5901 23RD DR W SEAWAY CORP 3C 2025-07-29

I I Narbeck Creek Drainage Detention Basin Design I Lot 3D is currently located on a ridge line, with approximately half of its area draining towards 111 the east into Narbeck Creek and half towards the west into Powder Mill Gulch. The project proposes to maintain the same overall drainage pattern for the area,while flattening out the ridge for recreational uses. To maintain that drainage pattern,the lot will have a crest from south to north, with slopes to the east and west. The discharge to the west will be routed to the proposed R/D pond in the northwest corner of the project area, which has been sized to account for this II area's drainage. The drainage to the east will be to another proposed pond in the northeast corner of the project area that will discharge into Narbeck Creek. The contributing area of Lot 3D to the Narbeck Creek drainage basin prior to development is approximately four acres. To ensure the overall drainage pattern of the area remains the same, 111 the developed portion of Lot 3D that contributes to the Narbeck Creek drainage will also be four acres,as demonstrated in Figure 1. The drainage will be sloped to the north and discharge into the proposed Narbeck Creek detention pond. Attached are the data used to develop the storm 1111. water runoff model of the Narbeck Creek drainage basin,as well as the stage/storage data for the proposed Narbeck Creek detention basin and the flow control structure characteristics. Figures 2 and 3 show the flow paths and T0's for the existing and developed conditions, respectively. The Narbeck Creek detention and is four feet deep(inclu ding( dmg one foot of freeboard), has trapezoidal geometry,side slopes of 3H:1 V,and a foot print of 44' x 104'. Because there will be 0 no impervious area in the drainage basin after development,neither a percent impervious volume correction factor nor additional volume for water quality treatment are required. Hence, the pond was sized using only detained volume (7,481 cf)generated by the 100 year storm event over the IIentire Norbeck Creek drainage basin and no additional pond volume was provided. Discharge from the pond is through a flow control structure at 50%of the two year pre- developed, 100%of the 10 year pre-developed, and 100%of the 100 year pre-developed peak II discharges. From the flow control structure,the discharge passes through another catch basin and into Narbeck Creek. The second catch basin is to collect storm water from future development to the south in the Narbeck Creek drainage basin. A gabion type flow dissipator is I_ designed for the discharge into Narbeck Creek. The flow dissipator will be placed at the bottom of the conveyance system from the pond,prior to discharge to Narbeck Creek. lI Orifice Release to Powder Mill Gulch and Stub The overflow riser diameter is specified in the plans as 18", not 12"as described in the drainage report. Splitting the 100 year developed peak discharge of 10.6 cfs between the two risers leads to 5.3 cfs in each. From Figure 1-5.16 of the COE Stormwater Management Manual, an 18" riser passing 5.3 cfs will only require 0.5 ft of head,which still leaves an additional 0.5ft of freeboard. Table I presents the total discharge from the project site, and Tables 2 and 3 demonstrate the 74%and 26%flow splitting of the total discharge to the Powder Mill Gulch system and the 18"storm sewer stub, respectively. I C:1My Documents\Northshore(97-070)Worthshore supplemental.doc