Date of Award

9-1975

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geography

Supervisor

S. B. McCann

Abstract

The Malpeque barrier chain is a 43 km long sandy barrier shoreline, consisting of three islands and a spit at the present time. This shoreline has limited fetch distances to the north, northeast, and east, and storm waves dominate the coastal environment in the 8 to 10 month period of ice-free conditions. Longshore sediment transport is to the southeast for the exposed eastern section of the field area, at rates generally between 160,000 and 250,000 m³ yr⁻¹, but transport rates are lower in the sheltered northwestern section. The longshore sediment transport rates are comparable with moderate rates for open ocean coasts.

The age of the barrier shoreline is between 1,500 and 3,000 years. The oldest section of shore, Hog Island in the east, incorporates a progradational series of ridgos formed earlier than 1765, which possibly relates to the initiation of the barrier island chain. This western section of coast is a transgressive barrier shore, owing much of its present form to coastal actions in the last 200 years. The barrier islands occur as a shallow sediment body over bedrock, and are generally less than 15 m thick in the duno area, thinning seawards to the outcrop of bedrock at depths between 9 and 17 m.

The occurrence of numerous temporary inlets within the last 200 years has resulted in 12.5 km of the present barrier island chain's morphology being affected by actions associated with inlet development and closure. The temporary inlets provide the setting for much of the landward movement of sediments within the island system. Over 90% of the sand moving landwards in the 1935-68 period in the Malpeque barrier chain occurred at contemporary and former inlet locations. Inlets appear to be more important in this regard than on transgressive barriers along some ocean coasts, where widespread overwash and wind action are also significant in moving sand landwards.

The restricted nature of overwash results in the common occurrence at narrow barrier island sections, less than 200 m in width. The combination of closed inlets, washover passages, and narrow sections gives rise to the unusual highly irregular lagoon margin characteristic of the transgressivity barrier shore here.

The average shoreline erosion rate for the 1935-68 period is low - 0.26 m yr⁻¹. Erosion was discontinuous in location along the shore in that period and in the 200 year period, occurring mainly in the lower energy western half of the field area and along eastermost Hog Island. The bread alongshore variations in the occurrence of erosion imply sediment deficits in the littoral zone of the eroding sectors: the irregular discontinuous nature of the shoreline retreat in those locations over years to decades indicates the marginal mature of the deficits. Shoreline erosion in the eroding sectors appears to result from the short-lived formation of 400-600 m lengths of narrow foreshore, causing accelerated erosion under storms encountered each winter.

Shoreline erosion at Malpeque is a result of the limited sand reserves in the nearshore and on the shoreface, in relation to the energy levels of the coastal processes moving sand seawards and alongshore. Sea-level rise is not the primary cause of shoreline retreat on this shore; evidence also suggests that sea-level rise is not significant on barrier coasts elsewhere.

\,~ut, ,nd l4tormwtw\'l!I dl~n\tl\nt" tl(\~ ('oat'\~Al,~nvtl'onml'nt il\'~lW a tl.'·lO u\Qu.th ('dod I.'f tC(\"'f~l't' ,,(\nd t ~tN'N. 1,~~nf'\~hol'~' I,itl"\HIMl\t tt"nn[~p\l1.·t bt t\.) th0 I outhet'\8t for th,'I ~xposyd "'l\!it,\rn tltl'Ct lon,or thl~ fh'lh\ l\f\"l.l. ftt r{\tllf\ I (\n,\r«l1y b('t"''''''N\ lb0000 n~\d r~o,ooo m) yr-t • but trftuf'l'\wt ,~fttl\!i nt'" OW0r in th0 ·8ht.'ltl.~rJd northw"'l.i\tl'l"U (4\"ction. 'Ihl\ r'ongt"lt\,'\n' tl0d'l\ynt < ... ' rnl\t\port rat(>t' nr~ ~omlH\t'Ab)l' ....1th mNh'rnu' t'~t~~f\ for (.l\Wn o~"t\ C\)(\8tf'. Tbo"&\Stl of thll bnrri,'r 8ho1'l'll1\('1 16 h..'twNm \t~OO ~\ld 3.0ll0 '-'arN. Th('l oldNlt ~l\ction of flbon't UOR ll\~l\t\d in th\' ""at tincot\po.t'&\t\.'l\ . I ' . .. pr"l\rndat io••l .ortao 0 f rids". forUled o.r11or tim. 17C~' whie11 I possibly rcl'l.lte9to the initiation of thl' barrier island ch.:lin. Th~\ . ! !o ~et"t~rn l"(let!on of COl.lftt iN n trft1\~Brl'lf\t\lv\.l \larril'lr flhon\, owing Illl,cb ~f ito presont form to cont'\tnl action6 in the Ift~t 20B y~ar~. Th~ . I barrior i81~~d8 occur as a shallow sedimQnt body.over bl'drock; and ar~ , . 1.U

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