Longmire Drainage Addendum
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4200 Sixth Ave SE, Surte 301
Lacey, WA 98503 ,
Jim Gibson ~ ,
Development Review Engineer
(360) 49,3-6002
(360) 493-2476 Fax City of Yelm `
PO Box 479
sca@scaconsultinggroup com ~ Yelm, WA 98597
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www thescagroup com
~ "Addendum"
for Final Drainage Report for Longmire Street SE
Dear Jim: -
- As you are aware we have been burring the midnight oil in a effort to complete the
design and construction of the project on schedule. ,As such we hadn't had the
opportunity to thoroughly review the initial drainage design report. Therefore after
giving the drainage report a 'second look" we believe that there are some -
refinements we would like to make and use this opportunity to amend the above
mentioned report.
Our findings are as follows: _ '
The correct size of drainage areas for'~Longmire Basin" is 4.45 acres of "
• impervious surface, not 3.32 acres shown previously in report. This area
includes:
' 47 lots x 500 sf of impervious surface per lot = 0.54 ac
We have 2,900' of Longmire St. in our Basin: '
2,000' of S/W on one side'x 5' = 0.23 ac
900' of S/W on both sides x 5' = 0.21 ac
S/W on internal streets~in the basin = 0.12 ac
Total 0.56 ac of sidewalk; ,
2,900' of road x 33' = 2.19 ac
Internal streets road in the basirS = 0.94 ac
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Total 3.13 ac of road; ,
0.22 ac for pond;
4.45 acres of impervious surface total for the "Longmire Basin" for developed
• condition.
We do not expect disturbed pervious areas to generate any appreciable runoff
due to the flat grades and high soil permeability rate.
• The peak volume for 6-month/24-hour storm is 17,100 cf, that's the volume
needs to be treated.
100-year/24-hour event will have peak volume of 61,000, cf.
As you know Longmire is an existing road with about 20 feet wide paving.
Pre-developed runoff for 2-year/24-hour event is 21;300 cf. Per 1992 Edition
' of DOE Manual (The Technical Manual) we allowed to release 50% of that
amount = 10,600 cf. or 0.3 cfs.
Per chapter I-2.9 of The Manual:
"by limiting the peak rate of runoff from individual development sites to 50
percent of the existing condition 2-year, 24-hour design storm"
DESIGN/BUILD CIVIL AND TRANSPORTATION ENGINEERING PLANNING SURVEYING
Runoff will be conveyed to wetpond for treatment and storage. Wetpond with
bottom elevation 327'feet will have 3 feet of "dead" storage for treatment, 3
feet of "live" storage for storage of active volume for larger events including
100-year/24-hour event.
3 feet bf "dead storage" will provide 17,400 cf of capacity, which is adequate
- treatment for 6 month event (6-month/24-hour storm peak volume 17,100
cf). 3 feet of "dead" and 3 feet of "live" storage will provide 48,700 cf of
capacity., Full wet pond capacity with 1 foot overboard is 62,550 cf, which is
more than 100-year/24-hour peak,volume of 61,000 cf. "Live"storage is
detained normally no longer than 24 hours. It should or dry or flow out.
Active volume to be detained for 100-year/24-hour event with design
infiltration rate of 20 iri/hour is 13,300 c_ f. Wet pond stage capacities are•
included in this addendum. ,
The wet pond will have a single orifice. 34 If long 6" diameter PVC pipe with
invert in of 330:0 and slope of 0.5% will have maximum discharge capacity of
0.57 cfs and will connect wet pond and 4 feet deep infiltration pond with
bottom area of 1,000 sf and bottom elevation of 329.5 feet. The infiltration
capacity of infiltration pond using Darcy's approach is:
At the bottom Q=0.83*(0+10)/10*1000=830 cf/hr
At elevation 330.5 Q=0.83*(1~+10)/10*1000=913 cf/hr
At elevation 331.5 Q=0.83*(2+10)/10*1000=996 cf/hr
At elevation 332.5 Q=0.83*(3+10)/10*1000=1,079 cf/hr
1,079cf/hr *48 hrs '= 51,792 cf •
The infiltration pond spillway will have elevation of 332.5 and will discharge
9,200 cf in case of 100-year/24-hour event into Thompson Creek or
constructed wetland as needed.
Soil report suggest that winter ground water table in well #10 is 12 feet from
existing grade. Infiltration pond is 4 feet,deep that gives us healthy 8 feet,
from bottom,of infiltration pond to possible•,winter (high) ground water table.
The attached sketch illustrates the revised size and shape change we are now
proposing for the for the Longmire ponds.
We believe that the revised design will adequately handle the storm water runoff. '
Therefore we request you concur with the revise design and allow the construction to
continue with the as built drawings reflecting the revised design.
Should you have any questions please call me. _
Yours truly,
SCA CONS~TING GROUP
ain, P.E.
(K:
5.doc)
DESIGN/BUILD CIVIL AND TRANSPORTATION ENGINEERING PLANNING SURVEYING
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Worksheet
Worksheet for Circular Channel
Project Description
Project File c \haestad\fmw\zalupa.fm2
Worksheet na huh
Flow Element Circular Channel
Method Manning's Formula
Solve For Discharge
Input Data
Mannings Coefficient 0.009
Channel Slope 0.005000 ft/ft
Depth 0 50 ft
Diameter 6.00 in
Results
Discharge 0 57 cfs
Flow Area 0 20 ft2
Wetted Perimeter 1.57 ft
Top Width 0 15e-7 ft
Cnt~cal Depth 0 39 ft
Percent Full 100 00
Critical Slope 0.005648 ft/ft
Velocity 2 92 ft/s
Velocity Head 0 13 ft
Specific Energy 0 63 ft
Froude Number 0 14e-3
Maximum Discharge 0 62 cfs
Full Flow Capacity 0 57 cfs
Full Flow Slope 0 005000 ft/ft
Flow is subcritical
FlowMaster v5 15
10/ 14/05
03 45 23 PM Haestad Methods, Inc 37 Brookside Road Waterbury, CT 06708 (203) 755-1666 Page 1 of 1
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