preliminary storm drainage report

Project Manager: James A. Olsen, P.E. Prepared by: Robert H. Stevens, P.E. Reviewed by: Holli H. Heavrin, P.E. First Submittal: May 30, 2017 Revised: August 22, 2017, November 14, 2017 Core No: 17008

Preliminary STORM DRAINAGE REPORT

FOR

HOLMGREN SHORT PLAT

REDMOND, WASHINGTON

Core Design, Inc. HOLMGREN SHORT PLAT STORM DRAINAGE REPORT Page i

Table of Contents 1. Project Overview ................................................................................................................................. 1-1 2. Conditions and Requirement Summary ............................................................................................... 2-1 3. Off-Site Analysis ................................................................................................................................. 3-1

Upstream ....................................................................................................................................... 3-1 Downstream Resource Review ..................................................................................................... 3-1

Redmond Sensitive Areas Maps ............................................................................................. 3-1 FEMA – Flood Insurance Rate Map (FIRM #53033C0369) ................................................. 3-1 USDA Natural Resources Conservation Service Soil Survey ................................................ 3-1

Downstream Field Investigation: .................................................................................................. 3-3 4. Flow Control and Water Quality Facility Analysis and Design .......................................................... 4-1

Design Standards .......................................................................................................................... 4-1 A. Existing Conditions .................................................................................................................. 4-1 B. Upstream Conditions ................................................................................................................ 4-2 D. Developed Conditions .............................................................................................................. 4-5 E. Detention Calculations ............................................................................................................. 4-8 F. Water Quality Calculations ..................................................................................................... 4-16

5. Conveyance System and Analysis and Design .................................................................................... 5-1 6. Special Reports and Studies ................................................................................................................ 6-1 7. Other Permits ....................................................................................................................................... 7-1 8. CSWPPP Analysis and Design ............................................................................................................ 8-1 9. Bond Quantities, Facility Summaries and Declaration of Covenant ................................................... 9-1 10. Operations and Maintenance Manual ................................................................................................ 10-1

Core Design, Inc. HOLMGREN PROPERTY STORM DRAINAGE REPORT Page 1-1

1. Project Overview

The Holmgren property is a proposed residential development consisting of single-family detached units. The Holmgren Property project is located in Section 14, Township 25 North, Range 5 East, W.M, just north of the NE 51st Street uphill from West Lake Sammamish Parkway (see Figure 1-1 - Vicinity Map). The project property totals 0.88 acres. The total area to be developed is 0.99 acres; including frontage improvements. The project is surrounded by single family residential areas. Access to the site will be from NE 51st Street. This project is located within the City of Redmond. The proposed site consists of two parcels as follows:

Table 1-1

KC Parcel # Site Area (SF)

1425059128 15,892

1425059181 22,988

Drainage from the property discharges into an existing rear yard drainage system near the NE corner of the site. Proposed development of the property will include the demolition of all structures on the property and construction of three single-family units on three lots, along with associated roadway, utilities, and drainage detention. Frontage improvements will consist of replacing a small portion of curb with a driveway ramp. Flow control is provided with approximately 11.5 feet of live storage and will match developed discharge durations to 50% of the pre-developed 2-year peak flow to the full 50-year peak flow as determined by King County Runoff Time Series (KCRTS). A single detention vault will be located within tract A on the east side of the subject property and designed per Redmond’s design criteria. The detained flows will then be discharged east to a rear yard drainage pipe system with subsurface drainage pipes where it flows east in the existing rear yard storm drain system. Wellhead Protection Zone Limitations The project site is within Wellhead Protection Zone 4, per the City of Redmond’s Wellhead Protection Zone Map (see Figure 1-2).


Figure 1-1: Vicinity Map


2. Conditions and Requirement Summary

The proposed project is classified as a “Large Project” per Section 3.5, page 52, within the City of Redmond Clearing, Grading, and Stormwater Management Technical Notebook, Issue No. 7A dated March 1, 2017 (Technical Notebook). Per Sections 3.4 and 3.5, this project is classified as a Large Project, since it will exceed 5,000 square feet of new impervious area. Therefore, minimum Requirements #1-9 will be triggered in accordance with Washington State Department of Ecology’s 2012 Stormwater Management Manual for Western Washington as amended in 2014 (2014 SWMMWW). The City of Redmond Clearing, Grading, and Stormwater Management Technical Notebook, Issue Number 7A discusses wellhead protection within the City and corresponding restrictions and limitations. Section 2.5.5, beginning on page 24 of the Technical Notebook, provides the following summary regarding Groundwater Protection in Wellhead Protection Zone 4:

“In Wellhead Protection Zone 4, runoff from pollution generating impervious surfaces can be infiltrated without treatment provided the soil profile provides treatment per Chapter 3.3 of Volume III of the 2005 Ecology Manual. Infiltration of runoff from non-pollution generating impervious areas considered to be clean, including most roofs and sidewalks, is strongly encouraged where feasible.”

As noted above, infiltration is encouraged in Wellhead Protection Zone 4 provided the soil profile supports the applicable water quality treatment requirements. Minimum Requirement #1: Preparation of Stormwater Site Plans: Civil Plans separately and Final Storm Drainage Report herein have been prepared for the subject project. The proposed project is classified as a “Large Project” per Section 3.5, page 57, within the Technical Notebook. Minimum Requirement #2: Construction Stormwater Pollution Prevention: All new development and redevelopment shall comply with the Construction SWPPP Element #1 through Element #13 listed in the 2014 DOE Stormwater Manual. The Holmgren Property project disturbs more than 2,000 square feet of land and will require a Construction SWPPP. An NPDES permit is not required as the total site disturbance is less than one acre. Minimum Requirement #3: Source Control of Pollution: RMC 13.06.066 requires that applicable adopted source control BMPs (operational and structural) be used on all sites except Single Family Residential sites. As this project is a Single Family Residential Site Source Control BMPs do not apply. Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls: Natural drainage patterns shall be maintained, and discharges from the project site shall occur at the natural location, to the maximum extent practicable. How runoff is discharged from the project site must not cause a significant adverse impact to downstream receiving waters and down gradient properties. Discharge from the site is to the existing natural discharge location. See Section 3 of this Report for the downstream analysis and discussion of the natural discharge location. The storm water runoff for the project site will be conveyed to the existing tight line pipe system located in the rear yards of downstream properties.


Minimum Requirement #5: On-Site Stormwater Management: Per the Technical Notebook Appendix B Minimum Requirement 5 Map, the site falls within the Standard LID zone (aka “2014 SWMMWW LID Required”). Projects are required to implement On-site Stormwater Management BMPs to infiltrate, disperse, and retain stormwater runoff onsite to the maximum extent feasible without causing flooding or erosion impacts. Project proponents shall use Ecology’s 2012 Stormwater Management Manual for Western Washington: Volume I of Section 2.5.5, Chapter 3 of Volume III, and Chapter 5 of Volume V to determine which on-site stormwater management BMPs shall be employed on a particular site, and document the determination in the Stormwater Site Plan (Drainage report) Per Figure 2.5.1 in Volume I of the 2012 DOE Manual, this project shall be required to utilize LID techniques for all impervious surfaces. Per Figure 2.5.1 (shown below) this project shall comply with List #2 per Volume I of the 2012 DOE Manual.


Per list #2 the following BMPs were considered for the site: Lawn and Landscaped areas:

• Option 1: Post construction Soil Quality and Depth in accordance with BMP T5.13 in Chapter 5 of Volume V (2012 SWMMWW).

o Response: This site will employ amended soils in lawn and landscape areas and will follow guidelines in Appendix Q of the 2012 (Issue #6) Redmond Stormwater Technical Notebook, “Guidelines for Landscaping with Compost-Amended Soils,” the 2012 City of Redmond Standard Drawing #632, “Soil Amendment and Depth,” or DOE BMP T5.13, “Post-Construction Soil Quality and Depth.”.

Roofs:

• Option 1: Full Dispersion in accordance with BMP T5.30 in Chapter 5 of Volume V, or Downspout Full Infiltration Systems in accordance with BMP T5.10A in Section 3.1.1 in Chapter 3 of Volume III.

o Response: Full dispersion requires 65% of the site be protected in a forest or native condition. The proposed development on this site will be unable to retain 65% in its native condition and is therefore not feasible.

• Option 2: Bioretention (See Chapter 7 of Volume V) facilities that have a minimum horizontally projected surface area below the overflow which is at least 5% of the total surface area draining to it.

o Response: Bioretention requires an infiltration rate of 0.30 in/hr. During site exploration and soil sampling the geotechnical engineer determined the soil to have an infiltration rate of 0.00 in/hr. which is less than the min. required and therefore bioretention is not feasible.

• Option 3: Downspout Dispersion Systems in accordance with BMP T5.10B in section 3.1.2 in Chapter 3 of Volume III

o Response: Splash blocks require a 50-foot vegetated flow path from the downspout to the property line. The site cannot provide the required flow path and is therefore not feasible.

o Response: Dispersion Trenches require a 25-foot vegetated flow path from the trench to the property line and 700 square feet of roof area per 10 feet of trench length. Approximate roofs of the proposed homes are 3,200 square feet and would require a trench of approximately 46 linear feet. Due to the size of the proposed lots, this space is unavailable while maintaining required setbacks from property lines and structures and therefore is not feasible.

• Option 4: Perforated Stub-out Connections in accordance with BMP T5.10C: Perforated Stub-out Connections (p.905)

o Response: Per the geotechnical report the soils on site are not suitable for infiltration therefore perforated stub connections are not feasible. Per City of Redmond detail 673 the required setbacks are 15 feet minimum from structures and 10 feet minimum from property lines. Due to site layout and connection locations, perforated stub connections are not feasible.

Other Hard Surfaces:

• Option 1: Full Dispersion in accordance with BMP T5.30 in Chapter 5 Volume V.


o Response: Full dispersion requires 65% of the site be protected in a forest or native condition. The proposed development on this site will be unable to retain 65% in its native condition and is therefore not feasible.

• Option 2: Permeable pavement in accordance with BMP T5.15 in Chapter 5 of Volume V. o Response: Areas of pavement on site include the access road and driveways. These

surfaces are considered pollution generating and per the Redmond Technical Notebook Section 7.1, page 46, the use of permeable pavement on pollution generating surfaces is not allowed. Therefore, permeable pavement is not feasible. A revision was made to the technical notebook on February 17, 2016 that states “For public hard surfaces where pervious pavement is determined to be required, a “functionally equivalent” alternative design such as draining to a landscape strip or using trench drains and infiltration trenches shall be employed. “Functionally equivalent” means the hard surface and associated infiltration BMP is modeled to meet the 2012 Ecology Manual's “LID Performance Standard”. Per the geotechnical report the soils on site are not suitable for infiltration therefore trench and infiltration drains are not a feasible alternative to permeable pavement.

• Option 3: Bioretention BMPs (See Chapter 7, Volume V) that have a minimum horizontally

projected surface area below the overflow which is at least 5% of the total surface area draining to it.

o Response: Bioretention requires an infiltration rate of 0.30 in/hr. During site exploration and soil sampling the geotechnical engineer determined the soil to have an infiltration rate of 0.00 in/hr. which is less than the minimum required and therefore bioretention is not feasible.

• Option 4: Sheet Flow Dispersion in accordance with BMP T5.12 o Response: Sheet Flow dispersion will be applied to pathways and rear patios in areas

where a 10-foot minimum vegetated flow path can be accomplished while maintaining clearances from property lines.

Minimum Requirement #6: Runoff Treatment: Per the Technical Notebook Appendix C, Minimum Requirement 6 Map, the site is located within the “Basic or Enhanced” Runoff Treatment Custom Area. The following require construction of stormwater treatment facilities (see Table 2.1 included below):

• Projects in which the total of pollution generating impervious surface (PGIS) is 5,000 square feet or more in a threshold discharge area of the project, or

• Projects in which the total of pollution-generating pervious surfaces (PGPS) is three-quarters (3/4) of an acre or more in a threshold discharge area, and from which there is a surface discharge in a natural or man-made conveyance system from the site.

This project exceeds the thresholds for water quality and therefore does require runoff treatment. Runoff treatment will be provided by a combined detention/water quality vault. Design of the water quality facility is described in Section 4 of this Report. Placement of the water quality facility is shown on the Civil Plans. Minimum Requirement #7: Flow Control: Per the Technical Notebook Appendix D, Minimum Requirement 7 Map, the site is located within the “Flow Control Required” Flow Control Requirements


Custom Area. Per Section 2.5.7 of the Technical Notebook, the following require construction of flow control facilities and/or land use management BMPs that will achieve the standard flow control requirement for western Washington (see Table 2.2 included below):

• Projects in which the total effective impervious surface is 10,000 square feet or more in a threshold discharge area; or

• Projects that convert 0.75 acres or more of native vegetation to lawn or landscape, or convert 2.5 acres or more of native vegetation to pasture in a threshold discharge area, and from which there is a surface discharge in a natural or man-made conveyance system from the site; or

• Projects that disturb one acre or more, that through a combination of impervious surfaces and converted pervious surfaces cause a 0.1 cubic feet per second increase in the 100-year flow frequency from a threshold discharge area as estimated using the Western Washington Hydrology Model or other approved model.

This project results in an excess of 10,000 square feet of effective impervious surfaces and therefore requires construction of a flow control facility. Per Section 2.5.7 of the Technical Notebook, the Standard Flow Control Requirements applies with the pre-developed condition to be modeled as forested land cover. Stormwater discharges shall match developed discharge durations to pre-developed durations for the range of pre-developed discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. Design of the flow control facility is described in Section 4 of this Report. Placement of the flow control facility is shown on the Civil Plans. Minimum Requirement #8: Wetlands Protection: This requirement applies only to projects whose stormwater discharges into a wetland, either directly or indirectly through a conveyance system. There are no wetlands located on site or off site in proximity to the project. No additional protection measures are required. Minimum Requirement #9: Operation and Maintenance: Per Section 2.5.9 of the Technical Notebook, an operation and maintenance manual that is consistent with the provisions in Volume V of the 2014 DOE Manual is required for all proposed public and private stormwater facilities including flow control and treatment facilities, conveyance systems, constructed source controls, and green infrastructure. The


operations and maintenance manual shall be a stand-alone document prepared in accordance with the City of Redmond O&M Manual Template (see Appendix N of the Technical Notebook). The development proposal shall include provisions for maintenance of facilities in perpetuity.


3. Off-Site Analysis

Upstream This project has two upstream areas tributary to the site. The open space tract west of Lot 3 is approximately 2/3 of an acre forested valley that slopes to the west property line. The vacant lot and rear yards west of Lots 1 and 2 is also approximately 2/3 of an acre forested area. A bypass storm drain line will be provided to both of these areas.

Downstream Resource Review The following resources were reviewed in preparation of this report. Redmond Sensitive Areas Maps These maps were reviewed for Erosion Hazard Areas, Wetlands, Landslide Hazard, and Seismic Hazard. The project site does not fall within or near any sensitive areas noted on the City Sensitive Area maps. We also reviewed the Streams Classification Map which shows a Class IV stream in the northern portion of the site. The City of Redmond has written assess this stream and have confirmed that it is not a class IV stream. See the Wetland Reconnaissance report by Wetland Resources, December 28, 2016. FEMA – Flood Insurance Rate Map (FIRM #53033C0369) This site is reported to be in Zone X, with no flood areas identified on or near the site. This was also confirmed on the City of Redmond Parcel Viewer online interactive GIS map system. USDA Natural Resources Conservation Service Soil Survey The soil for this property is classified as AgC (Alderwood gravelly sandy loam, 8 to 15 percent slopes) from the Natural Resources Conservation Service Soil Survey.

47° 39' 27'' N12

2° 7

' 37'

' W47° 39' 27'' N

122°

7' 2

3'' W

47° 39' 20'' N

122°

7' 3

7'' W

47° 39' 20'' N

122°

7' 2

3'' W

N

Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84

Feet

MetersMap Scale: 1:1,370 if printed on A landscape (11" x 8.5") sheet.

Soil Map may not be valid at this scale.


Downstream Field Investigation: The field investigation was completed on May 11, 2017. The weather was cloudy and raining. The project site lies within the Sammamish River drainage basin. See Downstream Drainage Exhibit at the end of this section. Runoff on site generally drains to a small ravine running through the middle of the northern half of the project site (Point A). The ravine slopes to the east. At the property line, runoff flows down through gravel and enters one of two pipes completely buried in the gravel. One pipe is an 8-inch corrugated plastic pipe and the other pipe is a 4-inch perforated pipe. Both pipes direct flow to a Type I catch basin with solid lid (Point B) that is located east of the project site. Flow leaves this catch basin through one of two pipes, both flowing to the east. One is an 8-inch corrugated plastic pipe and the other is a 4-inch corrugated plastic pipe. The two pipes leaving Point B direct flow east to another catch basin (Point C) located in the back-yard of the single family home on the corner of 162nd Avenue NE and NE 51st Street, according to as-builts from the City of Redmond. No one was home during the field investigation so this catch basin could not be verified during the field investigation. From Point C, runoff continues east in a 12-inch CPEP pipe that flows to a Type I catch basin with solid lid located on the west side of 162nd Avenue NE (Point D). From Point D, flow goes southeast in a 12-inch PVC pipe to a Type I catch basin with solid lid (Point E) located in the middle of 162nd Avenue NE just before the road meets NE 51st Street. Flow leaves Point E and goes east in a 12-inch PVC pipe to a Type I catch basin located on the eastern corner of the intersection of 162nd Avenue NE and NE 51st Street. Flow leaves the Type I catch basin and heads south to a stormwater manhole located in the middle of NE 51st Street (Point F). This manhole was not able to be opened during the field investigation. According to city of Redmond as-builts, runoff leaves Point F and flows northeast along the middle of NE 51st Street through 24-inch pipe. It passes through four more stormwater manholes along NE 51st Street after Point F and then reaches West Lake Sammamish Parkway NE. From the intersection of NE 51st Street and West Lake Sammamish Parkway NE, stormwater continues to head northeast to a large naturally vegetated area directly east of West Lake Sammamish Parkway NE. The outlet of the 24-inch mainline pipe was found east of West Lake Sammamish Parkway NE. This pipe outlets into a concrete channel that directs the flow southeast. The flow exits the concrete channel through a pipe heading southeast that flows to a very deep Type II catch basin with solid lid (Point G). Flow leaves Point G and discharges to the naturally vegetated area. The ¼ mile downstream mark is reached in the large naturally vegetated area (Point H), terminating the field investigation. Based on King County iMap, runoff moves north and east through the large naturally vegetated area and flows into the Sammamish River approximately 400 feet beyond the quarter mile downstream point. The downstream system appears, in general, to be stable. The following pages show photos of the downstream path. See Downstream Drainage Exhibit on the following page for location of photos.


1 – Looking east where runoff leaves the project

site.

2 – Looking northwest at Type I CB with solid lid.

3 – Looking north at Type I CB with solid lid on the

west side of 162nd Ave NE

4 – Looking southeast at Type I CB with solid lid

in the middle of 162ns Ave NE just before NE 51st St.


5 – Looking southeast at Type I CB at intersection

of 162nd Ave NE and NE 51st St.

6 – Looking northwest at stormwater manhole

in the middle of NE 51st Street.

7 – Looking southeast at stormwater manhole at

intersection of NE 51st St and W Lake Sammamish Pkwy NE

8 – Looking southwest at outlet end of 24” pipe

running down the middle of NE 51st St


9 – Looking southeast runoff leaving concrete

channel through the pipe shown.

10 – Looking west at deep Type II catch basin

with solid lid.

11 – Looking west at pipe flowing into naturally

vegetated area.


4. Flow Control and Water Quality Facility Analysis and Design

Design Standards The detention facility will be designed per the Washington State Department of Ecology’s Stormwater Management Manual for Western Washington, December 2014 Edition (DOE Manual) and the City of Redmond Clearing, Grading, and Stormwater Management Technical Notebook Issue 7A. The drainage analysis for flow control was modeled using the King County Runoff Time Series (KCRTS) software as allowed per Chapter 2.1.1 within Volume III of the 2014 DOE Manual. The vault will match developed discharge durations to 50% of the pre-developed 2-year peak flow to the full 50-year peak flow and provide dead storage The site is located in the Sea-Tac rainfall region with a location scale factor of 1.0. Per the NRCS Soil Survey, the site soils are entirely Alderwood gravelly sandy loam, (AgC), KCRTS group Till and Hydrologic Group C.

A. Existing Conditions The existing basin boundary area is defined as that area that will be improved through development of the subject property including the frontage improvement area. The existing basin boundary area, 0.99 acres, will include the 0.89-acre project site, plus 0.1 acres of offsite access roadway area. Existing conditions for the project area was analyzed assuming 100% pervious ground cover consisting of forested land cover as mapped in the Technical Notebook, Appendix E, Map of Historical Land Cover.

Table 4-1: Existing Conditions Areas

Land Cover Area (acres)

Till-Forest 0.99

The KCRTS hourly pre-developed peaks are shown below.


Flow Frequency Analysis Time Series File:17008_existing.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.063 2 2/09/01 18:00 0.080 1 100.00 0.990 0.017 7 1/06/02 3:00 0.063 2 25.00 0.960 0.046 4 2/28/03 3:00 0.048 3 10.00 0.900 0.002 8 3/24/04 20:00 0.046 4 5.00 0.800 0.027 6 1/05/05 8:00 0.041 5 3.00 0.667 0.048 3 1/18/06 20:00 0.027 6 2.00 0.500 0.041 5 11/24/06 4:00 0.017 7 1.30 0.231 0.080 1 1/09/08 9:00 0.002 8 1.10 0.091 Computed Peaks 0.074 50.00 0.980

B. Upstream Conditions The small valley that slopes to the west property line of lot 3 is not expected to convey concentrated flows, but a conveyance system will be provided for any extreme events. An existing perforated pipe is located within the valley. This perforated pipe will be connected to CB 9 as shown in the engineering plans. Additional perforated pipe will run north and south of CB 9 to collect any drainage from the valley. This conveyance system will not be routed through the site, avoiding the onsite detention system, but will discharge to the same location as the detention system. Additionally, a forested vacant lot lies west of lot 2; a stub will be provided to the bypass system for any future development of this lot. These areas combined are delineated below.

Table 4-2: Upstream Conditions Areas


Till-Forest 1.3


The KCRTS hourly and 15 minute pre-developed peaks are shown below. Flow Frequency Analysis Time Series File:17008_upstream.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.082 2 2/09/01 18:00 0.105 1 100.00 0.990 0.022 7 1/06/02 3:00 0.082 2 25.00 0.960 0.061 4 2/28/03 3:00 0.063 3 10.00 0.900 0.002 8 3/24/04 21:00 0.061 4 5.00 0.800 0.036 6 1/05/05 8:00 0.053 5 3.00 0.667 0.063 3 1/18/06 20:00 0.036 6 2.00 0.500 0.053 5 11/24/06 4:00 0.022 7 1.30 0.231 0.105 1 1/09/08 9:00 0.002 8 1.10 0.091 Computed Peaks 0.097 50.00 0.980

Flow Frequency Analysis Time Series File:17008_upstream_15min.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.085 2 2/09/01 17:15 0.142 1 100.00 0.990 0.025 7 1/06/02 2:30 0.085 2 25.00 0.960 0.060 4 3/06/03 18:45 0.065 3 10.00 0.900 0.003 8 3/08/04 18:45 0.060 4 5.00 0.800 0.040 6 1/05/05 7:45 0.056 5 3.00 0.667 0.065 3 1/18/06 20:15 0.040 6 2.00 0.500 0.056 5 11/24/06 5:15 0.025 7 1.30 0.231 0.142 1 1/09/08 9:30 0.003 8 1.10 0.091 Computed Peaks 0.123 50.00 0.980


D. Developed Conditions Proposed development of the property will include the demolition of all structures on the property and construction of three single-family units on three lots, along with associated roadway, utilities, and stormwater facilities. No frontage improvements will be necessary, rather a new driveway ramp will be constructed to allow vehicular access to the entry road. A single detention vault will be located within Tract A on the eastern side of the property and designed per Redmond’s design criteria. The detained flows will then be conveyed east to the existing rear yard drainage system east of the site. The developed basin boundary, 0.99 acres, is the same as the existing basin boundary, consisting of the project site and access road. The impervious coverage calculations for the developed site are delineated below.

• Impervious Coverage for the offsite access road including new sidewalk The offsite impervious area of the access road and sidewalk is 4,200 SF or 0.1 acres.

• Impervious Coverage for the Lots

The impervious coverage within the lots areas are determined to be 80% of the allowable impervious area per zoning code. The project is in R-4 zoning, which is allowed 60% impervious coverage. With a total lot area of 0.79 acres, the total lot impervious coverage is 0.79 acres * 80% * 60% = 0.38 acres.

See Table 4-3 and Figure 4-2: Developed Conditions for a summary of the developed conditions areas.

Table 4-3: Developed Conditions Areas


Impervious 0.48

Till-Grass 0.51

Total Area 0.99

The KCRTS hourly developed peaks are shown below. Flow Frequency Analysis Time Series File:17008_dev.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.137 6 2/09/01 2:00 0.263 1 100.00 0.990 0.111 8 1/05/02 16:00 0.174 2 25.00 0.960


0.159 3 2/27/03 7:00 0.159 3 10.00 0.900 0.119 7 8/26/04 2:00 0.147 4 5.00 0.800 0.142 5 10/28/04 16:00 0.142 5 3.00 0.667 0.147 4 1/18/06 16:00 0.137 6 2.00 0.500 0.174 2 10/26/06 0:00 0.119 7 1.30 0.231 0.263 1 1/09/08 6:00 0.111 8 1.10 0.091 Computed Peaks 0.234 50.00 0.980


E. Detention Calculations The King County Runoff Time Series software (KCRTS) was utilized to size the detention portion of the vault per the Department of Ecology requirements. A detention vault will be located within Tract A on the eastern side of the site. The detained flows will then be discharged through a new conveyance line, easterly into an existing tight-lined storm drainage system located in the rear and side yards of downstream lots. Criteria for flow control per the DOE Manual is equivalent to the KCRTS Level 2 Flow Control Standard, where the 2-year and 10-year peak flow rates must not exceed the corresponding existing rates while also matching the durations from ½ of the 2-year through the 50-year storm event without exceeding any single point by more than 10% or for 50% of the time. KCRTS will be utilized to size the detention vault for this project. KCRTS documentation (input and output) is included at the end of this section. The allowable release rates/durations are calculated as 17008_existing.. The total developed flows discharging to the vault are calculated as 17008_Dev.tsf. The provided vault will have an inside total width of 13 feet, an inside length of 64 feet, an outlet elevation of 141.00, a 100-year maximum water surface elevation of 152.51 (provided volume of 9,577 CF) with a min. 0.5 feet of freeboard. Calculations provided on the following pages show the minimum required dimension of the vault, 13’x64’. Retention/Detention Facility Type of Facility: Detention Vault Facility Length: 12.00 ft Facility Width: 100.00 ft Facility Area: 1200. sq. ft Effective Storage Depth: 8.00 ft Stage 0 Elevation: 144.00 ft Storage Volume: 9600. cu. ft Riser Head: 8.00 ft Riser Diameter: 12.00 inches Number of orifices: 2 Full Head Pipe Orifice # Height Diameter Discharge Diameter (ft) (in) (CFS) (in) 1 0.00 0.44 0.015 2 5.80 1.13 0.051 4.0 Top Notch Weir: None Outflow Rating Curve: None Stage Elevation Storage Discharge Percolation (ft) (ft) (cu. ft) (ac-ft) (cfs) (cfs) 0.00 144.00 0. 0.000 0.000 0.00 0.01 144.01 12. 0.000 0.001 0.00 0.02 144.02 24. 0.001 0.001 0.00 0.03 144.03 36. 0.001 0.001 0.00 0.04 144.04 48. 0.001 0.001 0.00


0.17 144.17 204. 0.005 0.002 0.00 0.31 144.31 372. 0.009 0.003 0.00 0.44 144.44 528. 0.012 0.003 0.00 0.58 144.58 696. 0.016 0.004 0.00 0.71 144.71 852. 0.020 0.004 0.00 0.85 144.85 1020. 0.023 0.005 0.00 0.99 144.99 1188. 0.027 0.005 0.00 1.12 145.12 1344. 0.031 0.006 0.00 1.26 145.26 1512. 0.035 0.006 0.00 1.39 145.39 1668. 0.038 0.006 0.00 1.53 145.53 1836. 0.042 0.006 0.00 1.66 145.66 1992. 0.046 0.007 0.00 1.80 145.80 2160. 0.050 0.007 0.00 1.93 145.93 2316. 0.053 0.007 0.00 2.07 146.07 2484. 0.057 0.007 0.00 2.21 146.21 2652. 0.061 0.008 0.00 2.34 146.34 2808. 0.064 0.008 0.00 2.48 146.48 2976. 0.068 0.008 0.00 2.61 146.61 3132. 0.072 0.008 0.00 2.75 146.75 3300. 0.076 0.009 0.00 2.88 146.88 3456. 0.079 0.009 0.00 3.02 147.02 3624. 0.083 0.009 0.00 3.16 147.16 3792. 0.087 0.009 0.00 3.29 147.29 3948. 0.091 0.009 0.00 3.43 147.43 4116. 0.094 0.010 0.00 3.56 147.56 4272. 0.098 0.010 0.00 3.70 147.70 4440. 0.102 0.010 0.00 3.83 147.83 4596. 0.106 0.010 0.00 3.97 147.97 4764. 0.109 0.010 0.00 4.10 148.10 4920. 0.113 0.011 0.00 4.24 148.24 5088. 0.117 0.011 0.00 4.38 148.38 5256. 0.121 0.011 0.00 4.51 148.51 5412. 0.124 0.011 0.00 4.65 148.65 5580. 0.128 0.011 0.00 4.78 148.78 5736. 0.132 0.011 0.00 4.92 148.92 5904. 0.136 0.012 0.00 5.05 149.05 6060. 0.139 0.012 0.00 5.19 149.19 6228. 0.143 0.012 0.00 5.32 149.32 6384. 0.147 0.012 0.00 5.46 149.46 6552. 0.150 0.012 0.00 5.60 149.60 6720. 0.154 0.012 0.00 5.73 149.73 6876. 0.158 0.012 0.00 5.80 149.80 6960. 0.160 0.013 0.00 5.81 149.81 6972. 0.160 0.013 0.00 5.82 149.82 6984. 0.160 0.014 0.00 5.84 149.84 7008. 0.161 0.015 0.00 5.85 149.85 7020. 0.161 0.017 0.00 5.86 149.86 7032. 0.161 0.019 0.00 5.87 149.87 7044. 0.162 0.022 0.00 5.88 149.88 7056. 0.162 0.022 0.00 5.89 149.89 7068. 0.162 0.023 0.00 6.03 150.03 7236. 0.166 0.029 0.00 6.16 150.16 7392. 0.170 0.034 0.00 6.30 150.30 7560. 0.174 0.037 0.00


6.44 150.44 7728. 0.177 0.041 0.00 6.57 150.57 7884. 0.181 0.043 0.00 6.71 150.71 8052. 0.185 0.046 0.00 6.84 150.84 8208. 0.188 0.049 0.00 6.98 150.98 8376. 0.192 0.051 0.00 7.11 151.11 8532. 0.196 0.053 0.00 7.25 151.25 8700. 0.200 0.055 0.00 7.39 151.39 8868. 0.204 0.057 0.00 7.52 151.52 9024. 0.207 0.059 0.00 7.66 151.66 9192. 0.211 0.061 0.00 7.79 151.79 9348. 0.215 0.063 0.00 7.93 151.93 9516. 0.218 0.065 0.00 8.00 152.00 9600. 0.220 0.066 0.00 8.10 152.10 9720. 0.223 0.375 0.00 8.20 152.20 9840. 0.226 0.939 0.00 8.30 152.30 9960. 0.229 1.670 0.00 8.40 152.40 10080. 0.231 2.460 0.00 8.50 152.50 10200. 0.234 2.750 0.00 8.60 152.60 10320. 0.237 3.000 0.00 8.70 152.70 10440. 0.240 3.240 0.00 8.80 152.80 10560. 0.242 3.460 0.00 8.90 152.90 10680. 0.245 3.660 0.00 9.00 153.00 10800. 0.248 3.860 0.00 9.10 153.10 10920. 0.251 4.040 0.00 9.20 153.20 11040. 0.253 4.220 0.00 9.30 153.30 11160. 0.256 4.390 0.00 9.40 153.40 11280. 0.259 4.560 0.00 9.50 153.50 11400. 0.262 4.710 0.00 9.60 153.60 11520. 0.264 4.870 0.00 9.70 153.70 11640. 0.267 5.020 0.00 9.80 153.80 11760. 0.270 5.160 0.00 9.90 153.90 11880. 0.273 5.300 0.00 Hyd Inflow Outflow Peak Storage Target Calc Stage Elev (Cu-Ft) (Ac-Ft) 1 0.26 0.08 0.10 8.01 152.01 9612. 0.221 2 0.14 ******* 0.06 7.61 151.61 9135. 0.210 3 0.14 ******* 0.04 6.29 150.29 7547. 0.173 4 0.16 ******* 0.05 7.02 151.02 8418. 0.193 5 0.15 ******* 0.02 5.85 149.85 7014. 0.161 6 0.09 ******* 0.01 4.90 148.90 5884. 0.135 7 0.11 ******* 0.01 4.03 148.03 4831. 0.111 8 0.12 ******* 0.01 2.84 146.84 3412. 0.078 ---------------------------------- Route Time Series through Facility Inflow Time Series File:17008_dev.tsf Outflow Time Series File:rdout Inflow/Outflow Analysis Peak Inflow Discharge: 0.263 CFS at 6:00 on Jan 9 in Year 8 Peak Outflow Discharge: 0.097 CFS at 11:00 on Jan 9 in Year 8 Peak Reservoir Stage: 8.01 Ft Peak Reservoir Elev: 152.01 Ft


Peak Reservoir Storage: 9612. Cu-Ft : 0.221 Ac-Ft Flow Duration from Time Series File:rdout.tsf Cutoff Count Frequency CDF Exceedence_Probability CFS % % % 0.001 30566 49.847 49.847 50.153 0.502E+00 0.003 10648 17.365 67.211 32.789 0.328E+00 0.004 8197 13.368 80.579 19.421 0.194E+00 0.006 5747 9.372 89.951 10.049 0.100E+00 0.008 2088 3.405 93.356 6.644 0.664E-01 0.009 2091 3.410 96.766 3.234 0.323E-01 0.011 1216 1.983 98.749 1.251 0.125E-01 0.013 554 0.903 99.653 0.347 0.347E-02 0.015 46 0.075 99.728 0.272 0.272E-02 0.016 19 0.031 99.759 0.241 0.241E-02 0.018 5 0.008 99.767 0.233 0.233E-02 0.020 4 0.007 99.773 0.227 0.227E-02 0.021 2 0.003 99.777 0.223 0.223E-02 0.023 11 0.018 99.795 0.205 0.205E-02 0.025 8 0.013 99.808 0.192 0.192E-02 0.026 8 0.013 99.821 0.179 0.179E-02 0.028 14 0.023 99.843 0.157 0.157E-02 0.030 6 0.010 99.853 0.147 0.147E-02 0.031 7 0.011 99.865 0.135 0.135E-02 0.033 5 0.008 99.873 0.127 0.127E-02 0.035 8 0.013 99.886 0.114 0.114E-02 0.037 9 0.015 99.901 0.099 0.995E-03 0.038 4 0.007 99.907 0.093 0.930E-03 0.040 3 0.005 99.912 0.088 0.881E-03 0.042 9 0.015 99.927 0.073 0.734E-03 0.043 6 0.010 99.936 0.064 0.636E-03 0.045 6 0.010 99.946 0.054 0.538E-03 0.047 5 0.008 99.954 0.046 0.457E-03 0.048 3 0.005 99.959 0.041 0.408E-03 0.050 4 0.007 99.966 0.034 0.342E-03 0.052 6 0.010 99.976 0.024 0.245E-03 0.053 3 0.005 99.980 0.020 0.196E-03 0.055 1 0.002 99.982 0.018 0.179E-03 0.057 3 0.005 99.987 0.013 0.130E-03 0.059 2 0.003 99.990 0.010 0.978E-04 0.060 3 0.005 99.995 0.005 0.489E-04 ---------------------------------- Route Time Series through Facility Inflow Time Series File:17008_dev.tsf Outflow Time Series File:rdout Inflow/Outflow Analysis Peak Inflow Discharge: 0.263 CFS at 6:00 on Jan 9 in Year 8 Peak Outflow Discharge: 0.097 CFS at 11:00 on Jan 9 in Year 8 Peak Reservoir Stage: 8.01 Ft Peak Reservoir Elev: 152.01 Ft


Peak Reservoir Storage: 9612. Cu-Ft : 0.221 Ac-Ft Flow Duration from Time Series File:rdout.tsf Cutoff Count Frequency CDF Exceedence_Probability CFS % % % 0.001 30566 49.847 49.847 50.153 0.502E+00 0.003 10648 17.365 67.211 32.789 0.328E+00 0.004 8197 13.368 80.579 19.421 0.194E+00 0.006 5747 9.372 89.951 10.049 0.100E+00 0.008 2088 3.405 93.356 6.644 0.664E-01 0.009 2091 3.410 96.766 3.234 0.323E-01 0.011 1216 1.983 98.749 1.251 0.125E-01 0.013 554 0.903 99.653 0.347 0.347E-02 0.015 46 0.075 99.728 0.272 0.272E-02 0.016 19 0.031 99.759 0.241 0.241E-02 0.018 5 0.008 99.767 0.233 0.233E-02 0.020 4 0.007 99.773 0.227 0.227E-02 0.021 2 0.003 99.777 0.223 0.223E-02 0.023 11 0.018 99.795 0.205 0.205E-02 0.025 8 0.013 99.808 0.192 0.192E-02 0.026 8 0.013 99.821 0.179 0.179E-02 0.028 14 0.023 99.843 0.157 0.157E-02 0.030 6 0.010 99.853 0.147 0.147E-02 0.031 7 0.011 99.865 0.135 0.135E-02 0.033 5 0.008 99.873 0.127 0.127E-02 0.035 8 0.013 99.886 0.114 0.114E-02 0.037 9 0.015 99.901 0.099 0.995E-03 0.038 4 0.007 99.907 0.093 0.930E-03 0.040 3 0.005 99.912 0.088 0.881E-03 0.042 9 0.015 99.927 0.073 0.734E-03 0.043 6 0.010 99.936 0.064 0.636E-03 0.045 6 0.010 99.946 0.054 0.538E-03 0.047 5 0.008 99.954 0.046 0.457E-03 0.048 3 0.005 99.959 0.041 0.408E-03 0.050 4 0.007 99.966 0.034 0.342E-03 0.052 6 0.010 99.976 0.024 0.245E-03 0.053 3 0.005 99.980 0.020 0.196E-03 0.055 1 0.002 99.982 0.018 0.179E-03 0.057 3 0.005 99.987 0.013 0.130E-03 0.059 2 0.003 99.990 0.010 0.978E-04 0.060 3 0.005 99.995 0.005 0.489E-04 Duration Comparison Anaylsis Base File: 17008_existing.tsf New File: rdout.tsf Cutoff Units: Discharge in CFS -----Fraction of Time----- ---------Check of Tolerance------- Cutoff Base New %Change Probability Base New %Change 0.013 | 0.96E-02 0.30E-02 -68.9 | 0.96E-02 0.013 0.012 -11.2 0.017 | 0.65E-02 0.24E-02 -63.4 | 0.65E-02 0.017 0.012 -30.3 0.021 | 0.50E-02 0.23E-02 -54.9 | 0.50E-02 0.021 0.012 -42.7 0.025 | 0.38E-02 0.19E-02 -49.4 | 0.38E-02 0.025 0.013 -49.4


0.029 | 0.29E-02 0.15E-02 -47.7 | 0.29E-02 0.029 0.014 -50.4 0.032 | 0.22E-02 0.13E-02 -40.0 | 0.22E-02 0.032 0.022 -32.6 0.036 | 0.15E-02 0.10E-02 -28.9 | 0.15E-02 0.036 0.030 -17.6 0.040 | 0.10E-02 0.88E-03 -14.3 | 0.10E-02 0.040 0.036 -9.0 0.044 | 0.62E-03 0.59E-03 -5.3 | 0.62E-03 0.044 0.044 -0.3 0.048 | 0.34E-03 0.44E-03 28.6 | 0.34E-03 0.048 0.050 5.2 0.051 | 0.21E-03 0.28E-03 30.8 | 0.21E-03 0.051 0.053 3.5 0.055 | 0.16E-03 0.18E-03 10.0 | 0.16E-03 0.055 0.056 0.8 0.059 | 0.82E-04 0.82E-04 0.0 | 0.82E-04 0.059 0.059 0.2 0.063 | 0.16E-04 0.00E+00 -100.0 | 0.16E-04 0.063 0.061 -2.5 Maximum positive excursion = 0.003 cfs ( 6.6%) occurring at 0.046 cfs on the Base Data:17008_existing.tsf and at 0.050 cfs on the New Data:rdout.tsf Maximum negative excursion = 0.015 cfs (-53.2%) occurring at 0.028 cfs on the Base Data:17008_existing.tsf and at 0.013 cfs on the New Data:rdout.tsf ---------------------------------- Route Time Series through Facility Inflow Time Series File:17008_dev.tsf Outflow Time Series File:rdout Inflow/Outflow Analysis Peak Inflow Discharge: 0.263 CFS at 6:00 on Jan 9 in Year 8 Peak Outflow Discharge: 0.097 CFS at 11:00 on Jan 9 in Year 8 Peak Reservoir Stage: 8.01 Ft Peak Reservoir Elev: 152.01 Ft Peak Reservoir Storage: 9612. Cu-Ft : 0.221 Ac-Ft Flow Duration from Time Series File:rdout.tsf Cutoff Count Frequency CDF Exceedence_Probability CFS % % % 0.001 30566 49.847 49.847 50.153 0.502E+00 0.003 10648 17.365 67.211 32.789 0.328E+00 0.004 8197 13.368 80.579 19.421 0.194E+00 0.006 5747 9.372 89.951 10.049 0.100E+00 0.008 2088 3.405 93.356 6.644 0.664E-01 0.009 2091 3.410 96.766 3.234 0.323E-01 0.011 1216 1.983 98.749 1.251 0.125E-01 0.013 554 0.903 99.653 0.347 0.347E-02 0.015 46 0.075 99.728 0.272 0.272E-02 0.016 19 0.031 99.759 0.241 0.241E-02 0.018 5 0.008 99.767 0.233 0.233E-02 0.020 4 0.007 99.773 0.227 0.227E-02 0.021 2 0.003 99.777 0.223 0.223E-02 0.023 11 0.018 99.795 0.205 0.205E-02 0.025 8 0.013 99.808 0.192 0.192E-02 0.026 8 0.013 99.821 0.179 0.179E-02 0.028 14 0.023 99.843 0.157 0.157E-02 0.030 6 0.010 99.853 0.147 0.147E-02


0.031 7 0.011 99.865 0.135 0.135E-02 0.033 5 0.008 99.873 0.127 0.127E-02 0.035 8 0.013 99.886 0.114 0.114E-02 0.037 9 0.015 99.901 0.099 0.995E-03 0.038 4 0.007 99.907 0.093 0.930E-03 0.040 3 0.005 99.912 0.088 0.881E-03 0.042 9 0.015 99.927 0.073 0.734E-03 0.043 6 0.010 99.936 0.064 0.636E-03 0.045 6 0.010 99.946 0.054 0.538E-03 0.047 5 0.008 99.954 0.046 0.457E-03 0.048 3 0.005 99.959 0.041 0.408E-03 0.050 4 0.007 99.966 0.034 0.342E-03 0.052 6 0.010 99.976 0.024 0.245E-03 0.053 3 0.005 99.980 0.020 0.196E-03 0.055 1 0.002 99.982 0.018 0.179E-03 0.057 3 0.005 99.987 0.013 0.130E-03 0.059 2 0.003 99.990 0.010 0.978E-04 0.060 3 0.005 99.995 0.005 0.489E-04

Flow Frequency Analysis Time Series File:rdout.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) (ft) Period 0.061 2 2/09/01 20:00 0.097 8.01 1 100.00 0.990 0.010 7 1/07/02 4:00 0.061 7.67 2 25.00 0.960 0.052 3 3/06/03 22:00 0.052 7.02 3 10.00 0.900 0.009 8 8/26/04 7:00 0.037 6.29 4 5.00 0.800 0.012 6 1/08/05 5:00 0.016 5.85 5 3.00 0.667 0.016 5 1/19/06 13:00 0.012 4.90 6 2.00 0.500 0.037 4 11/24/06 8:00 0.010 4.03 7 1.30 0.231 0.097 1 1/09/08 11:00 0.009 2.84 8 1.10 0.091 Computed Peaks 0.085 8.01 50.00 0.980

Probability Exceedence

Dis

char

ge (C

FS)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

-510 -410 -310 -210 -110 010o

oo

R

rdout.durtarget.dur


F. Water Quality Calculations Basic Treatment is required for the subject site and a combined detention/wetpool vault will be designed and located in Tract A near the northeast corner of the project site. Per page 10-4 in Volume V of the 2012 Stormwater Management Manual for Western Washington, basic water quality treatment volume is defined as the volume of runoff from the water quality design storm – the 6-month, 24-hour storm event. Per page 4-1 in Volume V of the 2012 Stormwater Management Manual for Western Washington, the 6-month, 24-hour precipitation amount is assumed to be 72% of the 2-year, 24-hour amount.

The drainage analysis for water quality sizing was modeled using Western Washington Hydrology Model. The proposed vault will be utilized for both detention and water quality treatment.

The required volume of dead storage for the developed basin is 0.0748 acre feet or 3,258 CF. See WWHM Water Quality Volume report below; the project will use the on-line facility volume. The dead storage will be provided in the bottom 4 feet of the vault below the outlet elevation of 144.0. Dead storage will be 4 feet deep, 12 feet wide and 100 feet long. The total dead storage volume provided is therefore, 4’x 12’x 100’ = 4,800 CF.

Water Quality BMP Flow and Volume for POC #1 On-line facility volume: 0.0748 acre-feet On-line facility target flow: 0.0774 cfs. Adjusted for 15 min: 0.0774 cfs. Off-line facility target flow: 0.043 cfs. Adjusted for 15 min: 0.043 cfs.


5. Conveyance System and Analysis and Design

Conveyance calculations will be submitted at the time of final design.


6. Special Reports and Studies

The reports listed are provided under separate cover.

Geotechnical Report Prepared for: Holmgren Short Plat Prepared by: Theodore Schepper, PE and Alexander Dendy, EIT Dated: March 9, 2017 Revised: Terra Associates, Inc. 12220 113th Ave NE, Suite 130 Kirkland, WA 98034


7. Other Permits

Additional permits include the following:

• Right-Of-Way Use Permit


8. CSWPPP Analysis and Design

Design of the Erosion and Sediment Control plan was completed in conformance with Minimum Requirement #2 per the 2005 DOE Manual. Compliance with the 12 minimum requirements is demonstrated in the Construction Stormwater Pollution Prevention Plan report provided under separate cover. The site will follow Chapter 4 of Volume II in the 2014 DOE Manual for the erosion and sedimentation control design to reduce the discharge of sediment-laden runoff from the site. Clearing limits will be established prior to any earthwork on the project site. Perimeter protection will be provided by silt fencing along the downstream perimeter of the disturbed areas to limit the downstream transport of sediment to streams, wetlands and neighboring properties. Dust control, if required, will be provided by a water truck. A Certified Erosion and Sediment Control Lead inspector will be present onsite during earthwork activities. The inspector shall determine frequency of watering of the project site and will authorize and direct any additional erosion and sediment control measures as needed during all construction activities. The erosion control plan will be comprised of temporary measures (stabilized construction entrance, silt fence, interceptor swales, etc.) as well as permanent measures (detention vault, hydroseeding, etc.). In general, construction activities will be sequenced such that the permanent detention vault will be constructed and used for temporary erosion and sedimentation control. Runoff from the site will sheet flow across cleared areas into temporary interceptor swales and into the permanent detention vault that will serve as a temporary sediment pond. If a permanent detention vault is not installed and areas are cleared, a sediment pond shall be constructed to meet the minimum surface area requirements for the cleared area. TESC pond area was calculated for the project site using the 2-year storm event per BMP C241 Temporary Sediment Pond (page 4-11 of Volume II in the 2014 DOE Manual). The corresponding 2-year developed condition peak flow rate used for TESC pond area sizing are as follows:

• Developed 2-year Storm, 15 min interval: Q2 = 0.228 cfs • Developed 100-year Runoff, 15 min. interval: Q100 = 0.824 cfs


Flow Frequency Analysis Time Series File:17008_dev_15.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.228 6 8/27/01 18:00 0.824 1 100.00 0.990 0.174 8 1/05/02 15:00 0.544 2 25.00 0.960 0.544 2 12/08/02 17:15 0.356 3 10.00 0.900 0.184 7 8/23/04 14:30 0.327 4 5.00 0.800 0.356 3 11/17/04 5:00 0.289 5 3.00 0.667 0.289 5 10/27/05 10:45 0.228 6 2.00 0.500 0.327 4 10/25/06 22:45 0.184 7 1.30 0.231 0.824 1 1/09/08 6:30 0.174 8 1.10 0.091 Computed Peaks 0.731 50.00 0.980

TESC Pond Surface Area Calculations TESC Pond minimum surface area, SA, is determined by the following equation: SA = 2080 sf/cfs * (Total Q2-developed) (page 4-103, Volume II DOE Manual)

Where: SA = minimum TESC pond surface area (sf) Q2-developed = 2-year developed peak flow (0.228 cfs)

SAmin = 474 sf The design surface area of the proposed vault is 832 square feet, more than enough to provide adequate erosion and sedimentation control. Principal Spillway (Riser Pipe) The TESC pond and permanent detention vault is designed to function with 0.5 feet (6 inches) of head. The riser pipe must pass the pre-developed condition 10-year peak flow. Using Figure 4.2.21 – Riser Inflow Curves (page 4-109, Volume II DOE Manual) the minimum riser pipe diameter for pond/vault is less than 10 inches. The riser pipe diameter for the permanent detention vault is 12 inches, more than enough to provide adequate erosion and sedimentation control.


Emergency Overflow Spillway The TESC pond (permanent detention vault) must safely convey the developed condition 100-year peak flow. Since the permanent detention vault will be installed and used as the TESC pond, it will provide appropriate erosion and sedimentation control and it has been sized according to the City of Redmond minimum flow control requirements (including overflow standards). Emergency overflow spillway calculations follow to verify minimum design value. The minimum length of emergency overflow spillway for the TESC pond/vault follows the guidelines in Section 3.2.1 (page 3-36) of Volume III in the DOE Manual, and is calculated based on the following equation (Equation 3, page 3-40, Volume III DOE Manual):

𝐿𝐿 =𝑄𝑄100

3.21𝑥𝑥𝐻𝐻3/2 − 2.4 ∗ 𝐻𝐻

Where: L = minimum length of spillway (greater of calculated L or 6 feet) Q100 = developed 100-year peak flow H = height of water above spillway (0.2 ft minimum)

Q100 = 0.824 cfs

H = 0.25 ft

LA = 2.15 ft

Therefore, the design minimum emergency overflow spillway for the detention vault is 6 feet.


9. Bond Quantities, Facility Summaries and Declaration of Covenant

A bond estimate will be included for the improvements within the existing right of way upon final design.


10. Operations and Maintenance Manual

The Operations and Maintenance Manual will be included upon final design.

preliminary storm drainage report

Documents