is only 4m at the top of the reef. Four different structures are proposed by the designer Orbicon A/S The Western and Central structures are areas where the sea bed to 1.5 m below mean sea level (MSL). level is increased approximately 1 m with singular pillars reaching up The Eastern structure is stabilizing the shallowest part of the existing reef areas with plateaus of boulders in different levels up to 1 m below MSL. The Wall structure is stabilizing the northern part of the shallowest area reaching to 4 m below mean sea level

11 Physical conditions for new layout Wind rose (hindcast wind data) north of Læsø (LONG ;LAT ) for the period Wind direction is defined as coming from.

12 Model considerations at the reef Calm wheather flow may be stratified and use of 3D model required with no wave influence. Storms Flow will be uniform and use of 2D model including wave driven currents will be used. This includes wave modelling. 3D surface flow during storm 2D depth averaged flow during storm

13 Validation currents regional model Comparison of measured and modelled current in January 2005 from Læsø Rende West

14 Validation waves

15 Simulation plan 10 years of 3D hydrodynamics and waves for design purposes in global model. 10 years of 3D hydrodynamics and waves in the local model. 5 hydrodynamic scenarios in 3D for the local model. For each layout 4 hydrodynamic scenarios in 2D and 4 wave scenarios in the local model. For each layout. Sediment transport calculations for the selected 9 scenarios in 2D and 3D. For each layout.

16 Design data Simulation period (10 years) Current speed statistics for at position A-F Current Speed [m/s] Max Mean Std. Dev. Point A surface Point A bottom (4m) Point B surface Point B bottom (9m) Point C surface Point C bottom (7m) Point D surface Point D bottom (8m) Point E surface Point E bottom (8m) Point F surface Point F bottom (9m)

17 Design data extrema values The key results of the extreme value analysis of depth integrated current with 10- and 50- year return periods for the 6 positions A-F on Læsø Trindel Return Period [years] A B C D E F Depth [m] Current speed [m/s] Current speed [m/s] The key results of the extreme value analysis of surface current with 10- and 50-year return periods for the 6 positions A-F on Læsø Trindel Return Period A B C D E F [years] Depth [m] Current speed [m/s] Current speed [m/s]

18 Sediment transport approach 9 events choesen. Four storms and four conditions with gentle weather and one purely tidal case. Events chosen on the basis of the 10 years data of wind, waves and currents at the reef. For calm periods a 3D approach was used due to stratification For storm events a 2D approach was used assuming fully mixed water column at the reef All simulations carried out using MIKE21/3 FM fully coupled hydrodynamic model. Sediment transport model was MIKE 21 ST based on a tabular approach using a quasi 3D description. Based on surveys 4 grain size diameters were considered. 0.2mm, 2mm, 20mm and 100mm representative for different water depths.

19 Period Case 1: Strong wind from W 29/11/ :00 AM - 03/12/ :00 PM Case 2: Medium-strong wind from S-SW 26/12/ :00 PM - 29/12/ :00 PM Case 3: Medium-strong wind from S-SE 9/1/ :00 PM - 12/1/ :00 PM Case 4: Medium wind from W 17/2/1998 0:00 AM - 20/2/1998 0:00 AM Peak wind speed [m/s] Wind direction at peak wind speed [degr. N] Selected events Average wind speed [m/s] Average wind direction (wind speed > 8 m/s) [degr. N] Period Case 5: Strong NW current in upper layer, varying direction in lower layer 13/10/ :00 AM - 14/10/ :00 PM Case 6: Medium W-NW current in upper layer, varying direction in lower layer 05/10/2005 4:00 PM - 06/10/2005 4:00 PM Case 7: NW-SE current. Uniform flow direction over depth. Weak-medium current speeds 10/1/2005 3:00 AM - 11/1/2005 3:00 AM Case 8: NW-SE current. Medium current speeds 27/3/2005 4:00 AM - 28/3/2005 4:00 AM Maximum current speed, surface [m/s] Current directions, surface [degr. N] Period Varying tidal variation Maximum tidal amplitude [m] Minimum tidal amplitude [m] 28/08/2005 0:00 AM /09/2005 0:00 AM

20 Sediment distribution Water depth Finest sediment fraction present Sediment grain size in modelling <7 m 4-5 ( mm) 20 mm 7-11 m 6 (2-20 mm) 2 mm >11 m 7 (< 2 mm) 0.2 mm

21 Hydrodynamic results Instantaneous flow field for existing conditions (upper figure) compared to the situation including design 1 (lower figure) at the peak of the case 1 storm.

22 Hydrodynamic results Deviation in flow speed flow speed between the situation including the new structures and the existing conditions at the peak of the case 1 storm

23 Sediment results Magnitude of sediment transport at locations A (upper figure, water depth approximately 9m) and B (lower figure, water depth approximately 4 m) at LæsøTrindel. Medium strong wind from S-SE for grain sizes d50=0.2 mm, 2 mm, and 20mm.

24 Sediment results First reef design Existing conditions Area covered Accumulated net sediment transport rates during the severe storm for water depths above 11 m (sediment grain size 0.2 mm, upper figures), water depths 7-11 m (sediment grain size 2.0 mm, middle figures) and water depths below 7 m (sediment grain size 20.0 mm, lower figures). Triangle indicates area, where submarine structures created by leaking gases have been found.

25 Sediment results Erosion and deposition pattern in reef area for calculation with sediment grain size 2.0 mm. Existing conditions (upper figure) and including reef design 1 (lower figure). Representative for water depths between 7 and 11 m.

26 Conclusions Design data based on 10 years of model data was produced including waves and 3D currents. A comprehensive study was carried out for 9 weather conditions to determine impacts of reef layouts. Accumulated sediment transport rates are low. Annual sediment transport rates is 5-7 m 3 /m on the plateau and 5-15 m 3 /m in the deeper areas. Little impact from the proposed structures on the transport. Only very locally a decrease in the transport rates are seen. It was shown that the designers have large degrees of fredom to design the reef favourable for the flora and fauna. The model was proven to be a very powerfull design tool in areas of great complexity.