MORPHOLOGICAL AND SEDIMENTOLOGICAL EFFECTS OF SAND AND GRAVEL EXTRACTION IN POLISH MARITIME AREAS - AN OVERVIEW Szymon Uścinowicz, Piotr Przezdziecki, Wojciech Jegliński, Urszula Pączek (Polish Geological Institute - National Research Institute) Kazimierz Szefler, Jarosław Nowak (Maritime Instituite - Gdansk) EMSAGG Seminar: Reclamation & other uses of marine aggregate 22 April 2016, Gdańsk, Poland Historical Record of Marine Sand and Gravel Extraction in Poland Puck Lagoon Sea Sand for beach nourishment Słupsk Bank Southern Middle Bank ca. 830 000m 3 of gravel was extracted between 1985-1989 from Słupsk Bank Gravel (construction aggreagates) 1
Projects and Papers Related to Environmental Impact of Sand and Gravel Extraction Projects: 1/ 1988-1999: Hydrological, sedimentological, morphological and biological changes caused by gravel extraction from Słupsk Bank (Słupsk Bank case study 1) 2/ 2003-2004: The stability and the degree of renewability of marine aggregate deposits and investigation of the condition of the environment within the area of agregate exploitation. (Słupsk Bank case study 2) 3/ 2007-2009: Development of the program of remediation for the areas of post-dredging pits at the Puck Bay. (Grant No. R 14 042 03) 4/ 2008-2010: Impact of sand extraction from the bottom of the Southern Baltic Sea on seabed structure and meio- and macrobenthos communities (Grant No. 305/NCOST/2008/0) Papers: 1/ Gajewski L. Uścinowicz Sz., 1993 Hydrologic and sedymentologic aspects of mining aggregate from Słupsk Bank. Marine Georesources and Geotechnology, vol. 11: 229-244. 2/ Szefler K., Opioła R., Rudowski S., Kruk-Dowgiałło L., 2012. Post dredging pits in Puck Bay. Warsztaty Zagrożenia naturalne w górnictwie. Mat. Symp.: 412 423 3/ Uścinowicz Sz., Jegliński W., Miotk-Szpiganowicz G., Nowak J., Pączek U., Przezdziecki P., Szefler K., 2014 Impact of sand extraction from the bottom of the southern Baltic sea on the relief and sediments of the seabed. Oceanologia No. 56 (4): 1-24 Post dredging pits in Puck Lagoon: morhological effects of sand extraction HEL PENINSULA BALTIC SEA by-pas system Władysławowo: diameter ca. 100 m area 6519 m 2 volume 16 500 m 3 depth max. 7.7 m Chałupy size, ca. 930 x 430 m area 205068 m 2 volume 2050683 m 3 depth max. 9.2 m acc. to Szefler et al. 2012 POST DREDGING PITS 1992 PUCK LAGOON Large-scal, deep dredging has created a permament morphological features in lagoonal environment. 2014 2
Słupsk Bank case study 1: 1988-1999 Hydrological, sedimentological, and morphological changes caused by gravel extraction from Słupsk Bank test field Methods: hydrological measurements single-beam echosounding vibrocoring sea bed observations and measurements (sedimentological & morphological) by scuba divers datum points vibrocores echosounding lines geological cros-section line Observations and measurements were carried out before, during and after exploitation 3
Słupsk Bank case study 1 concentration of suspended matter in water (mg/l) within the test field (average from 12 samples) Time of sampling Min. Max. Av. content content content May 1988 0.7 2.1 1.2 August 1988 1 day before extraction 0.9 3.3 2.0 1 day after extraction 1.0 1.9 1.5 October 1988 1.6 3.9 2.4 The width of the zone of disturbance did not exceed 50 m. Temperature and light extinction fields quickly return to equilibrium state. After two hours there was no traces of dredging operations. ca.50 m Changes of temperature and light extinction coefficient along profiles perpndicular to dredger s course Concentration of suspended matter flowing with water off the dredger were between 1510 and 11250 mg/l. The grain-size distribution was mainly fine (71.9-25.3%) and very fine (15.5-3.9%) sand. Finer grains were observed in amounts smaller than 1%. Settling of sand on seabed resulting of aggregate extraction In traps located 50 m away from the dredger s route the maximum settling rates was 1 221 g/m 2. At distances larger than 50 m the amount of suspensions settling in the seabed decreased very quickly. Fine sand (81.1-70.1%) and very fine sand (27.9-17.7%) was found in traps. 4
Trough depth measured directly after dredging was 0-2-0.7 m. After 2 months the troughs became partly filled. After 9 months traces of the troughs were still visible, and their depth was 0.04-0.12 m. Troughs were filled partly by material sliding down from edges and partly by bottom-migrating fine sands. Słupsk Bank case study 2: 2003-2004 The stability and the degree of renewability of marine aggregate deposits and investigation of the condition of the environment within the area of aggregate extraction Gravel extraction in the area: 2001: 147,000 t 2002: 202,000 t There was no gravel extraction in years 2003-2004 2 km Methods: multibeam echosounding, side scan sonar, seismoacoustic (boomer, sub-bottom profiler, X-star), limited vibrocoring. Surveys were carried out in April 2003 and in July 2004. 5
Słupsk Bank case study 2: post-dredging pits Bathymetric map April 2003 3D model April 2003 Słupsk Bank case study 2: X-star record of sand waves side-scan mosaic, April 2003 6
Słupsk Bank case studies 1 and 2: Sea bed dynamics: Most of changes are within the measurement errors (c. ±0.2 m) Bigger changes (±0.2 0.4 m) are related to the bed forms migration and to filling up the post-dredging pits Changes in bathymetry (April 2003 June 2004) Sand and fine gravel (up to 32 mm in diameter) are transported during storms Changes in the seabed relief are related to the bed forms migration: 0.4 m thick layer of fine sands and 0.2 m thick layer of coarse sands and fine gravel are mobile Gravel deposits are stable Impact of sand extraction from the bottom of the southern Baltic Sea on seabed structure The test field was located at water depth 15-17 m, 6-7 km north from the coast near Władysławowo resort. The area of the research was chosen with regard to planned sand dredging for beach nourishment on the Hel Peninsula. 7
The area of investigation directly before sand extraction (March 2009): a) plan of the test field; b) sonar mosaic of the sea bottom; c) bathymetric map of the sea bottom Area of investigation directly after sand extraction (May 2009) a) bathymetric map of the sea bottom; b) sonar mosaic of the sea bottom; c) gradient of slopes of dredging pits and furrows Four pits were formed with diameters of about 80 120 m and depths of 3 to 4.5 m in the northern part Of the area. The maximum gradient of their slopes was 55. The total volume of the pits left by stationary extraction was about 58 500 m 3. Several irregularly shaped double furrows of different lengths were formed, and several pits were left by unplanned stationary dredging. The lengths of the furrows varied from 30 to 150 m, their width from 5 to 10 m and their depth from 0.3 to 1.9 m. The gradients of the furrow slopes were between 5 and 15. The distance between the furrows of 25 30 m. The total volume dredged by trailer suction dredging and of the small, irregular dredging pits was about 52 500 m3. 8
All the furrows formed by trailer suction dredging had disappeared completely after 11 months except for one depression 70 80 m in diameter and with a maximum depth of 0.5 m left by the deepest pair of furrows, initially 1.9 m deep. The depths of the dredging pits were between 2.5 and 3.0 m, i.e. they had become 2 2.5 m shallower, and the bottoms of the pits were flattened. The diameters of the pits were between 120 and 170 m, i.e. they had increased by 40 50 m. The gradients of the dredging pit slopes were also reduced. The maximum gradient was no steeper than 10. Total volume of the 4 pits from stationary dredging was about 2 000 m3 smaller than directly after the dredging. Impact of sand and gravel extraction from the bottom of the southern Baltic Sea on seabed morphology and seiments Conclusions The thickness of the currently mobile layer of sand, as determined by measurements of the 137Cs content, is between 0.4 and 0.8 m and depends on the grain size distribution. Slope slipping was the main process causing the morphological changes in the pits. The pits also acted as sediment traps for sandy material transported by waves and currents. However, the small changes in pit volume indicate that the supply of deposits from neighbouring areas played a secondary role during the study period. The spatial extent of the changes in type of sedimentary cover was limited to just a few dozen metres around the post-dredging pits following the settlement of the fine sandy suspension. A year after the extraction operations, the thin layer of fine sand had dispersed, and the surface of the sea bottom was covered by deposits of a grain size similar to that prior to extraction. This can be explained by very small amount of fine fraction (<0.063mm) in extracted sediments and the relatively high hydrodynamics of the areas. 9
Thank you for attention szymon.uscinowicz@pgi.gov.pl 10