Designing a Storm Sewer

To the layperson, designing and constructing a storm sewer system may seem incredibly complex. However, this work is usually undertaken by civil engineers who have a solid degree of education and practical experience to match their knowledge. They also have access to a host of modern technologies that make their task easier. We provide a simplified overview of storm sewer design that indicates why we leave it to the professionals.

The Storm Sewer Structure

At its simplest, a storm sewer is a network of closed pipes that join together several surface inlets. The former are placed laterally and are discharged into a trunk line. Surface drainage moves through the surface inlets via a manhole to where the outlet is located. When there is only one inlet and a maximum of two pipes, this is known as a culvert and will always be labeled as such on the plan sheets. You have probably heard of this term. However, this description applies to only one possible configuration for storm sewer designs. Engineers have manuals that provide the requirements for different configurations. As citizens, we can appreciate the knowledge and hard work that goes into storm sewer design.

Design Criteria

The criteria for storm sewer design are bound by set parameters. The parameters provide the limits for potential runoff treatment issues, potential construction problems, pipe strength, soil characteristics, hydraulic grade line, pipe flow capacity, and runoff rates. 

These factors start to give a clearer picture of the rigors of the job. Additionally, a lot of calculations and formulae are involved. For example, the pipe velocity is worked out by using Manning’s equation, as shown in this video which makes it more accessible to the layperson. Fortunately, there are computer programs that work out all these calculations nowadays.

What we can take from this is that the capacity of a pipe (the amount of water it can hold) is dependent on the diameter of the pipe, its roughness, and its slope. Thus, it is clear that a pipe must have sufficient capacity for the water flow that is expected to move through it. It also needs to accommodate unusual amounts of water in bad weather conditions, such as five-year floods. From the name, it is noted that bigger-than-average floods typically occur every five years. It is logical to build in spare capacity for this but not to account for one-in-a-hundred years floods.

Why the Design Criteria Are Important

A stormwater drainage system is how we design the correct hydraulic configurations for storm sewer lines by calculating their depth, slope, and diameter. A storm sewer system is only functional if it can handle all the water from rains and groundwater, excepting rare weather events. We consider it well-designed and in line with the principles of good stormwater management.

Stormwater Management

A Stormwater Management Plan (SMP) is prepared by every department responsible for managing stormwater in its area of jurisdiction. It is designed to protect road infrastructure so that roads are not swept away or become impassable and to protect the rest of the environment from heavy rains and runoff. It has to take into account the capacity of wetlands, dams, and rivers. Additionally, it is required to limit soil erosion and prevent sedimentation from polluting waterways. Natural habitats must be sustained. 

Each section of storm sewer is part of an integral whole.