Stormwater Management Manual

ADDENDUM 1

The policies and standards in this addendum are now in effect and, where applicable, supersede the respective portions of the Stormwater Management Manual. The addendum will be incorporated in revisions to the manual at a later date.

1. Overland Flow Parameter Guidelines
2. Overland Flow Path
3. Collector Flow Equation (Clarification)
4. Default Response Times for Common Land Uses
5. Hydraulic Grade Line Options 6. Rectangular Manhole Loss Coefficients
7. Roughness Coefficients for Culverts, Pipes and Channels
8. Effects of Woody Debris
9. Effects of Future Channel Vegetation Growth
10. Detention Requirement
11. Allowable Street Encroachments
12. Small Watershed Method Examples Pre and Post Development (less than 200 acres)

1. Overland Flow Parameter Guidelines (Supplemental Guidelines, Chapter V)
The values indicated below will be accepted for conditions that are the same or similar to those described. These values are used generally in the Districts watershed models. A single overland flow element may be used to represent to reflect the predominant land use in the watershed. A second element may be used to represent a second major land use if applicable.

Land Use	Parameter	Value	Remarks
Natural	n	0.8 0.6 0.4	Dense cover of trees and bushes Oak grasslands, open grasslands Poor grass cover, moderately rough surface
	L		Typically, about 600 feet, but can vary depending on local topography In mountainous areas, can be much longer: look for convergence of contours on topographical map
Parks	n	.035	Maintained lawns
	L		Determined by grading
Rural Residential (1-10 acre lots)	n	0.4	Maintenance or grazing assumed.
	L		Typically determined by natural topography.
Urban Residential	n	0.3 0.2 0.15	Maintained lawns assumed. Effects of landscaping, driveways, roofs included in combined value. 1-3 units/acre 3-10 units/acre >10 units/acre
	L		Use depth plus 1/2 width of representative lot.
	s		Typically graded to 1% slopes
Commercial/Industrial	n	0.11	Effects of landscaping, driveways, roofs included in combined value
	L		Varies depending on grading
	s		Varies depending on grading

2. Overland Flow Paths (Additional Requirement)

All drainage reports shall provide a map indicating the 100-year overland flow path and direction using arrows. These will simplify understanding of drainage patterns and help identify potential problem areas.

Overland release points shall be shown on grading plans to assure inspection.

3. Collector Flow Equation (Clarification)

Manning's equation, indicated on page V-10 and VI-5 may be substituted for Equation 5-4 (Page V-10; SWMM) to estimate response times (tr_c ) in gutter and pipe flow. The velocity computed for open channel flows using Manning's equation shall be increased by an adjustment factor to account for celerity (page V-13, SWMM).

4. Default Overland Response Times for Common Land Uses (Supplemental to Chapter V)

The following values will be accepted in lieu of a detailed evaluation using the equation or figures in the manual. In addition, these are the maximum values that will be accepted if a detailed evaluation is used.

Land Use	Overland Response Time
Single Family Residential > 4 units/acre	15 minutes to gutter
Multi-Family Residential	10 minutes to gutter
Commercial (to first inlet)	10 minutes to first inlet

5. Hydraulic Grade Line Options (Supersedes Section VI D 2 d (3) (g), p VI-9)

Two options are provided to account for minor losses at junctions in underground drainage systems with the 10-year flood:

i) Minor losses may be ignored if the hydraulic grade line is a minimum of one foot below all inlet grates and manhole covers of all structures of the upstream system except when required at the discretion of the agency or where:

   a) inlet conduit(s) enter the junction at an angle greater than 45 degrees to the outlet conduit.

   b) the velocity difference between the inlet conduit(s) and outlet conduit is greater than 5 feet/second.

ii) Minor losses shall be calculated at each junction to design the hydraulic grade line at least six (6) inches below all manhole covers, gratings, and inlets.

6. Rectangular Manhole Loss Coefficients (Supersedes Figure 6-1 p VI-8)

The scale for the head loss coefficients is corrected with this change.


7. Roughness Coefficient for Culverts, Pipes and Channels (Additional Requirement Pertaining to Section VI D 2 d (2), p VI-5 and Table 6-3)

A minimum "n" value of .015 shall be used in the Manning's formula for sizing conduits.

8. Effect of Woody Debris (Additional Requirement, Chapter VIII)

Experience indicates that open channels which drain watersheds with substantial tree coverage - especially natural, unmaintained watersheds - frequently generate and convey substantial quantities of floating woody debris. This debris is trapped against other fixed vegetation and man-made features, such as fences, effectively increasing the roughness of the conveyance. Where these conditions apply, nominal Manning n values for open channels determined using other criteria in the SMM shall be increased by at least 15%.

9. Effect of Future Channel Vegetation Growth (Additional Requirement, Chapter VII)

Increases in watershed development can result in extensive growth of riparian vegetation in previously dry, bare channels, raising n-values to between .1-.2. Manning n values used in evaluating flow and depth in these channels shall take the potential for future growth of vegetation into account. The Manning n value may used to determine floodplain boundaries may assume maintenance is provided if evidence of an acceptable maintenance program, including funding, is accepted. However, for determining elevations of adjacent structures, an Manning n value which assumes the channels to be unmaintained shall be used.

10. Detention Requirement (Supplemental Policy, Chapter VII)

Detention facilities shall be used to control the post-development 2-yr, 10-yr and 100-yr peak flow to pre-development objective outflows, as provided in VII D1. Exceptions are:

1. A specific District watershed plan supersedes this requirement.
   
   
2. An acceptable alternative is agreed upon by the approving agency and District. This would include instances where it would be infeasible to construct or operate the necessary facility. Acceptable alternatives include the contribution of funds, in additional to ordinary fees, for regional mitigation, such as regional detention facilities.

11. Allowable Street Encroachments (Supersedes Table 6-1)

The table on the following page supersedes Table 6-1.

Table 6-1 Allowable Street Encroachments
Type	Profile	10-year Storm	25-year Storm	100-year Storm
Local	Continuous grade.	Traveled way remains clear and does not convey runoff.	Maximum depth at flow line shall not exceed 6 inches. The center line depth shall be zero.	Maintain HGL at least 1 foot below building pads.
	Sag points.	Traveled way remains clear and does not convey runoff.	Maximum depth at flow line shall not exceed 6 inches. The center line depth shall be zero.	Maintain HGL minimum of 1 foot below building pads.
Collector	Continuous grade.	Traveled way remains clear and does not convey runoff.	Traveled way remains clear and does not convey runoff.	Flow is contained within the right-of-way. The center 12 feet of roadway shall remain clear.
	Sag points.	Traveled way remains clear and does not convey runoff.	Traveled way remains clear and does not convey runoff.	Flow is contained within the right-of-way. The center 12 feet of roadway shall remain clear.
Arterial and Expressway	Continuous grade.	Traveled way remains clear and does not convey runoff.	Traveled way remains clear and does not convey runoff.	All traveled lanes remain clear. Bike lanes may flood. Flow is contained within the right-of-way.
	Sag points.	Traveled way remains clear and does not convey runoff.	Traveled way remains clear and does not convey runoff.	All traveled lanes remain clear. Bike lanes may flood. Flow is contained within the right-of-way.

12. Small Watershed Method Examples (less than 200 acres):

Example 1: Pre-development Watershed

• Average Elevation of site is 100 feet.
• Poor grass cover on moderate rough surface.
• Return Period = 10
• Class D soils.
• Total area = 100 acres (30% impervious, 70% pervious).

Step 1- Determine overland response time, t_ro as follows:
Overland flow length, L = 500'

Poor grass cover, n, (Table 5-5, Page V-10) = .4

Slope, s = 1% = .01

Figure 5-1 (Page V-11) or
Equation 5-3 (Page V-10) gives:

tro =	0.355(nL)0.6
	s0.3

= 34.0 minutes


Step 2 - Determine collector flow #1 response time. t_rc#1,. as follows:

Collector is swale.

Roughness coefficient, n, (Table 6 3, Page VIII-4) = .08

Length, L = 200'

Slope, s = .03 (foot vertical, feet horizontal)

Side Slope, Z, 10 (feet horizontal/ foot vertical)

Contributing Area, Ac = 2.0 acres

Equation 5-4 (Page V-10) gives:

trc =

= 1.2 minutes

Step 3 - Determine collector flow #2 response time t_rc, repeat step #2.

Collector is channel.

Roughness coefficient, n, (Table 6-3, Page VI-6) = .06

Length, L = 800'

Slope, s = .02 (foot vertical, feet horizontal)

Side Slope, Z, 10 (feet horizontal/ foot vertical)

Contributing Area, Ac = 20.0 acres

Equation 5-4 (Page V-10) gives: trc = 2.6 minutes

Step 4 - Determine collector flow #3 response time

Collector is channel.

Roughness coefficient, n, (Table 6-3, Page VIII-4) = .06

Length, L = 1700'

Slope, s = .001 (foot vertical, feet horizontal)

Side Slope, Z = 10 (feet horizontal/ foot vertical)

Contributing Area, Ac = 100.0 acres

Equation 5-4 (Page V-10) gives: trc = 11.4 minutes

Step 5 - Sum response times

t_r = t_ro + t_rc#1 + t_rc#2 = t_rc#3 = 49.2 minutes

Step 6 - Determine unit peak discharge, q. from Figure 5-3A (Page V-14)

Elevation = 100'

Unit Peak Flow, q = .50 cfs/ac

Step 7 - Determine infiltration factor. Fi (cfs/acre)

Elevation = 100'

Class D soils, residential landscaping with good cover,

Infiltration rate, I (inch/hr) = 0.10 (Table 5-3)

Equation 5-5 (Page V-13) gives: Fi = 0.17 (cfs/acre)

Step 8 - Compute total peak flow. Qp (cfs) using Equation 5-6

Pervious area = (1 - .3)(100 acres) = 70 acres

Qp = 100(.5) - (70)(.17) = 38.1 cfs

Example 2: Post-development Watershed



For this example, the following assumptions were made for post development conditions:

• Lots are 100 feet deep and 60 feet wide.
• Lots have constant slope of one percent.
• Lots have Bermuda grass ground cover.
• Average elevation of subdivision is 200 feet.
• Class D soils.
• Area = 65% impervious, 35% pervious.
• Inlets are spaced at 300 feet.
• The contributing area to each inlet is 1.4 acres.

See figure on right for a typical lot detail.

Step 1 - Determine overland response time t_ro as follows:

Overland flow length, L = 130'
Bermuda Grass Cover, (Table 5-5, Page V-10), n = .4
Slope, s = 1% = .01
Figure 5-1 or Equation 5-3 gives:

tro =	0.355 (nL)0.6	= 15.1 minutes
	s0.3

Step 2 Determine collector flow response time, t_rc

Collector flow to inlet is assumed to be gutter flow (assume v-gutter)

Roughness coefficient, n = .015

Length, L = 300'

Slope, s = .01 (foot vertical, feet horizontal)

Assume v = 2.0 ft/s for this problem; Celerity Adjustment Factor c = Triangle = 1.33

trc =	__L__	=	_____300_____	= 1.9 minutes
	c x 60		2 x 1.33 x 60

Step 3 - Sum response times

t_r = t_ro + t_rc = 17.0 minutes

Step 4 - Determine unit peak discharge from Figure 5-3A (Page V-14)

Elevation = 200'

Unit Peak Flow, q = 1.9 cfs/ac

Step 5 - Determine infiltration factor:

Elevation = 200'

Class D soils, residential landscaping with good cover,

Infiltration rate = .12 (Table 5-3, Page V-6 & 7, SWMM)

Equation 5-5 (Page V-13, SVVMM) gives: Fi = .21

Step 6 - Compute total peak flow. Qp (cfs):

Pervious area = (.35)(1.4 acres) = .49 acres

Qp = 1.4(1.9) - (.49)(.21) = 2.55 cfs

This establishes flow into the inlet. At locations in the drainage system downstream from the inlet, the response time is added, the area is increased, the unit flow is revised and conduits are sized appropriately.