ABSTRACT
The purpose of a bridge is to carry traffic over an obstacle. In doing so, it must also be
affordable, buildable, safe, durable, and pleasing to the eye. The four main types of
bridge are as follows.
15.1.1 Fords
A ford is a dip in a road which passes through (rather than over) a stream, with a
submerged pavement able to carry traffic and a water level low enough not to impede
traffic. As a ford will be overtopped in any flow, there is no value in raising the pavement
level above the level of the stream-bed. All vehicles will travel slowly through fords. The
critical water depth at which the road becomes impassable is commonly 250 mm for cars
and 450 mm for trucks. Fords are common when the traffic and/or the stream flows are
usually low. A vented ford is one in which routine, low stream-flows are carried under the
pavement by small pipes. The crossing becomes a ford when an uncommon, high stream-
flow exceeds the pipe capacity and overtops the pavement. Because it creates a greater
obstruction to water flow, a vented ford is more likely than a conventional ford to cause
local scouring of the streambed and banks and of the approach road.
