When a new development proposes to connect to an existing sewer system, the sewer district needs to know whether the downstream pipes, manholes, and pump stations can handle the additional flow. A sewer capacity analysis answers this question. It compares the existing peak flow plus the proposed new flow against the carrying capacity of each downstream pipe segment. If any segment is over capacity, the developer may be required to fund an upgrade before the project can connect.

When a Capacity Analysis Is Required

Most sewer districts require a capacity analysis for projects that generate significant additional sewer flow. Common thresholds include:

• Any new sewer connection exceeding a specified flow (e.g., 10,000 GPD or 50 equivalent dwelling units)
• Subdivisions creating 5 or more lots
• Commercial or industrial projects with high water use (restaurants, laundries, food processing)
• Redevelopment projects that increase the number of dwelling units or change the use to a higher flow generator
• Any project connecting to a sewer system that the district has identified as having known capacity constraints

Some districts require a capacity analysis for every new connection, regardless of size. Others only require it when the project adds more than a de minimis amount of flow. The district's will-serve letter or connection requirements will specify whether an analysis is needed.

What the Analysis Includes

Step 1: Determine the Project's Sewer Flow

Calculate the average daily flow (ADF) and peak wet-weather flow (PWWF) for the proposed development using the same generation rates the district uses. Common rates:

The peak wet-weather flow includes the peaking factor (typically 2.5 to 4.0 for the design population) plus an infiltration and inflow (I&I) allowance. The peaking factor and I&I rates are specified by the district.

Step 2: Map the Downstream Sewer System

Trace the sewer route from the project's point of connection downstream to the treatment plant or to a point where the district confirms adequate capacity exists. This involves reviewing the district's sewer atlas (GIS or paper maps) to identify every pipe segment, manhole, and pump station in the path. Record the pipe diameter, material, slope, length, and invert elevations for each segment.

Step 3: Determine Existing Flow in Each Segment

Estimating existing flow in each downstream segment is the most challenging part of the analysis. Methods include:

• Land-use-based estimation: Using the district's GIS data, identify the parcels tributary to each pipe segment and calculate the expected sewer flow based on land use and generation rates. This is the most common method for desktop analyses.
• Flow monitoring: Installing flow meters in manholes to measure actual flow over a period (typically 1 to 4 weeks, including a wet-weather period). This is more accurate but expensive ($5,000 to $15,000 per monitoring location). Large developments or projects in known problem areas may be required to fund flow monitoring.
• District model data: Some districts have hydraulic models (InfoSWMM, PCSWMM, SewerGEMS) that provide existing flow estimates for every pipe segment. Request this data if available.

Step 4: Calculate Pipe Capacity

For each downstream pipe segment, calculate the full-pipe capacity using Manning's Equation with the pipe diameter, slope, and Manning's n value (typically 0.013 for all pipe materials in a capacity study, to be conservative). The design standard for gravity sewers is that the peak wet-weather flow should not exceed a specified percentage of full-pipe capacity, typically:

• 50% full (d/D = 0.5): conservative standard used by some districts
• 75% full (d/D = 0.75): common standard for most districts
• Full pipe: some districts allow full-pipe flow for existing systems but not for new construction

Step 5: Compare Flow vs. Capacity

For each pipe segment, compare the total flow (existing + project) against the design capacity. Present the results in a table:

If any segment exceeds the design capacity (d/D > 0.75 in this example), that segment is identified as a bottleneck that must be addressed before the project can connect.

What Happens When the System Is Over Capacity

If the capacity analysis identifies deficient segments, several options exist:

• Pipe upsizing: Replace the deficient segment with a larger pipe. The developer may be required to fund the upsizing, potentially with a reimbursement agreement if other developments also benefit.
• Parallel relief sewer: Install a new pipe alongside the existing one to share the flow. More disruptive but avoids taking the existing sewer out of service during construction.
• Flow reduction through I&I rehabilitation: If the deficiency is caused by excessive I&I rather than dry-weather flow, the district may accept I&I reduction in the tributary area as an alternative to pipe upsizing.
• On-site flow equalization: For projects with high peak flows (restaurants, laundries), an on-site equalization tank can reduce the peak flow to the sewer, though this is uncommon and adds ongoing maintenance responsibility.
• Alternative connection point: Connect to a different part of the sewer system that has available capacity, if feasible.

Capacity Analysis vs. Sewer Master Plan

A capacity analysis is project-specific — it evaluates the impact of one development on the existing system. A sewer master plan is a system-wide evaluation by the sewer district that identifies all deficiencies, plans future improvements, and establishes a capital improvement program (CIP). Many districts have completed master plans that already identify deficient segments. If your project connects to a segment that the master plan identifies as deficient, the district may already have an improvement project planned and funded. In this case, the developer's contribution may be limited to a fair-share fee rather than funding the entire improvement.

Always check whether the district has a current sewer master plan before starting your capacity analysis. The master plan data can significantly reduce the effort required for your project-specific study.