Flood Assessment in the Nerang Thesis

The Gold Coast area has a reputation as a flood prone area, even without considering the effects of global climate change.

The Gold Coast area comprises seven major catchment areas including the Tallebudgera, Currumbin, Nerang River, Coomera River, Pimpama River, South Moreton Bay, Sandy Creek and Broadwater area (Mirfenderesk, 2009). The Nerang River catchment is adjacent to the Tallebudgers catchment to the South. It is bordered by the Broadwater and Coomera River area to the North. The Nerang River catchment is adjacent to the Pacific beach area as well (Mirfenderesk, 2009).

Catchment areas have different levels of tolerance before the concentration of water to sediment reaches saturation levels, creating the likelihood of flooding in the area. The Tallbudgers, Currubin, and Broadwater area have time concentrations of approximately 3 hours, creating conditions favorable to short duration local flooding (Mirfenderesk, 2009). The Nerang River and Coomera catchments have time of concentrations from 3 to 92, making them susceptible to regional scale flooding of longer duration as well as short duration local flooding (Mirfenderesk, 2009).

By definition, local flooding results from heavy rainfall over short time period in a confined area. This type of flooding typically lasts only a short time until the run-off is able to drain. Regional flooding results from heavy rainfall over a number of river catchments. The flooding covers a large geographic area and may trigger flooding in areas susceptible to local flooding. Regional flooding may take a number of days to subside. Riverine flooding only occurs in a single river catchment and is confined to that river system (Mirfenderesk, 2009). Design of the catchment system must take into account all of the possible flooding scenarios in the area. This creates a challenge for city planners and engineers alike.

CSIRO Sustainable Ecosystems developed a system that considers the impact of climate change on Australia using a matrix that examine the affect of climate change on the infrastructure. (CSIRO Sustainable Ecosystems, 2006). According to this assessment, the impact of climate change on infrastructure was assessed in terms of four different scenarios: buildings in coastal settlements, electricity distribution and transmission, water supply infrastructure in major cities, and port infrastructure and operations. The impact of climate change were evaluated according to the estimated economic shock associated with them. These estimates were assessed using models that extended the impacts from 2007-2030, 2031-2070, and 2071-2100. Conditions were assessed using seven different climate change scenarios and conditions.

According to the CSIRO Sustainable Ecosystems model, increased height and land penetration of storm surges would have a negative impact on the ability of current flood protection systems to prevent damage and harm to communities within 50Km of the Queensland coastline (CSIRO Sustainable Ecosystems, 2006). The magnitude of the potential damage is considered low to moderate at the current time, but is expected to increase to moderate to high during the years 2031-2070, with some areas experiencing moderate to extreme potential for severe flood damage. The failure of all major infrastructure systems along the Queensland coast was expected to suffer similar risk potential for damage from flooding (CSIRO Sustainable Ecosystems, 2006).

There are many variables that could affect the outcome of the modeling scenarios. However, this model agrees with many of the other modeling scenarios discovered throughout the course of this research. These models agree that the potential for damage due to coastal flooding is expected to increase significantly in the time period from 2030-2070. This makes the evaluation and improvement of existing flood protection systems essential to the sustainable development of Queensland coastal communities. Developers must take the predicted increase in coastal flooding into consideration as they plan for the future development and economic expansion of coastal areas.

2.2. Climate Change Prediction in south east Queensland (2030-2070)

Climate change is expected to increase the severity and frequency of floods in the Gold Coast areas, adding to the impact of an already flood prone area (Mirfenderesk, 2009). Many factors affect the accuracy of long-range predictions of the effects of climate change. The most important factor is that humans can take measures to reduce the effects of climate change. However, from an emergency planning perspective and flood protection planning position, one must design for the worst-case scenario.

There are many models and algorithms available that predict the worst-case scenario. Many of the models are based on opinion and it is difficult to find a realistic prediction. Estimates range from conservative to alarmist. According to a recent media release, by the year 2060 to 2070, tides are expected be 40-50 cm higher than current average springtime tides (Coulter, 2009).Kling Tides in spring of 2009 gave Gold Coast residents an idea of what future rises in tide would mean for many local landmark, piers, beaches, and homes (Coulter, 2009). The information gathered during these events will be helpful for the future modeling of flood plans and for city development planners.

The most relevant factors to consider in Gold Coast, Queensland flood prediction is the amount of average rainfall, the sea level, and the frequency and intensity of storms. These three factors have the greatest influence on local and regional flooding in the area. The exact effect on local areas depends on their proximity to the coast, and their proximity to the river basin. These three factors will be used to estimate the flood frequency and intensity in the current research model.

The changing climate will have a different impact on every part of the world. However, for everyone, the effects of climate change will first be felt through changes in water supply. In a recent report, the DHI Group reported that in Australia and New Zealand water security problems are expected to intensify by the year 2030. Shortages will be the result of reduced precipitation and increased evaporation. Coastal development will increase the risks of flooding from sea level rise and an increase in the frequency and intensity of coastal storms (DHI Group). Climate change is expected to increase both rainfall and the severity and intensity of storm in the Southeastern Queensland and Northern New South Wales coastal areas (Abbs). This will place communities in these areas at greater risk for major flood events.

2.2.1.

Rainfall Totals

Mean annual rainfall totals for Southeast Queensland average 1,354 mm per year. Some of the highest rainfall occurs in the Nerang River catchment (Gold Coast City Council, n.d.). Rainfall events cause significant damager in Australia due to flooding. Extreme rainfall events are most significant along Australia's Eastern coastline, which also corresponds with the highest population centers in Australia as well (Abbs). Trend analysis on rainfall in Southeastern Queensland and Northern New South Wales found that rainfall in the area has been steadily increasing over the past decade. However, it was also found that when larger regions were considered, the degree of the increase was decreased as a result of the averaging effect. Considering a smaller local area increased the amount of average rainfall in the area (Abbs). One must be cautious of this factor when analyzing major meteorological trends. Regression analysis of a specific selected area over time will provide the most accurate assessment of rainfall trends in a particular area.

Rainfall simulation for a selected area of Southeastern Queensland and Northern New South Wales projected current rainfall in the area and compared it to the rainfall in the year 2040. The model used the Regional Atmospheric Modeling System (RAMS) to produce a compressible non-hydrostatic model using data from CSIRO Mark 3 GCM. Terrain was interpolated from the USGS 30 second data set (Abbs). The system was tested using data from 1960-1999 to check for accuracy. The modern underestimated the intensity of extreme rainfall, but the predicted and actual rainfall estimates were close enough to be considered reliable for making future predictions. The 2040 prediction found that rainfall tended to increase over the mountainous regions, but decrease everywhere else (Abbs). However, several increases were found near coastal areas, such as north of Brisbane and south of Cape Byron (Abbs).

Long-term rainfall predictions are subject to a considerable amount of error. Many variables can affect the outcomes and accuracy of the predictions. These predictions are based on historical data and many factors can influence the final outcome and results. Trend analysis over the period from 1950-2004 that examined the amount of summer and winter rainfall in the Southeastern Queensland coastal area demonstrates that as global temperatures increased, the amount of rainfall in the area decreased (Whetton, McInnes, & Jones et al., 2005). Temperatures are expected to warm significantly in the area between now and 2070, but rainfall is expected to decrease regionally (Whetton, McInnes, & Jones et al., 2005).

Although rainfall has demonstrated a historical trend of increasing in over the past 100 years, future predictions indicate that rainfall is expected to decrease in the area in the next 60 years. General statements regarding rainfall and climate would lead the researcher to expect rainfall to increase in the area, but when one examines specific local data,.....