Stormwater Pipes Research Articles

Advancing illicit connection diagnosis of urban stormwater pipes: Comprehensive analysis with EEM fluorescence spectroscopy

Urban drainage systems are significant contributors to the issue of black-odorous water bodies. The current application of stormwater pipe inspection technologies faces substantial limitations, especially in industrial areas with diverse wastewater. This study introduced an innovative approach using excitation-emission matrix (EEM) fluorescence spectroscopy for rapid and accurate diagnosis, providing a new perspective for diagnosing illicit connections. In single wastewater-type areas like residential zones, the method achieved a remarkable 91.5 % accuracy solely through spectra observation and fluorescence peak intensity comparison, outperforming conventional NH3-N-based techniques, which reached an accuracy of only 68.1 %. For regions with complex wastewater scenarios, after EEM subtraction, the residual spectra can be roughly categorized into four distinctive categories based on characteristics. This provides a preliminary assessment and helps in initially identifying the types and sources of inflowing wastewater. Furthermore, the least squares (LS) method refines diagnosis results, offering calculated coefficients reflecting the probability and severity of suspected wastewater intrusion. Simulation experiments and field sample analyses validated the feasibility and accuracy of the EEM-based method, highlighting its advantages for diagnosing illicit connections in both single and mixed wastewater scenarios. The results can significantly narrow down the investigation scope and enhance the confirmation of wastewater sources, exhibiting promising application prospects.

Bioswales: As an Alternative Technique for Surface Groundwater Recharge Treatment in Urban Areas like Mumbai

The expeditious development and population growth in urban areas are connected to the environmental quality of the city, including the quality of water. Surface groundwater plays an immense role in the sustainability of urban areas. A variety of techniques are used and implemented for surface groundwater recharge treatment in urban areas. This study aims to assess the suitability of bioswales as an alternative technique for surface groundwater recharge treatment in urban areas like Mumbai. A bioswale is nothing but a narrow and shallow strip of a landscaped area that redirects and filters surface groundwater. It has a linear formation and is shallow in depth, and it is used to collect surface ground runoff water from non-porous surfaces such as structural rooftops of buildings, parking lots, tar and concrete roads, etc. The research methodology adopted is to study bioswales as an alternative system to enhance storm sewers, and traditional stormwater pipes in urban areas like Mumbai. These factors include construction techniques and Design, application/ Installation and maintenance, and the economic feasibility of the Bioswales system. The result indicates that bioswales can help by providing a way to conserve water, improve water quality, reduce pollution in waterways, and enhance biodiversity in urban areas like Mumbai. Keywords: Bioswale, Alternative technique, Surface ground water, Urban area, Mumbai.

Tracking the source of antibiotic resistome in the stormwater network drainage in the presence of sewage illicit connections

Stormwater pipes are illicitly connected with sewage in many countries, which means that sewage enters stormwater pipes and the drainage is discharged to surface water without any treatment. Sewage contains more pathogens and highly risky antibiotic resistance genes (ARGs) than surface runoff. Therefore, sewage may alter the microbial and ARG compositions in stormwater pipe drainage, which in turn leads to an increased risk of resistance in surface water. However, the effects of sewage on ARGs in the drainage of stormwater networks have not been systematically studied. This study characterized the microbial and ARG composition of several environmental compartments of a typical stormwater network and quantified their contributions to those in the drainage. This network transported ARGs and microorganisms from sewage, sediments in stormwater pipes, and surface runoff into the drainage and thus into the river. According to metagenomic analysis, multidrug resistance genes were most abundant in all samples and the numbers and relative abundance of ARGs in the drainage collected during wet weather were comparable to that of sewage. The results of SourceTracker showed that the relative contribution of sewage was double that of rainwater and surface runoff in the drainage during wet weather for both microorganisms and ARGs. Desulfovibrio, Azoarcus, and Sulfuritalea were connected with the greatest number of ARGs and were most abundant in the sediments of stormwater pipes. Furthermore, stochastic processes were found to dominate ARG and microbial assembly, as the effects of high hydrodynamic intensity outweighed the effects of environmental filtration and species interactions. The findings of this study can increase our understanding of ARGs in stormwater pipe drainage, a crucial medium linking ARGs in sewage to environmental ARGs.

Stormwater discharge: An overlooked source of disinfection byproduct precursors

Discharge from the stormwater system is as an important pathway for contaminant transport, impacting the quantity and characteristics of dissolved organic matter (DOM) in surface water, and thus the formation of disinfection byproducts (DBPs) during downstream drinking water disinfection. In this study, DOM in stormwater pipes was characterized by size-exclusion chromatography, and the formation of 27 DBPs and halogen-specific total organic halogen (TOX) following chlorination was investigated. Overall, DOM in stormwater pipes was characterized by low molecular weight compounds and microbial-derived organics. Total DBP concentrations in chlorinated stormwaters were ∼1–15 times higher than in chlorinated surface waters. DBPs formed in stormwaters were dominated by trihalomethanes and haloacetic acids. Moreover, the DBP-associated toxicity of chlorinated stormwaters was ∼1–38 times higher than in chlorinated surface waters, and mainly due to the presence of large amount of haloacetaldehydes and haloacetonitriles. Sampling during a rainfall event suggested that stormwater discharge significantly increased DBP precursors in the surface water. The high formation and estimated toxicity of DBPs in stormwater discharge indicates this is an overlooked source of DBP precursors, posing a threat to the aquatic environment and potentially drinking water quality.

The impact of the nonlinear concrete behaviour on the required liner thicknesses of spray-repaired concrete pipes

Buried reinforced concrete pipes (including sewers, stormwater pipes and other gravity flow structures) that experience unsafe levels of deterioration are commonly rehabilitated using trenchless solutions, such as the application of cementitious liners. Cementitious liners are centrifugally cast onto the inner surface of the damaged pipes with a spinning head that travels along the length of the pipe. Once cured, the liner increases the strength of the damaged pipe to resist applied earth and surface loads. A design method previously proposed by the authors provides guidance for selecting the required liner thickness to rehabilitate deteriorated concrete pipes situated at deep burials. However, this design method considers the linear elastic behaviour of the soil and pipe materials. Neglecting the nonlinear effects of crushing concrete within the host pipe may impact the predicted liner thicknesses for repair. Thus, the current study evaluates the impact of incorporating nonlinear compressive material behaviour on the required liner thicknesses to repair damaged concrete pipes. The required liner thicknesses obtained from the nonlinear version of the design method for rehabilitations of several pipes selected from the ASTM C76-11 Standard experiencing varying levels of deterioration are compared to thicknesses obtained using the authors' linear elastic solutions. The comparisons indicated that considering the nonlinear material response of the concrete generates slightly larger estimates of required liner thickness. The discrepancies between the estimated liner thicknesses were observed to be insignificant for pipes experiencing cracking and losses of concrete cover, demonstrating that the linear elastic model is suitable for design purposes. However, for damaged pipes situated at deep burials experiencing losses of tensile reinforcement, the discrepancies between the liner and nonlinear calculations become more significant. It is recommended that the results of the linear elastic model be checked with the nonlinear model for the repair of pipes experiencing loss of tensile reinforcement, or for deeply buried pipes.

Stormwater Infrastructure Resilience Assessment against Seismic Hazard Using Bayesian Belief Network.

Resilient stormwater infrastructure is one of the fundamental components of resilient and sustainable cities. For this, the resilience assessment of stormwater infrastructure against earthquake hazards is crucial for municipal authorities. The objective of this study is to develop a resilience assessment framework for stormwater pipe infrastructure against seismic hazards. A Bayesian belief network (BBN)-based stormwater infrastructure resilience model is constructed based on the published literature and expert knowledge. The developed framework is implemented in the city of Regina, Canada, to assess the city's stormwater pipe infrastructure resilience. The outcome of the model indicates that proposed BBN-based stormwater infrastructure resilience model can effectively quantify uncertainties and handle the nonlinear relationships between several reliability and recovery factors. The model is also capable of identifying the most sensitive and vulnerable stormwater pipes within the network.

Direct discharge of sewage to natural water through illicitly connected urban stormwater systems: An overlooked source of dissolved organic matter

The illicit connection of sewage pipes to stormwater pipes commonly occurs in urban stormwater systems. This brings problems that sewage might be directly discharges into natural water and even drinking water sources without treatment, posing risks to ecological safety. Sewage contains various unknown dissolved organic matter (DOM), which could react with disinfectants and lead to the formation of carcinogenic disinfection byproducts (DBPs). Thus, understanding the impacts of illicit connections on downstream water quality is of significance. This study firstly investigated the characteristics of DOM using fluorescence spectroscopy and the formation of DBPs after chlorination in an urban stormwater drainage system in the case of illicit connections. The results found that the concentrations of dissolved organic carbon and dissolved organic nitrogen ranged from 2.6 to 14.9 mg/L and from 1.8 to 12.6 mg/L, respectively, with the highest levels occurring at the illicit connection points. Concerning DBP precursors, pipe illicit connections introduced considerable precursors of highly toxic haloacetaldehydes and haloacetonitriles into the stormwater pipes. Furthermore, illicit connections introduced more contents of tyrosine-like and tryptophan-like aromatic proteins, which may be related to foods, nutrients, personal care products, etc. in the untreated sewage. This indicated that the urban stormwater drainage system was a significant input source of DOM and DBP precursors to natural water. The results of this study are of great significance for protecting the security of water sources and promoting the sustainability of urban water environment.

Identification of high oxygen-consuming substances in stormwater drainage systems illicitly connected with sewage system

The dissolved oxygen content in water is an important indicator for assessing the quality of the water environment, and maintaining a certain amount of dissolved oxygen is essential for the healthy development of the ecological environment. When a water body is anoxic, the activity of anaerobic microorganisms increases and organic matter is decomposed to produce a large number of blackening and odorizing substances, resulting in black and odorous water bodies, which is a very common and typical phenomenon in China. Presently, there is still a relatively universal occurrence of illicitly connected stormwater and sewage pipes in the urban drainage pipe network in China, which makes oxygen-consuming substances be directly discharged into rivers through stormwater pipes and consume the dissolved oxygen in the water bodies, resulting in an oxygen deficiency of the water. This induces seasonal or year-round black and stink phenomena in urban rivers. Hence, identifying high oxygen-consuming substances, which lays the foundation for the subsequent removal of oxygen-consuming substances, is essential. Through a series of comparisons of water quality indicators and analysis of organic characteristics, it was found that the oxygen consumption capacity of domestic sewage was higher than that of industrial wastewater in the selected area of this study, and the oxygen-consuming substances of domestic sewage were small molecular amino acids. By comparing 20 conventional free amino acids, it was found that seven of them consumed oxygen easily, and compared with chemical oxygen consumption, biological oxygen consumption was in a leading position.

A Deterioration Model for Sewer Pipes Using CCTV and Artificial Intelligence

Sewer pipeline failures pose significant threats to the environment and public health. To tackle these repercussions, many deterioration models have been developed to predict the conditions of sewer pipes, most of which are based on CCTV inspection reports. However, these reports are prone to errors due to their subjective nature and human involvement. More importantly, there are insufficient data to develop prudent deterioration models. To address these shortcomings, this paper aims to develop a CCTV-based deterioration model for sewer pipes using Artificial Intelligence (AI). The AI-based model relies on the integration of an unsupervised, multilinear regression technique and Weibull analysis. Findings derived from the Weibull deterioration curve indicate that the useful service life for concrete and vitrified clay pipes are 79 years and 48 years, respectively. The regression models show that the R2 value for vitrified clay sewer pipes, concrete sewer pipes, and ductile iron sewer pipes are 71.18%, 71.47%, and 81.51%, respectively, and 73.69% for concrete stormwater pipes. To illustrate the impact of various factors on sewer pipes, sensitivity analyses under different scenarios are conducted. These analyses indicate that pipe diameter has a significant influence on sewer pipe deterioration, with little impact on stormwater pipes. These findings would guide decision makers in identifying critical pipes and taking necessary precautionary measures. Further, this provides a sound basis for prioritizing maintenance actions, which would pave the way for designing sustainable urban drainage systems for cities.

Physical Model–Based Failure Prediction of Concrete Stormwater Pipes Subjected to Rebar Corrosion

Physical Model–Based Failure Prediction of Concrete Stormwater Pipes Subjected to Rebar Corrosion

Microfibers shed from synthetic textiles during laundry: Flow to wastewater treatment plants or release to receiving waters through storm drains?

Microplastic particles (MPs), which have been considered as emerging contaminants, are widely found in water environments. In southern China, many apartment owners place washing machines on their balconies, where the separated drain system usually leads to the storm-water pipes, resulting in releasing MPs directly into the environment. In the present study, we firstly obtained the laundry preference of residents in the study area based on a questionnaire survey and then designed a simulated laundry experiment to determine the MFs shed in the laundry process. The approximate amount of MFs discharged into the environment under various scenarios was estimated. The results showed that type of washing machine and detergent had a great influence on the MFs shedding. The use of a front-load machine and liquid laundry detergent pods (LDPs) can reduce MFs shed in the laundry process. If the laundry effluent generated by washing machines placed on apartment balconies flows into the municipal pipe network and is treated by wastewater treatment plants (Andrea Menéndez-Manjón), the environmental load of MFs can be reduced. Taking Nanning, the capital city of Guangxi Zhuang Autonomous Region, as a case study, the calculated result shows that the number of MFs discharged into the environment decreased by 93% at the optimized laundry mode. This study provides a reference for the control of MFs and the rational revision of the Architectural Design Guidelines in subtropical cities equipped with the diversion drainage system.

Concentrations and risk assessment of metals and microplastics from antifouling paint particles in the coastal sediment of a marina in Simon's Town, South Africa.

Maintenance of maritime vessels includes the removal of paint from hulls that are sources of metals, antifouling paint particles (APPs) and microplastics (MPs) that end up in the coastal environment. Simon's Town is a small urban town in False Bay, Cape Town, South Africa, where maritime activities take place (there is a naval harbour, marina and shipyard). The aim of this study was to measure metals, APPs and MPs in Simon's Town, to assess the impact of maritime activities and a storm water pipe in a sheltered marina. Sediment samples were collected from six sites during winter 2018. Sediment and extracted APPs were analysed for metal concentrations (Al, As, B, Ba, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Se, Sb, Sn, Sr, V and Zn) and MPs characterised based on type (shape and polymer), colour and size. Highest average metal concentrations in sediment for all sites were Fe (32228 ± SEM 4024), Al (12271 ± 1062) and Cu (1129 ± 407). Metals in paint particles were highest for Fe (80873 ± 19341), Cu (66762 ± 13082) and Zn (44910 ± 1400µg/g). Metal and MP fragment concentrations were highest at the slipway of the shipyard, decreasing with increased distance from the slipway. MP filaments were highest close to the storm water outfall pipe. Our results suggest that shipyards are potential sources of metals and MP fragments (mainly APPs), with storm water pipes potential sources of MP filaments. Various indices applied to assess the potential impacts of metals and MPs suggest that these contaminants have the potential to adversely impact the intertidal ecosystem investigated.

Mapping shallow groundwater salinity in a coastal urban setting to assess exposure of municipal assets

Study regionChristchurch, New Zealand. Study focusLow-lying coastal cities worldwide are vulnerable to shallow groundwater salinization caused by saltwater intrusion and anthropogenic activities. Shallow groundwater salinization can have cascading negative impacts on municipal assets, but this is rarely considered compared to impacts of salinization on water supply. Here, shallow groundwater salinity was sampled at high spatial resolution (1.3 piezometer/km2), then mapped and spatially interpolated. This was possible due to a uniquely extensive set of shallow piezometers installed in response to the 2010–11 Canterbury Earthquake Sequence to assess liquefaction risk. The municipal assets located within the brackish groundwater areas were highlighted. New hydrological insights for the regionBrackish groundwater areas were centred on a spit of coastal sand dunes and inside the meander of a tidal river with poorly drained soils. The municipal assets located within these areas include: (i) wastewater and stormwater pipes constructed from steel-reinforced concrete, which, if damaged, are vulnerable to premature failure when exposed to chloride underwater, and (ii) 41 parks and reserves totalling 236 ha, within which salt-intolerant groundwater-dependent species are at risk. This research highlights the importance of determining areas of saline shallow groundwater in low-lying coastal urban settings and the co-located municipal assets to allow the prioritisation of sites for future monitoring and management.

Markov Chain–Based Inspection and Maintenance Model for Stormwater Pipes

AbstractInspection and maintenance (M&R) of a buried network of stormwater drainage pipes is a challenging task for asset engineers due to a large number of pipes and restricted access. A Markov-ba...

Riparian Land Cover, Water Temperature Variability, and Thermal Stress for Aquatic Species in Urban Streams

Thermal regime warming and increased variability can result in human developed watersheds due to runoff over impervious surfaces and influence of stormwater pipes. This study quantified relationships between tree canopy, impervious surface, and water temperature in stream sites with 4 to 62% impervious land cover in their “loggersheds” to predict water temperature metrics relevant to aquatic species thermal stress thresholds. This study identified significant (≥0.7, p < 0.05) negative correlations between water temperature and percent tree canopy in the 5 m riparian area and positive correlations between water temperature and total length of stormwater pipe in the loggershed. Mixed-effects models predicted that tree canopy cover in the 5 m riparian area would reduce water temperatures 0.01 to 6 °C and total length of stormwater pipes in the loggershed would increase water temperatures 0.01 to 2.6 °C. To our knowledge, this is the first time that the relationship between stormwater pipes and water temperature metrics has been explored to better understand thermal dynamics in urban watersheds. The results highlight important aspects of thermal habitat quality and water temperature variability for aquatic species living in urban streams based on thermal thresholds relevant to species metabolism, growth, and life history.

Development of radio-frequency identification (RFID) sensors suitable for smart-monitoring applications in sewer systems

Sanitary and stormwater sewers are buried assets that play important roles in the prevention of diseases and the reduction of health risks for our societies. Due to their hidden nature, these assets are not frequently assessed and maintained to optimal conditions. The lack of maintenance can cause sewer blockages and overflows that result in the release of pathogens into the environment. For cities, monitoring sewer conditions on a large-scale can be costly, time-consuming, and labor-intensive if using current low-throughput technologies, such as dye testing or closed-circuit television. Alternatively, smart sensor systems can provide low-cost, high-throughput, and automatic data-driven features for real-time monitoring applications. In this study, we developed ultrahigh-frequency radio-frequency identification (UHF RFID)-based sensors that are flushable and suitable for sanitary and stormwater pipes quick surveys. 3D printed RFID sensors were designed to float at the water-air interface and minimize the water interference to RF signal communications. The optimal detection range was also determined to support the design and installation of the reader in various utility holes. Field trials demonstrated that the UHF RFID system is a low-cost, high-throughput, and robust solution for monitoring blockage, illicit-connection, and water flow in sewer networks.

Study on Settlement and Deformation of Urban Viaduct Caused by Subway Station Construction under Complicated Conditions

With the rapid development of urbanization, the speed of constructing urban underground space and ground foundation engineering is accelerated. With the shortage of land resources, the distance between underground engineering and surface engineering is getting closer and closer, and the contradictions and problems between projects are becoming more and more serious. Based on the new subway station, this paper passes the existing urban viaduct in a short distance. The settlement and deformation of pile foundation and buildings caused by the excavation of the subway station is studied by using ROCSCIENCE software and site monitoring method. The potential risk sources around the station are pile foundations 18.5 m away, residential buildings 15 m away, storm water pipes 7.3 m away, and high‐pressure gas pipes at 5.6 m. According to the construction requirements of pile foundation settlement deformation during metro station excavation, we set up construction standards. The settlement of the buildings and pipelines around the metro station should not exceed 3 mm, and the horizontal displacement should not exceed 2 mm. The diaphragm wall is adopted to surround the subway station, and the ground is reinforced by advanced grouting. The results show that, during the excavation of the foundation pit of the metro station, the ground and the building will have obvious settlement deformation and the depth of deformation can reach 9 mm, which is more than the allowable value. The settlement deformation of the ground and the surrounding buildings can be controlled within 3 mm by strengthening the foundation pit support and ground advance‐grouting reinforcement. The control of construction deformation can meet the construction requirements.

The case of Mar Menor eutrophication: State of the art and description of tested Nature-Based Solutions

The Mar Menor (SE Spain), the largest hypersaline coastal lagoon of the Mediterranean basin, suffers a severe eutrophication crisis due to the nutrients that receives from the Campo de Cartagena watershed, mainly nitrate from intensive agriculture. This paper updates the state of the art in relation with nutrient discharges to the Mar Menor, and summarizes results from different tested Nature-Based Solution (NBS). Specifically, we show i) results from a pilot plant with woodchip bioreactors for nitrate-enriched brine denitrification, and ii) the first results obtained in a pilot plant with bioreactors and constructed wetlands for treatment of agricultural drainage water and leachates, as well as other effluents. Nutrient discharges to the lagoon are highly variable and occur via drainage network, drains, stormwater pipes, direct groundwater discharges from the Quaternary aquifer, and others. For instance, between January 2017 and January 2018 measured daily superficial discharge (floods excluded) ofN-NO3− amounted from 119 kg d−1 to 1084 kg d−1. Estimations subsurface discharges ranged 815 to 3836 kg N-NO3− d−1 in 2018–2020. Field studies in coastal wetlands (e.g. inflow ≈20–30 mg N-NO3− L−1, removal ≈80–90%) and results from pilot plants with bioreactors (e.g. inflow ≈30–40 mg N-NO3− L−1, removal ≈90–95%) and constructed wetlands (e.g. inflow ≈30–40 mg N-NO3− L−1, removal ≈60–70%), showed the good performance of these systems for nutrient retention. Four strategies are considered for reducing nutrient inputs into the Mar Menor, which include a combination of nature-based solutions and best management practices. (i) Reducing the leaching of nitrate to the aquifer and export of nutrients and sediments following heavy rains by improving fertilization, and irrigation routines, and soil conservation measures in the agricultural fields. (ii) Development of effective and scalable tools for denitrification of nitrate-rich brine produced by on-farm desalination plants. (iii) Capture and treatment of polluted water discharged to the Mar Menor via hydrologic networks, subsurface flow, drainage ditches, and others. (iv) Preservation and restoration of coastal wetlands.

Examining coastal dynamics and recreational water quality by quantifying multiple sewage specific markers in a North Carolina estuary

Fecal contamination is observed downstream of municipal separate storm sewer systems in coastal North Carolina. While it is well accepted that wet weather contributes to this phenomenon, less is understood about the contribution of the complex hydrology in this low-lying coastal plain. A quantitative microbial assessment was conducted in Beaufort, North Carolina to identify trends and potential sources of fecal contamination in stormwater receiving waters. Fecal indicator concentrations were significantly higher in receiving water downstream of a tidally submerged outfall compared to an outfall that was permanently submerged (p < 0.001), though tidal height was not predictive of human-specific microbial source tracking (MST) marker concentrations at the tidally submerged site. Short-term rainfall (i.e. <12 h) was predictive of E. coli, Enterococcus spp., and human-specific MST marker concentrations (Fecal Bacteroides, BacHum, and HF183) in receiving waters. The strong correlation between 12-hr antecedent rainfall and Enterococcus spp. (r = 0.57, p < 0.001, n = 92) suggests a predictive model could be developed based on rainfall to communicate risk for bathers. Additional molecular marker data indicates that the delivery of fecal sources is complex and highly variable, likely due to the influence of tidal influx (saltwater intrusion from the estuary) into the low-lying stormwater pipes. In particular, elevated MST marker concentrations (up to 2.56 × 104 gene copies HF183/mL) were observed in standing water near surcharging street storm drain. These data are being used to establish a baseline for stormwater dynamics prior to dramatic rainfall in 2018 and to characterize the interaction between complex stormwater dynamics and water quality impairment in coastal NC.

Numerical Assessment of Shear Boundary Layer Formation in Sewer Systems with Fluid-Sediment Phases

Numerical and empirical studies of soil slurry transport and deposition in urban stormwater sewers are few, presumably due to the difficulty of direct observation of soil slurry flow in stormwater pipes. Slurry in a sewer system includes both suspended load and bedload, but few studies have attempted to demarcate these two components. A boundary layer is a crucial determinant of sediment transport capacity. Stormwater runoff enters the sewer in turbulent flow, mostly mixed with soil slurry generated by rainfall. In this paper, we attempt analysis using ANSYS Fluent commercial CFD software. We describe the development of a numerical analytical methodology capable of predicting the flow of soil slurry in stormwater pipes, and propose a method for estimating the sediment–flow boundary layer. Using this model, we simulated stormwater runoff with a large content of soil slurry during a rainfall event. We investigated soil slurry transport and predict the formation of shear boundary layer by varying the inlet conditions (volume of soil slurry entering the stormwater sewer system) and by analyzing the flow velocity field and soil slurry volume fraction in the pipes under various experimental flow conditions. Based on the shear and settling velocity of sediment particles, we propose criteria for the formation of a shear boundary layer in stormwater pipes.