HydroCAD® Stormwater Modeling - Since 1986
Land-use reporting provides a detailed break-down of how the land within a given project is being used. This can include industrial, residential, and other uses. Land-use reporting is frequently used in conjunction with pollutant loading calculations, as described below.
Note: Land-use reporting was introduced in HydroCAD-9.0, and is not available in earlier versions.
Setting up Land-Use Reporting
Land-Use reporting is configured on the Settings|Land-Use screen. The Land-Uses tab is used to view the land-use catalog for the current project. The initial catalog will contain all land-uses from the current curve-number lookup table, plus any additional land uses that may be present in the current project. (See sample at right.)
You may delete, rename, or add land-uses as required for your specific purposes. (See the right-click menu for related options.) Note that if you delete or rename a land-use that is already assigned to a sub-area, the sub-area assignments are not automatically changed.
The table is also used to define pollutant concentrations. To define a new pollutant, click Add Pollutant and enter the appropriate concentration for each land-use. Blank cells are equivalent to zeros. To import pollutant data from another project, click Import Pollutants and select the desired project file. This will replace any current pollutant definitions. To rename or delete a pollutant, right-click the desired column and select an operation from the menu.
You can also use the right-click menu to save or load from a spreadsheet CSV file. When loading from a file, the pollutant names (columns) must appear in the same order as the spreadsheet columns, otherwise the concentrations will be assigned to the wrong pollutant.
Pollutant loading calculations are used to estimate the quantity of various pollutants that will occur in site runoff over a given period of time. This procedure is typically used to calculate the total weight of Phosphorus, Nitrogen, and Suspended Solids (TP, TN, & TSS) that will be discharged from a site over an entire year. However, the method is equally applicable to other time periods and pollutants.
To simplify the calculation of pollutant loading, each sub-area is assigned to a specific land-use category, and pollutant concentrations are defined for each category. Pollutant loads are calculated according to the equation:
L = R C A (eq.1)
For single-event analysis, loading can be calculated based on the actual runoff depth for each subcatchment. To calculate loading for a longer time period, runoff can also be estimated with the Simple Method, as described below.
The Simple Method is used to estimate the quantity of various pollutants in site runoff over an extended time period. Rather than using the single-event runoff depth, the Simple method uses the following equation to predict runoff over a longer time period:
R = P Pj Rv (eq.2)
The runoff coefficient is calculated by:
Rv = 0.05 + 0.90 I (eq.3)
When evaluating these equations by hand, care must be taken to use consistent units throughout. The Simple Method is often applied with the following equation, which consolidates equations 1 and 2 along with the most common conversion factors:
L = P Pj Rv C A 0.2266 (eq.4)
HydroCAD provides a number of land-use and pollutant loading reports, ranging from subcatchment detail to project-wide analysis. Several of these are available on the Project Reports window:
When applying the Simple Method to multiple subcatchments and land-use areas, HydroCAD provides several options for how the exact calculations are performed: (see sample screen at right.)
Option 1: Calculate imperviousness for the overall project, and use the resulting (same) runoff coefficient to calculate the pollutant loads for all land-uses.
Option 2: Calculate imperviousness and pollutant loads separately for each land-use, without regard to subcatchment delineation.
Option 3: Calculate imperviousness for each subcatchment, and use the resulting runoff coefficient to calculate the pollutant load for all land-uses within that subcatchment.
Option 4: Calculate imperviousness for each land-use within each subcatchment, and use the resulting runoff coefficient to calculate loading for that land-use and subcatchment.
Option 4 is recommended for maximum accuracy. The other options are provided for compatibility with pre-existing calculation procedures.
Option 5: This option uses the actual runoff depth as calculated by HydroCAD, rather than the runoff predicted by the Simple Method. Pollutant loading is calculated by equation 1 above.
What Pollutant Concentrations Should I Use?
Your local stormwater regulations should specify what pollutants you need to study, as well as appropriate concentrations for each land-use. Although there have been many studies of pollutant concentrations, there is considerable variation in the results, so there are no "standard" values.
If local rules and procedures are not available, the following documents may be of interest:
To see samples of the land-use reporting and pollutant loading calculations offered by HydroCAD, open the "Mountain View" project that is pre-installed with HydroCAD. Then click the "Land Use" button in the right-hand toolbar and make sure both of the reporting buttons are checked.
The pollutant loading analysis will now appear in each subcatchment summary. To see the project-wide pollutant analysis, select "Project Reports" from the toolbar and click the desired tab.
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