BANKFULL DISCHARGE RECURRENCE INTERVALS AND REGIONAL HYDRAULIC GEOMETRY RELATIONSHIPS: PATTERNS IN THE PACIFIC NORTHWEST, USA¹

Abstract

The model bankfull discharge recurrence interval (annual series) (Ta) in streams has been approximated at a 1.5‐year flow event. This study tests the linkage between regional factors (climate, physiography, and ecoregion) and the frequency of bank‐full discharge events in the Pacific Northwest (PNW). Patterns of Ta were found to be significant when stratified by EPA Ecoregion. The mean value for Ta in the PNW is 1.4 years; however, when the data is stratified by ecoregion, the humid areas of western Oregon and Washington have a mean value of 1.2 years, while the dryer areas of Idaho and eastern Oregon and Washington have a mean value of 1.4 to 1.5 years. Among the four factors evaluated, vegetation association and average annual precipitation are the primary factors related to channel form and Ta. Based on the results of the Ta analyses, regional hydraulic geometry relationships of streams were developed for the PNW, which relate variables, such as bank‐full cross‐sectional area, width, depth, and velocity, to bankfull discharge and drainage area. The verification of Ta values, combined with the development of regional hydraulic geometry relationships, provides geographically relevant information that will result in more accurate estimates of hydraulic geometry variables in the PNW.

Streamflow monitoring and statistics for development of water rights claims for Wild and Scenic Rivers, Owyhee Canyonlands Wilderness, Idaho, 2012

“…Peak streamflows with a 66.7-percent annual exceedance probability (AEP) (1.5-year recurrence interval) generally are considered representative of bankfull conditions (Leopold, 1994;Hortness and Berenbrock, 2004). Similarly, Castro and Jackson (2001) completed a regional study and determined that the average AEP for bankfull streamflows in rivers in the Pacific Northwest states (Oregon, Washington, and Idaho) is 71.4 percent (1.4-year recurrence interval). Castro and Jackson (2001) suggested a 66.7-percent AEP (1.5-year recurrence interval) for use in Idaho.…”

“…Similarly, Castro and Jackson (2001) completed a regional study and determined that the average AEP for bankfull streamflows in rivers in the Pacific Northwest states (Oregon, Washington, and Idaho) is 71.4 percent (1.4-year recurrence interval). Castro and Jackson (2001) suggested a 66.7-percent AEP (1.5-year recurrence interval) for use in Idaho. A streamflow with a 66.7-percent AEP has a 66.7 percent chance of occurring at any time in any given year.…”

Influences on flood frequency distributions in Irish river catchments

“…At higher return periods, conveyance rather than storage is likely to be the dominant influence and flood peaks will tend to be transferred downstream with less attenuation. Given that many bankfull recurrence intervals are in the order of 1-3 yr (see for example Richards, 1982;Petit and Paquet, 1997;Castro and Jackson, 2001) and that mean and median annual flows have return periods of 2.33 (assuming GEV type I distributions) and 2 yr respectively, flows in natural channels can frequently be subjected to a combination of these complex storage and attenuation effects.…”

How Well Do the Rosgen Classification and Associated ''Natural Channel Design'' Methods Integrate and Quantify Fluvial Processes and Channel Response?

“…Nevertheless, considerable resources are being expended by state and federal agencies as well as by academic institutions to determine bankfull channel dimensions and regional curves (based on channel and basin characteristics) to predict the bankfull discharge (eg. Harman et al, 1999;Odem et al, 1999;Castro and Jackson, 2001). Although data collection efforts are applauded, the overriding emphasis on determining bankfull dimensions at the expense of other data-collection programs that is troubling (ie.…”