Rain Barrel LID Control for InfoSWMM and InfoSWMM Sustain w/ SWMM5

This is an introduction along with images of the LID Control and data require3ments in InfoSWMM. Every InfoSWMM Control uses from one to 5 layers of data - each with different data requirements. You can use the Siting Manger of InfoSWMM Sustain to find LID locations and the LID Optimizer to find the optimized number of units, cost of units, area and thickness of the LID layers based on your runoff and water quality control objectives.

The Rain Barrel LID Control has five parameters:

· The Rain Barrel Height in inches,

· A drain coefficient in inches per hour or millimeters per hour,

· A drain exponent (dimensionless),

· A Drain Offset Height (inches or millimeters) and a

· Drain Delay (hours)

The Surface Process Layer of the Rain Barrel consists of:

· The Storage Depth in inches or millimeters

· The Vegatative Cover Fraction (0 to 1),

· Surface Roughness (Manning’s n)

· Surface Slope (percent).

The Underdrain Process Layer of the Rain Barrel consists of:

· Drain Coefficient (inches/hour or millimeters/hour)

· The Void Ratio as a Fraction (0 to 1),

· Drain Exponent,

· Drain Offset Height (inches or millimeters)

The two layers used in a simulation for a Rain Barrel LID are shown in the following image.

Excerpt from the EPA manual Storm Water Management Model Reference Manual Volume III – Water Quality (PDF) which can be found here

6.5.1 Rain Barrels Parameter Values

The Rain Barrel LID control can be used to model both rain barrels and cisterns. Rain barrels are typically 50 to 100 gallons in capacity and are used at individual home lots to collect roof runoff for possible landscape irrigation. Cisterns have much larger capacity, typically from 250 to 30,000 gallons, used to harvest rainwater from both homes and commercial facilities for non- potable indoor use. The parameters required for Rain Barrels/Cisterns are the height of the storage vessel (D3), its volume (from which its surface area ALID can be derived), its drain parameters, and possibly its drain delay time.

The height and volume of the rain barrel/cistern would be determined by commercially available sizes. The drain offset is typically 6 inches from the bottom (to trap sediment). Alternatively, one could use an offset of 0 and reduce the vessel height accordingly.

The drain flow parameters can be established from the orifice equation (Equation 6-38). The flow exponent would be 0.5 and the flow coefficient would be 4.8 times the ratio of the drain diameter to the barrel diameter squared. The latter quantity has units of ft0.5/sec. To convert to the in0.5/hr (or mm0.5/hr) used in SWMM’s input data set multiply by 12,471 (or 62,768).

As an example, a 2-foot diameter rain barrel with a 3/4 inch spigot would have a drain flow coefficient of 4.8 ´ (0.75 / (2´12))2 ´ 12,471 = 58.5 in0.5/hr. This is high enough to drain 4 feet of captured water (94 gallons) in less than 15 minutes. A slower release rate for landscape irrigation can be achieved by leaving the spigot valve only partially open or by using a soaker hose. This action can be simulated by using a reduced drain diameter when computing a drain flow coefficient.

The drain delay time is the period of time after rainfall ceases until the rain barrel is allowed to drain. If the delay time is set to 0 then the drain line is considered to be always open. This option might be appropriate for modeling rainwater harvesting with larger cisterns. Otherwise a choice of delay time will depend on what assumptions one makes about homeowner behavior.