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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