Hydraulics 101: If A 100-Year Rainfall Doesn't Always Cause a 100-Year Flood - Then What Does?

Hydraulics 101: If A 100-Year Rainfall Doesn't Always Cause a 100-Year Flood - Then What Does?

100 -Year Flood Explained: Over the last several years there have been numerous 100-year or greater floods in Texas. This includes Austin’s Onion Creek on Halloween of 2013, the Blanco River in Wimberley over the 2015 Memorial Day weekend, Houston’s Tax Day flood in April of 2016, and Hurricane Harvey in August of 2017.

Similar to the 100-year rainfall, a 100-year flood doesn’t only happen once every 100 years. The title “100-year flood” is meant to help simplify the statistic, which states there is a one-percent chance (or 1-in-100 chance) that a flood event of a certain magnitude will happen within a given year.

Placing flooding into a context like the 100-year flood allows for risk to life and property to be considered and addressed for insurance purposes. In the 1960s the U.S. government defined the 100-year flood by placing it within a framework where risk levels can be assessed in an understandable and consistent manner. This framework is now used by FEMA as a basis for the National Flood Insurance Program (NFIP).

A 100-year flood is measured by how high water levels get in pre-determined locations after a storm event.

Does every 100-year rainfall cause a 100-year flood?

No. Several factors can independently influence the cause and effect between rainfall and streamflow. This includes the amount of rainfall in the watershed that flows into a stream or river basin. Rainfall data is collected at specific points within the basin; it is unlikely that the exact same amount of rainfall occurs throughout an entire basin. Parts of the basin may even remain dry, supplying no additional runoff to the streamflow, which lessens the impact of the storm.

Existing conditions prior to a storm may influence the amount of stormwater runoff that goes into the stream system. Dry soil will absorb more of the rainfall, which reduces the amount of runoff. Equally, soil that is already wet from previous rainfall will not be able to absorb as much, which means more runoff will end up in the watershed.

Another factor is the duration of the storm in relation to the size of the stream basin where the storm occurs. For example, a 100-year rainfall that lasts for 30 minutes in a one-square-mile basin will have a more significant impact on streamflow than the same storm in a 50-square-mile basin. Streams with larger drainage areas typically require storms of a higher intensity, like the 100-year rainfall, and a longer duration for flooding to occur.

These and other factors determine whether a 100-year rainfall will produce a 100-year flood.

Can the 100-year flood level change?

The 100-year flood level is calculated using historical rainfall and existing stream geometry data. As stream geometry changes through degradation, erosion, or a project to improve the stream, geometric data may be updated and reevaluated through hydraulic modeling. The data allows for the estimates to become more refined, which may result in the 100-year flood level changing. Any updates to the 100-year flood level may cause changes in flood risk and consequently insurance rates. Therefore, it is important that the 100-year floodplains are mapped accurately.

If you would like to understand more about how flood maps are updated, please read our Flood Map Changes blog.

How do I know if I’m at risk?

A 100-year flood may or may not affect you. To find out if you are at risk for flooding, RiskMAP6 has interactive maps of flood-prone areas.

If your home is in a 100-year floodplain, there is a one-in-four chance it will flood at some point during a 30-year mortgage.

It is always best to take precautions whether you are in a mapped flood zone or not. For more information on flood insurance visit https://www.floodsmart.gov/ or contact your insurance agent.

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Glossary

100-year flood — Typically a riverine flood level that is based on the peak hydraulic level a watercourse reaches during a 100-year rainfall.

100-year rain — A rainfall event with a 1% probability of occurring in any given year.

100-year floodplain — An area of land that is predicted to be inundated during a 100-year flood (typically riverine).

Drainage Basin — An area of land where rainfall collects and drains into a common outlet, like a river or other body of water.

Flash flooding — Rapid flooding in low-lying areas.

Channel — The bed where a natural stream of water runs.

Floodplain — An area of low-lying ground adjacent to a river formed mainly of river sediment – it is subject to flooding.

Streamflow — The flow of water in streams, rivers, and other channels.

Watershed — The land that directs and channels rainfall and snowmelt to creeks, streams, rivers, and eventually to outflow points such as reservoirs, bays, and the ocean.

Watercourse — The flow path water takes along the watershed.

Riverine flooding — When excessive rainfall over an extended period causes a river to exceed its capacity.

Coastal flooding — Caused by storm surges, rising sea levels, or tsunamis in coastal areas.