Flood study process

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.