This means that the subcatchment can hold water within its boundaries, and release it over time. In terms of modeling architectures, these time-varying variables are called state variables because they capture the state of the system you are modeling and can be used to calculate the state of the system in a future time step in the absence of external forces.Įach subcatchment in PCSWMM is treated as a nonlinear reservoir. To design stormwater systems, we are usually most interested in the final runoff flowrate.
These variables are calculated internally and are necessary to simulate a physically-sound hydrologic model, but they are not necessarily applied to engineering design.
All of these flows of water into and out of each subcatchment are therefore updated through time, depending on previous and current conditions.
WHAT IS PCSWMM SOFTWARE
Given an input hyetograph (time series of rainfall depth), the software steps through time using small time increments, for example 30 seconds, calculating rainfall, infiltration, storage, and runoff at each time step. So how does PCSWMM predict runoff quantities from each subcatchment? PCSWMM performs a numerical simulation through time to predict runoff. The more subcatchments and drainage detail provided in PCSWMM, the more accurate the results can be however, this comes with the tradeoff of greater time and data input requirements. įigure 2: Three side by side maps of the same watershed divided into varying numbers of subcatchments. While it is important for you to know that you can set up multiple subcatchments within a larger subcatchment area, in this module we will use a single subcatchment and we will not require a network of conduits. The runoff produced from each subcatchment can then be linked together using conduits, which follow the natural channels or storm sewers that exist on site. In PCSWMM, a site or watershed can be modeled as one large area, or more commonly, be divided into many subcatchments. Sign in or register, and enroll in this course to view it. The red triangle is an outfall, where all flow leaves the model.Ĭheck Your Understanding is only accessible to enrolled learners. The yellow lines and blue dashed lines represent conduits which can be storm sewers (yellow) or natural channels (blue dashed). Junctions can represent storm sewer inlets (think a metal grate in a parking lot or a curb inlet on a street) or manholes where pipes are joined together. Each green area is a subcatchment with dashed red lines indicating where the runoff from the subcatchment goes.
SWMM can also simulate the effects of adding detention storage, low impact development (LID) structures such as rain gardens or permeable pavement, or any other stormwater mitigation.įigure 1: Example of SWMM model within PCSWMM įigure 1 provides an illustration of a SWMM model within the PCSWMM software that was introduced in Section 1 of this module. Ultimately SWMM predicts the hydrographs produced at points of interest in a watershed that can be used in the design of stormwater infrastructure. SWMM then routes the flow coming out of each of these subcatchments into a collection of channels and pipes, thereby solving the hydraulics of stormwater. SWMM performs numeric calculations for predicting the flow produced from small areas of land, called subcatchments, given rainfall inputs and site characteristics. SWMM has a vast array of modeling capabilities but we will focus on the dynamic simulation of the rainfall-runoff process. The stormwater management model (SWMM) is an open-source software maintained by the USEPA for the design and analysis of stormwater infrastructure The current version of SWMM is SWMM 5.0, with the first version dating back to 1971. About This SectionThe written content and questions presented in this section were adapted from another HydroLearn module that also uses the SWMM model within Rational Method to PCSWMM: Matching Models to SituationsRead through the written description, respond to the questions to check your understanding, and then watch the video produced by CHI - the company that developed the software - that provides a nice overview of the mathematical structure of the model.
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