Describe how excessive infiltration and inflow (I/I) specifically impacts the hydraulic design requirements for a gravity sewer system.

Excessive infiltration and inflow (I/I) fundamentally alters the hydraulic design requirements for a gravity sewer system by significantly increasing the total volume of wastewater the system must convey. Infiltration is groundwater entering sewer pipes through defects such as cracks, leaky joints, or faulty service connections. Inflow is stormwater entering the sewer system through direct connections like roof drains, foundation drains, or directly through manhole covers during wet weather. These extraneous flows combine with sanitary wastewater, leading to a much higher design flow rate than would be generated by sanitary flow alone.

This increased total design flow directly impacts several critical hydraulic design parameters. First, it necessitates larger pipe diameters. A gravity sewer's primary function is to transport wastewater by relying on the slope of the pipe, maintaining a free air surface within the pipe, not under pressure. When I/I substantially increases the flow, a pipe designed only for sanitary flow would become surcharged, meaning it flows completely full under pressure, leading to reduced capacity, potential overflows, and backups. Therefore, designers must select a larger pipe diameter to accommodate the combined sanitary and I/I flows while maintaining adequate capacity and free-surface flow conditions.

Second, the increased design flow impacts the required pipe slope and the resulting flow velocity. Gravity sewers must be designed with sufficient slope to generate a minimum self-cleansing velocity, typically around 0.6 meters per second (2 feet per second), which prevents solids from settling and accumulating within the pipe. However, the slope must also be limited to avoid excessive velocities, generally above 3 meters per second (10 feet per second), which can cause pipe scour, air entrainment, and premature wear. When I/I introduces highly variable flows, especially during wet weather, maintaining this narrow range of acceptable velocities becomes more challenging. A larger pipe diameter, chosen to handle peak I/I, might result in velocities below the self-cleansing minimum during dry weather when I/I is low, leading to solids deposition. Conversely, at peak I/I events, the greatly increased flow through the chosen larger pipe might still generate higher velocities than desired, though less so than in a smaller pipe. The designer must carefully balance diameter and slope to ensure velocities remain within the acceptable range across the full spectrum of expected flows, including dry weather minimums and wet weather peaks.

Third, higher flow rates due to I/I increase friction losses within the pipe. This translates to a steeper energy grade line, which represents the total energy of the flowing water. To maintain gravity flow and prevent surcharging, the sewer system may need to be laid at greater depths or with steeper slopes to provide the necessary hydraulic head, which directly impacts excavation costs and construction complexity. Failure to account for excessive I/I in hydraulic design leads to undersized pipes, chronic surcharging, basement flooding, and sanitary sewer overflows (SSOs), thereby degrading system performance and public health.