Land-Use Reporting
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.
Tip: To ensure that each sub-area is assigned to one of the custom land-use
categories, you should configure the categories before you begin to enter
subcatchment data. This allows each category to be automatically verified
during sub-area entry. If the pre-assigned category has been deleted or
renamed, you will be asked to select another land-use from the custom catalog.
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.
Tip:
The easiest way to define the
pollutant concentrations is to import from a previous project with similar
values. To get started on your first project, you can import from the "Mountain
View" project that is automatically installed with HydroCAD. However,
you must substitute appropriate concentrations for your project. The
values contained in the sample project are for demonstration purposes only and
are not recommendations for actual use.
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
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)
where:
L=Total pollutant load [pounds or kg]
R=Runoff depth [feet or meters]
C=Average Pollutant runoff concentration [pounds or kg per unit volume]
A=Runoff area [square feet or square meters]
Analysis Period
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.
Simple Method
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)
where:
R=Runoff depth [feet or meters]
P=Precipitation depth [feet or meters]
Pj=Fraction
of rainfall events that produce runoff
Rv=Runoff
coefficient
The runoff coefficient is calculated by:
Rv = 0.05 + 0.90 I (eq.3)
where:
I=Impervious
fraction of the runoff area
Hand Calculations
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)
where:
L=Total pollutant load [pounds]
P=Precipitation depth [inches]
Pj, Rv (as above)
C=Pollutant concentration [mg/liter]
A=Runoff area [acres]
Reporting
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:
Determining Imperviousness
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:
New York State Stormwater Management Manual:
The Simple
Method to Calculate Urban Stormwater Loads
Minnesota Stormwater Manual:
Simple Method for Estimating Phosphorous Export
Examples
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.
