scope:

INTRODUCTION: The highway facility should be designed to be compatible with existing drainage patterns. Concurrently, the highway and the traveling public should be protected from the hazards of flooding. In this regard, the drainage design for the highway facility has a threefold purpose:

□ prevent adverse interference to the existing drainage,

□ protect the traveled roadway surface from certain flood events, and

□ provide for the removal of water from the roadway subgrade and the embankment.

Storm drain system design should recognize that floodwater reaches the highway right-of-way in three main ways:

□ precipitation falling within the right-of-way;

□ stormwater that flows onto the right-of-way as surface or overland flow;

□ stormwater that comes to the highway in a defined watercourse. Water in a defined watercourse is usually conveyed as cross-drainage. In such cases, a primary design objective is to pass stormwater through the highway embankment without adverse effects to the highway or to the property on either side. Guidance for the design of cross-drainage facilities is contained in Chapters 4 and 7 (4, 7)1 and HDS 5 (39).

Although stormwater approaching the highway in a defined watercourse may be accommodated in a storm drain system, the primary concern of this chapter is floodwater from rain falling within the right-of-way and from overland flow. Guidance is given on how storm drain systems can be designed to collect, convey and discharge stormwater flowing within and along the highway right-of-way. This chapter covers most types of highway drainage excluding cross-drainage as defined above and most types of highway drainage facilities excluding culverts and bridges.

Storm drain systems include elements such as:

□ pavement drainage,

□ inlets,

□ storm drains,

□ roadside drainage channels,

□ stormwater storage facilities,

□ systems for draining subsurface water,

□ siphons,

□ pumping stations,

□ outfalls,

□ water quality facilities, and

□ hazardous spill containment systems.

In addition to discussion on the design of these elements, this chapter also discusses various factors affecting storm drain system design, computer modeling of storm drains, documentation and considerations pertaining to construction and maintenance.

Serious consideration and application of the subjects discussed in this chapter can have a positive effect on the efficiency, safety and cost of highway storm drain facilities.