Evaluation on the performance of a swirling-type hydrodynamic separator using physical and numerical models.

Hydrodynamic separators are commonly used to control the total suspended solid concentration in stormwater before being discharged to natural water bodies. The separator studied in this paper, featuring a swirling flow generated by tangential inlet and outlet connections, was analyzed for its sediment removal efficiency in relation to sediment and flow rates. For the separator studied in this paper, the numerical model showed that the flow field was favorable for the sediments to gather at the center and settle. A higher flow rate or a smaller sediment diameter corresponded to a lower removal rate and vice versa. The dimension improvement for increasing the sediment removal rate was also studied. It was found that increasing the diameter of the separator showed a higher sediment removal rate compared with corresponding increase in the height of the separator. A dimensionless parameter J was proposed to assess the sediment removal rate of a separator, which may be used for designing and optimizing such a device. The removal rate is positively correlated with the J value. When the J value reaches 0.5 or above, the sediment removal rate exceeds 80%, which is a good initial target value for designing this type of separator.

Can Blue-Green Infrastructure enhance resilience in urban drainage systems during failure conditions?

The need to enhance the resilience of urban drainage systems (UDSs) in view of emerging global climate change and urbanisation threats is well recognised. Blue-Green Infrastructure (BGI) provides a suitable strategy for building the resilience of existing UDSs. However, there are limited quantitative studies that provide evidence of their effectiveness for increased uptake in cities. In this research, coupled one-dimensional-two-dimensional (1D-2D) modelling is applied to assess the effectiveness of BGI that include rainwater harvesting systems, infiltration trenches, bioretention cells, and detention ponds using two case study UDSs located in Kampala that experience catastrophic pluvial flooding caused by extreme rainfall. The resulting flooding impacts are quantified considering 'failed' and 'non-failed' UDS initial states, using total flood volume and average flood duration as system performance indicators. The study results suggest that spatially distributed rainwater harvesting systems singularly lead to a reduction in total flood volume and average flood duration of 16-45% and 18-24% in the case study UDSs, respectively. Furthermore, the study results suggest that BGIs are more effective during moderate rainfall (T < 10 years). Based on the study findings, city scale implementation of multifunctional rainwater harvesting systems is recommended as a suitable strategy for enhancing UDSs' resilience.

A global analysis approach for investigating structural resilience in urban drainage systems.

Building resilience in urban drainage systems requires consideration of a wide range of threats that contribute to urban flooding. Existing hydraulic reliability based approaches have focused on quantifying functional failure caused by extreme rainfall or increase in dry weather flows that lead to hydraulic overloading of the system. Such approaches however, do not fully explore the full system failure scenario space due to exclusion of crucial threats such as equipment malfunction, pipe collapse and blockage that can also lead to urban flooding. In this research, a new analytical approach based on global resilience analysis is investigated and applied to systematically evaluate the performance of an urban drainage system when subjected to a wide range of structural failure scenarios resulting from random cumulative link failure. Link failure envelopes, which represent the resulting loss of system functionality (impacts) are determined by computing the upper and lower limits of the simulation results for total flood volume (failure magnitude) and average flood duration (failure duration) at each link failure level. A new resilience index that combines the failure magnitude and duration into a single metric is applied to quantify system residual functionality at each considered link failure level. With this approach, resilience has been tested and characterised for an existing urban drainage system in Kampala city, Uganda. In addition, the effectiveness of potential adaptation strategies in enhancing its resilience to cumulative link failure has been tested.