Clogging-up of a stormwater infiltration basin: a laboratory approach using image analysis

Abstract

The main function of stormwater infiltration basins is to favour stormwater drainage. However, the gradual clogging-up process caused by sediment accumulation inside these basins raises questions about their hydrodynamic functioning. Therefore, the objective of this work is to study the evolution of sediment pore distribution and its relationships with water retention and infiltration characteristics. Chevire basin (Nantes, France) ageing was simulated in the laboratory, using PVC columns (10 cm diameter, 32 cm height). Seven columns were first filled in with 20 cm of thick sandy Loire river alluvia, i.e. the same material as in the basin over which sediment accumulates. The objective of the experiment was to simulate 36 months of basin ageing within 9 weeks in the laboratory. Every day, the columns were submitted to 2 cycles composed of 4 h of rain separated by 8 h without rain. During the experiment, (1) the water flow at the bottom of the column, (2) the sediment layer thickness, (3) sediment water retention and hydraulic conductivity at saturation (Ks), and (4) pore space distribution by image analysis were measured. After 36 months of experimental simulation, 3 cm of sediment had accumulated (i.e. 1 cm year−1); this rate was representative of in situ observations. This progressive accumulation generated the formation of a water layer above the sediment, revealing early clogging-up by the sediment. Using HYDRUS 1D inverse resolution, a decreased Ks values from 25 to 6 × 10−6 m s−1 was observed after 6 and 36 months, respectively. The mean equivalent pore radius decreased 1.6-fold, from 606 to 380 μm after 6 and 36 months, respectively. These observations were confirmed by an image analysis study, whereby internal organisational changes were clearly evidenced in the sediment. Sediment particles, at first well individualised, progressively bound to one another, leaving hardly any voids. While the clogging-up process in stormwater infiltration basins has often been studied, very little has been done about sediment hydrodynamic properties. Pore space characterisation by image analysis is a major scientific progress and showed that the presence of high levels of organic matter did not favour sediment aggregation. On the contrary, sediment gradually constituted a barrier to water flow, leading to clogging-up.