## 1 Introduction  
River surfing, characterized by standing waves remaining stationary over time, has grown in popularity since its inception in 1972 by the [PERSON] brothers in Munich, Germany. Initially, natural river features created these waves, but modifications to riverbeds and the development of artificial systems have allowed the sport to spread globally. Notable examples include the Eisbachwelle in Munich and waves in the Snake River in Wyoming, USA. The recent trend of creating artificial standing waves in existing rivers, e.g., as seen in Hanover and Nuremberg (Germany), aims to provide surfing opportunities without direct ocean access [1, 2]. However, since building these artificial waves is still in its inception and each wave geometry is different, optimal inflow and outflow conditions have not been achieved yet. Laminar inflow conditions facilitate a glassy and desired wave surface. Outflow conditions behind the wave should exhibit a constant downstream flow carrying the surfer away from the wave. Recirculation areas (Figure 1) and back flow should be minimized or avoided since these may drag the surfer back into the wave, endangering them. Reducing the water flow rate (m\({}^{3}\)/s) is one way to reduce such potential endangering characteristics; however, this leads to reduced wave quality. Hence, in this study, we focus on improving outflow conditions by changing the bottom topology behind the wave by simultaneously improving the wave quality, i.e., enabling more water flow. This study simulates the surfing conditions at the _Fuchslochreille_ in Nuremberg whereby the concept can be transferred to any other standing waves. Despite the growing popularity of river surfing, research in this area remains limited compared to studies on ocean waves. Early investigations by [PERSON] and [PERSON] [4] focused on artificially generated waves to study the forces acting on surfboards. Subsequent research has expanded this notion to include numerical simulations [5; 6; 7] and experimental studies [8] on surfboard geometries and fin configurations [9; 10]. Although numerical research on surfboard and fin dynamics is well documented, studies on surf waves themselves, particularly standing river waves, are sparse. Historical research on waves dates back hundreds of years, focusing primarily on ocean waves and artificial reefs designed for coastal protection and tourism enhancement. [PERSON] et al. [11] provided an overview of surfing science, emphasizing the influence of coastal profiles on wave formation. Numerous studies [12; 13; 14; 15] have explored the design and optimization of artificial reefs, combining experimental and numerical approaches to improve wave quality and shoreline protection. For standing river waves, the literature is less extensive. According to [PERSON] [3], artificial river waves can be generated through various methods: sheet-flow on wave-like bathymetry (mapping of underwater topography), spatial wave tubes, hydraulic jumps at bottom drops, and hydraulic jumps with adjustable installations. The _Fuchslochreille_ utilizes the latter method, relying on hydraulic jumps--a phenomenon widely studied for energy dissipation and cavitation prevention [16; 17; 18; 19; 20; 21]. Optimal river surf waves aim to create waves with a wide or long surfa  
Figure 1: Exemplary surf wave illustration showing four distinct flow regions, inflow, surf wave, recirculation zone, and outflow, from (**A**) side view and (**B**) top view, with light blue arrows indicating flow direction, a dark blue vortex representing wave stabilization, and a red vortex depicting the undesired tumbling effect [3]. to generate speed and maintain control, while minimizing air mixture (i.e., glassy surface). This is often achieved through abrupt channel bed drops but should avoid being overly steep to ensure better maneuverability for the surfer. Froude numbers between 2.5 and 3 typically result in surfable waves [20], though adjustable deflectors can produce waves at lower Froude numbers.