IMPACT OF HYDRODYNAMIC PROCESSES ON SEDIMENTATION IN INNER PART OF BAY

Ants Erm, Jüri Kask, Victor Alari @ Inga Lips

Marine Systems Institute at Tallinn University of Technology, Akadeemia tee 21, 12618, Tallinn, . [email protected] Objectives

• It is shown by a lot of studies since 2003 that large sections of the coast of are influenced by wakes generated by high speed vessels. (Soomere, Rannat, Erm, Kask, Alari, Kõuts ,Parnell, Kurennoy, Didenkulova, Kelptshaite and many others were taking part in these studies)

• Based on combined in situ measurements of surface waves, underwater irradiance and the fluxes of resuspended sediment it is also shown, that the anthropogenic resuspension plays a key role in Tallinn Bay during the relatively calm spring and summer seasons. In some places at about 400 -1000 kg of sediment is resuspended and carried away from each meter of coastline annually.

• The real measurements of resupension are curried out in three coasts of the Bay ( Island, Beach and Katariina Beach).

• The question we try answer here is: what processes are going in other places. Measuring sites Keibu station and Shallow, weather stations near Keibu Bay and on the Tallinn Shallow are flagd.

a

t

i r i P Viimsi Tallinn Tallinn Bay Aegna

y a Paljassaar B i Kopli l p o K Longitude, E Naissaar Banks Uusmadal Bank

aar iss Na 24.4 24.5 24.6 24.7 24.8 24.9 Suurupi

59.6 59.5

59.65 59.55 59.45

N , e d u t i t a L

0

5 1

0

2

0 3 Longitude, E Longitude, (b) Keibu Bay Keibu (b) 23.6 23.65 23.7 23.75

59.3

59.28 59.26 59.24 59.22 Latitude, N Latitude, Latvia Estonia Longitude, E Longitude, Gulf of Finland of Gulf (a) Area of interest of Area (a) Finland Gulf of Gulf Riga 22 24 26 28 30 Baltic Proper

60 59 58 57

N , e d u t i t a L Investigatigations near the Katariina jetty have shown:

• the diameters of resuspended sediment particles ranged from 2--150 µm, remaining in 90% of cases in the range of 2--40 µm;

• The absolute majority of resuspension events was caused by ship wakes. They induced resuspension higher as the wind waves’ one about 50 times at the 0.2 m level and about 200 times at the 0.5 m level from the sea bed.

• the near bottom orbital velocities generated daily by fast ferries are equivalent to those induced by waves excited by at least 18 m s -1 southwestern winds and 12 m s -1 northern winds. About 400 kg of sediment is resuspended and carried away from each meter of coastline annually.

A quetion was coming up – how much the wind waves altogether act in resuspension processes in the Tallinn Bay? Measurements at the Katariina Jetty (pressure wave sonde was used) Wave amplitude (a) and Kd,502(t) (b) in the morning of 21

Tallink SuperStar, 21 july 2009 July 2009 (Katariina 2.5 Jetty) Kd,502

) 2.0 Radiometer Ramses-ACC- -1 1.5 VIS (Trios GmbH, (m

λ 1.0 Germany) was used to d,

K 0.5 monitor underwater light 0.0 field. 09:00 09:07 09:14 09:21 09:28 09:35 09:42 Distribution of diameters of sediment particles sampled from several horizons from the sea bed in 50.5 h period on 21--23 July 2009, ESD --- equivalent spherical diameter An imaging flowcytometer FlowCam of Fluid Images Technologys (USA) was used.

1000000 0.11m y = 5E+07x -2.4681 R2 = 0.9245 0.9 m y = 1E+07x -2.3308 R2 = 0.9148 100000 0.8 m y = 2E+07x -2.3813 R2 = 0.9197

-2.2658 2 ) 0.5 m y = 1E+07x R = 0.913

-1 10000 h -2 (m 1000 n

100

10 0 50 100 150 200 ESD (µm) A question came up – how much altogether the natural waves act in resuspension processes in Tallinn Bay?

• Sub-surface pressure transducers used for measuring the waves and wakes measure the instantaneous pressure that is the sum of air pressure, hydrostatic pressure and wave-induced dynamical pressure. In case air pressure and hydrostatic pressure are assumed to remain constant, the dynamic pressure under water is expressed with equations derived from the linear wave theory.

• Two important characteristics are derived from these data: significant wave height and the period corresponding to the first moment of the spectrum.

The next question was, is the linear theory adequate to discribe the real situation? Some times later

• simultaneous wave pressure, near bottom velocity and turbidity measurements were curried out in Naissaar Shallow (Tallinn Bay) and in Keibu Bay (NE coast of Estonia) in 2009 and 2010. Thes data were used for the validation of wave sonde: • 1) a very good coincidence of in situ measured (by ADV Ocean Hydra, Sontek) near bottom velocities and calculated from the pressure wave gauge (PTR Group, Tallinn) data wave parameters;

• (2) that the critical near bottom velocity for starting resuspension in Tallinn Bay is about 20 cm/s; Time-series comparison of near-bottom orbital speeds derived from wave gauge measurements and the ADV (Ocean Hydra of Sontek). Wind and bottom current parameters (a), turbidity and waves’ orbital velocity (b) in the Naissaar Shallow.

50 6500

40 vorb (cm/s) Current (cm/s) Wind (m/s) Turbidity 6000

30 5500

20 5000

10 4500 Speed (m/s; cm/s) Turbidity (counts) 0 4000 2 2 2 2 3 0 03/01/2010 05/01/2 07/01/2 09/01/2 11/0 13/0 2/ 4/ 6/ 8/ 0/12/2009 1/01/2010 12/2009 12/2009 12/2009 12/2009 1 1 /2 /2 0 0 0 0 0 10 10 10 10 10 According to the HIRLAM wind fields between years 2006-2009

• 6 m/s represents the typical state of the Tallinn Bay in autumn and winter (usually in summer, the background of wind waves is really negligible as the mean wind speed in Tallinn Bay during summer months is 3-4 m s-1)

• frequency of winds >12 m/s was 6 %,

• frequency of winds >18 m/s was 0.1 %. A numerical wave model SWAN applied (by Victor Alari) under forcing conditions of 3 wind speeds (6, 12 and 18 m/s) and in case of 16 directions.

• Winds with speed of 6 m/s do not resuspend sediments, this means that under typical conditions the wind wave induced resuspension does not prevail. • Southern winds with speed 12 m/s do not induce any resuspension. • Eastern winds with speed 12 m/s resuspend sediments at the tip of Kopli Peninsula. • Northern and western winds induce resuspension at eastern coast of Tallinn Bay and in the Paljassaar Bay, also around tips of and Kopli peninsulas. In case of northern and western winds the resuspension depth does not exceed 10-15 m and most of the time not more than 10 m. • In the Kopli bay resuspension is possible only when wind is blowing from the sector west to north. The resuspension only occurs in the southernmost part of the Kopli bay where water depth is less than 10 m. Conclusions

• The diameters of resuspended sediment particles ranged from 90% of cases in the range of 2--40 µm.

• The critical nearbottom velocity starting resuspension is about 20 cm/s.

• The absolute majority of resuspension events near the Katariina Jetty in July 2009 was caused by ship wakes.

• Pressure sonds could be used to determine the near bottom velocities.

• Under typical conditions the wind wave induced resuspension does not prevail in the Tallinn Bay.