this about Shear Stress and Hydrodynamic Recovery over Bedforms of Different Lengths in a Straight Channel

1. The streams within a drainage basin form certain patterns,
depending on the slope of land, underlying rock structure as well
oShear Stress and Hydrodynamic Recovery over Bedforms of
Different Lengths in a Straight Channelf the area . These are
dendritic, trellis, rectangular, andradial patterns. The dendritic
pattern develops where the river channel follows the slope of the
terrain. The stream with its tributaries resembles the branches of
a tree, thus the name dendritic. A river joined by its tributaries, at
approximately right angles, develops a trellis pattern. A trellis
drainage pattern develops where hard and soft rocks exist parallel
to each other. A rectangular drainage pattern develops on a
strongly jointed rocky terrain. The radial pattern develops when
streams flow in different directions from a central peak or dome
like structure. A combination of several patterns may be found in
the same drainage basin.

2. Abstract: Pools and riffles are common morphological features in rivers that are
frequently used but poorly specified analogs in restoration design. Here, straight
two-dimensional (2D) bedforms are conceptualized as perturbations and flow
recovery is measured in a laboratory flume with an array of ultrasonic Doppler
velocity profilers (UDVPs). The objectives are to (1) assess the variation of skin
friction, turbulent stresses, and total stress; (2) assess the role of topographical
feedback on flow recovery; and (3) compare flow recovery in isolated and bedforms
in series. The results show that the total shear stress and near-bed turbulence
greatly exceed the skin friction in decelerating flow and the pool and that
hydrodynamic recovery tends to occur at length scales similar to geophysical scales
despite potential negative feedback from the bed. Repeating short bedforms can
push the flow to a more turbulent and laterally concentrated equilibrium condition.
Implications for sediment entrainment thresholds, existing models of riffle-pool
hydrodynamics, and the stability of constructed riffle pools are discussed.

3. Pools and riffles are formed in gravel-bed channels by local variations in boundary shear stress acting
on heterogeneous bed-surface particles during varying stream discharges (Lisle et al. 2000). This
general principle is well understood. Recent numerical simulations, for example, show that an
unsteady one-dimensional (1D) coupled flow and morphology model with bed-sorting is sufficient to
maintain pool depth downstream of riffles (de Almeida and Rodríguez 2011). In practice, however,
engineers report widespread ambiguity in the design, construction, and maintenance of instream
structures such as riffles and pools due to a lack of specification standards (Miller and Kochel 2009;
Radspinner et al. 2010). Despite a cost of over a billion dollars (US) spent per year in the United States
alone on stream restoration activities, with over 25% of this total on instream habitat improvement
and channel reconfiguration (Bernhardt et al. 2005), the practice of stream restoration has in many
ways preceded the science (Lave 2008). Successful examples show that instream structures can
improve the stability and ecological integrity of river channels that have been negatively affected by
human activities such as urbanization and channel straightening (Newbury 2013; Pasternack et al.
2008; Rhoads et al. 2008; Rosgen 2001), but expensive failures may result if they are poorly conceived,
designed, or constructed (Kondolf et al. 2007; Miller and Kochel 2009). Hydraulic studies are needed
that demonstrate how basic design parameters such as bedform length affect the hydro and sediment
dynamics of open channels

4. Experimental Apparatus Experiments were conducted in a 17 m long, 0.6 m wide recirculating flume
at a slope of 0.001 m=m. Modular bedforms were constructed from PVC sheets. Uniform depth
modules 0.4 m long and either low (0.025 m) or high (0.085 m) were added or removed to create
deep or shallow uniform sections of different lengths (Table 1). The nonuniform depth modules were
0.51 m long and fixed at an angle of 7.2° from the horizontal (Fig. 1). This slope ensures that
permanent flow separation does not occur (Simpson 1981), and is in the range of typical leeside
angles in macrobedforms (Best and Kostaschuk 2002; Carling and Orr 2000). The nonuniform
modules could be turned 180° to create either CAF or CDF sections