VOSWMM uses different tools and methods to model urban runoff water quality. Calibration data is essential for realistic results. Here’s a brief overview of how VOSWMM handles water quality.
Pollutants, which are user-defined contaminants, accumulate on surfaces and are carried away during runoff in VOSWMM. The software can simulate the generation, washoff, and transport of multiple user-defined pollutants, each identified by name and concentration units. You can input concentrations from external sources, like rain or groundwater, and VOSWMM calculates concentrations generated by runoff. It also allows establishing dependencies between pollutant concentrations, like making lead a constant fraction of suspended solids concentration.
Land uses represent different activities (like residential, commercial, industrial) within a subcatchment, affecting pollutant generation. They show variations in pollutant buildup/washoff rates and the impact of street cleaning, if applied. A subcatchment can have multiple land uses, independent of pervious/impervious division. The percentages of land uses don’t have to total 100%, and unassigned areas are assumed to not contribute to pollutant loads.
The buildup function for a specific land use defines how pollutants accumulate on the land surface during dry weather, ready for washoff during runoff. The total buildup in a subcatchment can be expressed as mass per area or curb length. VOSWMM provides three options (power, exponential, saturation) to simulate buildup, customizable with parameters for various buildup behaviors.
Alternatively, for single-event simulations, you can set an initial pollutant loading across the subcatchment, overriding any previous buildup calculations.
Washoff is the removal of pollutants from a subcatchment surface during rain. In VOSWMM, you can represent this process for each pollutant and land use using three options: event mean concentrations (EMCs), rating curves, and exponential functions. Details on these functions can be found in the SWMM 5 Users Manual.
- EMC assumes constant pollutant runoff concentration.
- Rating curves depend only on runoff rate, yielding consistent washoff regardless of when it occurs.
- Exponential curves factor in both runoff rate and remaining pollutant on the surface.
- Buildup functions aren’t needed with EMCs or rating curves. If used with any washoff function, buildup decreases continually until exhausted, stopping washoff.
- Rating curves produce higher pollutant loads at storm end compared to exponential curves. This is especially notable in large storms where much buildup washes off early.
Pollutants, when washed off the subcatchment surface, enter the conveyance system and are routed according to flow results. In this system, they may decay naturally or undergo reduction at defined treatment nodes.
Pollutant Reduction from Land Surfaces
VOSWMM offers two methods to reduce surface pollutant loads within subcatchments:
- BMP Treatment: This assumes a Best Management Practice (BMP) is in place, reducing the standard washoff load by a fixed fraction. We’ll present this in Example 6, not here.
- Street Sweeping: This can be configured for each land use. It happens before the first storm event and between subsequent events, governed by four parameters: (1) days between sweeps, (2) fraction of buildup available for removal, (3) days since the last sweep at the simulation start, and (4) street sweeping removal efficiency (as a percentage). These parameters are set for each land use, and the fourth one applies to each pollutant as well.