A rapid and cost-effective method for assessing sediment volumes and accumulation rates in stormwater infiltration facilities

As soil artificialization and climate change continue to accelerate, effective stormwater management has become essential to mitigate flooding and preserve water resources, leading to the widespread development of stormwater management facilities based on infiltration (e.g., basins, swales, trenches, raingardens). Runoff carries suspended particles, which act as vectors for various micropollutants that can be potentially harmful or toxic to aquatic life. These facilities promote the retention of such pollutants through sedimentation and/or filtration. However, the layer of deposited sediment can, over time, impair their functioning (e.g., hydraulic regulation and contaminant mitigation). Inadequate management of sediment can thus negate the benefits of these facilities and lead to higher maintenance costs. Given the increasing implementation of stormwater infiltration facilities, accurately characterizing sediment accumulation is crucial for anticipating future maintenance needs across urban territories. However, to date, most existing methods, based on continuous measurements of flow rates and turbidity and/or stormwater sampling, are unsuitable for routine assessments across multiple sites.

This study proposes a rapid and cost-effective approach to evaluate sediment accumulation rates in stormwater infiltration facilities. The total accumulated volume over a known period is estimated by measuring sediment height along a tailored grid, combined with geostatistical interpolation. A detailed analysis of the dry bulk density of stormwater sediments, ranging from 0.4 to 1.2 g/cm³, also enables mass estimation, while knowledge of the accumulation duration allows the calculation of the average annual accumulation rate. The reliability of the method in delivering accurate estimates of the average annual particle load for urban catchments was verified by (i) comparing the results with continuous monitoring data from a pilot site over several years, and (ii) applying the method to nine sites in France and comparing the results with literature data.

Particle load estimates from this dataset showed significant variability, typically ranging from 50 to 2000 kg/ha impervious surface/year. In areas with lower sediment accumulation potential (e.g., residential areas or low-volume parking lots), loads generally do not exceed 1000 kg/ha_imp/yr, while more productive areas (e.g., high-traffic roads or heavy industrial sites) can reach up to 2000 kg/ha_imp/yr. These values can be translated into filling rates for facilities (cm/yr) by considering the degree of system centralization, defined by the ratio of infiltration area to catchment area. This rate tends to be several times higher in a centralized basin (almost 10 cm/yr) than in a source infiltration system (up to 1 cm/yr). However, spatially distributed measurements revealed heterogeneous accumulation patterns linked to hydraulic functioning, enabling targeted sediment removal as a prudent and cost-effective solution.

This approach enables the estimation of sediment accumulation rates across various urban catchments and provides an indirect method for quantifying contaminants that tend to associate with particles. Efficient in terms of both time and cost, this method supports the strategic planning of maintenance operations across diverse urban contexts, including densely populated cities and environmentally sensitive areas. By enhancing the long-term effectiveness of stormwater infiltration facilities, it helps prevent water contamination and mitigate risks to fragile ecosystems.