Storm Drainage System

The purpose of the Storm Drainage Master Plan is to provide the City with a comprehensive planning document that provides basic information and guidance necessary for the sound stewardship of the municipal storm drainage system.  The Master Plan was prepared by Murray, Smith & Associates of Portland, Oregon. 

The study area consists of the planning area and the contributing area to the storm drainage system. The planning area for this plan is comprised of approximately 1,591 acres of land within the UGB. This includes an area of approximately 1,270 acres of residential, commercial, industrial and institutional land within the existing City limits boundary and 321 additional acres of land within the UGB designated for expansion outside of the existing City limits. The study period for this plan is 20 years, through the year 2028.


The flood plain and uplands area to the north of the South Yamhill River consists of 10 minor basins and incorporates a total of approximately 2,436 acres. The uplands area drains south through the stormwater collection systems within the City and discharges into the South Yamhill River.


The area south of the South Yamhill River is divided into 8 minor basins, totaling 979 acres, with drainage dominated by a single 832-acre subbasin. This large subbasin drains north to the South Yamhill River and extends from the flood plains adjacent to the river to higher elevations south of Highway 18.


The South Yamhill River is the dominate drainage feature of this area, and flows west to east through the City. The river bisects the City forming several separate and independent drainage areas to the north and south of the river.


Runoff within the City is generally collected through a combination of street gutters, road side ditches, storm pipes and open channels, which convey runoff to the river. The City owns and operates most of the stormwater improvements within the City limits. In many developed areas of the City, the City's storm drainage system works generally well. Some minor flooding in a few localized areas can occur during larger rainfall events.


The major flooding risk in the City is due to high flows in the South Yamhill River. A significant portion of the City is located within the. Federal Emergency Management Agency (FEMA) designated 100-year flood plain of the river. Flood conditions can impede the City's storm drainage system capacity. The City's storm drainage system may not work effectively during extreme flooding events. However, immediately following the storm, as floodwaters recede, it is important that water drains quickly away via the storm drainage system.


The Storm Drainage Master Plan, dated April 2009, has been uploaded to this site.  The first document is the ENTIRE plan.  The Plan is also availabe separated by Sections for faster downloading.