The Low Impact Development Controls Manager dialog is used to define a Low Impact Development (LID) control that can be deployed throughout a study area to store, infiltrate, and evaporate subcatchment runoff.

The design of the control is made on a per-unit-area basis so that it can be placed in any number of subcatchments at different sizes or number of replicates. The editor consists of a list pane on the left that displays all of the LID controls that have been defined for the model and a tabbed input data area on the right that displays attributes for the selected LID control.

The buttons in the upper left are as follows:

Low Impact Development Controls Tab

The sections and fields that appear on the Low Impact Development Control tab depend on which control type (see ”LID Control Types’) is chosen. Each section of this tab (e.g., Surface, Storage, etc.) corresponds to a layer in the LID control (see ”Hydrologic Model Representation of LID Controls’). The table below summarizes which layers are available for each LID control:

Available Layers by LID Control

• Low Impact Development Control Type: Allows you to select the type of LID Control.
• Soil Layer Attributes

• Soil Thickness: The thickness of the soil layer.
• Soil Porosity: The volume of pore space relative to total volume of soil (as a fraction).
• Field Capacity: Volume of pore water relative to total volume after the soil has been allowed to drain fully (as a fraction). Below this level, vertical drainage of water through the soil layer does not occur.
• Soil Conductivity: Hydraulic conductivity for the fully saturated soil.
• Conductivity Slope: Slope of the curve of log (conductivity) versus soil moisture content (dimensionless). Typical values range from 5 for sands to 15 for silty clay.
• Wilting Point: Volume of pore water relative to total volume for a well dried soil where only bound water remains (as a fraction). The moisture content of the soil cannot fall below this limit.
• Suction Head: The average value of soil capillary suction along the wetting front (inches or mm). This is the same parameter as used in the Green-Ampt infiltration model. Porosity, field capacity, conductivity and conductivity slope are the same soil properties used for Aquifer objects when modeling groundwater, while suction head is the same parameter used for Green-Ampt infiltration. Except here they apply to the special soil mix used in a LID unit rather than the site's naturally occurring soil.
• Pavement Layer Attributes

The pavement layer attributes are only available when the "Porous Pavement" LID Control Type is selected.

• Pavement Thickness: The thickness of the pavement layer (inches or mm). Typical values are 4 to 6 inches (100 to 150 mm).
• Pavement Void Ratio: The volume of void space relative to the volume of solids in the pavement for continuous systems or for the fill material used in modular systems. Typical values for pavements are 0.12 to 0.21. Note that porosity = void ratio / (1 + void ratio).
• Impervious Surface Fraction: Ratio of impervious paver material to total area for modular systems; 0 for continuous porous pavement systems.
• Permeability: Permeability of the concrete or asphalt used in continuous systems or hydraulic conductivity of the fill material (gravel or sand) used in modular systems (in/hr or mm/hr). The permeability of new porous concrete or asphalt is very high (e.g., hundreds of in/hr) but can drop off over time due to clogging by fine particulates in the runoff.
• Pavement Clogging Factor: Number of pavement layer void volumes of runoff treated it takes to completely clog the pavement. Use a value of 0 to ignore clogging. Clogging progressively reduces the pavement's permeability in direct proportion to the cumulative volume of runoff treated.
• If one has an estimate of the number of years it takes to fully clog the system (Yclog), the Clogging Factor can be computed as: Yclog * Pa * CR * (1 + VR) * (1 - ISF) / (T * VR) where Pa is the annual rainfall amount over the site, CR is the pavement's capture ratio (area that contributes runoff to the pavement divided by area of the pavement itself), VR is the system's Void Ratio, ISF is the Impervious Surface Fraction, and T is the pavement layer Thickness.
• As an example, suppose it takes 5 years to clog a continuous porous pavement system that serves an area where the annual rainfall is 36 inches/year. If the pavement is 6 inches thick, has a void ratio of 0.2 and captures runoff only from its own surface, then the Clogging Factor is 5 x 36 x (1 + 0.2) / 6 / 0.2 = 180.

• Storage Layer Attributes
• The storage layer attributes are available for all LID Control Types except Vegetative Swale.
• Height: This is the height of a rain barrel or thickness of a gravel layer (inches or mm). Crushed stone and gravel layers are typically 6 to 18 inches (150 to 450 mm) thick while single family home rain barrels range in height from 24 to 36 inches (600 to 900 mm).
• Storage Void Ratio: The volume of void space relative to the volume of solids in the layer. Typical values range from 0.5 to 0.75 for gravel beds. Note that porosity = void ratio / (1 + void ratio).
• Storage Conductivity: Hydraulic conductivity for the fully saturated soil (in/hr or mm/hr).
• Storage Clogging Factor: Total volume of treated runoff it takes to completely clog the bottom of the layer divided by the void volume of the layer. Use a value of 0 to ignore clogging. Clogging progressively reduces the Filtration Rate in direct proportion to the cumulative volume of runoff treated and may only be of concern for infiltration trenches with permeable bottoms and no under drains.
• Surface Layer Attributes
• The surface layer attributes are available for all LID Control Types except Rain Barrel.
• Surface Storage Depth: When confining walls or berms are present this is the maximum depth to which water can pond above the surface of the unit before overflow occurs (in inches or mm). For LID controls that experience overland flow it is the height of any surface depression storage. For swales, it is the height of its trapezoidal cross section.
• Vegetative Cover Fraction: The fraction of the storage area above the surface that is filled with vegetation (i.e., volume occupied by leaves, stems, etc.).
• Surface Manning’s n: Manning's n for overland flow over the surface of porous pavement or a vegetative swale. Use 0 for other types of LID controls.
• Surface Slope: Slope of porous pavement surface or vegetative swale (percent). Use 0 for other types of LID controls.
• Swale Side Slope: Slope (run over rise) of the side walls of a vegetative swale's cross section. This attribute is not present for other types of LID controls. If either Surface Roughness or Surface Slope values are 0 then any ponded water that exceeds the storage depth is assumed to completely overflow the LID control within a single time step.
• Underdrain Attributes
• An underdrain is available for all LID control types except Vegetative Swale. It is required for Rain Barrels, and optional for other LID controls.
• Drain Coefficient and Drain Exponent: C and exponent n that determines the rate of flow through the underdrain as a function of height of stored water above the drain height. The following equation is used to compute this flow rate (per unit area of the LID unit):

• where q is outflow (in/hr or mm/hr), h height of stored water (inches or mm), and Hd is the drain height. If the layer does not have an underdrain then set C to 0. A typical value for n would be 0.5 (making the drain act like an orifice). A rough estimate for C can be based on the time T required to drain a depth D of stored water. For n = 0.5, C = 2D1/2/T.
• Drain Offset Height: Height Hd of any underdrain piping above the bottom of a storage layer or rain barrel.
• Drain Delay: The number of dry weather hours that must elapse before the drain line in a rain barrel is opened (the line is assumed to be closed once rainfall begins). This parameter is only available with Rain Barrels.

• Drainage Mat Attributes
• Drainage Mat Thickness: The thickness of the mat or plate (inches or mm). It typically ranges between 1 and 2 inches.
• Drainage Mat Void Fraction: The ratio of void volume to total volume in the mat. It typically ranges from 0.5 to 0.6.
• Drainage Mat Manning's n: This is the Manning's n constant used to compute the horizontal flow rate of drained water through the mat. It is not a standard product specification provided by the manufacturer and must therefore be estimated. Previous modeling studies have suggested using a relatively high value such as from 0.1 to 0.4.

Notes Tab

This tab contains a text field that allows you to enter descriptive notes that will be associated with the currently highlighted LID control.

Library Tab

This tab displays information about the LID control that is currently highlighted in the list pane. If the LID control is derived from an engineering library, the synchronization details can be found here. If the LID control was created manually for this model, the synchronization details will display the message Orphan (local), indicating that the LID control was not derived from a library entry.