To address the absence of historical measured data on the tidal regime and subsequent water level variations within the lagoon a project was initiated to collect basic data on water level fluctuations within the lagoon. A network of 10 self-logging pressure transducers (In-Situ miniTROLL Professional) was deployed at key locations within the lagoon at various depths (1 to 8 m depth). These instruments recorded pressure at five minute intervals for a period of 9 days from 29th June to 7th July 2010. Barometric pressure during the same period was monitored at five minute intervals using a self-logging pressure transducer (InSitu BaroTROLL). The atmospheric pressure signal was removed from the submerged pressure transducer signal and the pressure signal normalised to the mean value recorded at each location before being converted into equivalent meters of water. This data was then analysed using the T_TIDE (Pawlowicz et al. 2002) software to provide basic information on harmonic components of the water level fluctuations. Briefly, tidal cycles were similar across all sites sampled with a tidal range of approximately 1.1 m. Flooding tides occurred over an approximate 13 h period whilst ebbing tides were slightly quicker at 11 h. Low tide occurred around 12:00 to 13:00 each day sampled (Figure 104).
Figure 104 Tidal heights at 10 sites in Roviana logged over a 9 day period.
Current patterns
Little information exists on the movement of water through either lagoonal system and a series of long-term drifter and drogue deployments were conducted to overcome this knowledge gap. The drifters consisted of a 1 m length of PVC pipe (40 mm diameter) permanently sealed at one end (Figure 105 A). The permanently sealed end was weighted internally using dry beach sand to a height of 25 cm within the pipe to ensure that when deployed 90 cm (90 % of pipe length) was submerged. This minimised surface wind directly affecting the drifter movement. Before deployment a GPS logger (Holux M241) was fixed to the inside of top cap and set to log position and time every 2 seconds.
Chapter 6 Building social and ecological resilience to climate change in Roviana, Solomon Islands
Figure 105 A) Design of drifter used to monitor water currents and B) profile of drifter during deployment.
A series of drifter deployments in areas of interest within both Vonavona and Roviana on two separate periods July 2010 and January 2011. Areas of interest included estuary mouths, inner lagoon, mid lagoon, ocean passages and within marine protected areas. Prior to deployment locals were recruited to monitors drifters during sampling. A brief workshop was held explaining the sampling strategy and advice was sought on local water circulation patterns and in selection of suitable deployment sites. Deployment times occurred early morning generally 7 to 9 am with retrieval scheduled between 3 to 5 pm. One local assistant was assigned to each drifter and followed it using a 2-3 m wooden canoe. A distance of at least 10 m was maintained around between canoes and the drifter unless the water depth became too shallow (causing drifter to contact the bottom) and the drifter would then be collected and redeployed in the nearest deep water channel (typically <50 m away) and the time at which this occurred noted. Drift tracks generally followed the tidal cycle moving seawards during ebb tides and landward during flood tides ( Figure 106).
Figure 106 Surface current drifter track adjacent to Hura River mouth-central Roviana.
After each deployment the units would were collected and GPS loggers would be downloaded and reset for
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and the direction and speed calculated. The mean current speed was then calculated using all deployments for each area of interest. The predicted drift extent was estimated using the drift rate from the drift tracks and tidal phase duration from the pressure transducer data (Figure 107). These data suggest strong connectivity within the centre of the Roviana – Vonavona system (Munda region), with higher average drift rates and larger associated predicted drift extent. Marine protected areas within this region will likely service a larger region relative to their size compared with more distal areas.
Figure 107 Initial drifter deployment site and average drift rate during ebbing tide. Units m s-1. Indication of predicted drift extent based on drifter track, note central site of Noro Passage has no drift extent as this is beyond the boundary of the figure.
H
YDRODYNAMIC MODEL DEVELOPMENTA core driver of hydrodynamics in shallow lagoon systems such as Roviana is bathymetry. As no nautical charts or bathymetric surveys have documented bathymetry of the Roviana and Vonavona lagoon, this program collected this baseline data. Bathymetry was surveyed using a single beam 200 Hz sounder linked to a GPS (Garmin Extrex) onboard 6 m fibreglass boats to enable high speed sounding over shallow waters (Figure 108).
Chapter 6 Building social and ecological resilience to climate change in Roviana, Solomon Islands
Figure 108 Tracks of single beam bathymetry collected in July 2010.
Bathymetric data from these surveys was combined with existing bathymetric data for offshore areas and topographic data to develop a series of digital elevation models (DEMs). These DEMs incorporate a coarse-scale whole of Solomon Islands DEM as well as more detailed Western Province and Roviana Lagoon DEMs.
A pilot hydrodynamic model has been established for the western province region. Whilst development of a fine-scale hydrodynamic model of Roviana during the project phase was not possible, work is continuing to source and refine appropriate input data (meteorological data, ocean current data and tide data) to support the next stages of development. The development plan includes the nesting of the Roviana Lagoon model within the Western Province and whole-of-Solomons model.
Figure 109 Digital Elevation Model (DEM) of Roviana region.
Roviana
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