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DESIGN FOR CLIMATE CHANGE: The Flood Proof House, Part 3

Updated: Apr 17, 2023


Landscaping / Site Adjustments

In Part 2 of Designing The Flood Proof House, I shared the variety of risks that flooding creates, and I begin to offer strategies for surviving a flood by coordinating the efforts of your community. Now that we understand how the water's going to come, and we have our systems in place, it's time to deal with the water physically. The arrangement of your property and your landscaping is your first physical line of defense against water traveling over land and can mitigate flooding or even eliminate some smaller flood risks.

The two main landscaping strategies are diversion and absorption. Swales and deflection channels physically direct surface water away from buildings. Bioswales that are lined with grass are less intrusive than paved channels. A paved channel is essentially a gutter but can be lined with protrusions that slow the flow of water to reduce erosion.

Absorption is achieved to varying degrees by permeable hard landscaping and by bioswales. Gravel beds have high flow rates that encourage rainwater to percolate down into the soil instead of rushing along the surface. Mulch beds have lower drainage rates but are better at soaking up water and holding onto it. Unit pavers (clay or basalt, for example) provide a more durable, even surface yet allow some rainfall to soak into the ground. Other systems such as grass block pavers provide permeability in driveway and walkway areas that would otherwise be paved with solid asphalt or concrete.

To maximize the amount of water that soaks into the ground, we create bioswales which combine absorption with diversion. Types of bioswales are loosely and inconsistently defined, but they can be broadly classified as one of three types:

  1. Grass-lined swales are shallow, grass-lined gulleys that simultaneously direct water and absorb it. Periodic obstructions called check dams slow down the water flow to give it more time to infiltrate the ground.

  2. Engineered bioswales are usually lined with gravel and generally have a perforated pipe buried underneath that helps distribute the water further into the soil.

  3. Wet bioswales are effectively shallow, linear reservoirs planted with species designed for standing water (e.g. bulrushes).

The roof of your house is designed to shed water to its perimeter, so it's important not to keep that accumulation away from your foundations to avoid overloading the drainage capacity there. Rain barrels are an inexpensive, low-maintenance tool to hold back the water. Both flat roofs and sloped roofs can be made into green roofs; the vegetation and soil slow down the rate at which water passes through and soften the impact of a rainburst that would otherwise create a flash flood around the home.

Above a certain amount of rain, topographic features help little to avoid water reaching your home, but there is still work to be done on your property that can prevent catastrophic damage to the house. Inflatable or fillable dams (e.g. Aquadam) are emergency structures designed to hold back water far better than sandbags could. However, they are comparatively expensive, require space to store, and take time to deploy.

photograph of Aquadam emergency flood barrier deployed on a riverside promenade

Permanent structures including passive, self-raising devices (e.g. FloodBreak) are even more expensive, intended mainly for points of entry, and are not a viable protection option for homes.

Any outdoor fuel tanks that are inadequately anchored will be carried by the floodwater; the fuel lines will be torn and contaminate the water further with fuel which will float and could subsequently be ignited. The tank itself becomes a hazard and will act as a battering ram.

Depending on the severity of the flood, other floating objects in the water such as logs or vehicles can destroy what the water itself could not. Install heavy-duty bollards, similar creative structures, or boulders in strategic locations to stop floating masses.

Dry Flood Proofing Features

If the water has reached the house, you can have measures in place that prevent the water from getting into the building. These are collectively referred to as "dry flood proofing".

The biggest risks to flooding are openings in your walls. Demountable flood barrier products are available to block doorways. Some are comprised of rigid panels that are slid down into permanent tracks; others (e.g. Dam-Easy) are watertight panels that are secured into place with inflatable gaskets. Vent guards are covers that are put in place over exhaust vents to prevent the exhaust ducting from allowing water to enter.

photograph of Dam-Easy removable flood barrier panel in doorway of house
Dam-Easy removable flood barrier

Once the openings are secured, one must address all the cracks, joints, seams in the wall assemblies and around the openings. Use sealant to fill cracks in basement walls to prevent water seeping in, if securing the exterior face is not an option. Waterproofing membranes - not dampproofing - applied to the outside of walls and foundations are the best means of blocking water and are designed to resist the pressure of the water ("hydrostatic head").

A watertight building enclosure will only function if it doesn't cave in from the hydrostatic pressure, so the foundations, the walls, and the floor slab must be reinforced to resist the additional stress.

In Part 4 of Designing A Flood Proof House, I explain the concept of wet flood proofing and how your house can be configured to survive being partly submerged, with minimal damage. My SAPPHR Strategy includes these strategies. If you would like to discuss what the flood risk is to your house or how your new or remodeled home could include these techniques to prepare for a flood, please book a free call with me using the button below.


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