Stormwater_Review_Comments2_WithAttachments.pdfCity of Edmonds
121 STET AVENUE NORTH
EDMONDS, WA 98020
(425)771-0220
FAX(425) 771-0221
Website: www.ci.edmonds.wa.us
DEVELOPMENT SERVICES DEPARTMENT
Planning • Building • Engineering
Stormwater Review Comments
Permit #: BLD20061302 Date: 10/31/2007
Project Name/Address: Emerson SFR/7211 Meadowdale Beach Road
Contact Person/Address/Fax: Larr E. Steward PE/24738 SE Mirrormont Drive
Issaouah. WA 98027/Email lestewardOhotmail.com/Fax /Fax (425) 392-6111
Reviewer: Blaine Chesterfield Division: Engineering
Daring review of the subject submittal it was round that the following information,
corrections, or clarifications will need to be addressed:
1. For the existing conditions the time of concentration (Tc) is 6.3 minutes. 6.3
minutes is the minimum and needs to be higher for the predeveloped
conditions. Please take a second look at the predeveloped conditions Tc and
adjust up to a more appropriate level. I took a look at the site and a quick
analysis suggests that the predevelopment Te should be around 20 minutes. In
order to determine this the February 1992, "Stormwater Management Manual
for the Puget Sound Basin" from the Department of Ecology was used. The
Tc for sheetflow of up to 300 feet was used on page III -1-14 (Manning's
kinematic solution). The manning's value used was from Table III -1.4 of
forest with light underbrush on page III -1-16. I have attached copies of both
pages to the comments.
Please resubmit three copies of the revised plans/documents to a Development
Services Coordinator. You may contact me at 425771-0220 if you have specific
questions regarding these plan corrections.
STORMWATER MANAGEMENT MANUAL FOR THE PUGET SOUND BASIN
where, Tt = travel time (minutes)
L = flow length (feet)
V = average velocity (feet/sec) and
60 = conversion factor from seconds to minutes
Time of concentration (TJ is the SUM of T, values for the various consecutive flow
segments.
Te = Tt + Tt + T
1 2 int
where: Tc time of concentration (minutes) and
M = number of flow segments
Sheet flow
Sheet flow is flow over plane surfaces. it usually occurs, in the headwater of
streams. With sheet flow, the friction value (n.) (a modified Manning's effective
roughness coefficient that includes the effect of raindrop impact; drag over the
plane surface; obstacles such as litter, crop ridges and rocks; and erosion and
transportation of sediment) is used. These n, values are for very shallow flow
depths of about 0.1 foot and are only used for travel lengths up to 300feet. Table
sheet flow for various surface conditions.
111-1.4 gives Manning's nvalues for
For sheet flow of up to 300 feet, use Manning's kinematic solution to directly
compute Tt.
T 2-42 nL)
t (P2) 0J27 (S.) 0.4
where:
Tt = travel time (min),
n, sheet flow Manning's effective roughness coefficient (from
Table, 111-1-4).
L flow length (ft),
P2 2 -year, 24-hour rainfall (in), and ft/ft)
so slope of hydraulic grade line (land slope,
Velocity Equation
A commonly used method of computing average velocity of flow, once it has measurable
depth, is the following equation:
V = k �,
where: V = velocity (ft/s)
k = time of concentration velocity factor (ft/s)
so slope of flow path (ft/ft)
Ilk" is computed for various land covers and channel characteristics with assumptic
made for hydraulic radius using the following rearrangement of Manning's equation:
k (1.49(R) ,667)/n
where: R an assumed hydraulic radius
n Manning's roughness coefficient for open channel flow
a
Table 111-1.4 'In" AND "k" Values Used in Time Calculations for Hydrographs
"n," Sheet Flow Equation Manning's Values (for the initial 300 ft« of travel) n,
Smooth surfaces (cord 7e_tt;_, a'sphalt, gravel, or-7bare hand packed
soil)
0.0111
Fallow fields or loose soil surface (no residue)
0.05
Cultivated soil with residue cover (s< 0.20 ft/ft)
0.06
Cultivated soil with residue cover (s> 0.20 ft/ft)
0.1.7
Short prairie grass and lawns
0.15
Dense grasses
0.24
Bermuda grass
0.41
Range (natural)
0.13
Woods or forest with light underbrush
0.40
Woods or forest with dense underbrush
0.80
*tanning values for sheet flow only, from Overton and Meadows 1976
(See TR -55,
1986)
I`k" Values Used in Travel Time/Time of Concentration calculations
Shallow Concentrated Flow (After, the initial 300 ft. of sheet flow,
R = 0.1)
1. Forest wTJT_h heavy -ground 1 itteiaru(! -meadows (n = 0. 1
3
2. Brushy ground with some trees (n = 0.060)
5
3. Fallow or minimum tillage cultivation (n = 0.040)
8
4. High grass (n = 0.035)
9
5. Short grass, pasture and lawns (n = 0.030)
11
6. Nearly bare ground (n = 0.25)
13
7. Paved and gravel areas (n = 0.012)
2*7
Channel Flow (intermittent) (At the beginning of visible channels R
= 0.2)
1�,
1. Forested Swale w 3. t h 1�_jv y -gr o u n d _71111 -t 67 _(n = 0.10)
5
2. Forested drainage course/ravine with defined channel bed (n =
0.050)
10
3. Rock -lined waterway (n = 0.035)
15
4. Grassed waterway (n = 0.030)
17
5. Earth -lined waterway (n = 0.025)
20
6. CMP pipe (n = 0.024)
21
7. Concrete pipe (0.012)
42
8. Other waterways and pipe 0.508/n
Channel Flow (Continuous stream, R = 0.4)
kc
9. Meandering stream with some pools (n = 0.040)
20
10. Rock -lined stream (n = 0.035)
23
11. Grass -lined stream (n = 0.030)
27
12. other streams, man-made channels and pipe 0.807/n**
FEBRUARY, 1992