Land Uses

Land Uses are categories of development activities or land surface characteristics assigned to subcatchments. Examples of land use activities are residential, commercial, industrial, and undeveloped. Land surface characteristics might include rooftops, lawns, paved roads, undisturbed soils, etc. Land uses are used solely to account for spatial variation in pollutant buildup and washoff rates within subcatchments. 

The GeoSWMM user has many options for defining land uses and assigning them to subcatchment areas. One approach is to assign a mix of land uses for each subcatchment, which results in all land uses within the subcatchment having the same pervious and impervious characteristics. Another approach is to create subcatchments that have a single land use classification along with a distinct set of pervious and impervious characteristics that reflect the classification.

The following processes can be defined for each land use category: 

• Pollutant buildup

• Pollutant washoff

• Street cleaning

Pollutant Buildup

Pollutant buildup that accumulates within a land use category is described (or “normalized”) by either a mass per unit of subcatchment area or per unit of curb length. Mass is expressed in pounds for US units and kilograms for metric units. The amount of buildup is a function of the number of preceding dry weather days and can be computed using one of the following functions: 

• Power Function: Pollutant buildup (B) accumulates proportionally to time (t) raised to some power, until a maximum limit is achieved,

$B = M \ln(C_1 C_2 t^{C_3})$ Here $C_1$ = maximum buildup possible (mass per unit of area or curb length), $C_2$ = buildup rate constant, and $C_3$ = time exponent

• Exponential Function: Buildup follows an exponential growth curve that approaches a maximum limit asymptotically, 

$B = C_1 \left(1 - e^{-C_2 t}\right)$ Here $C_1$ = maximum buildup possible (mass per unit of area or curb length) and $C_2$ = buildup rate constant (1/days).

• Saturation Function: Buildup begins at a linear rate that continuously declines with time until a saturation value is reached,

$B = \frac{C_1 t}{C_2 + t}$ Here $C_1$ = maximum buildup possible (mass per unit area or curb length) and $C_2$ = half-saturation constant (days to reach half of the maximum buildup).

• External Time Series: This option allows one to use a Time Series to describe the rate of buildup per day as a function of time. The values placed in the time series would have units of mass per unit area (or curb length) per day. One can also provide a maximum possible buildup (mass per unit area or curb length) with this option and a scaling factor that multiplies the time series values.

Pollutant Washoff

Pollutant washoff from a given land use category occurs during wet weather periods and can be described in one of the following ways:

• Exponential Washoff: The washoff load (W) in units of mass per hour is proportional to the product of runoff raised to some power and to the amount of buildup remaining,

$W = C_1 Q^{C_2} B$ Here $C_1$= washoff coefficient, $C_2$ = washoff exponent, q= runoff rate per unit area (inches/hour or mm/hour), and $B$ = pollutant buildup in mass units. The buildup here is the total mass (not per area or curb length) and both buildup and washoff mass units are the same as used to express the pollutant's concentration (milligrams, micrograms, or counts).

• Rating Curve Washoff: The rate of washoff W in mass per second is proportional to the runoff rate raised to some power, 

$W = C_1 Q^{C_2}$ Here $C_1$ = washoff coefficient, $C_2$ = washoff exponent, and $Q$ = runoff rate in user-defined flow units.

• Event Mean Concentration: This is a special case of Rating Curve Wash-off where the exponent is 1.0 and the coefficient C 1 represents the washoff pollutant concentration in mass per liter (Note: the conversion between user-defined flow units used for runoff and liters is handled internally by GeoSWMM).

Note that in each case, buildup is continuously depleted as washoff proceeds, and washoff ceases when there is no more buildup available.

Washoff loads for a given pollutant and land use category can be reduced by a fixed percentage by specifying a BMP Removal Efficiency that reflects the effectiveness of any BMP controls associated with the land use. It is also possible to use the Event Mean Concentration option by itself, without having to model any pollutant buildup at all.

Street Sweeping

Street sweeping can be used on each land use category to periodically reduce the accumulated buildup of specific pollutants. The parameters that describe street sweeping include: 

• Days between sweeping

• Days since the last sweeping at the start of the simulation 

• The fraction of buildup of all pollutants that is available for removal by sweeping 

• The fraction of available buildup for each pollutant removed by sweeping 

Note that these parameters can be different for each land use, and the last parameter can vary also with pollutant.