Use the attached impervious surface worksheet to assist you with your calculations. Impervious cover â Any structure, surface or improvement that reduces and/or prevents absorption of storm water into land. Calculation of Impervious Surface: Percentage of impervious surface shall be calculated by dividing the surface area of existing and proposed impervious surfaces on the portion of a lot or parcel that is within 300 feet of the ordinary high water mark by the total surface area of the lot or parcel, multiplied by 100. The ISR is calculated by dividing the square footage of the area of all impervious surfaces â¦ IMPERVIOUS SURFACE RATIO (ISR) means a measure of the intensity of hard surfaced development on a site. Vicinity sketch showing property location in relation to major â¦ Porous paving, paver blocks, gravel, crushed stone, crushed shell, elevated structures (including boardwalks), and other similar structures, surfaces or improvements are considered impervious cover. Calculate impervious surface area . Calculate impervious surface area Next, you will assess the accuracy of your classification by statistically comparing it to the original image. Features. Impervious surface and hardscape calculations show the square footage of all impervious surfaces and/or hard surface coverage on-site in relation to the lot area. Treated impervious surfaces; The impervious surface calculation may exclude any existing or proposed impervious surface where the property owner can show the county that runoff from the impervious surface is treated by existing or proposed devices such as storm water ponds, Impervious Surface Existing Ft2 Proposed Ft2 Total Ft2 Lot Area Ft2 Impervious Surface Ratio Buildings + Other Impervious Surface Percentage Maximum Allowable 25% + = ÷ = x 100 = LR: Online Permitting Forms 2016: Impervious Surface Calculation Worksheet 04-11-2016 Signature: Date _____ Additional information regarding impervious surface limits including those in the Swift Creek Watershed may be obtained by contacting Wake County Planning staff at 919.856.6335. On March 10, 2015, Harrisonburg City Council adopted the Stormwater Utility Fee Ordinance whereby a Submit the worksheet at the time of application or show calculations on the site plan. This downloadable suite of easy-to-use scripts for ArcGIS calculates the percentage of impervious surface area within user-selected geographic areas such as watersheds, municipalities, and subdivisions. For Neuse River Watershed impervious limits contact Wake County Environmental Services at 919.856.7400. Greenscape calculations show the square footage of living vegetation in the front setback in relation to the area of the front setback. Impervious surface calculations. Additional calculations may be requested for shoreline properties. After confirming that the classification has an acceptable accuracy, you will calculate the area of impervious surface â¦ Previously, you classified an image to show impervious surfaces. Impervious Surface Calculations Property Owner Impervious Surface Property Address Parcel Number Lot Size 21 acre = 43,560 ft 30% Impervious Surface ft2 Lot size x 0.3 15% Impervious Surface ft2 Lot size x 0.15 7.5% Impervious Surface ft2 Lot size x 0.075 Signature _____ Date_____ Impervious Area Calculations Fact Sheet Last Updated: April 2015 Impervious means surface area composed of material that significantly impedes or prevents natural infiltration of water into soil. the impervious area calculations unless the ground underneath them is considered 1 Modular-grid pavement is a general term meaning a brick or block surface with large spaces between the brick/blocks that is fill with soil and growing grass. Requires ArcGIS 10.x and Spatial Analyst Extension. The Drainage Manual defines impervious surface as a non-vegetated surface area which either prevents or retards the entry of water into the soil mantle as under natural conditions prior to development. A non-vegetated surface area which causes water to run off the surface in greater quantities or at an increased rate An impervious surface ratio is the relationship between the total impervious surface area on a site and the gross land area.