Approach route
Water may arrive from the road, rear lot, side yard, slope or neighbouring drainage system.
Overland flow and surface water
Surface water follows elevation, openings and drainage routes.
When rainfall or snowmelt exceeds local drainage capacity, water can move across streets and lots toward low doors, garages, window wells and foundation openings.
- Ontario-focused
- Plain-language guidance
- Risk reduction, not guarantees
The essential answer
Overland protection begins by identifying flow routes and low openings. Combine site drainage, opening protection, maintenance, warning and emergency action rather than relying on one temporary barrier.
Who this guide supports
Use this page when the property, question or decision matches one of these situations.
- Homeowners with basement walkouts, low garages or window wells
- Residents near creeks, valleys or low-lying streets
- Owners who have seen ponding near entrances
- People reviewing overland flood insurance
Understand the system
Four parts of the decision deserve separate attention.
Collection point
Depressions, reverse-slope driveways and blocked inlets can hold water beside the building.
Entry point
Doors, vents, windows, garages and penetrations can admit water once the level rises.
Response time
Fast events may leave little time to deploy barriers or move property.
Guidance 1
Map the route before changing the site
Observe high and low points, curbs, catch basins, swales, driveway slopes and visible high-water marks. Municipal flood maps and warning systems add context but do not replace site observations.
Do not build berms or redirect flows without understanding drainage rights and downstream effects.
- Photograph ponding after safe rainfall events
- Keep public inlets clear without entering traffic
- Protect legal swales and drainage easements
- Record past water depths at openings
Guidance 2
Protect vulnerable openings
Permanent elevation or drainage changes may be more reliable than a manually deployed barrier, but property constraints and permits matter.
Deployable barriers require storage, training, lead time, clean contact surfaces and a plan for water accumulating behind or beside them.
- Inventory all low openings
- Check door and window seals
- Maintain window-well drainage
- Practice barrier deployment before storms
Guidance 3
Pair property action with warnings and insurance
Monitor local weather and flood messages, especially where river, creek or shoreline conditions influence risk.
Insurance definitions for overland flooding, sewer backup and sump failure can differ, so coverage should be reviewed before an event.
- Save alert sources
- Define a safe evacuation threshold
- Keep documents above flood level
- Ask the insurer about separate endorsements
Practical workflow
Move from observation to a documented next step.
- 1
Map likely surface-water routes
- 2
Measure and photograph vulnerable openings
- 3
Review grading, drainage and legal constraints
- 4
Select permanent and deployable measures with qualified input
- 5
Create a warning, deployment and evacuation plan
Homeowner checklist
Record each completed item and create a follow-up action for anything unresolved.
Continue through the RSM system
Related guidance, programmes and professional pathways.
Official source pathway
Use current public sources for emergency, insurance, programme and technical context.
RSM summarizes and connects information but does not replace the issuing authority. Review the current official page before acting.
Frequently asked questions
Clear limits help homeowners make better decisions.
What is overland flooding?
It is water flowing across normally dry land and entering a property from the surface, often after intense rainfall, snowmelt or watercourse overflow.
Will a backwater valve stop overland water?
No. A backwater valve addresses reverse flow in connected plumbing, not water entering through doors, windows or walls.
Are sandbags a complete solution?
No. They can be one temporary measure but require time, placement, disposal and a broader safety plan.
Should I enter a flooded street to clear a drain?
No. Floodwater, traffic, hidden openings, currents and electrical hazards can make that unsafe.
Editorial and review note
This educational page was prepared by Regional Stormwater Management using official public sources and a risk-reduction framework. It does not provide engineering, legal, insurance or municipal approval. Content is reviewed when official guidance changes and on the RSM editorial schedule.
Recommended next step
Plan for the opening, the warning and the person deploying protection
Review flood-barrier and flood-door considerations for residential properties.