2610 VIEWCREST AVE 2022-04-13 - Laserfiche WebLink

As shown in the area summary above this project proposes to construct less than 5,000 sf of PGIS and is therefore below the threshold of requiring runoff treatment. 9. FLOW CONTROL Projects shall be exempt from flow control if the following thresholds are met: • Total effective impervious area less 5,000 sf, • Conversion of less than 3/4 acre of native vegetation to landscaping, or • Conversion of less than 2.5 acres of native vegetation to pasture, • Improvements that cause less than or equal to 0.15 cfs between the existing and developed 100-year peak discharge from the site. As detailed in Section 7 of this report, Runoff from the new roof surfaces of the SFR and approximately 1/3 of the driveway will be infiltrated and can be modeled as 'impervious infiltration'. Runoff from the concrete pad will be tightlined and has thus been modeled as 'Flat Driveway'. The existing conditions have been modeled as pasture and the converted pervious areas have been modeled as lawn. The inputs to WWHM12 are.as follows. PreDeveloped Conditions: C, Pasture, Mod: 0.29 acres Developed conditions: C, Lawn, Mod: 0.111 acres A B IMP INF FLAT: 0.128 acres DRIVEWAYS, FLAT: 0.051 Peak 100-year flow rate for Existing Condition: 0.0164 cfs Peak 100-year flow rate for Developed Condition: 0.0710 cfs Change in 100-year flow rate: 0.0546 cfs As all but 2,207 sf of hard surface runoff will be infiltrated for this project, this project has shown to construct less than 5,000 sf of effective impervious surfaces (EIS). This project also proposes less than 3/4 acre of land conversion. By infiltrating most of the runoff generated by the proposed roof and driveway surfaces, this project is shown to increase the peak flows by only 0.0545 cfs in the downstream system (See Appendix A for WWHM2012 printout). Therefore, flow control is not required for this project. 10. WETLAND PROTECTION By designing the on-site improvements to increase the peak flow rates from the site by less than 0.15 cfs and by maintaining the sheet flow condition, this will provide the necessary protection to any downstream NGPA. McCann SFR 19-0913 September 2019 Reprised:January2020 Page 11