HomeMy WebLinkAboutCA_Drainage Assessment_5002_190612_v3PR19000084
B19001026
V3
REVISION Received: 07/17/2019 / aalexander
BUILDING DIVISION
APPROVEDBUILDING DIVISION
THIS SET OF APPROVED PLANS MUST BE ON THE JOB AT ALLTIMES DURING CONSTRUCTION. THIS BUILDING IS NOT TO BEOCCUPIED UNTIL AFTER FINAL INSPECTION BY THE RENTONBUILDING DIVISION AND RECEIPT OF CERTIFICATE OFOCCUPANCY.
APPROVAL SHALL NOT BE CONSTRUED AS THE AUTHORITY TOVIOLATE OR CANCEL ANY PROVISIONS OF CODES, ORDINANCES,OR OTHER REGULATIONS ENFORCED BY THIS CITY.
DATE: 08/02/2019
Narrative:
The subject property is an existing parcel is 10,696 square feet (lot) which was created by the Plat
of Puget Colony Homes in May 1968. The subject property is overgrown with blackberries, heavy
understory, and several large trees. The property is very flat with a slight grade from the east to
the west property line. The property sheet flows from the east to the west into the existing
drainage channel. Along the west property line is an existing drainage channel which flows from the
north to the south towards NE 1st Street. There is an existing 36” pipe conveying the flow from the
channel to the public storm system within NE 1st Street. Along the north side of NE 1st Street is an
open ditch which conveys roadway runoff from the east to the west into the 36” pipe. A new 12”
culvert will be installed within the roadside ditch to allow for access to cross this drainage feature.
Access to the proposed new residence will be from NE 1st Street along the east side of the lot.
Public utilities are available within NE 1st Street. There is no existing sewer system along the
frontage of this property with NE 1st Street. A sewer main extension will be installed to provide this
public utility to the lot and the adjoining lot to the west.
Site Characteristics:
Lot Area: 10,696 square feet
Developed Conditions:
Roof Area: 2,485 square feet
Driveway: 764 square feet
Patio: 100 square feet
Total Proposed Hard Surfaces: 3,349 square feet
Total Proposed Pervious Surfaces: 7,347 square feet
Flow Control BMP Description:
Lot Area = 10,696 square feet; therefore, small lot.
Evaluate Small Lot BMP Requirements per 2017 CRSWDM, Section 1.2.9.1
1. Full Dispersion – There is inadequate space on the downgradient side of the lot to provide
100 feet of dispersion flow path – not feasible
2. Full Infiltration of Roof Area – Per the attached Geologist Report by Cobalt Geosciences, the
underlying soils are Glacial Tills and a high ground water table - not feasible
3. (i) Full Infiltration – See note 2 above, not feasible
(ii) Limited Infiltration – See note 2 above, not feasible
(iii) Rain Gardens – High ground water table, not feasible
(iv) Bioretention – High ground water table, not feasible
(v) Permeable Pavement – High ground water table, not feasible
due to poor draining soils, not feasible.
4. Basic Dispersion – There is available space in the back yard (north side of new house) to
provide basic dispersion for the entire roof area: feasible
a. Sizing: Gravel Filled Trench with notched board per C.2.1.D for the roof area:
i. Flow path length shall be minimum of 25 feet of vegetated area
ii. 700 square feet of roof area per 10 linear feet of trench
Roof area = 2,485 sq. feet
Therefore, trench length 2485/700*10 = 35.5 USE 36 feet
5. BMP of a minimum of 10% of Site Area -
Total Lot Area: 10,646 square feet
Minimum required BMP: 10,646 * 0.10 = 1,065 sq. feet – See item4 above – BMP mitigation
has been provided for 2,485 square feet of roof area.
2,485 sq. ft. > 1,065 sq. feet feasible
(i) Reduced Imperious Surface Credit – The proposed Impervious (hard) surface
coverage calculations (cannot exceed 50% of lot area): feasible
Proposed Hard Surfaces: 3,350 sq. ft.
Maximum Allowed Impervious Surface per zoning 50%:
Lot area * 0.50 = 10,646 * 0.50 = 5,323 OR 4,000 sq. feet whichever is less.
Additional BMP provided to Reduce Impervious Surface to 3,350 square feet.
12” CULVERT SIZING
Upstream basin from 5001 NE 1st was walked on February 5, 2019, the attached basin map reflects
the contributing area to the new culvert.
A(T) = 1.08 acres; there are 4 dwelling units within this basin; therefore, the Basin Characteristics
(Cn) is 4 DU/acres.
The Tributary Area is less than 10 acres; use Rational Method to per Table 3.2 to calculate the Flow
(Q) into the upstream end of the culvert.
Calculate the 25-year flow rate of upstream basin: Q(25) = C(n)*I(25)*A(t)
C(n) from Table 3.2.1.A is 0.48
I(25) from Isopluvial Map Figure 3.2.1.C is 3.4 inches/hour
Q(25)=0.48*3.4*1.08 = 1.76 cfs
Check 12” D.I. pipe capacity using Figure 4.2.1.F
Q(full) = 4.6 cfs
Pipe is flowing 40% full at 25–year event OKAY
XSITE
3,350
x
Erosion and Sediment Control Measures:
The property is currently covered with several trees, blackberries, and heavy understory; the lot is
overgrown. Erosion Control mitigation for the construction of the proposed project on this lot will
include: a stabilized construction entry will be installed from NE 1st Street w/a new 12” culvert; silt
fencing will be installed on the north, south and west property lines; and a filter sock will be
installed within the existing catch basin on the south side on NE 1st Street.
Once the foundation of the house has been installed, the portion of the exposed disturbed property
should be covered in straw mulch to avoid silt laden water from running off the property during the
winter months; any stockpile areas should be covered with plastic.
Upon completion of the house and only after final landscaping, the erosion control measures should
be removed from the lot and the public right-of-way.
1.CONDITIONS OF USE
1.1.PLASTIC COVERING MAY BE USED ON DISTURBED AREAS THAT REQUIRE COVER MEASURES FOR LESS THAN 30 DAYS.
1.2.PLASTIC IS PARTICULARLY USEFUL FOR PROTECTING CUT AND FILL SLOPES AND STOCKPILES.
1.3.CLEAR PLASTIC SHEETING MAY BE USED OVER NEWLY-SEEDED AREAS TO CREATE A GREENHOUSE EFFECT AND ENCOURAGE GRASS GROWTH.
CLEAR PLASTIC SHOULD NOT BE USED FOR THIS PURPOSE DURING THE SUMMER MONTHS.
1.4.THIS METHOD SHALL NOT BE USED UPSLOPE OF AREAS THAT MIGHT BE ADVERSELY IMPACTED BY RUNOFF. SUCH AREAS INCLUDE STEEP AND
UNSTABLE SLOPES.
2.DESIGN AND INSTALLATION SPECIFICATIONS
2.1.PLASTIC SHEETING SHOULD HAVE A MINIMUM THICKNESS OF 0.06 MILLIMETERS.
2.2.IF EROSION AT THE TOE OF A SLOPE IS LIKELY, A GRAVEL BERM, RIPRAP, OR OTHER SUITABLE PROTECTION SHALL BE INSTALLED AT THE TOE
OF THE SLOPE IN ORDER TO REDUCE THE VELOCITY OF RUNOFF.
2.3.TIRES, SAND BAGS, OR EQUIVALENT MAY BE USED TO WEIGHT PLASTIC.
2.4.SEAMS BETWEEN SHEETS MUST OVERLAP A MINIMUM OF 12 INCHES AND BE WEIGHTED OR TAPED.
3.MAINTENANCE STANDARDS
3.1.TORN SHEETS MUST BE REPLACED AND OPEN SEAMS REPAIRED.
3.2.IF THE PLASTIC BEGINS TO DETERIORATE DUE TO ULTRAVIOLET RADIATION, IT MUST BE COMPLETELY REMOVED AND REPLACED.
3.3.WHEN THE PLASTIC IS NO LONGER NEEDED, IT SHALL BE COMPLETELY REMOVED.
NOTES:
10' MAX..
10' MAX.
TOE IN SHEETING IN
4" x4" MIN. TRENCH
PROVIDE ENERGY DISSIPATION
AT TOE IF EROSION IS LIKELY
SANDBAG (TYP.)
PLASTIC COVERING
STD. PLAN - 213.30
PUBLIC WORKS
DEPARTMENT
APPROVED:
DATEGreggZimmerman
Public Works Administrator
UNLESS OTHERWISE NOTED, DRAWING IS NOT TO SCALE (NTS)
DocuSign Envelope ID: 6EECF5B6-602A-4E4A-B436-A317AA638878
9/28/2018 | 9:45 AM PDT
Soils Evaluation per Cobalt Geosciences