Sediment characteristics and accumulation in catch basins: a case study in Calgary, Canada.

Catch basins (CBs) are essential components of urban stormwater infrastructure by trapping sediments and contaminants from street runoff. However, poor maintenance can lead to sediment buildup, increasing the risk of urban flooding and downstream water pollution. This study investigates sediment characteristics and accumulation in CBs and proposes an evidence-based cleaning strategy through a field study in Calgary, Alberta, across 28 CBs in 2023 and 2024. The results indicate that sediment accumulation rates and metal concentrations vary with land use, being higher in CBs near houses and roads than in those near grassed areas. Sediment composition and metal concentrations exhibit vertical stratification, with smaller particles (i.e., clay and silt) having higher metal concentrations in the bottom layers, while larger particles form an armoring layer at the surface. This segregation, together with the armoring layer, helps to protect metal-rich small particles from scouring during storm events, potentially reducing the release of pollutants. Sediment scour analysis reveals a sharp increase in scour when the free depth ratio - the distance from the sediment surface to the CB lead (i.e., outlet pipe) invert divided by the CB diameter - falls below 0.2. Therefore, CBs should be cleaned when this ratio approaches 0.2.