- Q The project is being built in a flood prone area. What is it doing to ensure it doesn't make flooding worse?
- A
It is a key requirement of the Airport Link Project Deed that the project does not adversely impact the existing hydraulogy across the project corridor.
Extensive flood modelling of the project design and construction methods has been undertaken by TJH and submitted to Brisbane City Council for approval prior to the relevant stage of construction commencing.
Brisbane City Council has now approved the hydraulic models for Bowen Hills, Kedron and Toombul.
- Q What is a hydraulic event?
- A A hydraulic event is a storm/rain event, for example a Q1. The number following the Q indicates when an event is likely to return.
- Q How do floods impact the project area?
- A A flood’s volume is determined by catchment size and by rainfall amount. Discharge of flood waters is governed by conveyance (the flow of water through a drainage system), where as flood water extents are governed by conveyance and the amount of flood storage.
- Q What are the Project's Hydraulic objectives?
- A
The Project Deed specified the following objectives:
- provide Q10,000 flood immunity to the tunnels and connection ramps,
- provide design flood immunity in accordance with relevant authorities for roads/walkways/bikeways,
- no adverse impacts to any other properties up to Q100 flood events and
- no adverse impact to existing drainage structure.
- Q What does no 'adverse impacts' mean?
- A The Project is not required to improve flood levels in any given area. However, it is required not to make the flood levels worse. For example, if during a flood event prior to the Project, water rose to the floor boards of a house, the occupant can expect that in future water levels will come to the same level.
- Q What are the flooding impacts near Kalinga Park?
- A
Airport Link’s Eastern Connection, near Kalinga Park, is a bi-modal catchment with Eagle Junction Creek (Lewis Street Drain) and Kedron Brook. This was the only area modelled in the Environmental Impact Study for the Project.
Eagle Junction Creek is a fast responsive catchment of approximately 30 minutes where as Kedron Brook responds critically for over 5 hours. As well, there is a tidal effect at Widdop Street. Kedron Brook’s flood levels are the controlling factor for events that are in excess of Q2.
The baseline for the hydraulic model was the state of the creek in 2007, prior to any Project construction. The model was validated against the May 2009 flood event.
The model demonstrates that without mitigation there would be adverse impacts to properties in the area. The most efficient mitigation requires significant flow conveyance increase which can be achieved through excavation and vegetation removal.
- Q What Flood Mitigation is being put in place to ensure there are no adverse impacts?
- A
The Project is implementing the following flood mitigation measures at the Eastern Connection:
- Regrading of Sandgate Road above Q10,000,
- floodwall on the East West Arterial and the tunnel portal,
- excavation under Sandgate Road Bridge (increase of cross-sectional area by 35%),
- excavation between Sandgate Road and Widdop Street,
- mangroves will not be able to grow in this area as it is above the marine tidal zone and the water turns from brackish to fresh,
- excavation downstream of Widdop Street to increase the water flow and conveyance through this area,
- realignment of Eagle Junction Creek through Kalinga Park,
- re-vegetation between NCR and downstream of Widdop Street,
- mangrove removal especially on the southern bank of Shulz Canal and
- landscaped with vegetation such as sedges which can be easily knocked down as water levels are raised.
- Q Is much of the flood plain lost due to the Airport Link tunnel structure in Kalinga Park?
- A No, only a small amount is lost as the flood plain starts at the North Coast railway line. The flow around the bridge is too turbulent to be considered as part of the flood plain.
