# Channel Downcutting and Widening

              While sediment transport calculations have traditionally been made with the same channel dimensions throughout a simulation, SWAT+ will model channel downcutting and widening. When channel downcutting and widening is simulated, channel dimensions are allowed to change during the simulation period.

              Three channel dimensions are allowed to vary in channel downcutting and widening simulations: bankfull depth, $$depth\_{bnkfull}$$, channel width, $$W\_{bnkfull}$$, and channel slope, $$slp\_{ch}$$. Channel dimensions are updated using the following equations when the volume of water in the reach exceeds 1.4 × 106 m$$^3$$.

               The amount of downcutting is calculated (Allen et al., 1999):

                        $$depth\_{dcut}=358.6*depth*slp\_{ch}\*K\_{CH}$$                                           7:2.5.1

              where $$depth\_{dcut}$$ is the amount of downcutting (m), $$depth$$ is the depth of water in channel (m), $$slp\_{ch}$$ is the channel slope (m/m), and $$K\_{CH}$$ is the channel erodibility coefficient (cm/h/Pa).

                The new bankfull depth is calculated:

                      $$depth\_{bnkfull}=depth\_{bnkfull,i}+depth\_{dcut}$$                                            7:2.5.2

where $$depth\_{bnkfull}$$ is the new bankfull depth (m), $$depth\_{bnkfull,i}$$ is the previous bankfull depth, and $$depth\_{dcut}$$ is the amount of downcutting (m).

                 The new bank width is calculated:

                          $$W\_{bnkfull}=ratio\_{WD}\*depth\_{bnkfull}$$                                                   7:2.5.3

where $$W\_{bnkfull}$$ is the new width of the channel at the top of the bank (m), $$ratio\_{WD}$$ is the channel width to depth ratio, and $$depth\_{bnkfull}$$ is the new bankfull depth (m).

                  The new channel slope is calculated:

                              $$slp\_{ch}=slp\_{ch,i}-\frac{depth\_{dcut}}{1000\*L\_{ch}}$$                                                               7:2.5.4

where $$slp\_{ch}$$ is the new channel slope (m/m), $$slp\_{ch,i}$$ is the previous channel slope (m/m), $$depth\_{bnkfull}$$ is the new bankfull depth (m), and $$L\_{ch}$$ is the channel length (km).

Table 7:2-2: SWAT+ input variables that pertain to channel downcutting and widening.

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