Floodplain Management in the South of Brazil with a Risk Index Proposal for Delimitation of a Flood Storage Area

A Floodplain Management Study was developed in the South of Brazil, aiming to regulate the occupation of areas of the municipality of Santa Cruz do Sul. An important part of the studied region is formed by floodplain areas subject to frequent natural floods, where depths equal to or greater than 50 cm are the preponderant condition.

Floodplain Management Studies aim to define a set of rules to regulate land use, in order to minimize human and material losses resulting from floods. The methodology includes the elaboration of flood mapping, usually with the definition of three risk zones: (i) Floodways are those areas where a significant volume of water flows during floods - comprised of the channel of the river or stream and the adjacent land that must remain free from obstruction so that the 100-year flood can be conveyed downstream; (ii) Flood Fringe - defined by the portion of the floodplain that is not part of the floodway and which, due to the small water depths and/or low speeds, does not contribute significantly to the drainage of the flood, and the type of development allowed is regulated by local authorities, with restrictions to guarantee minimum damages; (iii) Low Risk Zone, including urban areas restricted to exceptional flood events (outside the floodplain associated with flooding with 100-year flood).

Due to the characteristics of the study area, it is proposed to subdivide the Flood Fringe, including a fourth zone of risk, as defined by the New South Wales Floodplain Development Manual: the Flood Storage Area, covering the parts of the floodplain that must be kept clear, as they are important for the temporary storage of floodwaters during the passage of a flood and which, due to the high water depths, may cause damage to property and danger to persons and vehicles. A hazard index was proposed, based on relationships between velocity and depth of flood, as an element to assist in the identification of regions with high danger that should be allocated primarily to storage and flood mitigation, seeking to avoid its use for residential and commercial.