ce 3372 inlets, junctions, conduits. useful references hdm == txdot hydraulic design manual 2011 ed....

CE 3372Inlets, Junctions, Conduits

Useful References• HDM == TxDOT Hydraulic Design Manual 2011 Ed. • HDS2 == FHWA-NHI-02-001 Highway Hydrology

• HEC-22 == FHWA-NHI-01-021 Urban Drainage Design Manual

• HEC-24 == FHWA-NHI-01-007 Highway Stormwater Pump Station Design

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!

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

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)

What Kind of Infrastructure?

Lift Station

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

Storm Drain Design

• Establish design parameters and criteria

• Decide layout, component location, and orientation

• Use appropriate design tools

• Comprehensive documentation

• The process is iterative

Streets and flow in streets• Curb-and-gutter sections

Curb

Gutter

Inlet (Curb Opening + Grate)

Cross slopeLongitudinal Slope, S

0

Streets and flow in streets• Ditch sections

Streets and flow in streets• Flow in curb-and-gutter sections

Equation 10-4

Tc applies where?

500

ft2.5 acres

4.0 acres

250 ft

475 ft

S0=0.01

S0=0.01

40%

Im

perv

ious

15% im

pervious

30 ft

700

ftUse NRCS or Kerby-Kirpich;

channel flow only if appropriate

Travel time based on conduit hydraulics

Selecting an AEP

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)

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)

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

Typical roadway section

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.

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.

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.

Typical roadway section

Flow in curb & gutter

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.

Increase in ponded width

Flow accumulation

Longitudinal profile grade• Occasionally may need to be undulated to

accommodate good drainage.

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.

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

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.

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.

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

Inlet placement to reduce width• Ponding width

Inlet placement to reduce width• Partial capture with carryover

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

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

Inlet on grade

• Compute length of inlet for total interception• Subjective decision of actual length• Estimate carryover

LR

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

Profile grade vs. inlet length

• Inlet length is proportional to longitudinal slope • As slope increases, required length increases

Length for complete capture Longitudinal slope

Profile grade vs. inlet length

• Inlet length is proportional to longitudinal slope • As slope increases, required length increases

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

feature.

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

increases

Ponded width vs. vertical curvature• Median inlet configuration

Ponded width vs. vertical curvature• Median inlet configuration

Inlets and inlet performance (Videos)• Grate On-Grade

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

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

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.

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

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

Curb opening inlet design variables

• Ponding width = T• Gutter depression = a• Gutter depression width = W

Determining Inlet Length• Use HDM Equations 10-8 through 10-16 we will go

through an example

Depressed section

Beyond depressed section

Normal depth

• TxDOT HDM Eq 10-1

where Q = design flow (cfs); n = Manning’sroughness coefficient; Sx = pavement crossslope; S = friction slope; d = ponded depth (ft).

8/3

2/124.1

S

QnSd

x

Ponded width

• TxDOT HDM Eq 10-2

where d = ponded depth (ft); Sx = pavement cross slope.

xSd

T

Ratio of depressed section flow to total flow

• TxDOT HDM Eq 10-8

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

ow

wo KK

KE

Conveyance• TxDOT HDM Eq 10-9

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

3/2

3/5486.1nP

AK

Area of the depressed gutter section

• TxDOT HDM Eq 10-10

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

aWW

TWSA xW21

2

Wetted perimeter of the depressed gutter section

• TxDOT HDM Eq 10-11

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

22 WaWSP xW

Area of cross section beyond the depression

• TxDOT HDM Eq 10-12

where Sx = pavement cross slope; T = ponded width (ft); W = depression width (ft); Ao = area of cross section beyond depression.

22

WTS

Ax

o

Wetted perimeter of cross section beyond the depression• TxDOT HDM Eq 10-13

where T = ponded width (ft); W = depression width (ft); Po = wetted perimeter of cross section beyond depression.

WTPo

Equivalent cross slope

• TxDOT HDM Eq 10-14

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.

oxe EWa

SS

Length of curb inlet required

• TxDOT HDM Eq 10-15

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

6.03.042.0 1

6.0

er

nSSQL

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

L

h

d

Orifice Flow

• d>1.4h• Use equation 10-19

Weir Flow

• d<1.4h• Use equation 10-18

Inlets and inlet performance • Determine allowable head (depth) for the inlet

location.• Lower of the curb height and depth associated

with allowable pond width

Inlets and inlet performance • Determine the capacity of the grate inlet opening as

a weir. • Perimeter controls the capacity.

Inlets and inlet performance • Determine the capacity of the grate inlet opening as

an orifice. • Area controls the capacity.

Inlets and inlet performance • Compare the weir and orifice capacities, choose the

lower value as the inlet design capacity.

Inlet standards

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

Inlet standards

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

Inlet standards

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

Inlet standards

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

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

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.

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;

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

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

Design discharge• HDM 10-47

500

ft

2.5 acres

4.0 acres

250 ft

475 ft

S0=0.01

S0=0.01

40%

Im

perv

ious

15% im

pervious

30 ft

700

ft

A1

A2

B1

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

Design Discharge• Size conduit using HDM 10-36

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.

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).

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.

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

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

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

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

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.

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

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

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

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

Location• Avoid placing junction boxes in a wheelpath;

between wheelpaths or within an auxiliary lane is good

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

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

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.

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.

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.

Junction box standards

Riser does not have to be different dimension than the box

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

Junction box standards

• Try to make elements identical and repeatable• 1 set inlet, 1 set laterals, 1 junction

Junction box standards

• Offsets are not a constraint. • Can have multiple offsets.• Can have multiple penetrations on a side.

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.

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)

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

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!!!)

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!

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

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)

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

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)

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?

Problem Statement

• Working schematic

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

General Information

Estimate Runoff

• 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

Estimate Inlet Capture

• On grade

• Sag

• Compute spreadwidth after capture

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

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