Stormwater Design – Before You Begin

Over the past couple of weeks I’ve spent a number of hours teaching staff in my office about stormwater design. The course was specific to some of the software we use in the office, but I felt that this before you begin summary was worth reproducing here.

Note that many of the requirements are in accordance with MMCD documents which are a Canadian Specification for residential construction.

I’d love to hear any comments on the content too. Thanks.


  • Review City bylaws for design guidelines, design storm events,
    details particular to local regulations, as well as
    options/alternatives to traditional curb/gutter/stormsewer system.
  • Determine requirements for onsite detention or stormwater
  • Procure contours of all areas uphill of the area in question

Site Visit

  • Identify natural drainage courses as well as low or wet spots
    requiring attention
  • Examine depth of existing flow in watercourses if applicable
  • Identify downstream constrains
  • Identify future upstream development that could impact on proposed
  • If onsite detention is likely to be required, determine likely
    disposal or storage locations.

Preliminary Subdivision Design

  • Following road and lot layout and preliminary road grading,
    identify contributing catchment areas, particularly offsite or uphill
    of road corridor.
  • Develop major flow path system for 100 year storm
  • Ensure road corridor is designed to allow any proposed stormwater
    BMPs such as swales, bioretention etc. The earlier this is suggested
    and adopted into the design, the easier this is for everyone.
  • Layout of stormsewer pipe network. Best done in Civil3D. This is
    reliant on the location of other services, as well as local bylaw
    regulations and road widths. Check MMCD, Interior Health and City
    Bylaws for regulations.
  • Manholes must be located every 150m or at changes of grade or
    direction and intersections of pipelines, (except 100 and 150mm
    service connections).
  • Spacing of Catchbasins. To drain maximum paved areas of 500sqm on
    roads with grades of up to 4%, 400sqm on roads with grades greater
    than 4%. Check inlet capacity and bypass flows. Note that standard
    grate of catchbasin has open area of 0.068sqm. Catchbasins should not
    be installed on curb returns or in driveways.
  • Curved Pipelines. Typically pipes can be installed on curves with
    radius greater than 60m, and in accordance with pipe manufacturers
  • Provide lawn drains, french drains, diversion swales, concrete
    dish drains as required to divert and collect water that would pond
    or flood outside of the road surface.
  • Maintain existing creeks and flow paths with culverts or bridge
    crossings. Adjust lot lines to avoid watercourses and keep as public
  • Avoid rear of lot sewerlines where possible, as easements and
    vehicle access to manholes must be provided.


Published by Mike Thomas

Mike Thomas P.Eng. ENV SP, is the author of and Director of Engineering at the City of Revelstoke in the Interior of British Columbia, Canada.