A model-based analysis for trapping suspended sediment in stormwater inlets of urban drainage network.

Raised awareness of environmental constraints in recent decades has led stormwater management to incorporate quality components and focus on the treatment of urban runoff water at pollutant source areas. This study evaluated the impact of a developed type of sediment trap, installed into stormwater inlets, on the total suspended solids (TSS) load in an urban city center catchment in Finland. The objective was to outline a modelling approach to assess efficiency of the traps to treat TSS originating from different land uses (green areas, pavement, parking, roof, street, and other areas not belonging to the main land uses). A Storm Water Management Model (SWMM) parametrization of a 5.87 ha catchment in the Lahti city center, Finland was utilized as the computation engine. The model had separate subcatchments for each land use, allowing the use of literature-based Event Mean Concentrations (EMC) to estimate the TSS pollutant washoff for the land uses. A method to assess the individual stormwater inlet pollutant loads and potential removal effect of the sediment traps was introduced. The hydrological and TSS load simulations covered a period of 6 months. The stormwater network inlets installed with sediment traps were ranked according to their potential removal of TSS. One out of five EMC sets was selected to be representative of the urban land uses in the study site (green areas 75 mg/l, pavement 46 mg/l, parking 44 mg/l, roof 20 mg/l, street 64 mg/l, other 46 mg/l). The simulation results showed the influence of land uses on the pollutant load and revealed the optimal set of locations for the sediment traps. Additionally, the effect of regular maintenance intervals on the pollutant load, given a maximum storage capacity of the traps, was explored. The results showed a large variation in TSS removal depending on the inlets chosen for the sediment traps, with removal rates ranging from about 0 % to 10 % of catchment TSS load. The maximum TSS removal was 63 %, which was the reported efficiency of the traps. These results highlighted the need for an informed decision when selecting trap locations. Streets and parking lots were the largest TSS contributors, with stormwater inlets on streets being the desired sediment trap locations. While the absolute level of simulated TSS load was found to be dependent on the EMCs, the ranking of sediment trap locations was similar for the simulations with different EMC data sets.

Efficient job list creation for long-term statistical modelling of combined sewer overflows.

The modelling of urban drainage systems is an important aspect of their design process and long-term statistical modelling using historical rain series is commonly used. The objective of this study is to determine whether logistic regression models that use rainfall event statistics can be a viable alternative to create job lists with fewer extraneous events. Two methods are used to develop a regression model; both use iterative stepwise algorithms to select the rain variables to include and both perform similarly. The resulting model is able to capture ∼90% of the relevant events with ∼50% fewer jobs compared to the reference job list. The results suggest that there is no right threshold to use, but instead this methodology facilitates balancing the number of jobs with the desired level of precision of the results. In all cases, it is possible to greatly decrease the number of jobs that need to be run. The methodology works relatively well on different nodes in the system, though node characteristics appear to impact the amount of CSOs captured.

Evaluating catchment response to artificial rainfall from four weather generators for present and future climate.

The technical lifetime of urban water infrastructure has a duration where climate change has to be considered when alterations to the system are planned. Also, models for urban water management are reaching a very high complexity level with, for example, decentralized stormwater control measures being included. These systems have to be evaluated under as close-to-real conditions as possible. Long term statistics (LTS) modelling with observational data is the most close-to-real solution for present climate conditions, but for future climate conditions artificial rainfall time series from weather generators (WGs) have to be used. In this study, we ran LTS simulations with four different WG products for both present and future conditions on two different catchments. For the present conditions, all WG products result in realistic catchment responses when it comes to the number of full flowing pipes and the number and volume of combined sewer overflows (CSOs). For future conditions, the differences in the WGs representation of the expectations to climate change is evident. Nonetheless, all future results indicate that the catchments will have to handle more events that utilize the full capacity of the drainage systems. Generally, WG products are relevant to use in planning of future changes to sewer systems.

A framework for performing comparative LCA between repairing flooded houses and construction of dikes in non-stationary climate with changing risk of flooding.

Sustainable flood management is a basic societal need. In this article, life cycle assessment is used to compare two ways to maintain the state of a coastal urban area in a changing climate with increasing flood risk. On one side, the construction of a dike, a hard and proactive scenario, is modelled using a bottom up approach. On the other, the systematic repair of houses flooded by sea surges, a post-disaster measure, is assessed using a Monte Carlo simulation allowing for aleatory uncertainties in predicting future sea level rise and occurrences of extreme events. Two metrics are identified, normalized mean impacts and probability of dike being most efficient. The methodology is applied to three case studies in Denmark representing three contrasting areas, Copenhagen, Frederiksværk, and Esbjerg. For all case studies the distribution of the calculated impact of repairing houses is highly right skewed, which in some cases has implications for the comparative LCA. The results show that, in Copenhagen, the scenario of the dike is overwhelmingly favorable for the environment, with a 43 times higher impact for repairing houses and only 0% probability of the repairs being favorable. For Frederiksværk and Esbjerg the corresponding numbers are 5 and 0.9 times and 85% and 32%, respectively. Hence constructing a dike at this point in time is highly recommended in Copenhagen, preferable in Frederiksværk, and probably not recommendable in Esbjerg.