Earth Surface Processes and Landforms Earth Surf. Process. Landforms 28, 99–110 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.433 INTERFERENCE PROCESSES ON SCOURING AT BED SILLS MARIO A. LENZI, 1 * ANDREA MARION 2 AND FRANCESCO COMITI 1 1 Department of Land and Agroforest Environments, University of Padova, Agripolis, via Romea, 35020 Legnaro (PD), Italy 2 Department of Hydraulic, Maritime and Geotechnical Engineering, University of Padova, via Loredan 20, 35100 Padova, Italy Received 30 July 2001; Revised 31 May 2002; Accepted 25 July 2002 ABSTRACT The local scouring downstream of bed sills forming a sequence for bed stabilization in steep channels has been investigated in a laboratory flume. The initial bed slopes ranged from 0Ð078 to 0Ð148. The bed alluvium was characterised by a non- uniform grain size distribution. The results show that when the ratio between the critical water depth h c and the sill spacing L rises above a characteristic value the scouring dynamics become heavily affected by the presence of the downstream sill, associated with the onset of a form of “interference” which renders the scouring process less effective. The difference with an “undisturbed” case is demonstrated. Self-affinity of scour holes is confirmed and the scour length appears to be the reference parameter from which the scour depth might be evaluated. Copyright  2003 John Wiley & Sons, Ltd. KEY WORDS: bed sills; local scouring; channel erosion; laboratory flume. INTRODUCTION High-gradient mountain streams are often prone to channel incision. One method to stabilize them is to use a staircase-like sequence of grade control structures; these are generally called ‘check-dams’ when they are more than 1Ð5–2 m high above the original bed level and ‘sills’ if less. Traditionally, the principle governing the prevention of channel erosion through bed sills or check-dams is to reduce the mean longitudinal bed slope S to a lower value S eq between the works, called the equilibrium or final slope (Goitom and Zeller, 1989; Biedenharn and Smith, 1997) which should represent a dynamic equilibrium between bed scouring and aggradation. Different methods have been proposed to evaluate S eq. They can be grouped (Porto and Gessler, 1999) into empirical relationships (Woolhiser and Lenz, 1965; Falciai et al., 1977; Della Lucia and Fattorelli, 1981) and analytical approaches based on incipient motion criteria (Gessler, 1970; Ferro and Giordano, 1988; Julien and Wargadalam, 1995). In spite of these efforts, a reliable determination of the equilibrium channel slope is still an open problem (Biedenharn and Smith, 1997). Referring to bed sills, once the final channel slope is evaluated, the spacing between bed sills L has be chosen and the drop a 1 at each sill may be estimated by the following equation (Gaudio et al., 2000): a 1 D ⊲S  S eq ⊳L ⊲1⊳ where S is the initial channel slope and S eq is the equilibrium channel slope (see Figure 1). Assuming at the equilibrium a steady and quasi-uniform flow along the staircase-like system (i.e. where the uniformity is at a macro-scale with flow kinetic energy equal at each sill), the product Z D SL ⊲2⊳ represents the stream potential energy which must be dissipated along the distance L between two sills. The height a 1 (hereafter called morphological jump as in Gaudio et al., 2000) is roughly the drop energy associated with the free jet impinging into the pool tailwater. Approximately, drop energy a 1 is locally dissipated in * Correspondence to: M. A. Lenzi, Department of Land and Agroforest Environments, University of Padova Agripolis, via Romea, 35020 Legnaro (PD), Italy. E-mail: marioaristide.lenzi@unipd.it Copyright  2003 John Wiley & Sons, Ltd.