20191014 Preliminary Drainage Report Resub.pdfPreliminary Drainage Report
Sundstone SP
for
Sundstone HOA
Attn: Lindell Graham
1414 9th Ave N, Apt. 301
Edmonds, WA 98020
206.717.2995
RECEIVED
OCT 14 2019
DEVELOPMENT SERVICES
COUNTER
SITE LOCATION:
1414 9th Ave N, Apt. 301
Edmonds, WA 98020
Tax Parcels: 00427800100100,...100200,...100300,...100400,...100500,...200100,
... 200200,...200300,...200400,...200500, 300100,...300200,...300300,...300400,
... 300500
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Prepared by:
Joseph M. Smeby, P.E.
Job No: 19-0117
June, 2019
Revised: Oct. 2019
TABLE OF CONTENTS
TABLEOF CONTENTS .......................................................................................................................... I
LISTOF FIGURES .................................................................................................................................. 2
1. INTRODUCTION & SITE CLASSIFICATION .................................................................................... 3
2. STORMWATER SITE PLAN ........................................................................................................... 4
3. SWPPP NARRATIVE ..................................................................................................................... 4
A. SITE GRADING/EROSION CONTROL RISK ASSESSMENT .............................................................. 5
B. SWPPP MINIMUM ELEMENTS ..................................................................................................... 5
4. WATER POLLUTION SOURCE CONTROL ...................................................................................... 7
5. PRESERVATION OF NATURAL DRAINAGE SYSTEM ...................................................................... 7
6. ON -SITE STORMWATER MANAGEMENT ..................................................................................... 7
7. RUN-OFF TREATMENT ................................................................................................................ 9
8. FLOW CONTROL .......................................................................................................................... 9
9. WETLANDS PROTECTION ........................ I ................................................................................... 9
10. OPERATIONS AND MAINTENANCE MANUAL .............................................................................. 10
APPENDIX A - To be provided with final engineering report at time of construction plan
submittal.
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LIST OF FIGURES
ITEM PAGE
FIGURE 1 VICINITY MAP ....................................................................................... 10
FIGURE 2 EXISTING DRAINAGE BASIN MAP ...................................................... 11
FIGURE 3 DEVELOPED DRAINAGE BASIN MAP ................................................. 12
FIGURE 4 SNOHOMISH COUNTY SOILS MAP ..................................................... 13
FIGURE 5 UPSTREAM/DOWNSTREAM TRIBUTARY AREA MAP ........................ 14
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1. INTRODUCTION & SITE CLASSIFICATION
This document is intended to provide preliminary engineering information necessary to support
the preliminary short plat permit application submittal to the City of Edmonds for the new 2 lot
short plat proposed on this parcel. The current property is approximately 51,922 sf. Access to the
existing condos on this site is from an access easement to the northwest. The proposed access for
the new SFR will be directly off of 9th Ave N along the western property line.
The total area of disturbance including clearing and grading does not exceed % acres and the new
total impervious surfaces does exceed 3,550 sf. Therefore, this project is classified as a Category 1
Small Site Project. This classification was completed using figure 2-1 of the 2017 Storm water
Code Supplement to Edmonds Community Development Code Chapter 18.30. Due to the existing
soil conditions, on -site infiltration is not feasible for this project and dispersion will be the selected
LID BMP for this project where feasible. Refer to Section 6 of this report for a detailed feasibility
analysis. As a result, only minimum requirements 1-5 will be required to be reviewed and
addressed for this project.
This proposed project will create approximately 3,550 sf of new impervious surfaces including the
proposed SFR and driveway. No impervious surfaces will be considered removed or replaced for
this project. All of the new impervious surfaces will be mitigated using on -site stormwater
management BIVIPs including dispersion and amended soils; both LID techniques. In addition,
another 3,550 sf of retrofit area will be mitigated on -site along the east side of proposed Lot 2 via
a level spreader trench. This will meet the requirements per ECDC 18.30.060.D.S.b.i.
The site is located at 1414 9th Ave N, Edmonds, WA 98020, and in Section 13, Township 27N,
Range 3E, Willamette Meridian. See Figure I - Vicinity Map.
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2. STORMWATER SITE PLAN
As noted in Section 1 of this report this project is classified as a Category I Small Site Project.
Therefore, this drainage design has been prepared to address the requirements contained in the
City of Edmonds Stormwater Supplement and applicable handouts. This document is intended to
provide the supporting information to justify the BIVIPs used and to detail how the design of the
selected BIVIPs meet the required standards.
For this project some LID techniques were used to aid in minimizing the project's impacts to the
neighboring properties and downstream system. The new NPGIS & PGIS runoff will be dispersed
where feasible on -site and the soils within the proposed area of disturbance will be amended to
landscaping conditions as per BIVIP T5.13 out of the 2014 DOE SWMMWW. This is done to reduce
the stormwater runoff generated by the proposed impervious surfaces.
3. SWPPP NARRATIVE
Clearing, grading, and temporary erosion and sediment control plans will be prepared as part of
the final engineering drawings. However, since a construction site is dynamic it will be necessary
to reassess the erosion control BIVIP's during construction and install additional measures when
necessary.
Proposed temporary measures possible for this project will include the following BIVIP's:
Installation of stabilized construction entrance.
Retention of Existing Vegetation
Straw mulch, hydroseed or other mulching and planting method to stabilized unworked
areas.
Silt fencing if necessary
Permanent measures io reduce or eliminate erosion or water quality degradation' will include the
following BIVIP's:
Paving all traffic areas (concrete/asphalt)
Permanent landscaping in pervious areas.
Limiting cut and fill slopes to 2:1 maximum and 3:1 maximum where exposed to standing
water.
Routine maintenance and inspection of the grounds and response to developing
problems.
The listed erosion control BMP's will be engineered for anticipated conditions in compliance with
City and DOE guidelines. With proper installation, maintenance and inspection the proposed
BIVIP's should result in minimal impact to the surrounding environment. The City retains the
authority by code to require additional measures should the existing BIVIPs prove insufficient.
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A. SITE GRADI NG/EROSION CONTROL RISK ASSESSMENT
Area proposed to be cleared/worked: 9,000 sf = 0.21 acres
Average slope for the site (w/in the area of disturbance): 10%
Erosion Hazard of Soil Moderate
Critical Areas downslope No
Site is upstream of an ESA Stream No
Based on the above information and the fact that the site will retain some existing vegetation and
construction site runoff will filter through the soil, and that if site conditions warrant, additional
BMP's can be implemented as corrective measures the Risk Category for this site is Low Risk.
B. SWPPP MINIMUM ELEMENTS
1: Mark Clearing Limits
The first step in the "Construction Sequence" included on the clearing and grading plan
sheets is for the limits of clearing to be flagged and to have construction fencing placed
along the limits prior to any other construction activity.
2: Establish Construction Access
The SWPPP calls for the proposed construction entrance to be installed as the second
step after the staking of clearing limits. A detail is provided on the plans.
3: Control Flow Rates
This project is below the thresholds requiring flow control for the project.
4: Install Sediment Controls
This site and SWPPP proposes to construct/maintain a construction entrance, retained
vegetation and silt fencing. These features are intended to minimize the opportunity for
sediment to leave the site via stormwater or on vehicles. The construction of these
features is one of the first items required in the "Construction Sequence". Mulch will also
be used on the exposed soil as necessary to limit erosion.
5: Stabilize Soils
The "Construction Sequence" calls for the stabilization of soils that remain unworked for
certain lengths of time based on the time of year. Stabilization techniques may include but
not limited to mulching, plastic sheeting or hydroseeding, notes have been added to the
plan regarding protection for the stockpile area if necessary. A stockpile area has been
identified on the SWPPP and is setback a minimum of 25-feet from any down slope
property line.
6: Protect Slopes
All disturbed slopes on site during construction are required to be protected with mulch or
other means as specified in the construction sequence. No concentrated runoff or
significant amounts of sheet flow will be directed to new cut or fill slopes during
construction.
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7: Protect Drain Inlets
No new or existing culverts are proposed or affected by this project.
8: Stabilize Channels and Outlets
No new channels or outlets are proposed for this site.
9: Control Pollutants
No outside chemicals are expected to be necessary for the construction of this project. All
vehicles working on and around the site would need to meet the State requirements for
emissions.
10: Control DeWatering
DeWatering will not be necessary for this project. However, the existing vegetation
retained on site would be available to spread any water from construction for disposal,
11: Maintain BMPs
The construction supervisor will be responsible for maintaining all BMPs during
construction and working with the City to relocate or add BMPs as necessary as site
conditions change.
12: Manage the Project
It will be the responsibility of the Contractor and Developer to manage this project and
coordinate with the City Inspector and Engineer.
Inspection and Monitoring:
Site inspections shall be done by a person who is knowledgeable in the principles and
practices of erosion and sediment control. The person must have skills to first assess the
site conditions and construction activities that could impact the quality of stormwater, and
second assess the effectiveness of erosion and sediment control measures used to
control the quality of stormwater discharges.
Whenever inspection and/or monitoring reveals that the BMPs identified in the
Construction SWPPP are inadequate, due to the actual discharge of or potential to
discharge a significant amount of any pollutant, appropriate BMPs or design changes shall
be implemented as soon as possible.
Maintaining an Updated Construction SWPPP:
The construction SWPPP shall be retained on -site or within reasonable access to the site.
The SWPPP shall be modified whenever there is a change in the design, construction,
operation, or maintenance at the construction site that has, or could have, a significant
effect on the discharge of pollutants to waters of the state.
The SWPPP shall be modified if, during inspections or investigations conducted by the
owner/operator, or the applicable local or state regulatory authority, it is determined that
the SWPPP is ineffective in eliminating or significantly minimizing pollutants in stormwater
discharges from the site. The SWPPP shall be modified as necessary to include
additional or modified BMPs designed to correct problems identified. Revisions to the
SWPPP shall be completed within seven days following inspection.
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4. WATER POLLUTION SOURCE CONTROL
The City of Edmonds Stormwater Supplement and DOE Drainage manual was reviewed to
determine if the proposed land -use for this project required any site -specific source control BMPs
to be constructed. SFR projects are not listed as a proposed use that requires source control.
Therefore, no site/development specific source control BMPs are required for this project.
5. PRESERVATION OF NATURAL DRAINAGE SYSTEM
The runoff from the on -site basin drains to the west towards the western property line and the
adjacent R/W as sheet flow. It was determined that the runoff from the site improvements will be
dispersed on -site, the maximum extent feasible, or directly connected to the existing drainage
system in the R/W. Both the proposed roof & driveway will disperse flows over amended soils
when the required flow paths are available, otherwise roof downspouts will be collected and
directed to a perforated stubout connection before discharging to the existing drainage system
within 91h Ave N. This will allow the maximum amount of runoff to be treated and naturally
absorb into the on -site soils while not concentrating any of the flows. By not grading or
compacting most of the pervious surfaces to remain after construction in the area of disturbance a
maximum amount of site runoff from the developed project will be absorbed into the existing on -
site soils and remain as sheet flow as it leaves the site which will match the existing conditions.
6. ON -SITE STORMWATER MANAGEMENT
The on -site soils for this project have been mapped by the web soil survey as Alderwood Urban
Land Complex. This soil type is typically a till soil in urban/developed areas where the existing
ground has been disturbed/graded. Typically, the depth to the top of the till (hard pan) layer is
between 2-3 feet below the existing grade. The soil unit is gravelly ashy sandy loam. Refer to
Figure 4 to view Soils map. This project proposes to construct approximately 2,250 sf of new plus
replaced roof surfaces and/or walks.
Using the City of Edmonds Appendix A — On -Site Stormwater Management BMP Infeasibility
Criteria the following describes the BM P selection process for this project.
Lawn and Landscaped Areas:
Post -Construction Soif Quality and Depth: This option was selected, and the specifications will be
provided on the future construction engineering plans.
Roofs:
Full Dispersion: Not selected since the 65:10 required ratio could not be met for this project since
the 100-foot retained/native vegetative flow path cannot be provided.
Downspout Full Infiltration:
Based on the disturbed nature of the site and the mapped soil type there is not more than 3-feet
of permeable soils available for this project. In addition, the project Geotech has prepared a
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report based on their site investigations and found the long-term infiltration rate for the existing
soils to be less than 0.05"/hr which is well below the infeasibility rate for infiltration.
Bloretention or Rain Gardens:
Based on the disturbed nature of the site and the mapped soil type there is not more than 3-feet
of permeable soils available for this project and based on Geotechnical findings noted above.
Downspout Dispersion Systems:
SELECTED — Refer to Preliminary Development Plan
No further Roof BMP evaluation for feasibility is necessary.
Other Hard Surfaces:
Permeable Pavement:
Due to site grades it is necessary for much of the proposed driveway to be installed in a cut
condition which causes the bottom of the lowest permeable layer to be within 1-foot of the
identified hard -pan layer and based on Geotechnical findings noted above.
Bio-Retention or Rain Gardens:
See discussion above.
Sheet Flow Dispersion:
SELECTED — Refer to Preliminary Development Plan.
The roof will require a minimum of four splash blocks or level spreaders with additional locations
allowed as needed based on the final roof/gutter layout. Each splash block will provide a
minimum flow path of 50-feet over amended or natural vegetation/topsoil and level spreaders will
provide 25' flow paths.
A total of approximately 1,305 sf of PGIS is proposed for this project. The driveway will be graded
to allow sheet flow dispersion along the north side of the new surface. The runoff will then flow
over amended soils and vegetation to the maximum extent practical before being collected along
the east side of the existing 9th Ave N road surface where that is not possible.
Any other walks or patios would be designed the same to allow runoff to sheet flow off the
surface and spread over amended soils.
All permeable surfaces that are graded or compacted during construction will be amended using
topsoil or compost as provided in the specifications provided on sheet 6 of the plan set.
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7. RUN-OFF TREATMENT
This project proposes to construct less than 5,000 sf of new PGIS and is therefore exempt from
any run-off treatment,
8. FLOW CONTROL
This project proposes to construct less than 5,000 sf of new impervious surfaces and is therefore
exempt from any flow control requirements.
9. WETLANDS PROTECTION
This project will not impact any wetland on -site or downstream of the project site.
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10. OPERATIONS AND MAINTENANCE MANUAL
The Property Owner will be responsible for maintaining the stormwater and landscaping facilities
within this development. Included in this manual are checklists for each feature specific to this
project. Copies should be made of the checklists as necessary during routine inspections and
required maintenance. Specific problems can be recorded along with the appropriate action
taken.
These checklists are a guide for inspections and maintenance. The frequency of the
inspections/maintenance is identified in the left-hand column with the following abbreviations:
A = Annual (March or April preferred)
M = Monthly
S = After Major Storms (Use 1-inch in 24 hours as a guideline)
Routine inspections and maintenance will improve the long-term performance of the stormwater
facilities. If at any time you are unsure if a problem exists or how to address a specific problem,
contact a Professional Engineer.
Refer to Appendix A for a list of each facility to be maintained and the appropriate maintenance
checklist (To be provided with final engineering report).
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APPENDIX A
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