Level 1 Downstream Analysis Newcastle Way Apartments Newcastle, Washington King County April 14, 2014 Revised July 10, 2014 Prepared by: Dean A. Furr, P.E. Reviewed by: Donald Scarberry, P.E. Coffman Engineering, Inc. #140031 07/10/14
NEWCASTLE WAY APARTMENTS NEWCASTLE, WA LEVEL 1 DOWNSTREAM 3.0 OFF-SITE ANALYSIS The new 80-unit mixed-use multi-family development, Newcastle Way Apartments, is located adjacent to the City of Newcastle City Hall, near the intersection of Newcastle Way SE and 129 TH Avenue SE. More generally the site is located at the NE ¼ of Section 28, Township 24N, Range 5 WM in the City of Newcastle, Washington. Task 1: Study Area Definition and Maps Contributory upstream drainage from the adjacent PSE transmission line easement is considered insignificant because most of the surface runoff is intercepted by the gravel trail and conveyed south around the project site. Surface runoff from the development to the west is intercepted by the ditch within the PSE easement and conveyed south around the site. All upstream drainage north of the site is conveyed by the existing drainage ditches and culverts located along Newcastle Way SE and within city right of way and converges with the sites runoff. The Newcastle Way Apartments project contributes only a small percentage of the tributary drainage within the Coal Creek Basin. All onsite runoff flows eastward and is intercepted by the unimproved ditch and culvert drainage system along Newcastle Way SE. Stormwater enters a catch basin at the end of the improved Newcastle Way SE curb/gutter/sidewalk located near the south east corner of the site. Stormwater continues southward through the improved storm drainage system where it flows southward within 129th Avenue SE. This exceeds the ¼ mile downstream analysis. The site contributes less than 15 percent of the total tributary sub-basin at the downstream analysis limit, in accordance with the King County SWDM. Task 2: Resource Review Adopted Basin Plans and Finalized Drainage Studies The project site is within the Coal Creek Basin Plan, published by King County, in 1980. The conditions noted and recommendations for development have been incorporated into current storm water control design requirements (KCSWDM) and the City of Newcastle Storm and Surface Water Comprehensive Plan. Basin Reconnaissance Summary Reports: Drainage patterns and conditions for much of King County were outlined in the King County Basin Reconnaissance Program in approximately 1987. Recommendations were incorporated into the current design. CEI Project 140031 Page 1
NEWCASTLE WAY APARTMENTS NEWCASTLE, WA Floodplain/Floodway (FEMA) maps: LEVEL 1 DOWNSTREAM There are no FEMA floodplains within the study area of the Coal Creek Basin Offsite Analysis Reports: The Newcastle Professional Center is located adjacent to the site to the east. The TIR (Hugh Goldsmith and Associates, Inc., October 2005) was reviewed for potential additional drainage related problems, existing or predicted, within the common downstream reach of this development. It is was found that this development is within the Lake Boren drainage basin and has no effect on the proposed project. Senstitive Areas Folio: Review of the Geologic and Hydrologic Maps on King County s IMAP the site is located within a: Upper tributary Basin - Coal Creek Drainage Basin High Basin Condition - Areas with low development intensity (e.g., substantial forest cover, relatively few roads crossing aquatic areas and wetlands, low amounts of impervious surfaces, and low amounts armoring and structures along shorelines) and a significant biological value (e.g., the presence or high use by critical species or the presence of rare, endangered or highly sensitive habitats). The site is located outside of an erosion, seismic, landslide, coal mine, susceptible to groundwater contamination, sole source aquifer, stream, channel migration hazard, 100year flood plain, Chinook distribution and wildlife network areas. Drainage Complaints: Review of the IMAP shows 8 drainage complaints occurred downstream from the project. The latest complaint date was on January 10, 2013. All drainage complaints have been closed. Wetlands Inventory: There is a wetland located beyond the downstream study area at the south end of an unknown tributary to Coal Creek and near the north end of QFC shopping. The wetland has been mapped by King County, map folio ID 0603 with a wetland rating of 2. Task 3: Field Inspection A field investigation was performed on April 9, 2014. The weather was approximately 60 degrees and partly sunny, am showers. Review of the historical data shows an accumulation of 0.43 inches for the month of April and 16.88 inches for the year. March had 8.33 inches of rainfall. No signs of erosion were observed for the downstream conveyance and the current system appears to have capacity. CEI Project 140031 Page 2
NEWCASTLE WAY APARTMENTS LEVEL 1 DOWNSTREAM NEWCASTLE, WA Task 4: Drainage System Description and Problem Description The existing downstream drainage course for the project area starts at the catch basin located at the end of the Newcastle Way roadway improvements near the southeast corner of the site. Stormwater enters a 12 inch culvert and flows into the aforementioned Type 1 catch basin. Stormwater continues to flow southward and turning eastward within the storm system where it enters a catch basin at the southeast corner of the Newcastle Way and 129th Ave NE intersection. This catch basin is located within the sidewalk and has a solid cover. From there the stormwater flows eastward where it enters a Type 2 SDMH with a solid cover located in the bus lane. The stormwater is conveyed northward under Newcastle Way SE through an 18 CMP where it enters a SDMH with a solid cover in the sidewalk. The stormwater is conveyed through a 24 pipe to a SDMH located in the Coal Creek Village parking lot near the northwest corner of the Newcastle Way and Coal Creek Parkway intersection. The stormwater flows northward to a SDMH with solid cover. The stormwater flows eastward under Coal Creak Parkway through a series of catch basins that have solid covers within the Arco gas station parking lot. The storm water is conveyed to a SDMH in the landscape area between the Arco and Bank of America. The stormwater is conveyed eastward within the drive lane of the QFC loading area. This terminates the ¼ mile of the study area. Stormwater eventually flows northward where it enters Coal Creek and ultimately discharges into Lake Washington. Task 5: Mitigation of Existing or Potential Problems There are no significant existing or predicted drainage problems with respect to the proposed project. Mitigation of potential development impacts will be achieved by conformance to the 2009 KCSWDM and the current city municipal code and Storm water Management Plan. CEI Project 140031 Page 3
EXHIBITS