A framework model for the dimensioning and allocation of a detention basin system: The case of a flood-prone mountainous watershed

- A model for the dimensioning and siting of a flood-detention system in a watershed.
- 1st module indicates a volume to retain and tracks the associated catchment area.
- 2nd identifies topographically suited locations for the retention basin using GIS.
- 3rd selects a optimal place based on pre-defined environmental objectives.
- Scenarios of decentralization using multiple retention basins are addressed.

SummaryA straightforward approach in flood management is the attenuation of peak discharges through an appropriate detention system. In this study, a flexible framework model was developed to optimize the dimensioning and site selection of a flood mitigation system based on detention basins. The general workflow can be summarized by three separate but interrelated modules: (i) the hydrologic module, which back tracks the detention basin contributing area based on the application of engineering formulae to historical information on local river floods and associated hydrometric data; (ii) the geomorphologic module, implemented in a Geographic Information System, which indicates all the potential locations with adequate contributing area as required for the detention system, by analyzing the flow accumulation within the river basin; and (iii) the environmental module, which comprises the implementation of a multi-criteria decision analysis for the selection of best location(s) for the detention basin system, addressing three different objectives: to minimize diffuse pollution; to minimize point-source pollution; to optimize landscape integration (by minimizing the dam height). The framework model was applied to the flood-prone Vez River, which is the main tributary of the Lima River in Northwestern Portugal. Although the expectations as regards diffuse and point-source pollution are optimistic, results show that detention of the largest flood in this river could only be accomplished with one very large dam or a number of decentralized large dams. Decentralizing the detention system with multiple basins installed in various branches of the Vez River did not reduce the mean dam height, because the catchment is located in a region of craggy topography and high annual rainfall. An extensive reforestation of the basin headwaters would increase evapotranspiration reducing runoff. Eventually, this would expand the alternatives for flood mitigation, namely through the construction of sustainable flood detention basins.