Capital Hill PUD, Tentative (14-020) <br />August 22nd, 2017 <br />The primary goal for stormwater management on Capital Drive is to match existing <br />drainage patterns and peak discharge rates through the design storm. <br />The new pavement and widening improvements needed to construct a 21' wide roadway <br />will have an offset crown matching the existing pavement width that sheet drains to the <br />west. The new additional pavement width will sheet flow to the eastern gutter line where <br />it will be collected in catch basins for detention. Additional detail for these systems will <br />be described shortly. <br />Due to the existing intermittent curb, a flush curb is proposed along a majority of the <br />western side of the new improvements with a 1-2' gravel shoulder. The gravel shoulder is <br />proposed to aid in absorbing and dissipating the sheet flow draining to the west. The <br />flush curb will be placed in areas where an existing curb either (1) doesn't exist, (2) is a <br />top set curb in poor shape and not functioning as a curb, or (3) isn't a curb at all but <br />rather logs along the side of the road. Again, the intent is to match the existing drainage <br />pattern which is for a majority of the roadway to sheet flow toward the neighbors to the <br />west (Destination #5), without increasing peak runoff rates toward the neighbors or <br />downstream systems. These improvements will end up reducing the runoff sheet flowing <br />to the west by directing upland areas toward the north as will be described in the <br />following sections. <br />To summarize the curb locations: <br />• A new curb & gutter will start at the southern limits of the new construction and <br />end at the second driveway which appears to be 2910 Capital Drive. <br />• The remaining curb to the north up to the existing end of pavement will be flush <br />with a 1-2-foot gravel shoulder. <br />• The remaining portion of the roadway that extends pavement further north <br />halfway through lot 2 will have standard curb and gutter on both sides of the <br />road. <br />The runoff from the eastern portion of the widened roadway south of the high point will <br />be collected in catch basin(s) and directed to a mechanical treatment device near the <br />southern intersection with the private roadway. The existing basin drains across Capital <br />Drive to the western gutter line. It is for that reason that after directing this runoff <br />through detention and flow control, runoff will be directed to re-enter the gutter line via <br />a bubbler. A bubbler is proposed in order to match the existing drainage patterns of <br />runoff into the gutter while also applying a way to disperse the water with minimal <br />disturbance. The post-developed flows into this gutter line (Destination #4) will match <br />the pre-developed flow into the gutter line so there is no increase in the gutter line <br />drainage anticipated. One last note for this basin east of the crown and south of the <br />roadway high point, is that there is a small area south of the catch basin that is draining <br />to the existing eastern curb line (Destination #1) under the post-developed condition. <br />The private roadway runoff will be designed to over detain in order to match the pre- <br />developed flows into the eastern gutter line. <br />The runoff from the eastern portion of the widened roadway north of the high point, as <br />well as the new pavement to extend the roadway will be collected in catch basins, and <br />directed into a series of underground storage tanks to meet flow control requirements. <br />There is an existing basin that drains into a ditch along the eastern side of a gravel path <br />north of the new pavement improvements (Destination #3). This discharge location was <br />suggested by and coordinated with Fred Lockhart, the City of Eugene's Stormwater <br />Branch Engineering, Inc. <br />3 <br />