3.5
The thresholds triggering specific minimum requirements are based on the amount of the project’s new and replaced hard surface and converted native vegetation. Note that open, uncovered retention or detention facilities shall not be considered as hard surfaces for the purposes of determining whether the minimum requirement thresholds are exceeded. However, these facilities shall be considered hard surfaces for the purposes of stormwater facility sizing. Permeable pavement, vegetated roofs and areas with underdrains (e.g., playfields, athletic fields, rail yards) shall be considered as hard surfaces for the purposes of determining whether the minimum requirement thresholds are exceeded.
Refer to Chapter 12.08 KCC and Appendix A for detailed definitions of these key terms. The amount of native vegetation that is removed and replaced with lawn, landscaping, and pasture groundcover must also be calculated. New plus replaced hard surface areas and converted native vegetation shall be quantified separately for work within and outside the right of way.
Figure I-3.1 illustrates an example of how to determine new and replaced hard surfaces for a hypothetical Single Family Residential redevelopment project. In this example, the existing single-story house (30′ x 50′, or 1,500 square feet existing roof area) will be demolished and replaced with a two-story house (40′ x 70′ = 2,800 square feet of new and replaced roof area). In order to calculate the new plus replaced hard surfaces in this step, existing and removed hard surfaces also need to be tabulated, as follows:
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Existing Hard Surface (476 square feet) – Includes the existing hard surfaces to remain after construction, including the existing driveway (10′ x 20′ = 200 square feet), shed (8′ x 12′ = 96 square feet), and the portion of the existing walkway to remain (180 square feet).
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New Hard Surface (1,425 square feet) – Includes the portion of the proposed project site that was not previously covered in hard surface, but that will be covered in hard surface as a result of proposed roof area expansion, extension of the existing walkway, and new permeable pavement patio. New hard surfaces include the new building footprint (2,800 square feet) minus the existing building footprint (1,500 square feet) minus the portion of the existing back deck and walkway to be replaced (120 square feet). In addition to the new roof area, new hard surfaces include the new permeable pavement patio (10′ x 12′ = 120 square feet) and the new extension of the existing walkway (125 square feet), extending from the existing walkway to remain to the new permeable pavement patio. Thus, the total new hard surface area in this example is 1,425 square feet (2,800 – 1,500 – 120 + 120 + 125 [square feet]).
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Replaced Hard Surface (1,620 square feet) – Includes the part of the proposed new house footprint that overlaps the existing house footprint. This includes the 1,500 square feet of existing building footprint plus the 120 square feet of patio and walkway area behind the existing house, for a total of 1,620 square feet of replaced hard surface.
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Removed Hard Surface (30 square feet) – Includes the portion of the existing walkway that will be removed when the walkway is extended. This piece of removed walkway is not replaced with any other hard surface.
The total new plus replaced hard surface in this example is calculated as 3,045 square feet (1,425 + 1,620). Note, there is zero (0) native vegetation conversion in this example.
As illustrated in this example and as may be typical for many development sites, the existing, new, replaced, and removed hard surface areas are irregular in shape, and they may overlap each other in irregular patterns. For example, see the irregular polygon shapes for the existing back patio and walkway behind the existing house, to be replaced by new roof, and the small portion of the existing walkway that will be removed when the extended walkway is connected to the existing walkway to remain. Due to these irregular shapes, calculate these areas using area take-off methods from scaled drawings or in AutoCAD to accurately delineate and calculate the respective areas for these various features.
Figure I-3.1. Example Determination of New and Replaced Hard Surfaces.
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