Introduction
In dual-drainage modelling, accurately representing the overland (or “major”) system is essential. This system simulates surface flow paths using nodes and conduits, with sag areas acting as storage. It handles larger storm events and complements the underground (or “minor”) system.
Traditionally, modellers mirrored the minor system to set up the major system, but this approach often missed surface flow complexities, especially at intersections. Without Digital Elevation Model (DEM) data, it becomes difficult to capture natural flow patterns. Integrating DEM data improves accuracy by reflecting how water truly moves across the terrain during heavy rainfall.
VO SWMM enhances this process by incorporating natural topography with DEM data, resulting in a more precise representation of surface flow and sag areas. Key tools in VO SWMM include:
1. Fill Sinks: Removes pits in the DEM to ensure smooth flow across the surface, preventing water from being trapped in surface depressions.
2. Build Walls: Redirects flow paths around buildings by incorporating structures into the DEM, ensuring realistic overland flow patterns.
3. Flow Path Tool: Generates flow direction and accumulation layers, helping modellers align nodes and conduits with natural drainage patterns.
4. Sag Layer Tool: Identifies potential sag areas (water collection points) using the DEM, allowing these areas to be modeled as storage units.
5. Create Sag Node Tool: Converts sag areas into nodes and generates depth-area curves, simulating real-world water accumulation and overflow in drainage models.
Together, these tools enable more precise and realistic overland flow simulation, making the process faster and more accurate.
Tool #1: Fill Sinks
The “Fill Sinks” tool is a crucial step in refining the DEM for accurate modelling, eliminating the need for additional GIS software.This tool corrects small imperfections, like pits where flow would otherwise get trapped due to lower elevation. By filling these pits, it ensures smooth surface flow.
This tool corrects small imperfections, like pits where flow would otherwise get trapped due to lower elevation. By filling these pits, it ensures smooth surface flow.
Once processed, the updated DEM automatically reflects on the map, providing a more accurate base for further drainage modelling.
Tool #2: Build Walls
The “Build Walls” tool in VO SWMM prevents flow paths from passing through buildings, which act as barriers to water flow.
In the examples below, some flow paths incorrectly pass through buildings. After using the tool, flow paths are properly redirected around buildings, ensuring the DEM reflects their presence.
Without this tool, the DEM may not recognize buildings as barriers since it only represents elevation data. The “Build Walls” tool integrates infrastructure into the DEM, improving the accuracy of the overland flow model, especially in built environments.
Tool #3: Flow Path
The “Flow Path” tool in VO SWMM is essential for setting up overland modelling by using the DEM to generate a flow path layer, which accurately simulates surface water movement.
This tool generates flow direction and accumulation layers, helping modellers align drainage models with natural water movement. By guiding the placement of nodes and conduits, the tool ensures more reliable simulations of overland flow.
In Figure 2 below, blue lines show the flow paths generated, while buildings (in red) act as barriers. These flow paths help align the overland system with natural drainage patterns.
Tool #4: Create Sag Layer
The “Sag Layer” Tool in VO SWMM automatically identifies sag areas using the Digital Elevation Model (DEM). Sag areas are locations where water is likely to accumulate. This tool simplifies the process of modelling these areas by identifying potential storage units in the drainage system, which can simulate the behavior of water during storm events. If specific data on ponds is unavailable, the Sag Layer Tool can model these areas as ponds, enhancing the realism and accuracy of the simulation.By plotting Length and Full Depth for conduits, along with Full Depth for nodes, this feature provides a clear, graphical view of your system’s profile, helping to identify potential issues and ensure that your design meets all necessary specifications.
By identifying sag areas and converting them into sag nodes, we can develop depth-area curves that more accurately represent the ponding areas.
To convert a sag layer into a sag node and generate the Depth-Area Curve, VOSWMM provides the “Create Sag Node” tool.
Tool #5: Create Sag Node
The “Create Sag Node” Tool generates sag nodes from identified sag areas, representing them as storage units within the drainage model. These nodes simulate how sag areas hold and release water based on varying depths, transitioning between weir flow at shallow depths and orifice flow at greater depths. The tool also creates depth-area curves, which reflect how much water a sag area can store before overflowing into the drainage system. This automation of sag node creation ensures detailed and reliable modelling of overland flow.
VO SWMM offers more tools to simplify the creation of overland systems tools, including:For more information, explore the Surcharge Analysis section of the VO SWMM manual.
1. Add Node/Link: Easily add nodes or links and specify their types.
2. Create Delineation: Separate catchments and connect flow paths to a Major Node.
3. Merge Overland Nodes: Efficiently merge two or more overland nodes.
4. Imperviousness Calculation: Calculate the imperviousness percentage of subcatchments using layers like roofs and roads.
5. DEM Transect: Assign road transects directly from the DEM to a major system conduit.
6. Road Transect: Combine DEM and survey data for accurate road transects..
7. Fix Irregular Transect: Correct shallow or irregular transects.
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