chapter 3 fundamentals of fluvial geomorphology and stream ...

Figure 3.4 – Typical Plan andCross-sectional Views of Poolsand Crossings (FISRWG 1998)Bars are depositional features that occur within the stream.The types, sizes, frequency of occurrence, and locations of barsare related to the quantity and caliber of the sediment load, localsediment transport capacity, and morphology of the reach. Themost common types of bars are point bars, middle bars, andalternate bars.Point bars form at the convex bank of bends in ameandering stream (Figure 3.5). The size and profile of the pointbar are influenced by the characteristics of the flow, degree ofsinuosity, and the quantity and caliber of the sediment deposited atthe bend. The development of a point bar is driven by reduction inthe sediment transport capacity at the inner bank and sedimentsorting due to the action of transverse flows and secondarycurrents (Dietrich et al. 1984), often coupled with flow separation atthe inside of the bend downstream of the apex (Leeder andBridges 1975). Middle bar is the term given to areas ofdeposition lying within, but not connected to the banks. Middle barsin meandering rivers may form at riffles, especially where thecrossing reaches between consecutive bends are long, and inbends due to the development of a chute stream that separatepart of the point bar from the inner bankline. Figure 3.6 shows atypical middle bar on the Mississippi River formed by this process.Alternate bars are regularly-spaced depositional featurespositioned on opposite sides of a straight or slightly sinuous stream(Figure 3.7a) and may be a precursor to meander initiation orFundamentals of Fluvial Geomorphology and Stream Processes 27