To understand the formulas and calculations used by SWMM for runoff, infiltration, and other parameters, I recommend referring to the SWMM 5 reference manuals. These manuals provide detailed explanations of the underlying equations and methodologies employed by the software.
The two main reference manuals for EPASWMM 5 are:
1. "Storm Water Management Model Reference Manual Volume I - Hydrology": This manual covers the hydrologic processes simulated by SWMM, including rainfall, evaporation, infiltration, and runoff generation. It provides the mathematical equations and descriptions of the various methods available in SWMM for modeling these processes.
2. "Storm Water Management Model Reference Manual Volume II - Hydraulics": This manual focuses on the hydraulic components of SWMM, such as flow routing through the drainage network, flow in open channels, and flow in closed conduits. It explains the equations and numerical methods used for hydraulic calculations.
You can find these manuals on the official EPA SWMM website: https://www.epa.gov/water-research/storm-water-management-model-swmm
For your specific interest in runoff calculation and infiltration, here's a brief overview:
Runoff Calculation:
SWMM calculates runoff using a nonlinear reservoir model. It treats each subcatchment as a rectangular surface that receives inflow from precipitation and generates outflow (runoff) once the depth of water on the surface exceeds the maximum depression storage. The key equation for runoff generation is the Manning's equation, which relates the flow rate to the surface roughness, slope, and water depth.
Infiltration Calculation:
SWMM offers three different methods for modeling infiltration: Horton's equation, Green-Ampt method, and Curve Number method. Each method has its own set of equations and parameters to estimate the infiltration rate based on soil characteristics, antecedent moisture conditions, and rainfall intensity.
1. Horton's Equation: It assumes that the infiltration rate decreases exponentially from an initial maximum rate to a minimum rate over the course of a long rainfall event.
2. Green-Ampt Method: This method assumes that a sharp wetting front exists in the soil column, separating soil with some initial moisture content below from saturated soil above.
3. Curve Number Method: This method is an empirical approach developed by the NRCS (formerly SCS) that estimates infiltration based on the soil's infiltration capacity, land use, and antecedent moisture conditions.
I recommend going through the reference manuals to gain a deeper understanding of these methods and their associated equations. The manuals provide step-by-step explanations and examples that will help you comprehend how SWMM performs these calculations.
Additionally, you can explore the source code of EPASWMM 5, which is available on GitHub: https://github.com/USEPA/Stormwater-Management-Model
The source code can give you further insights into the implementation of these equations within the software.
If you have any specific questions or need further assistance, feel free to ask. Good luck with your project!
