DRAFT Document

NPDES CODE AMENDMENTS – Definitions in Chapter 30.91

Definitions to be Added or Modified in 2015: Add (from Appendix I of the 2013 permit)… • Converted vegetation area: The surfaces on a project site where native vegetation, pasture, scrub/shrub, or unmaintained non-native vegetation (e.g., himalayan blackberry, scotch broom) are converted to lawn or landscaped areas, or where native vegetation is converted to pasture. • Hard surface: An impervious surface, a permeable pavement, or a vegetated roof. • Pervious surface: Any surface material that allows stormwater to infiltrate into the ground. Examples include lawn, landscape, pasture, native vegetation areas, and permeable pavements. • Pollution generating hard surface: Those hard surfaces considered to be a significant source of pollutants in stormwater runoff. See the listing of surfaces under pollution-generating impervious surface. • Replaced hard surface: For structures, the removal and replacement of hard surfaces down to the foundation. For other hard surfaces, the removal down to bare soil or base course and replacement.

Modify… • Effective impervious surface (30.91E.070): “…the portion of impervious surface producing runoff upon which runoff cannot infiltrate and that is connected via sheet flow or discrete conveyance to a drainage system. Impervious surfaces on residential development sites are considered ineffective if: 1) the runoff is dispersed through at least 100 feet of native vegetation in accordance with BMP T5.30 in volume V, chapter 5 of the Drainage Manual((.))(“Full Dispersion”); 2) Residential roof runoff is infiltrated in accordance with Downspout Full Infiltration Systems in BMP T5.10A in Volume III of the 2015 Drainage Manual or 3) approved continuous runoff modeling methods indicate that the entire runoff file is infiltrated. • Impervious surface (30.91I.010): “…a ((hard surface)) non-vegetated area that either prevents or retards the entry of water into the soil mantle as compared to infiltration under natural conditions prior to development. A ((hard surface)) non-vegetated area which causes water to run off the surface in greater quantities or at an increased rate of flow from the flow that was present under natural conditions, prior to development. Common impervious surfaces include, but are not limited to, roofs, walkways, patios, driveways, parking lots, storage areas, concrete or asphalt paving, graveled areas and roads, packed earthen materials, surfaces covered by oil, macadam, asphalt treated base material (ATB), bituminous surface treatment (BST), chip seal, seal coat or emulsified asphalt and cutback asphalt cement, and other surfaces which similarly impede the natural infiltration of stormwater. Open, uncovered retention and detention facilities shall ((not be considered impervious surfaces for purposes of determining whether the thresholds for applying minimum stormwater management requirements are exceeded pursuant to chapter 30.63A SCC. However, open, uncovered retention and detention facilities shall)) be considered impervious surfaces for purposes of runoff modeling.

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DRAFT Document

• Municipal separate storm sewer system (30.91M.22: "Municipal separate storm sewer system,” or “MS4" means all separate storm sewers that are defined as "large," "medium" or "small" municipal separate storm sewer systems pursuant to 40 CFR § 122.26(b)(18). The “existing” MS4 means all elements of the system that are in place and operating at the time of a project’s initial permit application. The “planned” MS4 includes future elements of the system that are either 1) under construction; 2) included in the county’s adopted 6-Year Capital Improvement Program; or 3) included in any private development project that has submitted a complete application. • Project site (30.91P.354): “…that portion of a property, properties, or right-of-way subject to land disturbing activities, new ((impervious)) hard surfaces or replaced ((impervious)) hard surfaces.” • Receiving waters (30.91R.027.1): “…bodies of water or surface water systems to which surface stormwater runoff is discharged via a point source or via sheet flow. Ground water to which surface runoff is directed by infiltration.”

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