3 Steps in Finding and Eliminating Sanitary Sewer I&I using Dual Drainage Analysis in VO SWMM

Dual Drainage Analysis

Background Knowledge

‘Dual’ drainage describes the simultaneous flow of stormwater on the road surfaces and storm sewers. Storm sewers provide the convenience of removing the stormwater during frequent storms, up to a certain point. Storm sewer capacities are typically designed to carry peak flows up to 1-in-5-year return period storms. With larger storms, the overland system carries the balance. Urban drainage design and analysis calculates the overland and sewer flow division according to inlet capacities.

Modern drainage standards separate storm sewers and sanitary sewers. Sanitary sewers carry sewage to treatment plants while storm sewers convey stormwater to stormwater management ponds and Low-Impact-Development (LIDs) technologies for treatment before discharging to rivers and lakes.

Both storm and sanitary sewer systems include “manholes” to access the underground pipes for inspection and maintenance. Engineers account for a nominal amount of stormwater inflow from various sources during sanitary sewer design. This ‘allowance’ of stormwater from multiple sources is typically 0.26 L/s/ha (2450 gallons/acre/day).

However, recent hydrologic and hydraulic investigations have shown that stormwater entering sanitary sewers during large storms can far exceed these design allowances. The main cause of this has been identified as stormwater overland flow running overtop of sanitary manholes and pouring through manhole pick holes. While this process interferes with sanitary inflow allowances, this overland flow path created during larger storms is part of the normal stormwater dual drainage design.

Importance of a new Dual Drainage Analysis

A new dual-drainage analysis method now identifies inflow and infiltration (I/I) in sanitary sewer systems due to overland flow exposure at sanitary manhole locations. This method can identify and quantify the inflow from the surface into the sanitary sewers.

The new dual-drainage analysis goes beyond dividing and quantifying the stormwater flow between the overland and sewered systems. The new approach not only identifies manholes (MH) in low-lying areas but also considers hydraulic conditions that lead to MH submergence along streets outside of low-lying areas.

This analysis is crucial for maintaining residual storm and sanitary sewer capacities, preventing basement flooding, and minimizing environmental spills.

Eliminating excessive I/I from sanitary sewer systems reduces pumping and treatment costs while improving effluent quality at wastewater pollution control plants.

Application in Simulation Modelling

Here are three steps that VO SWMM uses to empower municipal drainage managers to find and eliminate sanitary sewer I/I through Dual Drainage Analysis:

1. Vulnerability Assessment: The approach involves the use of detailed area topography and road geometry. It identifies locations most susceptible to surface water ponding and higher flow depths with the greatest potential for inflow at vulnerable locations.
2. Capture Curve Projection: It quantifies the condition at MHs vulnerable to submergence, using capture curves that project peak and volume of I/I. These curves assist in prioritizing MH sealing.
3. Dual Drainage Modelling: VO SWMM conducts dual drainage modelling to predict surface water flow over roads and into storm sewers under various storm conditions.

VO SWMM models consider runoff, capture capacities, and sewer conditions to predict submergence depth-duration curves at MH locations. This provides unparalleled insights to make informed decisions about optimal system improvements.

Dual Drainage Analysis with VO SWMM optimizes sanitary sewer systems by assessing for potential sources of I/I, ultimately reducing municipal wastewater management costs and environmental impact.

For more on Dual Drainage Modelling, check out SCW’s 2-part webinar series:

Dual Drainage Modelling

Experience the simplicity and power of VO as you create accurate models.

To request a VO license, click here