Flood Risk Management

Inherently, living along a stream or river presents landowners with the risk of flooding. Because stream flows typically fluctuate seasonally with higher flows during the spring runoff season and lower flows in the winter months, it is possible to anticipate when you are more likely to see a flood. However, floods are naturally occurring events that cannot always be predicted. Not only is it important to accept that floods will happen, but also to understand the effects that a large flood could have on your property. By doing this, you can take precautions to minimize the damage to your property and ensure your own safety as well as the safety of your upstream and downstream neighbors.

In terms of flood risk management, there are many topics that must be evaluated for your property. Two of the main concepts to consider when evaluating flood risk on your property are the floodplain and the floodway. The floodplain of a stream system refers to the overall area that becomes inundated or flooded during a given event, most commonly a 100 year event. A 100 year event does not mean that the water will only reach those areas once every 100 years. Instead, it means that there is a 1% chance every year that you will experience a flood that reaches the edge of the 100 year floodplain.

During a flood event, there is a narrower area of land through which the flood waters will flow efficiently. This area is called the floodway. Areas of standing water within the floodplain are not typically considered part of the floodway. The floodway was delineated to ensure development or building does not take place within the area of greatest volume and velocity of flow.

The first way to limit your flood risk is to keep structures such as buildings and roads out of the floodway and floodplain. Every structure that is in the floodplain is likely to be heavily damaged by floods. If you are able to keep structures out of the floodplain, your property will be far less prone to costly flood damages. Ideally, all structures should be outside of the floodplain; however, this may not always be feasible given the constraints of the site. While you may already have structures in the floodplain, you can consider if those structures are able to be relocated to a safer location or removed if they are not used. At a minimum, structures in the floodway should be secure enough to minimize the risk of becoming dislodged and turning into debris for downstream neighbors.

This cross-section graphic shows the relationships between the stream channel, floodway, and 100 year floodplain. The flood fringe is the area outside of the floodway, but still within the 100 year floodplain.

One type of ‘structure’ that is typically in a floodplain is a stream crossing. Stream crossings such as bridges and culverts play a critical role in determining the extents and the impacts of a flood. As an example, a crossing that does not have enough area to allow floodwater and sediment to move downstream will impact upstream properties. As water and sediment begin to back up behind the structure, the properties upstream of it can become flooded. Similarly, bridges and culverts that become clogged by debris from upstream will not be able to move as much of the flood flow downstream. This causes a significant upstream impact. As the flood and debris progresses and the clogged stream crossings cannot hold back any more water, they will fail and send a wall of water down the channel. In order to not minimize these flood risks, stream crossings must be carefully designed and continually maintained.

Some of the flood risk management strategies will require working in the stream and/or modifying the floodplain/floodway. Therefore, you may be required to work with outside help such as engineers, environmental consultants or river constructors to design, permit and install the project. This will ensure that your project is successful for you and for your upstream and downstream neighbors.