EDM Appendix 6Appendix chapter 6:
drainage design requirements
ENGINEERING DESIGN MANUAL
October 28, 2009
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
Table of Contents
APPENDIX - DRAINAGE DESIGN REQUIREMENTS
Subject Page
6.1 Sample Calculation of Area-Weighted Coefficient of Runoff 6-1
6.2 Sample Infiltration Basin Design 6-3
6.3 Time of Concentration for Initial Subarea Nomograph 6-5
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Chapter 6 appendix Page
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
Determining an area-weighted
Coefficient of runoff
Predeveloped Coefficient of Runoff:
Cpre = ____(0.95 x P1) + (0.95 x B1) + (0.45 x L1)____
P1 + B1 + L1
Postdeveloped Coefficient of Runoff:
Cpost = ____(0.95 x P2) + (0.95 x B2) + (0.45 x L2)____
P2 + B2 + L2
Postdeveloped Coefficient of Runoff: ^c = Cpost - Cpre
1
Appendix 6.1 Page 6-
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
Sizing an Infiltration Basin
Why an Infiltration Basin:
A detention basin assures that the Q (cfs) generated on the site of
concern and draining to the downstream property does not increase. The
discharge of the detention basin is designed such that the discharge rate
in the developed condition is equal to or less than the predevelopment
rate of runoff.
However, a detention basin is not mean to regulate low-flows to a
downstream property, such as irrigation and nuisance runoff. An
infiltration basin can fill this function and is pictured on the next page.
How the Necessary Volume is Determined:
1)Find the area-weighted predevelopment and post-development runoff
coefficients. See Attachments 1 and 2. Find ^C as:
^C = Cpost – Cpre (dimensionless)
2)Using P6 = 2.5 (inches) and assuming the minimum time of concentration, Tc, as
5 minutes, determine rainfall intensity, I (inches/ hr) as: (See Attachment 3)
-0.645
I = 7.44 * P6 * Tc
Given P6 = 2.5 inches and Tc = 5 min, I should be around 6.6 in/hr
3)Determine the increase in Q over the pre-development Q as:
Q= ^C * I (in/hr) * A (acres)
4)Determine the required volume for the infiltration basin, V (cf) as:
V = 1.5 * 2.65 * Q (cfs) * Tc (sec)
Where Tc is converted from minutes to seconds, 1.5 is a factor to account for the
volume occupied by the gravel in the basin, and 2.65 is an
empirical scale factor for the required storage volume.
In the equations above, the variables are as follows:
A = entire site acreage (acres)
c = coefficient of runoff (dimensionless)
^c = change in c from predeveloped to developed condition
I = rainfall intensity (in/hr)
P6 = 6 hour precipitation (in)
Q = rate of runoff (cfs)
Tc = time of concentration (minutes)
PAGE 6-2 Appendix 6.2
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
3
Appendix 6.2 Page 6-
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
Time of concentration for initial subarea
PAGE 6-4 Appendix 6.3
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
Appendix 6.3 Page 6-5
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
PAGE 6-6 Appendix 6.3
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
Appendix 6.3 Page 6-7
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
PAGE 6-8 Appendix 6.3
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
Appendix 6.3 Page 6-9
CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009CITY OF ENCINITAS ENGINEERING DESIGN MANUAL - 2009
PAGE 6-10 Appendix 6.3PAGE 6-10 Appendix 6.3