Philips, D., Mead, S., Black, K. & Healy, T. (2003). Surf Zone Currents and Influence on Surfability. In Proceedings of the 3rd International Surfing Reef Symposium, Raglan, New Zealand, June 22-25, 2003 (pp.60-82). Raglan, New Zealand: ASR Ltd.
Permanent Research Commons link: https://hdl.handle.net/10289/190
Surfing headlands are shallow and exposed coastal features that provide a specific form of breaking wave allowing a board-rider to ride on the unbroken wave face. The seabed shape and refraction of the waves in relation to depth contours provide the greatest influence on the quality of the surf break. The large scale and orientation of the Raglan headland allows only the low frequency swells to refract around the headland to create seven different surfing breaks. Each represents a compartmentalization of the shoreline along the headland. This creates variability in wave and current characteristics depending on the orientation and bathymetry at different locations. This provides not only potential access points through the surf-zone (ie: smaller currents), but greater surfability in a range of conditions that is not possible on small scale headlands. Headlands with surfing waves can be classified as mis-aligned sections of the coast, where the higher oblique angle of the breaking surf generates strong wave-driven currents. These currents are far greater than that found on coastlines in equilibrium with the dominant swell direction, where comparatively insignificant longshore drift is found. The strength and direction of wave-driven currents in the surf zone can influence the surfability of a break. At a surfing headland strong currents flowing downdrift along the shoreline make it difficult for a paddling surfer to get to the "take-off" location of the break, or maintain position in the line-up. In comparison currents flowing updrift along headlands makes getting "out the back" relatively easy, although surfers can be taken out to sea past the "take-off" point by a fast flowing current. Field experiments at Raglan, on the west coast of New Zealand have been conducted to measure current speed and direction during a large swell event. Observations of surfers attempting to paddle through the breaking-wave zone, confirms the strength of the wave-driven currents with surfers being swept rapidly down the headland. Results from the experiments at Raglan, have shown strong currents in the inshore breaking wave zone with burst-averaged velocities attaining 0.8 ms-1, and maximum bed orbital velocities of up to 2.0 ms-1. Interestingly, further offshore the currents have been found to flow in a re-circulating gyre back up the headland. Comparisons are made from observations of waves and currents found at other surfing headlands around the world. The effect that strong currents may have on the surfability of artificial surfing reefs needs to be considered in the design process, if the surfing amenity is to be maximised for large surf conditions.
(c) ASR Ltd 2003. Used with permssion.
Philips, D., Mead, S., Black, K. & Healy, T. (2003). Mzunguko wa Surf Zone na Uvutano juu ya Surfability. "Mkutano wa tatu wa kimataifa wa ""Surfing Reef Symposium"" uliofanyika huko Raglan, New Zealand, Juni 22, 2003 (p." Raglan, New Zealand: ASR Ltd. "Kama ilivyoelezwa hapo juu, ""Surfing headlands"" ni sehemu ya pwani iliyo wazi na isiyo na kina ambayo hutoa aina maalum ya mawimbi ya kuvunja kuruhusu mwendeshaji wa ubao kuendesha uso wa wimbi usiovunja." Umbo la sakafu ya bahari na refraction ya mawimbi kuhusiana na contours kina kutoa ushawishi mkubwa juu ya ubora wa kuvunja surf. Ukubwa mkubwa na mwelekeo wa Raglan headland inaruhusu tu swells chini ya mzunguko wa refract karibu headland kuunda saba tofauti surfing mapumziko. Kila moja inawakilisha mgawanyiko wa pwani kando ya kilima. Hii inaunda kutofautiana katika wimbi na sasa sifa kulingana na mwelekeo na bathymetry katika maeneo tofauti. Hii hutoa si tu pointi ya uwezekano wa upatikanaji kupitia surf-zone (yaani: mikondo ndogo), lakini surfability kubwa katika mbalimbali ya hali ambayo haiwezekani juu ya headlands ndogo kiwango. Sehemu za pwani zenye mawimbi ya kuogelea zinaweza kuainishwa kama sehemu zisizo sawa za pwani, ambapo pembe ya juu ya kuogelea ya kuvunja hutokeza mikondo yenye nguvu inayoendeshwa na mawimbi. Mtiririko huo ni mkubwa kuliko ule unaopatikana kwenye pwani zilizo katika usawa na mwelekeo wa mawimbi, ambapo mwendo wa pwani ni mdogo sana. Nguvu na mwelekeo wa mawimbi yanayoendeshwa na mawimbi katika eneo la kuogelea unaweza kuathiri uwezo wa kuogelea wa mapumziko. Katika kilele cha kuogelea, mikondo mikali inayotiririka chini kando ya pwani hufanya iwe vigumu kwa surfer paddle kufika mahali pa "kuondoka" ya mapumziko, au kudumisha nafasi katika mstari. Kwa kulinganisha, mikondo inayoelekea juu kwenye vilima hufanya iwe rahisi sana "kutoka nyuma", ingawa wavuvi waweza kuhamishwa baharini baada ya mahali pa "kuondoka" kwa mkondo wa haraka. Majaribio ya shamba huko Raglan, kwenye pwani ya magharibi ya New Zealand yamefanywa ili kupima kasi ya sasa na mwelekeo wakati wa tukio kubwa la kufurika. Uchunguzi wa wavuvi wakijaribu kupiga makasia kupitia eneo la mawimbi ya kuvunja, unathibitisha nguvu ya mikondo inayoendeshwa na mawimbi na wavuvi wakisukumwa haraka chini ya kilima. Matokeo ya majaribio ya Raglan yameonyesha mikondo mikali katika eneo la mawimbi ya kuvunja pwani na kasi ya wastani ya mlipuko kufikia 0.8 ms-1 na kasi ya juu ya mzunguko wa kitanda ya hadi 2.0 ms-1. Jambo lenye kupendeza ni kwamba, mbali zaidi na pwani, imegunduliwa kwamba mikondo hiyo hutiririka katika mzunguko wa mzunguko wa mzunguko wa mzunguko wa mzunguko wa mzunguko wa mzunguko wa mzunguko wa mzunguko wa mzunguko wa mzunguko wa mzunguko. Ulinganisho huo hufanywa kutokana na uchunguzi wa mawimbi na mikondo inayopatikana katika maeneo mengine ya kuteleza kwenye bahari ulimwenguni pote. Mageuzi ya nguvu ya bahari yanaweza kuwa na athari kwa surfability ya miamba ya uvuvi bandia inahitaji kuzingatiwa katika mchakato wa kubuni, ikiwa huduma ya uvuvi inapaswa kuongezeka kwa hali kubwa ya uvuvi. (c) ASR Ltd 2003. Inatumiwa na permssion.