1. CE 3372 Inlets, Junctions, Conduits

2. Useful References HDM == TxDOT Hydraulic Design Manual 2011 Ed.
HDS2 == FHWA-NHI Highway Hydrology
HEC-22 == FHWA-NHI Urban Drainage Design Manual
HEC-24 == FHWA-NHI Highway Stormwater Pump Station Design

3. What is Urban Storm Drain Design?
Urban Storm Drain Design is the hydrologic and hydraulic design of both surface and subsurface drainage features in highly/densely developed areas;
Primarily aimed at keeping infrastructure functioning
Provides pathways for complete drainage (prevents long-term ponding)
NOT aimed at preventing property damage due to flooding!

4. Storm Drains
A storm drain is a system of curbs and gutters, inlets, and pipe networks that receives runoff and conveys it to some point where it is discharged into a pond, channel, stream, or another pipe system.
A storm drain may be comprised of a closed-conduit, an open conduit, or some combination of the two

5. What Kind of Infrastructure?
Urban Storm Drain Design produces infrastructure such as:
Drainage ditches
Curb-and-Gutter with inlets to subsurface conduits
Median inlets
Lift stations (pumps)

6. What Kind of Infrastructure?
Lift Station

7. Urban Storm Drain Design Goals
Controlling the amount of water flowing along gutters or ponding at low areas to rates that will not interefere with traffic is the primary goal
Reduction of hazard and traffic delay caused by (unavoidable) ponding is a secondary goal
Prevention of deterioration of the roadway components is tertiary goal.
Preventing damage to surrounding properties is a concern, but not a motivating goal.
Module 1

8. Storm Drain Design Establish design parameters and criteria
Decide layout, component location, and orientation
Use appropriate design tools
Comprehensive documentation
The process is iterative

9. Streets and flow in streets
Curb
Curb-and-gutter sections
Gutter
Inlet (Curb Opening + Grate)
Cross slope
Longitudinal Slope, S0

10. Streets and flow in streets
Ditch sections

11. Streets and flow in streets
Flow in curb-and-gutter sections
Equation 10-4

12. channel flow only if appropriate
Tc applies where?
500 ft
2.5 acres
4.0 acres
250 ft
475 ft
S0=0.01
40% Impervious
15% impervious
30 ft
700 ft
Use NRCS or Kerby-Kirpich;
channel flow only if appropriate
Travel time based on conduit hydraulics

13. Selecting an AEP

14. Roadway Crown (1 of 2)
Roadways always exhibit slope of some type in the transverse direction
May be curved (parabolic or circular),
May be comprised of straight segments (rooftop crown)

15. Roadway Crown (2 of 2)
Typical transverse slope rates are .02 ft/ft (~ ¼ in/ft) to .05 ft/ft (~5/8 in/ft)
Rooftop crowns often exhibit increasing slope moving away from the centerline (or profile grade line)

16. Lanes – Driving, Parking
Typical 2-way urban cross sections may contain:
left-turn lanes in the center
one or more through (driving) lanes on either side
auxiliary lane near the curb
The auxiliary lane may vary between
Parking
Bicycle
Designated right-turn

17. Typical roadway section

18. Curbs Curbs are the usual roadway bounding feature in urban areas.
Vary in height from negligible to as much as 8 inches
Curbs serve multiple purposes
Minor redirection for errant vehicles
Bounding feature for water running in the roadway as an open channel
Curbs provide constraint that allows them to become a part of inlets.

19. Sidewalks
Sidewalks are a common feature in urban roadway cross sections.
Their primary intent is to act as pedestrian walkways/ADA access
Sidewalks influence drainage features by:
Need to meet ADA standards for cross slope, ramps, and access.
Often become a constraint on the geometry and location of drainage features.

20. Roadway ponding & ponding width
The primary design criterion for urban storm drainage systems is usually “ponded width” in the roadway.
Ponded width is the width of the roadway covered by ponded water
What remains is considered usable roadway
The portion with water ponded is considered to be a traffic hazard
In the design process, each side of the roadway must is considered separately with respect to ponding.

21. Typical roadway section

22. Flow in curb & gutter
Ponded width is a function of depth of flow in the gutter by way of the transverse slope (or slopes)
The steeper the transverse slope, the smaller the ponded width for a given discharge

23. Increase in contributing area
The area adjacent to the roadway contributes discharge to the roadway in such a way that it can be approximated as a uniform, distributed source
Proceeding downstream, with respect to the longitudinal slope, contributing area increases
As contributing area increases, discharge in the roadway increases
As discharge increases, depth and ponded width increase.

24. Increase in ponded width
Flow accumulation

25. Longitudinal profile grade
Occasionally may need to be undulated to accommodate good drainage.

26. Sag vertical curves
Sag vertical curves always involve diminishing slope, increasing depth and ponded width.
Sag inlets must be placed in the low point of the sag!
Inlets in sags perform differently than those on grade
Often necessary to provide inlets on grade prior to the sag to control ponding as the low point of the sag is approached.

27. Slope break Transverse slopes are in range of 2%-4%
(.02 ft/ft – 0.04ft/ft)
Typical is 2% increasing to 3% after some distance depending on longitudinal slope.
Slope breaks are normally accomplished at lane lines

28. Ponded width
Ponded width computations will usually involve all “Z” values in the typical section.
Z1 is usually the slope closest to the curb and gutter.

29. Ponded depth Ponded depth is the depth at the curb (or edge).
If at an inlet, the depth would be measured from the lip of the inlet.

30. Inlet placement to reduce width
Inlets are placed in low points
Consider intersections
Acceptable ponding widths

31. Inlet placement to reduce width
Ponding width

32. Inlet placement to reduce width
Partial capture with carryover

33. Inlet Placement
Locations dictated by physical demands, hydraulics, or both
Logical locations include:
Sag configurations
Near intersections
At gore islands
Super-elevation transitions
Allowable ponded width guides location selection

34. Allowable Ponded Width
Guidelines (from HDM) include:
Limit ponding to one-half the width of the outer lane for the main lanes of interstate and controlled access highways
Limit ponding to the width of the outer lane for major highways, which are highways with two or more lanes in each direction, and frontage roads
Limit ponding to a width and depth that will allow the safe passage of one lane of traffic for minor highways

35. Inlet on grade LR Compute length of inlet for total interception
Subjective decision of actual length
Estimate carryover LR

36. Inlet on grade Design guidance in HDM pp. 10-30 – 10-35.
Formula for estimating required length
Need geometry
Need desired flow (to capture)
Calculate equivalent cross slope
Inlet height used here
Apply formula for required inlet length

37. Profile grade vs. inlet length
Inlet length is proportional to longitudinal slope
As slope increases, required length increases
Length for complete capture
Longitudinal slope

38. Profile grade vs. inlet length
Inlet length is proportional to longitudinal slope
As slope increases, required length increases

39. Sag Inlets Inlets placed at low point of a vertical curve.
Various actual geometries, lowest point is the key feature.

40. Ponded width vs. vertical curvature
As slope of vertical curve decreases, spread width increases

41. Ponded width vs. vertical curvature
Median inlet configuration

42. Ponded width vs. vertical curvature
Median inlet configuration

43. Inlets and inlet performance (Videos)
Grate On-Grade

44. Inlets and inlet performance (Videos)
Grate with Ditch Block (Sag Condition)

45. Design discharge
The design discharge to the inlet is based on the desired risk (AEP), the surface area that drains to the inlet, and the time of concentration
The time of concentration in this context is also called the inlet time

46. Design discharge (1 of 2) The “steps” for the inlet are:
State the desired risk (typically 10-50% AEP)
Determine the area that drains to the inlet
Determine the Tc appropriate for the area
If Tc<10 min., then use 10 min as the averaging time.

47. Design discharge (2 of 2) The “steps” for the inlet are:
Compute intensity from Tc.
EBDLKUP.xls, or equation in HDM – be sure to check time units with either tool!
Estimate a reasonable runoff coefficient, C.
Apply rational equation to estimate design discharge, Q

48. Capacity computations
Based on the design flow, gutter geometry, longitudinal and cross slope, and inlet length and height.
Computations for Inlet On-grade
Computations for Inlet in Sag

49. Curb opening inlet design variables
Ponding width = T
Gutter depression = a
Gutter depression width = W

50. Determining Inlet Length
Use HDM Equations 10-8 through we will go through an example
Depressed section
Beyond depressed section

51. Normal depth TxDOT HDM Eq 10-1
where Q = design flow (cfs); n = Manning’s
roughness coefficient; Sx = pavement cross
slope; S = friction slope; d = ponded depth (ft).

52. Ponded width
TxDOT HDM Eq where d = ponded depth (ft); Sx = pavement cross slope.

53. Ratio of depressed section flow to total flow
TxDOT HDM Eq where Kw = conveyance in depressed section (cfs); Ko = conveyance beyond depressed section (cfs); Eo = ratio of depressed section flow to total flow.

54. Conveyance
TxDOT HDM Eq where A = cross section area (sq ft); n = Manning roughness coefficient; P = wetted perimeter (ft); K = conveyance.

55. Area of the depressed gutter section
TxDOT HDM Eq where W = depression width (ft); Sx = pavement cross slope; T = ponded width (ft); a = curb opening depression (ft); Aw = area of depressed gutter section.

56. Wetted perimeter of the depressed gutter section
TxDOT HDM Eq where W = depression width (ft); Sx = pavement cross slope; a = curb opening depression (ft); Pw = wetted perimeter of depressed gutter section.

57. Area of cross section beyond the depression
TxDOT HDM Eq where Sx = pavement cross slope; T = ponded width (ft); W = depression width (ft); Ao = area of cross section beyond depression.

58. Wetted perimeter of cross section beyond the depression
TxDOT HDM Eq where T = ponded width (ft); W = depression width (ft); Po = wetted perimeter of cross section beyond depression.

59. Equivalent cross slope
TxDOT HDM Eq where Sx = pavement cross slope; a = curb opening depression (ft); W = depression width (ft); Eo = ratio of depression flow to total flow; Se = equivalent cross slope.

60. Length of curb inlet required
TxDOT HDM Eq where Q = flow (cfs); S = longitudinal slope; n = Manning’s roughness coefficient; Se = equivalent cross slope; Lr = length of curb inlet required.

61. Capacity in Sag Placement
Depends on water depth at opening and opening height
Determine if orifice-only flow (d>1.4h)
If d<1.4h compute using a weir flow equation and orifice flow equation for the depth condition, then choose the larger length d h L

62. Orifice Flow
d>1.4h
Use equation 10-19

63. Weir Flow
d<1.4h
Use equation 10-18

64. Inlets and inlet performance
Determine allowable head (depth) for the inlet location.
Lower of the curb height and depth associated with allowable pond width

65. Inlets and inlet performance
Determine the capacity of the grate inlet opening as a weir.
Perimeter controls the capacity.

66. Inlets and inlet performance
Determine the capacity of the grate inlet opening as an orifice.
Area controls the capacity.

67. Inlets and inlet performance
Compare the weir and orifice capacities, choose the lower value as the inlet design capacity.

68. Inlet standards
D+1 minimum for inside height. Not intended to force invert elevation, sometimes want pipe deep.

69. Inlet standards
Drawing does not show that often pipe exits in same direction as inlet, back underneath the roadway

70. Inlet standards
Knockouts 12-18” typical. Determine what is critical element of depth. Try to set all the same height.

71. Inlet standards Extensions: Avoid extending off both sides of box
No slope required in flowline of extension means no added slope.

72. Inlet standards 10 degree connection is to minimize rebar cut.
Goal is to have enough steel to keep from crushing.
Follow detail when possible, can game a little by making deeper in dimension where pipe enters

73. Conduit Design
Conduit size is computed based on the discharge expected at the upstream node;
Typically is done by the rational formula, applied to the sum of the areas contributing to that node;
TxDOT traditionally designs conduits to flow as open channels (free surface inside the conduit) at the design discharge.

74. Conduit shape
Circular sections are the most economical, usually being ¼ to 1/5 the cost of box sections;
Boxes are used where headroom is constrained;

75. Trunk lines
Trunk lines should follow ground contour in only the most general way- trunk line profile should be dictated by velocity/energy/depth management needs.
Trunk lines run from junction box to junction box, they should not run through other appurtenances (inlet boxes) nor should there be hidden pipe junctions (Ts or Ys)
Trunk lines should enter and leave junction boxes such that there is no backwater in upstream conduits

76. Trunk lines
Trunk lines should always stay the same size or increase in size in the downstream direction, never decrease.
Velocity in trunk lines should stay the same or increase by small increments in the downstream direction, never decrease.
Trunk lines should be designed to maximize the length of runs of the same diameter, rather than changing diameter frequently

77. Design discharge 2.5 acres 250 ft 4.0 acres 475 ft HDM 10-47 A1 A2
40% Impervious
250 ft
S0=0.01 A1
4.0 acres
500 ft
475 ft
15% impervious A2
S0=0.01
30 ft B1
700 ft

78. Design Discharge
Use rational equation to estimate discharges to each inlet – based on drainage area to that inlet. Tc also called inlet time.
If Tc < 10 min, use 10 min. Keep track of Tc-actual.
Accumulate discharge and area as move downstream. Tc are added to conduit travel time – use largest Tc+travel for each node

79. Design Discharge
Size conduit using HDM 10-36

80. Velocity –travel time Calculate velocity using HDM 10-37
Storm sewers should be designed such that velocities are maintained at levels similar to those in natural overland flow.
Minimizes the effects of changing timing of contribution in receiving streams.

81. Flowing as open channel – D/d
TxDOT procedures assume conduit flow is as an open channel at design discharge
Ratio of depth to diameter (D/d) is an important metric of open-channel flow
Flow efficiency increases as D/d increases until D/d reaches .5
Flow efficiency diminishes as D/d increases past .5, but discharge still increases until D/d reaches .85
After D/d reaches .85, the computed discharge diminishes; in reality flow becomes unstable (oscillates and surges).

82. Surcharge flow
If the discharge must be greater than the pipe will carry as an open channel at D/d of .85, flow will become pressure flow by building up head (surcharging) in junction boxes.

83. Hydraulic grade line
The Hydraulic grade line should be relatively uniform within any conduit run, i.e. should not include an backwater effects
The HGL of laterals should be matched or above the HGL of the trunk line at junctions
Hydraulic drops of laterals into junctions in order to “disconnect” the laterals from trunk line influences are good practice and allow consistency of lateral construction

84. Profile grade
Profile grade of storm sewer trunk lines may be “stairstepped” to control energy in cases of significant topographic relief
Profile grade of storm sewer trunk lines should always include some sort of drop at junctions (where additional flow comes in) to at least match HGL upstream and downstream
Profile grade should be driven by hydraulic considerations rather than topographic considerations

85. Sizes
Laterals carry water from inlets into junction boxes, where it leaves by a trunk line.
The HGL of laterals can be independent of- but above- that of the trunk line runs
It may be cheaper in the long run to have some laterals oversize and consistent with others rather than specify a small quantity of smaller pipe
Laterals often spill into junction boxes much higher than trunk lines enter and leave; they may be allowed to protrude a small amount to facilitate construction

86. Road crossing
Laterals often completely or partly cross the roadway and may need to be constructed in phases
Should be located deep enough to clear pavement construction!
Module 10

87. Slope
If the trunk line is located reasonably deep, slope of laterals is fairly free.
It is much preferred to have laterals enter high, with relatively low velocity and plunging flow, than to bring them in low, with high velocity entering a larger stream (momentum distribution).
Laterals can often be planned such that the length and slope of many laterals is the same, facilitating construction.

88. Box exit The “bell” or “groove” end of pipe is oriented upstream
With most inlets, the lateral emptying the box exits from the “front”, or roadway, wall of the box (despite what is implied by the standard)
FLOW

89. Junction with other appurtenances
Small storm sewer systems occasionally flow into other drainage structures, such as box culverts
Consideration of lateral momentum is important if a storm sewer system enters a culvert low in the culvert wall
Beware of badly unbalanced flow in multiple boxes if a lateral flows directly into a multi-box culvert

90. Junction boxes/manholes
Junction boxes are connections between lines; they usually serve also as manholes
Manholes are points of access and ventilation in a system. They may coincide with junction boxes, but may also be located based solely on access and ventilation needs

91. Location
System-wise, junction boxes should be located wherever laterals join a trunk line, or where there is a need to change conduit size or configuration (there are few reasons to change conduit size unless there is a change in discharge, i.e. a lateral enters)
Geographically, junction boxes are typically located within the roadway
Plan trunk lines and junction boxes to ease construction of the entire project- for instance, within the roadway in the first phase of a multi-phase roadway reconstruction process

92. Location
Avoid placing junction boxes in a wheelpath; between wheelpaths or within an auxiliary lane is good

93. Entry/exit of trunk lines
Trunk lines will always exit junction boxes flush with the bottom of the box (Duh!)
Trunk lines should enter junction boxes at an elevation sufficiently high above the exit that the HGL of the entering line is at or above the HGL of the exit line. Some assumption about loss within the box should be made (there is a formula, use it if you want, but a general assumption is adequate)
Remember that the HGL of the exit line will include discharge entering from laterals in addition to that entering from the trunk line

94. Entry of laterals
Laterals may enter a junction box at any elevation such that the HGL of the lateral is at or above the HGL of the trunk line upstream
They may be considerably higher- and flow plunge into the box, to minimize trenching and standardize lateral configuration from case to case
If they will not physically conflict with through flow, they may protrude into the box slightly to ease construction

95. Size
Junction box planform size is implied by the standard, which refers to conduit sizes for all entering and exiting conduits
Junction box depth is determined by the roadway PGL and the exit conduit flow line elevation
Flow line elevations can sometimes be manipulated to ease construction, i.e. to accommodate precast boxes.

96. Spacing
In areas of rapid change in topographic elevation, junction boxes may need to be spaced relatively closely in order to “stairstep” and control energy (velocity) in the trunk line
Normally, spacing is set by the need for entry of laterals due to inlets; it is closely associated with inlet spacing
In cases of long trunk line runs with no junctions (such as long lines to a remote outfall), refer to the HDM for maximum manhole spacing for access and ventilation purposes. Use judgement.

97. Junction box standards
Similar to inlet considerations, rebar important.
Sometimes can join boxes to maintain orientation
If box over 6 feet wide -- special case , call BRG.

98. Junction box standards
Riser does not have to be different dimension than the box

99. Junction box standards
Make as many elements as possible identical
Enhance constructability
Min/max dimensions: maximum is structural limitation, if must exceed call BRG for guidance.
D+1 ft. becomes limiting with box structures – they can get wide

100. Junction box standards
Try to make elements identical and repeatable
1 set inlet, 1 set laterals, 1 junction

101. Junction box standards
Offsets are not a constraint.
Can have multiple offsets.
Can have multiple penetrations on a side.

102. Junction box standards
When have high topographic relief, use drops in junction boxes to control energy and velocity.
Try to keep time of travel in system similar to pre-development conditions.
d/D, Q, and V tells us where we need to change D as move through the system.

103. Outfalls
The “outfall” is the downstream end of storm sewer system; where it empties into a stream or other receiving water
Location, configuration, size, and details of the outfall may have many impacts (environmental, public safety, system performance, etc)

104. Location
Outfalls should ideally be located in places that are accessible for inspection and maintenance, but do not draw public notice or attention
Locations may be subject to environmental regulation (MS4) and permitting
Consider the effects of introducing flow at a particular spot- quantity, momentum balance, sediment transport, and function of the receiving water

105. Elevation
The nature of the receiving water is critical to the selection of the outfall flow line elevation
If sufficient topographic relief is available, a short outfall run with a junction box incorporating a significant drop from trunk line to outfall run is desirable
The outfall should preferably not terminate in a significant drop unless there is well-designed scour protection underneath it (steep paved slopes are not well-designed scour protection!!!)

106. Velocity control
Exit velocity and momentum direction are critical elements of outfall design
Velocity should be low enough to prevent scour, but high enough to prevent clogging
In sanitary sewer design, engineers try to achieve 2-10 feet-per-second at the daily peak flow to keep entrained solids moving towards treatment plant.
Similar velocity range may be useful in storm sewers – 10 feet per second is quite fast!

107. Velocity control
Consider momentum effects on the receiving water if it is flowing water
A rigorous hydrologic examination of the receiving stream may be necessary to estimate an expected stage and velocity during storm sewer outflow
The characteristic times are likely to be very different

108. Intrusion control (public safety)
Outfalls, particularly large ones, should not facilitate easy entry and traverse by the public (particularly children)
As stated earlier, a junction box with a significant drop (>6’) close to the outfall discourages entry (a ladder is very difficult to get into a junction box from the outfall)
Gates, grates, etc may be necessary, but should be avoided if at all possible (they trap debris and present a danger should anyone enter the system upstream)

109. Intrusion control (public safety)
In conflict with earlier advise on trunk line configuration, a short outfall run may be designed with a wide, low box shape to discourage easy entry
Any measure that completely prevents intrusion also completely prevents escape in the event that a person enters the system higher up. Consider “discouraging” rather than “excluding” intrusion

110. Pollution
Storm sewers provide an immediate and efficient flow connection between the roadway and receiving waters.
A high potential for exacerbating the environmental effects of a chemical spill on the roadway or adjacent property any pollutants that end up on the roadway (oil, coolants, deicing chemicals, etc)

111. Pollution Consider sensitivity of the receiving waters.
Hazmat traps are available, but effectiveness is unknown
Questions to consider:
What will happen in the event of a spill?
Normal contaminants?
Do you need to do primary treatment on outflow?

112. Problem Statement
Working schematic

113. Problem Statement
At node A8 outflow from the shopping mall is accepted into the storm drain system.
The storm drain outfalls into a channel just downstream of a culvert, which accomodates flow from a 2266 acre watershed.
Hydrology and inlet data on TAB1 in spreadsheet
Design the system, determine hydraulic grade line
Module 13

114. General Information

115. Estimate Runoff Can be in acres, just don’t mix units
Prepare system plan and trial layout
Initial runoff calculations – use rational equation
C values are composite where applicable
Can be in acres, just don’t mix units

116. Estimate Inlet Capture
On grade
Sag
Compute spreadwidth after capture

117. Size Conduits Accumulate CA as move downstream
Add conduit time (uses slope)
Compute required diameter
round up to commercial pipe size
Compute flow depth at upstream end each conduit
Compute velocity each conduit
Compute HGL from outfall and work upstream

118. Software
The illustration is performed in Excel. Appropriate for the example.
Complicated systems can be modeled in
Geopack-Drainage
WinSTORM
SWMM
Sophisticated tool, useful for system hydraulics where backwater will be a substantial issue
HEC-RAS
Like SWMM, but not really good for storm sewers or looped systems