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Urban water systems are being stressed due to the effects of urbanization and climate change. Although household rainwater tanks are primarily used for water supply purposes, they also have the potential to provide flood benefits. However, this potential is limited for critical storms, as they become ineffective once their capacity is exceeded. This limitation can be overcome by controlling tanks as systems during rainfall events, as this can offset the timing of outflow peaks from different tanks. In this paper, the effectiveness of such systems is tested for two tank sizes under a wide range of design rainfall conditions for three Australian cities with different climates. Results show that a generic relationship exists between the ratio of tank:runoff volume and percentage peak flow reduction, irrespective of location and storm characteristics. Smart tank systems are able to reduce peak system outflows by between 35% and 85% for corresponding ranges in tank:runoff volumes of 0.15–0.8. This corresponds to a relative performance improvement on the order of 35% to 50% compared with smart tanks that are not operated in real-time. These results highlight the potential for using household rainwater tanks for mitigating urban flooding, even for extreme events.

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<p>Stormwater infrastructure will require investments in the order of $100s of millions per local government area to maintain current levels of urban flood protection. This investment is likely to increase in the future as a result of the impact of climate change, population growth and increased urban densification. Traditional solutions aimed at increasing the capacity of stormwater systems have been directed towards pipe upgrades. An alternative approach is the use of smart storages, which have the following advantages:</p><ul><li>Extension of the lifespan of existing stormwater systems</li> <li>Provision of water supply</li> <li>Reduction in pollution levels in receiving waters.</li> </ul><p>The development of smart technologies enables the use of real-time control for increasing the effectiveness of storages. If forecasts of the timing and magnitude of impending rainfall events are available, storage outlet controls can be optimised to release stored water prior to and during the rainfall event to enable the peak flows to be reduced. In addition, by jointly controlling the outflows from multiple, distributed storages, rather than using a single storage or controlling multiple storages independently, coincident flood peaks from different sub-catchments can be minimised, further reducing peak flows at critical locations.</p><p>In this study, the potential benefits of real-time time control for distributed storages are compared with a system that uses storages without real-time controls. The impacts were assessed using a two-storage system, which is modelled using the software package SWMM with the real-time control schemes of the storages being optimised using a genetic algorithm. The case study was conducted for two storage sizes (2 and 10 m<sup>3</sup>) under a wide range of design rainfall conditions, with storm durations ranging from short (30mins) to long (24hrs), and annual exceedance probability ranging from frequent (50%AEP), to rare (1%AEP) for three different Australian climates (sub-tropical/Mediterranean). This results in a total of 75 different combinations. Results show there is a generic relationship between percentage peak flow reduction and the ratio of storage size to storm runoff volume irrespective of location and storm characteristics. The benefits of real-time control of smart storage systems identified were:</p><ul><li>Significant peak flow reductions ranging from 85% (for a larger storage size of 80% of storm volume) to 35% (for small storages size of 15% of runoff volume).</li> <li>Importantly, real-time control of storages significantly outperforms storages without real-time control, with additional peak flow reduction of between 35% to 50%.</li> </ul><p>These results highlight the potential for using distributed storages for mitigating urban flooding, even for extreme events. The potential benefits of smart storages in more realistic cases studies (uncertain rainfall forecasts and larger scales) are also discussed.</p>