Historically, navigable canals—either barge or shipping—have provided a convenient way to transport goods and people. In the United States, there are approximately 35 navigable canals in operation while approximately 78 have been abandoned due to convenient and cost effective alternatives such as railroads and interstate highways. Canals can also be costly to maintain, especially since damage due to floods can be quite high. Because of this, it's important that navigable canals are built above the floodplains of nearby bodies of water in order to avoid flooding.

Challenges of Developing Floodplains for Navigable Canals

With a traditional riverine floodplain analysis, the inflow to a stream and the flow within the stream is generally unregulated. However, the flow into and within a canal is regulated using guard gates, locks, bypass weirs, and waste weirs. Therefore, floodplains for canals cannot be modeled in the same way as free flowing rivers and streams. A thorough understanding of how the canal is operated is necessary in order to model potential flood elevations.

The Process

For a canal, during high flow events, the guard gates are closed, preventing flood damage to the canal. For FEMA floodplain mapping purposes, it's generally assumed that the 100-year flood will be contained within the canal and the floodplain is labeled "Zone A."

If the canal is part of a larger riverine system and it's decided that a detailed study should be done, the canal design 100-year flow information would need to be obtained from the owner so it could be used as input in a hydraulic model such as the HEC-RAS hydraulic model. The model would be run under steady flow conditions with all gates open and the base flood elevations (BFEs) would be obtained. A conservative way to establish a BFE in a canal would be to use the BFE just upstream of the guard gate if that information is available.

To model the floodplain for the canal's feeder stream or river, the control gates should be considered closed and the side cut—a lateral canal connecting the main canal with the adjacent river or stream—should be ignored. If overtopping of the control gate occurs, it could be considered an ineffective flow area.

Mohawk River Study

Authorized under FEMA's Hazard Mitigation and Technical Assistance Program, we provided technical review of a floodplain study for the Mohawk River system. Approximately 77 miles of the river and 21.5 miles of the Erie Canal, including 39 bridges, 13 dams, 11 locks, three control gates, and 8,000 feet of levee, were analyzed using MIKE 11 software.

Although the model was executed under a hydrodynamic regime, constant flows were assigned at the required flow change locations and the six-day simulation ensured that steady flow conditions existed. The model was run under conditions of the navigation season, since the river water level is maintained by dams and the flood damage potential is highest at this time. Past flood history also shows that most of the high floods take place during the navigation season.

The Mohawk River Study has resulted in information that will be included in future FEMA Flood Insurance Study (FIS) reports and Flood Insurance Rate Maps (FIRMs) that are used by the affected communities for watershed planning purposes.