Sanitary Sewer Capacity Planning along the Russian River: Considering the Impact of High River Levels on Sewer Capacity

Background and Objectives

The Russian River County Sanitation District (District), managed and operated by Sonoma Water, provides sanitary sewer conveyance and treatment for approximately 3,300 properties in Sonoma County, California. The District's service area of about 4 square miles encompasses several unincorporated communities, most notably Guerneville, a small town and popular vacation spot located on the Russian River. This area of California experiences dry summers, with the majority of annual rainfall occurring during a wet season between approximately November and April. The location of the District in the lower Russian River watershed is noteworthy, as river flooding impacts the sanitary sewer system. The Russian River watershed comprises over 1,350 square miles upstream of the District's (4 square mile) service area. Because the Russian River flows through the service area, Guerneville and surroundings have experienced multiple significant flooding events, with river cresting above the major flood stage (as defined by the National Weather Service) during five wet seasons in the last forty years. During major floods, portions of the sanitary sewer system can be inundated (submerged) with flood waters. Sanitary sewer overflows during the 2016/2017 and 2018/2019 wet weather seasons, concurrent with flood events on the Russian River, prompted the California's North Coast Regional Water Quality Control Board to issue requirements to study and address the potential for capacity limitations in the District's collection system. The District's prior capacity assessment had been completed in 2016, based on hydraulic modeling and flow monitoring from around 2014. That study had found no capacity deficiencies in the system. However, the model developed and used at the time did not include or consider the potential impact of elevated river levels on the sanitary sewer system. Key objectives of the new study included expanding, updating, and recalibrating the hydraulic model to incorporate impacts from elevated river levels and to more realistically simulate varied seasonal conditions; identifying reasonable design conditions that balance the need to provide adequate sanitary sewer capacity, while recognizing that extreme conditions occur during river flooding events; and identifying associated capacity improvements. This paper may be of interest to agencies with collection systems similarly impacted by river conditions in addition to rainfall-dependent inflow and infiltration (RDI/I), or to engineers tasked with developing practical and straightforward models that incorporate inflow and infiltration from multiple sources. The model was developed using InfoWorks® ICM hydraulic modeling software, and the modeling and study were essentially completed in 2025.

Modeling both RDI/I and I/I due to High River Levels Required a Practical Approach that Made Use of Available Data
Challenges in realistically modeling inflow and infiltration in the District's system included an evidently significant impact of antecedent conditions on RDI/I response, resulting in a relatively much larger flow response from later season rainfall events, plus an unknown impact of elevated and flooding river levels. The previous hydraulic model had simply calibrated RDI/I based on late season rainfall events, overestimating the response to early season events, in an attempt to simulate worst case conditions for the purposes of capacity analysis. However, that model calibration proved unable to predict the peak flows to the wastewater treatment plant (WWTP) in subsequent years when river levels were higher. To improve the model RDI/I calibration and add an I/I component to reflect the impact of high river levels, the model RDI/I was first completely re-calibrated to better represent the varying response to rainfall over entire wet weather seasons. As no new flow data were collected as part of this study, the previous temporary flow meters located throughout the system in 2014 provided the basis for this recalibration. That season proved to be ideal for calibrating rainfall response, as significant rainfall events occurred without accompanying high river levels. Incorporating initial losses combined with daily evaporation estimates in the RDI/I simulation resulted in a much improved representation of the varying response to rainfall over the entire wet weather season. This improved calibration of varying RDI/I response over a full season provided the basis needed to identify additional I/I that could be correlated to high river levels. For calibrating and verifying the response to high river levels as well as the combination of high river levels and rainfall over entire seasons, the model was run for entire wet weather seasons from 2013/2014 through 2024/2025 and compared to WWTP flow data. A river I/I component was added to the model using the InfoWorks ICM ground infiltration model based on time-varying river level data. This approach allowed for flow input from high river levels at any manhole along a gravity line, based on comparison of the river level to the hydraulic gradeline in the manhole. Although many simplifying assumptions are inherent in this approach, it proved effective in better predicting flows to the WWTP over a variety of wet weather seasons. Not insignificantly, this approach was also relatively straightforward to implement with the available data. [b]Provide System Capacity, but Not too Much: Considering the Design Condition[/b] The prior capacity assessment for the District applied a 10-year, 24-hour rainfall event to the model to generate design flows and assess sanitary sewer system performance, which is a fairly typical approach for separate sanitary collection systems in California. This updated study, as well as the several years of observed conditions in the system since that assessment was completed, make it evident that a river level component must also be included in the design condition. Flows in the sanitary sewer system do seem to be affected by elevated river levels during less extreme river flooding events and even during periods when river levels are elevated but still below flood stage thresholds. It is important that the sanitary sewer system be able to handle flows during elevated river stages that occur frequently. However, the design condition must be chosen with care. Even in a typical system, choosing events too extreme could lead to the identification of so many system capacity deficiencies that the cost of improvements would be prohibitive, especially in a relatively small system such as the District's. Additionally, sizing a system for an extreme event could mean that the system does not function well under typical conditions during much lower flows (due to slow velocities in oversized pipes, or oversized pump stations). The District has the added complication that under major Russian River flood events, many manholes, cleanouts, and even pump station wet wells may be submerged under river flood waters. Even though such floods are known to occur in the District's service area, there is a significant risk that attempting to size facilities to accommodate major flood events would lead not only to the issues noted above (prohibitive cost of improvements and impractical sizing of facilities), but could also lead to even more river floodwater entering the sanitary sewer system as capacity is made available. The selected design conditions aimed to balance these needs by using the same 10-year, 24-hour rainfall event as used for the previous study, but adding a 'Moderate Flood' river level event (as defined by the National Weather Service), based on river stage data from an actual moderate flood experienced in February 2025. A secondary design condition also considered a lesser river level below flood stage, but with more stringent capacity standards. Both design conditions identified the same capacity improvement needs, including pipeline upsizing and pump station upgrades.

Conclusion
A straightforward approach to modeling the impacts of elevated river levels in addition to RDI/I in the Russian River County Sanitation District sanitary sewer system resulted in significantly better prediction of flows and provided greater understanding of how the system responds to varied wet weather conditions. Realistic capacity improvement needs were identified. As additional data and observations are collected in the future, the updated model can be further refined to reflect more localized variations in I/I due to elevated river levels.