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«CHAPTER 13 STORM DRAINS TABLE OF CONTENTS 13.1 OVERVIEW 13.1.1 Introduction 13.1.2 Symbols And Definitions 13.1.3 Concept Definitions 13.2 GENERAL ...»

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CDOT Drainage Design Manual Storm Drains

CHAPTER 13

STORM DRAINS

TABLE OF CONTENTS

13.1 OVERVIEW

13.1.1 Introduction

13.1.2 Symbols And Definitions

13.1.3 Concept Definitions

13.2 GENERAL DESIGN CRITERIA

13.2.1 Introduction

13.2.2 Hydrology

13.2.3 Design Frequency And Spread Width

13.2.4 Inlet Spacing

13.3 PAVEMENT DRAINAGE

13.3.1 Introduction

13.3.2 Major Storm And Street Capacity

13.3.3 Longitudinal Slope

13.3.4 Cross Slope

13.3.5 Hydroplaning / Pavement Texture

13.3.6 Curb and Gutter

13.3.7 Concrete Median Barrier

13.4 INLETS

13.4.1 General

13.4.2 Types

13.4.3 Inlet Criteria and Design

13.4.4 Inlet Capacities

13.4.5 Inlet Debris

13.4.6 Maintenance Access (Manhole) Locations

13.4.7 Maintenance Access Spacing

13.4.8 Curved Alignment

13.5 STORM DRAINS

13.5.1 Introduction

13.5.2 Trunkline Criteria And Design

13.5.3 Trunkline Velocity

13.5.4 Hydraulic Design of Pipes

13.5.5 Special Considerations

13.6 PRELIMINARY SYSTEM PLANNING AND DESIGN

13-1 CDOT Drainage Design Manual Storm Drains 13.6.1 Basic Data

13.6.2 Preliminary Design

13.7 FINAL SYSTEM DESIGN

13.7.1 Hydraulic Gradeline Procedure

13.8 REFERENCES

13.9 Appendix A Design Nomographs

–  –  –

13.1 OVERVIEW 13.1.1 Introduction The primary aim of storm drain design is to limit the amount of water flowing along the gutters, or ponding at the sags, to quantities which will not interfere with the passage of traffic for a common design storm. The storm drain system consists of surface inlets structures connected to a underground pipe system. The inlets are located at points and at such intervals to intercept flows and control the water's spread width into the traveled lane.

Photo 13.1 Storm drain facilities should provide enough combined capacity in the storm drain and the street typical to convey the major storm runoff through the roadway right-of-way in a manner which adequately drains the roadway and minimizes the potential for flooding and erosion to properties adjacent to the right-of-way.

Storm drain design is one of the more cumbersome and difficult problems encountered in highway drainage. The complex network of inlets and conduits requires extensive evaluation to provide an efficient, balanced system.

Storm Drains should be designed by the CDOT Hydraulics Engineer or experienced drainage consultant.

Hydraulic design of projects contracted to consultants should be reviewed and approved by the CDOT Hydraulics Engineer.

The most serious effects of an inadequate roadway drainage system are:

• damaging adjacent property from the water overtopping the curb and gutter;

13-3 CDOT Drainage Design Manual Storm Drains

• risk and delays to the driving public caused by excessive ponding in sags or excessive spread along the roadway;

• deterioration of the pavement structure and subgrade due to saturation caused by frequent and long duration ponding; and

• creating hydroplaning conditions for motorists.

–  –  –

To provide consistency within this Chapter and throughout this Manual, the symbols in Table 13.1 will be used. These symbols were selected because of their wide use in storm drainage publications.

13.1.3 Concept Definitions Following are discussions of concepts that will be important in a storm drainage analysis and design.

These concepts will be used throughout the remainder of this Chapter in addressing different aspects of

storm drainage analysis:

Bypass Flow: Flow which bypasses an inlet on grade and is carried in the street or channel to the next inlet downgrade.

Check Storm: The use of a less frequent event (e.g., a 50-yr storm) to assess hazards at critical locations where water can pond to appreciable depths is commonly referred to as a check storm or check event.

Combination Inlet: A drainage inlet usually composed of a curb-opening inlet and a grate inlet.

Crown: The crown, sometimes known as the soffit, is the top inside of a pipe.

Culvert: A culvert is a closed conduit whose purpose is to convey surface water under a roadway, railroad or other impediment. It may have one or two inlets connected to it to convey drainage from the median area.

Curb-Opening: A drainage inlet consisting of an opening in the roadway curb. (ex. Type R inlet) Drop Inlet: A drainage inlet with a horizontal or nearly horizontal opening.

Equivalent Cross Slope: An imaginary straight cross slope having conveyance capacity equal to that of the given compound cross slope.

Flanking Inlets: Inlets placed upstream and on either side of an inlet at the low point in a sag vertical curve. These inlets intercept debris as the slope decreases and act in relief of the inlet at the low point.

Flow: Flow refers to a quantity of water that is flowing.

Frontal Flow: The portion of the flow that passes over the upstream side of a grate.





Grate Inlet: A drainage inlet composed of a grate in the roadway section or at the roadside in a low point, swale or channel. (ex. Type C or D inlet) Grate Perimeter: The sum of the lengths of all sides of a grate, except that any side adjacent to a curb is not considered a part of the perimeter in weir-flow computations.

Gutter: That portion of the roadway section adjacent to the curb which is utilized to convey storm water runoff. It may include a portion, or all, of a traveled lane, shoulder or parking lane, and a limited width, adjacent to the curb, may be of different materials and have a different cross slope.

Hydraulic Grade Line The hydraulic grade line is the locus of elevations to which the water would rise in successive piezometer tubes if the tubes were installed along a pipe run (pressure head plus elevation head).

Inlet Efficiency The ratio of flow intercepted by an inlet to total flow in the gutter.

Invert The invert is the inside bottom of the pipe.

Lateral Line A lateral line, sometimes referred to as a lead, has inlets connected to it but has no other storm drains connected. It is usually 2 ft or less in diameter and is tributary to the trunk line.

Lateral: The underground conduit that connects the inlet to the main trunkline of a storm drain.

13-5 CDOT Drainage Design Manual Storm Drains Major Storm: The 50 to 100-year runoff to be assessed for with a storm drain design for minimum ponding depth and property inundation.

Minor Storm: The common storm that is used for designing the inlet size and location, the trunkline size and the spread width.

Panline: The lowest point in the curb and gutter section.

Pressure Head: Pressure head is the height of a column of water that would exert a unit pressure equal to the pressure of the water.

Runby/Bypass: Carryover flow that bypasses an inlet on grade and is carried in the street or channel to the next inlet downgrade. Inlets can be designed to allow a certain amount of runby for one design storm and larger or smaller amounts for other storms.

Sag Point/Major Sag Point: A low point in a vertical curve. A major sag point refers to a low point that can overflow only if water can pond to a depth of 2 ft or more.

Scupper: A vertical hole through a bridge deck for deck drainage. Sometimes, a horizontal opening in the curb or barrier is called a scupper.

Side-Flow Interception: Flow that is intercepted along the side of a grate inlet, as opposed to frontal interception.

Slotted Drain Inlet: A drainage inlet composed of a continuous slot built into the top of a pipe that serves to intercept, collect and transport the flow. Two types in general use are the vertical riser and the vane type.

Storm Drain: A storm drain is a closed or open conduit that conveys stormwater that has been collected by inlets to an adequate outfall. It generally consists of laterals or leads and trunk lines or mains. Culverts connected to the storm drainage system are considered part of the system.

Splash-Over: Portion of frontal flow at a grate that skips or splashes over the grate and is not intercepted.

Spread: The width of stormwater flow in the gutter or roadway measured laterally from the roadway curb.

Trunk Line: A trunk line is the main storm drain line. Lateral lines may be connected at inlet structures or access holes. A trunk line is sometimes referred to as a “main.” Trunkline: The underground pipe portion of a storm drain system. Major conveyance element into which the smaller pipes or laterals drain into from the storm drain inlets.

Velocity Head: Velocity head is a quantity proportional to the kinetic energy of flowing water expressed as a height or head of water (V2/2g).

–  –  –

13.2 GENERAL DESIGN CRITERIA 13.2.1 Introduction Highway storm drainage facilities collect stormwater runoff and convey it through the roadway right-ofway in a manner that adequately drains the roadway and minimizes the potential for flooding and erosion to properties adjacent to the right-of-way. Storm drainage facilities consist of curbs, gutters, storm drains, channels and culverts. The placement and hydraulic capacities of storm drainage facilities should be designed to consider the potential for damage to adjacent property and to secure as low a degree of risk of traffic interruption by flooding as is consistent with the importance of the road, the design traffic service requirements and available funds.

Storm drain systems have two separate drainage systems. One is the minor drainage system to handle the ordinary recurring storm (2 to 10-year storm events). The other is the major system to handle the large infrequent storm flows (100-year storm event). The minor system consists of underground piping that is connected to inlets draining roadway or offsite areas. The major system includes street flow, urban storm channels and other overflow provisions to pass the infrequent, large flows without excessive ponding or property damage.

Following is a summary of policies that should be followed for storm drain design and analysis. For a general discussion of policies and guidelines for storm drainage, the designer is referred to Reference (1).

For more specific design and engineering guidance refer to the AASHTO "Drainage Manual", the Federal Highway Administration publication, HEC 21 and HEC 22. and the Denver Regional Council of Governments, "Urban Drainage and Flood Control District - Critenia Manual."

–  –  –

13.2.2 Hydrology The Rational Method is the suggested procedure to compute the peak flows for storm drain systems with drainage areas less than 160 acres. It is the method that applies to the vast majority of small watersheds that are to be handled by storm drains. For more information on the Rational Method and other hydrological methods refer to Chapter 7 - Hydrology.

Estimated peak flows will be based on the existing runoff conditions and an allowance for the reasonably foreseeable future developments and conditions. The future flow patterns and basin sizes should be based on present topographic conditions if specific plans for developed modifications are unknown.

13.2.3 Design Frequency And Spread Width The major consideration for selecting a design frequency and spread width is the highways classification, because it defines and reflects public expectations for finding water on the pavement surface.

Ponding should be avoided on the traffic lanes of high-speed, high-volume highways, where it is not expected to occur.

Highway speed is another major consideration, because at speeds greater than 45mph, even a shallow depth of water on the pavement can cause hydroplaning and safety problems to motorists.

Design speed is recommended for use in evaluating hydroplaning potential. When the design speed is selected, consideration should be given to the likelihood that legal posted speeds might be exceeded. It is clearly unreasonable to provide the same level of protection for low speed facilities as for high speed facilities.

For curb and guttered roadways with no parking, it is not practical to avoid all travel lane flooding when longitudinal grades are flat (0.3 to one percent). However, flow spread width shall never exceed the lane adjacent to the gutter for design conditions. Municipal bridges with curb and gutter should also use this criterion. For single lane roadways at least 8 ft of roadway shall remain unflooded for design conditions.

Storm drain systems are normally designed for full gravity flow conditions using the design frequency discharges.

The exceptions are depressed roadways and underpasses where ponded water can be removed only through pumps via the storm drain systems. In these situations, a larger design frequency is advisable for the inlets at the sag location and for sizing the main storm drain line.

Table 3.2 presents the Design Frequency vs.

Spread Width.

13.2.4 Inlet Spacing The time of concentration (tc) for inlet spacing is the time for water to flow from the hydraulically most distant point of the drainage area to the inlet, which is known as the inlet time. Usually this is the sum of the time required for water to move across the pavement or overland back of the curb to the gutter, plus the time required for flow to move through the length of gutter to the inlet. For pavement drainage, when the total time of concentration to the upstream inlet is less than 5 min, a minimum tc of 5 min should be used to estimate the intensity of rainfall. The time of concentration for the second downstream inlet and each succeeding inlet should be determined independently, the same as the first inlet. For a constant roadway grade and relatively uniform contributing drainage area, the time of concentration for each succeeding inlet could also be constant.

–  –  –

Note: These criteria applies to shoulder widths of 4 ft or greater. Where shoulder widths are less than 4 ft, a minimum design spread of 4 ft should be considered.

–  –  –



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