Vegetated swales are an integral component of Green Stormwater Infrastructure (GSI), designed to manage urban stormwater at its source by reducing volumes and peaks, retaining water within the urban landscape, and providing high-capacity runoff pathways. They facilitate the integration of vegetation-based stormwater solutions with traditional grey infrastructure, creating synergies and enhancing urban drainage. In light of contemporary urban drainage challenges, swales are now assigned multiple functions beyond stormwater conveyance, necessitating enhanced predictability and reduced uncertainties in their hydrologic performance.

This thesis investigates the hydrologic functions of vegetated swales in controlling urban stormwater. Vegetated swales are shallow, vegetated channels that manage runoff through infiltration, conveyance, storage, dissipation, and filtration, leading to reduced runoff volumes and attenuated peak flows. The study aims to advance the understanding of swale functions by examining their hydrologic and hydraulic performance under varying conditions. Key objectives include exploring the relationship between hydraulic and hydrological factors and swale hydrographs, such as soil moisture dynamics and swale characteristics, representing swale processes and spatial variability, and evaluating long-term hydrological behavior concerning soil water content (SWC).

The methodology involved field experiments and long-term monitoring at two swales in Luleå and a combined stormwater control measure (SCM) in Skellefteå, Northern Sweden. These swales, with differing topographies and vegetative covers, were subjected to controlled irrigation experiments to mimic runoff inflows. The combined SCM, consisting of a rocky slope, vegetated slope, and vegetated collector swale arranged in series, was monitored to assess hydrologic parameters and responses based on natural rainfall inflows. Data collection included rainfall events, inflow and outflow hydrographs, soil infiltration, and SWC using Time Domain Reflectometry (TDR).

The study highlights the influence of initial soil moisture conditions only on vegetated swale function. Low SWC leads to high runoff attenuation (up to 74%), whereas high SWC results in predominant conveyance function (attenuation as low as 17%). Runoff peaks were proportionally reduced, with outflow hydrograph lag times ranging from 5 to 15 minutes. Variability in soil properties, hydraulic conductivity, and topography significantly affected swale performance, with bottom slope irregularities impacting runoff dissipation. Double-ring infiltrometer measurements showed infiltration rates varying from 1.78 to 9.41 cm/hr across the swales.

For the example of a vegetated swale in combination with additional drainage features upstream, runoff volume reductions frequently exceeded those in studies on grassed swales or filter strips, attributed to large pervious areas and abundant depression storage. Hydrological reductions varied with site-specific conditions, such as soil properties and shallow groundwater interactions, resulting in a median runoff coefficient of 0.03 over 60 storm events. Groundwater interactions and soil moisture fluctuations influenced unsaturated zone dynamics, causing water exfiltration even during dry periods, leading to variable runoff travel times and delayed peak lag times.

Eight years of monitoring revealed high spatial variability in SWC, attributed to soil mixing during development. Vegetated slopes showed greater SWC variability than the downstream swale, influenced by lateral stormwater inflows. Seasonal trends indicated increasing site moisture, driven by vegetation maturation, which improved stormwater retention and site resilience.

Overall, this dissertation enhances the understanding of influential processes and environmental conditions impacting the function and effectiveness of vegetated swales, providing valuable information to reduce uncertainties in designing and predicting swale hydrological responses.

A paired-catchment study of two adjacent commercial areas in northern Sweden, one with Green Infrastructure (GI) storm drainage and the other with a conventional storm sewer system, served to evaluate the hydrological performance of both drainage systems and demonstrate advantages of GI. The GI catchment avoided directly-connected impervious areas by diverting runoff from a parking lot to a cascade of three infiltration features, a fractured rock strip draining onto a sloping infiltration area, followed by a collector swale. Both catchments were monitored over 4 years by measuring rainfall, runoff and, in the vicinity of the swale, soil water content and groundwater levels. For frequent storms, the median GI efficiencies in reducing runoff volumes and peak flows, and extending peak flow lags, were 96, 99 and 60%, respectively, compared to conventional drainage The storm rainfall depth, initial soil water content, increases in intra-event soil water storage and groundwater levels, had statistically significant effects on either runoff volume or peak flow reductions. No effects were found for storm rainfall intensity and duration, antecedent dry days, and initial groundwater levels. The study demonstrated that GI drainage can be successfully applied even in the challenging environment of a subarctic climate.

Swales are the oldest and most common stormwater control measure for conveying and treating roadway runoff worldwide. Swales are also gaining popularity as part of stormwater treatment trains and as crucial elements in green infrastructure to build more resilient cities. To achieve higher pollutant reductions, swale alternatives with engineered media (bioswales) and wetland conditions (wet swales) are being tested. However, the available swale design guidance is primarily focused on hydraulic conveyance, overlooking their function as an important water quality treatment tool. The objective of this article is to provide science-based swale design guidance for treating targeted pollutants in stormwater runoff. This guidance is underpinned by a literature review.

The results of this review suggest that well-maintained grass swales with check dams or infiltration swales are the best options for runoff volume reduction and removal of sediment and heavy metals. For nitrogen removal, wet swales are the most effective swale alternative. Bioswales are best for phosphorus and bacteria removal; both wet swales and bioswales can also treat heavy metals. Selection of a swale type depends on the site constraints, local climate, and available funding for design, construction, and operation. Appropriate siting, pre-design site investigations, and consideration of future maintenance during design are critical to successful long-term swale performance. Swale design recommendations based on a synthesis of the available research are provided, but actual design standards should be developed using local empirical data. Future research is necessary to identify optimal design parameters for all swale types, especially for wet swales.

Flow observations alone may not provide sufficient information for calibration of detailed hydrological models of urban drainage swales. Therefore this study investigated the added value of using soil water content (SWC) observations made throughout the swale. This can be done by (1) including SWC in the likelihood function that is used to quantify model performance or (2) by using the SWC observations to set initial conditions in the model. The results show that combining outflow and SWC in the likelihood function is necessary to obtain reliable and precise predictions for both variables, and that this increases the number of parameters that are identifiable from the data. Using SWC observations to set initial model conditions improves model performance and affects the degree to which soil hydraulic parameters are identifiable. Overall, it is concluded that SWC observations may be a valuable complement to outflow observations in the modelling of urbanswales.

Fully distributed urban drainage models can be used to analyse and predict the behaviour of green urban drainage infrastructure such as swales, but they need to be calibrated for specific study sites. Using only drainage outflow measurements may not provide enough information to do this in an optimal way, so additional types of measurements have to be considered. This study identifies different approaches to including soil water content (SWC) observations in the calibration process and investigates how they affect parameter identifiability and the predictive uncertainty of the calibrated model. This is done using the Generalized Likelihood Uncertainty Estimation methodology applied to a model of a large urban swale. It was found that setting initial conditions based on the SWC measurements improved the fit between observed and simulated SWC, but also reduced the accuracy of the simulated amount of infiltration. Including SWC observations allowed to identify one parameter (saturated moisture content of the swale bottom) that was not identifiable from outflow measurements alone. Including SWC observations in the derivation of predictive uncertainty bounds made those bounds narrower (more precise), but where SWC had been used to set initial conditions the uncertainty bound failed to capture the observations. It is concluded that SWC observations can provide useful information for the calibration of distributed urban drainage models.

Les modèles d'assainissement urbain entièrement distribués peuvent être utilisés pour analyser et prédire le comportement des infrastructures vertes d'assainissement urbain comme les noues, mais ils doivent être calibrés pour des sites d'étude spécifiques. Le fait de n'utiliser que les mesures des décharges issues de l'assainissement peut se révéler insuffisant pour y parvenir de manière optimale, d'où la nécessité d'envisager d'autres types de mesures. Cette étude identifie différentes approches pour inclure dans le processus de calibrage les observations sur la teneur en eau du sol (SWC) et examine comment elles affectent l'identifiabilité des paramètres et l'incertitude prédictive du modèle calibré. Pour cela, la méthode d'estimation généralisée de l'incertitude de probabilité est appliquée à un modèle d'une grande noue urbaine. Il s'est avéré que l'établissement de conditions initiales basées sur les mesures de la SWC améliorait la correspondance entre les SWC observées et simulées, mais réduisait également la précision du degré simulé d'infiltration. L'inclusion desobservations de la SWC a permis d'identifier un paramètre (la teneur en humidité saturée du fond de la noue) qui n'était pas identifiable par les seules mesures des décharges. L'inclusion des observations de la SWC dans la dérivation des limites d'incertitude prédictives a rendu ces limites plus précises, mais lorsque la SWC avait été utilisée pour établir les conditions initiales, la limite d'incertitude n'a pas reflété ces observations. Il est conclu que les observations de la SWC peuvent fournir des informations utiles pour le calibrage des modèles distribués d'assainissement urbain.

The feasibility of simulating the hydrological response of a grass swale to runoff inflows was examined using the hydrological model Mike SHE and the available input data from 12 irrigation events mimicking runoff from block rainfalls. The test swale channel had a trapezoidal cross-section, bottom slope of 1.5%, length of 30 m, and was built in loamy fine sand. The irrigation events consisted in releasing two equal constant inflows to the swale: a concentrated longitudinal flow at the upstream end and a distributed lateral inflow along the swale side slope adjacent to the contributing drainage area. The total inflows approximated runoff from two events with return periods of 2 months and 3 years, respectively, for durations of 30 min. Irrigation experiments were done for two states of the initial soil moisture, dry or wet antecedent moisture conditions (AMC). Mike SHE has been extensively used on catchments of various sizes, but rarely for small stormwater management facilities and their detailed topography investigated in this study. The latter application required high spatial and temporal resolutions, with computational cells of 0.2 × 0.2 m and time steps as short as 0.6 s to avoid computational instabilities. For dominant hydrological processes, the following computational options in Mike SHE were chosen: Soil infiltration – the van Genuchten equation, unsaturated zone flow – the one-dimensional Richards equation, and overland flow – the diffusive wave approximation of the St. Venant equations. For study purposes, the model was calibrated for single events representing one of four combinations of low and high inflows, and dry and wet AMC, and then applied to the remaining 11 events. This was complemented by calibration for two events, representing high inflow on wet AMC and low inflow in dry AMC. The goodness of fit was statistically assessed for observed and simulated peak flows, hydrograph volumes, Nash-Sutcliffe model efficiencies (NSE), and soil water content (SWC) in swale soil layers. The best fit (NSE > 0.8) was obtained for high inflows and wet AMC (i.e., when the primary swale function is flow conveyance); the least fit was noted for low inflows and dry AMC, when the primary swale function is flow attenuation. Furthermore, this observation indicates the overall importance of correct modelling of the soil infiltration. The effects of spatial variation of SWC on the swale discharge hydrograph could not be confirmed from simulation results, but high topographical accuracy was beneficial for reproducing well the locations of the observed water ponding. No significant increases in simulated SWC at 0.3 m or greater depths were noted, which agreed with field observations. Overall, the results indicated that Mike SHE was effective in process-oriented small-scale modelling of grass swale flow hydrographs.

Grassed drainage swales, which represent common elements of urban green infrastructures, are designed for different soils, flow capacities, dimensions, slopes and vegetation. Their design is often based on local experience rather than technical guidelines, and consequently, the design and performance of grassed swales, with respect to flow capacity and stormwater management objectives may significantly vary from one jurisdiction to another. To improve this situation and reduce design uncertainties, a field study of grassed swales was conducted by assessing their hydrologic performance. A 30-m section of an urban grassed swale in sandy soils, located in the City of Luleå (Northern Sweden), was equipped with a mobile water supply system and instrumented for measuring swale flow characteristics. The water supply system comprised five containers (~ 1 m3 each) providing controlled longitudinal and lateral inflows into the tested swale section. These inflows were selected to mimic stormwater runoff from a typical drainage area. At the first test site, 14 rainfall events of 30- minute duration were simulated and the resulting swale flows and soil moisture conditions were measured. The experimental variables addressed included wet and dry antecedent conditions, and three inflow rates. The preliminary results indicate that the degree of swale inflow attenuation depended on the magnitude of runoff inflow, on the initial soil moisture conditions and that significant volumes of water can be stored and transmitted during the stormwater drainage process.

For fast emerging Asian megacities, knowledge of water resource conditions is indispensable for sustainable water balance management and planning. Urbanisation results in the sealing of surfaces to different degrees in relation to the urban densities and structures developed and ultimately to an alteration of the urban hydrograph. In recent decades urban flooding in Ho Chi Minh City has become one of the most pressing issues. To support the Ho Chi Minh City’s planning authorities, within the frame of this the research project TP. Ho Chi Minh, the rainfall-runoff regime of the southern Vietnamese metropolis of Ho Chi Minh City was investigated. On the basis of high resolution digital databases as well with a previously generated urban structure type map, a German water balance model ABIMO was used to calculate the long-term annual means of individual water balance components for the entire administrative area of the city. Current conditions and further time-series of future urban development scenarios as set out in the draft land use plan up to the year 2020/25 over static climate conditions were modelled. The results were mapped for each of the individual 16,282 land-use blocks of the city’s official land use plan and construed to planning recommendations. The results showed that for the current conditions from a total annual precipitation input of 1573 mm, 117 mm or approximately 7 % is unable to infiltrate or evaporate and converts into surface run-off. Evidence, that urbanization is one of the main cause of increased flooding, could be given by the finding that currently 212 million m3 and based on the simulation for the year 2020/2025 overland flow of 586 million m3 will occur. Finally on the basis of modelled results, a planning recommendation map was compiled displaying zones as planning priorities, targets and measures.