New Tool Release: Sanitary Sewer Network Sizing & Peaking Factors
Designing a sanitary sewer network is a delicate balancing act. The pipes must be large enough to handle extreme diurnal peak flows without backing up into basements, yet steep enough to maintain self-cleansing velocities during low nighttime flows so solids don't settle and rot.
To help engineers thread this needle instantly, I am incredibly excited to introduce the newest addition to our hydrology suite: the Sanitary Sewer Sizing & Peaking Factor Tool! 🚽💧
This web-based worksheet automates the tedious process of routing cumulative populations, applying empirical Peaking Factors (Harmon, Babbitt, Ten States), and solving the complex geometry of partially-full pipe flow. It even generates a dynamic HGL profile and plots your network exactly where it sits on the Peaking Factor curve!
The Peaking Factor Problem
In storm sewer design, we worry about the sky. In sanitary sewer design, we worry about human behavior.
Wastewater generation is highly diurnal. At 3:00 AM, flow in a residential pipe is practically zero. At 7:00 AM, when everyone wakes up and turns on the shower, the flow spikes dramatically. The smaller the population contributing to a pipe, the more extreme this instantaneous peak is.
To design pipes that won't surcharge during these morning bursts, engineers apply a Peaking Factor (PF). Rather than designing for the Average Dry Weather Flow ($q_{avg}$), we design for the Peak Dry Weather Flow.
This tool natively supports the three most common empirical formulas used in North America for calculating this factor:
- The Harmon Formula: The classic standard. PF drops smoothly from ~4.0 for small populations down to 2.0 for large cities.
- The Babbitt Formula: Often preferred for very small populations because it yields slightly higher, more conservative peaks.
- Ten States Standards: The official design requirement across much of the midwestern United States.
The Challenge of Partially-Full Flow
Once you calculate your peak flow, you have to size the pipe. But unlike storm sewers which are often assumed to flow 100% full ($d/D = 1.0$), sanitary sewers are explicitly designed to flow as open channels. They should never be full.
Why? Because sanitary sewage creates toxic, corrosive, and explosive gases (like hydrogen sulfide and methane). If the pipe is 100% full, the gas cannot vent, and the pipe will rot from the inside out (Crown Corrosion). Most design standards limit the maximum allowable depth ratio to $d/D \le 0.80$.
Calculating the true velocity of a pipe that is 62% full requires solving complex circular segment geometry (Camp's Equations). This tool does it instantly. It tracks the $d/D$ ratio for every single pipe in your network, providing a visual progress bar and throwing a red "CAPACITY" warning if your flow exceeds the 0.80 safety limit.
How to Use the Tool
This worksheet was built from the ground up to replace your clunky spreadsheets. Here is how to route your first sanitary network in 5 simple steps:
Set Global Parameters
Start on the left sidebar by defining the specific flow characteristics for your city. Input the Base Generation Rate (e.g., 250 Liters per capita per day) and the Infiltration (I/I) Rate based on the age of your pipes.
Next, select your Peaking Factor Method (Harmon, Babbitt, or Ten States). The tool will automatically cap the multiplier using the Max and Min PF caps you provide to prevent physically unrealistic math!
Build the Network
In Section A, construct your pipe topology. You don't have to enter them in order—the internal Directed Acyclic Graph (DAG) algorithm sorts them from upstream to downstream automatically!
Data Entry: Enter the Pipe ID, Up/Dn Nodes, the Local Population entering that specific manhole, and the Local Area (used for Infiltration calculations). You can also click the Import CSV button to load an entire network from Excel instantly.
Review Sizing Results (The Magic)
The moment you add a pipe, Section B populates instantly. The tool automatically accumulates the population flowing into each manhole, dynamically calculates the exact Peaking Factor for that specific population, and determines the total Design Q.
Depth Ratio Check: Look at the $d/D$ column! The tool solves Camp's equations to find the exact partially-full flow depth. If your flow exceeds the 0.80 safety limit, it visually flags the pipe with a red CAPACITY warning.
Visualize the Profile
To visually guarantee that your pipe slopes and drops are correct, use the built-in Profile Viewer. Select a flow path from the dropdown, and it instantly generates a Civil 3D-style longitudinal profile.
It draws the physical pipe walls, plots the manhole shafts, and draws the actual physical water surface inside the pipe based on your calculated $d/D$ ratio.
Export & Report
When your network is perfectly sized and all your velocities hit the self-cleansing target, you are ready to report!
Click the Export Report button in the toolbar to download a pristine CSV file containing all your cumulative populations, peaking factors, and final pipe dimensions to attach directly to your engineering submittal.
Built-in Design References & Export
To make this a true one-stop workstation, I included a massive library of reference tables at the bottom of the tool. If you forget the standard Manning's $n$ for PVC, or need to look up the typical per capita flow generation for a hospital or school, it is right there on the page.
Once your network is fully sized and your velocities are safely above the 0.6 m/s self-cleansing limit, click the Export Report button to download a clean CSV file containing your entire routed network data.
Head over to the tool page and let me know how it handles your workflows. If you find a bug or have a feature request, drop a comment below!
Happy Designing!
- CivilSheets
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