Sub ma rine land slides on the slope of a sandy bar rier: a case study of the tip of the Hel Pen in sula in the South ern Bal tic
Stanisław RUDOWSKI1, *, Maria RUCIŃSKA-ZJADACZ2, Radosław WRÓBLEWSKI1, 3 and Patryk SITKIEWICZ3
1 Mar i time In sti tute in Gdańsk, De part ment of Op er a tional Ocean og ra phy, Długi Targ 41/42, 80–830 Gdańsk, Po land
2 Uni ver sity of Gdańsk, De part ment of Ma rine Ge ol ogy, Al. M. Piłsudskiego 46, 81–378 Gdynia, Po land
3 Uni ver sity of Gdańsk, De part ment of Geo mor phol ogy and Qua ter nary Ge ol ogy, Bażyńskiego 4, 80–952 Gdańsk, Po land
Rudowski, S., Rucińska-Zjadacz, M., Wróblewski, R., Sitkiewicz, P., 2016. Sub ma rine land slides on the slope of a sandy bar rier: a case study of the tip of the Hel Pen in sula in the South ern Bal tic. Geo log i cal Quar terly, 60 (2): 407–416, doi:
10.7306/gq.1291
The aim of the study is to pres ent the oc cur rence of land slides on the sub ma rine slope of a sandy bar rier. The scale and sig - nif i cance of this phe nom e non is dis cussed. The study area is the Hel Pen in sula. It is a 35 km long young sandy spit. The landform is prograding into the deep part of Gdańsk Bay, and the un der wa ter ex ten sion of its tip is a steep sandy slope. The tip is still poorly in ves ti gated, as it was a mil i tary area closed to re search ers un til 2003. The first de tailed sur vey of the tip on the sea bed was con ducted aboard the re search ves sel “IMOR”. A multibeam echosounder, a sub-bot tom profiler and a side-scan so nar were used, while grab and core sam ples of sed i ment were ob tained. Many forms, com monly cre ated as the re sult of un der wa ter mass move ment, were rec og nized and de ter mined (main scarps, main bod ies, land slide steps, land - slide foots, se ries of small slides, “sheet” slides and struc tural slides). On the one hand, the land slide pro cesses that oc cur in the tip of the Hel Pen in sula are re lated to the nat u ral de vel op ment of sandy spits. On the other hand, they threaten the shore’s sta bil ity. The gen e sis of the land slides is still not known.
Key words: un der wa ter mass move ments, sandspit, dig i tal ter rain model, side-scan so nar, sub-bot tom pro fil ing.
INTRODUCTION
The aim of the study is to pres ent the oc cur rence of land - slides on the sub ma rine slope of a sandy bar rier, and to re veal the gen eral scale of this phe nom e non and its sig nif i cance to eval u ate the state and de vel op ment of the Hel Pen in sula and other sub aque ous slopes of sandy bar ri ers, del tas and ex ca va - tion walls. The study is based on ma te ri als from pi o neer ing re - search con ducted at the tip of the Hel Pen in sula in 2006–2007 (Rucińska-Zjadacz, 2013).
The study area is the tip of the Hel Pen in sula (Fig. 1). The Hel Pen in sula is the most char ac ter is tic fea ture of the Pol ish coast line. It is a 35 km long sandy spit, 150 m wide at its base and 3 km wide at its head.
The hydro meteoro logi cal con di tions of the tip are spe cific and com plex (Rucińska-Zjadacz and Rudowski, 2009, 2015). It is ex posed to winds and waves from ev ery di rec tion, though the west erly winds are dom i nant (Miętus and Jakusik, 2005). The large depth (over 60 m) around the tip makes the waves from
the open sea come di rectly to the shore with very strong and di - rect ef fect. The crests of the waves are sub jected to strong and var ied re frac tion caus ing multi-fac eted in ter fer ence sys tems.
This is im por tant for the for ma tion of a com plex and dis tinct sys - tem of sur face and shelf/slope cur rents, in clud ing upwelling and downwelling. So far, no ad e quate spe cial ized hy dro dy namic re - search has been con ducted in the area of the tip. Ex ist ing in for - ma tion about hy dro dy namic con di tions of this re gion come from the stud ies deal ing with the Hel Pen in sula (e.g., Miętus et al., 2005; Stanisławczyk, 2005) or the Pol ish coast as a whole (Pilarski et al., 2011).
The de vel op ment of the Hel Pen in sula, like the whole South ern Bal tic coast, is mainly re lated to the pe riod of the Littorina Trans gres sion and the last rel a tive sta bi li za tion of the sea level (Uścinowicz, 2014). Dur ing the trans gres sion pe riod, the coast of mo raine pla teau was abraded and its sed i ment was trans ported long shore in an east ern di rec tion. The Hel Pen in - sula was formed as a spit fol low ing the de po si tion of this sed i - ment (Tomczak, 1995a). The em bry onic spit de vel oped to - wards the cen tre of Gdańsk Bay. The north ern shore of the spit was purely an ex ten sion of the land shore line. Due to a later sea-level rise, the South ern Bal tic sea shore was re tracted. The spit moved on to wards the south, un til it reached land sed i ment.
About 5500 years ago, the sea level be came rel a tively sta bi - lized. The Hel Pen in sula had been grow ing very fast, un til it reached a deep part of the Gulf of Gdańsk. Af ter that, the de vel -
* Corresponding author, e-mail: starud@im.gda.pl Received: July 22, 2015; accepted February 17, 2016; first published online: April 12, 2016
op ment of the spit be came slower (Tomczak, 1995a; Uścino - wicz, 2003). About 1760 years ago the tip of the Hel Pen in sula reached the place where the town of Hel is lo cated to day, and about 1000 years ago the shape of the spit be came sim i lar to the pres ent shape of the Hel Pen in sula (Tomczak, 2005).
A 100 m thick layer of Ho lo cene sed i ment, found in most phases of the Bal tic de vel op ment, is lo cated on the dis tal part of the Hel Pen in sula (Tomczak, 1995b). Li thol ogy and bio - stratigraphy of in di vid ual parts of the layer have been gen er ally de ter mined based on drill ing re sults. These are sandy and muddy de pos its from sub se quent phases of the Bal tic Sea de - vel op ment (Tomczak, 2005). These lay ers lie con sec u tively on Pleis to cene tills (ca. 10 m thick) and Cre ta ceous marly rocks (whose top is at about 100 m b.s.l.) (Tomczak, 1995b).
The un der wa ter slope of the Hel Pen in sula tip is com posed of a se ries of col lu vium de pos its, pri mar ily me dium- and fine - -grained sand. Peb bles, frag ments of peat, or ganic de tri tus and other el e ments also oc cur there (e.g., Musielak, 1989; Kra - marska, 1995; Tomczak, 2005). The sea-floor around the tip is com posed of fine-grained silty sand and sand-silt-clay sed i - ment (Kramarska, 1995). The un der wa ter part of the tip of the Hel Pen in sula is a steep slope (lo cally 20° or more). The foot of the slope is lo cated at a depth of nearly 60 m (Rucińska - -Zjadacz and Rudowski, 2009). As re gards non-tidal bas ins, there are few for ma tions of this type in the world. The most sim i - lar landform is Long Point on Lake Erie, but its slope is not so steep and its foot is lo cated at merely 40 m b.s.l. (Davidson- Ar - nott and Conliffe Reid, 1994).
Only a few stud ies have been con ducted in the re gion of the tip of the Hel Pen in sula, and all of them on the shore (Wünsche, 1904; Łęgowski, 1925; Bączyk, 1963; Grygorenko, 1972;
Graniczny et al., 2004). Re sults of these stud ies show that the tip is grow ing to wards the south-east; how ever, the west ern shore is eroded. There are some rea sons for the lack of stud ies in this re gion. First, the area was closed for re search ers un til 2003, be cause it had been a mil i tary area. Sec ondly, pre cise
meth ods of rec og niz ing sea bed fea tures have not been af ford - able in the past.
Sub ma rine mass move ment is hard to be ob served in ac - tion. Usu ally, only ef fects of the pro cess are re cord able (land - forms and de pos its). In our times, non-in va sive meth ods (mainly: a multibeam echosounder, a sub-bot tom profiler and a sidescan so nar) can be used to sur vey a sub ma rine land slide.
These meth ods are a good tool for ob tain ing the char ac ter is tics of the slide (Ca nals et al., 2004; GazioÈlu et al., 2005; Sichi et al., 2005; Bozzano et al., 2009). The re sults ob tained based on a clas sic anal y sis of sed i ments were of lim ited sig nif i cance be - cause grab sam ples are col lected from ran dom points with out the re la tion to the forms of sea-floor re lief. The main ba sis for de ter min ing the slope re lief and na ture of its sur face is full spa - tial (no in ter po la tion be tween mea sure ment points) reg is tra tion of the sea-floor sur face and seis mic pro fil ing. The re sults of par - ti cle-size dis tri bu tions and the de scrip tions of core sam ples could only be used aid ing with the geo log i cal in ter pre ta tion of geo phys i cal data.
The first stud ies con cern ing the un der wa ter tip of the Hel Pen in sula were con ducted in the years 2006–2007 (Rucińska - -Zjadacz and Rudowski, 2008, 2009, 2015; Rucińska-Zjadacz, 2013, 2015). A sim i lar work was per formed in the nearby area of the Hel Port (Rudowski et al., 2015).
METHODS
The re search area is a ca. 1 km wide poly gon sur round ing the tip and par al lel to the shore line. It in cludes the un der wa ter slope of the tip of the Hel Pen in sula (Fig. 2). The study is based on ma te ri als from the pi o neer ing re search car ried out in 2006 and 2007 (Rucińska-Zjadacz, 2013). Sur veys were con ducted aboard the re search ves sel “IMOR” by the staff of the De part - ment of Op er a tional Ocean og ra phy of the Mar i time In sti tute in Gdańsk.
Fig. 1. Lo ca tion of the study area
A – sit u a tion on the Bal tic Sea; B – sit u a tion on the Gulf of Gdańsk; C – sat el lite im age (DigitalGlobe af ter Google Ink., 2015, taken on 1 June, 2014); D – ae rial photo of the tip of Hel Pen in sula (taken by P. Domaradzki on 7 June, 2013);
P – view point of the ae rial photo (D) marked on the sat el lite im age (C)
A re lief sur vey was per formed us ing a multibeam echo - sounder (Seabat 8101). Based on the ob tained point cloud, a dig i tal model of the bot tom was gen er ated, and a full spa tial im - age of the sea-floor re lief was achieved.
The char ac ter of the sea bed sur face was re corded by a sidescan so nar (EdgeTech MF4200) and, lo cally, by an un der - wa ter TV (ROV TV sys tem).
Twelve seis mic pro files (Fig. 3) were per formed by sub-bot - tom profilers SeaBat Oretech 3010 and AG&G Boomer, to find out the in ner struc ture of the slope. The ar chived res o lu tion of
the seis mic re cord is ca. 0.3 m with 2–3 m of pen e tra tion. The deeper parts of the seis mic pro files are not use ful for in ter pre ta - tion be cause there are nu mer ous crossed lat eral and ver ti cal mul ti ple re flec tions, re lated to the sea-floor mor phol ogy and the con fig u ra tion of land slide lay ers.
Ad di tion ally, three core sam ples (by a vibrocorer VKG-3) and 62 grab sam ples (by a Van Veen Grab Sam pler) were col - lected (Fig. 3; Rucińska-Zjadacz, 2013). The mac ro scopic de - scrip tions and sieve anal y sis (1 phi) were per formed.
The re sults of the non-in va sive re cord ing meth ods were geo - log i cally in ter preted based on knowl edge of the re gion (e.g., Pikies and Jurowska, 1992; Uścinowicz and Zachowicz, 1992;
Tomczak, 2005) and ex pe ri ence in the meth ods (Fish and Carr, 1990; Blondel and Murton, 1997; Stoker et al., 1997; Blondel, 1999; Lurton, 2002; Sichi et al., 2005; Grządziel, 2008; Rudowski and Rucińska-Zjadacz, 2010). Sonograms were de ter mined from the sidescan so nar im ages. They iden ti fied ar eas that have a sim i lar char ac ter to the bot tom sur face, with re spect to the phys i cal fea tures of the sed i ment (grain-size, den sity and com - pac tion) and the re lief fea tures of the sea-floor. Seis mic fa cies were dis tin guished on the sub-bot tom profiler re cords by con sid - er ing the char ac ter of the re flec tions. The li thol ogy of the sed i - ment is con nected with the scat ter ing and ab sorp tion of acous tic en ergy (Badley, 1985; Stoker et al., 1997; Wicher and Necel, 2000; Przezdziecki, 2004; Rudowski and Rucińska -Zjadacz, 2010). Geo log i cal in ter pre ta tion of the seismo acoustic re cords was per formed with con sid er ation of the core and grab sam ples.
RESULTS
The anal y sis of 62 grab sam ples re veals a sig nif i cant vari - abil ity in sur face sed i ments of the sub ma rine slope. Based on the ob tained re sults, the fea tures of sed i ments have been de - scribed re lated to the main mor pho log i cal forms (macroforms) (Figs. 4 and 5), in clud ing a coastal shal low, slope edges, slope sur face, and slope fore land. The re sults are con cerned the east ern, the cen tral and the west pro files (Fig. 4).
Fig. 2. Re lief im age of the study area (Rucińska-Zjadacz and Rudowski, 2009) The pic ture of the land is the orthophotomap
(CODGiK, July 10, 2010)
Fig. 3. Doc u men ta tion map
The dif fer en ti a tion of sur face sed i ments is high lighted by the cha otic pres ence of var i ous el e ments de pos ited on the slope and fore ground (Fig. 6) due to abra sion of the Hel Pen in - sula shores. These are frag ments of trunks and branches, plant de tri tus, con crete rub ble, gravel and stones, frag ments of an i - mal bones (mainly fish) etc. The di verse na ture of the sed i - ments is also been ob served in the sed i ment bore holes (Fig. 7).
There are interlayers of rel a tively well-sorted fine sand and silty sand as well as un sorted sand. Con se quently, the na ture of the sed i ments is com pat i ble with the clas si cal col lu vium con cept, in clud ing downslope trans port and slides of the ma te rial from abun dant sup ply of full-term di ver si fied sed i ments on the slope and fore ground.
An ac cu rate over all pic ture of the sea bed sur face has been ar chived us ing non-in va sive meth ods in or der to rec og nize the sea-floor re lief. Nu mer ous, com mon sub ma rine mass move - ments, var ied in size and type, have been ob served. Se lected ex am ples are pre sented and described below.
Large fully de vel oped land slides oc curred in the cen tral part of the tip. Their main scarps and main bod ies were rec og nized
(Fig. 8). The main bod ies were com posed of fine sand and silty sand, with the ad di tion of coarse sand and clay. The sed i ment was poorly sorted. More fine-grained sed i ment is ob served on the un der side of the main bod ies than on the up per sides. A num ber of bricks, and pieces of con crete and wood were also found on the sur faces of the main bod ies. These in cluded frag - ments of de stroyed de fence coastal forms and build ings, as well as anthropogenic rub bish.
Three main seismoacoustic fa cies were dis tin guished within the in ner struc ture of the slope. These are: fa cies I – re cent sur - face sed i ment, fa cies II – young col lu vium, and fa cies III – older col lu vium. The ex am ples of the seis mic re cords are pre sented in Fig ure 9.
Fa cies I – re cent sur face sed i ment. It is a thin cover ly ing on a rel a tively flat slope area. Its thick ness is roughly from 20 to 50 cm. It arose from the grav i ta tional ac cu mu la tion of sus pen - sion, and is com posed of fine sand in the shal lower parts of the tip and fine silty sand in its deeper parts.
Fa cies II – young col lu vium. The seismoacoustic im age of the top bound ary of the fa cies owes its fea tures to the oc cur -
Fig. 4. Dif fer en ti a tion of grain-size pa ram e ters (in phi units) of sur face sed i ments within bathymetric pro files: 1, 2 and 3 (pro file lo ca tions on Fig. 3) MP – coastal shal low sed i ments, SK – edge zone sed i ments, St – slope sed i ments,
DM – slope base sed i ments, M1 – mean, M2 – stan dard de vi a tion, M3 – skew ness
rence of shells and peb bles that are com monly found in vibrocore sam ples. The lower bound ary of the fa cies is rag ged and ero sive. The thick ness of the fa cies ranges from a few decimetres on the up side of the slope to 6 m on the un der side.
The fa cies is in ter preted as be long ing to a group of young land - slide for ma tions with char ac ter is tic dis turbed sandy sed i ment.
Fa cies III – older col lu vium. The re corded im age is sim i lar to the Young Col lu vium Fa cies, but the re flec tions are stron ger.
The thick ness of the fa cies is >10 m. The layer be low the bound ary is not vis i ble in the de tail on the re cords. The land slide ma te rial is com posed of highly com pacted var i ously grained sand, in clud ing top pled rub ble.
A rel a tively vast, flat, shal low bot tom is lo cated near the steep slope of the tip. It is an area un der ac tion of waves and cur rents only, with out land slides.
Two main very steep (up to 35°) scarps are ob served (Fig.
8), with edges lo cated 7 m b.s.l. In ter pre ta tion of their in ner struc ture (pro file a–b in Fig. 9) con firms the cor rect ness of the iden ti fi ca tion of the form.
Only the up side of the main body “A” (Fig. 8), ge net i cally re - lated to the main scarp no. 1, has re mained. The rest of the main body may have been dis trib uted on the slope and/or the land slide that is ge net i cally re lated to the main body “B”. It may also be as so ci ated with the con vex parts of the slope lo cated 50–55 m b.s.l., which are in ter preted as a land slide foot.
The main body “C” (Fig. 8) is com posed, among oth ers, of ma te ri als that have slid from the main scarp no. 2. This land - slide is prob a bly the youn gest of all. It lies on the in side of a long nar row land slide fur row val ley. The dis tal part of the main body is an elon gated fan, and nu mer ous steps are on the sur face of the form.
The main scarp as so ci ated with the main body “D” (Fig. 8) has not sur vived. It was bur ied by ma te rial trans ported across the slope from the north-east to the south-west. The sur face of the main body “D” is thinly ridged. It is lo cated in side a wide land slide fur row val ley, and is split by an inlier trough. There are many steps on the inlier trough, which may be older than Fig. 5. Grain-size pa ram e ter val ues of sur face
sed i ments within the mor pho log i cal units (ver ti cal mark ers in di cate av er age val ues)
Units: P – beach, MP – coastal shal low, SK – edge zone; E – east ern part of cape, W – west ern part of cape, St – slope, DM – slope base;
M1 – mean, M2 – stan dard de vi a tion, M3 – skew ness (af ter Rucińska-Zjadacz and Rudowski, 2015)
Fig. 6. Ex am ples of un der wa ter pho tos (ROV TV) of the dif fer ent ob jects on the slope (part of a tree trunk and limbs, con crete and var i ous rub bish)
Lower edge of each photo is ca. 1 m wide
the sur round ing di chot o mous main body, or may have arisen as a re sult of slid ing in the main body sed i ment. The dis tal part of the main body is com posed of a se ries of wide foot fans (Figs. 2 and 8).
The east ern part of the tip (Fig. 8) is com posed mainly of me dium-grained and fine-grained sand. The rate of long shore sed i ment trans port from the north is high, while the cross-shore trans port is di verse. Three types of slide (Fig. 10) can be de ter - mined by con sid er ing their size and char ac ter. These in clude: a se ries of small slides, “sheet” slides (the forms of flat sur face, which are dis placed) and struc tural slides (the forms that are dis placed in con for mity with the bed ding plane and the struc - ture, but not with the trun ca tion of cross wise struc tures). These have been clas si fied by anal ogy to land slides that oc cur on land (vide Kleczkowski, 1955; Klimaszewski, 1978; Hampton et al., 1996; Einsele, 2000; Migoń, 2015).
Nu mer ous steps are found in the un der side of the slope (pro file g–h in Fig. 9). These are rel a tively firm, and their clear re lief is an in di ca tion of a low rate of sed i ment de po si tion. Be - tween the steps the depth dif fer ences are about 2 m. The for - ma tions are com posed mainly of fine sand.
A sin gle step is ob served in many places on the tip. These have arisen as a re sult of mass move ment, like a translational land slide, in con di tions of am ple sed i ment ac cu mu la tion. They are com posed mainly of fine sand, but also con tain coarse ma - te ri als, in clud ing frag ments of bricks, con crete el e ments, roots and branches of trees.
DISCUSSION
The pre sented sub ma rine land slides oc cur on the steep sandy slope of a bar rier mov ing on a deep non-tidal ba sin. So far, land slides of dif fer ent scales and lithologies have been de - scribed oc cur ring on con ti nen tal slopes (e.g., Hampton et al., 1996; Mulder and Cochonat, 1996; Ad ams et al., 1998;
McAdoo et al., 2000; Beyer et al., 2003; Ca nals et al., 2004;
Hühnerbach and Masson, 2004; GazioÈlu et al., 2005; Masson et al., 2006; Elverhoi et al., 2010), large river del tas (e.g., Prior and Suhayda, 1979; Nemec, 1990), and un der wa ter slopes of lakes (e.g., Gardner et al., 2000; Bozzano et al., 2009). Sub - aque ous land slides have been ob served in Po land on the bot - Fig. 7. Borehole sec tions
For lo ca tion see Fig ure 3
tom of Wigry Lake (Rutkowski et al., 2003) and at the ex ca va - tion walls cre ated as a re sult of sand ex plo ra tion in the Puck La - goon (Rudowski et al., 2009). Land slide pro cesses may also oc cur near the tip of the Hel Pen in sula – at the un ex plored west - ern slope of the Hel Pen in sula in the re gion of the Długa Shoal and the Bórzyńska Shoal. How ever, this ques tion re quires fur - ther in ves ti ga tion.
The clas si fi ca tion of land slide forms and phe nom ena, es pe - cially sub aque ous, re quires evolv ing. In this pa per, the slides,
“sheet” slides and struc tural slides have been clas si fied by anal ogy to land slides that oc cur on land. Pre sented re sults may be a start ing point to a dis cus sion on types and na ture of land - slide forms on sub ma rine slopes of sandy barriers.
The multibeam echosounder sur vey was re peated af ter 17 months. Dur ing this pe riod, nei ther re lief nor land slide de vel op - ment was ob served. No re lief changes to the tip were no ticed, ei ther. They were not oc ca sioned even af ter the in tended det o - na tion (in Feb ru ary, 2009) of a wreck from World War II (a barge with mines), ly ing in the west ern part of the slope (Rucińska - -Zjadacz, 2013). Nev er the less, the clear re lief and the in ner struc ture of the slope in di cate that changes oc cur oc ca sion ally and they may be sig nif i cant.
The or i gin for the land slides has not been un equiv o cally de ter mined. Many fac tors could cause in sta bil ity of the slope of the Hel Pen in sula tip, and there are many op tions to con - sider. Land slides oc cur pri mar ily as a re sult of sed i ment over - load (LÝseth, 1999). Wa ter mass move ment (wind waves, sea level changes, in ter nal waves) can also cause a slide (Hampton et al., 1996; Mulder and Cochonat, 1996; LÝseth, 1999; Hühnerbach and Masson, 2004). One of the re sults of wave ac tion is cy cli cal changes to the pore wa ter pres sure, which can cause sed i ment to loosen (LÝseth, 1999). Sea level changes can also ac ti vate changes in ground wa ter pres sure, and the re sult can be a slide. Wave-gen er ated vi bra tions are also an im por tant slide fac tor. Land slides at the tip of the Hel Pen in sula can also oc cur as the re sult of ex cep tion ally strong storm. Other prob a ble rea sons for land slides in the Hel Pen in - sula tip are earth quakes (such as the 2 trem ors that oc curred on 21 Sep tem ber 2004, 5.0 and 5.3 in mag ni tude, with their epi centre in the Kaliningrad Oblast; Wiejacz, 2006), and the loos en ing of sed i ment com pac tion due to the de cay of or ganic mat ter. The pres ence of great gas erup tions (e.g., gas vol ca - noes with crat ers over 200 metres in di am e ter; Rudowski et al., 2010) on the sea bed around the Hel Pen in sula tip should Fig. 8. Re lief im age of the Hel Pen in sula tip
The south-per spec tive is on the top and the east-per spec tive is be neath; land slide’s main scarps (1, 2) and the main bod ies (A–D) are marked by white lines; seis mic pro files a–b, c–d, e–f, g–h (Fig. 9) are marked by black lines; ex am ples of so nar im ages (Fig. 10) are con toured by white lines; the pic ture of the land is the orthophotomap (CODGiK, July 10, 2010)
also be con sid ered as a po ten tial ca pa bil ity to ac ti vate land - slides on the slopes.
On the one hand, the land slide pro cesses that oc cur on the tip of the Hel Pen in sula are con nected with the nat u ral de vel op - ment of a sandy spit. But on the other hand, they threaten the shore’s sta bil ity. This is es pe cially risky in the west ern part of the tip, with a highly ur ban ized coast and eroded shore (Rucińska - -Zjadacz, 2015; Rucińska-Zjadacz and Rudowski, 2015). So far, the im pact of the pro cesses that oc cur on the sub ma rine slope has not been taken into ac count in any stud ies of the dy namic
state of the Hel Pen in sula, in clud ing those ex am in ing the sed i - men tary bud get and the shore abra sion haz ards.
The prob lem re quires fur ther re search, es pe cially given that the area has been poorly in ves ti gated from a hy dro dy namic, geomorphologic and tec tonic point of view. The sub ma rine land slides are a chal lenge for en gi neer ing geology.
The mor phol ogy should be mon i tored, and the de vel op ment of the slope should be ob served. This is all the more sig nif i cant be cause the con di tion and the de vel op ment ten den cies of the whole landform – the Hel Pen in sula – are man i fested via the con di tion of the tip.
Fig. 9. Ex am ples of seis mic pro files: a–b, c–d, e–f, g–h (for lo ca tion see Figs. 3 and 8) Fa cies: I – re cent sur face sed i ment, II – young col lu vium,
III – older col lu vium; in ter pret able area is col oured
CONCLUSIONS
This is the first de tailed pre sen ta tion of the re lief of the sub - ma rine slope of a sandy bar rier known to au thors. The work is es sen tial for any dis cus sion on the struc ture de vel op ment and state of un der wa ter sandy slopes (e.g., bar ri ers, del tas, ex ca va - tion walls) for util ity and cognitive purposes.
The com monly ob served sub ma rine mass move ments, var - ied in size and type, have formed col lu vial sed i ments, tens of metres in thick ness.
The pre sented re sults show a clear de pend ence of the de - vel op ment of the sub ma rine slope on the lat eral sup ply of sed i - ment from the shore. The west ern part of the slope sur face, with lower sed i ment sup ply, is ero sive caus ing abra sion of the shore. The east ern part is a clear ac cu mu la tion area, and the cen tral part of the tip is in a rel a tive equi lib rium state with a pre - dom i nant deposition process.
The pre sented ma te rial shows a lack of ter mi nol ogy in dis - tin guish ing and clas si fy ing sig nif i cant dif fer ences of un der wa ter land slide forms, since the pre vi ous ex pe ri ence is re lated mainly to large forms on steep con ti nen tal slopes in deep wa ters. The re sults can be a con tri bu tion to work in this field.
The pre sented ma te rial pro vides a good base to carry out fur ther de tailed mon i tor ing and spe cial ist stud ies (geomorpho - logical, geo log i cal and hy dro dy namic) that are nec es sary for the proper di ag no sis of the causes and rate of change in the slope un der the in flu ence of cer tain fac tors and pro cesses, e.g.
the de ter mi na tion of geotechnical prop er ties to mon i tor the changes, de ter min ing the ac tion of wind waves in the ba sin, and oc cur rence of internal waves, currents, etc.
Ac knowl edge ments. We would like to thank J. Nowak and all of the staff of the De part ment of Op er a tional Ocean og ra phy of the Mar i time In sti tute in Gdańsk for per form ing the sur veys and pre par ing the dataset. Jour nal ref er ees N. Blažauskas and A. Wysocka are also ac knowl edged for their com ments.
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