18-010 Legacy Alderwood- Preliminary Storm Drainage Report … · Preliminary Storm Drainage Report RAM No. 18-010 Page G-1 . G. GRADING and EROSION CONTROL SUMMARY (Preliminary SWPPP

Legacy Alderwood 18601 33rd Ave W.

Lynnwood, WA 98037

Preliminary Storm Drainage Report

Prepared for:

Legacy Hospitality, Inc.

Date: February 7, 2020

2/7/2020 Prepared by: Rob Long, PE

RAM Engineering, Inc. 16531 13th Ave W, Suite A108, Lynnwood WA 98037

(425) 678-6960 RAMengineeringinc.com Job No. 18-010

kholdsworthReceived

kholdsworthText BoxLegacy Alderwood Hotel

kholdsworthText BoxPDR-008980-2020 and ERC-008981-2020

RAM Engineering, Inc. Legacy Alderwood

Preliminary Storm Drainage Report RAM No. 18-010 i

Legacy Alderwood

Introduction: This summary report provides site design information for a 150-unit hotel by Legacy

Hospitality, Inc. This report includes preliminary storm drainage analysis to support preliminary

permit review and approval. The property is located on the westerly edge of the Alderwood Mall

at 18601 33rd Ave W, in the northeast 1/4 of Section 15, T 27 N, R 4 E, W.M.

Site Address: 18601 33rd Ave W.

Lynnwood, WA 98037

Tax Parcel Number: 270410-004-018-00

(portion of)

Applicant: Legacy Hospitality, Inc.

150 120th Ave NE, Bldg F, Ste 305

Bellevue, WA 98005

Contact: Faizel Kassam

TABLE OF CONTENTS

SECTION PAGES

A. Project Summary ............................................................................................................................... 3

B. Parcel (Vicinity) Map .......................................................................................................................... 1

C. Site Plan ............................................................................................................................................. 1

D. Aerial Photo ....................................................................................................................................... 1

E. Stormwater Control Summary ......................................................................................................... 10

F. Drainage Basin Description and Maps ............................................................................................... 2

G. Grading and Erosion Control Summary (Preliminary SWPPP Evaluation) ......................................... 3

H. Operation and Maintenance Guidelines ............................................................................................ 5


Preliminary Storm Drainage Report RAM No. 18-010 Page A-1

A. PROJECT SUMMARY:

This report provides preliminary engineering information for the proposed construction of a 150-unit hotel;

the project is located on the westerly side of the Alderwood Mall east of 33rd Ave W in the City of Lynnwood.

The applicant, Legacy Hospitality Inc. proposes to construct a 150-unit hotel on an existing parking lot area

of the Alderwood Mall (west of JC Penney).

Existing Conditions. The proposed side development will be constructed on the westerly portion of the JC

Penney parcel. A concurrent short plat will create the project site parcel (west 186 ft of the existing JC

Penney parcel); the new project parcel will be 81,601 sf (1.873 ac) in size. The site is currently fully

developed as an existing parking lot with associated landscaping along the westerly boundary of the

Alderwood Mall (see attached aerial photo- section D).

The site generally slopes in the south/southeasterly direction. The site runoff sheet flows across the existing

parking lot and is collected in a catch basin and pipe system of the Alderwood Mall stormwater system. The

Alderwood Mall stormwater system routes runoff to a large open stormwater detention pond along the

south boundary of the mall. The existing pond is about 1,000 feet south of the project site. Controlled and

treated stormwater is discharged from the mall’s detention pond in a southerly direction under Interstate

5 to the headwaters of Golde Creek. Additional discussion of the local drainage basin and downstream path

is discussed in section F of this report.

Developed Conditions. The proposed project includes the construction a 150-unit hotel and associated

parking lot and utilities. Access to the new hotel will be from the internal driveway system within Alderwood

Mall, no direct access to 33rd Ave W is proposed. About 75,400 sf (1.731 ac) of the existing parking lot will

be redeveloped for the construction of the hotel, utilities, and altered parking lot area. The project will

create/replace about 64,650 sf (1.484 ac) of hard/impervious surface. To mitigate the new/replaced

hard/impervious surface an underground detention tank system will be installed in the southerly portion of

the parking lot area. The onsite new/replaced hard surfaces will be collected and routed to the onsite

detention tank prior to discharging to the Alderwood Mall stormwater pipe system.

Summary of Minimum Requirements: The following section summarizes compliance with the City’s

drainage manual, the current Department of Ecology stormwater manual, 2019 Stormwater Management

Manual for Western Washington (The 2019 SWMMWW):

Minimum Requirement #1 – Preparation of Stormwater Site Plan. The proposed site development consists

of disturbing about 75,400 sf (1.731 ac) of land and creating/replacing about 64,650 sf (1.484 ac) of hard

surface area. The site is currently completely developed as a parking lot associated with the Alderwood

Mall; currently there is about 69,200 sf of hard surface onsite (85%). The for the proposed project is

considered a “redevelopment project”. The civil site development plans and this report have been prepared

to address the projects impacts.

Minimum Requirement #2 – Construction Stormwater Pollution Prevention. A construction Stormwater

Pollution Prevention Plan (SWPPP) has be incorporated into the site development plans. A summary of the

site’s erosion control measures that evaluates the typical 13 elements of a SWPPP are included in section

G. The total site disturbance area of the project is over one acre (1.731 ac), thus a formal Notice of Intent

application for NPDES coverage will be made by the owners prior to construction.



Minimum Requirement #3 – Source Control of Pollution. No specific source controls measures are included

for the site development. The building use and parking lot area do not anticipate any special source control

needs. General requirements for the site include preventing the discharge of pollutants to the City’s storm

drainage system per Lynnwood City Code Chapter 13.45.025 (Illicit Discharges). This includes items such as

pesticides, herbicides, fertilizers, detergents and fluids from vehicle maintenance. Any potential pollution

generating activities created by the internal use of the hotel (food preparation, laundry, etc.) shall meet all

local, state and federal control requirements.

Minimum Requirement #4 – Preservation of Natural Drainage Systems and Outfalls. No natural drainage

systems are found on or adjacent to the site; the site is completely developed as a parking lot area

associated with the Alderwood Mall. The site generally slopes and sheet-flows runoff in a

south/southeasterly direction to the existing Alderwood Mall stormwater system. Runoff from the

developed building, landscape and parking lot areas will be first collected and detained in an underground

detention tank prior to discharging to the Alderwood Mall stormwater system; thus, maintaining the existing

drainage system outfall of the site.

Minimum Requirement #5 – Onsite Stormwater Management. The underlying soils found on site are listed

as Urban Land (map unit #78); see soil mapping provided in section G of this report. “Urban Land” soils

consist of developed or reworked soils compacted imported or native soils with runoff characteristics similar

to glacial till. Till soils have a high runoff potential and are typically classified as hydraulic soil group “C”.

The soil underlying soils are thus exhibit poor soil infiltration characteristics and no infiltration or dispersion

BMPs are proposed. All disturbed pervious (landscape) areas shall preserve or restore the health and water-

holding capacity of the soils by compost-amending (Per DOE BMP T5.13: Post-Construction Soil Quality and

Depth).

5.6 Minimum Requirement #6 – Runoff Treatment. The project will create/replace more than 5,000 sf of

pollution-generation hard surface (PGHS) onsite with the proposed parking lot redevelopment; thus,

meeting the threshold requirement to provide stormwater runoff treatment. It is estimated that the project

will create about 42,153 sf PGHS of parking lot and driveway areas. The existing site contains about 69,200

sf of parking lot area; therefore, the project will actually create a net reduction in PGHS. Additionally, the

existing Alderwood Mall has an open detention/water quality wet pond that has been sized to include the

hotel project site. The existing Alderwood Mall stormwater pond provides adequate water quality control

for the project site and therefore no additional runoff treatment measures are proposed on the hotel

project site.

5.7 Minimum Requirement #7 – Flow Control. The developed site conditions include new/replaced hard

surface for the hotel building roof top, driveways, parking and walkway/patio areas for the proposed site

redevelopment. The site redevelopment will create/replace about 64,650 sf (1.484 ac) of hard surface

area; thus, exceeding the 10,000 sf hard surface threshold to require flow control per the City’s drainage

manual (DOE 2019). The existing Alderwood Mall pond which collects and control the current site runoff

was designed and sized using an outdated single event stormwater modeling (DOE 1992). Therefore, to

meet the current DOE 2019 standards an onsite underground detention pipe system will be installed

onsite. Section E of this report contains a summary and sizing calculations of the onsite detention tank

system designed to meet the current flow control standards (DOE 2019).



5.8 Minimum Requirement #8 – Wetland Protection. There are no existing wetlands on or adjacent to the

project site.

5.9 Minimum Requirement #9 – Operation and Maintenance. An operation and maintenance summary is

provided in section H.


Preliminary Storm Drainage Report RAM No. 18-010 Page B-1

B. PARCEL (VICINITY) MAP


Preliminary Storm Drainage Report RAM No. 18-010 Page C-1

C. SITE PLAN:


Preliminary Storm Drainage Report RAM No. 18-010 Page D-1

D. AERIAL PHOTO (Google, 5/13/2018 photo)


Preliminary Storm Drainage Report RAM No. 18-010 Page E-1

E. STORMWATER CONTROL SUMMARY:

An underground detention tank will be installed in the southwest corner of the project site; the

detention tank will provide flow control of the project’s stormwater runoff (Minimum Requirement

#7). The proposed detention tank consists of 580 feet of 10 ft diameter pipe and provides 45,530 cf

of storage volume.

The following hydrologic analysis uses WWHM2012 software to model historical storm events in the

existing and proposed developed conditions. Per the City’s drainage manual (DOE 2019), the

project shall meet the site discharge duration for half the 2-year to 50-year storm events using a

continuous storm event model (WWHM2012). Below is a summary of the existing and developed

condition areas of the project.

Existing Conditions:

Project Redevelopment Area = 1.731 ac. (75,400 sf)

(Total Project Area modeled as forested-pervious)

Developed Conditions:

Hard Surface (Impervious):

Hotel Roof = 0.385 ac. (16,782 sf)

Parking/Driveways = 0.968 ac. (42,153 sf)

Sidewalks/Patios = 0.131 ac. ( 5,715 sf)

Impervious Total = 1.484 ac. (64,650 sf)

Pervious

Landscape Areas 0.247 ac. (10,750 sf)

Total Area = 1.731 ac. (75,400 sf)



The design WWHM2012 summary input and output printout is provided below.

WWHM2012 PROJECT REPORT __________________________________________________________________ WWHM2012 PROJECT REPORT ___________________________________________________________________ Project Name: 18010 PreEng onsite Pipe Site Name: Legacy Alderwood Mall Site Address: 18-010 City : Report Date: 2/8/2020 Gage : Everett Data Start : 1948/10/01 Data End : 2009/09/30 Precip Scale: 1.00 Version Date: 2018/10/10 Version : 4.2.16 ___________________________________________________________________ Low Flow Threshold for POC 1 : 50 Percent of the 2 Year High Flow Threshold for POC 1: 50 year ___________________________________________________________________ PREDEVELOPED LAND USE Name : Site Bypass: No GroundWater: No Pervious Land Use acre C, Forest, Flat 1.731 Pervious Total 1.731 Impervious Land Use acre Impervious Total 0 Basin Total 1.731 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ MITIGATED LAND USE Name : Site Bypass: No GroundWater: No Pervious Land Use acre C, Pasture, Flat .247 Pervious Total 0.247 Impervious Land Use acre ROOF TOPS FLAT 0.385 SIDEWALKS FLAT 0.131 PARKING FLAT 0.968 Impervious Total 1.484 Basin Total 1.731



___________________________________________________________________ Element Flows To: Surface Interflow Groundwater Tank 1 Tank 1 ___________________________________________________________________ Name : Tank 1 Tank Name: Tank 1 Dimensions Depth: 10 ft. Tank Type : Circular Diameter : 10 ft. Length : 580 ft. Discharge Structure Riser Height: 9.5 ft. Riser Diameter: 12 in. Orifice 1 Diameter: 0.625 in. Elevation: 4 ft. Orifice 2 Diameter: 0.75 in. Elevation: 7.5 ft. Orifice 3 Diameter: 1.5 in. Elevation: 8.5 ft. Element Flows To: Outlet 1 Outlet 2 ___________________________________________________________________ Tank Hydraulic Table Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) Infilt(cfs) 100.00 0.000 0.000 0.000 0.000 100.11 0.027 0.002 0.000 0.000 100.22 0.039 0.005 0.000 0.000 100.33 0.047 0.010 0.000 0.000 100.44 0.054 0.016 0.000 0.000 100.56 0.061 0.022 0.000 0.000 100.67 0.066 0.029 0.000 0.000 100.78 0.071 0.037 0.000 0.000 100.89 0.075 0.045 0.000 0.000 101.00 0.079 0.054 0.000 0.000 101.11 0.083 0.063 0.000 0.000 101.22 0.087 0.073 0.000 0.000 101.33 0.090 0.082 0.000 0.000 101.44 0.093 0.093 0.000 0.000 101.56 0.096 0.103 0.000 0.000 101.67 0.099 0.114 0.000 0.000 101.78 0.101 0.125 0.000 0.000 101.89 0.104 0.137 0.000 0.000 102.00 0.106 0.148 0.000 0.000 102.11 0.108 0.160 0.000 0.000 102.22 0.110 0.173 0.000 0.000 102.33 0.112 0.185 0.000 0.000 102.44 0.114 0.198 0.000 0.000 102.56 0.116 0.210 0.000 0.000 102.67 0.117 0.223 0.000 0.000 102.78 0.119 0.237 0.000 0.000 102.89 0.120 0.250 0.000 0.000 103.00 0.122 0.263 0.000 0.000 103.11 0.123 0.277 0.000 0.000 103.22 0.124 0.291 0.000 0.000 103.33 0.125 0.305 0.000 0.000



103.44 0.126 0.319 0.000 0.000 103.56 0.127 0.333 0.000 0.000 103.67 0.128 0.347 0.000 0.000 103.78 0.129 0.361 0.000 0.000 103.89 0.129 0.376 0.000 0.000 104.00 0.130 0.390 0.000 0.000 104.11 0.131 0.405 0.003 0.000 104.22 0.131 0.419 0.005 0.000 104.33 0.132 0.434 0.006 0.000 104.44 0.132 0.449 0.007 0.000 104.56 0.132 0.463 0.007 0.000 104.67 0.132 0.478 0.008 0.000 104.78 0.133 0.493 0.009 0.000 104.89 0.133 0.508 0.010 0.000 105.00 0.133 0.522 0.010 0.000 105.11 0.133 0.537 0.011 0.000 105.22 0.133 0.552 0.011 0.000 105.33 0.132 0.567 0.012 0.000 105.44 0.132 0.582 0.012 0.000 105.56 0.132 0.596 0.013 0.000 105.67 0.132 0.611 0.013 0.000 105.78 0.131 0.626 0.014 0.000 105.89 0.131 0.640 0.014 0.000 106.00 0.130 0.655 0.015 0.000 106.11 0.129 0.669 0.015 0.000 106.22 0.129 0.684 0.015 0.000 106.33 0.128 0.698 0.016 0.000 106.44 0.127 0.712 0.016 0.000 106.56 0.126 0.726 0.016 0.000 106.67 0.125 0.740 0.017 0.000 106.78 0.124 0.754 0.017 0.000 106.89 0.123 0.768 0.018 0.000 107.00 0.122 0.781 0.018 0.000 107.11 0.120 0.795 0.018 0.000 107.22 0.119 0.808 0.019 0.000 107.33 0.117 0.821 0.019 0.000 107.44 0.116 0.834 0.019 0.000 107.56 0.114 0.847 0.023 0.000 107.67 0.112 0.860 0.026 0.000 107.78 0.110 0.872 0.028 0.000 107.89 0.108 0.884 0.030 0.000 108.00 0.106 0.896 0.032 0.000 108.11 0.104 0.908 0.033 0.000 108.22 0.101 0.920 0.034 0.000 108.33 0.099 0.931 0.036 0.000 108.44 0.096 0.942 0.037 0.000 108.56 0.093 0.952 0.052 0.000 108.67 0.090 0.962 0.064 0.000 108.78 0.087 0.972 0.072 0.000 108.89 0.083 0.982 0.079 0.000 109.00 0.079 0.991 0.085 0.000 109.11 0.075 1.000 0.091 0.000 109.22 0.071 1.008 0.096 0.000 109.33 0.066 1.015 0.100 0.000 109.44 0.061 1.022 0.105 0.000 109.56 0.054 1.029 0.248 0.000 109.67 0.047 1.035 0.817 0.000 109.78 0.039 1.039 1.501 0.000 109.89 0.027 1.043 2.042 0.000 110.00 0.000 1.045 2.328 0.000 110.11 0.000 0.000 2.590 0.000



___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:1.731 Total Impervious Area:0 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:0.247 Total Impervious Area:1.484 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.037106 5 year 0.054922 10 year 0.06712 25 year 0.082852 50 year 0.094753 100 year 0.106784 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.021294 5 year 0.036771 10 year 0.051322 25 year 0.076062 50 year 0.100189 100 year 0.130254 ___________________________________________________________________



Stream Protection Duration Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.021 0.016 1950 0.041 0.019 1951 0.033 0.016 1952 0.026 0.015 1953 0.022 0.015 1954 0.082 0.018 1955 0.056 0.046 1956 0.049 0.069 1957 0.055 0.019 1958 0.037 0.018 1959 0.040 0.018 1960 0.035 0.018 1961 0.037 0.032 1962 0.032 0.015 1963 0.039 0.017 1964 0.033 0.014 1965 0.037 0.018 1966 0.020 0.016 1967 0.045 0.017 1968 0.053 0.019 1969 0.040 0.017 1970 0.029 0.017 1971 0.041 0.069 1972 0.036 0.016 1973 0.029 0.022 1974 0.051 0.020 1975 0.029 0.015 1976 0.028 0.018 1977 0.023 0.015 1978 0.029 0.015 1979 0.052 0.015 1980 0.033 0.016 1981 0.027 0.015 1982 0.035 0.020 1983 0.050 0.018 1984 0.036 0.064 1985 0.048 0.040 1986 0.119 0.093 1987 0.053 0.066 1988 0.029 0.024 1989 0.025 0.015 1990 0.038 0.019 1991 0.041 0.020 1992 0.031 0.019 1993 0.020 0.014 1994 0.019 0.020 1995 0.039 0.027 1996 0.068 0.024 1997 0.130 0.368 1998 0.025 0.017 1999 0.036 0.024 2000 0.020 0.029 2001 0.006 0.011 2002 0.037 0.036 2003 0.027 0.017 2004 0.043 0.024 2005 0.032 0.018 2006 0.073 0.036 2007 0.062 0.032 2008 0.099 0.072 2009 0.031 0.019



Stream Protection Duration Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.1297 0.3676 2 0.1187 0.0926 3 0.0993 0.0716 4 0.0821 0.0693 5 0.0733 0.0691 6 0.0679 0.0664 7 0.0624 0.0641 8 0.0557 0.0461 9 0.0546 0.0399 10 0.0530 0.0361 11 0.0528 0.0355 12 0.0522 0.0322 13 0.0508 0.0317 14 0.0496 0.0288 15 0.0492 0.0272 16 0.0481 0.0242 17 0.0453 0.0242 18 0.0429 0.0239 19 0.0410 0.0238 20 0.0409 0.0218 21 0.0407 0.0201 22 0.0398 0.0195 23 0.0397 0.0195 24 0.0391 0.0195 25 0.0387 0.0192 26 0.0384 0.0192 27 0.0375 0.0191 28 0.0369 0.0188 29 0.0369 0.0187 30 0.0367 0.0186 31 0.0365 0.0185 32 0.0362 0.0182 33 0.0357 0.0179 34 0.0353 0.0179 35 0.0351 0.0178 36 0.0332 0.0177 37 0.0327 0.0176 38 0.0327 0.0175 39 0.0321 0.0174 40 0.0316 0.0174 41 0.0312 0.0173 42 0.0311 0.0173 43 0.0292 0.0172 44 0.0292 0.0171 45 0.0290 0.0165 46 0.0290 0.0164 47 0.0289 0.0163 48 0.0278 0.0163 49 0.0273 0.0158 50 0.0267 0.0155 51 0.0264 0.0154 52 0.0247 0.0154 53 0.0246 0.0153 54 0.0233 0.0151 55 0.0218 0.0151 56 0.0210 0.0147 57 0.0203 0.0146 58 0.0201 0.0146 59 0.0196 0.0141 60 0.0190 0.0137 61 0.0065 0.0114



Stream Protection Duration POC #1 The Facility PASSED

The Facility PASSED. Flow(cfs) Predev Mit Percentage Pass/Fail 0.0186 22651 19472 85 Pass 0.0193 20533 11790 57 Pass 0.0201 18583 8767 47 Pass 0.0209 16803 8367 49 Pass 0.0216 15148 7965 52 Pass 0.0224 13706 7587 55 Pass 0.0232 12459 7219 57 Pass 0.0239 11293 6759 59 Pass 0.0247 10235 6348 62 Pass 0.0255 9300 5982 64 Pass 0.0262 8461 5604 66 Pass 0.0270 7687 5142 66 Pass 0.0278 6945 4678 67 Pass 0.0286 6314 4209 66 Pass 0.0293 5775 3741 64 Pass 0.0301 5285 3317 62 Pass 0.0309 4853 2935 60 Pass 0.0316 4442 2526 56 Pass 0.0324 4094 2212 54 Pass 0.0332 3702 1957 52 Pass 0.0339 3375 1747 51 Pass 0.0347 3054 1561 51 Pass



0.0355 2751 1237 44 Pass 0.0363 2505 999 39 Pass 0.0370 2301 813 35 Pass 0.0378 2108 760 36 Pass 0.0386 1949 744 38 Pass 0.0393 1821 722 39 Pass 0.0401 1698 702 41 Pass 0.0409 1581 680 43 Pass 0.0416 1480 666 45 Pass 0.0424 1398 652 46 Pass 0.0432 1329 642 48 Pass 0.0440 1260 629 49 Pass 0.0447 1197 609 50 Pass 0.0455 1138 582 51 Pass 0.0463 1081 560 51 Pass 0.0470 1024 552 53 Pass 0.0478 952 542 56 Pass 0.0486 914 532 58 Pass 0.0493 879 527 59 Pass 0.0501 845 517 61 Pass 0.0509 807 507 62 Pass 0.0516 767 498 64 Pass 0.0524 732 492 67 Pass 0.0532 700 483 69 Pass 0.0540 676 471 69 Pass 0.0547 655 460 70 Pass 0.0555 639 447 69 Pass 0.0563 620 433 69 Pass 0.0570 604 415 68 Pass 0.0578 588 402 68 Pass 0.0586 573 389 67 Pass 0.0593 560 370 66 Pass 0.0601 551 359 65 Pass 0.0609 539 348 64 Pass 0.0617 523 338 64 Pass 0.0624 510 331 64 Pass 0.0632 496 322 64 Pass 0.0640 473 309 65 Pass 0.0647 457 295 64 Pass 0.0655 448 282 62 Pass 0.0663 438 262 59 Pass 0.0670 426 244 57 Pass 0.0678 417 231 55 Pass 0.0686 402 216 53 Pass 0.0694 396 190 47 Pass 0.0701 385 175 45 Pass 0.0709 374 168 44 Pass 0.0717 362 158 43 Pass 0.0724 355 156 43 Pass 0.0732 349 154 44 Pass 0.0740 338 150 44 Pass 0.0747 329 147 44 Pass 0.0755 320 146 45 Pass 0.0763 310 143 46 Pass 0.0770 306 141 46 Pass 0.0778 300 137 45 Pass 0.0786 296 135 45 Pass 0.0794 288 132 45 Pass 0.0801 283 128 45 Pass 0.0809 276 125 45 Pass 0.0817 270 122 45 Pass



0.0824 260 120 46 Pass 0.0832 252 118 46 Pass 0.0840 245 116 47 Pass 0.0847 239 112 46 Pass 0.0855 234 110 47 Pass 0.0863 227 107 47 Pass 0.0871 215 105 48 Pass 0.0878 205 101 49 Pass 0.0886 200 98 49 Pass 0.0894 194 96 49 Pass 0.0901 188 94 50 Pass 0.0909 184 92 50 Pass 0.0917 176 89 50 Pass 0.0924 170 85 50 Pass 0.0932 165 82 49 Pass 0.0940 158 81 51 Pass 0.0948 152 80 52 Pass ___________________________________________________________________ Water Quality BMP Flow and Volume for POC #1 On-line facility volume: 0.0501 acre-feet On-line facility target flow: 0.0277 cfs. Adjusted for 15 min: 0.0277 cfs. Off-line facility target flow: 0.0159 cfs. Adjusted for 15 min: 0.0159 cfs. ___________________________________________________________________ LID Report

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Preliminary Storm Drainage Report RAM No. 18-010 Page F-1

F. DRAINAGE BASIN DESCRIPTION:

The site is located within the City of Lynnwood Golde Creek watershed basin. The Golde Creek basin is a

centralized urban basin that collects stormwater runoff from commercial and residential areas in the City

Lynnwood and ultimately discharges flows south/southeasterly to Swamp Creek (see attached watershed

map).

The site generally slopes and directs runoff south to the Alderwood Mall stormwater detention pond located

on the south boundary of the mall. The Mall’s detention pond discharges south under Interstate 5 and to

the headwaters of Golde Creek (0.6 miles from the site). Golde Creek continues in a southerly direction

merging with Scriber Creek and eventually discharges into Swamp Creek about 2.6 miles from the site. See

attached drainage basin map and stream basin map for the local downstream path.

No apparent flooding or system inadequacies within a ¼ mile downstream of the site were discovered during

site visits or research of the downstream system.

F-2. Drainage Basin Map (City of Lynnwood Compressive Drainage Basin Map)


Preliminary Storm Drainage Report RAM No. 18-010 Page F-2

F-3. Stream Map (City of Lynnwood Stream Map)


Preliminary Storm Drainage Report RAM No. 18-010 Page G-1

G. GRADING and EROSION CONTROL SUMMARY (Preliminary SWPPP Evaluation):

The project will require grading to construct the proposed hotel building, parking lot and utilities. Standard

erosion control measures are proposed to be used during construction. The primary erosion and sediment

control BMP during construction will be proper soil stabilization methods. Exposed soils shall be stabilized

by application of effective BMPs that protect the soil from the erosive forces of raindrops, flowing water,

and wind. Applicable practices include, but not limited to, temporary and permanent seeding, sodding,

mulching, plastic covering, erosion control fabrics, matting, the early application of gravel base on areas to

be paved, and dust control. The contractor shall select a soil stabilization method best suited for the

particular situation. Stock piles must be stabilized and protected with sediment trapping measures. See

the site development plan for details.

Soils: Soils found on site are listed as Urban land (map unit #78). USDA Natural Resource Soil Conservation

Service (NRCS) Web Soil Survey soils map is included at the end of this section. “Urban Land” soils consist

of developed or reworked soils compacted imported or native soils with runoff characteristics similar to

glacial till. Till soils have a high runoff potential and are typically classified as hydraulic soil group “C”.

Construction Stormwater Pollution Prevention Plan (SWPPP): Due to the fact the project site disturbance

area (1.9 ac) is greater than one acre, a full SWPPP report using the DOE standard template will be prepared

during final construction plan design. Prior to start of construction, a NPDES notice of intent (NOI)

application will be submitted to Department of Ecology (DOE) by the applicant. The following is a

preliminary summary of the site’s erosion control measures that evaluates the typical DOE 13 elements of

a SWPPP:

Element 1: Mark Clearing Limits: Clearing limits have been delineated on the engineering site development

plans.

Element 2: Establish Construction Access: A construction access has been delineated on the engineering

site development plans.

Element 3: Control Flow Rates: The site is currently developed as a parking lot that flows the Alderwood

Mall’s stormwater detention pond system. The existing mall detention pond will provide flow

control for the site.

Element 4: Install Sediment Controls: Catch basin socks and inlet protection will be provided to control

sediment during construction. Due to the fact the current site is developed as a parking lot;

limiting the amount of exposed soils and properly covering regraded during construction will

limit the amount of sediment control.

Element 5: Stabilize Soils: The site is currently completely developed as a parking lot; existing pavement

will be maintained as much as possible during construction to help stabilize site soils. Exposed

soils will be protected by rock or plastic sheeting once graded/reworked. Soils will be stabilized

per the TESC notes listed on the engineering site development plans.



Element 6: Protect Slopes: Exposed slopes shall be stabilized per the TESC notes listed on the engineering

site development plans.

Element 7: Protect Drain Inlets: Drain inlet protection will be installed on all catch basins that will receive

sediment-laden runoff. See the engineering site development plans for locations and detail.

Element 8: Stabilize Channels and Outlets: This element is not applicable since there are no temporary

channels or outlets proposed.

Element 9: Control Pollutants: Pollutants shall be managed as described in the TESC notes listed on the

engineering site development plans.

Element 10: Control De-Watering: Highly turbid or contaminated de-watering water shall be handled

separately from stormwater. The water from all de-watering systems for trenches and

foundations shall be treated or disposed prior to discharging from the site.

Element 11: Maintain BMP’s: BMPs shall be maintained and removed at the end of the project as follows:

i. All temporary and permanent erosion and sediment control BMPs shall be inspected,

maintained and repaired in accordance with the Drainage Manual or as approved or

required by the City to assure continued performance of their intended function in

accordance with BMP specifications.

ii. The applicant may remove temporary BMPs when they are no longer needed.

iii. All temporary erosion and sediment control BMPs shall be removed within 30 days after

construction is completed and the City has determined that the site is stabilized.

Element 12: Manage the Project: The owner of the site is responsible for managing the installation and

maintenance of the site BMPs.

Element 13. Protect Low Impact Development BMPs: No infiltration or dispersion BMPs are proposed for

the project.

Conclusion: The final site development construction plans include specific grading and drainage

improvement notes and details. With proper installation, maintenance and inspections of the proposed

construction the project should have minimal impact to the surrounding environment.



G-1 Soils Map:


Preliminary Storm Drainage Report RAM No. 18-010 Page H-1

H. OPERATION AND MAINTENANCE GUIDELINES:

These guidelines are intended to provide operation and maintenance instructions for the project’s storm

drainage control facilities. The owner is responsible for maintenance of storm drainage facilities within

the property (private property owner system); the owner is not responsible for maintenance within the

public right-of-way (City system).

Private Property Owners: Private property owners are responsible for properly maintaining the stormwater

infrastructure on their property to ensure it operates as designed.

City System: City crews perform maintenance activities on the entire storm drainage system, including

inspection and cleaning of catch basins, street sweeping, emergency flooding response, creek

maintenance, inspection and monitoring of private stormwater detention systems.

Operation and Maintenance Requirements for private property owner system:

This manual is not comprehensive; although it explains the intended operation of the various

components of the drainage system, and suggests a routine of inspection and maintenance, it cannot

anticipate every problem. Once a historical record of maintenance is established, it may be prudent to

alter the routine. It is recommended that maintenance records be kept, and that the records be

reviewed periodically.

Conveyance Systems. The design objective in pipe sizing was to convey the large storm events without

the water surface exceeding the ground elevation. Frequent overtopping of the pipe system in a particular

area might indicate a downstream blockage. Overtopping of the drain systems at the catch-basins is an

indication that maintenance is required.

The following pages outline standard general maintenance criteria for the project’s drainage facilities.

For additional and updated maintenance information visit the Washington State Department of

Ecology’s web-site at: http://www.ecy.wa.gov/programs/wq/wqhome.html



Appendix V-A: BMP Maintenance Tables

Ecology intends the facility-specific maintenance standards contained in this section to be conditions for determining if maintenance actions are required as identified through inspection. Recognizing that Permittees have limited maintenance funds and time, Ecology does not require that a Permittee perform all these maintenance activities on all their stormwater BMPs. We leave the determination of importance of each maintenance activity and its priority within the stormwater program to the Permittee. We do expect, however, that sufficient maintenance will occur to ensure that the BMPs continue to operate as designed to protect ground and surface waters.

Ecology doesn’t intend that these measures identify the facility's required condition at all times between inspections. In other words, exceedance of these conditions at any time between inspections and/or maintenance does not automatically constitute a violation of these standards. However, based upon inspection observations, the Permittee shall adjust inspection and maintenance schedules to minimize the length of time that a facility is in a condition that requires a maintenance action.

Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults)

Maintenance Component

Defect Conditions When Maintenance is Needed Results Expected

When Maintenance is Performed

Storage Area

Plugged Air Vents One-half of the cross section of a vent is blocked at any point or the vent is damaged.

Vents open and functioning.

Debris and Sediment

Accumulated sediment depth exceeds 10% of the diameter of the storage area for 1/2 length of storage vault or any point depth exceeds 15% of diameter.

(Example: 72-inch storage tank would require cleaning when sediment reaches depth of 7 inches for more than 1/2 length of tank.)

All sediment and debris removed from storage area.

Joints Between Tank/Pipe Section

Any openings or voids allowing material to be transported into facility.

(Will require engineering analysis to determine structural stability).

All joint between tank/pipe sections are sealed.

Tank Pipe Bent Out of Shape

Any part of tank/pipe is bent out of shape more than 10% of its design shape. (Review required by engineer to determine structural stability).

Tank/pipe repaired or replaced to design.

Vault Structure Includes Cracks in Wall, Bottom, Damage to Frame and/or Top Slab

Cracks wider than 1/2-inch and any evidence of soil particles entering the structure through the cracks, or maintenance/inspection personnel determines that the vault is not structurally sound.

Cracks wider than 1/2-inch at the joint of any inlet/outlet pipe or any evidence of soil particles entering the vault through the walls.

Vault replaced or repaired to design specifications and is structurally sound.

No cracks more than 1/4-inch wide at the joint of the inlet/outlet pipe.

Manhole Cover Not in Place Cover is missing or only partially in place. Any open manhole requires maintenance.

Manhole is closed.



Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults)


Defect Conditions When Maintenance is Needed Results Expected

When Maintenance is Performed

Locking Mechanism Not Working

Mechanism cannot be opened by one maintenance person with proper tools. Bolts into frame have less than 1/2 inch of thread (may not apply to self-locking lids).

Mechanism opens with proper tools.

Cover Difficult to Remove

One maintenance person cannot remove lid after applying normal lifting pressure. Intent is to keep cover from sealing off access to maintenance.

Cover can be removed and reinstalled by one maintenance person.

Ladder Rungs Unsafe Ladder is unsafe due to missing rungs, misalignment, not securely attached to structure wall, rust, or cracks.

Ladder meets design standards. Allows maintenance person safe access.

Catch Basins See Table V-A.5: Maintenance Standards - Catch Basins

See Table V-A.5: Maintenance Standards - Catch Basins


Table V-A.4: Maintenance Standards - Control Structure/Flow Restrictor


Defect Condition When Maintenance

is Needed Results Expected When

Maintenance is Performed

General

Trash and Debris (Includes Sediment)

Material exceeds 25% of sump depth or 1 foot below orifice plate.

Control structure orifice is not blocked. All trash and debris removed.

Structural Damage

Structure is not securely attached to manhole wall.

Structure is not in upright position (allow up to 10% from plumb).

Connections to outlet pipe are not watertight and show signs of rust.

Any holes - other than designed holes - in the structure.

Structure securely attached to wall and outlet pipe.

Structure in correct position.

Connections to outlet pipe are water tight; structure repaired or replaced and works as designed.

Structure has no holes other than designed holes.

Cleanout Gate Damaged or Missing

Cleanout gate is not watertight or is missing.

Gate cannot be moved up and down by one maintenance person.

Chain/rod leading to gate is missing or damaged.

Gate is rusted over 50% of its surface area.

Gate is watertight and works as designed.

Gate moves up and down easily and is watertight.

Chain is in place and works as designed.

Gate is repaired or replaced to meet design standards.

Orifice Plate

Damaged or Missing Control device is not working properly due to missing, out of place, or bent orifice plate.

Plate is in place and works as designed.

Obstructions Any trash, debris, sediment, or vegetation blocking the plate.

Plate is free of all obstructions and works as designed.

Overflow Pipe Obstructions Any trash or debris blocking (or having the potential of blocking) the overflow pipe.

Pipe is free of all obstructions and works as designed.



Table V-A.4: Maintenance Standards - Control Structure/Flow Restrictor


Defect Condition When Maintenance

is Needed Results Expected When

Maintenance is Performed

Manhole See Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults)

See Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults)

See Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults)

Catch Basin See Table V-A.5: Maintenance Standards - Catch Basins



Table V-A.5: Maintenance Standards - Catch Basins


Defect Conditions When Maintenance is Needed Results Expected When

Maintenance is performed

General

Trash & Debris

Trash or debris which is located immediately in front of the catch basin opening or is blocking inletting capacity of the basin by more than 10%.

Trash or debris (in the basin) that exceeds 60 percent of the sump depth as measured from the bottom of basin to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of six inches clearance from the debris surface to the invert of the lowest pipe.

Trash or debris in any inlet or outlet pipe blocking more than 1/3 of its height.

Dead animals or vegetation that could generate odors that could cause complaints or dangerous gases (e.g., methane).

No Trash or debris located immediately in front of catch basin or on grate opening.

No trash or debris in the catch basin.

Inlet and outlet pipes free of trash or debris.

No dead animals or vegetation present within the catch basin.

Sediment

Sediment (in the basin) that exceeds 60 percent of the sump depth as measured from the bottom of basin to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of 6 inches clearance from the sediment surface to the invert of the lowest pipe.

No sediment in the catch basin

Structure Damage to Frame and/or Top Slab

Top slab has holes larger than 2 square inches or cracks wider than 1/4 inch. (Intent is to make sure no material is running into basin).

Frame not sitting flush on top slab, i.e., separation of more than 3/4 inch of the frame from the top slab. Frame not securely attached

Top slab is free of holes and cracks.

Frame is sitting flush on the riser rings or top slab and firmly attached.

Fractures or Cracks in Basin Walls/ Bottom

Maintenance person judges that structure is unsound.

Grout fillet has separated or cracked wider than 1/2 inch and longer than 1 foot at the joint of any inlet/outlet pipe or any evidence of soil particles entering catch basin through cracks.

Basin replaced or repaired to design standards.

Pipe is regrouted and secure at basin wall.

Settlement/ Misalignment

If failure of basin has created a safety, function, or design problem.

Basin replaced or repaired to design standards.



Table V-A.5: Maintenance Standards - Catch Basins


Defect Conditions When Maintenance is Needed Results Expected When

Maintenance is performed

Vegetation

Vegetation growing across and blocking more than 10% of the basin opening.

Vegetation growing in inlet/outlet pipe joints that is more than six inches tall and less than six inches apart.

No vegetation blocking opening to basin.

No vegetation or root growth present.

Contamination and Pollution

See Table V-A.1: Maintenance Standards - Detention Ponds

No pollution present.

Catch Basin Cover

Cover Not in Place Cover is missing or only partially in place. Any open catch basin requires maintenance.

Cover/grate is in place, meets design standards, and is secured

Locking Mechanism Not Working

Mechanism cannot be opened by one maintenance person with proper tools. Bolts into frame have less than 1/2 inch of thread.

Mechanism opens with proper tools.

Cover Difficult to Remove

One maintenance person cannot remove lid after applying normal lifting pressure.

(Intent is keep cover from sealing off access to maintenance.)

Cover can be removed by one maintenance person.

Ladder Ladder Rungs Unsafe

Ladder is unsafe due to missing rungs, not securely attached to basin wall, misalignment, rust, cracks, or sharp edges.

Ladder meets design standards and allows maintenance person safe access.

Metal Grates (If Applicable)

Grate opening Unsafe

Grate with opening wider than 7/8 inch. Grate opening meets design standards.

Trash and Debris Trash and debris that is blocking more than 20% of grate surface inletting capacity.

Grate free of trash and debris.

Damaged or Missing.

Grate missing or broken member(s) of the grate.

Grate is in place, meets the design standards, and is installed and aligned with the flow path.

Washington State Department of Ecology 2019 Stormwater Management Manual for Western Washington (2019 SWMMWW)

Publication No.19-10-021

Documents

18-010 Legacy Alderwood- Preliminary Storm Drainage Report … · Preliminary Storm Drainage Report RAM No. 18-010 Page G-1 . G. GRADING and EROSION CONTROL SUMMARY (Preliminary SWPPP