**Descriptive Analysis of the Performance of a Vegetated Swale through Long-Term Hydrological Monitoring: A Case Study from Coventry, UK**

**Luis A. Sañudo-Fontaneda 1,2,\*, Jorge Roces-García 1, Stephen J. Coupe 2, Esther Barrios-Crespo 1, Carlos Rey-Mahía 1, Felipe P. Álvarez-Rabanal <sup>1</sup> and Craig Lashford <sup>2</sup>**


Received: 15 August 2020; Accepted: 30 September 2020; Published: 6 October 2020

**Abstract:** Vegetated swales are a popular sustainable drainage system (SuDS) used in a wide range of environments from urban areas and transport infrastructure, to rural environments, sub-urban and natural catchments. Despite the fact that vegetated swales, also known as grassed swales, have received scientific attention over recent years, especially from a hydrological perspective, there is a need for further research in the field, with long-term monitoring. In addition, vegetated swales introduce further difficulties, such as the biological growth occurring in their surface layer, as well as the biological evolution taking place in them. New developments, such as the implementation of thermal devices within the cross-section of green SuDS for energy saving purposes, require a better understanding of the long-term performance of the surface temperature of swales. This research aims to contribute to a better understanding of these knowledge gaps through a descriptive analysis of a vegetated swale in Ryton, Coventry, UK, under a Cfb Köppen climatic classification and a mixed rural and peri-urban scenario. Precipitation and temperature patterns associated with seasonality effects were identified. Furthermore, a level of biological evolution was described due to the lack of periodical and planned maintenance activities, reporting the presence of both plant species and pollinators. Only one event of flooding was identified during the three hydrological years monitored in this research study, showing a robust performance.

**Keywords:** biological evolution; ecosystem services; low impact development (LID); stormwater best management practices (BMP); stormwater control measures (SCMs); sustainable drainage systems (SuDS); water sensitive urban design (WSUD)

## **1. Introduction**

Sustainable drainage systems (SuDS) are nature-based solutions (NBS) utilised to manage water, both in urban and rural environments, as well as in transport infrastructures. They are often referred to as stormwater best management practices (BMP), water sensitive urban design (WSUD), stormwater control measures (SCM) and low impact developments (LID), amongst other terminology [1].

Swales are SuDS that are mainly utilised in transportation infrastructure and in urban and sub-urban environments to capture pollutants and attenuate runoff volumes [2–5]. Furthermore, they are used in rural environments and farms to manage stormwater [6]. These techniques also provide landscape features, as well as an improvement in biodiversity and amenity [7]. In addition, swales have been utilised in permaculture practices showing a robust long-term performance, as highlighted by Abrahams et al. [8]. These authors, along with Winston et al. [9], related the ecosystem services provided by swales, to those delivered by wetlands, especially when vegetation growth is allowed under a low maintenance condition. In other words, allowing nature to take ownership of the system up to some degree.

Vegetated swales have been treated in scientific analyses as conventional or standard swales, as indicated by Fardel et al. [10]. Therefore, they were included in the same category as swales, grassy swales, vegetated roadside swales, planted swales and grassy media, amongst others. Other categories refer to dry, wet and bio-swales. However, dry swales are often described as swales able to completely drain stormwater runoff between two consecutive storm events by authors such as Hunt et al. [11], which also includes some vegetated swales in this category.

Fardel et al. [10] gathered the main parameters influencing swale performance in the literature up to 2019. The authors distinguished between those variables affecting the drainage area, such as the discharge area, the discharge ratio and the main concentration at the swale inlet; those variables associated with the swale itself, such as the swale length, slope, type of soil, vegetation and operational life. This research also revealed the limitations of previous work, emphasizing the need for continuous and long-term monitoring alongside later work such as Purvis et al. [12]. Most of the investigations carried out in the literature show a limited number of storm events which introduces a certain level of uncertainty, as they miss the major hydrological effects influenced by the climate on the location, the rainfall and temperature patterns and the seasonality. To minimise this problem, McCarthy et al. [13] proposed a minimum range of 15–20 storm events in order to capture robust data from a water quality stand point. Therefore, the hydrology should also reach this threshold in order to be reliable and representative for comparison with other international studies. Recent research, such as Purvis et al. [12], monitored 39 storm events over 12 months in a bio-swale in North Carolina, USA, which also followed the described conditions.

Temperature relationships within swales were highlighted as an important factor, especially when considering potential energy applications like the implementation of a ground source heat pump (GSHP) as outlined by Charlesworth et al. and Rey-Mahía et al. [14,15]. Both reports indicated that more research is needed to understand the long-term patterns and their role in the hydrological impact on pervious pavements and vegetated swales.

The most up-to-date reviews and scientific research on vegetated swales, such as Gavri´c et al. [16], pointed out the need for improved modelling in grass–soil media, being underpinned by a better knowledge of physical processes taking place in this SuDS technique. Furthermore, complete facility descriptions ideally are required, to fully describe the functions and ecosystem services provided by vegetated swales.

Design guidance for new developments should specify the implementation of SuDS treatment trains for stormwater management, based on recent studies carried out by Williams et al. [17] regarding user perception of SuDS benefits. Additionally, treatment trains can connect to further social and ecological elements of the urban landscape, due to their comprehensive and holistic design features, as pointed out by Lähde et al. [18]. Treatment trains containing green roofs and grassed swales promote hydrological processes of detention and conveyance, including infiltration within the swale, when designed for that purpose [19].

Once the knowledge gaps were identified in the literature, the research presented in this paper aimed to provide further information about the long-term hydrological processes, occurring in a vegetated swale performing under real weather conditions in the field, showing the seasonality effect as well as the evolution of the water temperature of the system. This research also highlights the hydrological impact of a vegetated swale within a treatment train, when associated with an extensive green roofs.

The specific objectives of this research are cited as follows:


Long-term hydrological and temperature monitoring alongside operation and maintenance monitoring allows detailed investigation of the performance of vegetated swales in the field, to develop a better understanding of this SuDS technique.
