.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 WORDS: 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 coasts defined linear bars along its ma rgin , whil e th e outer channel inlet effects dom inated shoreline cha nge wit hin 4.3 km of the terminates in a depositional lobe. Ma rgin al flood-tid e cha n­ inlet and influ en ced the coast for up to 6.8 km on the updrift nels feed into the inlet from both sides betw een th e delta side of th e inlet and 5.4 km on th e downdrift side. On wave platform and th e adjacent shore. Wave-built linear ba rs front dominated coasts they found th at the scale of dominan ce was the north ern side of th e platform, mergin g near th e shore . th e sa me, but th at th e total spa n of influe nce extended up to with bars th at are sub- parallel to Waihi Beach. On the south­ 13 km on th e downdrift side of th e inlet. east side of the platform, linear bars lie sub-pa ra llel to the A key finding from thi s past work is t hat tida l delta-inl et inlet shore , wh ile the platform front ofte n conta ins la rge t ran­ syste ms disrupt th e longshore continuity of th e littoral drift sitory arcua te bars. Th ese linear and arcua te bars periodi­ strea m by bypassin g sa nd in discontinuous packet s. Thus ca lly weld onto th e cuspate forela nd tha t marks th e southern whe n th e ebb delta is accumulating sand, whi ch may occur ent ra nce to th e inlet. Th e morphod yn ami cs of the ebb delta over severa l years, th e downdrift shore mu st erode to mak e and sa nd transpo rt pathways a re detai led by HUME et al . (in up th e sa nd supply deficit a nd a n erosion 'wave' may be ini­ pr epa ration ), Th e delta is compose d of fine sand, slightly fin­ tiated. When t he delta relea ses sand, as when ba rs weld to er in texture and similar in composition to th at on the adja­ th e shore, a migrating sa nd 'slug' is initiated te.g., BRUUN, cent beach es. 1954 ; DOLAN, 1970 ). Such perturbat ions potentiall y induce Waihi Beach, on th e north-west side of the inlet, extends 8 erosion/accret ion cycles well alongs hore from the inlet but, km between th e Katikati headland and the beginning of a becau se th ey probabl y occur over multi-year t ime scales a nd rocky cliffed shore at Wa ihi. Histo ricall y, its average shore­ the ir magnitudes attenuate alongsh ore, th ey may be difficult line position appea rs to hav e changed little since th e first to det ect ag ainst th e larger amplitude, more frequent storm­ cada stral plans wer e surveyed in 1870 m .K. SMI TH, personal induced cha nges that characte rise beaches far from th e inl et . communicat ion; GlRH, 1994 ), although short -term shore line In thi s pap er, we investigate patterns of beach change over fluctuations up to 70 m wide have been observe d ( H A RI~A Y, a four yea r peri od along 25 km of shore cent red on a natural. 1976 l. It comprises fine to medium sand but with the sa nd mixed-en er gy, tid e-dominated inlet with a large ebb delta. close to the ebb delta bein g noti ceabl y coarser th an elsewhere Our a ims a re to isolate th e magnitudes, spatia l patterns, on the beach. characterist ic time scales , a nd ca uses of th e main erosion a nd Matakana Island, th e barrier island on the south-east side accretion signals, both within th e sha dow of th e ebb delta a nd of th e inlet, has a notabl e cuspate foreland or 'hu mpback' further alongsh ore . A compa nion pap er (Hu MEet al., in prep­ opposite th e south-easte rn margin of th e ebb delta, resulting arat ion ) describes the morphodyn amics of th e ebb delta in in th e 'dru mstick' plan view th at is ty pica l of barrier inlet greate r detail. sys tems (l-/AYES and KANA, 1976). Historicall y, th e Matak­ ana Island open-coast shoreline has experienced short-te rm PHYSICAL SEITING fluctuations of 20-30 m but little net change (G IBB, 1994 ). Close to the inlet, between th e humpback and the inlet Katikati Inlet is located in th e western Bay of Plen ty on throat, th e Matakana shore has experienced se vere spates of New Zeal and's North Island (Figure 1). It form s th e northern erosion and accretio n, with migrations up to 300 m over pe­ entrance to Tauranga Harbou r, a large (- 200 km ") meso­ riods of 3-5 years. tidal es tuari ne lagoon wh ich is sepa ra ted from the sea by th e In th e west ern Bay of Plenty, th e ast ronomical tides are

.Iourn al of Coas tal Research. Vol. 15. No. 1,1999 222 Hicks et al.

176' OO'E 175 °E TASMAN SEA

3JOS 6 Wave buoy

SOUTH PACIFIC OCEAN

NEW ZEALAND f)

~ 3 r 3 0 'S

'------v

'b '?

'b '?

Tauranga

) Harbour

) (J~- j / ( /- Kil ometres

Figure 1. Map showing Kat ikati Inlet and ebb tid al delt a , Waihi and Mat ak an a Island Beaches, beach profiles (Wand M number s), nearshore bathym­ etry, and wave buoy location.

se midiurnal and the mean neap and mean spring tid al ranges 1 m for 70% of th e time, averaging 0.8 m and peaking at 4.7 for t he open coast are 1.26 and 1.64 m respectively. Sea-l evel m (MACKY el al., 1995 ). Significant wave periods averaged 6 se t-ups associated with storm surges ran ge up to 700 mm , seconds, whil e most of th e wave energy was found at periods whil e th e barometric effect causes local sea level to cha nge bet ween 7 and 13 seconds and arrived from th e northeast­ by betw een - 4 and - 10 mm .hPa- 1 (BELL and GORING, easterly secto r, which st raddles the regional shore-norma l di­ 1996 ). rection. Th e long-term net littoral drift on th e Bay of Plenty Nears hore currents (5- 10 m water depth) on the open coast coast appea rs to be gene ra lly towards the southeas t but, at are dominantly shore parall el, with speeds ran gin g from 0­ least in th e western part of the Bay, is small compared to th e I 0.30 ms- and a median speed of around 0.05- 0.07 ms I gross drift (EWART, 1961 ; Gmn, 1977; HARRAY and HEALEY, (WILLIfuVI S and BELL, 1991 ; HUME et al. , in preparati on ). Th e 1978; MACKY et al., 1995). tidal constituents account for about 25% of th e va riation in The wave and tid al regime at Katikati Inl et classifies th e the current records in the longshore direction. Significant inlet as "mixed energy, tid e-dominated"(HAYES , 1979; NUM­ wave heights over the four year study peri od were less th an MEDAL and FISCHER, 1978). The ebb delta also rates a "mixed

J ourn al of Coasta l Research, Vol. 15, No. 1, 1999 Sh oreline Change in New Zealand 223

FIgure 2. Aerial photograph f 1988 1of th e ebb tid al delt a a nd adjacent beaches, s howing th e morp hologrca l features on th e ebb delta and the nea r field profile locauons,

energy. Group 2" clas sification in th e sche me of G lBEAUT an d delta: (ii i ext rac ting pattern s of shoreline a nd sa nd volu me D AVI S (1993 ), owing to its arcuate shape. th e asy mmetry in­ change from th e profile dat aset, including identifying th e duced by th e orient ation of th e main ebb channel. a nd t he longshore exte nt of th e inlet effects : (iii) monitoring waves well-developed landward migrating swash bars on the ebb and sea level;(iv) correlating th ese forcing functio ns with th e delta platform. The rati o of net annual littoral drift to spring shore chan ge in order to identify th e main process es drivi ng peak tid al discharge is approxima tely equal to 12 (HUME et th e shore change; and (v) matching these processes wit h mor­ al., in pr eparation), which su ggests, from the scheme of phological evidence on and aroun d the ebb delta. B RUUN (1990 ), th at tidal processes are likely to be the main mean s of bypassing and transferring sand aro und th e ebb Beach Profile Surveys and Analysis delta. In October 1990, 25 beach profiles were estab lished along METHODS the 25 km span of the study shore (Figure 1). Th e profile General Approach spacing increased away from the inl et. All profiles were re­ The study included five tas ks: (i) monitoring a network of surveyed at approximately monthl y intervals from August profiles along th e beach es adjacent to and beh ind the ebb 1991 until January 1995. Th e survey met hod was a variant

Journal of Coasta l Research . Vol. 15. No.1, 1999 224 Hicks ct al. of EMERY'S (1961) technique, with two 1.6 m poles used to origin al variables. When used to identify longsh ore patterns, measure hei ght differences a nd a ta pe to measure distances. each mode has a cha ra cterist ic 'loading' at ea ch site along­ Th e surveys, conducted about th e low tid e, exte nde d from th e shore and a varying 'score' with time. Since th e first few prin­ dunes (up to 7.8 m abo ve mean sea level) to below low tid e cipal components ty pically account for 90',!, or more of th e level (down to 1.2 m below mean sea levelJ. In December variance in th e or iginal dataset, th e tec hnique tends to be 1991, th e survey of th ese beach pr ofiles was combined with very effective at condens ing the original data into a few plot s. a detail ed bathym etric survey of th e ebb delta and th e near­ PCA was applied to th e beach volu me (Y*) dataset using th e shore region off th e adjacent beaches (HICKS and HUME, softwa re pack age STATISTICA (STATSOFT, 1991). The anal­ 1997 ). yses were conducted for th e whole span of shore and for th e Th e ana lys is of th e beach profile data set was based on t wo four sub-reaches described previously. Cross-correlat ion anal­ parameters: the sand volume (Y) above th e MSL datum and ysis was used to establish how each principal compone nt the excurs ion distance (X) from the landward end of the pro­ score related to th e records of waves and sea level. file to th e MSL contour. Th ese parameters were normalised While the re is some redundancy in th e results from tech­ (to y * and X*) by taking th e deviation from th e mean values niques (il-t iv), th ey are mutually support ive since one meth­ for each profil e over th e study period. Thus X* 'J ' for survey i od may provide a ready explanation for features in a nother. of profile j, was th e gai n or loss in sa nd volume above MSL Th ey all, however, offer unique results. For exa mple, the sim­ from the average volume at profil e j. y * was found to be high­ ple sta tistics of X" convey th e most information on the mag­ ly correlated with X*, and so most of th e subse que nt a na lys is nitudes of shoreline cha nge, th e trends in V" within sub­ focused on expla ining th e V" signa l. reaches provide sa nd bud get information, the dispersion di­ Excluding th e three profiles in th e inlet throat, th e spatia l agra ms highli ght wave-like morphological features, whil e and temporal patterns shown by y * and/or X"' were ext ra cte d PCA is th e only technique whi ch shows th e proportion of th e from : (i l th e range a nd variance over th e study period at eac h total shoreline variance associated with indi vidual processes. site ; (ii) the time t re nds in YOi' accumulate d along severa l sub­ reaches of the study shore; (iii ) dispe rsion diagrams; a nd (iv l Waves and Sea level: The Forcing Functions pri ncipal compone nts ana lysis. Fou r sub-reaches of th e open -sea-facing shore were de­ Directional wave data were record ed over 17 minutes at fined, representing th e 'near field ' (in th e 'lee' of th e ebb del­ a -hour interva ls for a period of four yea rs by a n END ECO ta) a nd 'far field ' areas of influ ence of th e ebb delta on both buoy located 8 km seawa rd of th e ebb delta in 34 m wa te r sides of the inlet. Th eir boundaries were fine-tuned from th e depth (MAC KY et al ., 1995). Th ese data wer e used to compute longshore pattern of V" and X*. Net sa nd volu me changes in a record of th e dim en sionless 'fall speed' para meter, Hj wT eac h sub-reach a nd along th e whole shore we re determined (where H", is th e significant wave heigh t in deep water, T is from th e cha nges at th e compone nt profiles by in tegrating th e wave period, and w is th e beach-sand fall speed), whi ch th e average change between adjacent pro files multiplied by we used as an indica tor of whether th e shore should have th eir sepa rat ion distance. been gene ra lly eroding or accreting. Essen tially, th is param­ 'Dispers ion dia grams' are useful for vis ually identifying ete r compares t he heigh t to whi ch waves disper se sa nd above temporal and longshore signa ls in sa nd volume or oth er pa­ the bed with th e distance the sa nd can fall back down during ram eters ie.g., HASHIMOTO and UnA, 1982; HICKS and IN­ a wave period. Net sand transport is expecte d to be onsh ore MAN, 1987 ). Th ey are particularly good for identifyin g sa nd when thi s ratio is low, du e to the asy mmet ry of shoaling wave migrations, whi ch show up as diagonal trends (he nce waves, an d offshore whe n th e ra tio is high a nd waves stee per th e term 'dispersion diagr arn' L In this study, disp ersion dia­ m EAN, 1973 ). Laboratory and field st udies of beach erosion! grams were pr epa red using grids of y * and X"' constructed at accretion under irregul ar wav e conditi ons (see LARSEN and intervals of 500 m a longs hore and 14 days in tim e usin g a KI{AllS, 1989, for a review ) suggest em pirica lly th at beaches linearly-interpolating triangulation algorit h m with a x20 should accre te when HjwT < :1.2 a nd th ey should erode weighting assigned to the tim e valu es to correct for th e ani­ when HjwT > a. 2 (whe re it is assumed th at for a Rayleigh sot rophy induced by the di fferen t t ime and distance sca les distribution of wav e heights, H", equa ls 1.6 times the mean (Go Lm:N SOFTWARE, 1994 ). Similar dispersion diagram s wave height). were also prep ared for th e rate-of-cha nge of volu me and MSL Thi s emp ir ical criterion is based on the assumption t ha t excu rs ion distance, dY/dt and dXldt , respectively. It wa s hy­ the exist ing beach profil e Is reason abl y close to an equilibri­ pothesised that th ese latter diagram s would better depi ct th e um condition (LARSEN and KRAllS, 1989 ). Neve rt heless, a signa ture of discrete events (su ch as coastal storms a nd ac­ time se ries of HjwT is a useful qualitative index of whether cre t ion phases ) while th e X" and V" diagrams would better t he beaches should have been eroding or accre t ing over th e depict cumulative t re nds . study period . A representative fall speed was tak en as 0.oa6 Principal compone nts analys is (PCA), whi ch resolves th e rns I based on a median (d c,o) sa nd size of 0.28 mm (HICKS variance structure of a mul ti-variate da ta set, is commonly and HUME, 1996) and equat ion 4- 8 of CERC (1984 ). When used to interpret record s of cross-shore or longshore bea ch th e wind was offshore and th e rep resentative wave condition change (e.g., WINANT et al., 1975; AUBREY, 1979 ; DOLAN et at th e buoy was offsh ore ii.e., when th er e wer e no waves at al., 1982; HASIM OTO and UOA , 1982; MEDINA et al., 1991) . th e shore), Hj wT was assign ed a value of 3.1, which is th e Th e approach isola tes characteristi c, ind ependent modes of avera ge value for all onshore waves recorded over th e 4 years . covariance or correla tion (norma lised cova riance) among the This value was assumed to represen t conditions when th e

Journal of Coast al Resear ch. Vol. Hi. No. 1, 1999 S hnn-li n« Cha ng" in Now Zeala nd 22" beach wa s neither accret ing or eroding du e to cross- shore assume d as a start ing point with numeri cal sho re line mod­ tran sport, and is very close to th e 'n eutral' value of :3.2 pre­ elling te.g, CRAVENS et aI., 1991J. Tan[3 was est imate d from sente d by LAHSEN and KRA US (1989) . empirical rel ationsh ips given by HANSON a nd KRAUS (1989) Regional longshore transport was estimated as suming a whi ch relate tan[3 to th e deep-water wave height a nd steep­ shore with st raight and parall el contours trending :ll o West ness and th e medi an beach sa nd size. Equation (2 ) includes of North and applying lin eal' wav e theory and Sn ell's law to a term for th e effect of a long sh ore gra dient in wav e height. shoa l and refr act th e wave condit ions mea sured at th e bu oy Thi s term may become significa nt wh ere the nearshore to­ shorew ard to th e br eaker zone. Th e resulting time-seri es of pogr aphy changes shar ply, as around an ebb delta , or where br ea kin g wave conditio ns was combined with th e CERC bathym etric irregul arities induce wave focusing. 11984) 'wave ene rgy flux factor' equ ation to compute th e po­ The longshore gr adient in longshore transport wa s det er­ tential longshore tran sport. Th e longshore transport rate (Q" mined by differencing th e trans port rate between adjacent in m-d ,) is . beach profile sites. Th e diver genc e patterns generated were used to identify th e wav e conditions wh en longshore trans­ ( 1) port was stall ed by the ebb delta, a nd to help expla in th e where H." is th e significant br eak er height a nd a " is th e br ea­ longsh ore load ings of the principal compone nts . ker a ngle. A permanent open-co ast sea -level record ing station, located A more detail ed retraction-diffracti on study wa s ca rried out 25 km south-east from Katikati Inl et at Moturiki Island off to identify th e changes in th e wave field du e to local-scal e Tauranga Inl et (Figu re 1), pro vided a 15-minute digital rec­ bathym etric features, notabl y th e ebb delta , and th e conse­ ord of sea level through th e study peri od (CORING and BELL, quent effects on th e rates a nd longshore gr adi ents of long­ 1996 l. A 28-day running-mean sea level was ext racte d from shore tran sport. Thi s employed a variation of th e numer ical this record for compa rison with th e bea ch changes. wave refraction mod el RCPWAVE (E BI';HsoLE, et al., 1986 ; C RH:N, 1994) to pred ict th e breaking wave condit ions at eac h Ebb Delta and Shore Morphology of th e ocean-facin g beach pro file site s. To cover much of th e obse rve d deep water wave climate , wa ves 01'2.0 m significant The morphology of the study shore and the shallower parts wave hei ght with periods ranging from 4.5 s to 10.5 sand of the ebb t ida l delta were monitored over the study period source directions ranging from 15° cas t of north to 95° east through a combination of ground a nd a ir photographs a nd of north were processed . Th e rectangular bathym etry-grid visual obse rvations . Th e migrations of bars, eros ion 'holes', used for th e refr action model was gen er ated from th e 1991 and t idal channels were traced to derive a conceptual a ppre ­ bath ym etric and beach survey datasets (HICKS and H UM E, ciati on of the processes respons ible for sand ga ins and losses. 1997J. Wave ene rgy dissip ation was ign ored in th e refr action model runs. RESULTS Th e pot en tial longshore trans port ra te at each beach pro­ Morphologic Change file site was est imated usin g th e equation of HANSON and KRAUS (19H9): Th e obse rved changes in morphology on the far field beach­ es over the study peri od were typi cal of ope n-coast beaches Q, = IH, "C),,[a'Sin 2a - a"cos ex ilI'I ,] (2) responding to wave events . A sma ll progressiv e sa nd loss was c1 x " observed on Matakana Island. More noticeabl e changes oc­ wh er e H, is th e signi ficant wave height, the subsc ript b de­ curred on th e near field beaches. Within the inlet throat, notes br eak er conditions, a is th e br eak er ang le with resp ect Cave Bay and the tip of Matakana Island spit tsites WI and to t he nearshore contours at th e br eak-point, and c, is the M1 on Figure 2) tended to accrete in unison at th e same time wave group speed (estimate d as IgH,)''''), a , and a" are di­ as t he nearby ocea n-fac ing beaches (W2 and M2 ) eroded, Th is men sionl ess param et ers defined by change wa s obse rve d afte r periods of high ene rgy easte rly waves.We infer th at th ese waves swept sa nd into the inl et, K, some of it being ca ught on th e throat beaches and som e being 1)(1 p )(1.416 )"" inject ed into the main tidal channe l. At th e easte rn , ncar field end of Waihi Beach (profiles (3) W2-W5 on Figure 2), an erosion trough was see n to migrate westward during th e study period whil e an accreti onary wh ere pjp is th e den sity ra t io of sa nd a nd wa te r I tak en as pul se began at W2 in 1994. Th e trough related mainly to th e 2.65), p is th e poro sity of sa nd on th e seabed Itak en as 0.4 ), posit ion of th e west ern marginal flood ti de cha nne l, whi ch K, and K" are empirical efficiency paramet ers for th e tran s­ was forced to move by a sand bar migrating shore ward from port th at is driven by oblique ly inc ide nt waves and th e long ­ off th e western margin of th e ebb delta . Accreti on followed shore gradient in wave height, resp ecti vely, tan[3 is th e av­ whe n the flood cha nne l j umped seaward and the bar welded erage slope across th e longshore transport zone, a nd th e fac­ to th e beach . Th e erosio n trough was also a ffecte d for a t ime tors involvin g 1.416 are to conv ert from significant wave by wave focusing through a ga p in th e offshore bars (Figure height to root-mean-squa re wav e height. K, wa s taken to be 31 . In similar fash ion , a trough migrating along the Matak­ 0.77, as origina lly det ermined by KOMAH a nd INM AN (1970), a na near field beach (profiles M5-M 2 ), followed by accre tio n, K" was assumed to be 0.:38, equa l to half of K, as is ofte n related to th e scouri ng action of the easte rn flood channe l,

.lou rnal of Coa stal I{espa rch . Vol. l:i . No . I. 1!J!J!J 226 HIcks pi al

I'

~ . ~ ;..;. i.' ~ ~ ; .,

\ . .--

------.. l _...... w....,.,', \ . \

Figure 3. Erosion hole on Waihi shore betw een W2 a nd W3 showing wave focusing through a gap in th e inner offshore bar 0 3 .July 1991). which wa s forced against this shore for a tim e by sand bars ditions record ed at th e wave buoy were H, = 3.9 m, T = 7.4 migrating shoreward from the eastern ebb delta pla tform s, and th e an gle of approach to th e regional shoreline trend (Figure 4). Th e accretion ph ase began wh en th e bars weld ed was 25°). Thi s 'flip-flop' pattern is primarily related to onto th e cuspate foreland (at M5). changes in th e direction of waves, most of which a pproach Morphologic changes on the Katikati ebb delta through th e very nearly normal to the shore. Systematic period s of net study period are detailed in a companion pap er (H UME et al., drift only become a pparent in th e record of accumulated long­ in preparation). shore transport (Figure 5d). Thi s shows something like a three-year cycle in the net drift, with an extend ed phase of Waves and Sea level: Patterns and Correlation's net ea ster ly drift (albeit with some temporary reversals) from Among the Forcing Functions May 1991 to October 1992 , th en generally westward trans­ Tim e-series of wav e height, fall speed parameter, potential port until May 1994, then generally eastwa rd transport longshore transport rate, and accumulated net potential long­ again. Whil e th e net calcu lated drift rate over th e 4 year shore transport are plotted in Figure 5 along with th e 28-day study period was 57,000 m3y -l to th e west, it is clear from mean sea-level record . Figure 5a shows no clear regularity or Figure 5d th at this average figur e is very dependent on the seaso na lity in th e occurrence of storm wav es, although time span (e.g ., our figure differs from the 70,000 m 3y- l re­ storms often occured in spa tes with exte nded relatively calm ported by MACKY et al., 1995 , whose calculation used th e periods between (e.g., September 1991 to June 1992). Th e fall same formul ae but only the first 3 years of th e wave record ), speed parameter plot (Figu re 5b ) suggests episodes of erosion For another period, the average could well be in th e reverse (Hj wT high s-corresponding to steep storm wave s ) and lon­ direction. ger periods of accr eti on (H,j wT lows-corresponding to Further caution should be attached to the estima te of av­ swell). erage net drift rate due to its sensitivity to syste ma tic errors Figure 5c shows th at the longshore tran sport potential var­ in th e bre ak er angle. An error of a few degrees is possibl e in ied cons iderably in magnitude and direct ion, even from day thi s from th e compas s setting on th e wave buoy an d/or in to day, with 'instantaneous' rates sometimes exceeding defining th e regionally representative shorelin e orientation.

100,000 m'td I te.g ., on 23 October 1992 , when th e wave con- While such errors induce min or un certainty on th e gross

Jo urna l of Coasta l Research, Vol. 15. No. 1, 1999 Shoreline Cha nge in New Zealand 227

Figur e 4. Er osion embayment cut in th e Mat ak an a shore by th e flood tid e margin al cha nnel which is dir ected at the shore by bars on the delt a platform .

drift, th e average net dr ift is a classic exa mple of a small occur red, albeit with episodes of more inten se tran sport in number with large un certainty obtained by differencing large th e direction to which th e bias was applied. numbers with small un certainty ii.e., the total drift to ea st Th e 28-day ave rage sea level (Figure 5e) varied over a and west) . A sens itivity ana lysis, whi ch involved systema ti­ ran ge of 0.2 m through th e study period. While this ran ge cally changing the br eaker angle by ::':: 1°, 2°, and 5°, induced was small relative to th e spring tidal range (1.65 rn ), th e sea­ respective chan ges of only approximately::':: 1%, 2%, and 5% level anomalies lasted typ ically of the ord er of 6 months, in th e average gross drift (1,290,000 m-y - I) but ::':: 70%, 140%, which should be an adequate period to initiate some degree and 350% in the aver age net drift (57,000 m-y - 1 west). Thus of 'Bruun-ru le' type shoreline adjustment (BRU UN, 1962). we conclude th at over th e four year record period th e gross Auto - and cross-correlation a na lyses were conducted on drift was large rela tive to the net drift, and any net dri ft was th ese forcing-function param et ers to qua ntify any regula ri­ of indeterminate direct ion. Th e sens itivity ana lys is showed, ties and int er- conn ections in th e signa ls shown in Figure 5. however, that th e inter -annual pattern of net dri ft reversal , Unless specified ot herwise, th e level of statistica l significa nce as described above, wa s not signi ficantly influenced by sys­ for t he correlation coefficient , r, was set at 5%. The auto­ tematic changes in the break er angle- the 3-year cycle still correlation results showed weak but significant a nnual cycles

Jo urn al of Coasta l Resear ch, Vol. 15, No. I, 1999 228 Hicks ct al.

a 6

o c-, I_(/)E

o

6 tl------"------"------"------"- --, b I- { (/) -!1IIf~rwr\Yt I Ih't Jfi\i lfbJ l-J\i 1-i' ·H\ I'-. I L I lJL Nl

o I ~_ . . •• . _~. ~M " "'M I , ... n .

150000 t-I------>- ---- '------+------+----, c I-~ (f)>' ....J.g5 ~ ~ o 'ro E f~To East ' I rr.,".., 0- -k-o-~~- -150000 -JAN·92~~--~ ~ AN · 9 3 Y.N'-94 I- (f) 5600 00 +1 ------t- - - - - t------+-- --- +----, d ....J (lJ >~ ~ To Wes t ~~ '~ C") ~E o ~~------:::J- E :::J .400000 ~ AN - 92 o 1·JAN-91 JAN.93 IJAN·94 e ....J 1.6 (f)~ 1.7E ~.s 1.5~e51 e_ ~. --:~ . . . ~ - ~ - :.. --:.. -- 1.4 01-JAN-91

f o (f) : ~1 ·3 0WAN~~JAN:94

Figure 5. Tim e se ries plots of: (a ) Deep-water sign ifica nt wav e heigh t . (h i Fa ll s pee d parameter (7·da y av erage). (c ) Dai ly longshore transport pote nti a l (positive tra ns port is to the west ), (d ) Cumulative ii.e., netl longshore trans port since th e beginn ing of the wa ve record , (e ) Mea n sea level (28-dayaverage J, an d ( f) South ern Oscillation Index (monthly av erage).

both in sea level (a seasonal sea-level signal in the Bay of with sea level lagging th e SOl by some 9 months and being Plen ty was also identified by BELL and GORING (1996» and low when the Southe rn Oscillation is in its negative, El Nin o, th e fall speed parameter. Sea level te nds to be higher du ring phase (BELL and GORING, 1996 ). There -is an indication also the autumn equinox, whil e the fall speed parameter tends to of correla tion of both sea level a nd fall speed param et er with be higher in the late winte r-spr ing and lower in summ er­ the SOl for monthly fluctuations (compare Figures 5b and 5e autum n. Th e maxi mum cross-correlation between sea level with Sf), with the fall speed pa rameter showing a significant and fall speed parameter (r = 0.57) was achieved wit h a lag corre la tion with th e SOl (r = -0.56) at a lag of 4 months. of 6-7 months. Thus, on a seasonal basis, th e fa ll speed pa­ No seasona l signal is evident in th e pote ntial longshore ram et er (which is essentially an ind ex of the beach erosion transport sign al, However it is possibl e th at the quasi 3-yea r pot ential of the incid ent waves ) te nds to be higher when th e cycle suggested in Figure 5d is associa ted with an inter-an­ mean se a level is lower - a fortui tous combina tio n for beach nu al mechanism such as the Southern Oscillation. Th er e is sta bility along the study shore . Mean sea level in the Bay of an inverse correlation (r > - 0.31; significant at 6% leve l) be­ Plenty is well correlated with t he Southern Oscillation Index tween th e SOl and net longshore transport at mon thly scales, (SOl - Figu re 50 at a qu asi -l-yea r in te r-annua l time sca le, with tran sport tending north-westward when the SOl is

J ourna l of Coast a l Resea rch. Vol. 15, No. 1, 1999 Shorc hn e Cha nge in New Zea la nd more nega tive ii.e., towards EI Nin o conditions , whe n south­ ren ourish the delta margin erosion zone s a nd to scour th e erly- westerly wea ther is more pr eval en t in the mid-lat itudes ) deposit ion zones closer toward th e inl et. Later discussion sug­ a nd sout h-eastward when th e SOL tends more positive ii.e.. gests th a t the se processes are landward migrating sand hal'S towa rd s La Nina condit ions, whe n north-easterl y weather is a nd scouring flood-tidal flows, resp ectively. more common). It should be born e in m ind that through mu ch of the four-year study period the SO I was negative (Figure Beach Profile Volumes and Excursion Distances 50, with th e Southe rn Osci lla tio n in its EI Nino mode. Variance and Near- and Far Field Definitions

Littoral Drift Divergence Th e range and va riance of the mean sea level excurs ion distances a nd heach volu mes for th e ocea n-faci ng pr ofile site s Th e effects of wav e refraction on the longsh ore gradie nt in (Figure 7) show high est va riatio n, particul arly in beach longshore transport (Figure 6 ) indicate th at the significant volum e, in th e lee of the ebb delt a, between profil es W4 a nd effects of th e delta extend between profiles W9 a nd M9. Out­ M5 IV* changes up to 275 m'm 1 a nd X" change s up to 165 side this zone, th e pr ed icted trans port rates va ry relatively m ). In compa rison, in the far field th e beach cha nges were little alongshore . Within this zone, th e influ en ce of the ebb more un ifor m a nd smalle r (V* up to 70 m-m 1 and X* up to delta va ries according to the period and direction of th e in­ 50 rn l. W4 marks the wes tw ard extent of majo r sand bars on cident waves. Waves wit h peri ods up to 8.5 seconds tend to the Waih i side of the ebb delta (Figure 2). M5 mark s the ap ex result in a consiste nt longsh ore transport pa ttern , wit h th e of the cuspate forela nd on th e Matak ana side of th e delta , refraction becoming gr ea te r, a nd the longsh ore transport di­ where sa nd ha l'S migrating shore ward from the delta tend to vergence consequently becomin g greate r, as t he period in­ attach to the beac h. However, the ne arshore morph ology creases. Longer peri od wa ves divert from this patter n; ap ­ sugges ts t hat distincti ve inl et-re lated sand bar activity parently, their longer wavelengths a re more st rongly influ­ exte nds a lmost as far west as W5 a nd as far ea st as M7 enced by th e ebb delta morphology. Transport direction re­ (Figure 2 ), 2 km and 3 km from the inlet centre-line, versa ls are common on th e easte rn , Ma tak a na side of th e resp ecti vely. Thus from t he comb ined evidence of the beach inlet bu t are ra re on the weste rn side. For inci den t wav es va ria nce and the nea rsh ore morphology, for the purposes of a rriving from 55° East of North, wh ich is essent ially normal the ana lyses of bea ch volumes a nd principal components we to th e regiona l shore line trend, the fill' field transport rates re-defined the near field of th e inl et-delta influen ce as lying are close to zero wh ile in th e nea r field th e refract ion-modi­ between W5 and M7. fied transport rates a re mu ch high er and a re directed to­ Th e Pearson corre lation coefficients hetween beach volu me, wards the inlet from both sides (Figure Gb), V*, a nd excurs ion distance, X,\ at eac h profile ran ged from For a given wav e period, th e a mp litude of th e longsh ore 0.4 1 to 0.97, averaging 0.74. All of th ese correlation coefficien ts gradients in longsh ore tran sport and th e asymmetry about were significant at the 5'h level, thus confirming that analys is the inl et both increase as th e inciden t wave travelling direc­ could be concentrated on one pa rameter or th e oth er. tion rotates clockwise (Figure 6d I. For exa mple, B.5 S waves from 15° East of Nort h indu ce rela t ively little change from Beach Volumes the far field tra nsport rate on th e weste rn wing of th e ebb delta and have a slightly grea ter effect on the easter n wing. Uni t beach sand volu mes t mrn 1) were det ermined for four When th e waves a rc normally inciden t, a rriving from 55°, the sub-reaches spa nning th e ocea n-faci ng shore : Wa ih i fill' field effect is greater on both sid es but more on the eastern side. (WI 1-W6 ); Waihi nea r field (W5-W2); Matakana nea r field While the actual longsh ore t ran sport rates a nd directi ons (M2-M7 l; and Ma takana far field (M8-M12). Th e average un it vary considera bly as th e incident wave directi on changes volume for th e ent ire shore (F igure Sa) showed a qu asi-sea­ I Figu re Gd), th eir longshore gra dien ts , or diverge nces, main­ sonal pattern of sa nd gains and losses sup er im posed on a tai n a rem arkab ly consi stent pa tte rn (Figure 6e ). Thi s in di­ t rend of net sand loss. Sand ga ins tended to occur in the ea rly cates th at irr espectiv e of whether the regional longsh ore sum mer and losses in t he winter. Th e volume cha nges in th e transp ort is to th e west, cast, or th e waves a re normal to t he separate sub-reaches (Figure Sh) showed firstl y th at t he shore, longsh ore transport should cause erosion on the ebb short-term fluctu ations in volume were in-phase alongshore, delta marg ins, accre t ion closer towards th e inlet , a nd rela ­ a nd secondly th at th e net overall sa nd loss was du e to sand tively little change on th e far field beaches. When the trans­ losses in the nea r field sub-rea ches, notabl y on th e Matakana port is eas twa rd (e.g .. directi on = 15° on Figu re 6d ) the delta shore. In contrast, the far field sub-reaches experience d little margin erosion is assoc iate d with a local acceleratio n in th e net sa nd volume change. Th e sand losses from th e near field eastward tra nsport; when th e wav es a rr ive norm all y (e.g .. di­ beac hes a re expected to be matched by sand ga ins on the ebb rection = 55°) a local t ran sp ort syste m is set up t hat drives delta and ins ide the inlet. Certainly, t he beach volume s at sand towards th e inl et from both sides ; and when th e tran s­ Cave Bay on th e western shore of th e inlet t hroat (WI ) and port is westward (e.g .. direct ion = 95°) th e delta margin ero­ on th e ins ide of the Matak ana spit (M1) ten ded to increa se sion is as sociated with a local acceleration in th e westward whe n the nearby ocea n-fac ing beaches (W2 and M2 ) eroded tran sport. (Figure Sc). Obvio usly, such a cons istent longshore transport diver­ Sign ificant corre lations wer e found between the fall speed gence pattern for all wave directi ons ca nnot persist without param eter (Figure 5b ) and sa nd volu me for th e whole sh ore shoreline per tu rbations growing unl ess other pro cesses act to and for the two far field compartments, indi cating th e direct

J ournal of Coas ta l Research, Vol. 15. No. 1, 1999 230 Hicks et al.

o a iF 50000 f'K::------~ Period c 4. <: -." ..... ~ilr - c:j ro o- _- Il ~ &..a -cJ:::'-i.e vr=" 4 .5 ~~ - ' ::::i '_ _ - .- 6 .5 IW11 W ID WB W5 W2 M2 M5 MB MI D MI l C -50000 ' I .:: 0 4000 8000 12000 16000 20000 o 0­ 50000 ,------, Period b (J) c Direction =55 ~~ ., 4.5 I-~ ~ - - .- 6 .5 /'- ... -c -- :--'~ ~ ---. . - ~---~ -50000 IL-_L-_~_~_~_~_~_~_~_~_____' o -l o 4000 8000 12000 16000 20000

'§ 0­ o,.------.;;------, Period c (J) Direction 95 C = 4 .5 co j~ $ .= ~ -s - .- 6 .5 - 3 00 00 ~ ~' ._~ ~.gJ .. . ,/l -::: - : ' - .- 8 .5

0 :;::' ~ Q 0 ' .r:: ~ ~ .~ ,/ 1 0 .5 (J) OJ ~ c , I , I I o -l 4000 8000 12000 16000 20000

Direction s 50000 ,----, ------::----;---;----=-::------, ., Period = 6.5 5 d 5t 15 co>­c ­ ~-= 4-" ~ -.:.. " ~ -<>-<>-e ~ ~__ :-=- =-=-- ~ co 35 1-'0 O ~ -_=-- ~- -<,..: ~-7'~~-----~ Ql M ~ - - 5 5 <5 E " ...... :/::..--: .r::: ­ '- I . " - .- - 75 (/) ....-...... 0) 50000 c - . - - 95 o ..J o 4000 8000 12000 16000 20000

t:: Direction o e 5tc Accre tion - . - 15 ~ ~>: b--~;;fJ - - 3 5 I- c CO 0 1- QlQl'O -~- 5 5 ~ 0) - Erosion o ~ E s: Ql ­ -20 - 75 (/) > M 95 g o E- WI D W8 W5 W2 MI D o - 4 0 ~ ..J o 4000 8000 12000 16000 --fr- W S 3 Distance Alongshore Southeast from Waihi (m)

Figure 6. Longshore variation in potential longshore tran sport rate and divergence for varying wa ve dir ection s and periods, computed at the open-ocean ­ facin g profile locations. Katikati Inlet is located approximately 9,000 m from th e distan ce orig in, whi le selected profile locations are labelled Wll, W10, etc. Positi ve tr an sport rat es indicat e tran sport toward the sout heast. Positive tr ansport divergence values indi cat e accretion. (a-e): Longshor e transport rate for waves arriving from 15° ca st of north (wave thrust directed south-eastward), 55° cas t of north (wave thrust dir ected norm al to the regiona l shoreline orientation ), and 95° cast of north (wave thrust dir ected north-west ward). (d-e): Longshore transport rate and divergence for 8.5 second waves of various directi ons. Triangles on e show the loading pattern of th e third principal compon en t obtain ed from analysis of the 'whole shore' dat aset (WS3L On all plots, points ar c omitted where th e wave refraction analysis failed to provide accepta ble breaking wave conditions (often, th is occurred at profile W4, where a flood-tid al cha nnel lay off the beach ).

effects of storm-waves and swell on beach erosion and accre­ Dispersion Diagrams tion , respectively. No such correlation was found for the near field compartments, indi cating th at th e direct sign ature of the The patterns shown by the sub-reach volumes become incident wave s on cross-shore tran sport was masked by other clearer on the V" dispersion diagram, which shows the processes which are later shown to relate to sand bars mi­ cha nges with time at all of the ocea n-facing profiles (Figu re grating from the ebb delta and to shifting flood-tidal cha nnels. 9). Thi s illustrates a marked contrast in the near- an d far

Journal of Coastal Research, Vol. 15, No. 1, 1999 Shoreline Cha nge in New Zealand 231

a 100 "- -~ " "1 - '" -_ ..:..._.t....i····i····i··.. ·~ ··.f .--!- ..r ..' ··· _ ~ · · · - !· · · · 1 · ..~ ... ",1-,-,

--' [ ~i ~! ~I~l ~[~i* (f) :2 Ql j.... > -50 .l .. ..i.... j...L.)...i.... o .c -c Ql -100 E oOJ > -150 .':....:....:.... ~ .....:.....~ ...0;.... ::r Max Min 0 Mean+SO »»»»»»»»»»> ~omoo~m~~MNNMv~m~oomO~N Mean-SO ~~~~~~ ~~~~:2:2:2:2:2:2:2:2~~~ 0 Mean Profile

80

b . : : : ~ : : i; ! I 40 Ql ::;C --' (f) 0 :2 ~ !~ i ~ : ~ : ~ : .~! ~ ;~ i ~l ~! ~! ! l · : ~ . : ~.i.~ i ~ : ~: ~ ' ~ E ij! Ql u c -40 I1l iii is c 0 -80 •.• ~ .••. ~• •••~_ ••••• • -j •. •.••. ••.:-••••:.• • ·f. .-!- .. ~ "-' : " "'i" -. ~ . -. -~ "'!" ' -l-" ·00 ;, ::r Max o Min x w -120 0 Mean+SO X XXXXXXX XX XXXXXXXXXXX ~ o m oo~m~vM NN Mv~ m~ ro rno~N Mean-SO ~~~~~~~~~~:2:2:2:2:2:2:2:2:2:2:2 0 Mean Profile Figure 7. Plots showing ra nge and sta ndard deviat ion abo ut mean va lues of (a) excursion volume above MSL dat um and (b I excurs ion dist anc e to MSL line at eac h of th e open-ocean-facing profile locations over the study per iod.

field changes, and ind eed sugges ts th at th e near field might fixed over profiles M5-M9 and th e trough fixed over profiles be better defined extending between pr ofiles W7 and M9. In W5-W7 (Figu re 9). th e far field, th e qu asi-seasona l pattern of sand gains and Other near field features have migrated , as recorded by losses shows up well (as 'horizontal morphology'). In contra st, oblique 'morphology' on th e dispersion diagr am . A good ex­ in th e near field th e predominant features on the dispersion a mple is th e series of oblique 'troughs' and 'ba rs' th at lie be­ diagram run 'vertically', indicating features that spa n rela­ tween profile s M2 a nd MS. Th ese 'slope-up' to th e left on th e tively short distances alongshore but persist in tim e. Thes e diagr am , an d are the signatur e of two sa nd bars an d inter­ patterns rela te to the development of bars and holes span­ vening trough s migrating westward toward s th e inl et. Th ese nin g alongshore dist an ces of about 500 m to severa l km and features were also trac ked by visua l observa tion during th e persisting for severa l yea rs . st udy peri od and are associated with the shoreward migra­ Some of th ese near field features a ppea r to be fixed in po­ tion th en welding of bars to th e cus pate foreland , which in sition, at least over th e time sca le of the study, which prob­ turn forced th e eas te rn margin al flood-tid e cha nnel to im­ ably relates to 'standing' effects on th e waves and currents pinge agains t and scour th e shore on the inlet side of th e imposed by th e general geome t ry of the ebb delta. Ind eed, forela nd. Th e consequent progr essive loss of sand along this th eir position is consistent with th e pattern of deposition and shore is clearl y shown on Figure 9. erosion predicted by th e longshore-transport divergen ce ana l­ A simila rly migrating trough was observed to move along ysis (Figure 6e)- for exa mple, th e trough-b ar-trough pattern the beach betw een W2 and W5 over th e study period (its ap-

Journ al of Coasta l Research, Vol. 15, No. 1, 1999 232 Hicks et al.

a 20 15

10 .§ 5 1:

Q) 0 E :::l -5 0 > -10

-15 -20 1-Jan -9 1 l-Jan-92 31-Dec-92 31-De c-93 31-Dec-94 b 80 Waihi Far- field 60 --- Waihi Near-field ---Matakana Near-f ield .§ 40 _...... Matakana Far-field 1: 20

Q) E 0 :::l 0 > -20 .> -40

-60 1-Jan -9 1 1-Jan-92 31- Dec-9 2 31-Dec-93 31-Dec-94

C 100 --W 1 80 -- M1 60 - - W2 .§ 40 - - M2 1: 20 Q) E 0 :::l 0 -20 > -40 -60 -80 1-Jan-91 1-Jan-92 31-Dec- 92 31-D ec· 93 31-Dec -94

Figure 8. Ch an ges in sand volum e (pe r unit shore len gt h ) abo ve MSL da tum for (a l whole span of open-ocea n-facing shore, (b l the four near- and far field compa rtments (sec text for bound ar y definit ions I, and (c) the profiles within the inlet (W1 at Cav e Bay and M1 in side th e Matak an a spit tip ) and imm ediately beside the inl et (W2 and M2 1. All volum es are deviations from th e mean va lues over th e stu dy per iod.

pearance on Figure 9 is partly masked by th e superimposed one end of Waihi Beach to th e oth er. This appears on Figure quasi-seasonal changes), Th is trough was associated with 9 as a trend for erosion at th e western end of the beach and shoreward migration of a sa nd bar from th e ebb delta , which accretion at th e eas te rn end whil e th e region al longshore forced the western margin al flood cha nne l to migrate shore­ transport was eas twa rd (e.g., mid 1991 through to late 1992), ward and westward, scouring th e beach as it migr ated. Fig ­ with th e reverse trend occurring when th e net transport was ure 9 records this migr ating sa nd bar welding to th e beach westward (1993 through 1994). in th e area of W2 in 1994 . The erosion/accreti on cycles pr eval ent in the far field Th e patterns along Waihi Beach also show th e effect of th e regions of Figure 9 correlate reason ably well with the fall inter-annual fluctuation in net longshore transport (Figu re speed param eter signal (Figure fib), confirming that they re­ 5d), which appears to have induced a migration of sand from late to storm-wave/swell driven cross-shore transport.

J ourn al of Coastal Research, Vol. 15, No.1, 1999 Shorelin e Cha nge In New Zea la nd 2:1 3

Profile

Wl 1 W1Q W9 W8 WI W5 W2 M2 0,15 M I M9 M1Q M11

1400 -e 150 en '" 80 ""E

15 ~'''h c; Q M 10 ... c» ;? ~ 1000 (J" >- '"- x ~-----=- W Cl'" ... ;;; c 0 <: Ql ~~- '"- ::;: ·5 ..,:> 8 00 eE. . 10 c Q '" ·20 '" '!! '" > 600

Distance Alongshore East from Waihi (m) Trace of migrating bar or trough ?

Figure 9. 'Drspersion diagr am ' showing alongshore patterns of chango In sa nd volume I per urut shore length J above MSL datum over th e study pen od. Tria ngles locat e surveys of indi vidu al beach profiles . Dashed lmes sugg est the tr aces of alongsbore-rmgra ung eros ion trough s and sand bars. Th e volum es are deviau ons from th e mea n val ues over th e study period.

Th e equivalent dispersion diagrams for excursion distan ce, Th e second principal compone nt (WFF2, 20% varian ce) X*, and for th e rates of chan ge of v* and X* lead to th e sa me showe d a phase reversal alongshore a nd an a pproxima te ly inte rp reta tions and so are not reproduced here. 3-year cycle th at correlated very well with th e cumulative longsh ore tra ns port potential (Figure I Ib ). Th e longshore Principal Components Analysis phase reversal indicates pivoting of sa nd ga ins/losses about Principal components analyses were conducte d on th e a centra l nodal point. Thi s function thus capt ures th e sa nd beach volume (V*) data for each of the four sub-reaches and migr ation reversals on Waihi Beach dri ven by inter- annual for the whole study shore . Th e sub-reaches were ana lyse d variati ons in the directi on of net longshore tra ns port ob­ sepa ra tely to ens ure that far field signals were not masked se rved in this st udy. WFF 3 (explaining only 8% of th e vari­ by t he lar ger amplitude near field ones (e.g., sec Figu re T). a nce) is possibly th e signature of a sta nding sa nd wave which The longshore 'loadings' a nd 'scores ' over time for th e main induces subtle fluctu ations in Waihi Beach's planform: du r­ principal components found for each reach of shore arc plot­ ing storms sa nd tend s to conve rge alongs hore towards the ted in Figure 10, whil e the correlations between th e scores centre of th e beach (st ra ightening th e shoreline overa ll) and and th e records of sub-reach volume, longsh ore tran sport po­ during swell it returns to either en d (increasing th e shorelin e tential , fall speed parameter, and mean sea level are listed curvat ure). in Tabl e 1. Along th e Waihi near field (WNF) sub-reac h (profiles Th e principal components results varied among th e sub­ W2-W5), th e first th ree principal components (Figure l Obl reaches. For th e Waihi far field (WFF) sub-reach (profiles conta in 90% of th e varian ce in V*. Th e load ings of the firs t W11-W6), th e three largest principal components explained principal compone nt (WNF1, 50% va riance ) ind ica te th at W2 91% of th e V* variance. Th e first (WFF1, 63% variance) was a nd W3 respond in opposite phase to W4 and W5, whil e the in-phase all alon g the shore (i.e., when one profile gai ned or scores show a progressive increase with a su peri mposed in­ lost volum e, all the oth ers did th e same) and showed a qu asi­ ter- annual variation. Thi s pattern is consisten t with th e vi­ seasona l cycle th at correlat ed significantly with th e sub­ sua l record of a trough migrating slowly westward along this reach volume and th e fall speed parameter (Table 1, Figu re segment of shore through th e st udy period, followed by the 11a ). Th is functio n clearl y relates to changes on the sub-ae­ accretion of a sand bar in th e region of W2-W3. Th e WNFI rial beach associated with stor m-waves and swell. scores correlate with the regional cumulative longsh ore

Journal of Coas ta l Resear ch. Vol. 15, No.1, 1999 234 Hicks et al.

a 4 0 Waihi Far Field '6

30 Waihi Far Fi eld PC Scores 1.2 I Wah; Far Field PC Load ing s 20 o 10 __WFFl (63% vat 1 I 0. 8

c% 00 --WFF2 (20% var , ~ O . 4 ~ ~ AA .10 WFF3 (8% var .l ....:....:...:. ~ .. .20 .:: .. •. . t ..•. A . . . .. ;>;:..:) .3.0 '- --.J , .Jan.9 1 '.Ja n.9 2 31. 0 ec-92 31.0ec.93 1. Jan .9 5 0 2000 4000 6000 BOC Date Distance Alo ngshore S E fro m Wl 1 (m)

Waihi Near Field b 2.0 20 1 5 l Wahi NearField PC Load ing s

1 0 10 ~ . 8 00 --WNF1 (50% var ) ~ 0.5 ~ __WNF2 (29% va t 1 .3 , ' 0 WNFJ (11% var ] 0.0 I ,/ . ,. ·0 5 ·2 0 . 10 L· ----=:..- --.J

, .Jan.91 1.J an-92 31.0ec-92 31.0 9c-93 ,.Jan. 95 7000 7200 7400 760 0 7800 8000 8200 840 Date Distance Alon gshore SE from W11 (m)

Matakana Near Field

C 3.0 ' 5 ,-, ------, Mal akana Near Fi eld PC Lo adings 1.0 ..,II. 1 0 --MNFl (55%var ) 0 5 __MNF2 (2B% var.) ~ 1 ~-- . ~ ~ ~ ~( . • . • • • . MNF3 (12% var ] t , , , ·00' 0 ·05 . .. > '1 .--- ' ·l~2 0 .. - . , 0 IL..... --'

1·J an· 91 1·J an· 92 31-0 ec-92 31· 0e c-93 , ·J an·95 9500 10000 10500 11000 11500 1201 Date Dist anc e Alon gs ho re SE Irem Wl 1 (m)

Matakana Far Field

1 5 ,------, d 40 '-' ------, Malakana Far Fial d PC Lo ad ings 30 1.0 A

20 ~ 0.5 --MH 1 (54°k va r l ~ 1 0 --MFF2 (21·JoVaI ) '"~ 0 0 I l. ..A. _=--~ I ~ 00 I '1'. ..1- -:'i.\".£\:Ie \0( ' .\ . MFF3 (13"'{'var l

·10 ·0 5 [ . 1.0 -----J

.20 IL ----- ­ t -Jan-s t 1·Jan·92 31·Dec·92 31·Dec·93 , ·Jan·95 10000 15000 20000 2500 ' Da le Disla nce Al ong shore SE fro m Wl1 em)

Whole Shore

e 0 4 2.0 Whole Shore PC Load ing s

' 0 " ~ .•.. --WSl (37% vat ) ~ o o l 'I-\~ --WS2 (21% va t ) .... WS3 (13% var.) ...I -0 2 ••...•. ..: \.:: ·10 f " == ~' ·2 a I I · 0 4 j ·Jan·91 ' ·Jan·9? 31· Dec·92 31· Dec·93 1·Jan·9S 5000 10000 15000 20000 250 0 Dat e Dist an c e Alon g shore SE fr om W11 (m)

Figur e 10. Temporal 'scores' and longshore 'loadin gs' for th e th ree largest pr incipal compone nt s for (a ) Waih i Beach far field profiles, (b) Waih i Beach near field profiles, (c) Matakana Island nea r field profiles,(d I Matakana Island fa r field profiles, and (e) all open-ocea n-facing profiles along the study shore. Th e % figures show the proportion of the total variance in sa nd volume explained by each prin cipal componen t.

Jo urnal of Coastal Research, Vol. 15. No. 1. 1999 Shorcliru- Chu ngc in New Zea la nd

Table 1 Coellicien fs oirorrclotion bct uven princi pal com pon rn !.... and /l '(f l' l' fiJ,.C'in~ poratncter» and sand rulu nu: in Huh-reaches. ASfef'ishs m arh correlations th at aI''' s i~ lIi/i('(l ll t at till' p ' 0.0,'; /,,/,,,1.

Correlat ion ('(wIJicief1! u-ith

Principal I{pa"h Fa ll sl,,'c d Curnu lut ivo 28 ·da~· moun I{,'ach cum ponent , ; v. u-iu ncc vol ume parumou.r longsh oro tran sport spa h-vcl

Waihi far field 1 n4 0 ,77 " - 0,47 " 0 .11' 0 .15 .wn .ws. 2 IH 0.:15' 0 ,01 0 .8 1 ' - 0 , 14 :1 7 0 ,0:1 0 .10 O.OH 0 . 11' Wai h i ncar lip id I 50 0 .:11" 0 .07 O,4IF - 0 . 1l; , W5· W2 1 2 2!) O,f>7' O.4:r 0 .2(; (J.()4 :1 11 (!.O4 0 . 11 O.I H O,:lO Mat a ka nu near field 1 :'lii O,HI;' 0 .2:1 O,7n ' 0 .:1H" IM2-:\171 2 21' 0 ,07 0. 1:1 O,2H ·0 .0:1 :1 12 0 .20 O.On 0 ,:11' 0 .:12 Mat a ka na far field 1 ;)4 (U l:l ' 0,44 ' 0 .5 7 ' 0,41' ' 1:\1H-i\112 1 2 2 1 0 ,2 1 0.0 4 0 ,:15" 0 .24 :1 I f> 0 . 18 0 . 11; 0 .41; 0.14 Wh ole shoro I :17 0 .7:1 0 ,40 ' 0 .54 ' 0.4 1 ' I\V II -MI 2 ' 2 2 1 0 ,01; O,:lli ' O,nH' o.o : :1 1:1 0 22 O.o:l - O.:I:!" 0 .0:1 tran spo rt signal (Ta ble 11. Th e sign of thi s correlation sug­ eroding whil e th e easte rn end accre te d, and va ried over some­ ges ts th at W4 and W5 tended to accrcte a nd W2 and W:3 to thing like a 2-year time sca le. Thi s had no significant cone­ erode when th e net regional transport wa s eas tward (a nd lation wit h an y of th e forcing fun ctions, alt houg h a significant vice-ver sa when th e transport was westward l. Thi s longsh ore correlat ion with th e cumulative longshore tran sport ( I' = tran sport influ ence may arise from a comb ination of effects : - 0.6:3) was ach ieve d afte r a 6 month lag. This suggests a wave refr action caus ing long shore transport divergenc e (Fig­ signa l linked to sand accu mulation on swash bars offs hore on ure 6e ); an exte ns ion of th e sa nd 'sloshing' between th e two th e ebb delta platform. Th e origina l sa nd depos it ion would Waihi Beach headlands; and sa nd losses into th e inlet during be associated with longshore transport events but it wou ld eastwa rd tran sport condit ions. re quire of th e ord er of 6 months for it to translate shore wa rd WNF2 (29'lt va ria nce) had loadings in-phase alongshore a nd affect th e beach . MNF3 (] 2 r;, va riance ), showing another a nd showed a quasi-annual cycle which correlated both with 'centrally inverted ' loading and an irregular time se ries , a p­ th e fall speed parameter and th e sub-reac h sa nd volume (Ta­ pears to ca pt ure th e welding of a sa nd bar to th e cus pa te ble I ). Thi s clearly relates to cross-shore transport effects as ­ foreland (in th e are a of pr ofiles W4 a nd W5 ) during 1994. socia te d with stor m and swell wav es . Th e notabl e differen ce Th e MNF:3 scores correlate significantly with th e cumulat ive with th e Waihi far field, though, is that in th e near field this longshore transport pot en ti al afte r a 12 month lag. Again, cross-shore transport factor is subordina te to oth er effects. this suggests that th e bea ch accre tion was initiated by an Th e near-zero loading ofWNF2 at W2, nearest the inlet , sug­ ea rlier longshore-transport-linked deposition event on t he ges ts that this incid ent wave signal diminishes progressively delta platform, with a lag period required to tran slate this in th e lee of th e ebb delta . WNF3 (11'It variance ) ma y capt ure bar form shore wa rd onto th e bea ch . th e welding to the bea ch in 1994 of th e sand bar tha t had A key feature of th ese Matak an a near field principal corn­ been migr ating shorewa rd from the delt a margin . ponents is th at they all involved inter-annual to longer time For th e Matakana near field (MNF l sub-reach (profiles scales . None correlated with the sea sonal storm-wa ve/swell M2-M7 l, th e three largest pr incipal components (Figure 10c) signal. Thi s is cons istent with the Matakan a near field beach­ expla ine d 95r,1 of the varian ce in V*. Th e first (MN F1, 55'lt es bein g well pr otect ed from waves from the N-NE by the variance) had loadings in-phase alongshore a nd showed a ebb delta bars. pr ogr essive decrease in V* with time. This pattern is cons is­ For th e Matak ana far field (MFF) sub-reach (profiles tent with th e progressive erosion observed to accompa ny th e M8-M 12 ), th e three largest pr incipal compone nts (Figure migration toward th e in let of a scour hole associate d with t he l Od) expla ine d 88'lt of th e variance in V*. Th e MFF1 (54'lt easte rn marginal flood-tidal chan nel, as describ ed earlier. va riance) load ings were in-phase alongshore , and th e scores MNFI correlate d well with the sub-reac h sa nd volume a nd showed a qu asi-season al fluctuati on whi ch correla te d wit h with th e cumula tive regional long shore transport (Fi gure 11c the sub-reach sa nd volu me a nd th e fall spee d paramet er ITa ­ and Tabl e t i Th e latter correlat ion involves a decrea se in ble l ), As at Wai hi Beach , this ind icates th e pr edominan ce of beach sa nd volum e associate d with westward transport, sug­ th e incident wa ves. MFFI also correla te d significantly with ges ting th at easte rly (of normal ) waves tended to dri ve Ma­ the cumulative longsh ore transport ('I'ablc 1), indicating ac­ tak ana Bea ch sa nd into t he inlet entrance from where it was cre t ion during epis odes of eastwa rd transport and erosion ca ught in th e tid al flows th en recir culated out onto t he ebb with westward tran sport. Th e latter connectio n is sugg estive delta . of' west er ly-directed wav es driving sa nd from th e beach into MNF 2 (28' ,1 va riance: had th e inl et end of th e sub-reach th e in let. East erl y-di rected waves may simply stop this pro -

Journal 01' Coasla l H" st 'arch . Vol. Ifl. No , I. I !)!l~l 236 Hicks et al.

a -2

-1 ;£ '" '"E E:o ",­ ~ ~ ~ -o o ._ 0 o > -;;; a...'" ~'" - E -0 co -1 u:u.. 0 '" E~ z '"0. 0 :;:: - (J)Z -2 -es - u, - - Fall Speed Parameter -3 3 l -Ja n-91 l- Jan- 92 31-Dec -92 31-Dec-93 31-Dec-94

b to ~ 2 '"c; ~ f­ ", -0 ~ '" ..co :.='" o "'E'" '" c ~ o 0 - 1 -'z - - --WFF2 (20% va r. ) z '" -2 --Net Longshore Transport N u, u, :;:: -3 1-J a n-91 l -J a n-92 31-0ec-92 31-0ec-93 31- 0ec-94

c to 2 -2 o a. ---MNFl (55% va r.) '"c ...... Net Longshore Transport ~ -1 -e f- ---Volume ~ - w '"OO 0-0 E '-= .r:'" o o ~E'" o~ '"Cl:'='" C co >0 o E Z -' ~ _ 0 ", z -1 Z - . " ' ., u, Z -2 2 :2 I·J an -91 l -Jan -92 31-Dec-92 31-0 ec -93 31-0 ec ·94

d to o a. '"c; ~ f- '"~ -0 o '" .r: '" V> :.= o I J \ '"C E'" o ~ -' 0 Z -1 Z'" N (J) :;:: -2 l -Ja n-91 1-J a n-92 31-0ec-92 31-0 ec -93 31-0ec-94 Figu re 11. Selected pr incipal compnnent s time-series (scores ) overla id on various forcing functio n a nd beach volum e time-se ries. (a l Waihi Beach far field first pri ncipal component (WFF l) a nd beach volum e with fall spe ed pa ram eter, ( h) Waihi Bea ch far field second principal componen t (WFF2) with net region al longshore tra nsport, (c) Mata kan a Island near field first principal compone nt (MNF 1) and bench volume with net regional longshore transport , and (d ) whole-shore second pr incipal component (WS2) wit h ne t regional longsh ore tran sport.

J ournal of Coas ta l Resea rch, Vol. 15, No. I, 1999 S hore-line Chungo in New Zeal and 237 cess . Our wave refraction analysis (Figure oj suggests that or in association with morphologic chan ge on th e ebb delta no sign ificant 'drumstick' effect (i.e., whereby transport would ba r/channel syste ms or with bars migrating shorewa rd offthe sta ll on the downdrift sid e of th e ebb delta du e to wave re­ delta and weld ing to th e beach . Day to day, storm to storm fracti on effects) occurs th is fa r alongshore with easterly-di­ beach cha nges associate d with t he incident wave conditions rected waves. a re sma ll compared with th ese longer-term chan ges. The MFF2 and MFF3 (21'It a nd 1:3'1t of variance, respec­ Thi s contrasts wit h the far field sit uation where th e pri­ tivelyJ loadings vary in sign a longs hore, showing patterns mary beach cha nges show storm to seasona l time sca les a nd sugges tive of discontinuous sand bars. Th e MFF2 scores sug­ a re phase-locked to th e incident wa ve conditions. Th ese gest a :3-4 year cycle and correlate weakl y but significantly cha nges reflect variations in wave stee pness and corres pond with th e cumulative longshore tran sport (Ta ble 1J. A much to cross-shore sand excha nges bet ween th e sub-ae rial beach better corre lation with th e longshore tran sport is ac hieved (r and offshore bars. Lesser far field cha nges are linked to inter­ = - 0.72) after a 14 month lag, sugges ting th at thi s signal is annual variations including sa nd migrati on from one end of assoc iated with th e effect of longshore transport on offshore a headl and-bound beach to a nothe r and sand 'pumping' from bars, possibly associate d with bar-bypassing a round th e ebb the beaches into th e inlet/ebb-d elta system. delt a margin. Th e correlation of th e more irregul ar MFF 3 Th e spa tial scales of cha nge found in th is st udy a re consis­ scores with the longshore tran sport does not improve notabl y ten t with tho se reported by FENSTER and DOLAN (996) from after a lag peri od, suggesting a direct influ en ce of obliqu ely mixed energy , tid e dominated natural inlets on th e Virginia incident waves on th e beach volum e. Possibly , thi s signa l re­ barrier isla nds . Using rate-of-chan ge of shoreline position to lates to offshore bar effects on wave refr action. define t he exte nt of inlet effects on adjacent beaches, t hey As ant icipated, th e principal compone nts a na lysis was less found th at inlet effects dominated shore line cha nges up to success ful at se para ting varia nce in V* along the whole 4.3 km from th e inlet a nd exerte d lesser influ ence up to 6.8 length of open-ocean-facing shore (the first three comp onents km upd rift and 5.4 km downdrift. At Katikati, inlet domi­ explai ned 71'j, of th e vari anc e ) du e to th e weighting effect of nan ce, defined in terms of th e magnitude of the shore line th e more closely spaced near field profiles. Nonetheless, th e fluctuations (Figure 7I, spanned a pproxima te ly 2 km either first whole-shore principal compon ent (WS l, 37'It vari an ce ) side of th e inlet. However, by cons idering temporal patterns patterns do suggest somet hing like a 3-4 yea r cycle of beach of cha nge as well (e.g., Figure 9 ), it is clea r th at th e spa n of change involving th e near field of th e ebb delta a nd th e Ma­ influen ce exte nded for 3-4 km on the eastern (Mata ka na l side takana far field. Th e WS1 scores correlate d well with the rec­ and 2.5 km on th e western (Wa ihi: side. Sin ce th e tid al pr ism ord of beach volum e weighted by profile spacing, but since of Katikati Inl et is 0.95 X 10" 01" while th e tid al pr ism s of th ey also correlated significantly with all of th e forcing func­ FENSTER and DOLAN'S Virgini a inlet s ranged from 2.6-5.6 tions, th e cause of t he overall beach volume cha nge is con­ x 10' 01", and since ebb delta size is proportional to tidal fuse d. Most likely, th e cycle ca ptured by WSI reflects th e prism volum e to the power of 1.4 (HICKS and HUM E, 19961, long-term circulation of sa nd betw een th e beaches a nd th e we would ex pect th at the linea r sca les at Katikati inlet would ebb delta. The loadings ofWS2 (2 1'1t vari an ce ) a re weighted be approxima te ly (0.95/5.6)"""" = 0.44 times th e largest re­ to Waihi Beach and M12 on Matak ana Island, and its scores ported by FENSTI.;R an d DOLAN, which is so. Although our correlate well with th e cumulative longsh ore transport (Fig­ a pproach differed from FENSTER and DOLAN'S in th at we a n­ ur e l l d I and less so with th e fall speed param et er (Ta ble 1). alyse d normalised cha nges in beach volume and shoreline po­ Th is appea rs to re prese nt se veral far field effects, such as sition, since th e time ste ps between our surveys were reason­ reversin g sa nd migrations and th e qu asi-annual storm-wa ve abl y uniform (avera ging one month ) thi s is essentially th e influence. WS:3 (13'1t varia nce) has a loading pattern very sa me as work ing with th e integr al of the rates of chan ge. simila r to th e tran sport divergenc e pattern found from th e refraction ana lys is (Figure fie), a nd its scores show a reason­ Inlet and Ebb Delta Near-field Effects ably distinct 2-year cycle a nd correlate only with the cumu­ Th e tidal inlet and ebb delta sys te m at Katikati affect th e lative longsh ore tra ns port. WS:3, th en , appears to represent near field shore through a number of processes. Th ese include th e conse que nces of th e ebb delta's int erruption of the re­ wave she lte ring, locali sed wave focusing, scour by flood-t idal gional longshore tra ns port field . The WS3 loading pattern cha nnels, bar migration and welding, and divergence in th e suggests th at thi s effect migh t exte nd as far west from th e longshore tran sport du e to wave re fraction. Over tim e scales inlet as W9 and as far east as M11- further than sugges te d of se vera l years , th ese processes produ ce a superposition of by th e refr action analysis. positionally fixed, migrating, and transient shore features. DISCUSSION Wave Sheltering Scales of Coherent or Similar Change Th e ebb delta, with its broad dissip ative platform areas, Th e above results depict clea r differences in th e spatial a nd margin al bars, and obliquely-inclined rows of lin ear bars pre­ te mporal sca les of sa nd volum e a nd shoreline change on sents a n effective breakwater for th e near field beaches with­ beaches behind and adjacent to th e Katikati ebb delta. Th e in its lee. Thi s she lte ring is substantial along the Matak an a near field changes are larger and mainly occur over multi­ shore betw een th e inlet and th e cus pate foreland, at least as yea r tim e sca les, either in ph ase with (or laggin g) inter- an­ regards erosion du e to offshore tran sport. Th e main direct nu al fluctu ations in th e directi on of net longshore transport effect of incident waves on thi s shore a ppea rs to be easte rly

J ourna l of Coastal Research, Vol. Hi, No. 1,1999 2:3 8 Hicks et al. wa ves driving sand alongs hore towa rds th e inlet throat. A circulation cells on either side of th e inlet may be ind epen­ stor m wave signa l was recogni zabl e on th e Waihi near field den t. A conceptua l model of sa nd tra ns port paths around th e beaches. However, th e principal compone nts a nalysis showed Katikati ebb delta und er different wave directions is given in th at this storm wave erosion was subordinate to th e effects HUM r; et al. (in preparation). Similar processes have been or longshore t ra ns port and bar/channel migr ation and that it described at mixed energy inlets elsew here (e.g ., FITZGER­ diminish ed towards th e inlet, almost disappearing at profile AL D, 1988 ), W2. Locally and for irregula r periods, wh ere ga ps developed From thi s study and from inspection of ea rlier ae rial pho­ in the sys tem of offshore bars or wave field distortion oc­ tographs , the time fram e of th e Katikati cycles a ppears to be curred over th e bar/channel topography of th e ebb delta typi cally about 4- 5 yea rs . Th eir longsh ore extent depends on flank s, more wave ene rgy reached th e beach and transient the length of bars shed by th e ebb delta. During our study scour holes developed (e.g., Figure 3 l. ba r accretion occurred at th e cuspate foreland, but HUME et Generally, at lea st over th e four yea rs of th e study, th e al.'s (in preparation) historical record indicates that bars br oad extent of th e wave sheltering effect appea red to be come onshore up to 3 km further south-east (i.e., as far as fixed in space by th e shape or th e delta. Major changes in th e profile MlOon Figure 1). ebb delta shape mad e by large rare storms would alte r this pattern or wave protection, thus inducing a long-term cycle Longshore Transport Divergence or nea r field beach chan ge while th e ebb delta recovered its Our investigation of th e divergence induced in the regional normal shape. HUME et al. (in preparation ) describe th e Ka­ longsh ore transport field by wave refraction over the sub­ tikati ebb delta bein g "flattened agains t th e shore" during merged ebb delta pr edicted a divergence pattern th at was major storms in 1975 and 1982, inducing decad al-scale vari­ rem arkably similar for a wide ran ge of incident wave direc­ ations in th e ebb delta sha pe. FITZGERALD and NUMM EDAL tions (Figure 6e ). Thi s essentially standing effect indi cated (1983) describ ed cyclical episodes or bar growth on the ebb deposition very close to the inlet on both sides th en erosion delta at Pr ice Inl et, South Carolina , which led to varying ex­ holes extending alongshore for approximately 3 krn. Th e pr in­ posure to wave energy and va rying beach erosion. cipal components a na lysis suggested th at th is effect was in­ deed occurri ng, waxin g and wa ning accord ing to the regional Moving Flood-Tidal Channels, Accreting Bars, and longsh ore tra ns port potential , but was partly masked by oth­ Sand Circulation er' processes such as bar and channel migration. These would tend to smooth-out the shoreline perturbati ons th at would Flood-tid al cha nnels a nd bars migr ating shorewa rd from oth erwise develop from th e tran sport divergence pattern. th e ebb delta platform were major influ ences on th e near field Figure 6e shows that th e tra ns port divergence effect beach cha nges record ed at Katikati . Erosion occurred when spa nned some 9-10 km between profiles WI 0 and MI 0. Thi s bars on the ebb delta platform forced the flood-tidal flow close exte nds well into what we have defined as th e far field , em­ aga inst the beaches. Thi s erosion was maximised on th e Ma­ phasizing that the wave field is distorted by th e whole area tak an a side of th e inlet during episodes of energetic ea ste rly of submerged ebb delta, not ju st the shallow platform area waves, when th e flood current and the highly obliqu ely inci­ that is easily visibl e on a ir photographs (the bathym et ric sig­ dent wave s combined to scour the beach face a nd dri ve sa nd nature of th e Katikati ebb delta extends offs hore to about th e towards th e inlet throat. Changes in th e position, orientati on, 20 m isobath and spa ns some 6-7 km alongs hore- H ic x s and and degree of development of th e flood cha nnels followed from HUl\1 E,1 9971. changes in the bar patterns, which in turn were link ed to the Th ere is no indic ation on Figure 6e th at littoral tran sport wave condit ions. Afte r a bar migrat ed shoreward a nd welded divergence is th e ca use of th e drumsti ck sha pe of th e Mata­ to th e beach, th e flood channel tended to reform sea ward of kan a Island barrier. This a ppea rs to be more to do with th e th e bar and the beach pa ssed int o an accretiona ry phase. attachm ent of bars at th e cuspate foreland. Possibly when Th ese nea r field erosion/accretion cycles a re the shoreline thi s forela nd is in a more accreted sta te th an it was during signatu re of sand circulation cells within th e inlet/ebb-d elta our study (whe n th e bath ym etry for th e refr action a na lysis system. In these, sa nd is circulated at intervals from tem ­ was surveyed), it would indu ce a different divergence pattern porary storage on th e near field beaches, moving via the flood involving more exte nsiv e sa nd t ra pping. At that state, too, cha nnels a nd the main ebb cha nn el, to storage on bars flan k­ one would expect more bar-bypassing of sand aro und th e del­ ing the ebb channel and a round th e delta margins. The loop ta. During most of our study, the foreland was not parti cu­ is closed when th e bars migrate shorewa rd, often as ba r com­ larl y accret ed a nd extended period s of easterly waves kept it plexes, to weld to the near field beach (see HlTM E et al .. in so, trimming sa nd off th e foreland and pumping it into th e prepa ration, for det ail s ). Thi s latter stage is th e slowest a nd inlet. appea rs to be th e main cont rol on the time scale of the cycle. However, th e history of wave conditions also matter s, since Far Field Effects: Shoreline Pivoting, Littoral Drift thi s determines th e modification of bars and bar complexes Capture and Bypassing and th eir rate a nd directi on of migration. Large storms, which can qui ckly create new bar fea t ures a nd induce shifts Alt hough much of th e far field beach cha nge was associated in the main ebb cha nnel, can interru pt or acce lerate th e cycle. with storm waves and cross-shore sa nd tran sport, th e long­ Also, va riations in th e wave direction a nd th e preva ilin g di­ shore tran sport regim e also induced change. Different mech­ rection of longshore t rans port (Figure 5dJ mean th at th e sa nd a nisms dominated th e longshore tra nsport effect on opposite

.Iou rnal or Coasta l Research. Vol. I;' , No. I. 1!J!J!) Shoreline Cha nge in New Zealand 239 sides of th e inlet. On Waihi Beach, th e inter-annual rever sa ls er far field cha nges were link ed with longshore transport at in th e net regional longshore transport potential a ppeared to inter-annual tim e sca les . The longshore transport was man ­ induce sa nd migration betw een th e Waihi and Bowentown ifest as sand migr ati on rever sa ls between th e small head ­ headlands, which cause d th e Waihi shoreline to subtly pivot lands bounding th e weste rn beach and pumping of bea ch over inter -an nual tim e peri ods. sand into th e inlet/ebb delta sys te m from eithe r sid e. On Matakana Island, th e far field beach showed a weak (3) On th e near field beaches, erosion du e to storm waves trend of sand loss superimposed on th e storm-swell induced was severely damped by th e breakwater action of th e ebb changes. Net sa nd losses appeared to be as sociated with ep­ delta. The predominant near field beach changes occurred at isodes of inten se westward tran sport (compare Figures 5e mu lti-year tim e scale s and related mainly to th e shore ward and Sb). This westward transport continued through th e Ma­ migration of sa nd bars off the ebb delta and scour ing by th e takan a near field, being most inten se on th e inlet side of the marginal flood-tid al flows as these were deflected and forced cus pate foreland sediment move­ CONCLUSIONS me nt. Journal or Geophys ical Research , 84, 634 7-6354 . BI-:I.I.. R.G. and G' lI{!:--:(;, D.G. 1996. Tec hniques for a na lyzing sea The main conclus ions of this study of a la rge mixed-en er gy level record s a roun d New Zeal and. Marine Geodesy. 19, 77- 98. ebb delta and its adjacent beaches a re: BHl : N :--: , P. 1954. Migratin g sand waves or sand hum ps, wit h special referen ce to inv est iga tio ns ca rrie d out on th e Da nish Nor th Sea (1) Cha nges to sa nd volume a nd shore line position on near coast. Proceedings or the Sth Coasta l Engineering Conference field beaches were 3-4 times larger than on the far field I ASe E, Grenoble I, pp. 269-294. beaches. Bm ux, P. 1962. Sea level rise as a cau se of shore eros ion. Journal Walen cays and Harbours Division, 881 WW 11, 117- 130. (2) Cha nges on th e far field beaches wer e dominated by or Bill I ll:", P. 19f1O . Morph ological and naviga tional aspects of tida l in­ cross-shore tran sport assoc iated wit h alternatin g storm a nd lets on litt ora l drift shores. Journal or Coastal Research , 2, 123­ swell episodes th at showed a weak quasi-annual cycle. Sma ll- 145.

•Journal or Coast al Research. Vol. 15, No. I. 199H 240 Hicks ct al.

CEHe 1984. Sh ore Protection Manual. Coastal Engineering Research HANSON . H. a nd KI{Al IS. N.C. 1989. GE NESIS: Generali zed model Centre, U.S. Army Corp s of Engineer s, Waterways Experiment for simulating shoreline cha nge. Report 1,"Technical Reference ". Station. Technical Report CERC-89-19, U.S. Army Waterw ays Experiment DEAN, RG. 1973. Heurist ic model s of sand transport in the surf Station, Coast al Engineering Research Center , Vicksburg. zone. Proceedings of the Conference on Eng ineering Dynamics in HAHIMY . KG. 1976. Beach erosion and sediments at Waihi Beach. Un­ the Surf Zone (Sydney, Australia i, pp. 208-214. published MSc th esis . University of Waik ato , New Zealand, 79p. DAVI S, RA. Jr., HUM E, T.M., a nd HICKS, D.M., in preparation. Geo­ HAHHAY . KG. and HE AL~;Y, T.R 1978. Beach erosion at Waihi morphology and clas sification of 'flood tid al deltas' in a headland Beach. 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