Button 42 (CBI: Cost-to-Benefit Index)

Computes a cost-to-benefit index (CBI) as a ratio of estimated flood damage potential versus cost to upgrade the local drainage system. The user defines a street flood depth versus flood damage cost potential (or “penalty” cost) for various land use types. The CBI program computes unimproved “penalty” costs and divides by drainage system improvement cost opinions for each link used in the drainage system network. Upon ranking in increasing order, the highest CBI value corresponds to the highest cost-to-benefit ratio. The CBI results aid in prioritizing storm drain system improvements.

To define the penalty function, the user establishes approximately at what flood depth in the street flooding initiates for structures; for example, a 1-foot depth may be appropriate. For the selected land use being studied, the corresponding damage, or “penalty”, value may be assigned to be “1.0”. Similarly, for another land use type, a 1.5-foot depth in the street may be the suitable flood depth in the street at which structures begin to be flooded; however, in comparison to the previous land use type, the “penalty” value is say 2.5 – that is, this second land use type has about 2.5 times the flood damage potential as the previous land use type. The user then defines the beginning flood depth and the “relative” damage potential for each hydrologic land use type.

For lesser flood depths, the user also defines flood damage potential with respect to the “relative” damage versus land use types.

Next, the hydrology is rerun without new systems in-place, and assuming all flows remain within street section boundaries. For each link in the model, a street flow depth is estimated for the “unimproved” condition in order to obtain flood damage potential or penalty values. These values are then divided by the cost-to-improve, on a link-by-link basis, to obtain the CBI values.

Program requirements: RATSCx, HydroLINK.

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