Redland City has not experienced major flood crisis in its history, which is relatively short as an urban populated area, having been originally settled as framing land. There were no major instances of loss during the 1974 or 2011 floods, primarily due to Redland City not having large waterway systems apart from Tingalpa Creek.
Tingalpa Creek forms the boundary with Brisbane City and located a few kilometres upstream from its mouth into Moreton Bay is the Leslie Harrison Dam which forms the Tingalpa Reservoir. SEQ Water manages Leslie Harrison Dam which is an earth filled embankment with a concrete gravity spillway located on the left abutment. The spillway is capable of passing 1660m3/s of flow with all four gates (extending the entire width of the spillway at a height of 3.3m) fully opened before overtopping of the embankment occurs. When the gates are closed, the dam has a full supply level of RL 18.315m and a total storage capacity of 24,800 mega litres.
Leslie Harrison Dam has been added to list of SEQ managed dams, along with Somerset, North Pine and Wivenhoe dams, as it is a managed gated release dam. Wivenhoe and Somerset dams have the capacity to store additional water over 100% for flood mitigation. Leslie Harrison Dam does not; that is 100% of the dam’s capacity is maintained for water usage. This means that there could be significant flooding impacts downstream if the dam was already at 100% and a significant rainfall event occurred, which required the dam managers to release large volumes of water quickly. Since Ex-tropical Cyclone Oswald in January 2012 the dam management strategy has changed from releasing water once the dam is 100% full to releasing water once the dam reaches 88% capacity. This process is done as a slow trickle release designed to coincide with the low tide to reduce the likelihood of adverse impacts for the residents and businesses downstream.
There appears to be a belief that Redland City is not at serious risk from flooding. This is based on the fact that there has been no major history of flooding within the area since settlement and no major river catchments within the city’s boundaries, apart from Tingalpa Creek. Whilst Tingalpa Creek is considered the major waterway system in Redland City, other local waterways which are also subject to tidal impacts have been known to cause flash flooding. This is of particular concern for the considerable residential population of Redland City which live on the coastline where the waterways enter Moreton Bay. When the 1974 floods impacted South-East Queensland, less than 28,000 people lived in the Redlands area. The population at the 2006 census had grown to 131,200 and the population is predicted to rise to 181,000 by 2030. Other local waterways within Redland City include:
- Tarradarrapin Creek
- Hilliards Creek
- Ross Creek
- Eprapah Creek
- Moogurrapum Creek
- Weinam Creek
- Serpentine Creek
- California Creek
- Native Dog Creek.
Flood prone, storm tide and drainage constrained land mapping (which appears for each of the islands in Parts 5-10 of this plan) has identified large areas of flood prone land on Southern Moreton Bay Islands (SMBI), the majority of which do not have a dwelling built on them. However, there are dwellings on SMBI that have been built on land designated as drainage constrained land and there are a large number of dwellings that border directly onto flood prone land. Similar mapping for Coochiemudlo Island identifies the risk of flooding is in relation to the affects of storm tide which primarily impacts the golf course, the length of Victoria Parade South which includes the island’s jetty and boat ramp and Victoria Parade East over Council owned land that does not contain infrastructure.
Similarly to Coochiemudlo Island, potential flooding on North Stradbroke Island is limited to the affects of storm tide primarily around Amity Point and the Point Lookout end of Flinders Beach. Current mapping indicates that during a storm tide event, Amity Point could be heavily impacted including transport routes being cut, and Point Lookout would become inaccessible from Flinders Beach.