Vol I - Chapter 13 Storm Drainage Systems AASHTO pp

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CHAPTER 13

STORM DRAINAGE SYSTEMS

FINAL DRAFT
Storm Drainage Systems 13-i

1 CHAPTER 13 – TABLE OF CONTENTS
2 Section Page
3
4 13.1 OVERVIEW ......................................................................................................................... 13-1
5
6 13.1.1 Introduction .......................................................................................................... 13-1
7 13.1.2 Inadequate Drainage ............................................................................................. 13-1
8 13.1.3 General Design Guidelines ................................................................................... 13-2
9 13.1.4 Detention Storage ................................................................................................. 13-2
10
11 13.2 GENERAL CONSIDERATIONS ........................................................................................ 13-2
12
13 13.2.1 Introduction .......................................................................................................... 13-2
14 13.2.2 Hydrology ............................................................................................................. 13-3
15 13.2.3 Bridge Decks ........................................................................................................ 13-3
16 13.2.4 Inlets ..................................................................................................................... 13-3
17 13.2.5 Storm Drains ......................................................................................................... 13-4
18 13.2.6 Roadside Channels and Ditches ........................................................................... 13-4
19 13.2.7 Flood Hazard ........................................................................................................ 13-4
20 13.2.8 Property Development Drainage Policy ............................................................... 13-5
21 13.2.9 Hydroplaning ........................................................................................................ 13-5
22 13.2.10 Access Holes......................................................................................................... 13-5
23
24 13.2.10.1 Location ............................................................................................ 13-5
25 13.2.10.2 Spacing.............................................................................................. 13-6
26
27 13.3 DESIGN FREQUENCY AND SPREAD ............................................................................. 13-6
28
29 13.3.1 Selection Considerations ...................................................................................... 13-6
30 13.3.2 Design Frequency ................................................................................................. 13-7
31 13.3.3 Review Flood Frequency ...................................................................................... 13-7
32 13.3.4 Allowable Water Spread ....................................................................................... 13-7
33
34 13.4 ROADWAY GEOMETRICS ............................................................................................... 13-7
35
36 13.4.1 Introduction .......................................................................................................... 13-7
37 13.4.2 Roadway Cross Section ........................................................................................ 13-8
38
39 13.4.2.1 Width................................................................................................. 13-8
40 13.4.2.2 Cross Slope ....................................................................................... 13-8
41
42 13.4.3 Vertical Alignment ............................................................................................... 13-9
43
44 13.4.3.1 Longitudinal Slope ............................................................................ 13-9
45 13.4.3.2 Sag Vertical Curves .......................................................................... 13-9
46 13.4.3.3 Crest Vertical Curves ........................................................................ 13-9
13-ii AASHTO Drainage Manual, Volume One “Policy”

1
2 13.4.4 Pavement Texture ................................................................................................. 13-9
3 13.4.5 Curb and Gutter .................................................................................................... 13-10
4 13.4.6 Medians ................................................................................................................ 13-10
5
6 13.5 REFERENCES ..................................................................................................................... 13-11
7
Storm Drainage Systems 13-iii

1 List of Tables

2 Table Page
3
4 Table 13-1. Access Hole Spacing ............................................................................................................ 13-6
5 Table 13-2. Allowable Water Spread for Roadways ............................................................................... 13-7
6
Storm Drainage Systems 13-1

1
2 Chapter 13
3 STORM DRAINAGE SYSTEMS

4 13.1 OVERVIEW

5 13.1.1 Introduction

6 This chapter provides policy guidance on all elements of storm drainage design: system planning,
7 pavement drainage, gutter flow calculations, inlet spacing, pipe sizing and hydraulic grade line
8 calculations, maintenance, and control of runoff from future development. The quality of the final
9 constructed system usually reflects the attention given to every aspect of the design and that accorded to
10 the construction and maintenance of the facility. Suggested policy and guidelines are given to encourage
11 user agencies to develop their own policies and design criteria.

12 The design of a drainage system should address the needs of the traveling public and those of the local
13 community through which it passes. The drainage design for a roadway traversing an urbanized region is
14 more complex than for roadways traversing sparsely settled rural areas. This is often due to

15  wide roadway sections, flat or steep grades (both in longitudinal and transverse directions),
16 shallow water courses, and absence of side or outfall channels;
17  more costly property damages that may occur from ponding of water or from flow of water
18 through built up areas;
19  the roadway section must carry traffic, but also act as a channel to convey the water to a disposal
20 point. Unless proper precautions are taken, this flow of water may interfere with or possibly halt
21 the passage of highway traffic;
22  uncontrolled runoff from developed property and the potential increase in runoff due to future
23 development making design more difficult;
24  coordination with the master drainage plans of cities and counties that add to the difficulty of
25 design to provide compatibility and the timing of the construction of drainage system
26 components;

27  funding and cost sharing agreement with cities and counties to construct storm drainage systems;
28 and
29  compliance with the city drainage and floodplain ordinances that add to the difficulty of design.

30 13.1.2 Inadequate Drainage

31 The most serious effects of inadequate roadway drainage systems are
13-2 AASHTO Drainage Manual, Volume One “Policy”

1  damage to adjacent property resulting from water overflowing the roadway curb and entering
2 such property,
3  risk and delay to traffic caused by excessive ponding in sags or excessive spread along the
4 roadway, and
5  weakening of roadway base and subgrade due to saturation from frequent ponding of long
6 duration.

7 13.1.3 General Design Guidelines

8 A storm drain is defined as that portion of the storm drainage system that receives runoff from inlets and
9 conveys the runoff to some point where it is then discharged into a channel, water body, or piped system.
10 A storm drain may be a closed-conduit, open-conduit or some combination of the two. They may be
11 designed with consideration for future development, if appropriate. A higher design frequency (or return
12 interval) should be used for storm drain systems located in a major sag vertical curve to decrease the
13 depth of ponding on the roadway and bridges and potential inundation of adjacent property. Where
14 feasible, the storm drains should be designed to avoid existing utilities. Attention should be provided to
15 the storm drain outfalls to ensure that the potential for erosion is minimized. Drainage system design
16 should be coordinated with the proposed staging of large construction projects to maintain an outlet
17 throughout the construction project.

18 This chapter discusses design guidelines for storm drainage design and analysis, which are based on
19 HEC-22 (3). For additional guidance, refer to the AASHTO Highway Drainage Guidelines, Chapter 9
20 (1).

21 13.1.4 Detention Storage

22 The reduction of peak flows can be achieved by the storage of runoff in detention basins, storm drainage
23 pipes, swales and channels, and other detention storage facilities. These should be considered where
24 existing downstream conveyance facilities are inadequate to accommodate peak-flow rates from highway
25 storm drainage facilities. In many locations, agencies or developers, or both, are not permitted to increase
26 runoff when compared to existing conditions, thus necessitating detention storage facilities. Additional
27 benefits may include the reduction of downstream pipe sizes and the improvement of water quality by
28 removing sediment and/or pollutants. See Volume One, Chapter 14 “Storage” for a discussion on
29 detention storage.

30 13.2 GENERAL CONSIDERATIONS

31 13.2.1 Introduction

32 Highway storm drainage facilities collect stormwater runoff and convey it through the roadway right-of-
33 way in a manner that adequately drains the roadway and minimizes the potential for flooding and erosion
34 to properties adjacent to the right-of-way. Storm drainage facilities consist of curbs, gutters, storm drains,
35 channels, and culverts. The placement and hydraulic capacities of storm drainage facilities should be
Storm Drainage Systems 13-3

1 designed to consider the potential for damage to adjacent property and to secure a degree of risk of traffic
2 interruption by flooding as is consistent with the importance of the road, the design traffic service
3 requirements, and available funds. Following is a summary of policies that should be followed for storm
4 drain design and analysis.

5 13.2.2 Hydrology

6 The Rational Method is the most common method in use for the design of storm drains when the
7 momentary peak-flow rate is desired. Its use should be limited to systems with drainage areas of 200 acres
8 or less. A minimum time of concentration of seven minutes is generally acceptable. Drainage systems
9 involving detention storage, pumping stations, and large or complex storm systems require the
10 development of a runoff hydrograph. The Rational Method and hydrograph methods are described in
11 Volume One, Chapter 9 “Hydrology.”

12 13.2.3 Bridge Decks

13 Many short bridges may not require any drainage facilities at all. Longer and wider bridge decks may
14 require drainage facilities to provide adequate traffic passage for the desired level of service. Bridge
15 designs with sags, flat, or zero gradients should be avoided, otherwise an extensive drainage system will
16 be required. The level of service for the design is based on the roadway classification and ADT.
17 Limitation of street spread is the primary factor in bride deck design. The street spread is to be limited to
18 that shown in the design criteria and procedure portion Volume Two, Chapter 17 “Bridges.”

19 13.2.4 Inlets

20 The term “inlets” refers to all types of inlets (e.g., grate inlets, curb inlets, slotted inlets). Drainage inlets
21 are sized and located to limit the spread of water on traffic lanes to tolerable widths for the design storm
22 in accordance with the design criteria specified in Section 13.3.4. The width of water spread on the
23 pavement at sags should not be substantially greater than the width of spread encountered on continuous
24 grades.

25 Grate inlets and depression of curb opening inlets should be located outside the through traffic lanes to
26 minimize the shifting of vehicles attempting to avoid them. All grate inlets shall be bicycle safe where
27 used on roadways that allow bicycle travel. Curb inlets are preferred to grate inlets at major sag locations
28 because of their debris handling capabilities. When grate inlets are used at sag locations, assume that they
29 are half plugged with debris and size accordingly.

30 In locations where significant ponding may occur (e.g., underpasses, sag vertical curves in depressed
31 sections), recommended practice is to place flanking inlets on each side of the inlet at the low point in the
32 sag.
13-4 AASHTO Drainage Manual, Volume One “Policy”

1 13.2.5 Storm Drains

2 A storm drain is defined as that portion of the storm drain that receives runoff from inlets and conveys the
3 runoff to some point where it is discharged into a channel, waterbody, or piped system. It consists of one
4 or more pipes connecting two or more inlets. A storm drain may be a closed-conduit, open-conduit, or
5 some combination of the two. Strom drains should have adequate capacity to accommodate runoff that
6 enters the system. They should be designed with future development or extension in mind if it is
7 appropriate. The storm drain for a major vertical sag curve should have a higher level of flood protection
8 to decrease the depth of ponding on the roadway bridges. Where feasible, the storm drains should be
9 designed to avoid existing utilities.

10 Attention should be given to the storm drain outfalls to ensure that the potential for erosion is minimized.
11 Drainage system design should be coordinated with the proposed staging of large construction projects in
12 order to maintain an outlet throughout the construction project.

13 The placement and capacities should be consistent with local stormwater management plans. A minimum
14 velocity of 3 ft/s is desirable in the storm drain in order to prevent sedimentation from occurring in the
15 pipe.

16 The trunk line and lateral are to be designed to convey runoff intercepted by the inlets. Surcharging may
17 be allowed if accounted for in the design analysis. A hydraulic gradeline analysis, including minor and
18 major losses, should be performed for systems having large potential for junction losses.

19 13.2.6 Roadside Channels and Ditches

20 Large amounts of runoff should be intercepted before it reaches the highway in order to minimize the
21 deposition of sediment or debris on the roadway and to reduce the amount of water which must be carried
22 in the gutter section. Slope median areas and inside shoulders to a center depression to prevent runoff
23 from the median area from running across the pavement. Surface channels should have adequate capacity
24 for the design runoff and should be located and shaped in a manner that does not present a traffic hazard.
25 Where permitted by the design velocities, channels should have a vegetative lining. Appropriate linings
26 may be necessary where vegetation will not control erosion, see Volume One, Chapter 10 "Channels".
27 Right-of-way restrictions/costs in urban areas often render impracticable the provision of roadside
28 ditches.

29 13.2.7 Flood Hazard

30 The storm drain system design must be reviewed to assess the potential flood hazard or for use in design
31 of the major storm drainage. The flood hazard to adjacent properties upstream and downstream must be
32 assessed. The increase in runoff due to impervious pavement may be an issue. The flood hazard for the
33 lower storm frequencies should be considered as well. The coincidental occurrence of flooding of the
34 receiving waters should be considered.
Storm Drainage Systems 13-5

1 13.2.8 Property Development Drainage Policy

2 Drainage from potential future development should be considered. It is difficult to project the amount of
3 potential future development that may affect highway drainage systems. Agency regulations often require
4 developers to limit runoff for various levels of storm frequencies. Most drainage regulations require a
5 flood-hazard assessment for the 100-yr storm frequency review and design to limit property damage and
6 impact to human life. The major storm drainage system should be designed for the 100-yr storm event.
7 Detention ponds are often utilized to control runoff to acceptable conditions. Increase in outfall channel or
8 pipe capacity is another means to control increase in runoff due to development. Developers must provide
9 drainage design plans, analysis, and flood hazard assessment.

10 13.2.9 Hydroplaning

11 Hydroplaning conditions can develop for relatively low vehicular speeds and at low rainfall intensities for
12 storms that frequently occur each year (4). Analysis methods developed through this research effort
13 provide guidance in identifying potential hydroplaning conditions. Unfortunately, it is virtually
14 impossible to prevent water from exceeding a depth that would be identified through this analysis
15 procedure as a potential hydroplaning condition for wide pavements during high-intensity rainfall and
16 under some relationship of the primary controlling factors of:

17  vehicular speed;
18  tire conditions (pressure and tire tread);
19  pavement micro and macrotexture;
20  roadway geometrics (pavement width, cross slope, grade); and
21  pavement conditions (rutting, depressions, roughness).

22 Speed appears as a significant factor in the occurrence of hydroplaning; therefore, it is considered to be
23 the driver’s responsibility to exercise prudence and caution when driving during wet conditions (1). In
24 many respects, hydroplaning conditions are analogous to ice or snow on the roadway.

25 Designers do not have control over all factors involved in hydroplaning. However, remedial measures can
26 be included in development of a project to reduce hydroplaning potential, see Design Guidelines for
27 Reducing Hydroplaning on New and Rehabilitated Pavements (5).

28 If suitable measures cannot be implemented to address an area of high potential for hydroplaning, or an
29 identified existing problem area, consideration should be given to installing advance warning signs.

30 13.2.10 Access Holes

31 13.2.10.1 Location

32 Access holes are used to provide entry to continuous underground storm drains for inspection and
33 cleanout. Some highway agencies use grate inlets in lieu of access holes when entry to the system can be
13-6 AASHTO Drainage Manual, Volume One “Policy”

1 provided at the grate inlet, so that the benefit of extra stormwater interception can be achieved with
2 minimal additional cost. Typical locations where access holes should be specified are:

3  where two or more storm drains converge,
4  at intermediate points along tangent sections,
5  where pipe size changes,
6  where an abrupt change in alignment occurs, and
7  where an abrupt change of the grade occurs.

8 Access holes should not be located in traffic lanes; however, where it is impossible to avoid locating an
9 access hole in a traffic lane, care should be taken to ensure that it is not in the normal vehicular wheel
10 path.

11 13.2.10.2 Spacing

12 The spacing of access holes should be in accordance with Table 13-1.

13 Table 13-1. Access Hole Spacing

Size of Pipe (in.) Maximum Distance (ft)
12–24 300
27–36 400
42–54 500
60 1000

14 13.3 DESIGN FREQUENCY AND SPREAD

15 13.3.1 Selection Considerations

16 The major considerations for selecting a design frequency and spread are type of highway and highway
17 speed. The public does not expect to find water on the pavement surface of Interstate and control access
18 highways. Highway speed is another major consideration, because at speeds greater than 45 mph, even a
19 shallow depth of water on the pavement can cause hydroplaning (Section 13.2.9). Design speed is
20 recommended for use in evaluating hydroplaning potential.

21 Ponding should be minimized on the traffic lanes of high-speed, high-volume highways (Interstates)
22 where it is not expected. It is clearly unreasonable and not cost effective to provide the same level of
23 protection for low-speed facilities as for high-speed facilities.

24 Other considerations include inconvenience, hazards and nuisances to pedestrian traffic and buildings
25 adjacent to roadways that are located within the splash zone. These considerations should not be
26 minimized and, in some locations (e.g., commercial areas), may assume major importance.
Storm Drainage Systems 13-7

1 13.3.2 Design Frequency

2 The design storm frequency for pavement drainage is the normally the10-yr return period for surface
3 drainage. Other components of the storm drain system may use other frequencies. For example, a 10-yr
4 return period may be selected to limit spread on grade and a 50-yr return period may be used at a sag
5 location to design the storm drain or pumping system. The following applies to storm drainage systems:

6  If a storm drain provides the outlet for a cross drain, then the design frequency of the cross drain
7 should be used for the storm drainage system downstream from the cross drain inlet.
8  If local drainage facilities and practices have provided storm drains of lesser standard, to which
9 the highway system should connect, provide special consideration to whether it is realistic to
10 design the highway system to a higher standard than available outlets.
11  For major sag points on Interstate, United States and State highways, the design frequency should
12 be 50 years where water can pond 2 ft deep or more on the travel lane and where projected 2-way
13 ADT is greater than 5000.

14 13.3.3 Review Flood Frequency

15 The review flood frequency is typically the 100-yr return period.

16 13.3.4 Allowable Water Spread

17 In general, the water spread for the design storm frequency should be held to the allowable width shown
18 in Table 13-2. For storms of greater magnitude, the spread can be allowed to utilize “most” of the
19 pavement as an open channel. For multi-laned curb and gutter, or guttered roadways with no parking, it is
20 not practical to avoid travel-lane flooding when longitudinal grades are flat (0.2 percent to 1 percent).
21 However, flooding should not exceed the lane adjacent to the gutter (or shoulder) for design conditions.
22 Municipal bridges with curb and gutter should also use this criterion. For single-lane roadways, at least
23 8 ft of roadway should remain unflooded for design conditions.

24 Table 13-2. Allowable Water Spread for Roadways

Type of Facility Allowable Water Spread

Interstate Edge of traveled way

United States and State Highways, Local Roads, Ramps Greater of 8 ft or shoulder width

25 13.4 ROADWAY GEOMETRICS

26 13.4.1 Introduction

27 This Section discusses the role of roadway geometrics on pavement drainage applicable to the hydraulic
28 design of storm drainage systems. Where applicable, the discussion extracts information from or
13-8 AASHTO Drainage Manual, Volume One “Policy”

1 references the AASHTO Green Book (2). This Section does not discuss the following pavement drainage
2 considerations:

3  bridge decks (see Volume One, Chapter 17 “Bridges”);
4  roadside channels (see Volume One, Chapter 8 “Channels”); and
5  fill slopes, see AASHTO Green Book (2).

6 Roadway geometric features that impact gutter, inlet and pavement drainage for storm drainage systems
7 include:

8  roadway width and cross slope (Section 13.4.2),
9  vertical alignment (Section 13.4.3),
10  pavement texture (Section13.4.4),
11  curb and gutter sections (Section 13.4.5), and
12  presence of median barriers (Section 13.4.6)

13 The pavement width, cross slope, profile and pavement texture control the time it takes for stormwater to
14 drain to the gutter section. The gutter cross section and longitudinal slope control the quantity of flow that
15 can be carried in the gutter section. Each of these is discussed in the following Sections.

16 13.4.2 Roadway Cross Section

17 13.4.2.1 Width

18 In general, the wider the roadway width (i.e., traveled way plus shoulder/curb offset width), the greater
19 the quantity of water that can be accommodated by the curb and gutter storm drainage system.

20 13.4.2.2 Cross Slope

21 The design of pavement cross slope is a compromise between the need for reasonably steep cross slopes
22 for drainage and relatively flat cross slopes for driver comfort. The AASHTO Green Book (2) reports that
23 cross slopes of 2 percent have little effect on driver effort in steering, especially with power steering or on
24 friction demand for vehicular stability. Use of a cross slope steeper than 2 percent on pavements with a
25 central crown line is not desirable. In areas of intense rainfall, a somewhat steeper cross slope may be
26 necessary to facilitate drainage. In such areas, the cross slope may be increased to 2.5 percent.

27 When three or more lanes are inclined in the same direction on multi-lane pavements, it is desirable that
28 each successive pair of lanes, or the portion thereof outward from the first two lanes from the crown line,
29 have an increased slope. The two lanes adjacent to the crown line should be pitched at the normal slope
30 and successive lane pairs, or portions thereof outward, should be increased by approximately 0.5 percent
31 to 1 percent. Where three or more lanes are provided in each direction, the maximum pavement cross
32 slope should be limited to 4 percent.
Storm Drainage Systems 13-9

1 It is desirable to provide a break in cross slope at two lanes, with three lanes being the upper limit.
2 Although not widely encouraged, inside lanes can be sloped toward the median. This should not be used
3 unless four continuous lanes or some physical constraint on the roadway elevation occurs, because inside
4 lanes are used for high-speed traffic and the allowable water depth is lower. Median areas should not be
5 drained across traveled lanes. A careful check should be made of designs to minimize the number and
6 length of flat pavement sections in cross slope transition areas, and consideration should be given to
7 increasing cross slopes in sag vertical curves and crest vertical curves, and in sections of flat longitudinal
8 grades. Where curbs are used, depressed gutter sections can be effective at increasing gutter capacity and
9 reducing spread on the pavement.

10 13.4.3 Vertical Alignment

11 13.4.3.1 Longitudinal Slope

12 A minimum longitudinal gradient is more important for a curbed pavement than for an uncurbed
13 pavement because curbs limit the spread of stormwater. Flat gradients on uncurbed pavements can also
14 lead to a spread problem if vegetation is allowed to build up along the pavement edge.

15 Desirable gutter grades should be greater than 0.5 percent for curbed pavements with a minimum of 0.3
16 percent. Minimum grades can be maintained in very flat terrain by use of a rolling profile.

17 13.4.3.2 Sag Vertical Curves

18 On curbed facilities, sag vertical curves should be sufficiently "sharp" to prevent in adequate drainage
19 near the bottom of the vertical curve. This can be achieved by designing the sag vertical curve to provide
20 a minimum longitudinal slope of 0.3 percent at the two points 50 ft from the bottom. This yields a
21 maximum value of K = 167 for the vertical curve, which is typically called the drainage maximum.

22 13.4.3.3 Crest Vertical Curves

23 Drainage considerations are not as critical on crest vertical curves as sag vertical curves. However, good
24 design practice is to design crest vertical curves based on a maximum K = 167.

25 13.4.4 Pavement Texture

26 The pavement texture is an important consideration for roadway surface drainage. Although the
27 hydraulics engineer will have little control over the selection of the pavement type or texture, it is
28 important to know that pavement texture does have an impact on the buildup of water depth on the
29 pavement during rain storms. Macrotexture provides a channel for water to escape from the tire/pavement
30 interface and, thus, reduces the potential for hydroplaning.

31 A high level of macrotexture may be achieved by tinning new portland cement concrete pavements while
32 it is still in the plastic state. Re-texturing of an existing portland cement concrete surface can be
33 accomplished through pavement grooving and cold milling. Both longitudinal and transverse grooving are
34 very effective in achieving macrotexture in concrete pavement. Transverse grooving aids in surface runoff
13-10 AASHTO Drainage Manual, Volume One “Policy”

1 resulting in less wet pavement time. Combinations of longitudinal and transverse grooving provide the
2 most adequate drainage for high-speed conditions.

3 13.4.5 Curb and Gutter

4 Curbing at the outside edge of pavements is normal practice for low-speed, urban highway facilities.
5 Curbs serve several purposes:

6  containing the surface runoff within the roadway and away from adjacent properties,
7  preventing erosion,
8  providing pavement delineation, and
9  enabling the orderly development of property adjacent to the roadway.

10 Curbs may be either barrier or mountable type, and they are typically portland cement concrete, although
11 bituminous curb is used occasionally. Barrier curbs range in height from 6 in. to 10 in. with a batter of 1
12 in. per 3 in. of height. Mountable curbs are less than 6 in. in height and have rounded or plane-sloping
13 faces. Gutters are available in 1 ft through 3 ft widths.

14 A curb and gutter forms a triangular channel that can be an efficient hydraulic conveyance facility that
15 can convey runoff of a lesser magnitude than the design flow without interruption of the traffic. When a
16 design storm flow occurs, there is a spread or widening of the conveyed water surface. The water spreads
17 to include not only the gutter width, but also parking lanes or shoulders and portions of the traveled
18 surface. This is the width the hydraulics engineer is most concerned with in curb and gutter flow, and
19 limiting this width becomes a very important design criterion. Section 13.3.4 discusses the allowable
20 water spread.

21 Where practicable, it is desirable to intercept runoff from cut slopes and other areas draining toward the
22 roadway, before it reaches the highway, to minimize the deposition of sediment and other debris on the
23 roadway and to reduce the amount of water that must be carried in the gutter section. See Section 13.4.5.

24 13.4.6 Medians

25 Medians are commonly used to separate opposing lanes of traffic on divided highways. It is preferable to
26 slope median areas and inside shoulders to a center depression to prevent drainage from the median area
27 from running across the traveled pavement. The following applies to surface drainage considerations on
28 facilities with medians that are not depressed:

29 1. Flush Medians. Flush medians consist of a relatively flat paved area separating the traffic lanes
30 with only painted stripes on the pavement. Flush medians should be either slightly crowned to
31 avoid ponding of water in the median area or slightly depressed (with median drains) to avoid
32 carrying all surface drainage across the travel lanes.
33 2. Curbed Medians. Curbed, raised medians are most commonly used on lower-speed urban
34 arterials. The roadway is typically crowned to transport a portion of the pavement drainage to the
Storm Drainage Systems 13-11

1 outside and a portion to the median, which then requires a collection and conveyance system for
2 the median drainage.
3 3. Median Barriers. With narrow medians on high-speed facilities (e.g., Interstates), a median
4 barrier may be used to prevent out-of-control vehicles from crossing into opposing traffic lanes.
5 When median barriers are used, it is necessary to provide inlets, especially on horizontal curves
6 with superelevation, and connecting storm drains to collect the water that accumulates against the
7 barrier.
8 13.5 REFERENCES

9 (1) AASHTO. Storm Drain Systems. Chapter 9 in Highway Drainage Guidelines, 4th Edition,
10 American Association of State Highway and Transportation Officials, Washington DC, 2007.

11 (2) AASHTO. A Policy on Geometric Design of Highways and Streets, 5th Edition. Task Force on
12 Geometric Design. American Association of State Highway and Transportation Officials,
13 Washington, DC, 2004.

14 (3) FHWA. Urban Drainage Design Manual. Hydraulic Engineering Circular No. 22, Third Edition,
15 FHWA-NHI-10-009. Federal Highway Administration, U.S. Department of Transportation,
16 Washington, DC, 2009.

17 (4) National Cooperative Highway Research Program Web Doc 16 Improved Surface Drainage of
18 Pavements: Final Report. NCHRP, Transportation Research Board, Washington, DC, 1998.

19 (5) National Cooperative Highway Research Program, Research Results Digest, Issue Number 243:
20 Proposed Design Guidelines for Reducing Hydroplaning on New and Rehabilitated Pavements.
21 NCHRP, Transportation Research Board, Washington, DC, 1999.