This article was downloaded by: 10.3.98.104 On: 28 Sep 2021 Access details: subscription number Publisher: CRC Press Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place, London SW1P 1WG, UK Handbook of Engineering Hydrology Modeling, Climate Change, and Variability Saeid Eslamian Bankfull Frequency in Rivers Publication details https://www.routledgehandbooks.com/doi/10.1201/b16683-4 Carmen Agouridis Published online on: 21 Mar 2014 How to cite :- Carmen Agouridis. 21 Mar 2014, Bankfull Frequency in Rivers from: Handbook of Engineering Hydrology, Modeling, Climate Change, and Variability CRC Press Accessed on: 28 Sep 2021 https://www.routledgehandbooks.com/doi/10.1201/b16683-4 PLEASE SCROLL DOWN FOR DOCUMENT Full terms and conditions of use: https://www.routledgehandbooks.com/legal-notices/terms This Document PDF may be used for research, teaching and private study purposes. 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The publisher shall not be liable for an loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. 3 Bankfull Frequency in Rivers 3.1 Introduction.........................................................................................36 3.2 Identifying Bankfull............................................................................37 Field Indicators • Minimum Width-to-Depth Ratio 3.3 Determining Bankfull Discharge.....................................................40 Gaged Sites • Ungaged Sites 3.4 Computing Bankfull Frequency.......................................................43 Example • Solution Carmen Agouridis 3.5 Summary and Conclusions................................................................46 University of Kentucky References.........................................................................................................48 Au tHOR Carmen Agouridis is an assistant professor in the Biosystems and Agricultural Engineering Department at the University of Kentucky. A licensed professional engineer in Kentucky and West Virginia, Dr. Agouridis has expertise in stream restoration and assessment, riparian zone management, hydrology and water quality of surface waters, and low-impact development. She is the recipient of over $5 million in grants, has authored a number of publications related to streams and riparian manage- ment, and is the director of the Stream and Watershed Science Graduate Certificate at the University of Kentucky. Having received training in Rosgen Levels I–IV along with courses at the North Carolina Stream Restoration Institute and various conference workshops, she teaches Introduction to Stream Restoration, which is a senior- and graduate-level course at the University of Kentucky. Preface Bankfull discharge is often used as a surrogate for channel-forming or dominant discharge— the morphologically significant discharge that shapes the river. Because of this, understanding the magnitude and frequency of bankfull discharge is important for river management and restoration. While an average return period of 1.5 years is often cited for bankfull discharge, this event can occur at intervals of less than one year to more than a decade. Determining bankfull discharge magnitude and frequency requires the ability to identify bankfull elevation in the field, transform this elevation into a discharge, and then compute the frequency of the resultant discharge. 35 Downloaded By: 10.3.98.104 At: 06:12 28 Sep 2021; For: 9781466552470, chapter3, 10.1201/b16683-4 36 Handbook of Engineering Hydrology 3.1 Introduction Bankfull discharge represents the maximum flow that a river can convey without overflowing its banks [5,19,42,77]. This discharge is considered morphologically significant as it represents the separation between river formation processes and floodplain processes [19,42,57]. Bankfull discharge is considered deterministic and as such is frequently used to estimate the channel-forming or dominant discharge of alluvial rivers [19,27,66]. Channel-forming discharge is a theoretical discharge that if maintained for an indefinite period of time (i.e., held constant) would produce the same river morphology as that of the long-term hydrograph [2,19,66,69]. Bates and Jackson [9] define channel-forming discharge as the “dis- charge of a natural channel which determines the characteristics and principal dimensions of the chan- nel.” The concept of channel-forming discharge is applicable to stable rivers [19]. As channel-forming discharge is theoretical, it is not measured directly; rather it is indirectly esti- mated using bankfull discharge although effective discharge, the discharge that transports the maximum annual sediment load, is sometimes used [1,5,11,19,25,26,62,78,79]. Soar and Thorne [69] describe effec- tive discharge as the “integration of sediment transport with flow-duration.” As seen in Figure 3.1 with curves (i) and (ii), frequent but small discharges transport small amount of the sediment, and infrequent but large discharges transport large amount of sediment. However, when considering the effectiveness of a given discharge, as seen in curve (iii), it is the intermediate discharges that transport the greatest fraction of the average annual sediment load [5,56,69]. Computing effective discharge requires the use of long-term discharge and sediment data, of which obtainment of the latter can be especially challenging. Few monitoring stations collect sediment data, and of those that do, it is the suspended fraction that is sampled. Juracek and Fitzpatrick [36] note that very few US Geological Survey (USGS) gage sites have bed load transport curves. The type of sediment data required to compute effective discharge depends on the river of interest. For rivers dominated by suspended load, effective discharge had been calculated using just this fraction [56]. For gravel-bed rivers, effective discharge has been computed using only bed load data although the bed load transport rates were calculated instead of measured given the difficulty in collecting bed load data [5,60]. In cases Effective discharge (iii) (i) (ii) (iii) Sediment transport effectiveness curve (i) × (ii) (i) Discharge frequency curve (ii) Sediment transport rating curve Discharge F igure 3.1 Effective discharge curve (iii) developed from discharge frequency curve (i) and sediment transport rating curve (ii). (Adapted from Soar, P.J. and Thorne, C.R., Channel Restoration Design for Meandering Rivers, ERDC/CHL CR-01, U.S. Army Corps of Engineers, Coastal and Hydraulics Laboratory, U.S. Army Engineer Research and Development Center (ERDC), Vicksburg, MS, 2001.) Downloaded By: 10.3.98.104 At: 06:12 28 Sep 2021; For: 9781466552470, chapter3, 10.1201/b16683-4 Bankfull Frequency in Rivers 37 where rivers have a significant bed and suspended loads, the total bed material load is recommended [5,6,69]. Details regarding the procedure for calculating effective discharge are provided in Biedenharn et al. [11] and Soar and Thorne [69]. As determining bankfull discharge is less data intensive than computing effective discharge, and since it can be determined on both gaged and ungaged rivers, bankfull discharge is more commonly used by scientists, engineers, planners, and other environmental professionals than effective discharge. Estimations of bankfull discharge magnitude and frequency are particularly important in river restora- tion projects, which have increased dramatically in the United States [10,70], as bankfull discharge is a critical design parameter [2,14,68,74]. Williams [77] noted the frequency of bankfull discharge is not common across rivers. While the one–two year recurrence interval is often cited as the mean frequency of bankfull discharge [15,22,41], Williams [77] found it could vary widely from 0.25 to 32 years. Table 3.1 contains a summary of bankfull discharge return periods throughout the United States and in some locations in Europe, Caribbean, Australia, and Middle East. 3.2 Identifying Bankfull Computation of bankfull discharge first requires locating bankfull elevation. Identification of bank- full elevation is done in the field [31,64] though limited efforts have examined techniques for remotely determining bankfull characteristics [12,13]. Identifying bankfull elevation requires practice with the degree of uncertainty in identifying bankfull elevation decreasing with increasing experience [31]. The degree of difficulty in identifying bankfull stage is also related to the stability state of the river and its location in the watershed. Bankfull elevation is often difficult to identify in unstable rivers [35], which are the very rivers for which restoration efforts are focused. Identifying bankfull elevation is more challenging with rivers without well-developed floodplains such as those in more mountainous regions [64]. 3.2.1 Field Indicators Identification of bankfull elevation is best done through the use of multiple indicators, if possible. These indicators should identify a consistent bankfull elevation throughout the project reach [37,64]. For unstable rivers or those without well-developed floodplains, the presence of reliable