Note: Deposition/dredging of tidal ferry channel in Dutch Wadden Sea Date: 23 June 2017

Transcription

1 Deposition and dredging of a small tidal ferry channel through sand-mud flats in Dutch Wadden Sea 1. Introduction 2. Ferry channel and long-term morphology 3. Tidal flow and sediment transport 3.1 Tidal flow 3.2 Sediment transport 4. Sediment sources and composition 4.1 Sediment sources 4.2 Sediment composition 5. Deposition, consolidation and bulk density 5.1 Deposition processes 5.2 Consolidation and bulk density 6. Dredging volumes, composition and methods 6.1 Dredging volume and composition 6.2 Dredging methods 7. Deposition predictions 7.1 Mud sources 7.2 Longitudinal supply through channel 7.3 Lateral supply from tidal flats 7.4 Lateral supply from marshland zone 8. Conclusions 9. References 1

2 1. Introduction This paper analyzes the severe deposition and dredging of a small and windy tidal ferry channel in the Dutch Wadden Sea. The ferry channel with a length of about 12 km is situated between the village of Holwerd at the Frisian coast and the (barrier) island of Ameland, see Figure 1.1. The channel consists of various subchannels with a decreasing width in landward direction. Wide and flat sloping (1 to 700) marshlands are present near the Frisian coast which are intersected by narrow and shallow tidal channels with natural depths of about 1 m below LAT. The landing site of the ferry is situated at about 2 km seaward of the dike at Holwerd along the Frisian coast. As a result of increasing tourist visits, the ferry boats have become larger (length of about 75 m) and faster. Whereas the old ferry boats equipped with a single screw were able to sail through the soft mud of the upper bed layer, the modern ferry boats with multiple propulsion systems consisting of pumpjets require a relatively large keel clearance to prevent the intake of muddy water. Nowadays, the minimum required channel depth is about 4 m below mean sea level (MSL). Since 2000, the annual dredging volume of mud has gone up from about 0.2 million m 3 to about 1.8 million m 3 in Most of the dredging is taken place in the narrow (50 m wide at bottom) tidal channel with a length of about 4 km close to the mainland (Traject AC, Figure 1.1). The total thickness of the annual deposition volume spread out as a uniform layer in the channel is /(50x4000) 9 m, which is more than twice the channel depth of 4 m. D cross-section Figure 1.1 Ferry channel between Holwerd and island Ameland, Dutch Wadden Sea 2

3 Height (cm) to NAP Note: Deposition/dredging of tidal ferry channel in Dutch Wadden Sea 2. Ferry channel and long-term morphology The ferry channel (total length of about 12 km; minimum channel width at bottom= 50 m; minimum depth below mean sea level= 3.8 m) between the village of Holwerd at the Frisian coast and the island of Ameland consists of two major trajects (see Figure 1.1): channel traject DA of 8 km, which is relatively wide, deep and sandy; dredging is minimum; channel traject AC of about 4 km which is relatively narrow and shallow and runs partly parallel to the shore-connected tidal flats/marshlands; most of the dredging is taken place along this traject. The natural bed level along the channel gradually increases in landward direction from about -4 m below NAP (about mean sea level) just seaward of location A to about -1 m below NAP near the landing site. Figure 2.1 shows a cross-section perpendicular to the Frisian coast at about 500 m from (south-west) the landing site. The marshland zone consists of a high zone at about 1.5 m above NAP (about mean sea level MSL) and a pioneer zone with a slope of 1 to 700 between -0.5 m NAP and +1 m NAP over a distance of about 1000 m. The upper edge of landward channel side is at -0.5 m NAP at a distance of 1.8 km from the dike (Figure 2.1). The natural sedimentation in the pioneer zone in the period was about 0.01 to 0.02 m/year which is equivalent to about 10 to 20 m 3 /m over a distance of about 1 km. The coastal zone along the Frisian mainland with an alongshore length of about 25 km and a cross-shore length of 3 km (where Holwerd is situated) shows a long term sedimentation pattern (Rijkswaterstaat 1980). The observed sedimentation is about 45 mm/year in the period of and about 25 mm/year in the period The reduced sedimentation after 1975 is most likely caused by reduced dumping of dredged materials in the nearshore zone. Dike high zone pioneer zone Channel landward side Distance (m) Figure 2.1 Cross-section perpendicular to Frisian coast Table 2.1 and Figure 2.2 show the cross-sectional area below low water (LW) at various locations of traject AC based on earlier studies (Arcadis (2005, 2008, 2012), as follows: Traject BC: area reduces from about 330 ( 70) m 2 in 1988/1989 to about 200 ( 70) m 2 in 2011, which is a reduction of 40% in 20 years; Traject AB: area reduces significantly over 20 years. The minimum required area for safe navigation of the ferry is about 50x3,8 = 190 m 2 or about 140 m 2 with respect to LW. 3

4 Depth to NAP (cm) Cross-section area (m 2 ) Note: Deposition/dredging of tidal ferry channel in Dutch Wadden Sea Channel cross-sections 1988/ / BC Section 20 Arcadis 2008, Section 187 Arcadis 2008, Section 1 Arcadis A Section 2 Arcadis A Section IV Arcadis A AB Section 16 Arcadis 2008, Section 3 Arcadis A Section V Arcadis A NAP mean sea level (MSL) Table 2.1 Cross-sectional area at various locations (below LW at about -1 m NAP) Figuur Transect 20 Transect 187 Transect 1 Transect 2 Transect IV Mean trend line variation interval Time (years) Cross-sectional area as function of time at various locations of Traject AC The channel developments in the period are, as follows (see also Figure 2.3): landward migration of tidal flats in period 1989 to 2005; about 50 m in 15 years or 3 m/year or about 15 m 3 /m/year (layer thickness of 4 m); channel sides are stable after 2005 due to dredging; marshland sedimentation of about 0.25 m in period 1990 to 2016 or about 0.01 m/year; thin layers (0.1 to 0.2 m) of fluid mud have been observed in the ferry channel occasionally; particularly near the landing site due to agitation by ferry boat movements. Distance (m) Seaward Landward Figure 2.3 Cross-sections between B and C (see Figure 1.1) of ferry channel in period 1989 to

5 3. Tidal flow and sediment transport 3.1 Tidal flow The tide in the ferry channel is semidiurnal with a tidal range of about 1.8 m at neaptide and about 2.5 m at springtide, see Figures 3.2 to 3.4. Measured tidal velocities at locations A, B and C are shown in Figures 3.2 to 3.4. Computed peak tidal velocities based on the DELFT3D-model (Arcadis 2008) are given in Table 3.1. The peak tidal velocities are in the range of 0.45 to 0.5 m/s during springtide. Figure 3.1 shows peak flow velocities along the channel over a length of about 8 km based on DELFT3Dmodel (Deltares 2016). During flood and ebb, the peak velocities are about 1 m/s at location A and about 0.7 m/s at location C. Channel section Ebb Flood Waterlevel (m) Flow velocity (m/s) Waterlevel (m) Flow velocity (m/s) Traject AB Section m NAP m NAP m NAP m NAP m NAP , m NAP Traject BC Section m NAP m NAP m NAP m NAP m NAP m NAP NAP mean sea level (MSL) Table 3.1 Computed depth-averaged flow velocities in Trajects AB and BC Holwerd x=0 x=l Figure 3.1 Computed values of depth-averaged and neap-spring averaged peak flow velocities (vertical axis) in channel (relative distance on horizontal axis x/l; L= channel length AC); upper=flood; lower=ebb 5

6 3.2 Sediment transport Field measurements of flow velocities (ADCP-instrument) and mud concentrations (OBS and pump samples for calibration) have been done on 14 April 2016 in the channel between locations A and C, see Tables 3.2 to 3.4 and Figures 3.2 to 3.4. The most important results are: flood flow has a duration of about 7 hours with maximum depth-averaged flow velocities of about 0.7 m/s in location A, and 0.6 m/s in B and C; ebb flow has a duration of about 5.5 hours with maximum depth-averaged velocities of about 0.6 m/s in A; 0.8 m/s in B and 0.4 m/s in C; the maximum flood velocity occurs about 3 hours before high water (HW); the depth-averaged mud concentrations increase rapidly to about 1000 to 1500 mg/l in the period before maximum ebb flow and maximum flood flow; the mud concentrations decrease to minimum values of 150 to 250 mg/l in the period of 3 hours after maximum flood flow; the mud concentrations near the bed (< 0.5 m) are much larger than the depth-averaged mud concentrations (factor 2 to 5), which is an indication of relatively large settling velocities in the nearbed layer; fine sediments (< 8 m); concentrations are larger than elsewhere in the Wadden Sea (generally in the range of 50 to 150 mg/l). Figure 3.5 shows mud concentrations during maximum flood flow in the Holwerd channel in comparison to mud and fine sand concentrations in other estuaries (Europe, South-Amerika and Asia). Most of the data are in good agreement with the saturation formula of Bagnold (1962), Winterwerp (2006) and Van Rijn (2007). The mud concentrations at Holwerd are significantly larger than the upper limit values of the other data, which may be an indication that the mud concentrations at Holwerd are severely affected by the intensive dredging activities. Time Water level to NAP Water depth -0.5 m m m m m m m Depthmean values Table 3.2 Mud concentrations (mg/liter); location A 6

7 Water level to NAP (m) and flow velocity (m/s) Mud concentration (mg/l) Note: Deposition/dredging of tidal ferry channel in Dutch Wadden Sea Time Water level to NAP Water depth -0.5 m m m m m m m Depthmean Table 3.3 Mud concentrations (mg/liter); location B Time Water level to NAP Water depth -0.5 m m m m m m m 700 Depthmean Table 3.4 Mud concentrations (mg/liter); location C Figure Ebb Flood Depth-averaged flow velocity A Waterlevel landing site Holwerd Depth-averaged mud concentration A Time (hours) Waterlevel, depth-mean flow velocity and depth-mean mud concentration in location A

8 Depth-averaged concentration of mud/fines (mg/l) Water level to NAP (m) and flow velocity (m/s) Mud concentration (mg/l) Water level to NAP (m) and flow velocity (m/s) Mud concentration (mg/l) Note: Deposition/dredging of tidal ferry channel in Dutch Wadden Sea Figure Ebb Flood Depth-averaged flow velocity B Water level landing site Holwerd Depth-averaged mud concentration B Time (hours) Waterlevel, depth-mean flow velocity and depth-mean mud concentration in location B Figure 3.4 Flood Depth-averaged flow velocity C Water level landing site Holwerd Depth-averaged mud concentration C Ebb Time (hours) Waterlevel, depth-mean flow velocity and depth-mean mud concentration in location C Variation ranges Silt and fine sand Nessmersiel Fresh mud Nessmersiel Germany Silt and fine sand Huanghe River china 1987 Mud Amazone mouth, Brasil 1990 Mud Ems tidal river Germany Mud Ems tidal river Germany 1990 Mud Elbe tidal river Germany 2002 Mud Yangtze estuary, China 1991 Mud Thames tidal river UK,.. HOLWERD Channel, Wadden Sea, NL Saturation formula Bagnold 1962 (h=5 m; wso=0.5 mm/s) Saturation formula Bagnold 1962 (h=15 m; wso=0.5 mm/s) Depth-averaged flow velocity (m/s) Figure 3.5 Mud and fine sand concentrations in tidal channels of Europe, South-America and Asia 8

9 4. Sediment sources and composition 4.1 Sediment sources Sediment sources contributing to the channel deposition are (see Figure 4.1): natural deposition of mud and fine sand by landward asymmetrical tidal flow velocities (mud supplied by flood flow) in combination with increasing mud concentrations during flood period; continuous deposition in marshland zone; supply of fine sand and mud from tidal flats on both sides of the channel stirred up by wind waves and ship waves at the shallow tidal flats; lateral supply of sediment is about 10 to 20 m 3 /m/year (Chapter 7); supply of mud from lower zone of marshlands stirred up by wind waves and ship waves; natural deposition of marshland zone is about 0.01 to 0.02 m/year; or 10 to 20 m 3 /m/year; marshland zone near landing site shows no deposition over about 1 km which is indication of local supply of 10 to 20 m 3 /m/year towards channel due to cross-shore transport from marshland into channel (see Figure 2.1); recirculation of mud due to dredging; dredged material disposed during ebb flow will move seaward over a distance of about 3 to 5 km during the ebb period but the mud cloud may return party during flood flow; soft freshly deposited mud at location A (after disposal) will be eroded during flood. Dumpsite flood Supply of mud by flood flow Figure 4.1 Dumpsite ebb VA20-22 Mud sources along ferry channel Holwerd-Ameland A B Lateral supply from tidal flats Lateral supply from marshlands C landing Holwerd 4.2 Sediment composition Data from borecores in the ferry channel near the landing site show the presence of a layer of relatively soft muddy sediment with a thickness of about 1 m on top of more consolidated layers. Bed samples have been taken at two dates along channel traject AC: 23 August 2002 (Table 4.1): sandy bed material with d 50-values in the range of 100 to 150 m and mud percentages < 63 m in the range of 5% to 25%; the mud percentage is largest near the ferry landing location; 14 April 2016 (Tables 4.2, 4.3 and Figure 4.2): silty bed material with d 50-values in the range of 20 to 70 m; percentage of fines < 63 m is up to 80%; the dry bulk density of bed samples is in the range of 750 to 950 kg/m 3 ; settling velocities are in te range of 1 to 2 mm/s. 9

10 The bed material (2016) consists of: sandy bed: 10% clay, 45% silt and 45% sand. silty bed: 15% clay, 60% silt and 25% sand. The suspended sediment (2016) consists of: 10% fine sand, 60% silt and 30% clay. The silt fraction is dominant in most bed samples and the suspended samples. The dry bulk density of the bed material can be represented by: dry= 400p clay + 800p silt p sand Sediment properties Transect Transect Transect d 50 southwest side ( m) d 50 in middle of channel ( m) d 50 northeast side ( m) Percentage mud southwest side (%) Percentage mud in middle of channel (%) Percentage mud northeast side (%) Table 4.1 Bed sample data of 23 August 2002 in channel traject AC Bed samples d50 Percen tage sand Percen tage mud < 63 m (%) Wet bulk density Dry bulk density Settling velocity of flocculated sediment (concentration of bout 1 kg/m 3 ) w10 (mm/s) w35 (mm/s) w50 (mm/s) w75 (mm/s) ( m) (%) (kg/m 3 ) (kg/m 3 ) Location A KG07 (11.04 hrs) KG13 (18.05 hrs) Location B KG05 (10.41 hrs) KG06 (10.41 hrs) KG09 (16.34 hrs) KG14 (18.15 hrs) , Location C KG15 (18.35 hrs) Table 4.2 Bed sample data of channel traject AC; 14 April 2016 Sediment fractions Size class Settling velocity Stokes (at 10 o ) (mm/s) Suspended sample (%) Silty bed sample (%) Sandy bed sample (%) Sample from hopper (%) ( m) sand > fine sand very fine sand coarse silt medium silt fine silt very fine silt clay/lutum < Table 4.3 Size composition of bed, suspended (14 April 2016) and hopper sample (20 May 2016). 10

11 Figure 4.2 Cumulative particle size distribution based on Laser-Diffraction instrument (Malvern); deflocculated samples (bed samples 14 april 2016; hopper-beun sample 20 May 2016) 11

12 5. Deposition, consolidation and bulk density 5.1 Deposition processes Mud is mainly supplied by the tidal flood flow. During slack tide, most of the mud is deposited onto the channel bed. As consolidation proceeds rather quickly, the dry bulk density of the mud bed increases to about kg/m 3 after 2 to 3 hours and hence the critical stresses for erosion also increase (increase of bed strength) so that the mud can hardly be eroded by the weak ebb currents. Sand transport is almost absent as the flood velocities are not much larger than about 0.6 to 0.7 m/s. Most of the fine sand is eroded from the tidal flats by wind and ship waves. The total active top layer of the channel bed has a volume of about 4000x30x m 3 (80 dredging tracks with layer thickness between 0.5 m and 1 m; completed in 9 days). Most likely, this volume of mud is recirculated many times over the year by dredging, ebb disposal into the channel and return of mud clouds during flood. The total supply of mud during flood of 6 hours is estimated to be about 5000 m 3. Hence, the active layer can be replaced in 20 tides or about 10 days, which corresponds well to the dredging cycle time of about 9 days. 5.2 Consolidation and bulk density Analysis results of bed samples of channel traject AC shows dry bulk densities in the range of 750 to 950 kg/m 3 (see Table 4.2). These relatively high values suggest a dominating effect of the silt fraction. Bed sample material was used to perform settling-consolidation tests in cylindrical columns with a height of 0.55 m and a diameter of 0.11 m. Subsamples were taken and diluted with saline water from Holwerd. The tests are done at sediment concentrations of 20 g/l, 40 g/l and 60 g/l. The sediment-water mixture is gently stirred (to prevent breaking of the flocs) to get a uniform distribution over the settling column. Over time, the sediments settle in the column and an interface between the watersediment mixture and the clear water above it becomes visible, see Figure 5.1. Figure 5.1 Consolidation tests of bed material 12

13 mud % in water column heigth Note: Deposition/dredging of tidal ferry channel in Dutch Wadden Sea The process consists of various phases: hindered settling phase with particles and flocs settling to the bed; consolidation phase: - initial consolidation phase with concentrations larger than the gelling concentration and dominant effect of soil permeability; - final consolidation phase, where deformations are very small and effective stresses are dominant. Figure 5.2 shows the results of consolidation tests for samples KG-14 (location B, Figure 1.1). The initial mud concentrations are 20, 40 and 60 g/l. The gelling concentration can be roughly estimated from the mud height at the transition from settling to consolidation. The estimated gelling concentrations for KG-14 varies between 110 and 200 g/l. A rough estimate of the effective settling velocity can be obtained from the linear settling process of the test with the lowest initial concentration, resulting in: Sample KG-14; effective settling velocity=settling height/time 340 mm/150 s 2.3 mm/s; dry bulk density after about 2000 s in the range of kg/m 3 ; Sample KG-15; effective settling velocity=settling height/time 2 mm/s; dry bulk density after about 2000 s in the range of kg/m 3. The consolidation test results show that the deposited sediments quickly consolidate within 1 hour to dry bulk density values in the range of 300 to 400 kg/m KG g/l 40 g/l 60 g/l Time (s) Figure 5.2 Consolidation test results of bed material sample KG-14 (location B) 13

14 6. Dredging volumes, composition and methods 6.1 Dredging volumes and composition The bed of the ferry channel is dredged regularly by a small hopper dredger. Figure 6.1 shows the annual dredging volume of the channel between locations A and C (length of about 4 km) in the period 1990 to The annual dredging volume has gone up dramatically from about 0.1 millions in 1990 to about 1.8 million m 3 in The seasonal variation is not very large; slightly larger values in winter period. Up to 2007, the dredged material was brought to various dumping sites at a distance of about 3 km. After 2007, most of the dredged material (65%) is discharged into the ebb flow just seaward of location A. Possible causes of increased channel deposition are the gradual growth of the marshland zone reducing the tidal velocities through the channel (less erosion capacity during ebb flow) and the modified dredging strategy of mud disposal close to the dredging site during ebb flow. Shoal area Dumpsite flood Sandy dredging locations Shoal area Dumpsite ebb VA20-22 A B C landing Holwerd trend line disposed during flood disposed during ebb Figure 6.1 Annual dredging volume in m 3 per year (vertical axis) in channel between locations A and C Table 6.1 presents the dredging volumes and dry bulk densities of the hopper loads in the period 2008 to The dry bulk densities vary in the range of 250 to 650 kg/m 3. The composition of hopper samples is shown in Figure 4.2. The dredged materials consist of very muddy materials with a mean particle size of about 10 m (15% fine sand). 14

15 Year Dredging volume (m 3 ) Dredging mass (tons) Dry density (kg/m 3 ) Table 6.1 Dredging volumes and dry bulk densities based on Rijkswaterstaat (2008) Mud samples were taken from the hopper load on 20 May 2016, see Table 6.2. The hopper load was a thick slurry of mud (see Figure 6.2) with a d 50 of about 10 m. The dry bulk densities are in the range of 350 to 450 kg/m 3 in good agreement with the values of Table 6.1. Sample Description Dry bulk density Percen tage sand (%) Percen tage silt (%) (kg/m 3 ) 1 Upper layer of hopper load (no 415 internal circulation by pumps) 2 Near inflow zone of dredged 385 sediments into hopper 3 well-mixed hopper load % 70% 15% (internal circulation by pumps) Table 6.2 Composition of dredged sediment from hopper load, 20 May 2016 Percen tage clay < 4 m (%) Figure 6.2 Mud slurry of hopper load, 20 May Dredging methods On 20 May 2016, the hopper dredging process was studied by a site visit of various people to the hopper dredger at work. The hopper characteristics are: length= 60 to 70 m, width= 9 to 10 m, draft=1.5 to 3.3 m; hopper volume= 700 to 800 m 3 ; pipe suction width= 1 to 1.2 m. Most of the channel dredging is done over a layer thickness of about 1 m along the silty-muddy side parts of the channel (Figures 6.3 and 6.4) over a length of about 4 km. No or minor dredging is done in the sandy middle part of the channel which remains quite stable at a required depth of about 4.5 m below NAP (mean sea level). The latter is most likely caused by the pump jets of the ferry boat which sails through the middle part at an interval of 1 hour. The required channel width is about 50 m and the depth is 3.8 m below NAP. 15

16 Dredging process and locations : 1. hopper dredging of silt and mud during sailing along the side parts of the channel (Figure 6.3); mud overflow is about 10% to 30%; suction mouth (width= 1.2 m, height= 0.5 m) moves continuously between -3.8 and -4 m NAP (visible at monitor); during each track, a trench is made of 0.5 m deep and 1.2 wide over a length of 1 km yielding a hopper load of about 700 m 3 ; dredging time is about 15 minutes (suction capacity of 1 m 3 /s); total bed area to be dredged (side parts of channel) is about 4000x30= m 2 with an anual production of about 1.5 to 1.8 million m 3 (annual deposition is 12 to 15 m); 2. irregularities on both sides of the dredged trenches are smoothed out by a small tugboat with plough attached (plough width = 2 m); production goes down if the hopper tries to dredge more accurately; 3. deposited soft mud in the landing area is moved to the channel by water injection dredging over an area of 100x50=5000 m 2 ; total production is about m 3 /year (deposition is about 10 m/year); 4. hopper dredging of sand during sailing at various sandy locations along the channel (within bends and transitions from bends to channel); annual volume of about to m 3 /year. Hopper dumping and cycle times : 1. during flood, the muddy hopper load is dumped in the channel Zuiderspruit (see Figure 1.1 and 6.1) by opening the bottom doors of the hopper vessel; cycle time of dredging, sailing and dumping= 2 hours; 2. during ebb, the muddy hopper load is dumped by pumping (at the bow; at about 1.5 m above the local bed) into the water column at location A ( near bouys VA20-VA22, Figure 6.1) where the depth is about -4.7 m below NAP; this channel section is quite stable without dredging; pump capacity of 1 m 3 /s (pump time = 15 minutes); cycle time = 1 hour; 3. during ebb flow with velocities of about 0.4 m/s, the mud slurry with dry density of about 500 kg/m 3 will partly be dispersed in the water column and partly settle to the bed as a thin layer of about 0.2 to 0.3 m thick, 2 to 3 m wide and 1 km long with dry density of about 200 kg/m 3 ; this layer is a source of very soft mud for erosion during flood and transportation to the channel; 4. overall, about 35% of the dredged material is dumped during flood at the channel Zuiderspruit and 65% is dumped during ebb at location A; no major recirculation of dumped mud (mud clouds) is seen in the channel area). Hopper dredging production and return times : 1. channel dredging area along the sides is about 4000x30 = m 2 ; about 20 trenches with a depth of 0.5 m and length of 1 km are dredged (each trench width=1.2 m); in all, 4x20= 80 trenches are dredged to cover the channel of 4 km; 2. flood cycle time is 2 hours and 3 tracks can be completed; ebb cycle time is 1 hour and 6 tracks can be completed; in all, 9 trenches per 12 hours (working day= 12 hours and 7 days per week); day production is 9x700 m m 3 /day; maximum year production is 2 million m 3 for 365 days; 3. all 80 trenches can be completed in 9 days; thus, the return time is 9 days; in practice the return time is about 14 days in summer and 7 days in winter depending on the storm intensity; if necessary a second hopper dredger is available (bed soundings are done every 2 weeks); 4. given a dredging layer thickness of 0.5 m and a return time of 9 days, the estimated mud deposition at each location is 30/9x0.5=1.5 m/month or 18 m/year. Density and composition of dredged materials: 1. the dry density of the dredged materials (mud, silt and fine sand) is determined from the hopper volume and the water displacement (kg) of the loaded vessel; 2. dry bulk densities of the hopper load are in the range of 450 to 500 kg/m 3 (see Table 6.2); the mud concentration is measured in the internal pipeline system (gamma-radiation method; wet bulk density of 1.3 ton/m 3 equivalent with dry bulk density of 0.5 ton/m 3 on 20 May 2016); occasionally 16

17 Height to NAP (m) Depth to NAP (m) Note: Deposition/dredging of tidal ferry channel in Dutch Wadden Sea the dry bulk density is as low as 300 kg/m 3 in areas with severe deposition and cycle times smaller than 9 days; 3. the in-situ dry density of the deposition layer along the sides of the channel is much higher (750 to 950 kg/m 3 ) than the dry bulk density of the dredged sediments as 30% to 50% water is added during the dredging process; the excess water is partly pumped out of the hopper as overflow; Figure 6.4 shows the vertical distribution of the mud concentrations; 4. the dredging return time of 9 days is sufficient for consolidation of the deposited sediments from about kg/m 3 to about kg/m 3 (enhanced by fine sand coming from the tidal flats); 5. the dry bulk density in sandy areas is about 1100 to 1300 kg/m 3 ; 6. the mud slurry from the upper layer of the hopper load is very thin with estimated dry bulk densities of 200 to 300 kg/m 3 ; a long stick of 4 m can easily be pushed down to the hopper bottom; no firm layer is formed at the hopper bottom during sailing of 1 hour. Figure 6.3 Seaside Landside Distance from mid of channel (m) slope 1 op channel bed (consolidated mud) -5.5 hard sand bed -6 soft mud Schematized channel Sediment concentration required depth -3.8 m hard sand bed soft mud consolidated mud Dry sediment density (kg/m 3 ) Figure 6.4 Sketch of vertical distribution of sediment concentrations at channel side close to Holwerd 17

18 7. Deposition predictions 7.1 Mud sources The mud is supplied from (see Figure 4.1): 1. supply of mud during flood at channel location A; deposited mud during flood cannot easily be eroded by the relatively low ebb velocities m/s (consolidation proceeds relatively rapid within 9 days resulting dry bulk densities values > 750 kg/m 3 with increased critical stresses and velocities for erosion; critical velocity for erosion is estimated to be about 0.7 m/s; Van Rijn et al. 2017) 2. lateral supply of mud and fine sand from the tidal flats; 3. lateral supply of mud from the marshland zone. The total active top layer of the channel has a volume of about 4000x30x m 3 (80 dredging tracks with layer thickness between 0.5 m and 1 m; return time of 9 days). Most likely, this volume of mud is recirculated many times over the year by dredging, ebb disposal to the channel and return of mud clouds during flood. 7.2 Longitudinal supply through channel The flood flow at location A is determined by: volume of water to fill the layer between -1 m NAP and 0.5 m NAP; the flood flow mainly goes through the channel; volume of water to fill the surrounding area of tidal flats (approximately 3x 6 km 2 ) on both sides of the channel. The depth-averaged and tide-averaged flood flow velocity is: U flood= V fill/(a channel t) with: V fill= h L channel b channel = fill volume between -1 m and 0.5 m NAP ( 1.5x4000x million m 3 ), h = water level difference ( 1.5 m); t = time period ( 3 hours s); b channel = upper channel width at 0.5 m NAP ( 200 m); L channel= length of channel ( 4000 m), A channel= cross-section area of channel (below 0.5 m NAP), ( 300 m 2 ). This yields: U flood= 0.4 m/s; the peak flood flow is about 1.5x0.4= 0.6 m/s. In practice, the flood flow will be somewhat higher (0.7 m/s) as the area above the tidal flats will be partly filled with water supplied through the channel (total filling volume 3000x6000x1 18 million m 3 ). The ebb velocities are of the same order of magnitude. These estimated values are in good agreement with values from 2DH-model computations (Table 3.1; Figure 3.1). The mud supply at channel location A during flood can be estimated, as follows: V mud,flood= b channel h channel u flood c mud T/ b,dry. with: b channel= mean channel width= 100 m; h channel = channel depth to NAP= 4 m; u flood = depth-averaged and tide-averaged flow velocity during flood= 0.4 m/s; c mu = depth-averaged and tide-averaged mud concentration= 1 kg/m 3 ; 18

19 T= effective flood duration= 3x3600=10800 s; b,dry = dry density of consolidated mud = 400 kg/m 3 This yields: V mud,flood= 4000 m 3 for 1 flood period or about 3 million m 3 per year for 730 flood periods per year. The supply of fine sand during flood flow can be neglected as the flood velocities are too small to cause significant sand transport. The mud discharge during ebb is: u ebb = depth-averaged and tide-averaged ebb flow = 0.4 m/s; c mud = depth-averaged and tide-averaged mud concentration = 0.5 kg/m 3 ; T= effective ebb duration= 3x3600=10800 s; This yields: V mud,ebb = 1.5 million m 3 per year. The net longitudinal mud supply (V mud,net = V mud,flood V mud,ebb) is of the order of 1.5 million m 3 per year and the trapping efficiency of the channel is about 50% (50% of the supply during flood is returned during ebb). 7.3 Lateral supply from tidal flats The lateral supply of fine sand from the tidal flats strongly depends of wind waves and ship waves and can be estimated by: q sand,lateral,flood= p sand p wave h flat u lateral c sand T flood N tide/ b,dry with: p sand = percentage of sand in toplayer of tidal flats = 0.8; p wave = percentage of time with waves = 0.5; h flat = waterdepth at edge of tidal flat during flood= 0.5 m; u lateral = lateral flow velocity = 0.05 m/s; c sand = 0.1 kg/m 3 ; T flood= 3 hours = s, N tide= 730 tides per year, b,dry= 400 kg/m 3. This yields: q sand,lateral,flood= 20 m 3 /m/year ( 10). Thus, the lateral supply of fine sand from the tidal flats into the channel is of the order of 10 to 30 m 3 /m/year. Given a length scale of 4 km, this yields a total deposition of about 0.1 million m 3 /year. The lateral supply of sand can also be estimated form the landward migration of the tidal flats, see Figure 2.3. The migration rate is about 50 m/year in the period 1989 tot 1999/2005 or 5 m/year over a layer thickness of about 4, which is equivalent to a lateral sediment supply of 20 m 3 /m/year. From 1999/2005 onwards, the channel sides are stabile due to dredging activities (lateral suppy is dredged away). 7.4 Lateral supply from marshland zone The lateral supply of mud from the marshland zone into the channel (south-west of the landing site) is mainly determined by the return flow of shoaling/breaking wind waves and ship waves. q mud,lateral,ebb= p mud p wave h edge u lateral c mud T ebb N tide/ b,dry 19

20 with: p mud = percentage of mud in toplayer of marshland zone = 1; p wave = percentage of time with waves = 0.5; h edge = waterdepth at edge of marshland zone = 0.3 m; u lateral = lateral flow velocity = 0.05 m/s, c mu= 0.3 kg/m 3, T eb= 3 hours = s, N tide= 730 tides per year, b,dry= 400 kg/m 3. This yields: q mud,lateral,ebb= 50 m 3 /m/year ( 20). The natural deposition in the marshland zone can also be estimated from the sounding data in de period , which yields a value of about 0.02 m per year (Alkyon 2005, 2008). The total deposition in the marshland zone with lateral length of about 1 km is about 20 m 3 /m/year. Figure 2.1 shows that the marshland zone south-west of the landing site is quite stabile (no long term deposition) in the period 1999 to 2004, which is an indication that the annual supply of mud into the channel at that section is about 20 m 3 /m/jaar by lateral transport processes (wave-dominated). Given a channel length of about 4 km, the total lateral supply of mud is of the order of 0.1 million m 3 /year. Overall, the channel mud deposition is dominated by tidal filling processes (order of 1.5 million m 3 /year); the lateral supply of mud from the tidal flats and the marshland zone is of the order of 0.1 to 0.2 million m 3 /year. SEDHAR-model The channel deposition can also be estimated by using the simple SEDHAR.xls model, see Figure 7.1. The input data are given in Table 7.1. The most important data are: - settling velocity= 1 mm/s; - tidal amplitude= 1.2 m; - mud concentration= 0.5 kg/m 3 with tidal variation of 0.25 kg/m 3 ; - dry bulk density of mud-sand mixture= 500 kg/m 3 ; - lateral sand influx= kg/m/s or 15 ton/m/year (30 m 3 /m/year); - lateral mud influx from marshland=0.001 kg/m/s or 30 ton/m/year (60 m 3 /m/year). The annual computed deposition = 1.5 million m 3 /year (mainly mud). The deposition of sand is about 0.1 million m 3 /year from lateral influx at the tidal flats. The annual deposition layer as a uniform layer over the channel area is /(4000x50)= 7.5 m. In practice, most of the mud is deposited along the sides of the channel. The channel trapping efficiency is about 60%. Figure 7.2 shows the tidal water level and tidal velocities involved. The peak flood velocity is about 0.7 m/s and the peak ebb velocity is about 0.4 m/s (in agreement with measured values). Figure 7.3 shows the tidal variation of the mud concentrations. The mud concentrations vary between 0.1 and 0.6 kg/m 3 inside the channel; deposited mud cannot be eroded. 20

21 Mud concentration (kg/m3) Water level (m) and Velocity (m/s) Note: Deposition/dredging of tidal ferry channel in Dutch Wadden Sea Lateral inflow fine sand Inflow of fresh water and mud (river 2) Outside Cmud,river Uo Sand+Mud Mud inflow C2 Co Tide+Exchange C Tide PLAN VIEW River 1 Lateral inflow mud Entrance Exit tide Deposition ho CROSS-SECTION Outside Basin 1 Extra storage Basin 2 Figure 7.1 Channel schematization for SEDHAR.xls model Figure Tidal water level and peak tidal velocities Velocity entrance based on tidal storage equation Water level Velocity exit basin based on tidal storage equation Time (hours) Mud concentration outside basin Mud concentration inside basin 0.01 Figure Time (hours) Tidal variation of mud concentrations 21

22 Time step (can not be changed; tide=12 hours=constant) 300 (s) Tidal period (= 12 hours cannot be changed) 12 (hours) Surge level above MSL 0 (m) Bed level below MSL inside (negative value if bed above MSL) 4.5 (m) Bed level below MSL outside (negative value if bed above MSL) 4.5 (m) Significant wave height inside basin 0.3 (m) Significant wave height outside basin 0.3 (m) Peak wave period 8 (s) Width of entrance of basin 1 50 (m) Width of exit of basin 1 30 (m) Area of basin 1 (where dredging is required) (m2) Extra storage area landward of basin (m2) Width of basin 1 60 (m) Peak tidal velocity outside basin (parallel entrance) 0 (m/s) Steady river velocity outside entrance (parallel entrance) 0 (m/s) Steady flow velocity through entrance 0.15 (m/s) Wind-driven velocity inside basin 1 0 (m/s) Tidal amplitude 1.2 (m) Phase shift of tidal velocity outside basin (0 to 3 hours) 0 (hours) Initial (t=0) concentration in harbour basin 0.1 (kg/m3) Constant concentration outside 0.5 (kg/m3) Variational concentration outside 0.25 (kg/m3) Reduction of mud concentration in extra storage basin 2 (0 to 0.5) 0.5 (-) Maximum density difference (range 0 to 3) 0 (kg/m3) Mean fluid density outside basin 1025 (kg/m3) Sediment density 2650 (kg/m3) Settling velocity of mud in still water (m/s) Critical velocity for deposition (0.3 to 0.5 m/s) 0.5 (m/s) Dry bulk density of deposited material 500 (kg/m3) Percentage sand of bed material >63 um (range 0 to 1) 0.7 (-) median sand size (m) 90% value of sand (m) Horizontal exchange coefficient flood (=0.025) 0 (-) Horizontal exchange coefficient ebb (mostly 0) 0 (-) Fresh river water discharge into basin (river 2) Mixing factor fresh water and salt water (1.5 to 3) 0 (-) Percentage of time with fresh water input (0 to 1) 0 (-) Mud concentration of river flow into basin Lateral inflow of mud along bank per unit width ( kg/m/s) Length over which there is lateral mud inflow 2000 (m) Lateral inflow fine sand along flat per unit width ( kg/m/s) Length over which there is lateral fine sand inflow 3000 (m) Number of tides per year 730 (-) Table 7.1 Input data of SEDHAR.xls model 0 (m3/s) 0 (kg/m3) (kg/m/s) (kg/m/s) 22

23 8. Conclusions This paper is focussed on the deposition and dredging of a small and windy tidal fery channel with a total length of 12 km between the village of Holwerd at the Frisian coast of the Wadden Sea and the barrier island of Ameland. Severe deposition occurs in the most landward channel part of 4 km which intersects tidal flats and shoals and is bordered by marshlands of the mainland southwest of Holwerd. The depth of the channel is about 4 m below mean sea level and the width is about 50 m at the bottom of the channel. The annual dredging volume has gone up dramatically from about 0.1 million m 3 in 1990 to about 1.8 million m 3 in The seasonal variation is not very large; slightly larger values in winter period. The annual dredging volume of 1.8 millions m 3 is equivalent with a layer of 9 m (more than twice the channel depth), if the deposition volume is spread out as a uniform layer over the channel area of 4000 x 50 m 2. The bed of the ferry channel is dredged regularly by a small hopper dredger. The dredged materials consist of very muddy materials with a mean particle size of about 10 m (15% fine sand). The silt fraction between 8 and 63 m is the dominant fraction. The dry bulk densities of the dredged materials vary in the range of 250 to 650 kg/m 3. Up to 2007, the dredged material was brought to various dumping sites at a distance of about 3 km. After 2007, most of the dredged material (65%) is discharged into the ebb flow just seaward of the entrance of the channel of 4 km. Possible causes of increased channel deposition are the gradual growth of the marshland zone reducing the tidal velocities through the channel (less erosion capacity during ebb) and the modified dredging strategy of mud disposal close to the dredging site during ebb flow. Deposition computations show that the annual deposition can be quite well explained by supply of mud by the flood flow, and wave stirring of mud and fine sand by wind waves and ship waves at the tidal flats and marshlands. 9. References Alkyon, Present and future deposition in Kikkertgat (in Dutch). Report A1448. Alkyon, Feasibility study ferry channel Ameland; Phase 1: selection of alternatives (in Dutch) Arcadis, Quickscan dredging volumes ferry channel Ameland (in Dutch). Arcadis, Contribution mud to sedimentation Wadden Sea. Deltares, Analysis of ferry channel Holwerd-Ameland (in Dutch). Project Delft, The Netherlands Rijkswaterstaat, Directorate Friesland, Erosion and sedimentation Oosterbierum to t Abt. Note WWKZ-80.H253 Van Rijn, L.C., et al Erodibility of sand-mud mixtures. www. leovanrijn-sediment.com 23