CE 3372 Water Systems Design Lecture 014 Storm

CE 3372 Water Systems Design Lecture 014 Storm Sewer and Inlet Hydraulics

Review Supercritical, Critical, or Subcritical Location A ? Location B ? Location C ? 2

Storm Sewers Inlets to capture runoff Conduits to convey to outfall Lift Stations if cannot gravity flow to outfall Detention and diversions Outfall release back into environment 3

Storm Sewer Systems Inlets Lift Station Conduits Module 1

Storm Sewer Inlets Spread width Combination Inlet Curb+Grate Carryover Flow that passes beyond the inlet (none in this picture – complete capture) 5

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 closedconduit, an open conduit, or some combination of the two Module 1

Design Challenges Drainage in urban areas is challenging because of: Heavy traffic and subsequent higher risks Wide roadway sections Relatively flat grades, both longitudinal and transverse Shallow water courses Absence of side ditches Concentrated flow Module 1

Design Challenges Drainage in urban areas is challenging because of: Potential for costly property damage from water ponding or flow through built-up areas Roadway section must carry traffic and serve as a channel to carry water to a discharge point Limited ROW to place drainage infrastructure Outfalls not convenient Infrastructure impacts multiple jurisdictions Water quality 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 Module 1

Streets and flow in streets Curb-and-gutter sections Curb Inlet (Curb Opening + Grate) Transverse (cross) slope Gutter Lo ng itu din al Sl op e, S 0

Streets and flow in streets Ditch sections Module 2

Streets and flow in streets Flow in curb-and-gutter sections Equation 10 -4 Module 2

Rational & Modified Rational equation The “Rational Equation” is an equation that is used in the vast majority of urban storm drain designs. The basic equation (HDM) is: Z is a dimensions correction coefficient C is a “runoff coefficient” I is rainfall intensity for an appropriate duration and frequency A is contributing area, in acres. Module 2

Intensity-Duration-Frequency Intensity is the ratio of an accumulated depth to some averaging time usually the time of concentration. Called “inlet time” for inlet design.

Runoff coefficients are tabulated, and selected from a land use description. Module 2

Use NRCS or Kerby-Kirpich; 2. 5 acres 250 ft S =0. 01 channel flow only if appropriate 0 4. 0 acres 500 ft 47 15% impervious t f 5 S 0=0. 01 700 ft 40% Impervious Tc applies where? 30 ft Travel time based on conduit hydraulics

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.

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.

Manning’s Eq & Izzard’s form About 85% of the discharge is in the deepest half (closest to the curb) of the section. 85% of Q

Increase in ponded width ation l u m u c c a Flow Module 4

Velocity and travel time As average velocity of contribution increases, travel time for a given distance decreases. All other things being equal, as travel time decreases, critical duration decreases, and the intensity associated with it increases While it may appear that getting water conducted away from features of interest as quickly as possible is desirable, decreasing travel time typically is counter to reducing peak flow rates (because of the relationship between intensity and time). Module 4

Ponded width computations will usually involve all “Z” values in the typical section. Z 1 is usually the slope closest to the curb and gutter. Module 5

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 Module 5

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 Module 5

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

Inlet on grade Value of carryover Uses more of inlet open area – hence may be able to use shorter inlet (if there is compelling need) Module 5

Profile grade vs. inlet length Inlet length is proportional to longitudinal slope As slope increases, required length increases Length for complete capture Longitudinal slope Module 5

Profile grade vs. inlet length Inlet length is proportional to longitudinal slope As slope increases, required length increases Module 5

Sag Inlets

Sag 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 Module 6

Ponded width vs. vertical curvature Median inlet configuration Module 6

Ponded width vs. vertical curvature Median inlet configuration Module 6

Inlets and inlet performance Grate On-Grade (Videos) Module 6

Inlets and inlet performance Grate with Ditch (Videos) Block (Sag Condition) Module 6

Inlets and inlet performance (Videos) Tandem Grate Inlets On Grade Module 6

Inlets and inlet performance (Videos) Tandem Grate Inlets with Ditch Block (Sag Condition) Module 6

Curb Inlets on Grade and Sag

Design discharge Module supports objectives 2 and 5: Describe components of an urban drainage system. Select locations and design storm drain inlets. 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 Module 7

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. Module 7

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 Module 7

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 Module 7

Curb opening inlet design variables • Ponding width = T • Gutter depression = a • Gutter depression width = W Module 7

Determining Inlet Length Use HDM Equations 10 -8 through 10 -16 we will go through an example Depressed section Beyond depressed section Module 7

Normal depth Tx. DOT 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). Module 7

Ponded width Tx. DOT HDM Eq 10 -2 where d = ponded depth (ft); Sx = pavement cross slope. Module 7

Ratio of depressed section flow to total flow Tx. DOT 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. Module 7

Conveyance Tx. DOT HDM Eq 10 -9 where A = cross section area (sq ft); n = Manning roughness coefficient; P = wetted perimeter (ft); K = conveyance. Module 7

Area of the depressed gutter section Tx. DOT 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. Module 7

Wetted perimeter of the depressed gutter section Tx. DOT 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. Module 7

Area of cross section beyond the depression Tx. DOT HDM Eq 10 -12 where Sx = pavement cross slope; T = ponded width (ft); W = depression width (ft); Ao = area of cross section beyond depression. Module 7

Wetted perimeter of cross section beyond the depression Tx. DOT HDM Eq 10 -13 where T = ponded width (ft); W = depression width (ft); Po = wetted perimeter of cross section beyond depression. Module 7

Equivalent cross slope Tx. DOT 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. Module 7

Length of curb inlet required Tx. DOT 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. Module 7

Capacity in Sag Placement Depends on water depth at opening and opening height Determine if orifice-only flow (d>1. 4 h) If d<1. 4 h compute using a weir flow equation and orifice flow equation for the depth condition, then choose the larger length d h L Module 7

Orifice Flow d>1. 4 h Use equation 10 -19 Module 7

Weir Flow d<1. 4 h Use equation 10 -18 Module 7

Summary Inlet on grade design equations Required length Example/Exercise Module 7

Urban Storm Drain Design: DES 602 Module 8 Drop Inlets on Grade and Sag Inlet Standards

Inlets and inlet performance Module supports objectives 2 and 5: Describe components of an urban drainage system. Select locations and design storm drain inlets. Grate Inlets in Sag Conditions Weir Flow at Low Depth Orifice Flow at Large Depth Inlet standards Module 8

Inlets and inlet performance Choose grate of standard dimension (e. g. Type. H, etc. from standards and specifications server) Module 8

Inlets and inlet performance Choose grate of standard dimension (e. g. Type. H, etc. from standards and specifications server) Module 8

Inlets and inlet performance Determine allowable head (depth) for the inlet location. Lower of the curb height and depth associated with allowable pond width Module 8

Inlets and inlet performance Determine the capacity of the grate inlet opening as a weir. Perimeter controls the capacity. Module 8

Inlets and inlet performance Determine the capacity of the grate inlet opening as an orifice. Area controls the capacity. Module 8

Inlets and inlet performance Compare the weir and orifice capacities, choose the lower value as the inlet design capacity. Module 8

Inlet standards Highlight various parts of Curb Inlet Type C and Extension Type E standards and specification sheet. Identify “things” that may confuse Feel free to mark on the sheet in your participant manual. Module 8

Inlet standards D+1 minimum for inside height. Not intended to force invert elevation, sometimes want pipe deep. Module 8

Inlet standards Drawing does not show that often pipe exits in same direction as inlet, back underneath the roadway Module 8

Inlet standards Knockouts 12 -18” typical. Determine what is critical element of depth. Try to set all the same height. Module 8

Inlet standards Extensions: Don’t extend off both sides of box No slope required in flowline of extension means no added slope. Module 8

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 Module 8

Conduit Design Module supports objectives 6 : Design layout and size storm drain conduits 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; Tx. DOT traditionally designs conduits to flow as open channels (free surface inside the conduit) at the design discharge. Module 9

Sizing Typically is done by the rational formula, applied to the sum of the areas contributing to that node; Subareas are pre-multiplied by their respective runoff coefficients (ΣC*A) Intensity is calculated for the travel time coinciding with the longest flow path leading to that node; Module 9

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; Module 9

Conduit shape Other section shapes are available, but should not be used unless there is a compelling reason (contact DES-HYD) Boxes are often used in an attempt to minimize trench protection cost by closely following ground contour. USUALLY NOT EFFECTIVE. Trenching cost is less than the difference in material cost. Module 9

Conduit materials Reinforced Concrete is the most common material for storm sewers. Corrugated metal is available, but is discouraged by Tx. DOT for storm sewer construction. Various plastic materials have been proposed in recent years; beware of “low Manning’s n” claims. RC pipe or precast boxes are the easiest to construct - backfilling lighter materials without “float” is much harder than it looks. Structural design/bedding condition is important. Module 9

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 Module 9

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 Module 9

Design discharge 2. 5 acres 250 ft S =0. 01 0 A 1 4. 0 acres 500 ft 47 15% impervious t f 5 A 2 S 0=0. 01 30 ft 700 ft 40% Impervious HDM 10 -47 B 1 Module 9

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 Tcactual. Accumulate discharge and area as move downstream. Tc are added to conduit travel time – use largest Tc+travel for each node Module 9

Design Discharge Size conduit using HDM 10 -36 Module 9

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. Module 9

Flowing as open channel – D/d Tx. DOT 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). Module 9

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. Module 9

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 Module 9

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 Module 9

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 Module 10

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. Module 10

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 Module 10

Junction with other Small stormappurtenances 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 Module 10

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 Module 10

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 multiphase roadway reconstruction process Module 10

Location Avoid placing junction boxes in a wheelpath; between wheelpaths or within an auxiliary lane is good Module 10

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 Module 10

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 Module 10

Losses There will be some loss of energy through a junction box. There is a formula, but such precision is rarely justified. Module 10

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. Module 10

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. Module 10

Junction box standards Highlight various parts of Manhole Type M standards and specification sheet. Identify “things” that may confuse Feel free to mark on the sheet in your participant manual. Module 11

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. Module 11

Junction box standards Riser does not have to be different dimension than the box Module 11

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 Module 11

Junction box standards • Try to make elements identical and repeatable • 1 set inlet, 1 set laterals, 1 junction Module 11

Junction box standards • Offsets are not a constraint. • Can have multiple offsets. • Can have multiple penetrations on a side. Module 11

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 predevelopment conditions. • d/D, Q, and V tells us where we need to change D as move through the system. Module 11

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) Module 12

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 (MS 4) and permitting Consider the effects of introducing flow at a particular spot- quantity, momentum balance, sediment transport, and function of the receiving water Module 12

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!!!) Module 12

Elevation The outfall should preferably not terminate low enough in a stream to promote blocking or clogging by sediment Outfall design usually involves a compromise between scour potential and clogging potential. Consider conditions other than the design conditions! Module 12

Submergence In some cases, outfalls must be located such that they are subject to submergence, either by streamflow or by tidal flow Every effort should be made to keep the line self-cleaning even when intermittently submerged (how? ) Consider stage/frequency and or tidal range/frequency Module 12

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! Module 12

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 Module 12

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) Module 12

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 Module 12

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) Module 12

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? Module 12

Urban Storm Drain Design: DES 602 Module 13 Conduit Design - II

Storm Drain Design Example Similar to earlier examples, but more extensive in scope. The example is intended as a workshop Module 13

Problem Statement Working schematic is provided as a Z-fold Module 13

Problem Statement At node A 8 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 TAB 1 in spreadsheet Design the system, determine hydraulic grade line Module 13

General Information Module 13

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 Module 13

Estimate Inlet Capture On grade Sag Compute spreadwidth after capture Module 13

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 Module 13

Software Complicated systems can be modeled in Geopack-Drainage/Win. STORM SWMM Sophisticated tool, useful for system hydraulics where backwater will be a substantial issue HEC-RAS Not very good for storm sewers or multi-path systems Module 13