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HomeMy WebLinkAbout03710 - Technical Information Report - Drainage .♦ ,
NE 7th North Short Plat
LUA13-001131
013005672
3506 NE 7th StreettlPc\l.I.
Renton, Washington 98056
pogevc4
DRAINAGE REPORT Ot(L'4 2
October 29, 2013
(Updated December 18, 2013)
(Updated January 14, 2014)
Prepared for:
KRRV Development, LLC
Attn: Kyle Miller
P.O. Box 908
Ravensdale, Washington 98051
(425) 432-5932 office
Prepared by: /
SAa�+
Offe Engineers, PLLC .44., 4,OF � ,'Y'
Darrell Offe, P.E. • (64
13932 SE 159th Place ci co ` ,, ..
Renton, Washington 98058-7832 ,A - 1141
�, ,.
425 260-3412 office 4' S" w
( ) �: '44'0/27,ASS" ,..
(425) 227-9460 fax -'9`'pQNALEtIG
darrelioffe(@comcast.net I
3 7/0
Table of Contents
• Technical Information Worksheet
• Section 1: Project Overview
• Section 2: Conditions and Requirements Summary
• Section 3: Offsite Analysis
• Section 4: Flow Control and Water Quality Facility Analysis and Design
• Section 5: Conveyance System Analysis and Design
• Section 6: Special Reports and Studies
• Section 7: Other Permits
• Section 8: CSWPPP Analysis and Design
• Section 9: Bond Quantities, Facility Summaries, and Declaration of Covenant
• Section 10: Operations and Maintenance Manual
•
•
• J
City of Renton
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Part 1 PROJECT OWNER AND Part 2 PROJECT LOCATION AND
PROJECT ENGINEER DESCRIPTION
Project Owner: KRRV Development, LLC Project Name: NE 7th North Short Plat
Address: P.O. Box 908
Ravensdale,WA 98051 Location
Phone: (425) 432-5932 Township: 23 North
Project Engineer: Darrell Offe, P.E. Range: 5 East
Company: Offe Engineers, PLLC Section: 5
Address/Phone: 13932 SE 159th Place
Renton, WA 98058
(425) 260-3412
Part 3 TYPE OF PERMIT Part 4 OTHER REVIEWS AND PERMITS
APPLICATION
Subdivision —
1 DFW HPA — Shoreline Management
X Short Subdivision 1 COE 404 Rockery
❑ Grading ❑ DOE Dam Safety Structural Vaults
❑ Commercial ] FEMA Floodplain C Other
C Other
❑ COE Wetlands
Part 5 SITE COMMUNITY AND DRAINAGE BASIN
Community
North Renton
Drainage Basin
Maplewood Creek/Cedar River/Lake Washington
Part 6 SITE CHARACTERISTICS
C River ❑ Floodplain
Seeps/Springs
C Stream
_ High Groundwater Table
_ Critical Stream Reach
Groundwater Recharge
❑ Depressions/Swales
C. Other
❑ Lake
C Steep Slopes
•
Part 7 SOILS
Soil Type Slopes Erosion Potential Erosive Velocities
Qgt 5— 10% minor
1 Additional Sheets Attached
Part 8 DEVELOPMENT LIMITATIONS
REFERENCE LIMITATION/SITE CONSTRAINT
❑ Ch. 4—Downstream Analysis "Limited Dispersion"
C
❑ Additional Sheets Attached
Part 9 ESC REQUIREMENTS
MINIMUM ESC REQUIREMENTS MINIMUM ESC REQUIREMENTS
DURING CONSTRUCTION AFTER CONSTRUCTION
❑ Sedimentation Facilities ❑ Stabilize Exposed Surface
❑ Stabilized Construction Entrance ❑ Remove and Restore Temporary ESC Facilities
❑ Perimeter Runoff Control ❑ Clean and Remove All Silt and Debris
❑ Clearing and Grading Restrictions ❑ Ensure Operation of Permanent Facilities
❑ Cover Practices C Flag Limits of SAO and open space
❑ Construction Sequence preservation areas
❑ Other
Cl Other
C. r
Part 10 SURFACE WATER SYSTEM
El Grass Lined Tank Infiltration Method of Analysis
Vault
Channel _ _ Depression 2009 City of Renton
El Pipe System _ Energy Dissipater _ KCRTS
Wetland X Flow Dispersal
IA Open Channel Compensation/Mitigati
❑ Dry Pond _ Stream L Waiver on of Eliminated Site
Storage
— Regional
Detention
Brief Description of System Operation: Catch basins within curb line of street will convey
runoff to existing City system downstream, limited dispersion of houses, perforated
pipe connection to storm system, and sheet flow dispersion of driveways.
Facility Related Site Limitations
Reference Facility Limitation
Part 11 STRUCTURAL ANALYSIS Part 12 EASEMENTS/TRACTS
Cast in Place Vault Drainage Easement
Retaining Wall
X Access Easement
Rockery >4' High
❑ Structural on Steep Slope Tract
❑ Other _ Other
Part 13 SIGNATURE OF PROFESSIONAL ENGINEER
I or a civil engineer under my supervision my supervision have visited the site. Actual site
condi .. s as observed were incorporated into this worksheet and the attachments. To the best of
my . edge the information provided here is accurate.
Signed/Date
•
Section 1: Project Overview
The proposal is to create four individual single family lots from this 28,699 square foot parcel located in the
Renton Highlands near the Renton Vocational College. The property address is: 3506 NE 7th Street; King
County Tax parcel #801110-0045. The existing residence, and impervious areas, and out buildings on the
property will be removed to create these four new lots. The property is located within the"existing
conditions"area of the drainage basin map.
There are no sensitive areas on the project site. The property has a gentle slope of approximately 3%
towards the southwest corner of the property. The soils on the site have been identified by a Geotechnical
Engineers as"glacial consolidated till"(Qgt). These soils are not suitable for Full Infiltration of storm water
runoff. The soil logs indicate moist silty sands with no ground water within the holes at 4'. The drainage
calculations within this report will show that the developed runoff from the project is less than 0.10 cfs
during a 100-year storm event between the existing runoff and the developed (mitigated) runoff. Based
upon this review and analysis, the project does not require stormwater treatment. The project does not add
5,000 square feet of new impervious surface and therefore is except from water quality treatment.
Section 2: Conditions and Requirements Summary
2009 City of Renton Drainage Manual
Preliminary Conditions of Short Plat Approval
Section 3: Offsite Analysis
The downstream system was walked on April 19th, 2013. The site drainage currently sheet flows across the
property to the west and eventually entering the City storm system in NE 7th Place. The proposed
development is to convey the drainage form the site into NE 7th Street and then to the west into the City
storm system at Newport Court NE and NE 7th Street. This point of inflow is approximately 500 feet above
the current sheet flow point. The drainage system is completely tight lined from Newport Court NE/ NE 7th
Street through the City storm piping to an apartment site, Hilltop Homes Apartments downstream. The
network of pipes runs 810 feet to a storm water pond within the Hilltop Apartments. The storm pipe system
was visually inspected in three locations downstream; at CB #113111, CB #113118, and CB #113041.
These are City of Renton storm water codes for City catch basins. The system of pipes were 12"concrete
entering into
Type II catch basins. There appeared to be no overtopping of the basins or capacity issues. The roadway
area above these catch basins had no indication of overflow.
The storm water enters into a pond within Hilltop Apartments, City node #145531. This pond is heavily over
grown with trees and blackberries. A walk of the parameter of the pond shows no signs of overtopping or
erosion. The parking lot to the south of the pond is about 4' below the pond berm. There were no signs of
overtopping in this area either.
The drainage leaves the pond and enters into Monroe Avenue NE about 200 feet north of NE 4th Street. The
storm system is within the centerline of the road and was not inspected. The storm system continues to the
south in Monroe Ave until it enters the City Shop Area at NE 2th Street.
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Structures CtifJert(Private) Facility Pond " Surface Water Utility
Renton Maintained Diameter Renton Maintained 4 Sala!l6111Wrinlikil ,:
Mal 44 •`. 3431
4 ■ Type 1 Catchbasin or Inlet - - Unknown �6I
• Type 2Catehbaain-Manhole T� 8-24 • ` �1 �5' �a �
Tank `� _ Asset Inventory
4 Access Riser 25-48 ® Vault 4302 `4301 5306 1�cc �iCeN1�l��
© Vault 49-105 b \ ` -,I■- 1 q a,L7 iia ry
Unknown e) Bios.. �■ �►its
Pipe(Public) dal c�� r tv/ r l �"��
PrivatelyMaintainedOMMINTSIESplime aIt-omp May 2013
Privately Maintained Diameter■ Catchba sin or Inlet —Oa- Unknown `6�•�+�• i,-a ,O Type2Catchbasin-Manhole �►" 8"-2t' Tank uJl 'W�aiO1�� 53116 illMri ;l8 Access Riser a>_� 25'-48" ® Vault .giMill - a
0 Vault EINEM 49"+ co •323`� .ire ■
Bioswale �. aZ,5320 1 ■
Fel Unknown Pipe(Private) ® �ilv.dl>, �4�a�e7�Ir1 tt �
Rain In ai n ���
Unknown Maintenance Diameter 1 `, i '.�0� 1 50 300 600
Unknown Matntainence r 5329 •
n Catchbasin or Inlet —4► Unknown `
Pond k i die.X11® �1 :a Feet
Catchbasin-Type2MH mar 8'-24' _
Culvert(Public) �. 25 -46" Pumpsutlon
tri
,` RIO„41/41/gr932M �
Diameter a 49'+
I I li, ! l'RENTON Ml��'+'�� �� MIEN'
--° Unknown Other Conveyance I I PAA .1 4201 Y! 11515 � 5202 ,
Tr 8'-24” ConveyenceType tOeContourfill , �
� � Mil
1 TOE :Y4e■
25'-48" —0.- Pipe(Unknown Maini) ri ■Yrmi■•7memor � �
rm �
49'+ ------ Ditch :' 0 9 T23N RO5E S E 1 /4
Disclaimer: This Inventory information is schematic only.
It was compiled from numerous sources. It is the best
— City of information available at this time and should be used
enton
for spon ibl guidance or only. The City of Renton is not
neNrorl a"�DorT"ecuavE 5 responsible for errors or omissions. When this information
Public Works Department is used for planning,design,and/or construction purposes, 5309
• - v users are to field verify this information. E5S E
Pg. 48
1. .
Section 4: Flow Control and Water Quality Facility Analysis and Design
FLOW CONTROL REVIEW
Based upon the City Drainage Manual, the following steps are required to determine the mitigation of storm
water runoff of the developed project:
Full Dispersion — N/G (no good) —the property does not have 100 feet of flow path available on
each new lot;
Full Infiltration — N/G —the available soils on the property are silty sandy (Alderwood) soils and do
not provide adequate depth for full infiltration.
Limited Infiltration — acceptable — no ground water was found in the test holes below 6 feet,
therefore limited infiltration using gravel filled trenches will be utilized to treat 1,000 square feet of
developed impervious runoff. The gravel filled trenches will be installed at a depth to bottom of
trench of 3' (36").
The proposal is to mitigate all the Lots using gravel filled trenches located in the rear yards to treat 1,000
square feet of impervious area; to utilize sheet flow of the driveway across the front yards to mitigate the
runoff from the impervious area of the driveway; to utilize perforated pipe connection at the connection to
the storm conveyance system within the front yards. The frontage along NE 7th Street will be collected and
conveyed into an existing public storm system located across NE 7th and west approximately 250 feet. A
curb and gutter will be installed along the west side of the private access road (Olympic Avenue NE) to
collect the runoff from the road. This runoff together with the connection to each lot will be conveyed into
the NE 7th Street system. Based upon the attached analysis the following flows were developed to compare
to existing runoff.
Pre-Developed Condition (runoff) Mitigated Developed Condition (runoff)
2 Year 0.053 cfs 2 year 0.112 cfs
10 year 0.087 cfs 10 year 0.134 cfs
100 year 0.186 cfs 100 year 0.243 cfs
The difference between 100 year"Pre-Developed Condition"and "Mitigated Developed Condition" is
0.243 — 0.186 = 0.057. The"Mitigated Developed Condition" runoff if less than 0.10 cfs during a 100 year
storm event than the"Pre-Developed Condition" runoff. Therefore, based upon the City Drainage Manual,
no storm water detention is required for this project.
The calculations for the above analysis are located within this Section.
WATER QUALITY REVIEW
The City Manual requires water quality treatment for projects that add 5,000 square feet or more of"new"
pollution generating imperious surfaces (PGIS). The proposed developed (PGIS) condition consists of:
DEVELOPED (PGIS) EXIS 1'NG (PGIS)
Access Road 5,309 Sq. feet Driveway 1,088 sq. feet
Each Lot (D/W) 0 Sq. feet (mitigated) Asphalt in Road 1,406 sq. feet
Frontage Widening 1,536 Sq. feet Shoulder 240 sq. feet
Frontage Access Apron 160 Sq. feet
Developed PGIS = 7,005 Square feet Existing PGIS = 2,734 sq. feet
The proposed "Developed Condition"generates 4,271 square feet of NEW PGIS. Based upon the City
Drainage Manual this is less than 5,000 square feet and therefore water quality treatment is not required.
'
NE 7th North (2) (3)
Area Breakdown ** Impervious
(1) Mitigated Impervious Surface
Developed Condition Lot Area Impervious Surfaces Runoff Pervious
Area Breakdown (sa. feet) (sq. feet) jsa. feet) (so. feet) (so. feet)
(50% *(2)) (50% *(2))
Lots
Lot 1 6720
House, walkways, patio 2300 1000 1800 500
Driveway 300 300 150 150
Road (PGIS) 1400 0 1400
Landscaping 0 2720
3350 3370
Lot 2 6240
House, walkways, patio 2300 1000 1800 500
Driveway 300 300 150 150
Road (PGIS) 1460 0 1460
Landscaping 0 2180
3410 2830
Lot 3 7872
House, walkways, patio 1800 1000 1300 500
Driveway 300 300 150 150
Road (PGIS) 2449 0 2449
Landscaping 0 3323
3899 3973
Lot 4 7872
House, walkways, patio 2600 1000 2100 500
Driveway 300 300 150 150
Road 0 0 0
Landscaping 0 4972
2250 5622
Frontage
Sidewalk 330 330
Apron (PGIS) 160 0 160
NE 7th Street (PGIS) 1536 0 1536
Landscape strip 0 854
2026
14935 16649
0.34 0.39
Acres Acres
**-Lot mitigation includes "limited Infiltraion"of 1,000 square feet of roof area(credit 50%impervious runoff)
and "sheet flow"dispersion of driveway(20'x 15) -each lot. Mitigation of 1,300 square feet per lot
Existing Condition Impervious Pervious
Area Breakdown Total Areas (sq. feet) !sq. feet)
Parcel 28699
House 1820
Driveway (PGIS) 1088
Walkways 512
Sheds (outbuildings) 148
Landscaping 25131
Frontage 2880
Aspahlt (PGIS) 1406
Compacted garvel shoulder (PGIS) 240
Lawn 1234
5214 26365
0.12 0.61
Acres Acres
NE 7th North Short Plat (Pre Developed — pre-existing conditions)
7 land Use S —UxJ
Area
Till Forest 0.00 acres
Till Pasture 0.00 acres
Till Grass 0.61 acres
Outwash Forest 0.00 acres;
Outwash Pasture 0.00 acres
Outwash Grass 0.00 acres
Wetland 0.00 acres'
s
Impervious 0.12 acres!
Total !
1
0.73 acres!
Scale Factor : 1.00 Hourly Reduced
Time Series: NE 7th North Short Plat (pre-ex) »1
Compute Time Series I
Modify User Input
File for computed Time Series 1.TSF1
`'4ttetp 1 Fifes - KCR7`; =(oJ xJ
Flow Frequency Analysis
Time Series File:ne 7th north short plat (pre-ex) . 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
0.084 4 2/09/01 2 :00 0 . 186 1 100. 00 0.990
0.053 6 1/05/02 16:00 0. 104 2 25. 00 0. 960
0. 104 2 2/27/03 7: 00 0.087 3 10. 00 0. 900
0.041 8 8/26/04 2 : 00 0.084 4 5. 00 0.800
0.053 7 10/28/04 16 :00 0.080 5 3. 00 0.667
0.087 3 1/18/06 16: 00 0 .053 6 2. 00 0. 500
0. 080 5 11/24/06 3: 00 0.053 7 1 . 30 0. 231
0. 186 1 1/09/08 6:00 0 . 041 8 1. 10 0. 091
Computed Peaks 0. 158 50. 00 0. 980 d
NE 7th North Short Plat (Developed Conditions)
Q Land Use S j
Area .1
Till Forest 0.00 acres
Till Pasture 0.00 acres
Till Grassi 0.39 acres
Outwash Forest 0.00 acres?
Outwash Pasture 0.00 acres
Outwash Grass 0.00 acres/
Wetland 0.00 acres
Impervious 0.34 acres]
Total -,
0.73 acres]
Scale Factor : 1.00 Hourly Reduced
Time Series: NE 7th North (Developed Conditions) »I
Compute Time Series
Modify User Input
File for computed Time Series [.TSF]
17--Neap!Rtes- KCHT`' OJ J
Flow Frequency Analysis
Time Series File:ne 7th north (developed conditions) . 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. 117 5 2/09/01 2: 00 0.243 1 100.00 0.990
0.090 8 1/05/02 16:00 0. 142 2 25.00 0.960
0.142 2 2/27/03 7:00 0. 134 3 10.00 0. 900
0.092 7 8/26/04 2:00 0. 124 4 5.00 0.800
0. 112 6 10/28/04 16:00 0. 117 5 3.00 0.667
0.124 4 1/18/06 16:00 0. 112 6 2.00 0.500
0.134 3 10/26/06 0:00 0.092 7 1.30 0.231
0.243 1 1/09/08 6:00 0.090 8 1 . 10 0.091
Computed Peaks 0.209 50.00 0. 980
C.2.3 LIMITED INFILTRATION
C.2.3 LIMITED INFILTRATION
invited infiltration is the use of infiltration devices from Section C.2.2 in soils that are not as permeable
as the medium sands or coarse sands/cobbles targeted for full infiltration in Section C.2.2. These less
desirable soils include fine sands,loamy sands,sandy loams,and loams,which tend to be more variable in
permeability,more frequently saturated during the wet season,and more prone to plugging over time.
While full infiltration may be possible under the best of these soil conditions,in the long run,these
conditions will conspire to limit average infiltration capacity to something much less than that of full
infiltration. Therefore,using limited infiltration as specified in this section will not be credited the same
as using full infiltration as specified in Section C.2.2.
Applicable Surfaces
Limited infiltration may be applied to any impervious surface(e.g.,roof,driveway,parking area,or road)
subject to the minimum requirements and design specifications in this section.
Operation and Maintenance
See Section C.2.3.5(p.C-50).
C.2.3.1 REQUIRED SOILS REPORT
In order to properly design limited infiltration devices,a soils report is required to identify the depth to
impermeable layers(i.e.,hardpan)and to the maximum wet season groundwater level. See Section
C.2.2.1 (p.C-41)for more details on this report. In many cases,this report will have already been
prepared as required in Sections C.1.3.1 and C.1.3.2 for lots where full dispersion is not feasible or
applicable to target impervious surface per Section C.2.1.
C.2.3.2 MINIMUM DESIGN REQUIREMENTS FOR LIMITED INFILTRATION
The minimum requirements for limited infiltration are the same as those for full infiltration,except
infiltration depressions are excluded and existing soils in the location of the infiltration device may be fine
sands,loamy sands,sandy loams,or loams as opposed to only medium sands or better. Note that gravel
and medium sand soils used for full infiltration correspond to Soil Types 1A, IB, 2A and 2B in the Soil
Textural Classification system used for onsite septic system design;fine sands are Type 3;and loamy
sands,sandy loams and loams are Type 4 soils. Silt and clay loams, and cemented till(hardpan)are not
suitable for limited infiltration systems.
C.2.3.3 USE OF GRAVEL FILLED TRENCHES FOR LIMITED INFILTRATION
The specifications for use of gravel filled trenches for limited infiltration are the same as those used for
full infiltration,except the 1 im a surface requires different
trench lengths as follows a)75 feet if the soil is a fine sand/loamy sand, b) 125 feet if the soil is a
sandy loam,or(c) 190 fee.
C.2.3.4 USE OF DRYWELLS FOR LIMITED INFILTRATION
The specifications for use of drywells for limited infiltration are the same as those used for full infiltration,
except that every 1,000 square feet of tributary impervious surface requires different gravel volumes as
follows:(a)230 cubic feet if the soil is a fine sand/loamy sand,(b)380 cubic feet if the soil is a sandy
loam,or(c)570 cubic feet if the soil is a loam.
2009 Surface Water Design Manual—Appendix C 1/9/2009
C-49
•
SECTION C.2 FLOW CONTROL BMPs
C.2.3.5 MAINTENANCE INSTRUCTIONS FOR LIMITED INFILTRATION
If the limited infiltration flow control BMP is proposed for a project,the following maintenance and
operation instructions must be recorded as an attachment to the required declaration of covenant and
grant of easement per Requirement 3 of Section C.1.3.3 (p.C-18). The intent of these instructions is to
explain to future property owners,the purpose of the BMP and how it must be maintained and operated.
These instructions are intended to be a minimum; DDES may require additional instructions based on site-
specific conditions. Also,as the County gains more experience with the maintenance and operation of
these BMPs, future updates to the instructions will be posted on King County's Surface Water Design
Manual website.
TEXT OF INSTRUCTIONS
Your property contains a stormwater management flow control BMP (best management practice)called
"limited infiltration,"which was installed to mitigate the stormwater quantity and quality impacts of some or
all of the impervious surfaces on your property. Limited infiltration is a method of soaking runoff from
impervious area (such as paved areas and roofs) into the ground. Infiltration devices, such as gravel filled
trenches, drywells, and ground surface depressions,facilitate this process by putting runoff in direct
contact with the soil and holding the runoff long enough to soak most of it into the ground. To be
successful,the soil condition around the infiltration device must be able to soak water into the ground for a
reasonable number of years.
The infiltration devices used on your property include the following as indicated on the flow control BMP
site plan: ❑ gravel filled trenches, ❑ drywells. The size, placement, and composition of these devices as
depicted by the flow control BMP site plan and design details must be maintained and may not be changed
without written approval either from the King County Water and Land Resources Division or through a
future development permit from King County.
Infiltration devices must be inspected annually and after major storm events to identify and repair any
physical defects. Maintenance and operation of the system should focus on ensuring the system's viability
by preventing sediment-laden flows from entering the device. Excessive sedimentation will result in a
plugged or non-functioning facility. If the infiltration device has a catch basin, sediment accumulation must
be removed on a yearly basis or more frequently if necessary. Prolonged ponding around or atop a device
may indicate a plugged facility. If the device becomes plugged, it must be replaced. Keeping the areas
that drain to infiltration devices well swept and clean will enhance the longevity of these devices. For roofs,
frequent cleaning of gutters will reduce sediment loads to these devices.
1/9/2009 2009 Surface Water Design Manual—Appendix C
C-50
C.2.2 FULL INFILTRATION
fiAL(11.1.
FIGURE C.2.2.A TYPICAL TRENCH INFILTRATION SYSTEM
roof • -
,/ drain
PLAN VIEW
NTS 4"rigid or 6"flexible
perforated pipe
3 J
•
infiltration trench sump w/solid lid
PLAN VIEW roof drain
NTS II
overflow
4"rigid or 6"flexible splash block .1
perforated pipe
i :4 ° '
6
oQ �. l' illiml
' ° 0 washed rock ' ' \° Q o°p C 1'711 5.0'min I
12"j ;• — 1 1/2"3/4„ 'D.'"...°
0°°o 0: / I1' n
n Q.4__r� .r'
fine mesh CB sump w/solid lid
erroen
Loo. varies------ -- - -
A
filter fabric
i compacted backfill
6" I1 _
:�• 4"rigid or 6"flexible
° °°' ° °a perforated pipe
24" °9 ° °4 '
i`o o S ,0° o
' :0 4° 78'
V * washed rock
12" r o o a �
i 4. ,a0 ,,,, ,o oa° p ° 1 washed
/2"-3/4"
0.
.--• -24" ...I
SECTION A
NTS
2009 Surface Water Design Manual-Appendix C 1/9/2009
C-45
C.1.3 APPLICATION OF FLOW CONTROL BMPS
C.1.3 APPLICATION OF FLOW CONTROL BMPS
Flow control BMPs are methods and designs for dispersing,infiltrating,or otherwise reducing or
preventing development-related increases in runoff at or near the sources of those increases. Flow control
BMPs include,but are not limited to,preservation and use of native vegetated surfaces to fully disperse
runoff;use of other pervious surfaces to disperse runoff;roof downspout infiltration;permeable
pavements;rainwater harvesting;vegetated roofs;and reduction of development footprint.
For projects subject to Small Project Drainage Review,the application of flow control BMPs is mandatory
for individual lot projects(i.e.,projects not subdividing land)and is optional for subdivision projects
unless otherwise specified by DDES. For individual lot projects,flow control BMPs must be applied as
specified by one of the following two sets of BMP requirements,whichever is applicable based on the size
of site/lot:
• "Small Lot BMP Requirements"(for sites/lots<22,000 square feet),Section C.1.3.1
• "Large Lot BMP Requirements"(for sites/lots 22,000 square feet),Section C.1.3.2
These requirements specify both the order of preference for selection of flow control BMPs and their
extent of application to the developed surfaces of an individual lot project. This application of
requirements is illustrated by the flow chart in Figure C.1.3.A(p.C-123). In addition,the implementation
of flow control BMPs by projects on either size of site/lot must be in accordance with the"BMP
Implementation Requirements"detailed in Section C.1.3.3.
If the proposed project is a single family residential subdivision(i.e.,plat or short plat project),the
application of flow control BMPs for future anticipated improvements on the lots created by the
subdivision may be deferred until an individual lot project is proposed for permit approval on each lot.
Alternatively,the BMPs required for anticipated improvements on each lot may be installed as part of the
subdivision project if the applicant so chooses. In either case,the required application of BMPs,in terms
of order of preference and extent of application to developed surfaces,is the same as that for an
individual lot project(i.e.,as specified in Sections C.1.3.1 or C.1.3.2,and C.1.3.3).
Note:road right-of-way improvements constructed as part of a subdivision project may require
implementation offlow control BMPs or other mitigation as determined necessary by DDES.
C.1.3.1 SMALL LOT BMP REQUIREMENTS
Any proposed project that is on an individual site/lot smaller than 22,000 square feet must comply with
the flow control BMP application requirements in this section.
A. MITIGATION OF IMPERVIOUS SURFACE
For projects subject to small lot BMP requirements,flow control BMPs must be applied to the project's
target impervious surface according to the order of preference and extent of application specified in the
following requirements:
1. The feasibility and applicability of full dispersion as detailed in Section C.2.1 (p.C-26)must be
evaluated for the roof area(or an impervious area of equivalent size)on the site/lot. If feasible and
applicable,full dispersion of roof runoff must be implemented as part of the proposed project.
Typically,small lot full dispersion will be applicable only in subdivisions where enough forest was
preserved by tract,easement,or covenant to meet the minimum requirements for full dispersion in
Section C.2.1.1 (p.C-26). If this first requirement is met for the site/lot,no other flow control BMPs
are required for mitigation of impervious surface,and the remaining requirements below are optional.
2. Where full dispersion of roof runoff(or equivalent)is not feasible or applicable,or will cause
flooding or erosion impacts,the feasibility and applicability of full infiltration as detailed in Section
C.2.2(p.C-41)must be evaluated for roof runoff(note, this will require a soils report for the siteulot).
2009 Surface Water Design Manual—Appendix C 1/9/2009
C-13
•
SECTION C.1 SMALL PROJECT DRAINAGE REVIEW REQUIREMENTS
If feasible and applicable,full infiltration of roof runoff must be implemented as part of the
proposed project. If this requirement or the full dispersion requirement above is met for the site/lot,
no other flow control BMPs are required,and the remaining requirements below are optional.
3. Where full dispersion or full infiltration of roof runoff as specified in Requirements 1 and 2 above is
not feasible or applicable,or will cause flooding or erosion impacts,one or more of the following
BMPs must be applied to(or used to mitigate for)an impervious area equal to at least 10%of the
site/lot for site/lot sizes up to 11,000 square feet and at least 20%of the site/lot for site/lot sizes
between 11,000 and 22,000 square feet. For projects located within a critical aquifer recharge
area,1°these impervious area amounts must be doubled. The BMPs listed below may be located
anywhere on the site/lot subject to the limitations and design specifications for each BMP. These
BMPs must be implemented as part of the proposed project.
• Limited Infiltration(see Section C.2.3)
• Basic Dispersion(see Section C.2.4),
• Rain Garden(see Section C.2.5),
• Permeable Pavement(see Section C.2.6),
• Rainwater Harvesting(see Section C.2.7),
• Vegetated Roof(see Section C.2.8),
• Reduced Impervious Surface Credit(see Section C.2.9),
• Native Growth Retention Credit(see Section C.2.10).
4. Any proposed pipe connection of roof downspouts to the local drainage system11 must be via a
perforated pipe connection as detailed in Section C.2.11.
B. MITIGATION OF NEW PERVIOUS SURFACE
For projects subject to small lot BMP requirements, no flow control BMPs are required for new pervious
surface. Note, however, that KCC 16.82.100(G)requires amendment of the soil to mitigate for lost
moisture holding capacity in any area that has been compacted or that has had some or all of the duff
layer or underlying topsoil removed. The amendment must be such that the replaced topsoil is a minimum
of 8 inches thick, unless the applicant demonstrates that a different thickness will provide conditions
equivalent to the soil moisture holding capacity native to the site. The replaced topsoil must have an
organic content of 8-13%dry weight and a pH suitable for the proposed surface vegetation(for most soils
in King County, 4 inches of well-rotted compost tilled into the top 8 inches of soil is sufficient to achieve
the organic content standard). The amendment must take place between May 1 and October 1.
C. MITIGATION OF WATER QUALITY IMPACTS
For projects subject to small lot BMP requirements,most water quality impacts will be adequately
addressed through proper application of flow control BMPs to impervious surface as specified above.
However,if the project results in 5,000 square feet or more of new pollution-generating impervious
surface from which runoff is not fully dispersed in accordance with Section C.2.1 (p.C-26),then one of
the following actions must be taken:
10 Critical aquifer recharge area is the critical area designation,defined and regulated in KCC 21A,that is applied to areas where
extra protection of groundwater quantity and quality is needed because of known susceptibility to contamination and
importance to drinking water supply. Such areas are delineated on the King County Critical Aquifer Recharge Area Map
available at DDES or on the County's Geographic Information System(GIS). See the definition of this term in KCC 21A.06 for
more details.
11 Local drainage system means any natural or constructed drainage feature that collects and concentrates runoff from the site
and discharges it downstream.
1/9/2009 2009 Surface Water Design Manual—Appendix C
C-14
Section 5: Conveyance System Analysis and Design
The analysis of the new conveyance system is attached to this Section. The new conveyance system was
analyzed using the"Rational Method of Analysis" per section 3.2 King County Drainage Manual. The
calculated 25 year storm event was 3.96 cfs from runoff of the existing upstream area and the developed
site conditions (NE 7th North Short Plat).
Based upon this flow, a backwater analysis was performed on the new system (see attached). The analysis
concluded that the storm system is adequate to convey the 25 year event.
Rational Method
Runoff from Basin 25 year event
P(25) 3.40 inches (from attached Isopluvials Map)
A(T) 4.20 acres
Areas
A(1) 3.50 acres single family residential 1.4DU/GA - existing
A(2) 0.70 acres single family residential 5.7 DU/GA - development (NE 7th North)
A(T) 4.20 acres
Description of flow Path k®
L1 250 feet s1 0.05 Lawn/landscaping 7.0
L2 220 feet s2 0.10 Nearly bare ground 10.1
L3 170 feet s3 0.08 Asphalt 20.0
Composite Runoff Coefficient
A(1); C(1) 0.33
A(2); C(2) 0.54
C© 3.50*0.33 + 0.70*0.54/4.20 0.37
Time of Concentration
T(1) 250/60*{(7.0)*(0.05)**0.5)} 3 minutes
T(2) 220/60*{(10.1)*(0.10)**0.5)} 2 minutes
T(3) 170/60*{(20.0)*(0.08)**0.5)} 1 minute
USE 7 minutes
Compute i®
i®=(aR)*(Tc)
i® = 2.66*7**-0.65 0.75
Compute I(25)
I(25)=(P25)*(i25)
I(25)=3.40*0.75 2.55
Q(25) = C*I(25)*A
Q(25) = 0.37*2.55*4.20 3.96 cfs
3.2.1 RATIONAL METHOD
TABLE 3.2.1.A RUNOFF COEFFICIENTS- "C"VALUES FOR THE RATIONAL METHOD
General Land Covers Single Family Residential Areas*
Land Cover C Land Cover Density C
Dense forest 0.10 0.20 DU/GA(1 unit per 5 ac.) 0.17
Light forest 0.15 0.40 DU/GA(1 unit per 2.5 ac.) 0.20
Pasture 0.20 0.80 DU/GA(1 unit per 1.25 ac.) 0.27
ACI)
Lawns 0.25 1.00 DU/GA 0.30
Playgrounds 0.30 1.50 DU/GA 0.33 -
Gravel areas 0.80 2.00 DU/GA 0.36
Pavement and roofs 0.90 2.50 DU/GA 0.39
Open water(pond, lakes, 1.00 3.00 DU/GA 0.42
wetlands) 3.50 DU/GA 0.45
4.00 DU/GA 0.48
4.50 DU/GA 0.51 A(1)
5.00 DU/GA 0.54 •
5.50 DU/GA 0.57
6.00 DU/GA 0.60
•Based on average 2,500 square feet per lot of impervious coverage.
For combinations of land covers listed above, an area-weighted"C,:x A,"sum should be computed based on the
equation Cc: x A,=(C1 x A1)+(C2 x A2)+ ...+(C"x An),where A,=(A1+A2+ ...+219),the total drainage basin area.
TABLE 3.2.1.B COEFFICIENTS FOR THE RATIONAL METHOD"iR" EQUATION
Design Storm Return Frequency aR bR
2 years 1.58 0.58
5 years 2.33 0.63
10 years 2.44 0.64
25 years 2.66 0.65
50 years 2.75 0.65
100 years 2.61 0.63
TABLE 3.2.1.0 kR VALUES FOR T,USING THE RATIONAL METHOD
Land Cover Category kR
Forest with heavy ground litter and meadow 2.5
Fallow or minimum tillage cultivation 4.7
Short grass pasture and lawns 7.0
Nearly bare ground 10.1
Grassed waterway 15.0
Paved area(sheet flow)and shallow gutter flow 20.0
2009 Surface Water Design Manual 1/9/2009
3-13
SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS
FIGURE 3.2.1.0 25-YEAR 24-HOUR ISOPLUVIALS
�S �'— __ _ __ [SHcoYNO COUNTY
...../
2.8 o i
39 ® ® Dv
To 4•
C
333
Cli
,,off ') s,�
mt
a �` .. _.. ..
5.0
=> 110
SI
1 6, 0 \ N
a7
_ - - 1
t v
jii
cYj ,
CO 0
433.
oi �- ..,,t, _ --` _KING COUNTY
(L. PIERCE COUNTY ,
WESTERN JIs
KING COUNTY \ p 5s
N 50
4.5
25-Year 24-Hour , ://.,
Precipitation N �, ti,��rf
0� �Miles �• n3• - �
in Inches io,•
1/9/2009 2009 Surface Water Design Manual
3-16
STORMWATER CONVEYANCE BACKWATER CALCULATION SHEET
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
Barrel Entrance Entrance
Pipe 25 year Pipe Outlet Inlet Barrel Barrel Velocity TW Friction HGL Head
Segment Q Length Pipe "n" Elev Elev Area Velocity Head Elev Loss Elev Loss
CB to CB (cfs) (ft) Size Value (feet) (feet) (sq. feet) (fps) (feet) (feet) (feet) (feet) (feet)
#EX - #1 3.96 30 12" 0.011 382.07 382.37 0.7854 5.04 0.39 382.20 0.0054 382.21 0.08
#1 - #2 3.96 230 12" 0.011 382.37 384.67 0.7854 5.04 0.39 383.36 0.0054 383.36 0.08
#2 - #3 3.96 89 12" 0.011 386.30 391.12 0.7854 5.04 0.39 387.50 0.0054 386.11 0.08
#2 - #4 1.45 20 12" 0.011 384.67 385.60 0.7854 1.85 0.05 386.11 0.004 386.11 0.01
(14) (15) (16) (17) (18) (19) (20) (21)
Exit Outlet Inlet Approach Bend Junction
Head Control Control Velocity Head Head HW RIM ELEV
Loss Elev Elev Head Loss Loss Elev UPSTREAM
(feet) (feet) (feet) (feet) (feet) (feet) (feet) CB
0.39 382.68 383.75 0.39 0 0 383.36 385.40 OK!
0.39 383.83 386.50 0.39 0 0 386.11 389.30 OK!
0.39 386.58 392.00 0.39 0 0 391.61 394.12 OK!
0.05 386.18 387.50 0.39 1.3 0 388.41 390.50 OK!
• 4.2.1 PIPE SYSTEMS—METHODS OF ANALYSIS
FIGURE 4.2.1.K BEND HEAD LOSSES IN STRUCTURES
I I I I '
— _ c4j�2
— — frOM
— — Gia k
1.2
— —
r D
il,,....
1.0 — #4),
— 4,
Y 0.8
4.
c
°' — Bend at Manhole, —
o
no Special Shaping —
43 —
0
U — —
o — Deflector —
0.6 —
— .4 Curved —
Bend at Manhole,
Curved or Deflector
— I \YY
-
0.4
— Curved Sewer —
r/DI=2
i—
0.2
Sewer r/D>6
- ., 4
0.0 I I I I
0° 200 40° 60° 80° 90° 100°
Deflection Angle 7, Degrees
2009 Surface Water Design Manual 1/9/2009
4-27
SECTION 4.3 CULVERTS AND BRIDGES
TABLE 4.3.1.B ENTRANCE LOSS COEFFICIENTS
Type of Structure and Design Entrance Coefficient,Ke
Pipe, Concrete, PVC, Spiral Rib, DI, and LCPE C��)
Projecting from fill, socket(bell)end 0.2 .1
Projecting from fill, square cut end 0.5
Headwall, or headwall and wingwalls
Socket end of pipe(groove-end) 0.2
Square-edge 0.5
Rounded(radius= 1/12D) 0.2
Mitered to conform to fill slope 0.7
End section conforming to fill slope* 0.5
Beveled edges, 33.7°or 45°bevels 0.2
Side-or slope-tapered inlet 0.2
Pipe, or Pipe-Arch, Corrugated Metal and Other Non-Concrete or D.I.
Projecting from fill (no headwall) 0.9
Headwall, or headwall and wingwalls(square-edge) 0.5
Mitered to conform to fill slope(paved or unpaved slope) 0.7
End section conforming to fill slope* 0.5
Beveled edges, 33.7°or 45°bevels 0.2
Side-or slope-tapered inlet 0.2
Box, Reinforced Concrete
Headwall parallel to embankment(no wingwalls)
Square-edged on 3 edges 0.5
Rounded on 3 edges to radius of 1/12 barrel dimension or beveled 0.2
edges on 3 sides
Wingwalls at 30°to 75°to barrel
Square-edged at crown 0.4
Crown edge rounded to radius of 1/12 barrel dimension or beveled top 0.2
edge
Wingwall at 10°to 25°to barrel
Square-edged at crown 0.5
Wingwalls parallel (extension of sides)
Square-edged at crown 0.7
Side-or slope-tapered inlet 0.2
* Note: "End section conforming to fill slope"are the sections commonly available from manufacturers. From
limited hydraulic tests they are equivalent in operation to a headwall in both inlet and outlet control. Some end
sections incorporating a closed taper in their design have a superior hydraulic performance.
1/9/2009 2009 Surface Water Design Manual
4-42
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Section 6: Special Reports and Studies
Geotechnical Study attached
GEOSPECTRUM CONSULTANTS, INC.
Geotechnical Engineering and Earth Sciences
August 15, 2013
Mr. Mike Lombardo
KRRV Development, LLC
P. O. Box 908
Ravensdale, WA 98051
SUBJECT: GEOTECHNICAL EVALUATION
Proposed 4-Lot 7th Street North Plat
3506 NE 7th Street
Renton, Washington
Project No. 13-121-01
Dear Mike,
This report presents the results of our geotechnical evaluation for the site of your
proposed new residential development on the `subject property. The purpose of our
work was to provide geotechnical engineering evaluations of the site and geotechnical
recommendations for the residential development including design of foundations, site
grading, site drainage and erosion control. Our work was performed in accordance with
the scope and conditions of our proposal dated July 22, 2013.
A site topographic map and development plan (see Figure 2) was provided to us and
was used as a reference for our evaluations. Based on our review of the plan provided
and discussions with you, we understand that the property will be divided into 4 lots and
the development will include a new 2-to 3-story wood-frame residence on each of the
new lots. The structures will include ground level garages which will have slab-on-
grade floors and the remainder of structures may have raised floors with crawl space or
slab-on-grade.
We assume that bearing wall loads will be in the range of about 2 to 3 klf and maximum
column loads to be in the range of about 10 to 20 kips. If actual structural loads exceed
the above values by more than 25%, this office should be notified.
Review of the Renton online Sensitive Areas Maps indicates that the property is not
indicated to be within a Landslide Hazard, Erosion Hazard, Seismic Hazard or Coal
Mine Hazard area.
P.O. Box 276, Issaquah, WA 98027-0276 • Phone: (425) 391-4228 Fax: (425) 391-4228
a
KRRV Development, LLC August 15, 2013
SCOPE OF WORK
Our geotechnical evaluation included review of geologic mapping, site explorations,
engineering analyses and evaluations and the preparation of this report. The scope of
work included the following specific tasks:
o Review of published geologic mapping of the site vicinity and our recent
explorations on the adjacent property to the east.
o Performed a reconnaissance of the site as well as observations of the
adjacent developed lots to the east.
o Observed and logged four test pit explorations on the site (see Figure 2) to
depths up to 4 feet below existing ground. Logs of the test pits and
results of field and laboratory testing are presented in the Test Pit
summaries of Appendix A along with log% of adjacent explorations on the
adjoining property to the east.
o Performed geotechnical engineering evaluations of the proposed site
development and developed our geotechnical recommendations for
foundation design and site grading.
o Prepared this geotechnical report summarizing our findings and
recommendations.
OBSERVED SITE CONDITIONS
The property is bordered on the west, north and east by developed residential lots (we
previously performed geotechnical evaluations for new development along the south
side of 7tn Street). At the time of our field exploration the property was developed with a
residence and some small sheds and a hot tub as shown in Figure 2.
The topographic mapping included on the site plan of Figure 2 indicates that the
property generally slopes very gently down to the west/southwest at gradients that
range from less than 3 percent up to about 6 percent in localized areas and overall
average gradients across the property from northeast to southwest are less than 3
percent with a maximum elevation difference across the property of about 8 feet from
northeast to southwest.
The site was well vegetated primarily with grasses but also included shrubs, ivy,
elderberry and numerous evergreen trees to 3+ feet in diameter as shown on Figure 2.
We did not observe any seeps or springs or evidence of current or past erosion on the
site.
Project No. 13-121-01 Page 2
KRRV Development, LLC August 15, 2013
Subsoils
Our evaluation of the subsurface conditions was based on our observations of four
exploratory test pits within the property. Approximate locations of the onsite test pits
and the previous adjoining test pits are shown on Figure 2. Logs of the onsite test pits
are presented in Appendix A. Subsoils encountered in our test pits were found to
include some minor fill and natural glacially consolidated soils.
Fill soils were encountered in TP-4 located at the lower south side of the property as
shown on Figure 2. Fill soils at TP-4 were silty crushed gravel and were only about 1.5
feet thick at that location.
Natural soils encountered at the test pit locations were generally silty fine or very fine
sand with some medium to coarse sand and gravel. The upper soils were typically
loose to medium dense soils to depths of about 2 to 3 feet below the natural surface
becoming very dense and hard and cemented at greater depths.
Ground Water
No ground water or seepage was observed in any of the test pits. Typically the shallow
soils were classified as slightly moist and the deeper soils were classified as slightly
moist to moist. The measured moisture contents of the soils generally ranged from
about 5 to 9 percent.
Subsurface Variations
Based on our experience, it is our opinion that some variation in the continuity and
depth of subsoil deposits and ground water levels should be anticipated due to natural
deposition variations and previous onsite structures and grading. Due to seasonal
moisture changes, ground water conditions should be expected to change with time.
Care should be exercised when interpolating or extrapolating subsurface soils and
ground water conditions between or beyond our test pits.
Project No. 13-121-01 Page 3
•
KRRV Development, LLC August 15, 2013
SITE EVALUATIONS
General
The referenced geologic map of Figure 1 indicates the site vicinity to have surface
exposures of glacial till (Qgt) soils. The Qgt glacial till soils are highly consolidated,
heterogeneous mixtures of sand, silt, clay and gravel soils deposited during the
advance of the Vashon glaciation, the last glacial advance into the Puget Sound area,
approximately 13,000 to 16,000 years ago.
Based on the soils observed on the site and review of the referenced map, it is our
opinion that the natural very dense/hard silty sand with gravel soils underlying the
property and are most likely glacially consolidated till deposits (Qgt).
Based on the results of our field investigations combined with our own experience and
judgment, it is our opinion that the geotechnical site conditions are suitable for the
proposed development provided our recommendations are followed.
Hazard Assessment
Landslide: The Renton Sensitive Areas Maps indicate the site is not within a Landslide
Hazard area. In addition, the geologic map of Figure 1 indicates no mapped landslides
within the site vicinity and our site observations indicate the site is currently stable.
Considering the very gentle slope gradients and the observed glacial till soils, it is our
opinion that the potential for future instability on the site is very low to nil.
Erosion: The Renton Sensitive Areas Maps indicate the site is not within a Erosion
Hazard area. We observed that the site is well vegetated we observed no indication of
any seepage or concentrated water flow or current or past erosion on the site. Based
on our site observations and explorations it is our opinion that there is no unusual
erosion risk at this site and any potential erosion potential resulting from development
will be mitigated by our recommended grading procedures and drainage/erosion control
measures and by final re-vegetation/landscaping incorporated into the proposed
development plans. 3
Coal Mine: The Renton Sensitive Areas Maps indicate the site is not within a Coal Mine
Hazard area.
Seismic: The Renton Sensitive Areas Maps indicate the site is not within a Seismic
Hazard area, however the Puget Sound region is a seismically active area. About 17+
moderate to large earthquakes (M5 to M7+) have occurred in the Puget Sound and
northwestern Cascades region since 1872 (141 years) including the 2/28/01 M6.8
Nisqually earthquake and it is our opinion that the proposed structures will very likely
experience significant ground shaking during their useful life.
Project No. 13-121-01 Page 4
KRRV Development, LLC August 15, 2013
Based on a recently published study the site lies only about 2.5 miles south of the
southern mapped location of the Seattle fault and about 18 miles southwest of the
estimated trace of the South Whidbey-Lake Alice fault both of which have postulated
maximum credible magnitudes of 7.0 to 7.5. Another recent study of the Vashon-
Tacoma area presents evidence for the east-west trending Tacoma Fault which is
indicated to pass through the south end of Vashon and the middle of Maury Island
about 10.5 miles southwest of the site. The study suggests that the Tacoma Fault and
the Seattle fault may be linked by a master thrust fault at depth.
The Seattle fault has been documented to have moved at its west end (Bainbridge
Island) about 1000 to 1100 years ago and evidence of movement at the east end has
also recently been documented. Some experts feel that the recurrence interval
between large events on the Seattle Fault may be on the order of several thousands of
years but our calculations indicate it may be on the order of 1200 to 1400 years. The
activity of the documented Tacoma fault is considered to be on the same order as the
Seattle fault. The recurrence of a maximum credible event on the South Whidbey fault
is not known but some experts have assigned a recurrence of about 3000 years,
however smaller events will occur more frequently as evidenced by the 5.3 event on
May 2, 1996 which was attributed to that fault.
In addition to Puget Sound seismic sources, a great earthquake event (M8 to M9+) has
been postulated for the Cascadia Subduction Zone (CSZ) along the northwest Pacific
coast of Oregon, Washington and Canada. The current risk of a future CSZ event is
not known at this time. Published reports have indicated recurrence intervals for a CSZ
event to range from as little as 100-200 years to as long as 1000+ years and the time of
the last event is reported to have been about 313 years ago.
The 2009 International Building Code (IBC) adopted by the City of Renton requires
consideration of a spectral acceleration level with probability of exceedance of 2
percent in 50 years for seismic structural design. This corresponds to about a 2475-
year recurrence interval earthquake ground motion. Based on the short period spectral
response accelerations presented in Figure 1613.5(1) of the 2009 IBC, adjusted as per
equations 16-36, 16-38 and factored as per section 1803.5.12(2), we estimate the IBC
peak ground acceleration for soils and foundation design at this site to be about 0.38g.
Please note that the 0.38g peak ground acceleration includes the additional reduction
factor of section 1803.5.12(2) and is not intended for structural analyses. We
recommend the site be considered a Site Class D for structural design.
Secondary seismic hazards due to earthquake ground shaking include induced slope
failure, liquefaction, lateral spreading and ground settlement. Considering the very
dense/hard nature of the soils recommended for bearing at the site, it is our evaluation
that the potential for damage to the structures due to liquefaction, lateral spreading and
settlement is very low to nil. The potential for seismically induced shallow failures is
also considered very low to nil.
A
Project No. 13-121-01 Page 5
KRRV Development, LLC August 15, 2013
Structure Support Considerations
In our opinion the undisturbed very dense/hard natural glacial till soils encountered in
our explorations should provide good support for the proposed structure foundations.
Bearing soils are expected to be encountered at depths ranging from about 2 to 3 feet
below the natural ground surface at the site.
Foundations should penetrate through any existing fill, topsoil and loose/medium dense
soils to bear on undisturbed very dense/hard natural soils. Conventional spread
footings are considered appropriate for support of the proposed structures considering
that bearing soils are at shallow depths. However, if any deep fill areas are encountered
on the site, lean concrete footing extensions or pipe piles could be used to transfer
foundation loads to the deep bearing soils in those areas. We have included
recommendations for spread footings and lean concrete support in this report.
RECOMMENDATIONS
The following subsections present our recommendations for design of foundations, site
grading, drainage control and erosion control. Also included are recommendations for
plan review and observations and testing during construction.
Spread Footing Foundations
Conventional spread footings founded on undisturbed very dense/hard natural glacial till
soils should provide good support for the proposed structures. Bearing soils are
expected to be encountered at depths ranging from about 2 to 3 feet below the natural
ground surface of the site.
Continuous wall footings should be at least 18 inches wide. Square footings for column
support should be at least 24 inches wide. Footings supported on undisturbed very
dense/hard natural glacial till soils may be designed based on an allowable bearing
pressure of 2000 psf.
All footings should be founded at least 18 inches below the lowest adjacent final grade.
Where the natural bearing soils slope, the footing excavation should be stepped to
maintain a horizontal bearing surface.
If deep fill or other unsuitable soils are encountered, foundation loads may be
transferred from the recommended minimum foundation depths to the recommended
very dense/hard bearing soils by a monolith of lean concrete having a minimum
compressive strength of 1000 psi. The width of an un-reinforced lean concrete
monolith should be at least as wide as the footing or at least one-third of the monolith
height, whichever is greater. Reinforced monoliths should be designed by a structural
engineer. A suitable width trench should be excavated with a smooth edged excavator
Project No. 13-121-01 Page 6
KRRV Development, LLC August 15, 2013
bucket (no teeth) to expose the dense/very dense bearing soils under observation by
our office and backfilled as soon as possible with the lean concrete to the footing
elevation.
The estimated settlement of a 18-inch wide continuous footing carrying a load of 3 kips
per foot is on the order of 1/4 to 1/2 inch. Our settlement estimates assume that
foundations are supported on undisturbed very dense/hard natural bearing soils and
that all fill and loose/disturbed material is removed from the bearing surface prior to
concrete pour. Maximum differential settlement within the proposed structures is
expected to be 1/2 inch or less. Settlements are expected to occur primarily during
construction.
3
Resistance to lateral loads can be assumed to be provided by friction acting at the base
of foundations and by passive earth pressure. A coefficient of friction of 0.45 may be
assumed with the dead load forces in contact with on-site soils. An allowable static
passive earth pressure of 250 psf per foot of depth may be used for the sides of
footings poured against undisturbed medium dense to dense natural soils or properly
compacted structural fill. An allowable static passive earth pressure of 150 psf per foot
of depth may be used for the sides of footings or grade beams poured against existing
loose soils.
The vertical and lateral bearing values indicated above are for the total dead load plus
frequently applied live loads. For short duration dynamic loading caused by seismic or
wind forces, the vertical bearing values may be increased by 50 percent and allowable
lateral passive pressures may be increased by 33 percent.
Site Grading
Site grading is expected to consist primarily of excavation for proposed foundations and
subgrade preparation for slab and pavement areas and utility trenches. Onsite granular
soils cleaned of debris and organics are considered suitable for use in general
compacted fills but in our experience the onsite glacial till soils will be moisture
sensitive with regard to grading and compaction characteristics. Grading should be
scheduled for the late summer months if possible. Wet weather grading may require
the use of imported clean granular fill soils which are more easily compacted at higher
moisture levels. Recommendations for site preparation, temporary excavations,
structural fill, subgrade preparation, site drainage and trench backfill are presented
below.
Site Preparation: Existing vegetation, debris, fill soils, and loose or organic natural soils
should be stripped from the areas that are to be graded. During rough grading, excess
soils may be stockpiled for later use. Stripping in subgrade areas is expected to
average about 1 foot. Soils containing more than 1% by weight of organics may be
used in planter areas, but should not be used for structural fill. Stumps, debris and
trash, plus rocks and rubble over 6 inches in size, should be removed from the site.
Project No. 13-121-01 Page 7
KRRV Development, LLC August 15, 2013
Subsoil conditions on the site may vary from those encountered in our test pits.
Therefore, our office should observe the prepared areas prior to placement of any new
fills.
Temporary Excavations: Sloped temporary construction excavations may be used
where planned excavation limits will not undermine existing structures or interfere with
other construction. Where there is not enough room for sloped excavations, shoring
should be provided.
Based on the subsurface conditions encountered in the test pits, it is our opinion that
sloped temporary excavations may be made vertically to depths of 4 feet or less.
Excavations up to 10 feet in depth should be sloped no steeper than 1:1 within
loose/medium dense soils and no steeper than.' :1 (horizontal:vertical) within the un-
weathered, hard natural glacial till soils. It should be noted that the contractor is
responsible for safety and maintenance of construction slopes.
We recommend that cuts over 4 feet in depth be covered with visqueen tarp to help
control ravelling and sloughing. Surface drainage should be directed away from the top
edge of cut slopes. Surcharge loads should not be allowed within 5 feet of the top of the
slope or within a 1:1 (horizontal:vertical) plane extending up from the toe of excavation,
whichever is greater.
Structural Fill: Provided that soil moisture can be reduced and maintained near
optimum, excavated onsite soils cleaned of organics and debris may be used for
general structural fill but the onsite soils are expected to be moisture sensitive and
during the rainy season the soils may become too wet for practical compaction.
Therefore imported granular fill soils should be used if moisture conditions cannot be
adequately controlled.
Loose soils, formwork and debris should be removed prior to placing fill or backfill.
Structural fill should be placed in horizontal lifts not exceeding 8 inches in loose
thickness and compacted to at least 90 percent of the maximum dry density as
determined by the ASTM D1557 test method.
Imported granular fill should consist of clean, well-graded sand and gravel materials
free of organic debris and other deleterious material. Imported material for wet weather
grading should be a sand/gravel mixture with less than 5 percent fines based on the
sand fraction.
Slab/Pavement Support: Slabs-on-grade and pavement should be supported directly on
undisturbed dense natural soils or on properly compacted structural fill over medium
dense natural soils. Where unsuitable soils such as existing fill, loose/disturbed soils
(from tree stump removal) and organic soils exist at subgrade level, subgrade
preparation should include excavation of the unsuitable soils as required to expose
medium dense natural soils or to a maximum depth of 2 feet (or deeper as required to
remove all loose/disturbed soils from tree stump removal) and placement of structural
Project No. 13-121-01 Page 8
KRRV Development, LLC August 15, 2013
fill to final subgrade elevation. Subgrade fill should be placed in accordance with the
recommendations for structural fill except that the top 6 inches of the subgrade fill
should be compacted to at least 95 percent of the ASTM D1557 maximum dry density
in pavement and driveway areas.
It should be noted that where the proposed slabs cross a fill/natural contact line, there
will be a high risk of cracking. Risk of cracking can be reduced by placing construction
joints at the contact and by proper steel reinforcement of the slab. Interior concrete
slabs should be underlain by a capillary break consisting of a polyethylene vapor barrier
of at least 6 mil thickness.
Utility Trenches: Buried utility conduits should be bedded and backfilled around the
conduit in accordance with the project specifications. Bedding material should extend
from six inches below the pipe to six inches above the pipe. Where conduit underlies
pavement or slabs-on-grade, the remaining backfill above the pipe should be placed
and compacted in accordance with the recommendations for structural fill. If imported
granular fill is used for trench backfill it should be capped with 12 inches of onsite silty
soils.
Drainage Control
Surface drainage from the site and adjoining upslope areas should be controlled and
diverted around the development area in a non-erosive manner. Adequate positive
drainage should be provided away from the structures and on the site in general to
prevent water from ponding and to reduce percolation of water into subsoils. Granular
backfill should be capped with paving or 6 inches of onsite silty soils. A desirable slope
for surface drainage is 2% in landscaped areas and 1% in paved areas.
Roof drains should be tightlined into the storm drain system (no discharge on the
ground surface). A permanent perimeter drain, independent of the roof drain system,
should be placed adjacent to the base of the continuous exterior foundations. The
drain should consist of a four-inch diameter perforated PVC drain pipe placed in at least
one cubic foot of washed drain gravel per lineal foot along the base of the foundations.
The drain gravel zone around the pipe should be encapsulated with a membrane of
Mirafi 140 filter fabric or equivalent between the drainage zone and onsite silty soils.
Erosion Control
Although we observed no evidence of erosion, onsite soils are expected to be erodible
when disturbed and exposed to concentrated water flows. Siltation fences or other
detention devices should be provided around the downslope side of the disturbed site
area and soil stockpile areas during construction to control the transport of eroded
material. The lower edge of the silt fence fabric should have "J" shaped embedment in
a trench extending at least 12 inches below the ground surface.
Project No. 13-121-01 Page 9
KRRV Development, LLC August 15, 2013
Surface water flow should be collected in area drains and tightlined to the storm water
system, no water should be discharged on the site unless via a properly designed and
approved infiltration/dispersion system. Exposed final graded soil areas should be
planted immediately with grass and deep rooted plants.
Plan Review
This report has been prepared to aid in the evaluation of this site and to assist the
architect, structural and civil engineers in the design and construction of the project. It
is recommended that this office be provided the opportunity to review the final design
drawings and specifications to determine if the recommendations of this report have
been properly implemented and to make any supplemental design recommendations
which may be required.
Observations and Testing During Construction
Recommendations presented in this report are based on the assumption that soil
conditions exposed during construction will be observed by our office so that any
necessary design changes or supplements may be made. Foundation excavations
should be observed to verify that they expose undisturbed very dense/hard bearing
soils and that excavations are free of loose and disturbed materials.
All structural fill and slab/pavement subgrade areas should be observed by a
representative of this office after stripping and prior to placing fill. Drainage control
systems should be observed by our office to verify proper construction. Proper fill
placement and compaction should be verified with field and laboratory density testing
by a qualified testing laboratory.
Project No. 13-121-01 Page 10
•
KRRV Development, LLC August 15, 2013
CLOSURE
This report was prepared for specific application to the subject site and for the exclusive
use of KRRV Development, LLC and their representatives. The findings and
conclusions of this report were prepared with the skill and care ordinarily exercised by
local members of the geotechnical profession practicing under similar conditions in the
same locality. We make no other warranty, either express or implied.
Variations may exist in site conditions between those described in this report and actual
conditions encountered during construction. Unanticipated subsurface conditions
commonly occur and cannot be prevented by merely making explorations and
performing reconnaissance. Such unexpected conditions frequently require additional
expenditures to achieve a properly constructed project. If conditions encountered
during construction appear to be different from those indicated in this report, our office
should be notified.
Respectfully submitted,
GEOSPECTRUM CONSULTANTS, INC.
Va,tzwerd'ofilirri"kawytii •
p,. DOp
James A. Doolittl �`'of WAS, ,, �T `..,A0�0��'
Principal Engineer
C-44,
Encl: Figures 1 and 2
Appendix A 3623
oto, Fcrsr��
y 67oNAL G.�
Dist: 2/Addressee EXPIRES 3/19/ 20 t r'
Project No. 13-121-01 Page 11
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rRf: Geologic Map of the Renton Quadrangle
by D. R. Mullineaux, USGS Map GQ-405, 1965
Enlarged Scale: 1"=1000'
SITE VICINITY GEOLOGIC MAP
GEOSPECTRUM CONSULTANTS, INC..
Proposed
060NE 7thI Str street North Plat
Renton, Washington>>::1: ::: k :>
Geotechn/cc/Eng/nearing cnd Earth Sconce*
Proj. No.13-121 Date 8/13 Figure 1
. • I
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NE 7th STREET ', ,,,
ref: Site Plan provided by client,
undated, scale: lu= 40' SITE DEVELOPMENT & EXPLORATION PLAN
Proposed 4-Lot 7th Street North Plat
GEOSPECTRUM CONSULTANTS, INC. 3506 NE 7th Street
Renton, Washington
GEDcateachniccal ErmilneaearIng (=Ind Ecult)Scicancos
Proj. No.13-121 Date 8/13 Figure 2
APPENDIX A
FIELD EXPLORATION
Our field exploration included a site reconnaissance and test pit explorations. During
the site reconnaissance, the surface site conditions were noted, and the locations of the
test pits were approximately determined.
The test pits were approximately located using the existing structures as a guide. The
approximate locations of the test pits are shown on Figure 2. Test Pit elevations were
estimated based on the topographic mapping shown on Figure 2.
Test pits were advanced using a trackhoe excavator. Soils were continuously logged
and classified in the field by visual examination, in accordance with the ASTM Soil
Classification system.
Logs of the on-site test pits are presented on the test pit summary sheets A-1 and A-2.
The test pit summaries include descriptions of the soils and pertinent field data. Soil
consistency and moisture conditions indicated on the logs are interpretations based on
the conditions observed in the field. Boundaries between soil strata indicated on the
logs are approximate and actual transitions between strata may be gradual.
TEST PIT NO. 1
Logged by JAD
Date: 7/24/13 Elevation: 406'
Depth Blows Class. Soil Description Consistency Moisture Color W(%) Comments
0 OL Organic duff& ivy loose slightly brown
SM gravelly Silty Sand, fine
_
1 w/abundant roots to %"
5.1
o
2 — f
rae cturiceme
ng medium
&
brown
gray
3 —
very cemented hard , gray-brn 4.6
4
_ Maximum depth 3.5 feet.
No ground water observed.
5 -
6 -
7
TEST PIT NO. 2
Logged by JAD
Date: 7/24/13 Elevation: 403'
Depth Blows Class. Soil Description Consistency Moisture Color W(%) Comments
6 OL Organic duff loose mlisah ly brown
is
1 _ SM Silty fine Sand w/some m-c sand " to IIi ht n 7.3
moist
blgow
very cemented hard 7.5
3 — Maximum depth 2.5 ft.
No ground water observed.
4 -
5 —
6 —
7 —
GEOSPECTRUM CONSULTANTS, INC. Proposed 4-Lot 7th Street North Plat
3506 NE 7th Street
Geotechnical Engineering and Earth Sciences Renton, Washington
Proj. No. 13-121 Date 8/13 Figure A-1
TEST PIT NO. 3
Logged by JAD
Date: 7/24/13 Elevation: 404'
Depth Blows Class. Soil Description Consistency Moisture Color W(%) Comments
0 OL Organic duff& decayed wood loose slightly brown
1 —
SM Silty Sand, very fine
5.8g/ambcs anndt &ooravtel3"
2 —
with some cementation medium light
dn
3 —
_ very cemented hard gray-brn 5.8 ,
4 —
Maximum depth 3.5 feet.
5 —
No ground water observed.
6
7 —
TEST PIT NO. 4
Logged by JAD
Date: 7/24/13 Elevation: 400'
Depth Blows Class. Soil Description Consistency Moisture Color W(%) Comments
0 OLsod dry FILL
GM Silty Gravel, crushed to 3" dense sli�Stly a
light
1 —
tt 9 Y
2 — SM wigv srya i e Sand w/orq. & roots loose sli�fitly dk brn
brown
to9.0
3 _ moist
4 — m. dense
_ becomes cemented v. dense gray-brn 6 6
5 _ Maximum depth 4.5 ft.
No ground water observed.
6 —
7 —
GEOSPECTRUM CONSULTANTS, INC. Proposed 4-Lot 7th Street North Plat
3506 NE 7th Street
Geotechnical Engineering and Earth Sciences Renton, Washington
Froj. No. 13-121 Date 8/13 Figure A-2
Section 7: Other Permits
None applicable at this time
Section 8: CSWPPP Analysis and Design
The proposed development and improvements comprise of 0.73 acres of property. The actual exposed and
"worked" area on the project will consist of widening NE 7th Street and installing a private access road along
the west side of the property. This"open and worked"construction area will be 0.20 acres. All construction
activities will be concentrated on NE 7th Street. The entire property is NOT proposed to be graded and
worked. Erosion control for the project will consist of filter fabric fencing along the westerly edge of the new
private road. The existing paved driveway will be used for construction entry. The Developer constructing
the improvements on NE 7th North Short is also the same developer that is constructing the NE 7th (three)
short plats on the south side of NE 7th. The project to the south is under construction and properly
protected from erosion. All staging of materials and vehicle parking will be provided off site on the project
to the south. Therefore minimizing disturbance to the NE 7th north property can be accomplished. This will
provide for more un-disturbed surface areas to filtering and minimize erosion runoff during construction.
A temporary pond is not proposed due to allow for minimal disturbance of the property.
There is an existing house and crawl space on the property that will be demolished under a separate permit.
The crawl space area will provide a localize depression if additional filtration is necessary. This area could be
used as a small ponding depression.
Section 9: Bond Quantities, Facility Summaries, and Declaration of Covenant
A Declaration of Covenant (Operation, maintenance, and restrictions) is attached for the proposed BMP's
shown on the civil construction plans.
Section 10: Operation and Maintenance Manual
The Operation and Maintenance of the proposed BMP's provided will be included within the Declaration of
Covenant from Section 9. These documents will be provided for review on as part of the utility permit
submittal.
• • r
APPENDIX A MAINTENANCE REQUIREMENTS FOR FLOW CONTROL,CONVEYANCE,AND WQ FACILITIES
NO. 5-CATCH BASINS AND MANHOLES
Maintenance Defect or Problem Condition When Maintenance is Needed Results Expected When
Component Maintenance is Performed
Structure Sediment Sediment exceeds 60%of the depth from the Sump of catch basin contains no
bottom of the catch basin to the invert of the sediment.
lowest pipe into or out of the catch basin or is
within 6 inches of the invert of the lowest pipe
into or out of the catch basin.
Trash and debris Trash or debris of more than'/:cubic foot which No Trash or debris blocking or
is located immediately in front of the catch basin potentially blocking entrance to
opening or is blocking capacity of the catch basin catch basin.
by more than 10%.
Trash or debris in the catch basin that exceeds No trash or debris in the catch basin.
1/3 the depth from the bottom of basin to invert the
lowest pipe into or out of the basin.
Dead animals or vegetation that could generate No dead animals or vegetation
odors that could cause complaints or dangerous present within catch basin.
gases(e.g.,methane).
Deposits of garbage exceeding 1 cubic foot in No condition present which would
volume. attract or support the breeding of
insects or rodents.
Damage to frame Comer of frame extends more than%inch past Frame is even with curb.
and/or top slab curb face into the street(If applicable).
Top slab has holes larger than 2 square inches or Top slab is free of holes and cracks.
cracks wider than 1/4 inch.
Frame not sitting flush on top slab,i.e., Frame is sitting flush on top slab.
separation of more than%inch of the frame from
the top slab.
) Cracks in walls or Cracks wider than 1/2 inch and longer than 3 feet, Catch basin is sealed and
bottom any evidence of soil particles entering catch structurally sound.
basin through cracks,or maintenance person
judges that catch basin is unsound.
Cracks wider than 1/2 inch and longer than 1 foot No cracks more than 1/4 inch wide at
at the joint of any inlet/outlet pipe or any evidence the joint of inlet/outlet pipe.
of soil particles entering catch basin through
cracks.
Settlement/ Catch basin has settled more than 1 inch or has Basin replaced or repaired to design
misalignment rotated more than 2 inches out of alignment. standards.
Damaged pipe joints Cracks wider than'A-inch at the joint of the No cracks more than''h-inch wide at
inlet/outlet pipes or any evidence of soil entering the joint of inlet/outlet pipes.
the catch basin at the joint of the inlet/outlet
pipes.
Contaminants and Any evidence of contaminants or pollution such Materials removed and disposed of
pollution as oil,gasoline,concrete slurries or paint. according to applicable regulations.
Source control BMPs implemented if
appropriate. No contaminants
present other than a surface oil film.
Inlet/Outlet Pipe Sediment Sediment filling 20%or more of the pipe. Inlet/outlet pipes clear of sediment.
accumulation
Trash and debris Trash and debris accumulated in inlet/outlet No trash or debris in pipes.
pipes(includes floatables and non-floatables).
Damaged Cracks wider than'',cinch at the joint of the No cracks more than%-inch wide at
inlet/outlet pipes or any evidence of soil entering the joint of the inlet/outlet pipe.
at the joints of the inlet/outlet pipes.
.)
2009 Surface Water Design Manual—Appendix A 1/9/2009
A-9
f
APPENDIX A MAINTENANCE REQUIREMENTS FLOW CONTROL,CONVEYANCE,AND WQ FACILITIES
NO. 5-CATCH BASINS AND MANHOLES
Maintenance Defect or Problem Condition When Maintenance is Needed Results Expected When
Component Maintenance is Performed
Metal Grates Unsafe grate opening Grate with opening wider than'/,inch. Grate opening meets design
(Catch Basins) standards.
Trash and debris Trash and debris that is blocking more than 20% Grate free of trash and debris.
of grate surface. footnote to guidelines for disposal
Damaged or missing Grate missing or broken member(s)of the grate. Grate is in place and meets design
Any open structure requires urgent standards.
maintenance.
Manhole Cover/Lid Cover/lid not in place Cover/lid is missing or only partially in place. Cover/lid protects opening to
Any open structure requires urgent structure.
maintenance.
Locking mechanism Mechanism cannot be opened by one Mechanism opens with proper tools.
Not Working maintenance person with proper tools.Bolts
cannot be seated. Self-locking cover/lid does not
work.
Cover/lid difficult to One maintenance person cannot remove Cover/lid can be removed and
Remove cover/lid after applying 80 lbs.of lift. reinstalled by one maintenance
person.
1/9/2009 2009 Surface Water Design Manual—Appendix A
A-I()
APPENDIX A MAINTENANCE REQUIREMENTS FOR FLOW CONTROL,CONVEYANCE,AND WQ FACILITIES
NO. 6-CONVEYANCE PIPES AND DITCHES
Maintenance Defect or Problem Conditions When Maintenance is Needed Results Expected When
Component Maintenance is Performed
Pipes Sediment&debris Accumulated sediment or debris that exceeds Water flows freely through pipes.
accumulation 20%of the diameter of the pipe.
Vegetation/roots Vegetation/roots that reduce free movement of Water flows freely through pipes.
water through pipes.
Contaminants and Any evidence of contaminants or pollution such Materials removed and disposed of
pollution as oil,gasoline,concrete slurries or paint. according to applicable regulations.
Source control BMPs implemented if
appropriate. No contaminants
present other than a surface oil film.
Damage to protective Protective coating is damaged;rust or corrosion Pipe repaired or replaced.
coating or corrosion is weakening the structural integrity of any part of
pipe.
Damaged Any dent that decreases the cross section area of Pipe repaired or replaced.
pipe by more than 20%or is determined to have
weakened structural integrity of the pipe.
Ditches Trash and debris Trash and debris exceeds 1 cubic foot per 1,000 Trash and debris Geared from
square feet of ditch and slopes. ditches.
Sediment Accumulated sediment that exceeds 20%of the Ditch cleaned/flushed of all sediment
accumulation design depth. and debris so that it matches design.
Noxious weeds Any noxious or nuisance vegetation which may Noxious and nuisance vegetation
constitute a hazard to County personnel or the removed according to applicable
public. regulations. No danger of noxious
vegetation where County personnel
or the public might normally be.
Contaminants and Any evidence of contaminants or pollution such Materials removed and disposed of
pollution as oil,gasoline,concrete slurries or paint. according to applicable regulations.
Source control BMPs implemented if
appropriate. No contaminants
present other than a surface oil film.
Vegetation Vegetation that reduces free movement of water Water flows freely through ditches.
through ditches.
Erosion damage to Any erosion observed on a ditch slope. Slopes are not eroding.
slopes
Rock lining out of One layer or less of rock exists above native soil Replace rocks to design standards.
place or missing(If area 5 square feet or more,any exposed native
Applicable) soil.
2009 Surface Water Design Manual—Appendix A 1/9/2009
A-I1