4.5.4 Runoff Model Representation

4.5.4.1 Retained trees
Where locally allowed, runoff modeling adjustments for retained trees can be applied to reduce impervious surface areas that are entered into hydrologic models used to size flow control facilities (See Chapter 4 of the SWMMEW and Appendix C: Sizing of LID Facilities of this Manual). Adjustments are provided as a percentage of the existing tree canopy area. The minimum tree canopy necessary for an existing tree is identified in Table 4.5.1.



Runoff modeling adjustments are not applicable to trees in native vegetation areas used for flow dispersion. Runoff modeling adjustments are also not applicable to trees in planter boxes. The total runoff modeling adjustment for retained trees may not exceed 25 percent of impervious or other hard surface requiring mitigation.

4.5.4.2 Newly planted trees

Runoff modeling adjustments for newly planted trees are provided in Table 4.5.2 by tree type. Where locally allowed, runoff modeling adjustments for new trees may be applied to reduce the impervious surface areas that are entered into hydrologic models used to size flow control facilities (See Chapter 4 in the 2004 SWMMEW and Appendix C: Sizing of LID Facilities in this Manual). Runoff modeling adjustments are not applicable to trees in native vegetation areas used for flow dispersion. Runoff modeling adjustments are also not applicable to trees in planter boxes. The total runoff modeling adjustment for newly planted trees should not exceed 25 percent of impervious or other hard surface requiring mitigation.

Table4-5-2

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