Storm drains include only the cost of the pipe by length. Drainage is
site specific and varies significantly. We have only included the cost
of the pipes as a representative indicator of drainage costs. Complete
estimation of drainage costs would include as you mention, the
cost/number of the drain grates and excavation/fill requirements. Those
factors are both difficult to estimate at the planning level, hence the
cost is based solely on the length of the pipe.
At one extreme, no formal drainage may be provided. This could occur
on flat terrain with soil and vegetation along the edges to absorb and
retain run-off. At the other extreme, storm drains could include catch
basins with sumps, grates, and a network of pipes. Because of the wide
variance in drainage scope and cost, as determined by site conditions
and facility design, we have only included the cost of the pipes.
How the Default Unit Cost was Determined
The best information found on drainage costs was in the Dutchess County, New York 2002 Hopewell Hamlet Pedestrian Plan. This planning document included cost information on dozens of components of a village-wide pedestrian improvement project. Costs were identified as $113 per linear foot for drainage pipes.