C.12 LID Control Editor
The LID Control Editor is used to define a low impact development 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 of different sizes or number of replicates. The editor contains the following data entry fields:
· Control Name: A name used to identify the particular LID control.
· LID Type: The generic type of LID being defined (bio-retention cell, rain garden, green roof, infiltration trench, permeable pavement, rain barrel, or vegetative swale). Process Layers: These are a tabbed set of pages containing data entry fields for the vertical layers and underdrain that comprise an LID control. They include some combination of the following, depending on the type of LID selected: Surface Layer, Pavement Layer, Soil Layer, Storage Layer, and Drain System or Drainage Mat.
The Surface Layer page of the LID Control Editor is used to describe the surface properties of bio-retention cells, porous pavement, infiltration trenches, and vegetative swales. These properties are:
· 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 LIDs 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.
· Surface Roughness: Manning's n for overland flow over the surface of porous pavement or a vegetative swale. Use 0 for other types of LIDs.
· Surface Slope: Slope of porous pavement surface or vegetative swale (percent). Use 0 for other types of LIDs.
· Swale Side Slope: Slope (run over rise) of the side walls of a vegetative swale's cross section. This value is ignored for other types of LIDs.
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.
The Pavement Layer page of the LID Control Editor supplies values for the following properties of a porous pavement LID:
· Thickness: The thickness of the pavement layer (inches or mm). Typical values are 4 to 6 inches (100 to 150 mm).
· 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. Here, 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.
· 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.
The Soil Layer page of the LID Control Editor describes the properties of the engineered soil mixture used in bio-retention types of LIDs. These properties are:
· Thickness: The thickness of the soil layer (inches or mm). Typical values range from 18 to 36 inches (450 to900 mm) for rain gardens, street planters and other types of land-based bio-retention units, but only 3 to 6 inches (75 to 150 mm) for green roofs.
· 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.
· 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.
· Conductivity: Hydraulic conductivity for the fully saturated soil (in/hr or mm/hr).
· 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.
· 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.
The Storage Layer page of the LID Control Editor describes the properties of the crushed stone or gravel layer used in bio-retention cells, porous pavement systems, and infiltration trenches as a bottom storage/drainage layer. It is also used to specify the height of a rain barrel (or cistern). The following data fields are displayed:
· 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).
The following data fields do not apply to Rain Barrels.
· 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).
· Filtration Rate: The maximum rate at which water can flow out the bottom of the layer after it is first constructed (in/hr or mm/hr). Typical values for gravels are 10 to 30 in/hr (250 to 750 mm/hr). If the layer contains a sand bed beneath it, then the conductivity of the sand should be used. If there is an impermeable floor or liner below the layer, then use a value of 0. The actual exfiltration rate through the bottom will be smaller than the normal infiltration rate into the soil below the layer.
· 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.
LID storage layers can contain an optional underdrain system that collects stored water from the bottom of the layer and conveys it to a conventional storm drain. The Underdrain page of the LID Control Editor describes the properties of this system. It contains the following data entry fields:
Drain Coefficient and Drain Exponent: Coefficient C and exponent n 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
q = C(h - Hd )n
Where is outflow (in/hr or mm/hr),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 (inches or mm).
· Drain Delay (for Rain Barrels only): 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 ignored for other types of LIDs.