Hydrograph Methods
In virtually all cases except a sanitary sewer system with no wet weather inflows, it is necessary to directly enter wet weather flow or perform some type of hydrologic calculation to convert precipitation (or snow melt) into a flow rate. Methods can be described based on how they handle time, whether they apply to nodes, links or catchments, and whether they are based on SWMM or Bentley hydrology methods.
The first modeling decision is whether the analysis is to be conducted over time and needs a hydrograph or if it will be a peak flow analysis using the rational method or a fixed flow.
Calculations for a single flow rate are performed using the rational method [rational method link] in the GVF-rational solver. Steady flows can be specified as a known flow [known flow link], inflow at any node [inflow help] or in the GVF-convex solver as an infiltration on a conduit [infiltration help].
The user can directly input a hydrograph at any node [inflow collection dialog box] or in the case of the GVF-convex solver, along any conduit. These values are independent of and supplemental to precipitation.
To convert precipitation hyetographs into surface flow for a catchment, a variety of hydrologic methods are provide. Methods to calculate hydrographs can be divided into two categories
When SWMM hydrology is used, the catchment runoff method is set to EPA-SWMM Runoff and a loss method must be provided. If the SWMM-RTK method is used, no loss method is required and the runoff appears at a manhole node rather than at a catchment [link SWMM RTK Unit Hydrograph Dialog Box].
When Bentley hydrology is used, the user must first select a hydrograph method which can include Unit Hydrograph, Modified Ration Method [modified rational method] or User Defined Hydrograph [Runoff hydrograph]. If Unit Hydrograph method [unit hydrograph methodology] is selected, the user must select one of the Unit Hydrograph types: the SCS [Soil Conservation Service], RTK [RTK method] or Generic Unit Hydrograph [Generic unit hydrographs]. The SCS and Generic unit hydrograph methods require a loss method (fLoss, Green-Ampt, Horton, or SCS-CN. For most of the hydrograph methods, storm data is needed in the form of a hyetograph (precipitation vs. time). The exception is the modified rational method which is driven from IDF storm data [Storm Data and Runoff Methods].
It is also very important to be aware of whether the flow being calculated are to be used as runoff in a stormwater or combined sewer model or the wet weather inflow/infiltration to a sanitary sewer model. In general, EPA-SWMM and SCS methods are better for surface runoff while RTK method tend to be preferred for sanitary sewer I&I although a calibrated generic unit hydrograph is also acceptable.
Catchment hydrology information is stored in the Hydrology Alternative. Storm data is entered under Components > Storm Data and is stored in the Rainfall-Runoff Alternative.
An outline of the available hydrologic method is given below
- Hydrograph
- User defined hydrograph (conduit, catchment or node)
- SWMM hydrology
- Catchment SWMM runoff
- SWMM node RTK
- Bentley hydrology (catchment)
- Unit Hydrograph
- SCS
- RTK
- Generic
- Modified rational
- User define hydrograph
Other related help includes: