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Section 2: Preliminary Concept Development

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Design Checklist

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  1. Identify the problem.
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  3. Develop a system plan.
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  5. Establish design criteria including:
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    1. design frequency (design AEP),
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    3. allowable ponded width,
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    5. allowable ponded depth,
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    7. pressure or non-pressure flow,
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    9. critical elevations,
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    11. suitable materials and conduit shapes,
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    13. minimum and maximum allowable velocity, and
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    15. minimum cover.
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  7. Determine outfall channel flow characteristics.
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  9. Identify and accommodate utility conflicts.
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  11. Consider the construction sequence and plan for temporary functioning.
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  13. Design the system, including determination of runoff, design of inlets and conduits, and analysis of the hydraulic grade line.
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  15. Check the final design, including check flood, and adjust if necessary.
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  17. Document the design.
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1. Problem Identification

As with any kind of project, the hydraulic designer 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, while optimizing safety and minimizing potential adverse impacts.

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2. System Plan

Preliminary or working drawings featuring the basic components of the intended design are invaluable in the development of a system plan. After design completion, the drawings facilitate documentation of the overall plan.

The following items should be included in the working drawings:

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

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3. Design Criteria

Design criteria includes such elements as design frequency (design AEP), allowable ponded width and allowable ponded depth, pressure or non-pressure flow, critical elevations, suitable materials and conduit shapes, minimum and maximum allowable velocity, and minimum cover.

a. The design AEP for a storm drain system design is based on 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), traffic demand, and a realistic assessment of available funds for the project. Chapter 4, Section 6 provides a discussion on design AEP and includes a table of recommended design AEPs.

b. The flow in the gutter should be restricted to a depth and corresponding width that will neither obstruct the roadway nor present a hazard to the motoring public at the design AEP. The depth and width of flow depend on the rate of flow, longitudinal gutter slope, transverse roadway slope, roughness characteristics of the gutter and pavement, and inlet spacing. Section 4, Ponding, provides more discussion on allowable ponded width and depth.

c. The standard practice of the Department is to design for a non-pressure flow network of collector conduits in most storm drain systems.

d. Typical critical elevations are at the throats of inlets and tops of manholes. Should the backwater (hydraulic grade line) within the system rise to a level above the curb and gutter grade, manhole, or any other critical elevation in a storm drain system, the system cannot perform as predicted by the calculations. Water will back out onto the roadway or runoff will be impeded from entering the system. The hydraulic designer must identify the critical elevations where these problems most likely will exist and compare the resultant hydraulic grade line to the system. The hydraulic grade line must not exceed the critical elevation for the design AEP at any point in the system.

e. The choice of material and component shape should be based on careful consideration of durability, hydraulic operation, structural requirements, and availability.Both the shape and material of storm drain system components influence the system hydraulic capacity. Some shapes are more hydraulically efficient than others. 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.

All possible storm drain materials should be considered 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. Because these characteristics may vary from site to site, a rule of thumb to use one material exclusive of all others may not be cost-effective.

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 directly affect the choice of shape. Refer to the Bridge Division for design considerations associated with durability. The roadway project's geotechnical report should be consulted for factors that affect material durability.

When choosing shape and material, the limitations of cover, headroom, and anticipated loading must be considered. Transportation costs are also important, as well as product availability in the geographic area of the project.

f. Flow velocities within a conduit network should be no less than 2 fps and no greater than about 12 fps. At velocities less than 2 fps, sediment deposit becomes a serious maintenance problem. When low velocities cannot be avoided, access for maintenance must be considered. At flow velocities greater than about 12 fps, the momentum of the flow can inflict a damaging impact on the components and joints within the system. When design velocities greater than 12 fps are necessary, countermeasures such as strengthened components and joints should be considered.

g. Both minimum and maximum cover limits for conduit must be considered. Minimum cover limits are established to ensure the conduit's structural stability under live and impact loads. For highway applications, a minimum cover depth of 3.0 ft should be maintained where possible in order to distribute live and impact loads. Where this criterion cannot be met, the conduit should be evaluated to determine if it is structurally capable of supporting the imposed loads.

With increasing fill heights, dead load becomes the controlling factor. Tables addressing maximum height of cover are available from the Bridge Division.

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4. Outfall Considerations and Features

The outfall of the storm drain system is a key component which impacts the physical and hydraulic characteristics of the system. The hydraulic designer should 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:

Whether the outfall is enclosed in a conduit or is an open channel, the hydraulic designer should assess its ability to convey design flows. The hydraulic designer should consider that the outfall may need to be modified to minimize any significant impact to the receiving channel or the surrounding property. Detention upstream of the outfall may be an option to channel modification.

An outfall for a Department storm drain system must be operated for the life of the system. This implies that the Department must have access to all parts of the outfall for purposes of maintenance to ensure adequate operation of the drainage system. In some instances an outfall right-of-way (drainage easement) must be purchased to assure accessibility and that the discharge from the outfall will not be restricted.

Special Outfall Appurtenances

Backflow preventers such as flap gates may be installed when necessary to prevent the outfall tailwater from backing into a storm drain system. However, backflow preventers are also maintenance intensive items which should be avoided if at all possible. The best solution is to design the storm drain system to prevent backflow from causing damage.

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5. Utility Conflicts

Hydraulic designers should minimize conflicts with existing utilities and potential conflicts with future utilities. During design, the order of consideration is as follows:

a. Carefully identify each utility and associated appurtenance that may be in conflict with any part of the storm drain system. Consider 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 may be encountered in an urban situation:

b. Where reasonable, avoid utility conflicts.

c. Where utility conflicts cannot be avoided, arrange for the relocation or adjustment of the utility.

d. 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 siphon. The utility company may be on state right-of-way under the agreement that the Department may request utility adjustments. However, as a general objective, attempt to minimize the disruption to utilities within reasonable and feasible design alternatives.

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6. Construction

The system must function, perhaps to a lesser extent, during the time of project construction. Storm drain lines should be built from downstream to upstream. For example, when inlets and laterals are built before the trunk line, the stormwater is trapped in the laterals. Therefore, it is important to consider construction sequencing during the design process.

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7. System Design

System design includes issues such as determination of runoff, design of inlets and conduits, and analysis of the hydraulic grade line. System design is discussed in detail in the remainder of this chapter.

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8. Check Flood

Once the final design is completed, the designer must review the design to assure that all the design criteria in Step 3 are still being met.

The intent of a check flood is to verify that the system will not experience problems for a frequency higher than the design AEP. Department policy is to use the 100-year frequency (1% AEP) as the check flood. This check is more than simply running the storm drain analysis with all the flow forced into the system; it is considering what will happen to excess flow which can't go into the inlets. Will it safely flow down the gutter to the outfall, or will it flow down a driveway and flood a structure? Are contingencies made for these possibilities? These are only a few of the circumstances which may be found on a project.

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9. Documentation Requirements

The storm drain documentation requirements are presented in Chapter 3, Section 5 of this manual.

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