This paper presents some new experimental results drawn from a wider research about the influence of bed sills spacing upon local scouring downstream of such structures. A new phenomenon called morphodynamic interference was observed during six clear water flume tests featuring an initial slope of 0.030, and not occurring at higher gradients. Three distances between the bed sills were implemented for each run. The runs - presenting the same grain size distribution - were different for water discharge but they all were characterized by critical and slightly supercritical flow condition. The combination of flow rate and sediment size did not lead to equilibrium slopes remarkably lower than the initial value, thus producing very small drop height at the sills. At each drop standing waves were generated which shaped the bed with in-phase undulations, similar to fixed antidunes. Such bed forms resulted in some cases to alter substantially the scour holes downstream when they happened to superimpose in correspondence with a subsequent bed sill. For non-interefered conditions, scour holes were small and did not feature any apparent flow recirculation. Such an interference appears to be completely different from that described previously by the authors.
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