SWMM can use several different kinds of Interface Files that contain either externally imposed inputs, such as rainfall or inflow/infiltration hydrographs; or the results of previously run analyses, such as runoff or routing results. These files can help speed up simulations, simplify comparisons of different loading scenarios, and allow large study areas to be broken up into smaller areas that can be analysed individually. The different types of interface files that are currently available include:
The Rainfall and Runoff Interface files are binary files created internally by SWMM, which can be saved and reused from one analysis to the next.
The rainfall interface file collates a series of separate rain gauge files into a single rainfall data file. Normally a temporary file of this type is created for every SWMM analysis that uses external rainfall data files, and is then deleted after the analysis is completed. However, if the same rainfall data is being used with many different analyses, you can save time by requesting SWMM to save the rainfall interface file after the first run and then reusing this file in subsequent runs.
The rainfall interface file should not be confused with a rainfall data file. The latter are external text files that provide rainfall time series data to rain gauges. The former is a binary file, created internally by SWMM, that processes the rainfall data files used by a project.
The runoff interface file can be used to save the runoff results generated from a simulation run. If runoff is not affected in future runs, you can request that SWMM use this interface file to supply runoff results without having to repeat the runoff calculations again.
Hotstart files are binary files created by SWMM, which contain hydraulic and water quality variables for the drainage system at the end of a run. This data consists of the water depth and concentration of each pollutant at each node of the system, as well as the flow rate and concentration of each pollutant in each link. The hotstart file saved after a run can be used to define the initial conditions for a subsequent run.
Hotstart files can be used to avoid the initial numerical instabilities that sometimes occur under Dynamic Wave routing. For this purpose they are typically generated by imposing a constant set of base flows for a natural channel network, or set of dry weather sanitary flows for a sewer network, over some startup period of time. The resulting hotstart file from this run is then used to initialise a subsequent run where the inflows of real interest are imposed.
It is also possible to use and save a hotstart file in a single run, starting off the run with one file and saving the ending results to another. The resulting file can then serve as the initial conditions for a subsequent run if need be. This technique can be used to divide up extremely long continuous simulations into more manageable pieces.
The RDII Interface file is a text file that contains a time series of rainfall-derived infiltration/inflow flows, for a specified set of drainage system nodes. This file can be generated from a previous SWMM run when Unit Hydrographs and nodal RDII inflow data has been defined for the project, or it can be created outside of SWMM using some other source of RDII data such as through measurements or output from a different computer program. The format of the file is the same as that of the Routing Interface File, where Flow is the only variable contained in the file.
A Routing Interface file stores a time series of flows and pollutant concentrations that are discharged from the outfall nodes of drainage system model. This file can serve as the source of inflow to another drainage system model that is connected at the outfalls of the first model. A Combine utility is available on the File menu, which combines pairs of routing interface files into a single interface file. This allows very large systems to be broken into smaller sub-systems that can be analysed separately, and linked together through the routing interface file. The figure below illustrates this concept.
A single SWMM run can utilise an outflows routing file to save results generated at a system's outfalls; an inflows routing file to supply hydrograph and pollutograph inflows at selected nodes; or both.
RDII Interface files and Routing Interface files have the same text format:
The first line contains the keyword "SWMM5" (without the quotes).
A line of text that describes the file (can be blank).
The time step used for all inflow records (integer seconds).
The number of variables stored in the file, where the first variable must always be flow rate.
The name and units of each variable (one per line), where flow rate is the first variable listed and is always named FLOW.
The number of nodes with recorded inflow data.
The name of each node (one per line).
A line of text that provides column headings for the data to follow (can be blank).
For each node at each time step, a line with:
The name of the node.
The date (year, month, and day separated by spaces).
The time of day (hours, minutes, and seconds separated by spaces).
The flow rate followed by the concentration of each quality constituent.
Time periods with no values at any node can be skipped. An excerpt from an RDII/Routing interface file is shown below.
SWMM5
Example File
300
1
FLOW CFS
2
N1
N2
Node Year Mon Day Hr Min Sec Flow
N1 2002 04 01 00 20 00 0.000000
N2 2002 04 01 00 20 00 0.002549
N1 2002 04 01 00 25 00 0.000000
N2 2002 04 01 00 25 00 0.002549