Stormwater Management Manual

VI. DRAINAGE SYSTEMS

A. Purpose This chapter specifies various design criteria, standards, guidelines for the design of systems which collect and convey runoff to an outlet. These systems are typically found in developed areas, and they include streets, conduits, and their appurtenances. Culverts for transverse crossings of open channels are also included.

B. Principles and Policies

1. General The following general principles will be considered in the design of storm drainage facilities:

a. Storm drainage planning, design, and construction will avoid increasing the storm drainage problems in any area, or transferring drainage problems from one location to another. Watershed boundaries shall not be altered, and flows shall not be diverted from one watershed to another without compelling reasons.

b. Storm drains should use the natural drainage channel alignments whenever possible.

c. Development plans shall provide a secondary surface flow escape path for flows in excess of the capacity of the primary piped or channelized drainage system without damage to structures.

d. Storm drainage planning and design shall be consistent with the flood boundaries and floodways delineated and regulated by the National Flood Insurance Program or other studies, such as watershed master plans.

e. Public storm drainage facilities shall normally be located within public road right-of-ways, unless specifically approved by the local jurisdiction, and shall be designed as permanent facilities with minimal maintenance costs.

f. The points at which drainage enter and exit a project shall be at the same vertical and horizontal location as exists before the project except by written and recorded agreement between adjacent landowners in the form of an easement.

g. Fill or structures shall not be permitted to block drainage paths even if these paths function only in storms of rare occurrence.

h. Storm drainage systems shall incorporate best management practices for the protection of water quality when required by the local jurisdiction.

2. Design storms All new development shall be planned and designed so that no damages occur to structures or improvements during the 100-year event and no inundation of private property occurs during the 10-year event.

a. Local Drainage The 10-year event is the minimum design storm for new developments in all drainages and all dedicated drainage facilities will be sized for this event.

The development plan will also identify the effects of the 100-year event and provisions will be made in the plan to prevent loss of life and damages to property during a 100-year event.

b. Regional Drainage Regional drainage facilities are th ose identified as such in watershed plans, or have drainage areas greater than 200 acres.

Regional drainage systems will be planned and designed for a 100-year event except where the cost of the system is clearly unjustifiable economically. Variances from the design frequency will be approved by the Flood Control District.

Designs of major flood control facilities will consider the effects of events which are lesser in magnitude as well as greater than the design event.

3. Preliminary Drainage Plan Requirements A narrative drainage report and other pertinent information shall be submitted with any preliminary plan of development. Specific information is listed and described in a separate section at the end of this manual entitled Requirements for Submittals. Additional information may be required as appropriate.

4. Final Drainage Plan Requirements Construction plans shall be provided by the developer or his agent for review b y the approving jurisdiction. Specific information is listed and described in a separate section at the end of this manual entitled Requirements for Submittals. Additional information may be required as appropriate.

C. Streets and Gutters

1. Policy Streets may be used for the conveyance of storm drainage; however, the primary purpose and use of streets is for traffic. 10-year flows shall therefore be conveyed within the gutter, roadside ditches or swales, or underground within street areas.

2. Design Criteria

a. Encroachments Limitations for pavement encroachment are presented in Table 6-1. When any of these encroachments are exceeded, a means of draining off the excess flow must be provided.

b. Cross Street Flow Cross street flows shall not be allowed. Cross street flow may be runoff which has been flowing in a gutter and then flows across the street to the opposite gutter or to an inlet or may be flow from some external source, such as a drainage way, which will flow across the crown of a street when the conduit capacity beneath the street is exceeded.

c. Walkover Curbs Walkover curbs shall be used at intersections in highly concentrated areas where large volumes of pedestrian traffic are likely.

d. Alleys

(1) Use Alleys shall not be used as storm runoff channels, except to convey internal block runoff to the streets.

(2) Sections and Grade Alleys are typically designed with inverted crowns. The maximum and minimum grades and crown slopes shall be the same as for streets. Where an alley intersects a street, the alley grade shall be raised above the street grade so flow within the street cannot move into the alley for the 10-year event.

(3) Carrying Capacity The allowable carrying capacity of an alley shall be calculated as covered in the following sub-sections. If the quantity of flow exceeds that allowable, mid-block inlets or other methods must be utilized to remove portions of the flow from the alley surface.

(4) Design Runoff All runoff from the 10-year event must be conveyed within the alley right of way.

 

TABLE 6-1 ALLOWABLE STREET ENCROACHMENTS
Type	Profile	10-Year Storm	25-Year Storm	100-Year Storm
LOCAL	Continuous Grade (uphill or downhill)	Travelled way is open to travel and does not carry stormwater flow.	Stormwater elevation does not exceed the top back of sidewalk. Maximum depth in travelled way - 6"	Maximum stormwater elevation is 4" above the top of curb and the stormwater flow cannot exceed 3 ft/sec.
	Sag Points	Stormwater elevation does not exceed the top back of sidewalk. Maximum depth in travelled way - 6". Centerline shall by dry.	Stormwater elevation does not exceed 4" above the top back of curb. Maximum depth in travelled way - 6".	Stormwater is a minimum of one foot below building pads. Ponding does not extend more than 120 feet from inlet (2 std. residential lot frontages) along any street segment.
COLLECTOR	Continuous Grade (uphill or downhill)	Travelled way is open to travel and does not carry stormwater flow.	Stormwater elevation does not exceed the top back of sidewalk. Maximum depth in travelled way - 6".	Stormwater flow is contained within the right of way. The center 12 feet of roadway is clear of stormwater. The stormwater flow cannot exceed 3 ft/sec.
	Sag Points	Stormwater elevation does not exceed the top back of sidewalk. Maximum depth in travelled way - 6".	Stormwater elevation does not exceed 4" above the top back of curb. Maximum depth in travelled way - 6".	Stormwater flow is contained within the right of way. The center 12 feet of roadway is clear of stormwater. The stormwater flow cannot exceed 3 ft/sec. Maximum depth over sidewalks shall be 6".
ARTERIAL AND EXPRESSWAY	Continuous Grade (uphill or downhill)			All travel lanes are clear of stormwater flow. Bike lanes are allowed to be inundated. Stormwater flow contained within the right of way. Maximum depth over sidewalks shall be 6".
	Sag Points			All travel lanes are clear of stormwater flow. Bike lanes are allowed to be inundated. Stormwater flow contained within the right of way. Maximum depth over sidewalks shall be 6".

Note: Criteria reflect the possibility of a sidewalk separate from the curb and gutter.

(a) Capacity The capacity shall be calculated using Mannings formula with a suitable "n" value.

(b) Ponding See Table 6-1.

(c) Limitations at Sidewalks See Table 6-1.

i) Residential, Industrial, or Light Business Areas Where alleys are located in residential, industrial, or light commercial areas, flow from alleys may pass across the sidewalk for both the 10-year and 100-year events.

ii) Concentrated Business Areas. When alleys are located in concentrated business areas of the types where walkover curbs might be installed, runoff from the 10-year event shall not be allowed to cross the sidewalks. Inlets shall be placed as necessary to intercept the flow before it reaches the sidewalk. For the 100-year event, runoff from the alley may flow across the sidewalk.

d. Rural Streets In general, the majority of requirements for urban streets are applicable for rural streets. A major difference is that rural streets use roadside drainage ditches instead of curbs and gutters for drainage. Roadside ditches are planned and designed as open channels.

D. Drainage Conduits This section is specific to the planning and design of underground drainage systems consisting of conduits -- round pipes or box culverts -- and various other related components such as inlets and manholes.

1. Policies

a. Permanent Installations Underground drainage systems shall be designed as virtually permanent installations.

b. Maintenance Ease Underground drainage systems shall be designed for ease of maintenance and repair.

2. Criteria

a. General

(1) Hydraulic Grade Line Closed conduit sections (pipes, boxes, etc.) shall be designed as flo wing full, whenever possible. The hydraulic gradeline (HGL) shall at all points be at least six (6) inches below all manhole covers, gratings, and inlets when operating under a head at the 10-year design flow.

(2) Minimum Velocity To prevent sedimentation, minimum velocity allowable in any closed conduit shall be two and one half (2 1/2) feet per second. Velocities shall be computed by using Manning's formula for pipes flowing full or half full, using "n" values as shown in Table 6-3.

(3) Slopes Pipe slopes shall be less than 70 percent of critical slope or more than 130 percent of critical slope at design flow. The maximum design quantity of flow for any line steeper than critical slope shall be computed assuming that the flow is at critical slope and critical velocity.

b. Drop Inlets

(1) Spacing Max. spacing of drop inlets shall be 500 feet.

(2) Design Cal Trans Type GO inle ts with a bicycle-proof grate shall be used for all streets with gutters.

(3) Capacity Inlet capacity shall allow for 50% blockage.

(4) Hydraulic Grade Line The design HGL should be at least 6 inches below the gutter grade at the inlet to allow the inlet to function properly. The inlet should not be counted as accepting flow if there is a possibility the hydraulic grade will be above this level.

c. Manholes

(1) Pipe Location The location of pipes within a manhole should be designed to insure maximum efficiency.

(2) Spacing Spacing of manholes shall generally be as shown in Table 6-2.

TABLE 6-2 MANHOLE SPACING
Pipe Size	Maximum Spacing
24" or less greater than 24"	400 feet 600 feet

A manhole shall always be located at the end of short radius bends and at angle points of 10 degrees or more.

Any local pipes that enter the storm drainage system shall enter the system at a manhole.

(3) Direction Changes Short radius bends may be used on 24 inch and larger pipes when flow must undergo a direction change at a junction or bend. Reductions in headloss at manholes may be realized in this way.

d. Computation Procedures The wetted perimeter of a pipe increases more rapidly than the area as the pipe approaches full capacity. Therefore, the capacity of a circular conduit at a given grade is the same at 91% and 100% ratios of d/D. Because it is impractical to design for the theoretical range where capacity exceeds that for the full conduit, open channel flow should only be assumed for d/D ratios of less than 0.90.

(1) Basic

The basic computations of the hydraulic grade line and conduit capacities shall be based on the Manning equation:

     [6-1]
where

V=velocity, feet per second

R =hydraulic radius, feet: the cross sectional area divided by the wetted perimeter.

S=slope of energy grade line, feet vertical per foot horizontal

n=roughness coefficient (see Table 6-3)

(2) Roughness Values Values for n from Table 6-3 below will be used unless otherwise specified in this manual.

(3) Hydraulic Grade Line The hydraulic grade line or HGL is computed from a known water surface elevation, usually the outlet condition, and the summation of head (energy) losses occurring in elements of the system due to friction and turbulence. These losses will be accounted for using the following criteria unless otherwise specified in th is manual



a) Friction Losses The Manning equation will be used to compute friction losses by solving for a value of S_f, the energy gradient, then computing total friction losses as a product of the S_f and the length of the applicable segment.

(b) Entrance Losses The head loss at an entrance to a conduit from a reservoir or open channel shall be estimated as follows (9):

h_i =K_iV² /64.4     [6-2]

where

h_i=entrance head loss, feet
k_i=Loss coefficient in Table 6-4 below
V=velocity in conduit, feet/second

TABLE 6-4 ENTRANCE LOSS COEFFICIENTS (9)
Type of Entrance	Ki
Square corners, flush with head wall Square corners, projecting Rounded Entrance	0.5 0.9 0.2

This equation applies to full or partially full conduits.

(c) Losses Due to a Sudden Enlargement Head losses occurring when a smaller pipe transitions suddenly into a larger pipe or open channel shall be estimated using the following equation (9):

h_n =0.01705(V_s- V_L)^1.919    [6-3]

where

h_n =head loss, feet

V_s=velocity in smaller conduit, feet/second
V_L=Velocity in larger conduit, feet/second

When the transition is to an open channel, V_L may be ignored.

(d) Losses Due to Sudden Contraction Head losses resulting from a sudden transition from a larger to smaller conduit shall be estimated using the following equation (9):

h_c=K_cV_s² /64.4     [6-4]

where

h_c=head loss, feet
V_s=Velocity in smaller conduit, feet per second
K_c=Contraction Loss Coefficient from Table 6-5

This equation was specified for a circular pipe but may also be assumed to apply to other sections.

(e) Losses Due to Gradual Bends Head losses due to gradual bends shall be estimated by increasing the value of Mannings n by .004 for the section of conduit containing the curvature (10).

(f) Losses in Manholes The following head losses at manholes for conduits under pressure are in addition to transition or junction losses (8)

i) Rectangular Conduits Losses are negligible.




ii) Circular or Arch Conduits

h_m=0.05 V₂²/64.4    [6-5]

where

V₂ =velocity in exit conduit, feet per second

iii) Rectangular Structure For a rectangular structure with the manhole shaft joining circular conduits with or without a shaped invert.

h_m=k_mV₂²/64.4    [6-6]

where

k_m=manhole loss coefficient
See Figure 6-1
V₂=velocity in exit conduit, feet per second


D_l and D₂ are the diameters of the conduits entering and leaving the manhole.

(g) Junction LossesThe head loss at a junction is completed as follows:

h_j=y+(V₁²-V₂²)/64.4   [6-7]

where

h_j=head l oss, feet

An estimate of losses at a junction requires an evaluation of pressure and momentum at the upstream and downstream sections. See Figure 6-2 on next page. In Equation 6-8, the terms on the left are pressure and those on the right, momentum. The equation is solved iteratively by assuming various losses until one is found which satisfies these equations:

P=M   [6-8]

P=(A₁ + A₂) y/2



where

P=pressure term in cubic feet
M=momentum term in cubic feet
y=difference between upstream and downstream HGL
A₁=area of upstream conduit flow, ft²
A₂=area of downstream conduit flow, ft²
A₃ =area of lateral conduit flow ft²
Q₁=flow in upstream conduit, ft³/sec
Q₂=flow in downstream conduit, ft³/sec.
Q₃=flow in lateral conduit, ft³/sec
=angle of convergence of lateral conduit with main conduit

e. Circular Pipes

(1) Minimum Size The minimum inside diameter for circular pipes that are part of a storm drain system shall be 12 inches.

Exceptions will be considered under special circumstances.

(2) Alignment Storm drains shall be straight and of uniform slope between manholes insofar as possible. Where long radius curves are necessary to conform to street layout, the radius of curvature shall be no less than 100 feet. Radius of curvature specified should coincide with standard curves available in the typ e material utilized wherever possible.

f. Outfalls An outfall is a point at which a storm sewer system discharges into an open channel or a major drainage conduit.

(1) Location The Flood Control District shall approve the location of all outfalls , in regional channels.

(2) Design Tailwater The water level in the receiving major drainage way for computing the HGL, shall be that for the design storm frequency.

(3) Energy Dissipation and Erosion Control The outlet should be reviewed for possible erosion tendencies if the major drainage way is flowing at less than the design depth.

Erosion control measures must be taken when the possibility exists of affecting the outfall channel. These may vary from involved stilling basins to simple riprap. Flow direction of the input water should not be perpendicular to that of the receiving water, but should be as much in the downstream direction as possib le.

E. Culverts for Transverse Crossings This section states criteria for relatively short circular or box culverts for transverse crossings: typically, road or railroad embankments.



1. Policy

a. Role in Reducing Flood Peaks Transverse culverts shall be sized to pass the design storm peak flow. However, if freeboard is available and, detention of flood peaks is desirable and consistent with regional master drainage plans, and no other adverse consequences are probable, then headwater surcharge should be considered to help reduce peak flows downstream.

b. Culvert Replacement The replacement of a small culvert with a large one may create higher peak flows downstream. This downstream impact shall be investigated.

c. Coordination with Flood Control Distr ict Since culverts can have a significant effect on downstream peaks, proposals to install or replace a major culvert on a regional stream shall be coordinated with the Flood Control District.

2. Criteria

a. Size The minimum inside diameter for circular culverts shall be as follows:

Culverts under roads	15 inches
Culverts under driveways	12 inches

b. Minimum Velocity The minimum velocity shall be 2 feet per second if possible in order to avoid deposition of sediment.

c. Blockage Allowance should be made for the possibility of blockage if the culvert area is less than 5 square feet. A 50 % blockage should be assumed.

d. Erosion Control The embankment next to the upstream and downstream ends of the culverts shall be protected to prevent erosion.

e. Invert Depth An invert depth and alignment to permit fish passage in low flow conditions may be required for designated streams.

f. Computation of Flow (11) Inlet or outlet conditions usually control flow in a transverse culvert. In culverts operating under inlet control, only the entrance configuration and headwater depth affect the culvert capacity. Under outlet control, headwater depth, tailwater depth, entrance configuration, and barrel characteristics all influence the culv ert's capacity.

After anticipated runoff or design flow has been computed, the drainage channel downstream from the culvert should be investigated to determine normal depth of flow during peak runoff to estimate the tailwater depth and determine whether inlet or outlet control is likely. Anticipated downstream flow depth and allowable headwater depth will determine the probable available head on the culvert.

(1) Inlet Control Federal Highway Administration nomographs shall be used to compute the discharge capacity and necessary headwater depth when the inlet controls discharge through the culvert. The nomographs are presented in Appendix 6A.

For a culvert type not specially represented in the nomographs, use a nomograph for the type with the most similar edge and inlet form.

Note that the nomographs apply to a single culvert opening. Computations for multiple culverts are based on the amount of flow passing through each.

(2) Outlet Control Under outlet control the culvert acts as a long tube, with frictional resistance, invert slope and tailwater depth generally producing a full flow condition. Under these circumstances, modifications to the inlet configuration and headwall design contribute little to increased flow.

The head differential through a culvert flowing with submerged inlet and outlet shall be estimated with the following equation:

hL =	V2	(Ke + Kf + Kx)	[6-9]
	_______
	64.4

where

h_L =total head loss, feet
v=velocity, feet per second
K_e =entry loss coefficient from Table 6-4
K_f=friction loss coefficient from Equation 6-11 below
K_x=exit loss coefficient

K f=	29n2L	[6-10]
	_______
	R4/3

n=Manning's roughness coefficient
L =length, feet
R =hydraulic radius, feet

Notes 1.		End Section conforming to fill slope: made of either metal or concrete, are the sections commonly available from manufacturers. From limited hydraulic tests they are equivalent in operation to a headwall in both inlet and outlet control. Some end sections, incorporating a closed taper in their design have a superior hydraulic performance. These latter sections can be designed using the information given for the beveled inlet.
2.		Source: Federal Highway Administration HDS-S Hydraulic Design of Highway Culverts Report No. FHWA-IP-85-15, September, 1985

References

1. American Society of Civil Engineers, Design and Construction of Sanitary and Storm Sewers, Manual of Engineering Practice , #37 (W.P.C.F. Manual of Practice, No. 9) Prepared by a Joint Committee of the A.S.C.E. and the Water Pollution Control Federation, A.S.C.E., New York, New York, 1960.
   
2. CalTrans, Highway Design Manual, Fourth edition with 1990 revisions.
   
3. Chow, Ven Te, Open Channel Hydraulics, McGraw Hill, 1959.
   
4. City & County of Denver, Criteria for Storm Sewer Design, Department of Public Works, Denver, Colorado, 1963.
   
5. Colby, Ardis V., Kenneth J. Kieker and Arnold Lenz, Storm Drainage Practices of Thirty-two Cities, A.S.C.E. National Meeting on Water Resources Engineering. New York, New York, October 16-20, 1967.
   
6. Federal Aviation Agency, Airport Drainage, Washington, D. C., 1966.
   
7. Federal Highway Administration, HDS 5. Hydraulic Design of Highway Culverts. Report No. FHWA-IP-85-15, September, 1985.
   
8. Institute of Transportation and Traffic Engineering, Street and Highway Drainage, Volumes 1 & 2, University of California, Berkeley, CA 1982.
   
9. King, H.W. and E.F. Brater, Handbook of Hydraulics, McGraw-Hill, 1976.
   
10. Metcalf & Eddy, Inc., Wastewater Engineering: Collection. Treatment, and Disposal, McGraw-Hill, 1972.
    
11. National Association of County Engineers Action Guide Series, Drainage Volume XIV, National Association of Counties Research Foundation, 1972.
    
12. Portland Cement Association, Handbook of Concrete Culvert Pipe Hydraulics, Chicago, Illinois, 1964.
    
13. Rouse, Hunter, Elementary Mechanics of Fluids, New York, New York, 1946. John Wiley and Sons, Inc.
    
14. Soil Conservation Service, Engineering Division, Design of Open Channels. Technical Release No. 25, 1977.
    
15. Taylor, Edward H., Flow Characteristics at Rectangular n Channel Junctions at Open-Channel Junctions, Transactions, A.S.C.E., Vol 109, 1944, p.893.