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Current predictions for the Rottnest Channel swim Agi K. Gedeon1,2, E.M.S. Wijeratne1, Charitha Pattiaratchi1, Roger Proctor3

1School of Civil, Environmental and Mining Engineering and the UWA Oceans Institute The University of Western Australia, Crawley, WA; 2Western Australian Integrated Marine Observing System (WAIMOS);

3eMarine Information Infrastructure, IMOS, University of Tasmania

Introduction

Marker buoys define the northern boundary of the swim course. Yellow buoys mark 12, 14, 16 and 17km. The blue 10km, and red 15 and 18km buoys (see Fig 7) must be reached by a cut-off time. The latter, adjacent to Phillip Rock, forms a gate, which swimmers must pass through to reach the 19km buoy by 4:20pm. Swimmers report that when currents have been strong in past years they have had to swim sideways to stay south of the 15 and 18km marks. Since start times are staged in waves every 15mins from 5:45am with solo, then duo teams setting off by 6:30am, teams of four can experience rather different conditions with starts ranging from 6:45–7:45am.

Acknowledgements We warmly acknowledge Andrew Hunt who has criss-crossed the channel multiple times and developed MapSwim to aid other swimmers; our thanks also to Tim Lefroy, 2014 race director and WA Youth awardee, for his insights along with that of several other swimmers. Lucy Wyatt, David Griffin, Madeline Cahill were all most generous with their time and contribution to providing the best predictions for this event.

School of Civil, Environmental and Mining Engineering

Current fields were forecast for the Rottnest swim using the Regional Ocean Modelling System (ROMS). The model domain and topography are shown in Fig 4. HYCOM global ocean model forecast outputs of salinity, temperature, and horizontal velocities were used to specify the open boundary section 3-D tracers and transport. A combination of nudging and radiation conditions were used for 3-D transport and tracers at the model open boundaries. The model was driven with GFS forecast data of wind and air pressure. Forcing tides were derived from the TPX07.2 global tidal model and were provided as complex amplitudes of earth-relative sea-surface elevation and tidal currents for eight primary harmonic constituents (M2, S2, N2, K2, K1, O1, P1, Q1). These harmonics were introduced in ROMS through the open boundaries elevation using the Chapman and current ellipse variable and Flather condition. Initially, model hind-cast simulations were undertaken to obtain optimal results, and predicted surface currents were compared with radar currents on real time basis (Fig 5).

Figure 5: Model verification against HF RADAR.

Figure 1: (a) entrance to the starting pen; (b) 1km start channel and sighting to the Leeuwin icon vessel anchored at 1500m. All competitors (solo, duo and team) must be met by their paddlers and support boats in order to proceed past the icon vessel.

Figure 6: (a) Wind speed and direction 6am-noon average on racedays over past 6 years from Hillarys. (b) Scatter plot shows the fastest swimmer and average times over the same period. No apparent correlation of wind speed or direction with speed of swim; however a stronger southward current was noted in 2009.

Figure 7: (a) Snapshot of velocity fields @ 11:30, 22 Feb 2014; (b) Daily average velocity field for 22 Feb 2014. Race marker red and blue buoys shown — swimmers must pass to the south of each buoy by a cutoff time.

The Rottnest Channel Swim Association (RCSA), conducts an annual 19.7km open water swim from Cottesloe Beach to Rottnest Island in February each year. It is one of the largest open water swim competitions in the world, attracting many interstate and international swimmers. The safety and welfare of swimmers is paramount (Fig 1) and strong winds, rough and choppy seas can add to the challenge, as can currents; which in this channel are typically northerly, but can also switch southerly. The currents are strongest in the last few kilometres on the approach to Rottnest, exactly when the swimmers are most tired: “some years we’ve been swept to the north so that as you get close to Rottnest you spend as much time swimming south as you are west at exactly the time you are feeling the worst,” conversely: “in 2009 when we did the duo we spent a lot of time swimming north around Phillip Rock” (see Fig 2, also Fig 6). This year, we aimed to help predict the current conditions for the event.

Swimmers

Forecast for 2015

Analysis

Figure 4: ROMS model domain and topography. Inset shows the model grid of 560 x 560 cells and 30 sigma layers resulting in an average resolution of ~500 m.

The prediction for the Rottnest swim day on 22 February took in the most current weather forecasts and was presented as a daily average velocity field. It was posted as an Ocean News item on the IMOS OceanCurrent page http://oceancurrent.imos.org.au/news.htm with the phrase ‘Don’t expect the usual northward current flow’. The RCSA also posted it on their Facebook page and circulated it on social media in the days before the event. The public appreciated the user-friendly plot detail, but asked how well it reflected the reality of the race period.

In 2014, current prediction provided from the ROMS model showed an unremarkable weak southward current becoming northward towards the end of the race (Fig 7a). The results of the 2014 Rottnest Channel swim validated the model prediction that the current would not be strongly positive in any direction that would impact swimmers. A slight increase in the strength of the northward current from the 18km marker buoy was remarked upon by some. The MapSwim hot track comparison of 2013 (Fig 2) to 2014 (Fig 3) shows that swimmers were not fighting the current to stay to the south of the rhumb line this year. Anecdotally, several swimmers reported an easy swim; one passed the Phillip Rock gate at 10:50am with no current; later others remarked on the mild northward current in the last kilometre approach to the island.

The key information of value to swimmers is the direction and strength of the currents as near to realtime as possible on the day. For 2015, the intention is to provide hourly predictions, since conditions can change over the course of the race. Further experimental model simulations (with and without tidal forcing) clearly indicated that the contribution of the tides to the currents in the Rottnest region is significant. We believe hourly predictions will allow for strategic course setting and assist skippers to guide their swimmer(s) to keep just south of the markers and close to the rhumb line for the duration of the race.

Swimmers increasingly try to determine the conditions for the race day using any available resources. Mainly relying on local knowledge, seabreeze, wind and wave websites, SST subscription and the IMOS HF radar. For the last 4–5 years, one swimmer has used ACORN data through a visualisation link that he has created —http://m2.mapswim.com/currents/— and has also developed the MapSwim app. for real-time GPS tracking of individual swimmers via their race number and support boat position.

MapSwim

Wind

Officially more than 2500 people have completed the solo swim since 1956. The crossing record is 4:00:15, held since 2000. The speed of the swim cannot be compared across years, but it was observed that “swimmers sticking to the left side of the 2014 course got a little boost of perhaps up to half an hour on a westerly current on the LHS”.

Figure 3: MapSwim tracks 2014 heat map implies there was virtually no cross current for the first 14 km and then a slight push northward. Inset shows close-up of the last 6 km, but with no clumping around marker buoys, indicating negligible northward current, as affirmed by swimmers.

The Rottnest Channel is at the very edge of the IMOS HF radar coverage near Perth. Estimates of surface current (which are also shown on MapSwim) are least accurate at the edges of coverage and subject to many sources of interference. The mismatch in Fig 5 also reflects the edge effect, so the accuracy of the north/south component of the flow is very uncertain.

Year Fastest swimmer2008 4:41:272009 4:40:442010 4:41:352011 4:50:492012 4:29:222013 4:34:372014 4:14:04

The challenge of predicting currents is of great interest to competitive solo swimmers. Keen open-swimmers are aware that ocean currents are quite dynamic and that averaged predictions too far in advance mask time variability in seabreeze and tide. Those swimmers who used the information to target the pub straight down the rhumb line were very happy: “Thanks for the heads-up from IMOS, you saved us deviating too vigorously to the south (which served us well last year) and saved us about 20–30 mins overall!”; another was pleased to note that “the predictions held up nicely”.

Model configuration

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Loc-4

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Loc-6

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The course

Figure 2: MapSwim tracks 2013 heat map shows little evidence of cross current. Inset – on close-up, clustering to the south of identifiable marker buoy positions at 15,16,17,18km (arrows), suggests some force pushing northward causing participants to bunch up south of the markers. Phillip Rock location.

2013

2014

Predicted velocity fields

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