4.4.4 Runoff Model Representation

Modeling each bioretention facility and its individual contributing drainage area is preferable for estimating their performance. However, where site layouts involve multiple bioretention facilities, it may be necessary to “lump” the bioretention facilities and their contributing drainage areas in the model. In a lumped modeling approach, multiple bioretention facilities with similar design (e.g., soil depth, ponding depth, freeboard height, side slopes, and drainage area to ponding area ratio) and infiltration rates (within a factor of 2) may have their drainage and ponded areas summed and represented in the model as one drainage area and one bioretention facility. In this case, a weighted average on the longterm design native soil infiltration rates at each location may be used. The averages are weighted by the size of their drainage areas.

Using one of the procedures listed in Section Determining subgrade & bioretention soil media design infiltration rates, estimate the initial measured (e.g., short-term) infiltration rate of the native soils beneath the bioretention soil and any base materials. Because these soils are protected from fouling, no correction factor need be applied.

If using the default bioretention soil mix from Section Determining subgrade & bioretention soil media design infiltration rates, 6 inches per hour is the initial infiltration rate. The long-term rate is either 1.5 inches per hour or 3 inches per hour depending on the size of the drainage area into the bioretention facility. If using a custom imported soil mix other than the default, its saturated hydraulic conductivity (used as the infiltration rate) must be determined using the procedures described in Section Determining subgrade & bioretention soil media design infiltration rates. The long-term infiltration rate is ¼ or ½ of that rate, depending on the size of the drainage area.

If the long-term infiltration rate through the imported bioretention soil is lower than the infiltration rate of the underlying soil, the surface dimensions and slopes of the facility should be entered in to the model.

When defining the available storage in the bioretention facility, account for the storage in the voids of the bioretention soil mix and in the ponding zone. The procedure to estimate storage volume should account for the effects of longitudinal slope when the bioretention facility has greater than a 1 percent slope.

If designs include an under-drain, the designer must account for the reduced (or eliminated) infiltration benefits of the system when modeling flow control and runoff treatment performance. If using level-pool routing, as demonstrated in Appendix C: Sizing of LID Facilities, the stage-storage-discharge relationship would be redefined to include the ponding, bioretention soil mix, and under-drain layers for estimation of the stage-volume relationship. Infiltration would be reduced to account only for infiltration beneath the under-drain pipe invert elevation (or set to zero), and flow through the under-drain would be included in the stage-discharge relationship. Because discharge through the under-drain would be added to the total discharge from the site, the flow control benefits would be lower than corresponding designs without under-drains.