Screening of representative rainfall event series for long-term hydrological performance evaluation of grassed swales.

Evaluation of the hydrological performance of grassed swales usually needs long-term monitoring data. At present, suitable techniques for simulating the hydrological performance using limited monitoring data are not available. Therefore, current study aims to investigate the relationship between saturated hydraulic conductivity (Ks) fitting results and rainfall characteristics of various events series length. Data from a full-scale grassed swale (Enschede, the Netherlands) were utilized as long-term rainfall event series length (95 rainfall events) on the fitting outcomes. Short-term rainfall event series were extracted from these long-term series and used as input in fitting into a multivariate nonlinear model between Ks and its influencing rainfall indicators (antecedent dry days, temperature, rainfall, rainfall duration, total rainfall, and seasonal factor (spring, summer, autumn, and winter, herein refer as 1, 2, 3, and 4). Comparison of short-term and long-term rainfall event series fitting results allowed to obtain a representative short-term series that leads to similar results with those using long-term series. A cluster analysis was conducted based on the fitting results of the representative rainfall event series with their rainfall event characteristics using average values of influencing rainfall indicators. The seasonal index (average value of seasonal factors) was found to be the most representative short rainfall event series indicator. Furthermore, a Bayesian network was proposed in the current study to predict if a given short-term rainfall event series is representative. It was validated by a data series (58 rainfall events) from another full-scale grassed swale located in Utrecht, the Netherlands. Results revealed that it is quite promising and useful to evaluate the representativeness of short-term rainfall event series used for long-term hydrological performance evaluation of grassed swales.

Time-varying characteristics of saturated hydraulic conductivity in grassed swales based on the ensemble Kalman filter algorithm -A case study of two long-running swales in Netherlands.

Saturated hydraulic conductivity (Ks) of the filler layer in grassed swales are varying in the changing environment. In most of the hydrological models, Ks is assumed as constant or decrease with a clogging factor. However, the Ks measured on site cannot be the input of the hydrological model directly. Therefore, in this study, an Ensemble Kalman Filter (EnKF) based approach was carried out to estimate the Ks of the whole systems in two monitored grassed swales at Enschede and Utrecht, the Netherlands. The relationship between Ks and possible influencing factors (antecedent dry period, temperature, rainfall, rainfall duration, total rainfall and seasonal factors) were studied and a Multivariate nonlinear function was established to optimize the hydrological model. The results revealed that the EnKF method was satisfying in the Ks estimation, which showed a notable decrease after long-term operation, but revealed a recovery in summer and winter. After the addition of Multivariate nonlinear function of the Ks into hydrological model, 63.8% of the predicted results were optimized among the validation events, and compared with constant Ks. A sensitivity analysis revealed that the effect of each influencing factors on the Ks varies depending on the type of grassed swale. However, these findings require further investigation and data support.

Technical solutions and benefits of introducing rain gardens - Gdansk case study.

Nowadays, Nature-Based Solutions (NBSs) are developing as innovative multifunctional tools to maximize urban ecosystem services such as storm water preservation, reduction of runoff and flood protection, groundwater pollution prevention, biodiversity enhancement, and microclimate control. Gdansk is one of the first Polish cities to widely introduce rain gardens (one example of an NBS) in different areas such as parks, city center, main crossroads, and car parks. They involve different technical innovations individually tailored to local architecture, including historic buildings and spaces. Gdanskie Wody, which is responsible for storm water management in the city, adopted a pioneering strategy and started the construction of the first rain garden in 2018. Currently, there are a dozen rain gardens in the city, and this organisation's policy stipulates the construction of NBSs in new housing estates without building rainwater drainage. Various types of rain gardens can be created depending on location characteristics such as geo-hydrology, as well as local conditions and needs. Furthermore, each of them might be equipped with specific technical solutions to improve the rain garden's function - for example, an oil separator or setter can be included to absorb the initial, most polluted runoff. During winter, the large amount of sodium chloride usually used to grit the roads may pose the greatest threat to biodiversity and plants. These installations have been included in a large rain garden in Gdansk, located in the central reservation of the main streets in the city center. This work presents various technical considerations and their impact on ecosystem functions, and the urban circularity challenges provided by rain gardens operating in different technologies and surroundings. The precipitation quantity and the following infiltration rate were estimated by installing pressure transducers. Furthermore, mitigation of the urban heat island was analysed based on remote sensing images.