Designing Surface Water Runoff Controls for Paved Surfaces

By Vanessa Hatcher, P.E., and Jim Noll, P.E.

Learning Objectives

After reading this article you should understand:

  • The principle issues for designing runoff interceptors using standard procedures for predicting runoff volumes and, subsequently;
  • Learn the basic procedures for selecting and positioning interceptors to meet specific needs.

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Most paved streets, parking areas, and some highways require runoff controls to protect adjacent properties, manage surface water accumulations, and prevent pavement degradation. Curbs and gutters with inlet structures connected to storm drains are prevalent on urban streets and large paved areas such as parking lots. This paper discusses the central issues for designing runoff interceptors (inlets) for most common applications.

Runoff control on urban streets

For all streets, water encroachment onto traffic lanes is a dominant concern. Designing the most suitable control systems on curbed roadways can involve many variables and alternative approaches. The Federal Highway Administration's (FHWA) Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22 (HEC 22), available online at, is one of the most comprehensive informational resources in this regard and is referenced frequently in this paper.

Figure 1

The principle goals of pavement water management are to minimize traffic interference and contain runoff within the roadway confines. Controlling runoff volumes along gutters via inlets helps achieve these goals. The spread of water extending from the curb (encroachment) relates to the types and spacing of inlets within the curb/gutter system. Figure 1 illustrates the most common types of inlet configurations; Figure 2 shows how they influence encroachment. Since typical curb and gutter inlets are usually not 100 percent efficient in terms of intercepting gutter flow, temporary water encroachment upon the surface can result. Common practice allows up to 35 percent bypass at higher flow levels.

The allowable pavement encroachment for the design storm event (e.g., 50-year or 100-year rainfalls) is the first criteria needed for designing curb/gutter interceptors. ASCE Manual of Practice No. 77, Design and Construction of Urban Stormwater Management Systems, provides guidelines, based on allowing no curb overtopping, for the following street classifications:

  • local streets — flow may spread to crown of street;
  • collector streets — flow spread must leave at least one lane free of water;
  • arterial streets — flow spread must leave at least one lane free of water in each direction; and
  • freeways — no encroachment allowed on any traffic lanes.

Curb and gutter interceptors

There are four basic types of inlets for intercepting surface runoff — curb, grate, combination (curb and grate), and special-purpose inlets such as slotted drains. Their specific hydraulic capacity primarily depends upon inlet geometry, gutter flow characteristics, and whether they are used in conjunction with a local inlet depression. According to Johns Hopkins University studies (1959) for inlets on 1-percent longitudinal slopes, a local depression at a gutter grate inlet can increase inlet capacity by a factor of 4.3, while a depression can increase curb-opening performance up to 6.4 times. However, the benefit of local depressions for both inlet types steadily diminishes as longitudinal grade increases until — at about 10 percent grade — "splash-over" results in virtually no benefits from the depression.

Clogging from trash and debris should be factored into the design of grated inlets. While difficult to predict, many municipalities have rule-of-thumb guidelines that reflect their experiences and local conditions.

Grate inlets in gutters

While inlet geometry is a key performance factor, the hydraulic capability of inlets is affected by other matters, such as pavement cross and longitudinal slope, gutter flow depth, pavement/gutter surface roughness, and gutter flow velocities. At low flow velocities, all water flowing in the gutter section occupied by the grate (frontal flow) will likely be intercepted, and a portion of the flow along the grate's length (side flow) also will be intercepted. A lesser portion of frontal flow is intercepted as gutter slope increases. The inlet width should always match the gutter width.

Higher flow velocities, shorter grate inlets, and grate bars for bicycle traffic and pedestrian safety also contribute to lower gutter flow interception rates. Tests conducted by various agencies — such as the U.S. Bureau of Reclamation — provide data accounting for these reductions under most working conditions, and the resulting interception capacities are presented in HEC 22, as well as in many state and local specifications.

Designing the best system for gutter flow control for a specific application is often a trial-and-error process. Although this paper presents an example design process, not all design matters can be covered in this paper. Designers are encouraged to reference other publications.

Gutter flow hydraulics and inlet spacing

Calculating gutter flow is the usual starting point for designing street runoff interceptors. These calculations relate the gutter flow volume to the allowable water spread onto the pavement. The usual parameters, in addition to those listed previously, include design storm intensity, roadway width, roadway longitudinal and cross slopes, street classification, pavement texture, and gutter shape and size. Several relatively familiar equations, used in conjunction with hydraulic data from FHWA HEC 22, are used to design curb and gutter interceptors.

Gutter flow hydraulics are governed by the following modified Manning's formula:

Equation 1:

Q = [K/n] Sx5/3 S1/2 T 8/3

In which:
Q = gutter flow rate, cubic feet per second, ft3/s (m3/s),
K = 0.56 (0.376 for SI units),
n = Manning's roughness coefficient for gutter surface (dimensionless),
Sx = pavement cross slope, ft/ft (m/m),
S = longitudinal slope, ft/ft (m/m), and
T = allowable width of flow or spread, ft (m).

From which, the depth of flow at the curb face can be calculated using:

Equation 2:

d = T Sx

In which:
d = depth of flow at curb face, ft (m)
T and Sx are as defined for Equation 1.

Equation 1 was derived for curb and gutter profiles where the curb face is vertical and the gutter surface is uniform (Figure 3). V-shaped gutters, or those with multiple cross slopes, have different hydraulic properties because of their different cross-sections. Resources such as FHWA HEC 22 provide hydraulic data for various gutter profiles.

The following Modified Rational Method predicts runoff volumes from pavements:

Equation 3:

Q = CiA, (0.28CiA)

In which:
Q = runoff, ft3/s (m3/s),
C = runoff coefficient (dimensionless, typically 0.7 to 0.9 for paved surfaces),
i = rainfall intensity, inches per hour, in./hr (mm/hr), and
A = drainage area, acres (km2).


The following example relates to a paved street having a runoff distance from crown to curb of 26 feet, a vertical 6-inch curb face, uniformly sloped gutter, 2-foot-square grate inlets with parallel bars, and a storm intensity of 10.5 in./hr. Design criteria are:

  • Allowable spread, T = 8 ft,
  • Pavement/gutter section cross slope, Sx = 0.03 ft/ft,
  • Pavement/gutter longitudinal slope, S = 0.03 ft/ft, and
  • Pavement/gutter roughness coefficient, n = 0.016.

Step 1 — Calculate the longitudinal flow volume in the area of the allowable spread. Using Equation 1, Q = 4.5 ft3/s. Note: if the gutter and pavement surfaces have different roughness coefficients, Q will be different for flow above each surface (reference FHWA HEC 22).

Step 2 — Determine the initial inlet spacing using Equation 3 and the results from Step 1:

Q = CiA = (0.80) (10.5 in./hr) (26 ft) (L/43,560) In which: L = inlet spacing, ft; and 43,560 = 1 acre-foot (ft3).

Q = 0.005L = 0.005 ft3/s/ft
L = Q/0.005 = 4.5 ft3/s/0.005 ft3/s/ft = 900 ft
(the maximum distance for the first inlet from where downslope gutter flow begins)

Step 3 — Determine the ratio of flow at the grate width (2 feet) to the total gutter flow using Chart 1, Figure 4. Note: Figure 4 charts show only data from HEC 22 that is relevant to this example.


For W/T = 2/8 = 0.25, and Sw /Sx = 1

In which:
Sw = cross slope of a depressed gutter if present, ft/ft (m/m).
EO = 0.55 (from Chart 1, Figure 4)

Step 4 — Determine flow velocity (V) in gutter/pavement within spread limit:

(Equation 4)
V = [1.12/n] S 0.5 Sx 0.067 T 0.67
V = [1.12/0.016] (0.030.5) (0.030.067) (80.67)
V = 4.7 ft/s

Step 5 — Determine the frontal flow interception efficiency (Rf) of the inlet (ratio of intercepted frontal flow to total side flow):

Rf = 1 (from Chart 2, Figure 4; 2 ft x 2 ft, Type P-1-7/8 grate)

Step 6 — Determine the side flow interception efficiency (RS) of the selected inlet (ratio of intercepted side flow to total side flow):
For Sx = 0.03, L = 2 ft, and V = 4.7 ft/s,
RS = 0.052 (from Chart 3, Figure 4)

Step 7 — Determine the total interception efficiency (E) of inlet:

(Equation 5)
E = Rf EO + RS (1-EO)
E = 1 (0.55) + 0.052 (1-0.55) = 0.57


Step 8 — Determine the portion of total gutter flow intercepted by the inlet (Qi):

(Equation 6)
Qi = EQ
Qi = (0.57) (4.5 ft3/s) = 2.56 ft3/s

Step 9 — Determine the bypass or carryover flow, Qb (gutter flow not intercepted by the inlet):

(Equation 7)
Qb = Q-Qi = 4.5 ft3/s - 2.56 ft3/s = 1.94 ft3/s

(Note: 44 percent of total gutter flow is not intercepted by the inlet)


Step 10 — Determine the maximum inlet spacing beyond the first inlet, L, using the inlet's interception capacity, Qi (2.56 ft3/s):

Q = 0.005L (from Step 2), and L = 2.56 ft3/s /0.005 ft3/s/ft = 520 feet

Step 11 — Calculate the depth of flow at the curb face:
From Equation 2, d = T Sx = (8 ft) (0.03) = 0.24 ft = 2.9 in. (< 6-in. curb height)

In the above example, 2-foot x 2-foot grate inlets can satisfactorily manage anticipated gutter flows when the first inlet is located 900 feet or less from the crest and subsequent inlets are spaced no more than 520 feet. If the 44-percent bypass flow is unacceptable, lengthening the inlet to 4 feet increases the total intercepted flow to about 2.9 ft3/s and reduces bypass flow to about 35.5 percent. To check this, begin at Step 6.

Curb-opening inlets

Curb-opening inlets are an option whenever grate inlets would extend into traffic lanes or present hazards to pedestrians or cyclists. These offer little interference to traffic operations and usually have less clogging tendencies. Their interception capacity mostly depends upon flow depth at the curb and the length of the opening. Like grate inlets, curb openings lose capacity with steeper longitudinal pavement slopes and flatter cross slopes, yet they can be more hydraulically effective than gutter inlets in certain cases. Curb inlets work best for low longitudinal gutter slopes, steeper cross slopes, and the presence of an inlet depression. Their water-inlet capacities, in the right application, can surpass that of traditional square or rectangular gutter inlets.

The length of curb opening required for total interception of longitudinal gutter flow is determined by:

Equation 8:

LT = KQ0.42 S 0.3 [1/(nSx)]0.6

In which:
LT = curb opening length, ft (m), for intercepting 100 percent of the gutter flow,
Q, S, n, and Sx are as defined for Equation 1, and
K = 0.6 (0.817 in SI units).

For shorter curb openings (L) than that required for total interception (LT), the relative interception efficiency must be determined:

E quation 9:

E =1-(1-L/LT)1.8

In which:
E = the interception efficiency of a curb inlet,
L = actual curb-opening length, ft (m), and
LT = curb-opening length from Equation 8.

For curb openings in depressed gutter sections, the inlet length needed for total interception is determined by developing an equivalent cross slope (Se) as a substitute for Sx in Equation 8:

Equation 10:

Se = Sx + (S 'w • EO)

In which:
Sx = cross slope of gutter measured from the cross slope of the pavement, ft/ft (m/m),
EO = ratio of flow in the depressed section to total gutter flow (same as that used to compute frontal flow interception of a grate inlet (Chart 1, Figure 4), and

(Equation 11)
S 'w = a/(12W)

Where: a = gutter depression, in. (mm) and W = the width of the inlet depression, ft (m).

For an assumed case in which Sx = 0.03, S = 0.035, Q = 5 ft3/s, L = 10 feet, and n = 0.016, the curb opening length (LT) for intercepting all gutter flow is 43 feet (from Equation 8), with an interception efficiency of only 0.38 (Equation 9) and an inlet capacity [(0.38) (5 ft3/s)] of 1.9 ft3/s. Adding an inlet depression can improve inlet efficiency, which improves interception capacity.

Slotted drain inlets for streets

Special purpose inlets known as slotted drains, line drains, or trench drains are alternatives to grated gutter inlets and curb openings. These systems, consisting of either a continuous upright inlet (slotted drains) or horizontal grate opening (trench drains), typically provide continuous longitudinal inlets combined with a drainage component. Their most common street installations are within curbs/gutters as slot-on-grade flow interceptors, in curb/gutter sag or low points to intercept carryover from preceding slots on a grade, and to intercept surface runoff sloped toward the gutter.

Figure 5 illustrates the basic configurations of such interceptor-drains. "Trench" or "line" drains typically have a U-shaped, preformed drainage channel with bar or grate-type surface inlets. Available in various shapes and sizes from different manufacturers, each brand generally has different overall hydraulic properties.

Slotted Drains are fabricated from corrugated steel pipe with a longitudinal opening to which is attached an upright grate with spacers. The 1.75-inch-wide inlet is continuous along the pipe length (typically 10 or 20 feet). Vertical slots extending 2-1/2 inches or 6 inches above the pipe meet various site requirements.

Functionally, a slot inlet collects runoff and channels it to the drainage pipe (usually 12-inch through 36-inch diameter). The basic inlet configuration has parallel sides and vertical spacers as shown in Figure 6. An optional trapezoidal-shaped inlet with angled cross plates is designed for better hydraulic efficiency when flow is in the direction of the slanted spacers.

Slotted inlets in gutters perform like curb-opening inlets, behaving as weirs with flow entering from the side. FHWA tests show that the interception capacity of a slotted inlet can be determined in the same manner as curb openings using Equation 8. Similarly, Equations 9 and 10 are also applicable to slotted inlets.

In an earlier example, in which a 43-foot inlet was needed to intercept 100 percent of runoff, the construction of a conventional curb opening of such length would likely be cost-prohibitive. However, since long, slotted drain-type inlets are commonplace, interception and containment of all runoff is possible at reasonable cost with this type of special purpose inlet.

Grate Inlets for Non-Street Pavements

For paved parking lots and similar areas, the usual objectives are to contain all runoff on-site so it will not affect adjacent walks and properties while also ensuring pedestrian safety and accessibility. Grate inlets fitted to catch basins are often used on large pavements for this purpose. When fitted to curbs and gutters within the drainable area, the design process is the same as for streets. However, when used without curbs, inlets become an integral part of an overall drainage scheme that includes careful grading and surface contouring so that water is directed to the inlets.

In these applications, it is usually necessary to enhance grate inlet interception capabilities by creating sumps (depressed areas). Considerable attention should be given to the potential traffic and pedestrian hazards created by placing depressed inlets within parking lot surfaces. Grate inlets placed within sumps function as weirs only to certain depths (usually 0.4 feet maximum) and as orifices at depths over 1.4 feet. The inlet performance of inlets at intermediate depths is difficult to predict accurately.

Generally, inflow capacity of a depressed inlet serving as a weir can be calculated by:

Equation 12:

Q = Cw P d 3/2 )

In which:
Q = inlet capacity, ft3/s (m3/s),
Cw = 3.0 (1.66 in SI units),
P = perimeter of the grate opening, ft (m), and
d = depth of flow not above 0.40 ft (m).

The inflow capacity of a depressed inlet serving as an orifice can be calculated by:

Equation 13

Q = Co A (2gh)1/2

In which:
Q = inlet capacity, ft3/s (m3/s),
Co = 0.6 (approximate orifice coefficient for 2-ft x 2-ft inlet),
A = total opening area, ft2 (m2),
g = acceleration due to gravity, 32.16 ft/s2 (9.80 m/s2), and
h = head above the center of the orifice, not less than 1.4 ft (m).

If the grate inlet is flush mounted, a minimum grate length parallel to the flow direction is needed for complete interception. The required length is determined by:

Equation 14

L = 0.5 V(t+d)1/2
In which:
L = minimum length of clear opening, ft,
V = average velocity of the approach water, ft/s,
d = approaching water depth, ft, and
t = grate thickness, ft.

In SI units:
(Equation 14a)
L = 0.91 V(t+d)1/2

Slotted drain inlets for non-street pavements

Slotted drain systems are alternatives to grated inlets to fully intercept sheet flow. Normally, they do not require curbs and are flush mounted so that surface water flows perpendicular to the long opening. The design of runoff control systems is relatively straightforward and less complicated than with grated inlets. For instance, they eliminate most — if not all — of the surface contouring needed for grate inlets.

Once the anticipated runoff volumes are calculated (Equation 3), the required inlet capacities and corresponding lengths can be determined using procedures outlined earlier for inlets functioning as weirs. In many cases, however, lengths are set by site features such as driveway widths.


The design of runoff management systems has been intensely studied for more than 50 years, perhaps mostly because of the water problems created by the construction of larger homes and streets and the emergence of big shopping malls. Drainage plans for runoff control are required for gaining approval of most proposed developments since the consequences of inadequate management can be catastrophic.

Vanessa Hatcher , P.E., is a civil engineer for CONTECH Construction Products Inc., specializing in hydraulics. She has more than nine years experience in water resources design and construction.

Jim Noll, P.E., is manager of engineering services for CONTECH. He has 30 years experience in the corrugated metal pipe industry and is an active member of various technical organizations including ASCE, AREMA, and ASTM.