Under the Clean Water Act, the federal government has required cities to reduce and eliminate combined sewer overflows (CSOs). Cities are required to comply with those requirements to avoid penalization through fines and other legal actions. The Metropolitan Sewer District of Greater Cincinnati (MSDGC) is under a consent decree to respond to CSOs.
Stormwater management alternatives were assessed using the stormwater management modeling software XPSWMM to model watershed hydrology at the sub-basin scale, including hydraulics of open channels and enclosed systems of stormwater and wastewater flows. The hydrologic component of XPSWMM divided the Ludlow Run watershed into smaller sub-basins, using topographic data provided by MSDGC. Hydrologic processes, including infiltration, evaporation, ponding, and ground-surface water exchanges were included in the model.
Like many older cities throughout the Midwest, Cincinnati, OH, has a combined sewer system to manage stormwater and sanitary wastewater. Stormwater and sanitary waste are collected from their respective sources, combined into one pipe network, and conveyed to the municipal wastewater treatment plant. In Cincinnati, sections of the current combined sewer system were installed in the early 1900s and were sized to convey flows appropriate for that era. Over the decades, the city has continued to develop, increasing impervious surface area and overall water consumption. As a result, more stormwater runoff reaches the inlets on the streets, and more wastewater is discharged into the combined system. The existing sewer network was not designed to convey the increased volume of additional flows, causing the system to surcharge and back up. To relieve the surcharging, combined sewer overflows were installed in the system, discharging a mix of stormwater and untreated sewage into natural waterways during wet-weather events.
BMP Site Suitability Index
The Ludlow Run sewershed covers a large urban area, in excess of 2.5 square miles. An initial challenge to planning CSO separation in a watershed of this size is sifting through volumes of data to focus efforts on those areas suitable for regional stormwater BMPs. To remove biases in the site-selection process and maximize the efficiency of work in the field, AMEC developed a Site Suitability Index to prescreen each parcel in the watershed to determine suitability and identify potential BMP locations. A combination of factors was included in the evaluation, including slope, soils, and existing land use.
After the evaluation criteria were selected and applied to the entire watershed, the criteria were assigned point values, with higher points indicating a higher suitability for BMP placement. A color gradation map was used to interpret the range of point values, with green parcels indicating sites most suitable for regional BMPs and red indicating sites that are unsuitable for BMPs (Figure 3). Land use received the highest weighting, because this criteria has the greatest impact on both cost and construction feasibility. The scores were summed for each cell and ranged from a minimum of 100 points (other land use, HSG D, and >20% slope) to a maximum of 1,100 points (open space, HSG B, and 0% to 5% slope). Areas shown in green are the most suitable locations for BMP placement, while those shown in red are the least favorable. This process allowed for the identification of 10 potential BMP locations (areas A through J as shown in Figure 4) that were to be investigated further in the field.
Field Verification of Site Suitability
Upon selection of conceptual BMP locations (areas A through J), the suitability of individual sites was verified through field investigations. These investigations determined if the site could accommodate the logistics of potential BMPs. Several locations identified through the desktop screening exercise were eventually ruled out for various reasons (shown as yellow boxes in Figure 4). Location A proved difficult to route water to the location, as it is slightly higher than surrounding properties. Neighboring green space, depicted in a 2006 aerial photograph, had been recently converted into a parking lot, reducing the amount of available stormwater to support a viable BMP. Locations B, D, and J were determined unsuitable from the site investigation for similar reasons, including existing obstacles to stormwater routing and site access for construction.
Location H was determined to be suitable for floodplain storage BMPs. The floodplain in this area has signs of frequent inundation, which suggests that flows would often reach the floodplain and thus floodplain storage BMPs. The southern section of location I was determined to be suitable for green and grey BMPs, while the northern section of location I was determined to be suitable only for floodplain storage. In the southern section, the topography naturally drains from the CSO 162 sewershed down to location I, providing ideal conditions to convey and store stormwater. Due to the pipe configuration and topography, it was infeasible to route stormwater under gravity flow to the northern section of location I.
Development of Stormwater BMP Palette
There are multiple techniques available to store, convey, and treat stormwater. Many BMPs were considered and screened out based on field observations. This section discusses the selected BMPs included in our final stormwater BMP palette.
Stormwater management alternatives were assessed using the stormwater management modeling software XPSWMM to model watershed hydrology at the sub-basin scale, including hydraulics of open channel and enclosed systems of stormwater and wastewater flows.
Selected Alternative for Each CSO
An alternative for each CSO was selected based on the modeled ability to eliminate combined sewer overflows (Figure 9). Each alternative focuses on capturing and reducing peak flows to the combined sewer system.
CSO 24. The proposed separation alternatives for the CSOs described above rely on the removal of the largest combined sewer conveyance at the lowermost point in the watershed, CSO 24. This CSO location is the endpoint of Ludlow Run that discharges an estimated 318 million gallons into the Mill Creek every year. To completely separate CSO 24 (and the nested CSOs described above), the installation of a separate sanitary line would be required, thereby converting the existing Ludlow Run conveyance tunnel into a stormwater-only line (Figure 10). The CSO 24 overflow structure would then be effectively converted into a natural confluence with the Mill Creek, eliminating all CSO overflow events for a typical year.
The CSO 24 Separation Project illustrates the benefits of using a holistic watershed approach to CSO separation projects. By identifying existing constraints and opportunities early in the planning process, we were able to maximize schedule and budget to streamline the process. While there is an extensive menu of stormwater BMPs to choose from, each with particular requirements and limitations to function properly, determining a palette of BMPs suitable to a particular region of operation will focus design efforts. Equally as important is the process of field verification, getting “boots on the ground” to verify location and suitability of conventional and green stormwater BMPs. Regardless of the location, watershed, existing infrastructure, or proposed BMPs, a rigorous performance evaluation using XPSWMM or similar modeling software is an essential component to a watershed-scale management plan. By evaluating and modeling multiple alternatives, we were able to provide workable solutions to demonstrate separation and eliminate 328 million gallons of overflow to Mill Creek from the Ludlow Run watershed.