Sediment and Flocculation dynamics in the area of Zeebrugge Marc Sas, Alexander Breugem and Andrew Manning
Overview • Objectives • Overview of the measurement campaign • Global overview • Long term measurements • Through tide measurements • Coastal zone • Harbour of Zeebrugge • Near bed measurements using HCBS frame • INSSEV settling velocity measurements • Flocculation Dynamics • Conclusion
Objectives • The project :Langdurige monitoring van zout/zoet-verdeling in de haven van Zeebrugge en monitoring van zoutconcentratie, slibconcentratie en hooggeconcentreerde slibsuspensies in de Belgische kustzone • Waterbouwkundig Laboratorium • Maritieme Toegang
Objectives • Objective of the complete project • The first goal of the study and the survey is to detect the occurrence of near-bed high-concentration mud suspensions (referred to as high-concentration benthic suspensions - HCBS), their dynamic behaviour and the conditions and locations of their occurrence • The second goal is to gain insight in density currents between harbour and sea, and their relationship to siltation in the harbour of Zeebrugge. • Objective of the present presentation • Give overview of the measurements and some details about the flocculation dynamics inside the harbour of Zeebrugge.
Global Overview • Survey 1 : autumn-winter 2006-2007 • Survey 2 :summer 2007
Long term measurement locations • Period: • June 2006-Aug 2007 • RCM9 at: • MOW 1 • Sterneneiland • CTD at: • LNG terminal • Hermespier
Long term measurement setup • RCM-9 • Velocity • Salinity • SSC • Pressure • Temperature • CTD • Salinity • Pressure • Temperature
Through tide measurement locations • Harbour entrance: • One spring and one neap tide meas. • ADCP and SiltProfiler • Inside the harbour: • One summer and one winter meas. • SiltProfiler and CTD • Coastal zone: • Two transects in winter • Three transects in summer • SiltProfiler (summer and winter) • ADCP (summer only)
Through tide measurement instruments : ADCP + CTD • CTD: • Salinity (multiple depths) • Temperature • ADCP: • Velocity profile • Sediment concentration profile
Through tide measurement instruments : ADCP + CTD Harbour; Neap tide 3.30 h before HW Harbour entrance; Neap tide 2h before HW
Through tide measurement instruments : SiltProfiler • SiltProfiler: • Sediment concentration profile • Salinity profile • Temperature profile • Specifications • High Frequency (100 Hz) • Wireless Data Transfer via Bluetooth technology • Freefall Profiling • 3 Silt sensors: • 1 Seapoint BS sensor (0-700 mg/l) • 2 Transmittance Extinction Sensors (500-5000 mg/land 3500 – 50000 mg/l) • CTD sensor
Through tide measurement instruments : SiltProfiler Harbour; summer. Avg. tide; 0.20h after HW Navigation channel; winter. Avg. tide; 5h after HW
Near bed measurement locations • Campaigns: • 1 x Summer • 1 x Winter • Two different frames • Deployment: 2-4 weeks
Near bed measurements (HCBS Frame) • Valeport Midas and RCM-9: • Velocity (0.3 m and 1.0 m above bed) • Sediment concentration • Argus: Sediment concentration profile • Altus echo sounder: Bed level
Near bed measurements (HCBS Frame) : ARGUS MOW 1 during winter.
Near bed measurements (HCBS Frame) : Altus MOW 1; Winter
INSSEV Settling velocity measurements INSSEV floc camera surface electronics in laboratory on RV Lancaster survey vessel Example of floc images Hydrodynamic sensors on bed frame being deployed from RV Lancaster survey vessel 1.2 mm
15 INSSEV samples from Albert-II Dok Obtained 0.6m above the bed 8th November 2006 HW +1hr 16m deep Water Temp ~ 12.8oC Salinity = 32 U = 0.14 m/s SPM = 200-400 mg/l TMZ (17:00hr – HW+2.5hr) SPM rose to 5 g/l U = 0.03 m/s INSSEV Settling velocity measurements
Shear stress range: HW+1 = 2 Pa TMZ = 0.4 Pa LW = 0.15 Pa Dmean response: 130 mm375 mm Wsmean response: 2 mm/s at HW+1 Four-fold rise in TMZ to 8.5 mm/s INSSEV Settling velocity measurements
INSSEV Settling velocity measurements • HW+1: t = 2.1 Pa, SPM=333 mg/l • 621 flocs, 50-394 mm • Microflocs dominate high shear env. • D < 160 mm (SBs 1-4) • WsMACRO = 1.4 mm s-1 • only 29% of SPM • Wsmicro fell 0.8 mm s-1 FASTER • 71% of SPM • nfMicro : 2.6-3 • 80% of 643 mg.m-2s-1MSF(mass settling flux)
INSSEV Settling velocity measurements • TMZ: t = 0.4 Pa, SPM = 5.5 g/l • Floc population more bi-modal • Macroflocs dominate TMZ • D > 160 mm (SBs 5-12) • 2730 macroflocs, Dmax = 940 mm • WsMACRO = 10.2 mm s-1 • 81% of total flocs • 95% of SPM (1/3 in SB12) • 85-95% porous, nf ~ 2.5 • 99% of 61 g.m-2s-1 MSF • Wsmicro fell 9 mm s-1 SLOWER • 5% of SPM
Sediment & Flocculation Dynamics • A significant data set has been collected • There is no evidence for a tick HCBS layer in the navigation channels in the vicinity of Zeebrugge • Siltation of the harbour of Zeebrugge is strongly influenced by tidal filling and the eddy pattern in the harbour mouth. • Siltation of the harbour of Zeebrugge shows a strong influence on the neap-spring tidal cycle. • Density differences due to the fresh water discharge into the harbour have only a limited effect on the harbour siltation.
Conclusion • A significant data set has been collected on fine cohesive sediment characteristics in the vicinity of Zeebrugge, calling for: • A regular follow up: a series of fixed stations is highly recommended • Efforts should be put in characterizing the physical conditions leading to the siltation rate in the access channels and the port • Integration of wave/wind characteristics is necessary • Integration of biological parameters is recommended