2122 106TH ST SW 2025-03-12 - Laserfiche WebLink

BMP T5.16: Tree Retention and Tree Planting Purpose and Trees provide flow control via interception, transpiration, and increased Definition infiltration. Additional environmental benefits include improved air quality, carbon sequestration, reduced heat island effect, pollutant removal, and habitat preservation or formation. When implemented in accordance with the criteria outlined below, retained and newly planted trees receive credits toward meeting flow control requirements. The degree of flow control provided by a tree depends on the tree type (i.e., evergreen or deciduous), canopy area, and whether or not the tree canopy overhangs impervious surfaces. Flow control credits may be applied to project sites of all sizes. Tree Retention Setbacks of proposed infrastructure from existing trees are critical Design Criteria considerations. Tree protection requirements limit grading and other disturbances in proximity to the tree. Existing tree species and location must be clearly shown on submittal drawings. Trees must be viable for long-term retention (i.e., in good health and compatible with proposed construction). Tree size: To receive flow control credit, retained trees shall have a minimum 6 inches diameter at breast height (DBH). DBH is defined as the outside bark diameter at 4.5 feet above the ground on the uphill side of a tree. For existing trees smaller than this, the newly planted tree credit may be applied. The retained tree canopy area shall be measured as the area within the tree drip line. A drip line is the line encircling the base of a tree, which is delineated by a vertical line extending from the outer limit of a tree's branch tips down to the ground. If trees are clustered, overlapping canopies are not double counted. Tree location: Flow control credit for retained trees depends upon proximity to ground level impervious or other hard surfaces. To receive a credit, the existing tree must be on the development site and within 20 feet of new and/or replaced ground level impervious or other hard surfaces (e.g., driveway or patio) on the development site. Distance from impervious or other hard surfaces is measured from the tree trunk center. An arborist report may be required if impervious surface is proposed within the critical root zone of the existing tree. The critical root zone is defined as the line encircling the base of the tree with half the diameter of the dripline. If the arborist report concludes that impervious surface should not be placed within 20 feet of the tree and canopy overlap with impervious surface is still anticipated given a longer setback, the higher tree flow control credit may be approved. Volume V — Runoff Treatment BMPs — December 2014 5-27