Stormwater Management & Sewer Remediation

In order to mitigate Combined Sewer Overflows (CSOs) and their resulting waste, toxic materials and debris, the City of Fostoria started the development of the CSO Long-Term Control Plan (CSO LTCP) in 2007.

Avetin Engineering Ltd. worked with the City and developed a detailed hydrologic/hydraulic model using PCSWMM – with the objective of mimicking the sanitary and combined sewer system performance, determining the needed improvements and evaluating the mitigation effectiveness resulting from the proposed projects.

During the CSO LTCP development period, it was found that the City had experienced river intrusions through its CSO outfalls. A HEC-RAS model was then created to simulate the hydraulics of the East Branch Portage River.

In 2010, the City used PCSWMM’s river model importing tool to merge its HEC-RAS models into a practical watershed/sewershed model (which was updated two years later in 2012). The consistent updates to PCSWMM tools make it efficient for the City to evaluate the improvement results and re-prioritize the proposed projects if needed.

Shortly after the watershed/sewershed model became available, a flood control project located upstream to the City was developed. And, upon the completions of two major CSO LTCP projects, the integrated 1D-2D model was recently updated to incorporate all changes and improvements in 2016.

This model was then utilized as an important tool to develop system-wide dry/wet weather operational strategy for the City of Fostoria. The City takes the CSO LTCP as a living document and strives to comply with US EPA’s CSO policy as planned.

The Metropolitan Sewer District of Greater Cincinnati’s (MSDGC's) Mill Creek System Wide Model (SWM) is one of the largest 1D stormwater management model (SWMM) projects in the world, with 19 sub-watersheds divided into 6,550 subcatchments, 28,000 nodes and 28,400 conduits. MSDGC and their consultants are consequently faced with long simulation run times while tackling the challenging task of managing ongoing work on various sub-watersheds.

Wade Trim is using PCSWMM to enhance the Mill Creek SWM by reducing run durations, making it easier to update the model moving forward and improving overall performance in simulating system flows. PCSWMM’s auditing tools are being used to review the large number of model elements and identify areas for improved performance or representation.

Once complete, the model will then be reviewed to identify points in the system where the overall Mill Creek SWM could be broken into smaller, more manageable sub-watershed models as well as an interceptor model. Portions of the system may also be impacted by receiving stream water levels during annual simulations, which would require development of boundary conditions at any impacted outfalls.

A check-out and check-in system will then be developed, and inter-submodel communication will be provided using interface files. The Mill Creek SWM enhancements are expected to support MSDGC’s Wet Weather Improvement Program Phase 2 planning efforts.

Faced with the risk of surcharging and surface flooding, the City of Ottawa conducted an Integrated Dual Drainage Study. The study supports city-wide comprehensive asset and wet weather infrastructure management plans by characterizing existing drainage conditions, identifying existing levels of service and defining constraints and opportunities within the study area.

PCSWMM was used to develop and evaluate a pipe-by-pipe model, and GIS and spreadsheet analysis were used to prepare input data, perform complex analyses and render results. Sewer hydraulic grade line and percent pipe full capacity were used to characterize the minor system performance. Major system classification was based on ponding depths/extents, conveyance system, private property crossing points and level of service within Collector roads.

Input rainfall was based on the City’s design storms (various return periods), while impacts of climate change were simulated by increasing storm intensities by 20%. To qualitatively validate the PCSWMM model, simulations of historical storms were run and the results were tested against flooding reports documented by the City.

The results indicated the minor and major systems had average levels of service of 1:5 and 1:100 years, respectively. A systematic approach was used to evaluate various alternatives to improve system performance, starting with those most basic and cost-effective and then gradually increasing the cost and complexity until the desired balance was achieved.

By restricting flows to the minor system with inlet control devices, average inlet capture rates could be reduced by 40 - 70%, significantly reducing the risk of sewer surcharging conditions during a 1:100-year storm. Reduced inlet capture translated into increases in surface flows and ponding duration, although these were effectively accommodated within the existing capacity of the major system.

The Town of Huntsville undertook a multi-phase program to better understand the condition and performance of its storm sewer system, which includes over 1,400 pipes and 34 km of infrastructure. Following extensive CCTV inspections, surveying, and GIS database development, the Town required a hydraulic model to be built to translate this data into a system-level assessment of capacity and performance. Key considerations included building a model directly from verified GIS and inspection data, generating runoff based on defined drainage areas, and evaluating system performance against specific operational criteria such as pipe capacity, velocity, and cover depth.

Based on these needs, PCSWMM was chosen to develop a detailed hydraulic model of the storm sewer network. The model incorporated Town and District infrastructure data, allowing the full connected system to be assessed in a single model. The contributing subcatchments were explicitly defined and parameterized utilizing spatial weighting tools and inputs such as soil type, impervious area, slope, and curve numbers. These inputs were used to generate runoff and route flows into the system through connected catchbasins and inlet structures, reflecting how rainfall enters and moves through the network.

The model was then used to simulate system performance under various design storm conditions, with evaluation focused on three specific criteria: minimum pipe cover (1.5 m), minimum velocity (0.6 m/s), and surcharging during the 10-year storm event. PCSWMM’s post processing enabled direct identification of deficiencies, including 119 pipes experiencing surcharge and a subset of pipes with velocities below the required threshold, indicating potential sedimentation and maintenance concerns.

PCSWMM enabled iterative scenario testing of system upgrades. Pipes identified as surcharged were selectively upsized within the model to determine the extent of improvements required to eliminate capacity issues. This analysis showed that 133 pipes (approximately 16% of the network) would need to be enlarged to resolve surcharging under the 10-year event.

The resulting model provided a clear, quantified understanding of system deficiencies and upgrade requirements, linking field data directly to hydraulic performance. PCSWMM was used as a practical and integrated platform to convert inspection and GIS data into a calibrated hydraulic model, define watershed-driven inputs, and test targeted infrastructure improvements, supporting informed, data-driven planning decisions for the Town.