= Residual wall films in a receding gravity current
A falling <free surface> does not instantaneously dry previously wetted rigid walls. Thin films remain above the bulk liquid and drain under the component of <gravitational acceleration> parallel to the walls. For a film of thickness $b$ over a wetted distance $\ell$, <falling film flow> gives a speed of order $gb^2/\nu$, hence $b\sim(\nu\ell/(gt))^{1/2}$ at late times for an order-one wall angle. These films can be visible in a cross-section while holding asymptotically negligible volume. In a <V-shaped channel gravity current>, local bulk depth falls only as $t^{-1/7}$; at a fixed nonzero station with a finite previous maximum wetted height, film volume decays as $t^{-1/2}$ while bulk-sectional volume decays as $t^{-2/7}$. Thus the film does not change the leading spreading law.
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