Virtual Weir Phase 5 Investigations(Preliminary Results)

Canberra29 September, 2009

Talk Outline

• Background – Why we are doing these simulations

• Model Setup

• Extremal Wind Analysis

• Results from 2 dimensional TUFLOW-FV Model

• Issues with Existing Simulations

• Results from 3 dimensional ELCOM Model

• Discussion

Background: Virtual Weir Project OutlineProject Phase

Description

1Data Review and Analysis:Review available data for model boundary conditions and calibration data sets.

2Extended & Improved Model Calibration/Validation (Including Model Sensitivity Testing):Improve model calibration to increase confidence in model results.

3

Model Design Scenarios (including Worst Case Condition Testing):Application of 2D and 3D models to determine if the proposed “virtual weir” is able to prevent the formation of a salt wedge capable of transporting salt upstream towards the SA Water off-takes.

4Destratification Model Testing & Literature Review:If a salt wedge does form in the deeper parts of the River, what can be done to remove it?

5aPrognostic “Worst Case” Simulations (This Presentation):

Examine “worst case” conditions based on a number of proposed water level management objectives.

5b (?)

Prognostic “Worst Case” Simulations with Mitigation Options (Proposed):

Use a range of mitigation options to reduce impacts of “worst case” conditions determined in Phase 5a. Mitigation options include the use of salt traps, artificial mixers and environmental flows.

Investigation Outline

Modelling Objective

• Worst case conditions three years into the future.

• Three WL management objectives (-1.5, -1.2 and -0.8 m AHD) using sea-water inflow to maintain Lake level.

Modelling Approach

• Three years (1st September 2009 – 26th March 2012) modelled in 2D (only) using worst case conditions.

• 5 months (calibration/verification period, 26th March 2012 – 20th August 2012) modelled in 2D & 3D using observed 2009 meteorological conditions with winds scaled to 1 in 20 Year ARI magnitude.

Model Setup/Extents•TUFLOW-FV 2D (Blue) : All areas upstream of Tidal Barrages & Downstream of Lock 1, Lake Albert and Goolwa Channel disconnected

•ELCOM 3D (Green) : From Pomanda Island (upstream of weir location) to Murray Bridge

•TUFLOW-FV simulation executed first, then results fed to ELCOM (U/S & D/S Boundaries)

Model Extents and Boundaries (2D)

TUFLOW-FV (2D)

Model Extents and BC’s

Model Extents and Boundaries (3D)

ELCOM (3D)

Model Extents and BC’s

Adopted Boundary Conditions (1/9/2009 – 26/3/2012)

• 350 GL/Year (970 ML/Day) River flow into Lake Alexandrina

• 1982 Net Rainfall / Evaporation (-1.17m Net Loss)

• 2008 Pelican Point Wind Data

• 2008 SA Water Off-takes and Irrigation Extractions

• Addition River salt load (Groundwater / Salt Drain)

• Pumping 27.5 GL into Goolwa Channel Water Level Management Area (500 ML/Day from September – October 2009)

• Tauwitchere Seawater Inflows to maintain Lake Level and -0.8, -1.2 and -1.5mAHD (determined by a water balance spreadsheet model)

• Recorded Tauwitchere (Coorong) Salinity for 2008

Wind Extremal Analysis

ARI (1 in X Years)

Gumbel Distribution Design Wind Speed (m/s)

Weibull Distribution Design Wind Speed (m/s)

Partial Series DesignWind Speed (m/s)

~1 12.51 12.56~2 13.04 13.02~5 13.75 13.6210 14.25 14.28 14.0715 14.5 14.59 14.3420 14.7 14.81 14.53

• 10 years hourly Pelican Point wind data (1998 – 2009)

• An annual and partial (>12 m/s) series of high wind speeds were analysed

• Two distributions of probability were used to analyse the annual series:– The Gumbel distribution which was fitted using the software Windographer (Mistaya

Engineering).– A Weibull distribution which was analysed in EXCEL based on the methodology in

Pilgrim and Doran (1997).

• The partial series was analysed based on a methodology described in Smits et. al. (2005) which uses the plotting position formula of Gringorten (1963).

Pelican Point Design Wind Speeds using Three Extremal Analysis Methods

Wind Event Scale Factors

ARI (1 in X Years)

Weibull Distribution Design Wind Speed (m/s)

Ratio to Increase ANZAC Wind speeds to Design Event

~1 12.51 1.04~2 13.04 1.09~5 13.75 1.1510 14.28 1.1915 14.59 1.2220 14.81 1.23

Scaling Factors Required to Increase ANZAC Weekend 2009 to ARI Design Event

• In order to examine the impact of more extreme events on water quality within the lowest reach of the River Murray the 2009 ANZAC weekend event was scaled up to the magnitude of the 1 in 20 year ARI design event.

• The observed peak wind during the ANZAC event was 12.0 m/s.

• If the response of the system to a 1 in 20 year event is to be investigated the wind speeds should be increased to 14.81 m/s by factoring the wind speed upwards by 1.23.

Wind Event Scaling

TUFLOW-FV Results: Water Levels(maintained by letting water in from Tauwitchere Barrage)

TUFLOW-FV Results: Salinity

TUFLOW-FV Results – Spatial SnapshotsMarch 26, 2012

Summary Water Balance Results

GL/year 2009/2010 2010/2011 2011/2012

-0.8 394.4 255.2 249.8

-1.2 168.2 219.7 215.9

-1.5 16.2 176.4 174.7

Lock 1 350 350 350

Mt/year 2009/2010 2010/2011 2011/2012

-0.8 16.13 10.93 10.69

-1.2 7.34 9.44 9.28

-1.5 0.71 7.61 7.54

Lock 1 0.21 0.21 0.21

Annual Salt Load from Sea Water Inflow

Annual Sea Water Inflow Required to Maintain Water Level

ELCOM Results – Current Time Step Issues

• Previous models were run at 60s timestep with negligible model instability (CFL exceedance)

• Higher salinity gradient and increased velocities (due to extreme winds) creating model instability (CFL exceedance) at 60s timestep

• -1.2 mAHD scenario re-run at 45s step (still minor instability). – Run time actually slightly reduced due to improved convergence. – Significant increase in mass of salt transported upstream into River. – Concentrations throughout water column increased by > 50%.

• Model validation re-run at 30s – negligible change in results

ELCOM Results – Current Time Step IssuesTailem Bend Bed

ELCOM Results – Current Time Step IssuesSwanport Surface

ELCOM Results – March – August 2012(Salinity at Bed, Tailem Bend)

ELCOM Results – March – August 2012(Salinity at Surface, Tailem Bend)

ELCOM Results – March – August 2012(Salinity at Surface, Swanport)

ELCOM Results – March – August 2012(Salinity at Bed, Swanport)

Animations

• -0.8 m AHD Water Level

• -1.2 m AHD Water Level

• -1.5 m AHD Water Level

Discussion• Managing Lake level at lower level (-1.5mAHD) reduces amount of salt

transported upstream during backflow events

• Models currently being re-run with 30s time-step

• Worst Case scenarios shows salt > 1400 EC at Swanport Surface

• What is probability of 3 years of 1982 conditions?

• Are amelioration options likely to be effective?

Thank you