Introduction
In the field of urban stormwater drainage system design and analysis, understanding the difference between surface overland and storm sewered flows plays a pivotal role in evaluating the overall system performance and ensuring the resilience of the overall drainage infrastructure. The simultaneous analysis of both flows is referred to as “Dual-Drainage modelling.” The storm sewer flows are sometimes referred to as the ‘conveyance’ system as it is intended to remove the overland flow and ponding from the frequent storms at specific inlet locations. The overland flow that continues past individual inlets is the ‘major’ surface and overland flow, intended to occur only during infrequent, large storms.
VO SWMM is a hydraulic and hydrologic modelling application capable of analyzing surface and sewer flow (Dual‐Drainage) in urbanized areas. With its advanced Micro-Drainage analysis tools, VO SWMM provides an innovative Dual-Drainage modelling procedure that enhances the precision and speed of dual-drainage analysis. With detailed topographic and infrastructure network information, VO SWMM can predict the unintended consequence of too much inflow into the storm sewers or too little inflow.
This document is part of a series that explores the essential elements of the Dual-Drainage analysis and introduces VO SWMM’s unique and innovative tools. This describes how it predicts the overall storm drainage system performance and the unintended consequences of too little or too many inflows into storm sewers as well as potential impacts to sanitary sewers.
This first blog post describes the basic Dual-Drainage analysis concept.
What is Dual-Drainage Modelling?
The term ‘Dual-Drainage’ typically refers to the simultaneous analysis of storm water flow over roads and storm sewers. These analysis accounts for interactions between both systems as the overland flow is captured into the sewers at inlet locations, often at streetside catch basins.
Like most drainage system capacity analysis during wet-weather conditions, the calculations can be divided into two main elements:
1. Hydrology: Calculation of the flow or excess runoff produced in an area source (typically rain or snowmelt) due to
2. Hydraulics: The calculation of the movement of runoff over the ground surfaces and within the enclosed sewers.
For dual-drainage analysis, we will assume that the first part, the hydrology, is calculated using one of several standard rainfall-runoff methods. Dual-drainage focuses primarily on how the flow moves over the surface (component #1) and inside sewers (component #2), and whether this flow exceeds the capacity of the surface drainage systems (e.g. roads and ditches) or closed-conduits (e.g. sewers).
Component #1: Overland/Major System
As rainfall hits the ground, the water begins flowing down over the land surface and accumulates along the low points in the topography. This overland flow includes runoff from rooftops, grassy areas, sidewalks, boulevards, streets and other pervious and impervious surfaces. In urban areas, the overland, or “major system”, accumulates on the low point in the streets and roads, typically along the ‘gutters’. In VO SWMM, street/road segments are modelled as open channels.
Component #2: Underground Sewer/Minor System
As water continues along the gutters, the low point in the streets, the flow reaches storm sewer inlets, also known as catch basins. Catch basins are made of three main components: the metal grate and frame, the concrete structure containing a sump volume, and a lead that conveys the flows above the sump area to the sewers under the streets. The flow is conveyed along the underground sewer network. In dual-drainage terminology, the sewered flow is referred to as the “minor system”. Understanding (1) how much water is captured into the minor system and (2) how much remains in the major system is crucial for assessing potential impacts on the overall capacity.
By considering both surface and subsurface pathways simultaneously, managers can make informed decisions that optimize the drainage capacity and reduce risks like sewer surcharging and flooding.
An Important Side Note: Unintended Critical Sanitary Inflows
In a future blog post, we will cover one of the unintended consequences of urban storm drainage dual-drainage system designs – that larger stormwater flows along streets can accumulate and reach depths that rise over the sanitary manhole lids.
Most manhole lids have two holes used as pick holes that facilitate the opening to provide access to the underground sewer system. Due to the size and number of pick holes, surface water can rise over the lids and produce inflows through the holes (and other openings) that exceed the capacity allowance of the sanitary sewers. These inflows can cause the sanitary sewers to become surcharged. With other inflows, the surcharging can reach critical levels causing the sanitary sewers to become pressurized, so the sewage backs up into basements or causes spills into the environment.
In response to this challenge, VO SWMM conducts dual-drainage analysis of storm drainage systems and accounts for the sanitary sewer lid locations along the surface. This is the “Manhole I/I analysis” tool. The result is that VO SWMM can identify sanitary lids susceptible to submergence during various storms, allowing managers to select solid manholes lids or fully sealed manhole covers. More on this in a future blog post.
Dual-Drainage Modelling in VO SWMM
In VO SWMM Dual-Drainage can be summarized in five steps. It is not necessary these steps be completed in sequential order:
Step 1: Storm Sewer (Minor System) Network Building
Collect and review storm sewer asset data. The data may be available in GIS or in original engineering design drawings. This data can be imported with a shapefile or created directly within VO.
The image below displays a case study with a storm sewer network created in VO. Storm sewers are displayed with green lines and connecting nodes.
The Parameter table in the bottom panel shows the individual conduit properties. Users can cycle through the other available tabs that display data on other components.
Step 2: Digital Elevation Model (DEM) Processing
DEM Processing extracts and implements key information from a DEM file for the overland system, such as flow paths and sag area locations. This step is critical to achieve an accurate overland system representation.
The sag areas (in light blue) and the flow paths (in dark blue) in the image below have been layered overtop the storm sewer network. In VO SWMM, this is done using the Digital Elevation Model (DEM) Processing tool.
Step 3: Overland (Major System) Building
While the minor system refers to the underground sewer network, the major system refers to building a model of overland flow paths as open channels, utilizing nodes and conduits to replicate natural flow patterns.
As the major system is built in VO SWMM, Sag areas transform into storage units, using the “Create Sag Nodes” tool to secure rating curves. Strategic assignment of catch basins within the overland system, coupled with precise node placement, ensures an authentic representation.
This approach creates a detailed, accurate, and realistic overland system in VO SWMM.
The image below shows the overland system with brown lines. The section in red highlights an individual conduit of the overland system. Individual conduits can be further edited in VO SWMM with the “Shape Editor”. The window shown below displays the overland conduit transection.
Step 4: Major-Minor System Connection
Connect the overland and sewer systems using outlet links. These links, crucial for modelling flow from major to minor nodes, are governed by rating curves derived from capture curves or manhole cover types.
VO SWMM establishes this connection with tools like “Assign Catch Basin” and “Generate Capture Curves”.
The user can select these options within the “Micro-Drainage” ribbon (shown below). VO SWMM allows easy connection between these two systems with a simple click-and-drag. VO will identify the overland node and catch basins to establish the major-minor system connection.
Step 5: Simulation, Results Display, and Flood Analysis
Execute simulations, analyze results, and utilize tools to gain insights into sewer surcharge levels and produce flood mapping based on the overland system model.
In the plot results window (see labeled image below), users can access a list of runs showcasing various graphs, including depth, lateral inflow, and more.
Additionally, dual-drainage modelling with VO SWMM offers simulation summary results, detailed results, and elevation profiles tabs (see image labeled image below), providing users with a diverse range of information.
In VO SWMM, “Surcharge Analysis” and “Flood Mapping” are the tools that provide minor system information on different conduit surcharge conditions such as:
• No surcharge
• Surcharge Below Freeboard 2
• Surcharge Between
• Surcharge Above Freeboard 1
Notably, flood mapping generates a flood plain raster for user reference.
The 5th step completes the modelling cycle. Dual-Drainage modelling in VO SWMM provides engineers and planners with a powerful tool to accurately simulate and analyze the dynamic interaction between overland and underground drainage systems. Harmonizing these systems in a modelling approach produces results that contribute to resilient and sustainable urban water infrastructure.
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