June 2025 Webinar: Modeling Transient Life Loss: A Spatial Sampling Approach for Downstream Consequences

Transient populations can have a significant influence on risks posed by a dam, especially in locations with frequent water recreation downstream of the project. A key operating objective of many dams is to provide safe conditions for recreation within the vicinity of the dam. Dam Safety professionals must consider recreational populations when assessing emergency management plans and downstream consequences. These transient populations can be difficult to capture with conventional consequences modeling techniques which represent transient populations within a structure point dataset. Modeling methods often involve aggregating average annual population data and defining a set of points with fixed locations in the hazard area, potentially biasing risk estimates depending on the location of the point in relation to the terrain and hydraulics. These points do not capture uncertainty related to the transient population’s freedom of movement in the hazard area and do not allow for spatial variability or the representation of varying recreation populations near and within the river channel. An alternative methodology is presented here which involves random spatial sampling of transient population points within the hazard area, capturing the influence of population density and spatial variability. A population sampling grid is used to redistribute transient populations spatially within a study area hundreds of times, simulating the possibility of a range of population distributions and hazard exposures within a recreation area. This paper defines transient populations, details the importance of considering transient populations in dam risk assessments, and demonstrates alternative approaches used by Tennessee Valley Authority (TVA) and the United States Army Corps of Engineers (USACE) for modeling and estimating risk caused by transient populations below dams. The development of methodologies to better assess impacts to transient populations during breach events will advance risk assessment theory and practice and lead to better decision-making for risk estimators and emergency management agencies.