The Engineer's Role in Flood Plain Determination
Specialized Training Allows Compliance with FEMA Practices
Jun 25, 2009
FEMA publishes flood insurance rate maps (F.I.R.M.) that show various categories of flood hazard zones. However, the calculations, engineering, and surveying needed to determine those zones are done by engineers working in the private sector. This work might be done under these circumstances:
- An area-wide study (maybe an entire county) to update a set of F.I.R.M.s, such as for an urbanizing area. This will normally be contracted by the local community, often with Federal and State funding.
- Study of a specific river or stream, within a single community, to assist with planning and orderly development. The community will normally contract with the engineer.
- Study of a short stretch of a waterway that is to be developed (or has been developed) to determine the impact of the development on the flood-carrying capacity. This is typically contracted and paid for totally in the private sector.
- Occasionally FEMA itself, or another branch of the Federal or State government will contract with private sector engineers for a major flood study. This is most typically true for levee systems or flood control dams and reservoirs.
Hydrology and Hydraulics
For this flood plain work, the engineer must complete certain calculations. Hydrology calculations predict rainfall and compute the resulting stream flow. This is determined by the physical characteristics of the drainage basin—area, slope, shape, soil type, amount of development—and the regional climate—probable rainfall pattern and intensity based on years of historical records.
FEMA procedures require that all flood mapping be based on what is called the 100-year flood. This is the rainfall amount and associated stream flow that have a 1 percent chance of occurring in any given year (and thus statistically should occur once every hundred years). FEMA calls this the base flood. Calculations are also done for the 500 year flood, although that is less important in flood insurance issues than is the base flood.
Hydraulic calculations take the run-off determined from hydrology and compute the depth and spread of flood waters. The physical characteristics of the stream, such as slope, main channel dimensions, overbank dimensions, roughness, obstructions, and development on the overbanks, are all factored into the calculations.
The hydraulics of most interest are for the base flood. The calculations determine the height of flood waters from the 100 year rainfall, which is then called the base flood elevation (BFE). This is the elevation of greatest regulatory significance within the flood plain.
Mapping the Flood Hazard Zones
Flood hazard zones are then mapped based on the calculated BFEs and the survey of the waterway. For rural streams and rivers, the survey is often limited to a number of cross-sections through the flood plain perpendicular to the direction of flow. If available and accurate, results of aerial surveys may be used.
For urban streams, a full survey is normally required, with cross-sections being “cut” from the complete topography. The calculations are performed at each cross-section, the BFE calculated, and the horizontal spread of flood waters determined. Interpolation of these limits is done between cross-sections to provide a complete and smoothly-transitioning flood hazard zone on the map.
Determining the Floodway
After the limits of the flood hazard zone during the base flood is determined, the engineer will perform one additional analysis—to determine the floodway. This is not the main channel, but rather a defined zone within the flood plain that includes the main channel and some of the overbanks. It is determined by assuming development takes place in the outer limits of the flood plain, from the left and the right, encroaching onto the overbanks.
The calculations continue for continuingly larger encroachments until the water surface elevation rises to a target elevation, typically not more than 1.0 feet above the BFE. That is considered the floodway, which is the central core of the flood plain that must be protected from development to preserve the carrying capacity of the stream. Everything in the flood plain outside the floodway is called the “floodway fringe.” Development in this area might be possible, so long as local guidelines and policies are followed.
The engineer is an important member of the public-private partnership for providing flood insurance to at-risk property owners. Specialized training in rainfall, run-off, hydraulics, surveying, and mapping are all required to allow the engineer to effectively do this work.
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