Urban Storm Drain Design: Typical urban roadway characteristics

Roadway crown• Roadways always exhibit slope of some type

in the transverse direction• It may be curved (parabolic or circular), or

it may be made of straight segments (rooftop crown)

• 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 as you move 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 of that, and an auxiliary lane near the curb. This auxiliary lane may vary between parking/bicycle and designated right-turn

Typical roadway section

Curbs• Curbs are the usual roadway bounding feature

in urban areas. They may vary in height from negligible (in cases where a roadway has been repeatedly overlaid), and 8”.

• Curbs serve multiple purposes; they provide minor redirection for errant vehicles, as well as serving as a bounding feature for water running in the roadway as an open channel.

• Curbs may also 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.

• While not a primary drainage feature, sidewalks influence drainage features by the need to meet ADA standards for cross slope, ramps, and access. This is often 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 (as you might suspect), the width of the roadway covered by ponded water. What remains unponded is considered usable roadway;

• The portion with water ponded on it, while it may still be technically passable, is considered to be under a hazard to traffic.

• In the design process, each side of the roadway must be considered separately with respect to ponding.

Manning’s Eq & Izzard’s form• Manning’s equation

• Applied in a triangular section with following assumptions:• Width is substantially greater than depth – hydraulic radius well approximated by depth

€

Q =1.49

nAR2 / 3S1/ 2

Manning’s Eq & Izzard’s form• Break into two parts:

€

Q =1.49

n(d

4

zd

2)(d

4)

2

3 S1

2 +1.49

n(3d

4

zd

2)(

3d

4)

2

3 S1

2 =0.534

nzd

8

3S1/ 2

Manning’s Eq & Izzard’s form• Izzard’s form actually integrates the

section (many small divisions added together)

€

Q =1.49

n(x

z)

5

3 S1

2dxx= 0

x= zd

∫ =0.558

nzd

8

3S1/ 2

Manning’s Eq & Izzard’s form• About 85% of the discharge is in the deepest

half (closest to the curb) of the section.

€

Q =1.49

n(x

z)

5

3 S1

2dxx= 0

x= zd

∫ =0.558

nzd

8

3S1/ 2

Typical roadway section

Flow in curb & gutter

Ponded width vs. depth in gutter• Ponded width is a function of depth of flow

in the gutter by way of the transverse slope (or slopes).

• The steeper the transverse slope, the smaller the ponded width.

Increase in contributing area• Typically, area adjacent to the roadway

contributes to the roadway in such a way that it can be approximated as a uniform, distributed manner.

• As you proceed 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.

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).

Increase in ponded width

Flow accumulation

Longitudinal profile grade• The longitudinal profile of the roadway is

very critical to the performance of roadway drainage features. It should never be finalized without considering drainage.

• If too flat, water will stagnate and pond.

• If too steep, water is difficult to get into inlets.

Longitudinal profile grade• Occasionally may need to be undulated to

accommodate good drainage.

Crest vertical curves• Crest vertical curves that involve a change

in sign of the roadway profile grade always involve a region of effectively zero slope.

• These areas are larger for larger “K” value of the vertical curve.

• Crest vertical curves that do not involve a change in sign of the profile grade will result in an increase in velocity and decrease in ponded width.

Straight grades• Straight grade sections are assumed to reach

steady-state, non-uniform flow.• The non-uniformity is because of increasing

contributing area with distance down the roadway.

• This often resembles an idealized rectangular watershed

Sag vertical curves• Sag vertical curves always involve

diminishing slope, increasing depth and ponded width.

• Inlets in sags perform differently than those on grade

• Sag inlets must be placed in the low point of the sag!

• It is usually necessary to provide inlets on grade prior to the sag to prevent excessive ponding from small grades as the low point of the sag is approached.