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Section 2: System Planning and Design Considerations

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Design Checklist

  • Identify the problem.
  • Develop a system plan.
  • Establish suitable materials and conduit shapes.
  • Establish design criteria.
  • Determine outfall channel flow characteristics.
  • Identify and accommodate utility conflicts.
  • Consider the construction sequence and plan for temporary functioning.
  • Recognize other drainage facilities, and accommodate them.
  • Determine runoff.
  • Design inlets.
  • Design conduit.
  • Develop a hydraulic grade line analysis.
  • Check the final design, and adjust if necessary.
  • Document the design.
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Problem Identification

As with any kind of project, you must first clearly define the problem that the proposed design is going to address. For storm drain design, the goal is to provide adequate drainage for a proposed roadway, optimizing safety and minimizing potential adverse impacts.

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Preliminary or working schematics featuring the basic components of the intended design are invaluable in the design development. After design completion, the schematic facilitates documentation of the overall plan.

You may include the following items in the working schematic:

  • a general layout
  • basic hydrologic data
  • pertinent physical features
  • characteristics of flow diversion (if applicable)
  • detention features (if applicable)
  • outfall location and characteristics
  • surface features (topography)
  • utilities
  • tentative component placement.

The final drainage design schematic should include the existing physical features of the project area and indicate the location and type of the following:

  • streets
  • driveways
  • parking lots
  • bridges
  • adjacent areas indicating land use, such as undeveloped land, commercial land, industrial land, agricultural land, residential land, and park land.
  • detention facilities
  • pump stations
  • drainage channels
  • drainage diversions
  • off-site watershed boundaries.
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Material and Shape Selection

Consider all possible storm drain materials with regard to the local environment of the system site. The durability of a drainage facility depends on the characteristics of soil, water, and air. These characteristics may vary from site to site. It is not cost-effective to declare a rule of thumb that the storm drain system should be of one material exclusive of all others.

Base the choice of material and shape on careful consideration of durability, hydraulic operation, structural requirements, and availability.

Durability of drainage facilities is a function of abrasion and corrosion. Except in some mountainous areas of the state, abrasion is not a serious problem. As a rule, durability does not affect the choice of shape directly. Refer to the Conduit Durability section of Chapter 14 for discussions and design considerations associated with durability. You can usually consult the roadway project’s geotechnical report for factors that affect material durability.

The selection of both shape and material for storm drain system components influences the hydraulic capacity. Conduit roughness characteristics vary with conduit material; thus, the hydraulic capacity varies with the material type. For example, reinforced concrete pipe justifies a Manning’s n-value of 0.012 while conventional corrugated metal pipe requires the use of an n-value of 0.024 or greater.

When choosing both shape and material, consider cover limitations, headroom, and anticipated loading.

Choose materials, shapes, and components that require minimum transportation costs and that are readily available in the geographic region of the project. Items commonly manufactured in standard sizes include prefabricated pipe, inlets, and manholes.

Deviation from standard sized structures is rarely cost-effective. The pipe industry maintains current standard catalogs of nominal fabrication dimensions. Refer to fabricators’ catalogs for current lists of generically available sizes and shapes.

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Design Criteria

The design frequency is an indication of the level of flooding accommodated by the system without causing an undesirable impact to pavement, structures, traffic, and adjacent facilities and property.

Base the design frequency for a storm drain system design on the following:

  • the general nature of the system and the area it is to serve
  • the importance of the system and associated roadway
  • the function of the roadway
  • the traffic type (emergency/non-emergency) and demand
  • a realistic assessment of available funds for the project.

Chapter 5 provides a discussion on design frequency and includes a table of recommended design frequencies.

The allowable ponded width may vary within a single system. For example, an allowable ponded width of one lane of flooding on main lanes and one and one-half lanes for frontage roads may be acceptable. An allowable ponded width is the basis for locating points on the roadway surface at which runoff must be removed. Base the determination of allowable ponded width on such factors as width of roadway, number of lanes, and level of service desired during design frequency.

You may use the following recommended ponded widths with consideration for site specific parameters and limitations:

  • Limit ponding to one-half the width of the outer lane for the main lanes of interstate and controlled access highways.
  • Limit ponding to the width of the outer lane for major highways, which are highways with two or more lanes in each direction, and frontage roads.
  • Limit ponding to a width and depth that will allow the safe passage of one lane of traffic for minor highways.

The usual TxDOT practice is to design for a non-pressure flow network of collector conduits in most storm drain systems.

Critical elevations are used as comparative values to the key elevations on a developed hydraulic grade line. (See Chapter 6 for more information.) As a rule, a surface water removal system is designed to operate with no impedance or interruption of free fall into the system. Therefore, the system does not perform as predicted by the calculations if the backwater (hydraulic grade line) within the system rises to a level above a curb and gutter grade, a manhole, or any other critical elevation in a storm drain system. Water will either back out on the roadway or runoff will be impeded from entering the system as planned. You need to identify the critical elevations where these problems most likely will exist and compare the resultant hydraulic grade line. Typical critical elevations would be located at the throats of inlets and tops of manholes. For the design frequency, the hydraulic grade line should not exceed the critical elevation.

The usual preference is that flow velocities within the conduit network be no less than 2 fps (0.6 m/s) and no greater than about 12 fps (3.6 m/s). At velocities less than 2 fps (0.6 m/s), sediment deposit becomes a serious maintenance problem. Such slow velocities also indicate an inefficient drainage system. At flow velocities greater than about 12 fps (3.6 m/s), structural damage to the system components becomes a threat. The momentum of flow at higher velocities can cause a damaging impact on the structural components and connections within the system. There may be instances when design velocities outside the range of 2fps and 12 fps (0.6 m/s and 3.6 m/s) are necessary. If so, countermeasures such as greater access for maintenance or strengthened components may be in order.

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Outfall Considerations and Features

The outfall of the storm drain system is a key component, and you must coordinate with the demands of the physical and hydraulic characteristics of the system. Consider the requirements and characteristics of the area in which the outfall facility is located. Important considerations in the identification of an appropriate system outfall include the following:

  • the availability of the channel and associated right-of-way or easement
  • the profile of the existing or proposed channel or conduit
  • the flow characteristics under flood conditions
  • the land use and soil type through the area of the channel.

Whether the outfall is enclosed in a conduit or is an open channel, you should assess its ability to convey design flows. If necessary, modify the outfall to ensure minimizing the potential for significant impact.

An outfall for a TxDOT storm drain system must be operated for the life of the system. This implies that TxDOT must have access to all parts of the outfall for purposes of maintenance and to ensure adequate operation of the drainage system. If the outfall is by easement through private property, assure continuing TxDOT access to the outfall within that easement. In many instances, it is necessary to purchase an outfall right-of-way (drainage easement) so that continuing access by the TxDOT is assured.

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Special Outfall Appurtenances

When separate storm drain systems intersect, a bubble chamber may be useful to provide a means of connecting the systems. You can design the bubble chamber so that as the water level (hydraulic grade line) in one system rises to a certain level, flow in another system serves as a relief drainage facility.

If the outfall to be used by a storm drain system is permanently or temporarily inadequate to accommodate the flow from the system, you may need to install some type of flow restrictor. The flow restrictor must include a space for runoff detention, allowing a reduced runoff rate to exit into the inadequate outfall.

Flap gates are provided when an outfall might cause the storm drain system to back up. A flap gate allows flow out during lower outfall levels and prevents backflow when the water level is higher. For example, if the storm drain system is to outfall into a tidal basin in which the periodic fluctuation of tides represents a variation of possible outfall water levels, you may need to provide a flap gate at the end of the last downstream run of the system.

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Utility Conflicts

Direct consideration and planning toward minimizing conflicts with existing utilities and potential conflicts with future utilities. During design, the order of considerations is as follows:

  1. Carefully identify each utility and associated appurtenances that may be in conflict with any part of the storm drain system. Consider in the design any utility that intersects, conflicts, or otherwise affects or is affected by the storm drain system. Determine the horizontal and vertical alignments of underground utilities to properly accommodate potential conflicts. The following are typical utilities that you may encounter in an urban situation:
    • electrical
    • telephone or television transmission lines
    • water lines
    • wastewater lines
    • gas lines
    • irrigation ditches
    • high-pressure fuel facilities
    • communication transmission facilities.
  2. Where reasonable, relocate components of the storm drain system to avoid utility conflict.
  3. When relocation of the storm drain is not feasible, arrange for the relocation or adjustment of the utility. The entity responsible for the utility is usually cooperative in such cases.
  4. Make accommodations to the utility when adjustments are not feasible due to economics or other conditions. For example, it may be unreasonable to relocate a high-pressure gas line. In such a case, design an intersection of the unadjusted utility appurtenance and the subject component of the storm drain system. This may involve passing the utility through the storm drain component (e.g., through a junction box) or installing a syphon. The utility company may be on state right-of-way under the agreement that TxDOT may request utility adjustments. However, as a general objective, attempt to minimize the disruption to utilities.
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The construction sequence of the various storm drains can have a major influence on the design. The need to comply with the National Pollutant Discharge Elimination System (NPDES) General Permit for construction activities has increased the importance of proper sequencing.

The system must function, perhaps to a lesser extent, during the time of project construction. It must function adequately (but probably not optimally) both with the rest of the storm drain system and other project aspects. For example, it is usually recommended that storm drain lines be built from downstream to upstream in order to prevent “trapping” storm water during construction. Phase the storm drain system construction to accommodate the following:

  • sequences of roadway construction
  • traffic control
  • cut and fill operations
  • utility construction
  • structural operations.
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Identification of Other Drainage Facilities

You should attempt to identify any existing or proposed facilities that your proposed system is likely to affect or which may affect your proposed system. Examples include the following:

  • regional or local storm water detention facilities
  • proposed or recent changes to adjacent highway facilities
  • municipal master drainage plans
  • other major development.
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Design Documentation

Design documentation needs for the development of a storm drain design include the following:

  • watershed data
  • estimates of future development of watersheds
  • channel flow characteristics in outfall
  • logical inlet locations
  • curb and gutter slopes
  • transverse slopes
  • inlet calculations
  • times of concentration to each location (node)
  • rainfall intensity calculations
  • depth of flow and ponded width of curb/gutter flow
  • inlet sizing calculations
  • carryover rates
  • conduit slopes
  • conduit sizing calculations
  • conduit run travel times
  • critical elevations
  • hydraulic grade line elevations.
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Documentation Requirements

The design is not complete until the following are documented:

  • criteria
  • design parameters
  • considerations
  • calculations.

The documentation serves several important purposes including:

  • justification of the design
  • reference for review and checking
  • reference for potential field changes and future modifications
  • potential defense against litigation.

The Storm Drain Documentation Check List presents required documentation for storm drain systems.

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