## 1 Introduction  
The use of grassed channels as a sustainable urban stormwater management tool has increased rapidly compared to other types of green stormwater management tools in recent years [1, 2]. Grassedstormwater channel provides temporary storage and ecological water quality enhancement in addition to aesthetic benefits [3; 4; 5]. The most significant feature of grassed channels is the interaction between flowing water and the grass cover that diffuses the flow and lessens flow velocities [6; 7]. This helps in increased infiltration and reduction of runoff volume, and thus lessening of flood risk in the downstream [8]. The construction of check dams in grassed stormwater channel enhances the infiltration process by temporarily blocking the flowing water and increasing the detention time [7; 9]. Different materials are used for the construction of check dams such as soil, rocks, or bagged gravel or rock [4]. Check dams, therefore, can be (1) porous check dams--filtering water and trapping sediments, or (2) impervious check dams--dissipating flow energy and controlling water level [4; 10]. [PERSON]'s equation [11] is generally used for the designing of grassed channel. The effect of the infiltration process on flow in a grassed stormwater channel is usually included by considering the infiltration flow rate along the channel cross-section. However, [PERSON]'s equation is only applicable for the flow condition when the water level, cross-sectional area, and velocity remain constant, in addition to fully rough and turbulent flow [12]. In urban areas, these assumptions are difficult to fulfil for a grassed stormwater channel as the flow depends on rainfall events; hence, it is ephemeral and intermittent [13; 14]. Furthermore, the backwater, ponding, and energy dissipation resulting from check dam systems change the flow regime due to the rise in water depth and reduction of water velocity along the grassed channel [4; 15; 16]. Although some studies (e.g., [17; 18; 19; 20]) have been conducted to assess interrelation between the infiltration rate and the water depth, the effects of backwater and ponding resulting from check dams on channel infiltration have not been investigated and ignored in channel design procedures [21]. However, the effect of channel water depth on infiltration may be ignored for shallow depths but not for higher depth caused by check dams [20; 22]. Numerous models have been developed over the last century for prediction of infiltration rate (e.g., [17; 23; 24; 25; 26; 27]), each with different required parameters. Hence, each model has different prediction accuracy when satisfying the condition of required parameters [28; 29; 30; 31]. The modified Kostiakov model has been suggested in many studies (e.g., [28; 29; 31; 32; 33; 34]) for the estimation of infiltration rate in unlined irrigation and drainage channels. A comparison among the performance of five infiltration models, namely, the [PERSON], Horton, modified Kostiakov, Philip, and the Soil Conservation Service model (SCS), which conducted by [29] showed that none of the models consider the effects of variable channel cross-section. It has also been reported that adding three parameters of a channel trapezoidal section, namely, the water level, base width, and side slope to the modified [PERSON] model, it is possible to improve the prediction accuracy of the model [29]. Although the modified [PERSON] model showed significant improvement in the prediction accuracy of a channel infiltration capacity for a static condition, it has some limitations when used for flowing water conditions. It is important to improve the prediction accuracy of infiltration capacity in a grassed stormwater channel for better management of stormwater. In this study, laboratory experiments were conducted to collect data for the development of an empirical infiltration model for better estimation of infiltration capacity of a grassed stormwater channel with check dams.