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'versions can cause greater impacts than would otherwise occur by discharging runoff at the nat- ure .cation. Where • conveyance system exists at the adjacent downgradient property line and the discharge was previo► ly unconcentrated flow or significantly lower concentrated flow,then measures must be taken to prey-•t downgradient impacts. Drainage easements from downstream property owners may be needed .•d should be obtained prior to approval of engineering plans. The following dischar.- requirement is recommended: Where no conveyance sy -m exists at the abutting downstream property line and the natural(exist- ing)discharge is unconcentr. ed, any runoff concentrated by the proposed project must be dis- charged as follows: a. If the 100-year peak discharg- as estimated using an approved continuous runoff model using 15-minute time steps, is le than or equal to 0.3 cfs under existing conditions and will remain less than or equal to 0.3 cfs ► der developed conditions,then the concentrated runoff may be discharged onto outlet protects. with riprap, such as those described in V-1.4.3 Out- fall Systems, or to any other system that rves to disperse flows. b. If the 100-year peak discharge, as estimated .ing an approved continuous runoff model using 15-minute time steps, is less than or equal • 0.75 cfs under existing conditions and will remain less than or equal to 0.75 cfs under develop.. conditions,then the concentrated runoff may be discharged through a dispersal trench, such a hose described in V-1.4.3 Outfall Sys- tems, or other dispersal system, provided the applicant c.• demonstrate that there will be no significant adverse impact to downhill properties or drainage ystems. c. If the 100-year peak discharge, as estimated using an approved ..ntinuous runoff model using 15-minute time steps, is greater than 0.75 cfs for either existino or developed conditions, or if a significant adverse impact to downgradient properties or draina•- systems is likely,then a conveyance system must be provided to convey the concentrated runo .cross the down- stream properties to an acceptable discharge point(i.e., an enclosed draina•- system or open drainage feature where concentrated runoff can be discharged without signific. t adverse impact). Stormwater control or treatment structures should not be located within the expected 25-yea ater level elevations for salmonid-bearing waters. Such areas may provide off-channel habitat forju -n- ile salmonids and salmonid fry. Designs for outfall systems to protect against adverse impacts from concentrated runoff are included in V-1.4.3 Outfall Systems. 1-3.4.5 MR5: On-Site Stormwater Management • Projects shall employ Stormwater Management BMPs in accordance with the following thresholds,standards, and lists to infiltrate, disperse, and retain stormwater runoff on site to the extent feasible without causing flooding or erosion impacts. 2019 Stormwater Management Manual for Western Washington Volume I-Chapter 3-Page 115