HomeMy WebLinkAboutStormwater Plan - PLN General STORM WATER PLAN
ALLYN KNIT SHOP AND LOIS' BARBER SHOPPE
This plan is based on a small plot that is less than 5000 sq. ft.
The upper parking lot is 1600 sq. ft. Both the parking spaces and the approach to them are
slanted towards the curtain drain. Water is collected and then sent through a sump with a fine
mesh screen. From there water goes to a 2 ft. X 3 ft. deep and 66 ft. long drain rock filled
trench. This trench follows the contour of the land (a slight downhill slope). At the lower end if
overflow may occur, it then progresses to an existing swale that is parallel to State Hwy 3.The
Swale is a 20 ft. green belt 200 ft. long area located 10ft from the right of way.
1600 sq/ft. (.0013 x 53 - .0101)=94 ft. trench due to space restrictions a 66 ft. trench over
flowing into a 20 ft wide 200 ft. long existing swale per hand out L of the mason county
alternate infiltration elements.
The lower parking lot consists of about 1300 sq. ft. and 500 sq. ft. of roof run off. The water will
run through sumps with fine mesh and into the 64 ft. long infiltration trench with high capacity
gravel less chambers closest to State Hwy 3.
1800 sq/ft. (.00072 x 53 - .00303)= 64 ft. This comes from the Kitsap storm water plan page 115
table 5.1 and 5.2 see information provided.
The rest of the roof(1500 sq. ft.) will run through sumps with fine mesh and continue into the
other gravel less high capacity trench which is located 10 ft. from the building.This trench is
also 64 ft. long.
1800 sq/ft. (.00072 x 53 - .00303)= 64 ft. This comes from the Kitsap storm water plan page 115
table 5.1 and 5.2 see information provided.
During the construction all efforts will be made to control erosion by placement of straw on the
exposed ground and silt barriers to contain runoff. The natural slope of the building site is
towards the Swale located in the green belt this will aide in the water runoff during
construction.The temporay storge of dirt for back fill will be covered to help prevent erosion.
This plan has been discussed in detail with Loretta Swanson ext. 769
Table 5.1 -Recommended Infiltration Rates Based on%of Soil Retained by 00 Sieve
Short Term Infiltration rate Correction Facto,CF Estimated Long-term(Design)
(inches/hour) Infiltration Rate(inches/hour)
>87.5% 8 4 2.0
87.5%-75% 2 4 0.5
75%-55% 1 4 0.25
55%-45% 0.5 4 0.13
Table 5.2-Infiltration Trench Sizing Factors
BMP Native Soil Design Infiltration Regression Factors Regression Equation
Rate(inches/hour) M B
RockTrench 0.13 0.0244 0.4918 Length(feet)=Impervious Area(square feet)x
BMP 5.01 [M x Mean Annual Precipitation(inches)+B]
0.25 0.0097 -0.1171
0.5 0.0051 -0.0445
2.0 0.0013 +0.0101
Gravelless 0.13 0.0057 -0.0695 Length(feet)=Impervious Area(square feet)x
Chamber 0.25 0.0038 -0.0412 [M x Mean Annual Precipitation(inches)+B]
BMP 5.02
0.5 0.0021 -0.0104
2.0 0.00072 -0.00303
NOTE: (BMP 5.01=Figure 5.7)-Rock trench=2-feet wide
As an example,using table 5.2,the length of a rock trench receiving runoff from 1.000 square feet of impervious area at a site
with a native soil design infiltration rate of 0.5 inches per hour and a mean annual precipitation depth of 48 inches(from figure
5.4)would be calculated as:
Rock Trench Length (feet)=1,000 x[0.0051 x 48-0.0445]=200 feet
Similarly,the length of a gravelless chamber receiving runoff from 2,000 square feet of impervious surface area where the
native soil design infiltration rate is 0.25 inches per hour and the site mean annual precipitation depth is 34 inches(from figure
5.4)would be calculated as:
Gravelless Chamber Length(feet)=2,000 x[0.0038 x 34-0.0412]= 176 feet
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Figure 5.3—Gravelless Chamber Infiltration Trench
DOWNSPOUT DRAINAGE
TRENCH LENGTH PER DBIMJ
�CLEANDLIT
CHAMBER
CB SUMS'WITH SOLID LI
MIN 1 FT OF DEPTH BELOW ELBOW
III III III III III III
II III III III III III I Native Soil Backfill
III III III III III I I (Min 12"from top of the
II III � I III I chamber)
III III
GRAVEL TO TOP OF
LOUVERS
Min 3" OF GRAVEL
BELOW CHAMBER
CHAMBER
A gravelless chamber infiltration trench.Void space per linear foot shall be at least 2.6 cubic
feet. The infiltrative surface per linear foot shall be at least 2.8 square feet.
The following products are known to meet these criteria:
InfiltratorOc High Capacity by Infiltrator Systems, Inc.
EnviroChamberw High Capacity by Hancor, Inc.
StormtechVD SC-310 by Infiltrator Systems, Inc
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