Hydrologic Issues for Dams - Association of State Dam Safety Officials

credible estimates of extreme floods can be achieved by combining regional data from multiplesources. Thus, analysis approaches that pool data and information from regional precipitation,regional streamflow, and regional paleoflood sources should provide the highest assurance ofcredible characterization of low AEP floods.For many Reclamation dam safety risk assessments, flood estimates are needed for AEPs of1 in 10,000 and ranging down to 1 in 100,000, or even lower. Developing credible estimates at theselow AEPs generally require combining data from multiple sources and a regional approach. Table 1lists the different types of data which can be used as a basis for flood frequency estimates, and thetypical and optimal limits of credible extrapolation for AEP, based on workshop discussions orsubsequent communications. The limits presented in the table represent a general group consensus;however, opinions differed amongst workshop participants. In general, the optimal limits are basedon the best combination(s) of data envisioned in the western U.S. in the foreseeable future. Typicallimits are based on the combination(s) of data which would be commonly available and analyzed formost sites.Many factors can affect the equivalent independent record length for the optimal case. Forexample, gaged streamflow records in the western United States only rarely exceed 100 years inlength, and extrapolation beyond twice the length of record, or to about 1 in 200 AEP, is generallynot recommended (IACWD, 1982). Likewise, for regional streamflow data the optimal limit ofcredible extrapolation is established at 1 in 1,000 AEP by considering the number of stations in theregion, lengths of record, and degree of independence of these data (Hosking and Wallis, 1997). Forpaleoflood data, only in the Holocene epoch, or the past 10,000 years, is climate judged to besufficiently like that of the present climate, for these types of records to have meaning in estimates ofextreme floods for dam safety risk assessment. This climatic constraint indicates that an optimallimit for extrapolation from paleoflood data, when combined with at-site gaged data, for a singlestream should be about 1 in 10,000 AEP. For regional precipitation data, a similar limit is imposedbecause of the difficulty in collecting sufficient station-years of clearly independent precipitationrecords in the orographically complex regions of the western United States. Combined data sets ofregional gaged and regional paleoflood data can be extended to smaller AEPs, perhaps to about 1 in40,000, in regions with abundant paleoflood data. Analysis approaches that combine all types ofdata are judged to be capable of providing credible estimates to an AEP limit of about 1 in 100,000under optimal conditions.In many situations, credible extrapolation limits may be less than optimal. Typical limitswould need to reflect the practical constraints on the equivalent independent record length that applyfor a particular location. For example, many at-site streamflow record lengths are shorter than 100years. If in a typical situation the record length is only 50 years, then the limit of credibleextrapolation might be an AEP of about 1 in 100. Similarly, many paleoflood records do not extendto 10,000 years, and extensive regional paleoflood data sets do not currently exist. Using a recordlength of about 4,000 years, a typical limit of credible extrapolation might be an AEP of 1 in 15,000based on regional streamflow and regional paleoflood data.The information presented in Table 1 is intended as a guide; each situation is different andshould be assessed individually. The limits of extrapolation should be determined by evaluating thelength of record, number of stations in a hydrologically homogeneous region, degree of correlationbetween stations, and other data characteristics which may affect the accuracy of the data.129 Paper 18 – Swain, Bowles, Ostenaa