9509 19TH AVE SE 2018-01-02 MF Import

Everett LDS Parking Lot Expansion Storm Drainage Report 3. Off -Site Analysis Downstream Drainage Description Please refer to Figure 4A and 4b — Off -Site Conveyance Exhibits to aid in the following discussion. The proposal is to convert a portion of the site from pervious (lawn) to impervious. To mitigate for this change, a strcambank erosion control system (detent;on pine with control structure) is being installed to supplement the existing detention pipe system previously installed for the site. The results are that less flow will be released during large storm events to the existing downstream systcm. However just to be careful we have done some sizing calculations to establish that the existinp, system has capacity. The Upstream Basin of 30 acres +/- has the ability to generate significant flow. However some of that urbanized area is mitigated with stormwater systems. All of the flow from this area is concentrated at one Catchbasin on the North Side of 90" St. SG south of the church site. The potential flow from the this upstream basin is huge for a 100 year storm event (>30 cis). However it is limited as it all has to get into a single 12" pipe with about 3' of head available. The amount of water this inlet can handle is about 6.5 cis (See Figure 3A, page 11). At the south side of the property this pipe was upgraded to a 15 pipe, and this pipe size continues through this property and through the property directely to the north. The next constriction in the system happen at the last pipe in the system leaving the property. At this point the flow from the property is added to the system. The exit pipe is quite deep and is a 15" pipe, its inlet capacity is 15 cis (see second diagram of Figure 3). The available capacity is then 15 cis — 6.5 cis or about 8.5 cfs. The addition of flow from the existing portion of the property (ignoring the reduction effects of the existing detention system) is 3.25 cis and the flow from the proposed development is only 0.78 cis (again ingnoring the proposed detention system) which totals less than the 4 cis. The pipe system has available capacity even in the case where both systems wot:ld be completely non-functional. �-a f� f. a` • f Cniehhasin at Connection point to Existing system P:\2006\06113 Everett LOS Parkin0 LOt\Text\S1orm Reporlc\08113 OmmaOe Report R.Sea 01.10.0a.Goc Page 6 of 36