Hydrologic Issues for Dams - Association of State Dam Safety Officials

Figure 1. Diagramatic section across a stream channel showing a peak flood stageand various paleoflood features used as paleostage indicators (PSIs) such asslackwater deposits, gravel, cobble and boulder bars, trees scars, and erosional scars(Source: Jarrett and England, 2001).at various temporal and spatial scales (NRC, 1999; Jarrett and Tomlinson, 2000;Vecchia, 2001). Non-stationarity typically includes effects of natural andanthropogenic climatic variability, but also effects of land-use changes such asurbanization, detention storage, and wildfire. Paleoflood data are particularly usefulin providing upper limits of the largest floods that have occurred in long time spans(Enzel et al., 1993; Jarrett and Tomlinson, 2000). The extreme upper tail of a floodfrequency distribution is most important for design of high-risk structures (Vecchia,2001). Paleoflood data can provide unique insight for estimating the upper tails offrequency distributions.Because of the important role of paleoflood hydrology is increasingly being used in arange of water-resources investigations, the American Geophysical Union (House etal., 2001) published a book Ancient Floods, Modern Hazards, Principles andApplications of Paleoflood Hydrology that provides a state-of-the-art review ofpaleoflood hydrology. In 1999, the American Society of Civil Engineers began a taskcommittee on paleoflood hydrology as it relates to dam safety and risk-basedassessments as well as better use of historical data and paleoflood data in waterresourcesinvestigations. The task committee on Paleoflood Hydrology also ispreparing a monograph entitled Use of Paleoflood and Historical Data in WaterResources Applications (Swain and Jarrett, 2000), which will emphasize usingpaleoflood techniques and applications by practicing hydrologists, engineers, andscientists in related fields.Recent Paleoflood Research ResultsThis section summarizes two recent research studies of the PCC project. A main sourceof uncertainty in paleoflood reconstructions is maximum flood stage inferred from PSIs.151Paper 20 – Jarrett et al