Mixed flows in storm water systems (SWSs) generate a set of complex phenomena that are difficult to capture in modeling. Shock-fitting and shock-capturing techniques are the two main numerical approaches employed today, but these require a number of assumptions and simplifications that can compromise their accuracy. As a result, the current software and models are unable to adequately simulate the complex multiphase, multiregime, and flow instabilities that result from mixed flow. Significant contributions going back to the 1960s have used models to better understand mixed flows and sought to integrate the obtained data into the SWS design process. This paper reviews these attempts and compares and evaluates the models currently in use. It constitutes a valuable resource for new researchers in the field and professional readers looking for ways to make decision making in SWS more efficient.

The purpose of this study is to develop an optimization model for the selection and placement of best management practice (BMP) at the watershed level in order to control water quantity and quality. Linear programming (LP) was selected from among several mathematical optimization methods, with the objective function being to minimize the total cost of BMPs within the constraints of guaranteeing a free-surface flow for the whole network during heavy rainfall and limiting the peak flow generated at the outlet of the watershed at the interceptor capacity for frequent rainfalls. The model was used on two scenarios considering a 10-year return rainfall period for the water quantity control, and 25 mm rain for the water quality control. The impact of climate change and urbanization on the combined sewer performances is also discussed for each scenario. The results obtained from the model effectively demonstrate the viability of implementing BMPs in the goal of improving sewer performances and show that ...

Optimal assessment of the overall state of the degradation affecting sewer systems requires consideration of both the hydraulic and the structural aspects. This work identifies a set of 15 factors—pertaining to both degradation aspects—as primary and constituting the framework for hierarchical structuring of the condition states. The relative importance of these factors was determined by the analytic hierarchy process (AHP). The proposed model was successfully applied to assess the current degradation state of the sewer network in Saint-Hyacinthe, Quebec. The relative significance effect, assessing prevalence of either the hydraulic or the structural criteria in the degradation process, was established through sensitivity analysis. The outcome indicates that hydraulic factors have to be integrated in the pipe-deterioration model as well as the structural factors. With a reference case set at ws=0.5 (i.e., structural and hydraulic criteria having the same significance level), results remained conse...

Implementing best management practices (BMP) on watersheds could help mitigate the effects of urbanization and climate change on the hydrological cycle. Techniques such as retention ponds, rain gardens, infiltration trenches, and green roofs vary in technical performance, space requirements, and cost. The trade-offs between these present a challenge toward BMP selection and placement, therefore requiring optimization. Three optimization methods were applied for BMP implementation on a combined sewer: linear programming (LP); genetic algorithm (GA); and simulated annealing (SA). LP served as a reference point. The SA solution was only marginally better, 4.7% cheaper, whereas GA’s solution was 17.9% more expensive after computations froze at a local minimum; both methods required approximately 18 h of computational time. A second round of optimization used the solution from LP as a starting point. This modification significantly increased the performance of GA, providing a new solution that was 14% ...

The selection of stormwater Best Management Practices (BMPs) for mitigating the effects of urbanization on the hydrological cycle could be a complex process due to conflicting stakeholder views, and varying levels of performance of BMPs across a range of criteria (runoff reduction, erosion control, etc.). Part 1 of this article proposed a methodology based on the application of multi-criteria decision aid (MCDA) methods, which was tested here on a residential stormwater network in the Montreal area. The case study considered green roofs, rain gardens, rain barrels and pervious pavement over a range of economic, social, and water quality and quantity criteria by applying 4 MCDA methods under three different stakeholder views. The results indicated Elimination et Choix Traduisant la Réalité (ELECTRE) III to be the most appropriate method for the methodology, presenting flexibility concerning threshold values, criteria weights, and showing shared top choices across stakeholders (rain gardens, and rain gardens in combination with pervious pavement). The methodology shows potential for more formal applications and research opportunities. Future work may lie in the inclusion of multiple objective optimization, better stakeholder engagement, estimation of economic benefits, water quality modeling, long-term hydrological simulations, and estimating real BMP pollutant removal rates.