5.4.3 Preserving Native Vegetation

5.4.3.1 BMP Description

Preserving native vegetation (BMP T5.40 in Volume V, Chapter 2 of the Ecology Manual) on site to the maximum extent practicable will minimize the impacts of development on stormwater runoff. Preferably 65% or more of the development site shall be protected for the purposes of retaining or enhancing existing forest cover and preserving wetlands and stream corridors. See Title 17 KCC for native vegetation preservation where applicable.

5.4.3.2 Performance Mechanism

Forest and native growth areas allow rainwater to naturally percolate into the soil, recharging ground water for summer stream flows and reducing surface water runoff that creates erosion and flooding. Conifers can hold up to about 50% of all rain that falls during a storm. Twenty to thirty percent of this rain may never reach the ground but evaporates or is taken up by the tree. Forested and native growth areas also may be effective as stormwater buffers around smaller developments.

5.4.3.3 Applications and Limitations

New development often takes place on tracts of forested land. In fact, building sites are often selected because of the presence of mature trees. However, unless sufficient care is taken and planning done, in the interval between buying the property and completing construction much of this resource may be destroyed. The property owner is ultimately responsible for protecting as many trees as possible, with their understory and groundcover. It takes 20 to 30 years for newly planted trees to provide the benefits for which trees are so highly valued.

On lots that are 1 acre or greater, preservation of 65% or more of the site in native vegetation will allow the use of full dispersion techniques presented in Vol II–5.4.4 Dispersion BMPs. Sites that can fully disperse are not required to provide runoff treatment or flow control BMPs. See Table II-5.7 for a summary of which minimum requirements this BMP can be used to fully or partially satisfy.

Table II-5.7. Preserving Native Vegetation Applicability.

BMP	MR #5: On-site Stormwater Management		MR #6: Runoff Treatment				MR #7: Flow Control
	List	LID Performance Standard	Basic	Enhanced	Oil Control	Phosphorus
Preserving Native Vegetation		X					X

5.4.3.4 Site Considerations

Review and characterize the existing site vegetation, per Vol I–2.1.7 Vegetation, to identify valuable existing vegetation, like mature trees, that shall be preserved wherever feasible.

5.4.3.5 Design Information

Wherever feasible, designs shall incorporate the following with respect to preserved native vegetation areas:

• The preserved area shall be situated to minimize the clearing of existing forest cover, to maximize the preservation of wetlands, and to buffer stream corridors.

• The preserved area shall be placed in a separate tract or protected through recorded easements for individual lots.

• If feasible, the preserved area shall be located downslope from the building sites, since flow control and water quality are enhanced by flow dispersion through duff, undisturbed soils, and native vegetation.

• The preserved area shall be shown on all property maps and shall be clearly marked during clearing and construction on the site.

5.4.3.6 Operations and Maintenance Requirements

Vegetation and trees shall not be removed from the natural growth retention area, except for approved timber harvest activities and the removal of dangerous or diseased trees.