HEX Rolling Sunrise Engineer Rsp Part 3 051115
241 ERICKSEN AVENUE NE, BAINBRIDGE ISLAND, WA 98110 206.842.0605 info@BrowneWheeler.com
Revision 1 May 11, 2015
March 24, 2014
Michel Girard
BGH Development, LLC
Michel@BGHdevelopment.com
206.335.1649
Re: Rolling Sunrise Subdivision - Preliminary Plat
Dear Michel:
This letter presents our preliminary drainage, utility and road design for the proposed BGH Rolling
Sunrise Preliminary Plat. The proposed plat includes the subdivision of one 3.41-acre parcel into 7
single family residential parcels.
EXISTING CONDITIONS
The site is located south of the road end of Sunrise Drive, and north of Hyla Avenue. Currently the
site is undeveloped with grass, brush and trees. The site is served by a gravel access driveway
through a 20-ft easement from Sunrise Drive.
The soil is mapped by the United States Department of Agriculture as Harstine gravelly ashy sandy
loam. The site slopes generally to the south gently to steeply across the site.
SITE ACCESS
Existing
The site currently has a gravel driveway located in an easement which provides access to the site
from Sunrise Drive to the north.
Proposed
The existing access driveway to the north will be widened to meet road standards and fire access
requirements and extended to serve the north 4 lots. The driveway will be widened to 12-ft with 2
and 3-ft gravel shoulders as required by the City. The length of driveway with 2-ft shoulders rather
than 3-ft is 200-ft, from Sunrise Drive through the 20-ft wide access easement to provide space for
a 2-ft wide infiltration trench paralleling the driveway.
A new south access is proposed off of Hyla Avenue to the southwest in which an easement through
private property exists. The new access approach will be at a 90-degree turn in Hyla Avenue.
Because both access roads are gravel surfacing, the road approaches are proposed to be gravel.
REVISION 1
2
BGH- Rolling Sunrise Preliminary Plat, Bainbridge Island Browne Wheeler Engineers, Inc.
STORM DRAINAGE
Existing
Stormwater appears to infiltrate into the soils on-site or flows in a dispersed manner with no
indications of runoff on-site.
Downstream System
The site drains south toward Hyla Avenue and two private properties (see Drainage Map). Hyla
Avenue is a gravel road that does not have any stormwater management infrastructure installed.
Stormwater runoff appears to flow in a dispersed manner south to a closed depression along Hyla
Avenue, approximately 200-ft from the southwest of the subject property. The property owner
southwest of the project indicated that water frequently ponds on both sides of Hyla Avenue on
private and public property where it eventually infiltrates.
The property southeast of the project site also has an apparent closed depression along the south
edge but the owners have not indicated any issues related to stormwater runoff.
Proposed
The proposed project consists of providing storm drainage facilities for two access driveways to
serve the seven lots. The storm drainage system will be designed to infiltrate 100% of the
stormwater runoff from the driveways on-site through the installation of infiltration trenches along
the two access driveways. All new gravel access driveways will drain through a grass shoulder to
an infiltration trench along the driveway to allow stormwater runoff from the driveway area to
infiltrate into native soils. Check dams will be installed along the infiltration trenches as required to
disperse infiltration and prevent accumulation of water in one spot.
Threshold Determination
The impervious areas for the proposed access driveways were used to evaluate the Minimum
Requirements, as summarized in Table 1.
Table 1 - Impervious Areas
Proposed Total
Impervious Area (sf)
Existing Impervious
Area (sf)
New Impervious
Area (sf)
North Driveway 6,450 1,970 4,480
South Driveway 4,350 0 4,350
Total 10,800 1,970 8,830
A portion of the north access driveway overlaps the existing gravel driveway, about 9-ft wide, and
will be widened to meet the City road standard. The driveway overlap reduces the new impervious
area of the north driveway to 4,480-square feet (sf). A total of 8,830-sf new impervious area will be
created by the north and south driveways; therefore Minimum Requirements No.’s 1-10 apply to the
project as outlined by DOE in the Stormwater Management Manual for Western Washington
FILE NAME: DMAP.DWG PLOT DATE: 3/20/2014 10:04 AM PLOT DEVICE: DWG TO PDF.PC3 PAGE SETUP: ---- PLOTTED BY: KELSEYS
E N G I N E E R S , I N C
WHEELER
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P 2 0 6 . 8 4 2 . 0 6 0 5 I N F O @ B r o w n e W h e e l e r . C O M
ORB WNE
BGH - SUNRISE
DRAINAGE MAP
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SCALE: 1" = 300'
30075 150 600 900
PROJECT
LOCATION
HYLA AVE
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DUNCAN LN
TOPOGRAPHIC INFORMATION BASED UPON
KITSAP COUNTY LIDAR AERIAL SURVEY, 2-FT
CONTOURS. ELEVATION DATUM: NAVD88.
CLOSED DEPRESSION,
FREQUENT PONDING
WATER
CLOSED DEPRESSION,
NO KNOWN ISSUES
WITH WATER
APPROX. DRAINAGE
BOUNDARY FOR
CLOSED DEPRESSION
REVISION 1
4
BGH- Rolling Sunrise Preliminary Plat, Bainbridge Island Browne Wheeler Engineers, Inc.
(SWMMWW), 2005 edition (Figure 2.2). Table 2 describes how each of Minimum Requirements
No.’s 1-10 will be met.
Minimum Requirement No. 7, Flow Control requires construction of flow control facilities if any of
the following thresholds are exceeded:
1. If the project will have a total of 10,000-sf or more of “effective” impervious surfaces. Table
1, above, shows the project will result in greater than 10,000-sf. However this area is not
“effective” because the runoff will be fully infiltrated and not discharged off-site.
2. If the project converts ¾-acres or more of native vegetation to lawn or landscape, or
converts 2.5-acres or more native vegetation to pasture. We assumed 4,000-sf of
landscape per lot which results in approximately 0.64-acres, which is below the ¾-acres
threshold. There will be no conversion of native vegetation to pasture.
3. If the project will result in an increase in the 100-yr frequency peak flow of 0.1-cubic foot per
second (cfs) or greater, as estimated by Western Washington Hydrology Model Version 3
(WWHM3). WWHM3 was used to model the entire site, including the access driveways and
projected rooftop, driveway and landscaped areas for each of the seven lots. The model
predicted an increase of 0.05-cfs, which is below the 0.1-cfs threshold.
The attached calculations present a breakdown of land type areas used in the WWHM3
model for the predeveloped and the developed scenarios. The predeveloped scenario is a
moderately and steeply sloping forested condition with Type C soils. In the developed
scenario, the areas of driveway and rooftop which are infiltrated in either rain gardens or
infiltration trenches are removed from the area modeled in the developed scenario because
they are designed to infiltrate 100%.
Minimum Requirement No. 7 indicates “that portion of any development project in which the above
thresholds are not exceeded * shall apply Onsite Stormwater Management BMPs in accordance
with Minimum Requirement No. 5.” Following the SWMMWW, on-site BMPs will be utilized to
infiltrate water on-site because flow control facilities are not required.
* Deleted “in a threshold discharge area” per BIMC 15.20.060.B.
Soil Infiltration
The City determined that the on-site soil investigations performed by Daves Septic Services, Inc.,
dated 7/3/2013, is adequate soil exploration for Preliminary Plat submittal and approval. An
infiltration rate of 0.5-inches/hour was used for preliminary facility sizing, as outlined in our memo
dated November 6, 2014. Prior to Plat Utility Permit submittal, a geotechnical engineer will need to
perform soil investigations to verify site suitability of the soil for infiltration via trenches and water
quality treatment in accordance with Chapter 3.1 Roof Downspout Controls and SSC-6 Soil
Suitability for Treatment in Chapter 3.3.7 Site Suitability Criteria in Vol. III of SWMMWW. If the soils
are found to be unsuitable for infiltration trenches, rain gardens, with different site suitability criteria,
will be used.
REVISION 1
6
BGH- Rolling Sunrise Preliminary Plat, Bainbridge Island Browne Wheeler Engineers, Inc.
Individual Lot Development
The development area of each lot will be determined by the individual purchaser. All stormwater
runoff generated from each lot will be kept on-site to the maximum extent practicable through the
installation of rain gardens or other methods to infiltrate all stormwater from new impervious areas.
We evaluated the preliminary home layouts shown on the plans which range between 1,100-sf and
2,400-sf of rooftop area with driveways to each lot varying between 400-sf and 1,250-sf. Attached
is a WWHM3 output illustrating an average 2,000-sf rooftop area and 1,000-sf driveway area
combination infiltrating 100% of stormwater runoff through a 200-sf rain garden. This is NOT the
largest impervious lot coverage allowed by code but a reasonable coverage for the lot based on
preliminary plans. Drainage plans for individual lots will be developed during their respective
building permit applications. Disturbed landscaping areas will have the soil amended in accordance
with BMP T5.13 Post-Construction Soil Quality and Depth.
ON-SITE UTILITIES
WATER
Existing
There is no water utility connection currently to the property. Kitsap PUD’s water system is in the
vicinity of the project.
Proposed
A new 8-inch water main will be connected to the water system in Sunrise Drive. The new water
main will be extended south through the access easement and west and south to provide water
service to all seven lots. Two fire hydrants will be installed, one along the North access driveway
and one along the South access driveway and will be served by this new water main.
Water service for the proposed seven lots will be provided through seven new 5/8” water meters,
connected to the water main extension.
SANITARY SEWER
Existing
There are no sewer services currently serving the property.
Proposed
Each of the seven lots will have a private on-site septic system. The systems have not been designed
but space is allocated for drainfields to maintain required separations from stormwater facilities.
Power, telephone, and television cable will be designed by others.
REVISION 1
7
BGH- Rolling Sunrise Preliminary Plat, Bainbridge Island Browne Wheeler Engineers, Inc.
FRONTAGE IMPROVEMENTS
Existing
There are no frontage improvements required since the property does not front any right of way.
Proposed
The property does not front any rights of ways and no frontage improvements are proposed.
EROSION CONTROL
The Contractor will be responsible for maintaining erosion control facilities on the site during
construction and for ensuring that sediment does not leave the site. The general principles of
construction pollution prevention are:
Retain native vegetation
Prevent erosion rather than treat sediment laden water
Employ site specific best management practices (BMPs)
Divert upslope runoff around disturbed area
Phase construction operations to reduce total amount of disturbance at one time
Amend soils before seeding
Minimize the slope length and steepness of disturbed areas
Reduce runoff velocities
Prevent the tracking of sediment off site
Employ BMPs that address not only erosion but also other potential pollutants.
A detailed erosion and sediment control plan will be developed during final design.
Very truly yours,
BROWNE WHEELER ENGINEERS, INC.
5/11/2015 5/11/2015
REVISION 1
8
BGH- Rolling Sunrise Preliminary Plat, Bainbridge Island Browne Wheeler Engineers, Inc.
Table 2
Summary of Stormwater Minimum Requirements
Minimum Requirement Comment
1. Stormwater site plan A stormwater site plan will be presented.
2. Construction stormwater
pollution prevention
A detailed construction stormwater pollution
prevention plan will be developed during final
design.
3. Source control of pollutants Not applicable.
4. Preservation of natural
drainage systems and outfalls
All stormwater will continue to discharge in the
same location.
5. On-site stormwater
management
Stormwater will be kept on-site to the maximum
extent practicable through infiltration and the
amendment of all landscaping areas.
6. Runoff treatment Water quality treatment for the access
driveways will be provided through onsite best
management practices. Stormwater treatment
facilities are not required.
Water quality treatment requirements for
driveways on each lot will be determined during
development of their respective building permit
submittals.
7. Flow control Infiltration of stormwater runoff will reduce
runoff rates below thresholds to require flow
control facilities. On-site BMPs will be utilized
to infiltrate water on-site.
8. Wetlands protection Not applicable.
9. Basin and watershed planning Not applicable.
10. Operation and maintenance An operation and maintenance manual will be
prepared during final design.
REVISION 1
9
BGH- Rolling Sunrise Preliminary Plat, Bainbridge Island Browne Wheeler Engineers, Inc.
ENGINEERING CALCULATIONS