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Section 5: Storm Drain Inlets

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Inlet Types

You can divide inlets used for the drainage of highway surfaces into four major classes:

  • Curb opening inlets - See Figure 10-3.
  • Grate inlets - See Figure 10‑5.
  • Slotted drains - Slotted inlets function in essentially the same manner as curb opening inlets, i.e., as weirs with flow entering from the side. See Figure 10-6.
  • Combination inlets -- Combination inlets usually consist of some combination of a curb-opening inlet, a grate inlet, and a slotted drain. In a curb and grate combination, the curb opening may extend upstream of the grate. In a grate and slotted drain combination, the grate is usually placed at the downstream end of the slotted drain.
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Curb Opening Inlets

Figure 10‑3 illustrates a generic example of a typical curb opening inlet. Curb inlets are used in urban sections of highway along the curb line on continuous grades (on-grade) and at sag locations.

Curb Opening Inlet (click in image to see full-size image) Anchor: #i1001607grtop

Figure 10-3. Curb Opening Inlet

Most curb opening inlets depend heavily upon an adjacent depression in the gutter for effective flow interception (see Figure 10-4). Greater interception rates result in shorter (and probably, more economical) inlet lengths. However, a large gutter depression can be unsafe for traffic flow moving near the gutter line. Therefore, a compromise is in order when selecting an appropriate value for the gutter depression. The depth of the gutter depression should be:

  • 0 to 1 in. (0 to 25 mm) where the gutter is within the traffic lane
  • 1 to 3 in. (25 to 75 mm) where the gutter is outside the traffic lane or in the parking lane
  • 1 to 5 in. (25 to 125 mm) for lightly traveled city streets that are not on a highway route.

Curb Opening Inlet Depression (click in image to see full-size image) Anchor: #i1001609grtop

Figure 10-4. Curb Opening Inlet Depression

Some municipalities in the state prefer to recess curb inlets with significant depression to minimize interference with traffic flow. The inlet is recessed from the line of the curb and gutter such that the depression does not extend beyond the gutter line. This may improve driveability; however, the curb transition may pose a hazard to traffic.

Curb opening inlets are useful in sag and on-grade situations because of their self-cleansing abilities and hydraulic efficiency. Additionally, they are often preferred over grate inlets because the inlet is placed outside the travel way and poses less of a risk to motorists and bicycle traffic.

A drawback of curb opening inlets is that the flowline of the opening is fixed and not readily adaptable to changing pavement levels as occur in surface treatment overlays. Successive overlays can gradually reduce or even eliminate the original opening available for water removal, unless the pavement edge is tapered to the original gutter line.

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Grate Inlets

Figure 10‑5 illustrates a typical grate inlet. Water falls into the inlet through a grate instead of an opening in the curb. Designers use many variations of this inlet type, and the format of the grate itself varies widely as each foundry may have its own series of standard fabrication molds.

Grate Inlet Schematic (click in image to see full-size image) Anchor: #i1001649grtop

Figure 10-5. Grate Inlet Schematic

For the most part, use grate inlets in sag configurations in gutters adjacent to concrete traffic barriers or rails (where curb inlets would not be practicable), V-shaped gutters with no curb or barrier, and ditches. You may also use them in on-grade situations with curb inlets. Where you expect the grate inlet to intercept gutter flow in an on-grade configuration, the grate openings should be oriented parallel to the gutter flow in order to maximize hydraulic efficiency.

Grate inlets adapt to urban roadway features such as driveways, street intersections, and medians. When grate inlets are specified, assure that the grate configuration and orientation are compatible with bicycle and wheelchair safety. Consult with TxDOT’s State Bicycle Coordinator and the Design Division for additional information.

Access to the storm drain system through a grate inlet is excellent in that, usually, the grate is removable. On the other hand, maintenance of grate inlets can be a continuing problem during the life of the facility; their propensity to collect debris make grate inlets a constant object of maintenance attention. As such debris accumulates, it obstructs the flow of surface water into the inlet. Grate inlets also present potential interference with bicycles and wheelchairs.

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Slotted Drains

See Figure 10‑6 for an illustration of a slotted drain installation. The throat of a slotted drain inlet is ordinarily reinforced for structural integrity. The top of the throat is constructed flush with the surface of the pavement or the gutter.

Slotted Drain Inlet (click in image to see full-size image) Anchor: #i1001689grtop

Figure 10-6. Slotted Drain Inlet

Slotted drains may be an alternative to on-grade curb and grate inlets along curb lines. Also, they can be placed across driveways and street intersections.

Design for the removal of sheet flow from the roadway by strategically placing slotted drain pipe installations. Such installations may occur within the traveled way, either transversely or longitudinally. Where drainage is toward the inside of lanes and against median barriers, an installation of slotted drain pipe with appropriate outfall can be effective in removing accumulated runoff.

In asphalt concrete pavement applications, ensure structural integrity either by adequate structural characteristics of the slotted pipe or encasement in concrete such as illustrated in Figure 10‑7. Refer to the Bridge Division inlet standards, SD (M), concerning the proper type of slotted drain to use in these situations.

Slotted Drain Structural Integrity (click in image to see full-size image) Anchor: #i1001693grtop

Figure 10-7. Slotted Drain Structural Integrity

Slotted drains have the following advantages:

  • They are adaptable to intersections with urban roadway features such as driveways, street intersections, and sidewalks.
  • They can accommodate AASHTO HS 20 vehicular traffic as well as bicycles, wheel chairs, and some pedestrian traffic.
  • No depression is necessary for hydraulic efficiency.
  • Continuous sheet or gutter flow interception is possible at a relatively small cost.
  • Construction is very simple and proceeds quickly.
  • Pavement overlays or other surface treatment can be accommodated without any effect on the original intended hydraulic characteristics.
  • Slotted drain inlets in on-grade configurations are essentially self-cleaning.
  • They are aesthetically pleasing.

Disadvantages of slotted drain inlets include the following:

  • They have a high propensity to collect debris in sag configurations; therefore, do not use them in sag configurations.
  • Effective maintenance access usually requires an adjacent manhole or an adjacent curb opening or grate inlet.
  • Slotted drain pipes may have structural connector problems at locations where there are flexible joints in the roadway structure.
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Combination Inlets

Combination inlets such as curb and grate can be useful in many configurations, especially sag locations. Because of the inherent debris problem in sags, the combination inlet offers an overflow drain if part of the inlet becomes completely or severely clogged by debris. Maintenance of combination inlets is usually facilitated by the fact that the grate is removable, providing easy access to the inlet and associated storm drain system.

Combination inlets used on-grade are generally not cost-effective because of the relatively small additional hydraulic capacity afforded. Authentic data on such combinations are insufficient to establish accurate factors for determining the true capacity of a combination inlet.

For a combination curb and grate, assume that the capacity of the combination inlet comprises the sum of the capacity of the grate and the upstream curb opening length. Ignore the capacity of the curb opening that is combined with the grate opening.

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Inlets in Sag Configurations

An inlet in a sag configuration is the “end of the line” because the water and its debris load have no other place to go. Because of this, failure of an inlet in a sag configuration often represents a threat to the successful operation of a storm drain system, and you must consider some additional items. In a sag configuration, the controlling ponded width can be from one of three origins. The inlet itself may cause a head that translates to a ponded width. Furthermore, as water approaches the sag configuration inlet from each of two directions, the flow in the curb and gutter from each direction subtends its own ponded width. If the sag configuration inlet is in the trough of a vertical curve, the slope in the immediate vicinity of the sag inlet is equal to 0 %. Therefore, no specific slope is available for the computation of gutter flow characteristics. If the low point inlet is located at the intersection of two tangent approach slopes with no vertical curve, use the actual longitudinal slopes for the calculation of flow depths in the gutter.

Because the water or its debris load can go no other place, apply an appropriate safety factor to the inlet size. For grate inlets in sags, the usual safety factor is approximately two. For curb inlets, the ratio can be somewhat less. This is conventional practice for the TxDOT. For example, if a low point grate inlet requires an open area of 4.1 sq.ft. (2.1 m2) and the standard inlet open area is 4.0 sq.ft. (2.0 m2), provide two inlets for a total open area of 8.0 sq.ft. (4.0 m2) (safety factor = 1.9).

In addition, where significant ponding can occur such as in underpasses and in sag-vertical curves, it is good engineering practice to place flanking inlets on each side of the sag location inlet. Analyze flanking inlets as inlets on-grade at some specified distance away from the low point on the sag vertical curve. Often, the specified distance is 50 or 100 ft. (15 or 30 m). The on-grade inlets serve to relieve some or most of the flow burden from the inlet located at the low point. Place the flanking inlets so that they will limit spread on low gradient approaches to the level point and act in relief of the sag inlet if it should become clogged or if the design spread is exceeded.

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Median/Ditch Drains

Drains or inlets appearing in ditches and medians are usually grate inlets and are also termed “drop inlets.” Often, such an inlet is in a sag (sump) configuration. In sag configurations, drains have a high probability for maintenance problems. As with grate inlets in gutters, grate inlets used in medians or other ditches should usually have the grate bars aligned parallel to the flow. A concrete riprap collar that forms a type of bowl around the inlet will improve the operational characteristics of the facility. If the inlet in the median or ditch is in an on-grade configuration, you may need to provide a downstream dike or “ditch block” as illustrated in Figure 10‑8.

Median/Ditch Inlet (click in image to see full-size image) Anchor: #i1001805grtop

Figure 10-8. Median/Ditch Inlet

Over-side drains, also referred to as drainage chutes, are used when no inlet at the curb and gutter line connects to a storm drain system. An opening in the curb connecting to a scour-resistant channel or chute removes the concentrated flow in the curb and gutter from the roadway. In some instances, you may replace the channel or chute with a small pipe placed in the roadway embankment as illustrated in Figure 10‑9.

Over-Side Drains (click in image to see full-size image) Anchor: #i1003373grtop

Figure 10-9. Over-Side Drains

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Inlet Locations

The inlet location may be dictated either on the basis of physical demands, hydraulic requirements, or both. In all instances, you must coordinate the inlet location with physical characteristics of the roadway geometry, utility conflicts, and feasibility of underground pipe layout.

Establish logical locations early on as permanent and non-adjustable fixtures in the storm drain system. Determine their hydraulic characteristics in the ordinary trial and error process of storm drain design. Logical locations for inlets include sag configurations, near street intersections, at gore islands (see Figure 10‑10), and super-elevation transitions.

Inlets with locations not established by physical requirements should be located on the basis of hydraulic demand.

Inlet at a Gore Island (click in image to see full-size image) Anchor: #i1003413grtop

Figure 10-10. Inlet at a Gore Island

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Ponded Width Options

An on-grade inlet may be necessary to remove some or all of the flow at that point so that the basic design criterion, allowable ponded width, is not violated. For a given tentative inlet location, determine the ponded width to that point. Figure 10‑11 shows interdependence of inlet location, drainage area, discharge, and ponded width. If the calculated ponded width is greater than the allowable ponded width, you have two options:

  • Relocate the inlet at a point upstream in the curb and gutter section. This reduces the watershed area and, thus, the peak discharge. The lowered peak discharge causes a smaller ponded width. If this is done, the drainage area to the next downstream location is increased, thus increasing the discharge and ponding.
  • Locate an intermediate inlet at some point upstream in the curb and gutter section. This intermediate inlet defines a new watershed from which a reduced discharge flows, reducing the ponded width at the original inlet location.

Relation of Inlet Location to Design Discharge (click in image to see full-size image) Anchor: #i1003423grtop

Figure 10-11. Relation of Inlet Location to Design Discharge

If the calculated ponded width is less than or equal to the allowable ponded width, you must decide if it represents an efficient design. Compare the calculated ponded width to the allowable ponded width as a measure of efficiency. If you use all or most of the allowable ponded width, the location is probably efficient. If you use only a small portion of the allowable ponded width, a more efficient location may be possible. In extensive storm drain systems, it should be a design objective to minimize the number of inlets. You may do this effectively by using as much of the allowable ponded width as is possible.

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Carryover Design Approach

By using an on-grade inlet to intercept only a portion of the total flow in the gutter, you can make the inlet much more efficient than if all of the flow were to be intercepted. The rate of gutter flow not intercepted is called carryover. This design approach is recommended in those instances where it is not necessary to intercept all of the flow. The approach can be applied only in on-grade inlet configurations.

Figure 10‑12 illustrates (in profile) approximately what happens when the inlet is designed to intercept all of the approaching flow. Note the large portion of inlet opening that is not utilized efficiently.

Figure 10‑13 illustrates (in profile) approximately what happens when the inlet is designed to intercept less than all of the approaching flow. The remainder of the flow is the carryover. Note that the inlet opening is used much more efficiently for flow interception than the inlet illustrated in Figure 10‑12.

Inlet Designed with No Carryover (click in image to see full-size image) Anchor: #i1003433grtop

Figure 10-12. Inlet Designed with No Carryover

Inlet Designed with Carryover (click in image to see full-size image) Anchor: #i1003437grtop

Figure 10-13. Inlet Designed with Carryover

You must accommodate any carryover rates by ultimate interception at some other location (sometimes termed “bypass flow”). Furthermore, the gutter between the two points must accommodate the additional carryover rate. Carryover is not recommended upstream of intersections and driveways, at super-elevation transitions where the cross slope begins to reverse, and below entrance/exit ramps.

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Curb Inlets On-Grade

The design of on-grade curb opening inlets involves determination of length required for total flow interception, subjective decision about actual length to be provided, and determination of any resulting carryover rate.

For each on-grade inlet, determine early whether or not carryover is to be a valid design consideration. In some cases due to a logical location of the inlet, no carryover may be allowed. In other cases, while carryover is acceptable, there may not be a convenient location to accommodate the bypass flow.

Use the following procedure to design curb inlets on-grade:

  1. Compute depth of flow and ponded width (T) in the gutter section at the inlet.
  2. Determine the ratio of the width of flow in the depressed section (W) to the width of total gutter flow (T) using Equation 10-8. Figure 10‑14 shows the gutter cross section at an inlet.

    Equation 10-8.

    where:

    • E0 = ratio of depression flow to total flow
    • KW = conveyance of the depressed gutter section (cfs or m3/s)
    • K0 = conveyance of the gutter section beyond the depression (cfs or m3/s).

    Gutter Cross-Section Diagram (click in image to see full-size image) Anchor: #i1003481grtop

    Figure 10-14. Gutter Cross-Section Diagram

    Use Equation 10-9 to calculate conveyance, KW and K0.

    Equation 10-9.

    where:

    • K = conveyance of cross section (cfs or m3/s)
    • z = 1.486 for English measurements and 1.0 for metric
    • A = area of cross section (sq.ft. or m2)
    • n = Manning’s roughness coefficient
    • P = wetted perimeter (ft. or m).

    Use Equation 10-10 to calculate the area of cross section in the depressed gutter section.

    Equation 10-10.

    where:

    • AW = area of depressed gutter section (ft2 or m2)
    • W = depression width for an on-grade curb inlet (ft. or m)
    • SX = cross slope (ft./ft. or m/m)
    • T = calculated ponded width (ft. or m)
    • a = curb opening depression depth (ft. or m).

    Use Equation 10-11 to calculate the wetted perimeter in the depressed gutter section.

    Equation 10-11.

    where:

    • PW = wetted perimeter of depressed gutter section (ft or m)
    • W = depression width for an on-grade curb inlet (ft. or m)
    • SX = cross slope (ft./ft. or m/m)
    • a = curb opening depression depth (ft. or m).

    Use Equation 10-12 to calculate the area of cross section of the gutter section beyond the depression.

    Equation 10-12.

    where:

    • A0 = area of gutter/road section beyond the depression width (ft2 or m2)
    • SX = cross slope (ft./ft. or m/m)
    • W = depression width for an on-grade curb inlet (ft. or m)
    • T = calculated ponded width (ft. or m).

    Use Equation 10-13 to calculate the wetted perimeter of the gutter section beyond the depression.

    Equation 10-13.

    where:

    • P0 = wetted perimeter of the depressed gutter section (ft or m)
    • T = calculated ponded width (ft. or m)
    • W = depression width for an on-grade curb inlet (ft. or m).
  3. Use Equation 10-14 to determine the equivalent cross slope (Se) for a depressed curb opening inlet.

    Equation 10-14.

    where:

    • Se = equivalent cross slope (ft./ft. or m/m)
    • SX = cross slope of the road (ft./ft. or m/m)
    • a = gutter depression depth (ft. or m)
    • W = gutter depression width (ft. or m)
    • EO = ratio of depression flow to total flow.
  4. Calculate the length of curb inlet required for total interception using Equation 10-15.

    Equation 10-15.

    where:

    • Lr = length of curb inlet required (ft. or m)
    • z = 0.6 for English measurement and 0.82 for metric
    • Q = flow rate in gutter (cfs or m3/s)
    • S = longitudinal slope (ft./ft. or m/m)
    • n = Manning’s roughness coefficient
    • Se = equivalent cross slope (ft./ft. or m/m).

    If no carryover is allowed, the inlet length is assigned a nominal dimension of at least Lr. Use a nominal length available in standards for curb opening inlets. Do not use the exact value of Lr if doing so requires special details, special drawings and structural design, and costly and unfamiliar construction. If carryover is considered, round the curb opening inlet length down to the next available (nominal) standard curb opening length and compute the carryover flow.

  5. Determine carryover flow. In carryover computations, efficiency of flow interception varies with the ratio of actual length of curb opening inlet supplied (La) to length Lr and with the depression to depth of flow ratio. Use Equation 10-16 for determining carryover flow.

    Equation 10-16.

    where:

    • Qco = carryover discharge (cfs or m3/s)
    • Q = total discharge (cfs or m3/s)
    • La = design length of the curb opening inlet (ft. or m)
    • Lr = length of curb opening inlet required to intercept the total flow (ft. or m).

    Carryover rates usually should not exceed about 0.5 cfs (0.03 m3/s) or about 30% of the original discharge. Greater rates can be troublesome and cause a significant departure from the principles of the Rational Method application. In all cases, you must accommodate any carryover rate at some other specified point in the storm drain system.

  6. Calculate the intercepted flow. Calculate the intercepted flow as the original discharge in the approach curb and gutter minus the amount of carryover flow.
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Curb Inlets in Sag Configuration

The capacity of a curb inlet in a sag depends on the water depth at the curb opening and the height of the curb opening. The inlet operates as a weir to depths equal to the curb opening height and as an orifice at depths greater than 1.4 times the opening height. At depths between 1.0 and 1.4 times the opening height, flow is in a transition stage and the capacity should be based on the lesser of the computed weir and orifice capacity. Generally, for department design, this ratio should be less than 1.4 such that the inlet operates as a weir.

  1. If the depth of flow in the gutter (d) is less than or equal to 1.4 times the inlet opening height (h), (d<1.4H), determine the length of inlet required considering weir control. Otherwise, skip this step. Calculate the capacity of the inlet when operating under weir conditions with Equation 10-17.

    Equation 10-17.

    Rearrange Equation 10-17 to produce the following relation for curb inlet length required.

    Equation 10-18.

    where:

    • Q = total flow reaching inlet (cfs or m3/s)
    • Cw= weir coefficient (ft.0.5/s or m0.5/s)

      Suggested value = 2.3 ft.0.5/s or 1.27 m0.5/s. for depressed inlets

      Suggested value = 3.0 ft0.5/s or 1.60 m0.5/s without depression.

    • d = head at inlet opening (ft. or m), computed with Equation 10-1.
    • L = length of curb inlet opening (ft. or m)
    • W = gutter depression width (perpendicular to curb)

    If L > 12 ft. (3.6m), then W = 0 and Cw = 3.0 ft0.5/s or 1.60 m0.5/s.

  2. If the depth of flow in the gutter is greater than the inlet opening height (d > h), determine the length of inlet required considering orifice control. The equation for interception capacity of a curb opening operating as an orifice follows:

    Equation 10-19.

    where:

    • Q = total flow reaching inlet (cfs or m3/s)
    • Co = orifice coefficient = 0.67
    • h = depth of opening (ft. or m) (this depth will vary slightly with the inlet detail used)
    • L = length of curb opening inlet (ft. or m)
    • g = acceleration due to gravity = 32.2. ft./s2 or 9.81 m/s2
    • do = effective head at the centroid of the orifice (ft. or m).

    For curb inlets with an inclined throat such as Type C inlet, the effective head, do, is at the centroid of the orifice. This changes Equation 10-19 to:

    where:

    • Q = total flow reaching inlet (cfs or m3/s
    • Co = orifice coefficient = 0.67
    • h = depth of opening (ft. or m) (this depth will vary slightly with the inlet detail used)
    • L = Length of curb opening inlet (ft. or m)
    • g = acceleration due to gravity = 32.2 ft/s2 or 9.81 m/s2
    • y = depth of water in the curb and gutter cross section (ft. or m)
    • a = gutter depression depth (ft.).

    Rearranging Equation 10-19 allows a direct solution for required length.

    Equation 10-20.

  3. If both steps 1 and 2 were performed (i.e., h < d < 1.4h), choose the larger of the two computed lengths as being the required length.
  4. Select a standard inlet length that is greater than the required length.
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Slotted Drain Inlet Design

Use the following procedure for on-grade slotted drain inlets:

  1. Determine the length of slotted drain inlet required for interception of all of the water in the curb and gutter calculated by Equation 10-21.

    Equation 10-21.

    where:

    • Lr = length of slotted drain inlet required for total interception of flow (ft. or m)
    • z = 0.706 for English measurement or 1.04 for metric
    • Qa = total discharge (cfs or m3/s)
    • S = gutter longitudinal slope (ft./ft. or m/m)
    • E = function of S and Sx as determined by Equation
    • Sx = transverse slope (ft./ft. or m/m)
    • n = Manning’s roughness coefficient.

    Equation 10-21 is limited to the following ranges of variables:

    • total discharge 5.5 cfs (0.156 m3/s)
    • longitudinal gutter slope 0.09 ft./ft. (0.09 m/m)
    • roughness coefficient (n) in the curb and gutter: 0.011 n 0.017.

      Equation 10-22.

    • The longitudinal slope exponent (E) is determined with Equation 10-22:

      Because the equations are empirical, extrapolation is not recommended.

  2. Select the desired design slotted drain length (La) based on standard inlet sizes. If La < Lr the interception capacity may be estimated using Figure 10‑15, multiplying the resulting discharge ratios by the total discharge. Alternatively, the carryover for a slotted drain inlet length may be directly computed using Equation 10-23.

    Equation 10-23.

    where:

    • Qco = carryover discharge (cfs or m3/s)
    • Q = total discharge (cfs or m3/s)
    • La = design length of slotted drain inlet (ft. or m)
    • Lr = length of slotted drain inlet required to intercept the total flow (ft. or m).

Slotted Drain Inlet Interception Rate (click in image to see full-size image) Anchor: #i1003603grtop

Figure 10-15. Slotted Drain Inlet Interception Rate

As a rule of thumb, you can optimize slotted drain inlets’ economy by providing actual lengths (La) to required lengths (Lr) in an approximate ratio of about 0.65. This implies a usual design with carryover for on-grade slotted drain inlets.

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Grate Inlets On-Grade

The capacity of a grate inlet on-grade depends on its geometry and cross slope, longitudinal slope, total gutter flow, depth of flow, and pavement roughness.

The depth of water next to the curb is the major factor affecting the interception capacity of grate inlets. At low velocities, all of the water flowing in the section of gutter occupied by the grate, called frontal flow, is intercepted by grate inlets, and a small portion of the flow along the length of the grate, termed side flow, is intercepted. On steep slopes, only a portion of the frontal flow will be intercepted if the velocity is high or the grate is short and splash-over occurs. For grates less than 2 ft. (0.6 m) long, intercepted flow is small. Agencies and manufacturers of grates have investigated inlet interception capacity. For inlet efficiency data for various sizes and shapes of grates, refer to HEC-12.

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Bicycle Safety for Grate Inlets On-Grade

A parallel bar grate is the most efficient type of gutter inlet; however, when crossbars are added for bicycle safety, the efficiency is reduced. Where bicycle traffic is a design consideration, the curved vane grate and the tilt bar grate are recommended for both their hydraulic capacity and bicycle safety features. In certain locations where leaves may create constant maintenance problems, the parallel bar grate may be used more efficiently if bicycle traffic is prohibited.

Anchor: #LMFIFKLI

Design Procedure for Grate Inlets On-Grade

Use the following procedure for grate inlets on-grade:

  1. Compute the ponded width of flow (T). Use the outline provided in Section 4 ( Gutter Ponding Procedure for Continuous Grades).
  2. Choose a grate type and size.
  3. Find the ratio of frontal flow to total gutter flow (Eo) for a straight cross-slope using Equation 10-8. No depression is applied to a grate on-grade inlet.
  4. Find the ratio of frontal flow intercepted to total frontal flow, Rf, using Equation 10-24, Equation 10-25, and Equation 10-26.

    Equation 10-24.

    Equation 10-25.

    where:

    • Rf = ratio of frontal flow intercepted to total frontal flow
    • v = approach velocity of flow in gutter (ft./s or m/s)
    • vo = minimum velocity that will cause splash over grate (ft./s or m/s).

    For triangular sections, calculate the approach velocity of flow in gutter (v) using Equation 10-25.

    Equation 10-26.

    Otherwise, compute the section area of flow (A) and calculate the velocity using Equation 10-25:

    Equation 10-27.

    Calculate the minimum velocity (vo) that will cause splash over the grate using the appropriate equation in tables below.

    where:

    • vo = splash-over velocity (ft./s or m/s)
    • L = length of grate (ft. or m)
    Anchor: #i1010818Splash-Over Velocity Calculation Equations (English)

    Grate Configuration

    Typical Bar Spacing (in.)

    Splash-over Velocity Equation

    Parallel Bars

    2

    vo = 2.218 + 4.031L – 0.649L2 + 0.056L3

    Parallel Bars

    1.2

    vo = 1.762 + 3.117L – 0.451L2 + 0.033L3

    Transverse Curved Vane

    4.5

    vo = 1.381 + 2.78L - 0.300L2 + 0.020L3

    Transverse 45o Tilted Vane

    4

    vo = 0.988 + 2.625L – 0.359L2 + 0.029L3

    Parallel bars w/ transverse rods

    2 parallel/4 trans

    vo = 0.735 + 2.437L - 0.265L2 + 0.018L3

    Transverse 30o Tilted Vane

    4

    vo = 0.505 + 2.344L - 0.200L2 + 0.014L3

    Reticuline

    n/a

    vo = 0.030 + 2.278L - 0.179L2 + 0.010L3



    Anchor: #i1010854Splash-Over Velocity Calculation Equations (Metric)

    Grate Configuration

    Typical Bar Spacing (mm)

    Splash-over Velocity Equation

    Parallel Bars

    50

    vo = 0.676 + 4.031L - 2.13L2 + 0.598L3

    Parallel Bars

    30

    vo = 0.537 + 3.117L - 1.478L2 + 0.358L3

    Transverse Curved Vane

    115

    vo = 0.421 + 2.78L - 0.984L2 + 0.215L3

    Transverse 45o Tilted Vane

    100

    vo = 0.301 + 2.625L - 1.177L2 + 0.311L3

    Parallel bars w/ transverse rods

    50 parallel/100 trans

    vo = 0.224 + 2.437L - 0.869L2 + 0.192L3

    Transverse 30o Tilted Vane

    100

    vo = 0.154 + 2.344L - 0.656L2 + 0.155L3

    Reticuline

    n/a

    vo = 0.009 + 2.278L - 0.587L2 + 0.108L3



  5. Find the ratio of side flow intercepted to total side flow, Rs.

    Equation 10-28.

    where:

    • RS = ratio of side flow intercepted to total flow
    • z = 0.15 for English measurement or 0.083 for metric
    • Sx=transverse slope
    • v =approach velocity of flow in gutter (ft./s or m/s)
    • L = length of grate (ft. or m).
  6. Determine the efficiency of grate, Ef. Use Equation 10-29.

    Equation 10-29.

  7. Calculate the interception capacity of the grate, Qi. Use Equation 10-30. If the interception capacity is greater than the design discharge, skip step 8.

    Equation 10-30.

  8. Determine the carryover, CO. Use Equation 10-31.

    Equation 10-31.

  9. Depending on the carryover, select a larger or smaller inlet as needed. If the carryover is excessive, select a larger configuration of inlet and return to step 3. If the interception capacity far exceeds the design discharge, consider using a smaller inlet and return to step 3.
Anchor: #i1016888

Design Procedure for Grate Inlets in Sag Configurations

A grate inlet in sag configuration operates in weir flow at low ponding depths. A transition to orifice flow begins as the ponded depth increases. Use the following procedure for calculating the inlet capacity:

  1. Choose a grate of standard dimensions to use as a basis for calculations.
  2. Determine an allowable head (h) for the inlet location. This should be the lower of the curb height and the depth associated with the allowable ponded width. No gutter depression is applied at grate inlets.
  3. Determine the capacity of a grate inlet operating as a weir. Under weir conditions, the grate perimeter controls the capacity. Figure 10‑16 shows the perimeter length for a grate inlet located next to and away from a curb. The capacity of a grate inlet operating as a weir is determined using Equation 10-32.

    Equation 10-32.

    where:

    • Qw = weir capacity of grate (cfs or m3/s)
    • Cw = weir coefficient = 3 for English measurement or 1.66 for metric
    • P = perimeter of the grate (ft. or m) as shown in Figure 10‑16: A multiplier of about 0.5 is recommended to be applied to the measured perimeter as a safety factor.
    • h = allowable head on grate (ft. or m).

    Perimeter Length for Grate Inlet in Sag
Configuration (click in image to see full-size image) Anchor: #i1004597grtop

    Figure 10-16. Perimeter Length for Grate Inlet in Sag Configuration

  4. Determine the capacity of a grate inlet operating under orifice flow. Under orifice conditions, the grate area controls the capacity. The capacity of a grate inlet operating under orifice flow is computed with Equation 10-33.

    Equation 10-33.

    where:

    • Qo = orifice capacity of grate (cfs or m3/s)
    • Co = orifice flow coefficient = 0.67
    • A = clear opening area (sq. ft. or m2) of the grate (the total area available for flow). A multiplier of about 0.5 is recommended to be applied to the measured area as a safety factor
    • g = acceleration due to gravity (32.2 ft/s2 or 9.81 m/s2)
    • h = allowable head on grate (ft. or m).
  5. Compare the calculated capacities from steps 3 and 4 and choose the lower value as the design capacity. The design capacity of a grated inlet in a sag is based on the minimum flow calculated from weir and orifice conditions. Figure 10‑17 demonstrates the relationship between weir and orifice flow. If Qo is greater than Qw (to the left of the intersection in Figure 10‑17), then the designer would use the capacity calculated with the weir equation. If, however, Qo is less than Qw (to the right of the intersection), then the capacity as determined with the orifice equation would be used.

Relationship between Head and Capacity
for Weir and Orifice Flow (click in image to see full-size image) Anchor: #i1004605grtop

Figure 10-17. Relationship between Head and Capacity for Weir and Orifice Flow

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