.Journul of Coastal Research 220-2 40 Itoy,,1Palm I\"ach, Florida Wintor 19})}) Magnitudes, Spatial Extent, Time Scales and Causes of Shoreline Change Adjacent to an Ebb Tidal Delta, Katikati Inlet, New Zealand D. Murray Hicks, Terry M. Hume , And rew Swales, and Malcolm O. Green National Inst it ute of Wat er Na t iona l Inst it ute of Water a nd Atm ospheric Research and At mospheric Resea rch Ltd . Ltd . P.O. Box 8602 P.O. Box 11-115 Christchurch, New Zea land Hamilton. New Zealand ABSTRACT _ HICKS. D.M .: I!GME. '1' .1\1.: SWALES. A.. and GI{EEN. M.O.. J ~) 99. Magnitud es. Spat ial Extent. Time Scales and ,tllllllll:. Causes of Shore line Cha nge Adjacen t to an Ebb Tidal De-lta. Kati kati Inlet. New Zealand. •lourn al ofCoaet« ! Research. ~ 1fil ] l. 220-240. Royal Pa lm Heach I Florida I. ISSN 07 49 -020H. The ebb delta and adjacent beaches at a mixed energv Itide dominated I inlet were monitored to identify the influence eusus~ ~-'" S-- of the inletlebb-delta syste m on erosion and accretion of the udjaccnt beaches. Sub-aerial beach profi le dat a were collected c+ at 2fi locations at monthl y intervals over four yea rs. Beach excursion distances and volumes. dispersion diagrams. prin­ cipal components analysis. an d time-series corrolutions with wave pnram etors were used to extract till' magnitudes. time scales. and causes of the b"ach cha ngo». TIll' divergt-nce induced in tilt' regional longshore transport regime by wave refraction over the ebb deltn was also invest ignted. Cha ng"s on till' adjacent benches. beyond the wave shadow of the ebb delta and more tha n :l-4 km from till' inlet controlinc, were dominat ed by a quasi-an nual signal which reflected cross-shore sand tr ansport forced by storm waves. L"ss"r changes were linked with longshor« tra nsport. which reversed its prevailing direction at inu-r-annuul time sea les. TIll' longshore transport caused sand oscillation between the small headlands boundi ng till' wester» lu-nch. sand inputs to till' inlet/ehh-deltu system from t he beaches either side. and a standing patt ern of erosion and arcret ion about the inlet due to ref ractio n-induced tran sport divergence. Immediat ely behind the ebb delta till' quasi-annual storm-wave signal was either small or non-discern ible against much larger . multi­ year changes. TIll' multi-year changl's corrolatcd with till' longshor« transport potential and wen' interp reted as being associated with scouring by till' mar ginal flood-tidal flows. accretion of sa nd bars migrat ing shoreward from the ebb delt u pla t form and flan ks, and longshor« t ra nsport divergence. Most of these cha nucs appenrcd to bl' the on-shore signature of part of a cycle of sa nd circulatio n bet ween till' I)('aclll's and th e ebb deltn bar s. ADDITIONAL IN DEX WO RDS: Bccuh erosinn, eb b tida! del ta , tidu! i // /I'I . I)('I/(·h pm/il"s. clispersinn diagram»: prin ­ cipul componen t» ana lv sis , lo//gsho r" tra nsport . trut isp ort di/ '''''g"//(',,. /idl speed parameter. Sou thern Oecillatio» I l1 d".I" . coast al hazard s. INTRODUCTION Past st udies have s hown t hat the ebb-de lta/i nlet infl uence on beach es eit he r im med ia tely be hind the ebb delta or adja­ Ebb tidal deltas are known to in flu ence th e sta hility of cent to it is exe rted through various processes. These inc lu de beaches adja cent to thei r in lets Isee FITZl;EHAL\l, 1988, a nd wave s helte r ing by th e ebb tidal body (FrI'ZC ERALD et at.. F ENSTl';l{ a nd DOLAN, 1996, for reviews ). A thorough unde r­ 1979 ), wave refraction effect s wh ich ca n ca use littoral drift stand ing and quantification of this influen ce is required wh en to be locall y t ra ppe d on th e downdrift s ide of the ebb de lta assessing coastal eros ion ha zards in th e vicin ity of tidal in­ (HAYES a nd KANA, 1976), littoral drift bypassing effects, and lets . Key questions a re: Ii) "wh a t is th e magnitude of the temporary or permanent trappin g of beach sand on the deltas beach changes; Iii) h ow do these changes vary a longs hore (F r I'Zl :EI{AL\l a nd HAYES, Hl80; FrI' Z(; ~;I{ALll , 19841. Most of away from the inl et; (iii ) wh at time sca les a re represented in th is work ha s been focused on des criptions and con ceptual the va rious patterns of bea ch cha nge; and (iv l what processes mod el s for the various proc esses involved . are ca using th e changes a nd what a re the main forciru; fun c­ C haracteristic time a nd spatia l sca les of ch ange associa ted t ion s"? Wh en definin g eros ion hazard zones on th o ba si s of with various processes h a ve been reported . The wor k of FITZ­ s u rveyed hi storica l changes in s hore-line posi tion or s hore l;EI{AL\) and associa tes on mi xed ene rgy shores le.g .. Frrz­ sand volumes . th e monitoring frequen cies in s pan ' and time l;E){AL\1 and HAYES. l ~lH O ; FITZl;EHALll. 1988 1ha s s hown 3­ sho uld match th e ac t ua l temporal a nd al on gshore sca les of 1() yea r erosion and ac cretion cycles associa ted with th e beach cha nge. wh ile a ny a tte m pt to model th « e ros ion ha zard gro wt h of sa nd ba r complex es on the e bb delta platform. the requires th at th e effects of t he va r ious processes be identified s horewa rd mi gration of t hese bars, and their event ual weld­ and se pa ru ted. ing onto th e adjacent beach . Irres pect ive of whether th e inl et bypasses s ign ifica nt volu mes of sa nd alongshore or sim ply 9 70 1(; rcccii v d and (W('('pled ill rcrisiun I:' J ulv / 9!J7 circ u la tes sand around t he cbb-dc lta/ inlet sy ste m in a closed Shoreline Cha ngl' in New Zea land 22 1 loop, this sa nd t ran sfer occurs mainly in packet s, with sa nd 24 km long mainly Holocen e sand barri er of Matakan a Island accre t ion on th e adjace nt beach a nd/o r relea ses alongshore (MUNRO, 1994 ). Th e 80 km - of the lagoon that Kati kati inlet bein g limited by th e ra te at wh ich bar complexes ca n migra te drains has exte ns ive intert ida l areas. Th e spri ng tidal prism landward off th e ebb delta. Th e time sca le of th ese cycles is 95 X 10" m' and is dominated by tida l flows (l-/ICKS a nd tends to be shorter at sma ller inlet s, which have smaller ebb H UM E, 1996 ). It is a natural in let, without artificia l st ruc ­ deltas (WAI.TO N and ADAMS , 1976; HICKS a nd HUl\1E, HJ96 l. t ures, dr edgin g, or sand extrac tion. A rock headland on the Similarly , th e len gth of shore affecte d increases as the ebb northern shore provides positiona l stability to th e inlet , a sit­ delta size increases. Th e migr ating bars also extend further uation typi cal of tid al inlets on this part of th e New Zeal and alongshore when th e main ebb cha n nel is inclined at a lower coas t 01[JME and HEIWENDOH F, 1992). Th e inlet has only a angle to th e gros s shoreline t rend. Migr ating bar complexes small active flood-tide delta ca pping an older shoa l <DAV IS et ranging in len gth from 300 m to several km a nd inducing al., in preparation ). Th e ebb tid al delta is an order of mag­ shoreline erosion/accre tion shifts of ten s to over 100 m have nitude larger, extending offshore for about 3 km to 20 m been described ie.g., FITZUERAI.D and NUM MEDA L, 1983; depth, spanning 6 km alongshore, a nd storing some 30 X 10" FITZGERALD, 1984; SMITH and F ITZG ERALD , 1994 ). Larger m' of sa nd (HICKS a nd HUME, 1996 ). scale shore erosion/accretion cycles spanning periods of 7- 40 Morphologicall y, th e Katikat i ebb delta consists of a tri­ yea rs occur also. Th ese are governed by th e orienta tio n of th e a ngular-sha ped, largely subt idal swas h platform, offset main ebb cha nnel, which cyclically flips to either side of the sligh tly to th e south-ea st of th e inlet (Figure 2l. Th e main shore-normal. Shoreline shifts of 300- 400 m and extending ebb tid al cha nnel crosses th is platform, first at a southerly several km alongshore have resulted (e.g., FI'l'z<amALD , inclin ed a ngle to th e general shore-norma l direction becau se 1984 l.