HomeMy WebLinkAboutApp C Grading-DrainageGrading and Drainage
Existing Conditions
The project site is located in Jefferson County North of the Queets River and approximately one
half mile South of the Olympic National Park boundary. The project site is currently on 5
separate parcels with a total of approximately 23.5 acres.
Existing improvements on the site include a two lane asphalt entrance which transitions to a
single lane gravel drive that runs South, parallel to Highway 101. The gravel drive is
approximately 12-feet wide and approximately 1,090 feet long. The gravel drive contains
multiple driveways that connect to camp sites and 2 existing cabins. These areas are clear and
flat while the surrounding areas are forested with heavy vegetation. The gravel roadway ends at
the ocean, approximately 80 feet lower in elevation than the top of slope. Structures on the site
include the two rental cabins along with a well pump house and a shade structure. A large
portion of the property East of Highway 101 has been cleared and a gravel pad has been placed
on the site.
Currently, the majority of the runoff from the main gravel drive sheet flows into the swale
between the drive and Highway 101. The remaining roadway runoff sheet flows into existing
swales and into the vegetated area to the East. The majority of the remaining runoff from the
driveways flow towards the forested areas or in swales along the driveways. The area just to
the West of the main driveway has several low areas that naturally trap runoff. A small minority
of the runoff collects in natural swales and is conveyed down the slope towards the ocean and
wetlands below.
Using the 2-year, 24-hour rainfall precipitation, it was estimated that a total volume of 23,200
cubic-feet of runoff is produced from the estimated 5.40 acre watershed (as shown on Sheet 4
in Appendix A).
Proposed Site Improvements
Added impervious area has been minimized through use of the existing cleared areas and
improvements on site. New impervious areas will include the additional cabins and gravel
parking stalls, the pedestrian path. There are a total of 24 cabins being proposed with an
average of approximately 450 square feet each. Each cabin will require gutters, down spouts
and splash guards to help alleviate erosion. There will be a ratio of 1.5 new gravel parking stalls
per cabin.
The existing gravel drive is anticipated to have a 2-inch gravel overlay sloped away from the
centerline at a 1% to 3% slope. The new gravel parking stalls and paved path will have
approximately 2 ft of imported fill below them based on final recommendations from the
geotechnical engineer. The cabin foundations will consist of piers going into the soil
approximately 7 feet down. These improvements will produce approximately 27,000 square feet
of additional impervious area, increasing the impervious area from 14.5% to 17.1% of the total
project site.
The additional parking stalls, 24 cabins, a pedestrian path and the existing roadway
improvements will produce additional runoff. The 2 year, twenty-four hour rain event was used
to calculate the additional runoff on the 5.4 acre watershed. The preliminary calculations show
an estimated 8,400 cubic feet of additional runoff will be generated. In order to store the
additional runoff volume, vegetated swales running along either side of the gravel road and
driveways are being proposed. Additionally, small retention areas will be developed to collect
the excess runoff. The intent is to keep the runoff volume to pre-project conditions.
In areas of natural swales and drainage channels that run to the top of slope, there will be valley
gutters added to cross the proposed path. The proposed cabins and parking spaces that are
planned in areas of natural drainage patterns will provide for a redirected course back to the
existing drainage channels.