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 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, , 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 3 fps and no greater than about 12 fps. At velocities less than 3 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.