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9Gully pots are essential assets designed to relief the downstream system by trapping solids and 10 attached pollutants suspended in runoff. This study applied a methodology to develop a quantitative 11 gully pot sedimentation and blockage model. To this end, sediment bed level time series from 300 12 gully pots, spanning 15 months, were collected. A generalised linear mixed modelling (GLMM) 13 approach was applied to model and quantify the accumulation of solids in gully pots and to identify 14 relevant physical and catchment properties that influence the complex trapping processes. Results 15show that the retaining efficiency decreases as sediment bed levels increase. Two typical silting 16 evolutions were identified. Approximately 5% of all gully pots experienced progressive silting, 17 eventually resulting in a blockage. The other gully pots show stabilising sediment bed levels. The 18 depth of the sand trap, elapsed time since cleaning and the road type were identified to be the main 19properties discriminating progressive accumulation from stabilising sediment bed levels. 20

The behaviour of elongated air pockets in downward sloping pipes is investigated. The observed flow regimes in co-current air -water flow in downward sloping pipes are described. The effects of pipe diameter and surface tension on the air pocket motion are quantified and two criteria for the observed flow regime transitions are derived. One criterion stems from energy considerations and marks the transition from the presence of one elongated air pocket to multiple air pockets. The second criterion predicts the required water velocity to start the motion of elongated air pockets in the downward direction. Both criteria were validated with experimental data for a range of downward sloping pipe angles, lengths and diameters.