Hydraulic Report 090506the 3futcFi
Hydraulic Report
May 2006
Revised September 2006
Prepared For:
Yelm Property Development LLC
Attn: Dennis Balascio
P.O. Box
Yelm, WA 98597
Prepared by:
Skillings Connolly, Inc.
5016 Lacey Boulevard SE
Lacey, Washington 985
(360)491-3399
Jed
Rece 5 tioo6
Ee o
S
~l~ls.
TABLE OF CONTENTS
Section
Page
1 Proposed Project Description ................................................ ......... 1
2 Existing Conditions ................................................................ .........1
3 Infiltration Rates/Soils Report ................................................ ......... 1
4 Wells and Septic Systems ..................................................... ......... 1
5 Fuel Tanks ............................................................................. ......... 1
6 Sub-Basin Description ........................................................... .........2
7 Analysis of 100 year Flood ..................................................... ......... 2
8 Aesthetic Considerations for Facilities ................................... ......... 2
9 Facility Sizing ......................................................................... .........2
10 Covenants, Dedications, Easements ..................................... ......... 3
11 Property Owners Association Articles of Incorporation .......... ......... 3
12 Erosion Control Report .......................................................... ......... 3
ndices
A. Project Maps
Vicinity Map
SCS Soils Map
Regional Topography
FEMA 100-Year Flood Plain Map
B. Conveyance Calculations
Water Quality Calculations
Drywell Sizing
Computer Calculations
C. Supporting Data
Soils Data
SCS Soil Description
D. BMPs
SK/LL/NGS CONNOLLY, INC. 1 Velm Property Development LLC
Hydrolo9Y Report
SECTION 1 Proposed Project Description
The following hydrology report summarizes the storm drainage analysis and design fora 118-
lot development located between Mountain View Rd SE and Killion Rd SE. The land is
currently pasture with no trees. The project includes the addition of 118 residential lots for
single-family homes, new roadways and sidewalks, sewer, water services and stormwater
facilities to treat and dispose of the project's stormwater. The proposed roadway features and
utilities will be extended from both Mountain View Rd SE and Killion Rd SE.
The 19.48 acre site is located in Sections 20, Township 17 North, Range 2 East, W.M. The
Thurston County's tax parcel numbers associated with the project are 21713430200 and
21713430300.
The increased stormwater runoff resulting from the addition of impervious area will be treated
.and retained in accordance with the Washington State Department of Ecology's stormwater
Management Manual for the Puget Sound Basin (The Technical Manual) 1992.
SECTION 2 Existing Conditions
The site is presently covered with grass and evergreen trees with slopes ranging from 0-5
percent. Presently the site infiltrates directly into the ground with no off-site runoff.
Tax Parcel Nos.
21713430200 and 21713430300.
Legal Description:
THE SOUTH ONE-HALF OF THE SOUTHWEST QUARTER OF THE SOUTHEAST
QUARTER OF SECTION 13, TOWNSHIP 17 NORTH, RANGE 1 EAST., W.M.; EXCEPTING
THEREFROM COUNTY ROADS;
SECTION 3 Infiltration Rate/ Soils Report
Site soils are identified by the Soil Conservation Service Soil Survey of Thurston County,
Washington as a Spanaway gravelly sandy loam, a Type 'B' soil. This soil is characterized as
very deep, somewhat excessively drained and formed on terraces.
Soil test holes were dug in the vicinity of the proposed infiltration basins of the project, and
observations confirm that the soil types match the SCS soil description. A soil log map
showing the location of the test holes is attached in Appendix C. Based on observations of
the soil texture in these test holes, the sandy soil will have a subsurtace percolation rate in
excess of 100 inches per hour. A design rate of 10 inches per hour is used.
SECTION 4 Wells
One well was observed within 300 feet of the project site and is shown on the preliminary plat
map. This well is planned to be abandoned prior to final plat.
Each lot will be served by City of Yelm STEP collection system. The holding tank will be
maintained by the city and pumped on a regular basis. Domestic water will be provided by
the City of Yelm's water distribution system.
SECTION 5 Fuel Tanks
No fuel tanks were observed at the project site.
SKILL/NGS CONNOLLY, INC. 1 Yelm Property Oevelopmenl LLC
Hydrology Report
SECTION 6 Sub-Basin Description
Site topography contributes 0 acres of off-site storm runoff. The site is approximately 1.5 to 2
ft above the flat surroundings.
SECTION 7 Analysis Of The 100-Year Flood
Federal Emergency Management Agency Mapping does not indicate flooding in the
immediate area. Please see the exhibit in Appendix A.
SECTION 8 Aesthetic Considerations For Facilities
The stormwater treatment facilities and drainage basins will be located underground. Twp
separate rock percolation galler'es will be used to infiltrate the treated stormwater. The rock
gallery'es will have a flat grade above them and will be seeded for use as open space.
SECTION 9 Facility Sizing And Downstream Analysis
The basins were sized and analyzed using Waterworks software and the Santa Barbara Unit
Hydrograph method using a 100-yr, 24-hr SBUH Thurston County storm event. As previously
described, conservative infiltration rates were used for the design calculations.
Conveyance -
Stormwaterfor Basin 'A' and 'B' will be conveyed from the roadways to the treatment facilities
from Type I catch basin and through 12 inch ADS N-12 storm pipe
Basin Summary
Basin Pervious Area (ac 'Im ervious Area ac Roof Area ac '
A 3.25 5.71 3.36
B 3.29 ~ 1.91 1.93
Treatment
Basin 'A' will use AquaShield Aqua-Swirl Concentrator Model AS-5 stormwater treatment
system to treat the 6-month storm event of 0.97 cfs. The Aqua-Swirl Concentrator Model AS-
5 can treat up to 1.01 cfs (per WSDOE) which provides addional treatment of the runoff. See
appendix B for the Aqua-Swirl Concentrator Model AS-6 detail.
Basin 'B' will use AquaShield Aqua-Swirl Concentrator Model AS-3 stormwater treatment
system to treat the 6-month storm event of 0.30 cfs. The Aqua-Swirl Concentrator Model AS-
3 can treat up to 0.42 cfs (per WSDOE) which provides addional treatment of the runoff. See
appendix B for the Aqua-Swirl Concentrator Model AS-3 detail.
Treatment Summary
Treatment Basin Pervious Area (ac) 'Impervious Area (ac) MGSFIood On-Line '.
Treatment Flow 6-Mo cfs
A 3.25 5.71 0.97
B 3.29 1.91 0.30
Treatment flow is based on the MGSFIood 15-minute design discharge.
"The impervious area used to calculate the required for treatment does not include the
stormwater runoff generated by the roofs of the proposed homes; this stormwater will be
collected and infiltrated in individual dry wells.
SKILLINGS CONNOLLY, INC. 2 Yelm Property Development LLC
Hytlrology Report
Retention
After the stormwater is treated it will enter a rock gallery.
Basin 'A' will have a 4.0' deep rock gallery with a bottom area of 8,500 sf will be constructed
underneath the open space in Tract 'A'. The rock gallery will be constructed of 1 % - 2"
washed drain rock. The storage volume of the rock gallery is 11,220 cf (8,500 sf x 4.0' x
0.33). The rock gallery will have its sides and top lined with filter fabric to prevent sediment
from entering the facility.
Basin 'B' will have a 4.0' deep rock gallery with a bottom area of 4,000 sf will be constructed
underneath the open space in Tract 'B'. The rock gallery will be constructed of 1 % - 2"
washed drain rock. The storage volume of the rock gallery is 5,280 cf (4,000 sf x 4.0' x 0.33).
The rock gallery will have its sides and top lined with filter fabric to prevent sediment from
entering the facility.
Racin Summary
Basin 6-Month Elev 700-yr Elev Water Works (max
stared volume) (c Storage Volume
Provided (cf)
A 330.1 333.60 70,09fi 11,220
B 333.1 336.36 4,432 5,280
The two infiltration basins were sized in accordance with Washington State Department of
Ecology's stormwater Management Manual for the Puget Sound Basin (The Technical
Manual) t992 and exceed the required storage volumes.
Roof Runoff
stormwater for the roof area of the homes will be infiltrated in individual drywells. The
drywells will be sized in accordance with section III-3.6.6 BMP RI.15 Roof Downspout System
of Washington State Department of Ecology's stormwater Management Manual for the Puget
Sound Basin (The Technical Manual) 1992. See appendix 'B' for the roof downspout system
detail.
SECTION 10 Covenants Dedications, Easements
The storm management facility for this site will remain privately owned and maintained. A
maintenance agreement should be executed to ensure future maintenance of the facilities.
SECTION 11 Property Owners Association Articles Of Incorporation
Not applicable.
SKILLINGS CONNOLLY, INC. 3 Yelm Property Development LLC
nytlrolagy Report
SECTION 12 Erosion Control
SECTION 1 CONSTRUCTION SEQUENCE AND PROCEDURE
The construction sequence will be to install erosion control features, construct the
infiltration basin, and the roadway. Once the site has been hydroseeded or sod has
been installed and they are established, the temporary erosion control features will be
removed.
SECTION 2 TRAPPING SEDIMENT
Sediment control items are shown on the plans and include silt fencing, and a
construction entrance as required.
SECTION 3 PERMANENT EROSION CONTROL AND SITE RESTORATION
The disturbed areas will be hydroseeded or sodded.
SECTION 4 GEOTECHNICAL ANALYSES AND REPORT.
Not applicable
SECTION 5 INSPECTION SEQUENCE -
A final inspection will be performed to determine final grades and to verify the
installation and construction of other items, as required to complete as-built drawings.
The construction entrance and infiltration basin will be inspected during construction.
SECTION 6 CONTROL OF POLLUNTANTS OTHER THAN SEDIMENT
The amount of non-sediment pollutants will be so minimal or non-existent that this
section is not applicable.
SKILLINOS CONNOLLY, INC. 4 Velm Property Development LLC
Hydrolo9Y Report