PSI - Issue 84

Stefano Pagliara et al. / Procedia Structural Integrity 84 (2026) 1270–1277

1271

water surface elevations in the vicinity of the debris accumulation.  H represents a fundamental parameter governing pressure gradient across the structure (Palermo et al. 2025; Pagliara et al. 2025; Pagliara et al. 2026). An accurate estimation of h bw and  H is thus essential for flood risk assessment and structural design. Past experimental studies have demonstrated that h bw and  H depends strongly on the approach flow conditions commonly characterized by the Froude number ( Fr ) as well as on accumulation geometry, compactness, and wood characteristics (Parola 2000; Schmocker and Hager 2013; Panici and De Almeida 2018; Schalko et al. 2019; Wüthrich et al. 2020; Palermo et al. 2025, Pagliara et al. 2026). Further, studies on bluff bodies or submerged bridge decks have also emphasized the role of flow blockage in determining hydrodynamic actions (Naudascher and Medlarz, 1983; Picek et al. 2007; Qi et al. 2014; Kabir et al. 2023; Eames and Robinson 2024; Palermo et al. 2025). However, limited attention has been given to porous LWD accumulations under open-channel flow conditions. In particular, the combined influence of h bw ,  H and energy loss ( h L ) on hydrodynamic forces acting on debris remains insufficiently understood. This paper presents preliminary results of an experimental investigation of flow-induced modifications by woody debris. Specifically, we focus on quantifying the effect of  H and h bw on hydrodynamic forces acting on woody debris accumulations characterized by different porosities ( n ) and geometries. To this end, we critically analyze the relationship between  H , h bw and h L , validating the measured backwater against existing equations. Despite the outcomes are preliminary, they provide interesting insights into debris–flow interactions, which may support engineers in improving current criteria to assess debris-related hydraulic impacts on river infrastructures. 2. Materials and methods A laboratory study was conducted in a fixed-rough-bed rectangular channel to examine debris–flow interactions under low- Fr conditions. Fig. 1. shows the schematic sketch of the employed rectangular channel whose length l = 6 m, width w = 0.345 m and height h = 0.4 m. Discharge ( Q ) was delivered using a pump and was measured by a KROHNE® Optiflux 2000 electromagnetic flowmeter (accuracy,  1%). Flow stability and uniformity were maintained through a combination of an upstream headgate, a downstream sluice gate, and a honeycomb straightener at the inlet to minimize turbulence. Hydrodynamic forces on debris were measured using two orthogonally mounted strain-gauge load cells (CZL601, accuracy of  1 g) allowing simultaneous estimates of drag force ( F D ) and lift force ( F L ). Positive F D and F L are defined in the streamwise and downward directions, respectively (Fig. 1). Signals were conditioned and acquired using a National Instruments (NI-6000 series) device. Debris models were rigidly attached to the load-cell system via a steel frame using custom connectors to ensure precise alignment. Water-surface elevations were referenced to the bed datum ( Z 0 ). The undisturbed water level measured without debris is denoted as h 0 . After the debris was introduced, steady water levels were measured upstream and downstream of it and are denoted as h u and h d , respectively. These values represent averages of measurements taken at the channel centerline and near the sidewalls. Flow depths directly adjacent to the debris on the upstream and downstream sides are indicated by h u,nd and h d,nd respectively. These were obtained by averaging measurements taken at the channel centerline and along the debris edges. The backwater rise in the upstream portion of the channel is denoted as h bw = ( h u − h 0 ) (Schmocker and Hager 2013; Schalko et al. 2019), while h uB = h u,nd – h b indicates the flow depth at the upstream face of the debris. The corresponding water-level drop across the debris is represented by ΔH = ( h u,nd − h d,nd ). Three debris configurations were tested, including two permeable geometries (Db01 and Db02) and one fully impervious block (Db03). Debris length, width and height are denoted as Db l , Db w and Db h respectively. Table 1 provides the geometric and physical descriptions of the investigated debris.Debris are characterized by different porosities ( n ), geometries, solid wood volume ( V LWD ), total volume ( V t ), maximum projected frontal area ( A fmax ) and bottom area ( A lift ), with n = [1 – ( V LWD / V t )]. Following Ballio et al. (2024), projected frontal and bottom areas ( A drag and A lift , respectively) were obtained using the image-based analysis of binarized photographs. In particular, A drag = A fmax when the debris is fully submerged. The relative width ratio was in range of 0.49  Db w / w  0.55.