The urban drainage performance - how it plays with ARR guidelines, climate change and mitigation measures
Creators: name: AKTAR, Most. Nazneen
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400513
subject: Water resources engineering
classificationCode: 400599
subject: Civil engineering not elsewhere classified

description: The history of the storm drainage system in Australia is more than a hundred years old. In this country, drainage infrastructures have been formally designed following the guidelines developed by the Australian Rainfall and Runoff (ARR) since 1958. ARR guideline has been updated over the years. Majority of the existing drainage infrastructures were developed based on historical rainfall data and the methods suggested by the 1987 version of ARR (ARR 1987). That ARR suggested a single temporal pattern of design burst rainfall based on average variability method (AVM) of rainfall to runoff. On the other hand, the latest ARR (ARR 2016) is a probability-neutral transformation which suggests ensemble of events, Monte Carlo method, and continuous simulation approaches. The new methods minimize bias in the resulting annual exceedance probabilities. There is a significant difference between Intensity-Frequency-Duration (IFD) values of two ARRs. There are also deviations in storm durations and ARI (Average Recurrence Interval) or AEP (Annual Exceedance Probability). Besides, unlike ARR 1987 which was presented with a single average storm pattern, any storm is presented in ARR 2016 with a set of probable rainfall patterns. ARR 2016 recommends capturing the full hydrograph with continuous simulation and not only the peak value in designing drainage infrastructure. Therefore, there is a gap between ARR 1987 and ARR 2016 which would produce variation in drainage performance of the existing infrastructure. Apart from the transformation of ARR version, another concern of the infrastructure planners and designers is climate change. Australia has been experiencing climatic changes over the past few decades. The best performing climate change projections have forecasted both increased and decreased rainfall intensity, duration and frequency for this region, which would impact the current drainage capacity of urban city like Melbourne. <br><br> 
Furthermore, the increase in urbanization due to the rise in population is putting pressure on the existing drainage infrastructure. Past studies show that a 10-20% increase in impervious surface cover caused 20% increased runoff. Hence, the impact of urbanization is expecting on the drainage performance in a city like Melbourne which have a history and trend of increasing urbanization. The drainage capacity would be worse with further extension of urbanization.<br><br>
Therefore, this research aimed to address the above mentioned aspect along with examining some prospective mitigation measures. However, the drainage performance of each cluster of analysis was presented in terms of performance risk. A particular segment was considered to be under risk when its runoff capacity exceeded certain threshold value. This research provides valuable insight to planners and designers for designing new drainage infrastructure and selecting mitigation options if drainage performance is not satisfactory. <br><br>
If the designers were using ARR 1987 guidelines for designing a stormwater drainage network, they needed to consider the 30-min storm duration as the design event for designing underground minor infrastructure (pipes). In contrast, designers using ARR 2016 guideline should examine the 10-min storm for the same purpose. The most influencing factor found creating performance risk caused by minor event was the combined effect of flatter underground pipe slope and higher rainfall intensity which was followed by solely higher rainfall intensity and then slope itself.  <br><br>
For both ARRs, by 2050, the drainage performance would improve tremendously for CESM1-CAM5 model-data generated for RCP4.5 whereas it would not be impacted significantly for CNRM-CM5. Moreover, 10, 15, and 20% increase of rainfall would cause drainage problem to additional segments by 5.2, 10.3, and 19%, respectively for ARR 1987. On the other hand, for ARR 2016, a small portion of the upper catchment area would suffer higher drainage problem in the future, but the overall drainage performance caused by hypothetical scenarios (10, 15 and 20% increased rainfall in the future) would the same as present.<br><br>
Impacts of urbanization were investigated for the study area for 10% increased and 10% decreased of the existing imperious area. Though negative impacts were observed under different risk sub-classes due to 10% increase of urbanization, the overall impact was negligible. But reduced urbanization would positively impact the drainage capacity. Overall, 6.2 and 8.8% additional segments would be risk-free for decreased urbanization by 10% for ARR 1987 and ARR 2016, respectively.<br><br>
After investigating likely drainage issues caused by climate change and urbanization, some prospective mitigation options were examined under this study to explore their effectiveness. `Increasing pervious area¿ was found as a promising option. An increase of pervious area by further 10, 20 and 30% would make it would make 6.2, 9.1, and 10.1% additional segments out of at-risk drainage performances, for ARR 1987 whereas the overall contribution of making segments risk-free would be 8.8, 9.4, and 10.5% for ARR 2016. On the other hand, rainwater harvesting measure could make additional about 40% of segments completely risk-free.  Moreover, the entire network could be made completely risk-free by diverting 2,546 and 2,003 m3 of runoff for ARR 1987 and ARR 2016, respectively, to the underground reservoirs. However, the selection of mitigation options needs to investigate the actual geology, topography, land availability, and budget available.