## 1 Introduction  
It has been shown that the pollutants from non-point, or diffuse, sources contribute significantly to the pollutant load in the water system, with up to 37.6% of rivers, streams, lakes, reservoirs, ponds, bays, estuaries, and coastal shorelines having their water quality adversely affected by such pollutants [1]. In particular, for the Danjiangkou reservoir, China, the non-point source contributions to the chemical oxygen demand (COD\({}_{\text{Mn}}\)) and total phosphorus (TP) load were 68.4% and 82.9%, respectively [2]. In the Republic of Korea, the emission of COD and TP from non-point sources comprised 67.6% and 72.1% (i.e., 700 and 53 tons/day), respectively, of the total load to the water system in 2018, and was predicted to be increased by 2025 [3]. The control of pollutants in stormwater runoff is of prime importance for controlling the influx of non-point source pollutants to nearby water systems, particularly in urban areas. The pollutants of natural and/or anthropogenic origins in the air and on the surface are carried by stormwater runoff and deposited in lakes, rivers, wetlands, coastal waters, and groundwater [1]. The pollutants in stormwater runoff are not limited to particulates (i.e., suspended solids (SS)), but include both dissolved and particulate forms of organic matter [4; 5; 6], heavy metals [7; 8], and phosphorus [9]. Therefore, stormwater treatment technologies such as filtration, infiltration, and constructed wetlands have been developed and applied for the reduction of non-point source pollution. The use of granular media such as sand [10], gravel [11], and perlite [12] is a widely adopted runoff treatment process known to be most suitable in urbanized areas [10], with high removal efficiencies of SS, phosphorus, and heavy metals via several mechanisms including filtration and adsorption [13]. Granular filtration media are most frequently recommended due to their porosity, simple structure, easy operation, low price, chemical stability, and easy coating with metal (Fe or Mn) oxides/hydroxides for additional functions such as heavy-metal adsorption [14]. Various granular media have been reported, including recycled glass, foamed polymers, potting soil, coconut fiber, compost, water sludge, pumice/woodchip mixtures, and fibrous plastics [15; 16; 17; 18]. Although granular media filtration is an excellent alternative for the treatment of stormwater runoff, it must be more elaborated for the description of the system performance. The hydraulics of the system could well be described using several models, including a power model [16; 19; 20]. However, the evaluation and description of the SS removal of the systems remain a great challenge. This is primarily due to the high variation in the flow rate and quality of the influent (i.e., the stormwater runoff) [19]. The size of the SS is highly heterogeneous, the concentration of the SS fluctuates significantly, and the flow rate varies greatly, even during a single rainfall event [19; 21; 22; 23]. These variations can lead to phenomena that are not generally encountered in the granular media filtration systems used in water and wastewater treatment plants, e.g., migration, detachment, and re-dispersion of the trapped SS [17; 21; 22; 23; 24]. Moreover, the existence of a pretreatment chamber would induce more variation in the SS concentration in the influent of filtration chambers. A pretreatment chamber is generally a sedimentation chamber with a short retention time [25; 26] and is usually installed in stormwater runoff filtration facilities to reduce the SS load on the media layer in the filtration chamber [25; 26].