Investigating: Water Circulation

Channel Configuration vs Hydrodynamic Change

Introducing WeirsWater Turnover Time

Herve Damlamian , Jens Kruger

• Located 225 km from Rarotonga• 50 km2 lagoon• Max depth 11 m• Shallow water (0 m to 3 m) largely dominant, with only few areas > 5m

•Arutanga channel:- Narrow : 10 m to 20 m wide- Shallow : 1 m to 5m deep

Climate Analysis & Model

• Waves are Southerly dominant• Mean Significant Wave High, Hs = 2.4m

Wave & Wind Climate Analysis (ECMWF era interim model)

Climate ScenariosSoutherly Wave & Mean Easterly WindSoutheasterly Wave & mean easterly windNorthwesterly Wave & mean westerly windNo Wave & mean easterly wind

Numerical model & Water circulation

4 weather scenarios * 2 (Neap & Spring)

High tide Low tide

Water Circulation in AitutakiMean Condition

Low tide, Secondary pattern

Configuration 0, 1 & 2• Configuration 1: small boat and yacht- Channel width : 50 m- Channel depth : 4 m- Marina diameter : 150 m

• Configuration 2: international cargo ship - Channel width : 80 m - Channel depth : 8 m - Marina diameter : 150 m

Comparing Conf.0 vs Conf.1 vs Conf.2Water Circulation pattern

for main weather condition

• Similar main water circulation patterns

• Similar secondary water circulation pattern

• Increased occurrence of secondary pattern at Spring tide

Conf.0 : 1h50/day Conf.1 : 3h00/day Conf.2 :5h00/day

• New occurrence of secondary pattern at Neap tide.

Secondary pattern results from the dominancy of tidal forcing against (hydraulic gradient + wave) forcing. Opening of Arutanga channel increases tidal forces within the lagoon and strengthens that particular water circulation pattern.

(spring tide)

Comparing Conf.0 vs Conf.1 vs Conf.2Current Speed

Possible impact:Disturb sediment transport rate leading to erosion

Comparing Conf.0 vs Conf.1 vs Conf.2Surface Elevation

6

1

2

3

5

4

Possible Impacts:• Coral species sensitive to water level fluctuation such as Micro-atoll• Increase low tide exposure of reef and could potentially stress/kill coral communities•Increase in lagoon water temperature:

-Decrease in DO- Decline in seagrass

Introducing Channel Wallsto Mitigate Water Level Drawdown

Channel walls : Introducing Weir Structures

- Emerging at low tide- No flow passing through- Allow boat to travel between the Marina and the lagoon.

Impact of the Implementation of Weirs on the Surface elevation

Impact of the Implementation of Weirs on the Current Speed

- Only localized effect on Cur.spd- Water travels along the weir down to the channel entrance.

- Up to 300% increase in the marina’s entrance- Up to 20% decrease in the channel mouth

Possible impact: Increase sediment transport rate, erosion

Sc2

Conf 0 Conf 2Conf 1

T=420 h

T=420 h

T=420 h

T=135 h

Lagoon Water Turnover Time• Turnover time to quantify lagoon vulnerability to pollution scenario

and identify areas of low water quality potential

Transport model simulated a 20 days period.

•Eddy region: most vulnerable to pollution scenario.

• Opening the channel increase water turnover time (Double in Conf 2)

C0nf 0 Conf 1

Conf 2

T0 T0T0

Conclusion

Calibration

Low tide, Secondary pattern

Water Circulation in AitutakiNorthwesterly wave and wind field

High tide

Low tide

Channel Current vs Scenarios

- Flush out is dominant for each scenario as a result of surface elevation gradient

- Maximum flush out speed: southerly wave field.

- Minimum flush out speed: northwesterly wave field.

- maximum channel flush in: no wave condition.

- Flush in(Sc1)<Flush in(Sc2),due to Aitutaki geometry