Floodplain Mapping Update: Clark Fork River, Bitterroot River, Rock Creek

Missoula County residents are lucky to live in proximity to multiple rivers and creeks, but this also means increase risks of flooding. Missoula County is currently working with both state and federal agencies to update floodplain maps for the Clark Fork River, the Bitterroot River and Rock Creek. But what exactly is the floodplain, and why are updated maps needed?

Recently, the Missoula County commissioners spoke with Matt Heimel, floodplain administrator for Missoula County, and Adriene Beck, director of the Missoula County Office of Emergency Management. Adriene and Matt answer these questions and help decode the many acronyms around the floodplain mapping process.

Click here to listen to the episode, or find it wherever you get your podcasts under "The Agenda with the Missoula County Commissioners".

Flooding is the most common disaster in the United States. Severe flooding can cause immense damage. In many cases, entire livelihoods can be wiped out by a flood. It’s important to understand your flood risk; by understanding your flood risk you are better able to protect your family and property from harm. The Montana floodplain mapping program works with FEMA and communities across the state to identify flood risks. Identifying flood risk can help keep your home and local community safe from future flood events. Floodplain maps are one tool in learning and understanding flood risk.

Flood risk changes over time, and floodplain maps will require periodic updates. Several factors can change flood risk such as the age of the flood study, if there has been a lot of change or development in the area, or due to a flooding event. Flood studies today use the best available data and technology. This includes high-accuracy topographic information, updated hydrologic data and modern engineering methods.

Missoula County manages the regulated flood hazard under a principle that the actions of one property owner should not be allowed to adversely affect the rights of other members of the community. The adverse impacts of unwise development decisions can be measured by:

• increased flood levels
• higher flood velocities
• increased erosion and sedimentation
• deterioration of natural floodplain functions
• increased risk for emergency response personnel
• higher infrastructure maintenance costs
• other measurable adverse impacts to a community’s well-being

A concrete example of an adverse impact would be a homeowner in the floodplain having an unsecured structure that floats away in a flood and damages a public bridge downstream, impacting their neighbors’ ability to evacuate.

The approach of No Adverse Impact floodplain stewardship protects the rights of property owners and protects the natural systems that provide flood mitigation benefits to the community, such as slowing runoff and storing floodwater. Implementing strategies for no adverse impacts does not mean that no development can take place in the floodplain. It means that adverse impacts generated by land use activities are identified and mitigated on a case-by-case basis. This principle gives communities a way to promote responsible development measures through community-based decision making.

Developing regulatory floodplain maps is a complex process that uses the most accurate data available. The result of a flood study is a reliable map of the areas in a community that are at risk to flooding. Floodplain maps are critical tools for local officials to communicate and manage flood risk. To learn more about what goes into a flood study you can click here or visit FEMA's website.

Emergency responders, the insurance industry, community planners, developers, and mortgage lenders use floodplain maps. The maps are also critical tools for local floodplain managers and local, state and federal emergency management for communicating and managing flood risk.

Step 1 – Survey: Measurements are made of the topography around the river, along with any culverts, bridges and road crossings. LiDAR uses an airplane to collect ground elevation over a large area, and ground survey supplements the airborne data. The type of the survey depends on the size of the study area and type of study.

Step 2 – Hydrology: Determines how much water there will be in the river during a flood event. Data from stream gages will tell how many cubic feet of water per second the river will carry during the flood. Stream gage stations are an important tool to determine flow rates. If nearby stream gauges aren’t available, gage data from a similar location is used to determine the flow rate.

Step 3 – Hydraulics: Once the first two steps are complete, calculations can show where the water will go during the flood. The elevation data is combined with the flood flow data to determine where the water will go when it overflows the channel. There are five main components to the model:

1. Hydrology (stream flow data)
2. Cross Sections (measurements of the river bottom at key locations)
3. Roughness (thickness of vegetation, land cover, etc., determined by surveyors)
4. Structures (road crossings, culverts, bridges, etc.)
5. Downstream conditions.

Step 4 – Mapping (delineation): The results from Step 3 are combined with the elevation data and official maps to see how far the water will spread out. The result will be the floodplain boundary and a depth grid identifying the shallower and deeper areas of flooding. The area shown to be underwater during the flood is the regulatory floodplain.

Draft data is publicly available upon request. The draft engineering structure survey, hydrology, and hydraulics reports used for this project are available for download here.