HomeMy WebLinkAbout2004.06.08 - Trunk Sewer Replacement Project Phase 3 Discharge CalculationsCity of Port Townsend
Public Works Department
181 Quincy Street, Suite 301
Port Townsend, WA. 98368
Phone: (360) 385-72l2,Faxz (360) 385-7675
June 8,2004
Ms. Judy Surber, Senior Planner
City of Port Townsend
Building and Community Development
181 Quincy St., Suite 301
Port Townsend, WA 98368
Re: Trunk Sewer Replacement Project, Phase III - Chinese Gardens Lagoon
Dear Ms. Surber;
When considering Ms. Chebuhar's letter to you dated May 26, 2004, please include the
following in your consideration.
Using the Santa Barbara Urban Hydrograph method, I have calculated the anticipated
discharge quantity from the proposed storm drain pipe for six-month and 100-year storm
events (Note that storm events are referred to by their expected frequency; e.9., a 100-year
storm intensity can be expected to occur once every 100 years). After calculating these
discharge rates, I also calculated the anticipated incremental increase in the Chinese Gardens
Lagoon's water surface elevation that can be expected to result from these discharges. Please
reference the attached calculation sheets.
The information on the attached calculation sheets can be summarized as follows: The
increase in the Chinese Gardens Lagoon's water surface elevation due to the discharge from
the proposed storm drain pipe during a six-month storm is anticipated to be 0.016 inches, a
negligible quantrty. For a 100-year storm, this anticipated increase in the water surface
elevation is 0.28 inches; approximately one quarter of one inch.
Both of these quantities are insignificant relative to the existing fluctuations in the Chinese
Gardens Lagoon water surface during storm events. Also note that for purposes of wetland
delineations, other criteria are used rather than the 100-year water surface elevation.
A NATIONAL MAIN STREET COMMUNITY WASHINGTON'S HISTORIC VICTORIAN SEAPORT
The conclusion that results from evaluation of these figures and the attached calculation sheets
is that the impact to Ms. Chebuhar's property from the proposed storm drain pipe is
negligible, and damages, both financial and physical, will not be realized.
If you have any questions or comments regarding this letter or the attached calculation sheets,
please contact me at your convenience.
Best regards,
{
/';
Eric Project Engineer
City of Port Townsend Public Works
Discharge to Chinese Gardens Lagoon from the Proposed Twelve-inch Storm Drain Pipe:
Pond Areas and Volumes
Area of Pond Number One:
Overflow Depth of Pond Number One :
Overflow Volume of Pond Number One :
Area of Pond Number Two :
Overflow Depth of Pond Number Two :
Overflow Volume of Pond Number Two :
Storm Events
Six-month, 24 -hour PreciPitation :
1 00-year, 24-hour Precipitation :
Fairground Roof Runoff
1 O0-year, 24-hour Volume :
100-year, 24-hour Peak Discharge :
Impervious Area (Roofs) :
CN Impervious:
Time of Concentration :
Six-month, 24-hour Volume :
1 0O-year, 24-hour Volume :
S ix-month, 24 -hour Peak Disch ar ge :
100-year, 24-hovr Peak Dischatge:
Haonv Vallev Runoff
Total Area:
Pervious Area (Open Space) :
CN Pervious:
Impervious Area (Roofs and Streets) :
CN Impervious:
Time of Concentration :
Six-month, 24-hour Volume :
I 00-year, 24-hotsr Volume :
Six-month, 24-hour Peak Discharge :
10O-year, 24-hour Peak Discharge:
Combined Runoff (Happy Valleyand Fairground Roofs)
2,840 S.F
N/A
N/A
0.6Inches
2.5 Inches
1.54 Acres
98
10 Minutes
2,290 C.F.
t2,690 C.F
0.16 C.F.S.
0.89 C.F.S.
12.67 Acres
1 1.15 Acres
68 (Soil Type'A')
1.52 Acres
98
75 Minutes
2,230 C.F. (bypasses this system)
28,030 C.F.
0.08 C.F.S. (bypasses this system)
0.52 C.F.S.
40,660 C.F.
1.07 C.F.S.
7,500 s.F,
0.70 Feet
5,250 C.F
(The six-month storm runoff comes only from the fairground roofs. See above.)
Existing Discharge to the 51't Street Wetland at the Chinese Gardens Lagoon
Pond Areas and Volumes
Total Area: 600 Acres
This 600-acre drainage basin directs runoff to the Chinese Gardens Lagoon via a central
drainage corridor approximately two miles in length. Along this drainage corridor are a
series of local drainage basins where infiltration and detention occur. For the purpose of
this evaluation, it is assumed that only runoff 'downstream' of the last local drainage
basin contributes directly to the Chinese Gardens Lagoon immediately following a storm
event. This approach is conservative, as it decreases the percentage ofrunoffattributable
to existing "onditiotm,
thus increasing the percentage of runoff attributable to the
proposed storm drain PiPe'
Area of Pond Number One:
Overflow Depth of Pond Number One:
Overflow Volume of Pond Number One :
Area of Pond Number Two :
Overflow Depth of Pond Number Two :
Overflow Volume of Pond Number Two :
Storm Events
Six-month, 24 -hour Precipitation :
1 00-year, 24-hour PreciPitation :
Area Runqff
Total Contributing Area :
Pervious Area (Second Growth with Brush) :
CN Pervious:
Impervious Area (Roofs and Streets):
CN Impervious:
Time of Concentration :
Six-month, 24-hour Volurne :
1 0O-year, 24-hour Volume :
Six-month, 24 -hour Peak Disch at ge :
100-year, 24-hour Peak Dischatge :
Six-month, 24-hour Volume :
Six-month ,24-hour Peak Dischatge :
1 OO-year, 24-hotr Volume :
100-year, 24-hour Peak Discharge :
7,500 s.F
0.70 Feet
5,250 C.F
2,840 S.F
N/A
N/A
0.6Inches
2.5 Inches
7,100 c.F.
0.16 C.F.S.
279,740 C.F
4.57 C.F.S.
68.20 Acres
66.00 Acres
81 (Soil Type'C')
2.20 Acres
98
210 Minutes
4,820 C.F.
239,060 C.F.
0.09 c.F.s.
4.10 C.F.S.
Conclusions:
Six-month Storm
Under existing conditions, Pond Number One receives 4,820 cubic feet of runoff during a
six-month storm. Given that Pond Number One has a surface area of 7,500 square feet,
this equates to a water depth increase of 0.65 feet, just short of the overflow depth of 0.70
feet.
The additional runoff received from the fairground loofs will add 2,290 cubic feet of
water to the total runoff from a six-month storm. This additional runoff will cause Pond
Number One to overflow into Pond Number Two. Since Pond Number Two is connected
to the Chinese Garclens Lagoon via an open channel, the increase in the depth of Pond
Number Two will be negligible.
1OO-year Storm
During a 100-year storm under existing conditions, 239,060 cubic feet of water is
discharged to the Chinese Gardens Lagoon via Pond Numbers One and Two. Obviously
this volume of water quickly surpasses the capacity of Pond Number One, causing it to
overflow into Pond Number Two and the Chinese Gardens Lagoon.
The additional rtnoff will bring the total quantity of runoff entering the Chinese Gardens
Lagoon via Pond Numbers One and Two to 279,740 cubic feet. Given that the Chinese
Gardens Lagoon has a surface area of 1,766,000 square feet, this equates to a water depth
increase of 0.16 feet, or approximately two inches. Bear in mind that the Chinese
Gardens Lagoon receives runoff from a large drainage basin, of which the area addressed
in these calculations is a relatively small portion.
w Gardens
The incremental water surface elevation increases attributable specifically to discharge
from the proposed storm drain pipe are as follows (note that it is assumed that the
incremental discharge overflows Ponds One and Two and enters the Lagoon):
Six-month storm total discharge:
Chinese Gardens Lagoon area:
Incremental elevation increase :
100-year storm total discharge :
Chinese Gardens Lagoon area:
Incremental elevation increase :
2,290 C.F.
1,766,000 S.F.
0.001 3 Feet (0.01 6 Inches)
40,660 C.F.
1,766,000 S.F.
0.023 Feet (0.28 Inches)
Note that the incremeltal elevation increase from a six-month storm is negligible. The
incremental elevation increase from a 1O0-year storm is approximately one quarter of one
inch. Incremental elevation increases from two-year and l0-year storms, not calculated,
will be somewhere in between these figures.