Urban runoff remobilises solids and their associated pollutants from urban-built environments and transports them to drainage systems via gully pots. This study presents an extensive monitoring campaign on the solids loading to drainage systems, including 104 gully pots as sampling locations and lasting 2 years. The solids loading is modelled with Build-Up and Wash-Off (BUWO) models and a Regression Tree (RT). The performance of the RT is substantially better than the performance of the BUWO models, such that it is not recommended to use a single BUWO model to predict the loading of a set of gully pots/catchments. It is discussed whether the generally observed mismatch between monitoring data and wash-off models, both in this study and in literature, points to a fundamental misunderstanding of the underlying processes. Finally, the results show that an increased street sweeping frequency does not significantly reduce the solids loading to drainage systems.

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Runoff (re)mobilises solids and their associated pollutants from streets and transports them via gully pots to the drainage system. As the solids negatively impact the performance of the drainage systems, knowledge on the solids loading in terms of mass and composition is essential. However, monitoring data on the solids loading, in particular, covering all seasons and a number of sites, is scarce. This article presents the results of a monitoring campaign on the solids loading to a drainage system via 52 gully pots over a period of 2 years at a sampling rate of once per 3–4 weeks. The loading shows a maximum during the tree phases ‘leaf growth’ and ‘full capacity’ and is correlated with the rain intensity during these phases. The organic fraction and D₅₀ of the solids are correlated with leaf abscission. The settling velocity of the particles <1800 µm is strongly correlated with their organic fraction.

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A substantial part of urban surfaces is to some extent impermeable. Rainfall on these areas turns into runoff, which mobilises solids present on these surfaces. This runoff is removed from urban built areas by different type of drainage systems via gully pots. The objective of a gully pot is twofold, namely 1) to convey runoff to the drainage system with minimal hydraulic losses, while 2) to remove entrained solids to protect the downstream system. The continuous removal of suspended solids results in a growing sediment bed in the gully pot. This sediment bed can eventually reduce the hydraulic capacity of the gully pot and increase the probability of urban flooding due to rainfall. The increasing sediment bed also reduces the removal efficiency, which implies that more solids are transported to the downstream drainage system. Therefore, the objective of this study is to quantify the related processes, which are the build-up of solids on the street, the transport of solids to gully pots, and the removal of solids in gully pots. Which would assist the decision of the optimal cleaning interval of the sand trap. Four research questions have been formulated to meet the study objective: 1. What is the solids loading to gully pots in terms of mass and composition? 2. Does street sweeping reduce the solids loading to gully pots? 3. What is the removal efficiency of solids of a gully pot? 4. How do the in-gully-pot hydraulics influence the removal efficiency? @en

Urban runoff (re)mobilises solids present on the street surface and transport them to urban drainage systems. The solids reduce the hydraulic capacity of the drainage system due to sedimentation and on the quality of receiving water bodies due to discharges via outfalls and combined sewer overflows (CSOs) of solids and associated pollutants. To reduce these impacts, gully pots, the entry points of the drainage system, are typically equipped with a sand trap, which acts as a small settling tank to remove suspended solids. This study presents data obtained using Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) measurements in a scale 1:1 gully to quantify the relation between parameters such as the gully pot geometry, discharge, sand trap depth, and sediment bed level on the flow field and subsequently the settling and erosion processes. The results show that the dynamics of the morphology of the sediment bed influences the flow pattern and the removal efficiency in a significant manner, prohibiting the conceptualization of a gully pot as a completely mixed reactor. Resuspension is initiated by the combination of both high turbulent fluctuations and high mean flow, which is present when a substantial bed level is present. In case of low bed levels, the overlaying water protects the sediment bed from erosion.

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Runoff entering urban drainage systems contains suspended solids, which carry pollutants and may cause blockages in downstream parts of the system (for example infiltration facilities). Suspended solids inflow should, therefore, preferably be controlled by solids removal at gully pots. This paper presents the results of lab experiments on the solids accumulation in gully pots in a scale 1:1 setup. The accumulation process is initially dominated by settling in the gully pot. When a substantial sediment bed is created, the bed starts to interact with the flow, the removal efficiency of solids decreases, and the bed eventually reaches an equilibrium level. The effects of the discharge, sediment size, and geometry on these processes are assessed. The accumulation rate and equilibrium bed level are strongly affected by the flow pattern which is influenced by the combination of the position the jets impinge on the water and the gully pot’s outlet position.

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Gully pots are utilized for conveying runoff to drainage systems, as well as for reducing the system’s solids loading by retaining suspended solids. However, the accumulation of solids in gully pots reduces their removal efficiency, leading to an increase in solids transport towards the drainage system. This article aims to identify the main drivers of the solids accumulation in gully pots and, thus the relevant processes for wash-off models. The solids accumulation rates in 407 gully pots were monitored within a period of ~14 months and were analysed by means of a linear mixed model and a regression tree. The parameters vegetation factor, rainfall volume, and filling degree are the main drivers of the accumulation process. These parameters are linked to the solids build-up in a catchment, solids transport, and solids retention in gully pots, which means that none of these 3 processes is dominant.

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Sewer and urban drainage systems deal with the runoff of areas that lack infiltration capacity. During wet weather, solids that are present on the street are (re)mobilised and transported to the drainage system by the runoff. These solids and their associated pollutants can have detrimental effects on receiving water quality. This paper presents a new measurement device which has been developed to measure the inflow of solids in gully pots. This device has been applied to 100 gully pots over a period of a year, rendering a large dataset of solid inflows to the sewer. The results indicate that only 25% of solids is captured in gully pots without this device. This renders a huge potential for further optimisation of gully pot management, which is typically optimised towards prevention of blockage rather than removing a maximum amount of solids. @en