Groundwater Infiltration Forecasting for Sewer Network Resilience
Background
Groundwater infiltration into sewer networks presents a critical challenge to maintaining wastewater system integrity, particularly during periods of elevated groundwater levels. This can result in increased baseflows, reduced network capacity, and a heightened risk of storm tank discharges. A UK water company engaged GeoSmart Information to provide a forward-looking groundwater forecast service to support operational decision-making across a high-risk catchment.
The forecasting service formed part of a wider infiltration reduction programme. Its objective was to reduce the frequency and duration of storm discharges from the wastewater treatment works by providing up to 30 days’ warning of infiltration risk. This enabled operational teams to plan asset inspections, schedule interventions, and better manage sewer flows during groundwater-driven events.
Methodology
GeoSmart’s groundwater forecast service is underpinned by a validated national modelling framework that integrates real-time borehole telemetry, ensemble rainfall forecasts, and historical groundwater level behaviour. A conceptual model was developed to represent the shallow groundwater system in the catchment of interest, using data from nine monitoring boreholes, including several installed and maintained by the client.
The forecasting model employs a lumped-parameter water balance approach to track water movement through the soil, unsaturated, and saturated zones. It is updated daily using observed groundwater level data and rainfall inputs, and incorporates ensemble weather forecasts from the Global Ensemble Forecast System (GEFS) to generate probabilistic groundwater level projections.
Forecast outputs are delivered as ensemble hydrographs and interpreted against pre-defined groundwater thresholds linked to sewer invert levels. A Red-Amber-Green (RAG) status classification is then applied to indicate the forecasted risk of infiltration at key locations. These forecasts are accessed through an interactive, web-based platform designed for use by operational staff and drainage engineers.
Integration with Sewer Asset Management
The forecast outputs were integrated into the client’s ongoing asset management processes. These included prioritisation of CCTV inspections, identification of sewer relining candidates, and adjustment of wet weather response protocols. Where sewer flow monitoring data was available, correlations between groundwater levels and base sewer flows were observed, supporting the future development of infiltration-driven discharge forecasting.
The model’s architecture is designed for scalable integration, with capacity to incorporate sewer flow telemetry for real-time correlation and alarm triggering. This approach enables a transition from reactive to predictive network management, with the potential to trigger interventions in advance of groundwater-driven flow increases.
Study Findings
The conceptual model for the catchment identified several hydrogeological features influencing infiltration risk. Multiple superficial aquifers with permeable sand and gravel layers are interbedded with lower-permeability materials, overlying London Clay. These conditions support the formation of a basal water table at the contact with the clay. Where superficial deposits are thin, the groundwater table is close to the surface, raising the risk of interaction with sewers.
A principal Chalk aquifer underlies parts of the catchment and interacts with the superficial aquifer system. During winter, the Chalk contributes to rising groundwater levels through discharge into the surrounding valley superficial deposits and interaction with a nearby ephemeral river. This dynamic results in seasonal groundwater level rises, particularly in the northeast of the catchment.
The initial mapping indicated that a large proportion of the foul sewer network is always below the water table, a smaller portion is never below, and a significant portion is intermittently submerged. These results aligned with previous modelling which showed that dry weather infiltration contributes substantially to flow at the treatment works, with a further notable contribution from slow runoff during storm conditions.
High-risk areas include low-lying and western parts of the catchment influenced by shallow superficial deposits or surface water connectivity. Seasonal fluctuations impact areas to the east, while southern zones may be affected by proximity to historic gravel extraction features. The northeast is persistently impacted due to sustained inputs from the river and Chalk recharge.
In support of the initial findings, nine boreholes were installed across a range of hydrogeological settings to refine risk estimates and support forecast calibration. Monitoring from these installations, alongside Environment Agency data and real-time telemetry, now informs the operational forecasting system.
Outcomes
The forecasting system enhanced the client’s ability to monitor, predict, and respond to groundwater infiltration events. With up to 30 days of lead time, the service facilitated:
– Timely scheduling of investigations and maintenance
– Proactive management of sewer rehabilitation resources
– Improved operational planning at downstream treatment infrastructure
– Evidence-led justification for risk-based prioritisation
The tool enabled network managers to understand not only where infiltration was most likely to occur, but also when. This temporal dimension significantly improved the efficiency and cost-effectiveness of infiltration mitigation activities.
Why GeoSmart’s Forecast model works
GeoSmart brings extensive expertise in groundwater level forecasting, infiltration risk assessment, and SuDS design. The forecasting model is built to support operational teams and network modellers with clear, accessible intelligence on infiltration risk across multiple timescales.
Key features of the approach include:
– Real-time telemetry and rainfall forecast integration
– Ensemble forecasting with 30-day prediction horizons
– Defined infiltration thresholds linked to sewer asset data
– Flexible deployment across catchments and site-specific locations
– Compatibility with GIS platforms and existing hydraulic modelling workflows
– Sensitivity to seasonal and long-term climate variability
Conclusion
This case study demonstrates the role of groundwater infiltration forecasting as a valuable operational tool in managing wastewater infrastructure under variable environmental conditions. By reducing uncertainty and enabling proactive intervention, the service supports long-term drainage and wastewater planning, enhances resilience, and aligns with regulatory performance expectations.
Image references
This image shows sewer depths below the surface. Understanding the level of the water table and how it interacts with the sewers allows a prediction of the infiltration potential, which informs risk management interventions.
This map provides an overview of those river valleys within which groundwater flooding might be experienced. The point locations show where monitored boreholes are located and their forecast status. Red markers show where flooding is predicted. The green regions on the map represent chalk bedrock whilst the orange shows limestone.
Images showing the monitoring of borehole telemetry and the predicted forecast of future rise and fall in water levels based on rainfall forecasts. These examples show water levels well above seasonal norms and exceeding alert threshold levels for sewer infiltration and flooding.
An image of a borehole water level plot for Chilgrove House in West Sussex. It shows the logging of water levels as the red line and a black line showing a predicted rise above threshold, along with a cone of uncertainty in grey lines based on historic rainfall patterns.
