**5.6 Channel sinuosity (Sch)**

The channel sinuosity (Sch) can be indirectly estimated from width and depth of channel and ref. [33] has convincingly developed relationship between them in following regression equation:

$$\mathbf{S}\_{\rm ch} = \mathbf{3.5} \left( W\_{\rm c} / D\_{\rm c} \right)^{-0.20} \tag{27}$$

Where *W*<sup>c</sup> is the bankfull width and *D*<sup>c</sup> is the Bankfull channel depth.

When previously estimated values of *W*c and *D*<sup>c</sup> are substituted, Eq. (27) indicates that the channel sinuosity was about 1.361 for Brahmini sub basin and progressively increased to 1.632 for the Hurra sub basin through 1.517 for the Pachwara sub basin.

Additional estimates of channel sinuosity can be indirectly from dispersion of paleocurrent directions obtained from the pooled orientation data between channel sinuosity and consistency ratio (L) [41] as

$$\text{Log S}\_{\text{ch}} = \text{3.68-0.0684 L} + 0.00032 \text{ L}^2 \tag{28}$$

The consistency ratio (L) is concentration measure of orientation vectors and is (magnitude of vector mean/Number of observation) \* 100. Paleocurrent analysis of the Barakar sandstone in four sub basins of Rajmahal suggests consistency ratio (L)

82% for the Brahmini, 77% for the Chuperbhita, 74% for the Pachwara and 71% for Hurra respectively. Setting the values of L in Eq. (28) the channel sinuosity comes out to be 1.383, 1.570, 1.663 and 1.728 which signifies low sinuous (braided) for the southern part and moderately sinuous (meandering) in the northern part of the basin. Both Eqs. (27) and (28) yield similar values suggesting that the channel sinuosity increases as the sedimentation progressed from southern to northern part of the Rajmahal Gondwana master basin.

Very recently [37] presents following equation which can be used to estimate the maximum channel sinuosity P (max) of the depositing river:

$$\mathbf{P}\_{\mathrm{(max)}} = \mathbf{1.22}^\* \left( \mathbf{900} / \sqrt{\mathbf{10} \ \mathbf{D}\_{\mathrm{50}}}^{0.42} \right)^{0.09} \tag{29}$$

By substituting value for D50 (0.40 mm), the maximum channel sinuosity P(max) of the Barakar River in the study area is calculated to be 1.955.
