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Modeling the morphodynamic equilibrium of an intermediate reach of the Po River (Italy)
No full textThe Po River, in the last century, has undergone significant altimetrical and planimetrical changes, mostly
induced by a progressively increasing human pressure. The extensive protection and regulations works
carried out to reduce the risk of flooding, the narrowing of the river for improving the navigation, the
local interruption of sediment transport caused by a large mobile barrage built for hydropower purposes
and the intense sand mining caused huge alterations of the river morphology. These changes were ini-
tially very fast and determined a significant and generalized deepening of the middle water course. In
the last few decades, however, the pressure induced by human activities on the river decreased signifi-
cantly and, consequently, a dynamic equilibrium condition tended to be re-established along most of the
reaches, as suggested by topographic surveys spanning a period of about twenty years. The present con-
tribution investigates this equilibrium condition by means of a one-dimensional movable bed model,
with reference to a 98 kmlong reach located between the confluence with the Oglio stream and the gaug-
ing section of Pontelagoscuro, for which an up to date stage-discharge relationship is available. Consid-
ering steady forcing conditions, we estimate the formative discharge producing the observed river
topography and the corresponding sediment transport capacity. The field surveys of cross section geom-
etry used to investigate the possible existence of an equilibrium morphology span a period (1982–2005)
of about twenty years. In the presence of fixed banks, the rived bed morphology appears to be controlled
by relatively moderate discharges, quite close to the mean yearly discharge and significantly smaller than
both the ordinary flood discharge and the maximum annual discharge. Even though significant deviations
from equilibrium are produced by the sediment waves triggered by larger floods, deposition occurring
during lower stages and the continuous reworking of the bed due to less intense but more frequent dis-
charges implies a tendency of the river to recover its equilibrium profil
Spatial width oscillations in meandering rivers at equilibrium
No full textIn canaliform rivers channel width at bankfull stage is fairly uniform though, at bend apexes, it is typically smaller than at crossings. Conversely, in sinuous point bar rivers bankfull width peaks at bend apexes. Why? Is there any mechanistic constraint that forces this different behavior? We provide an answer to these questions investigating how bankfull width must vary in a sequence of sine-generated meanders in order for the constraints of equilibrium (constant flow discharge and sediment flux) to be satisfied. With the help of a 3-D fully nonlinear analytical model of flow and bed topography in meandering rivers with variable width, we show that, in a meandering channel characterized by a constant longitudinal free-surface slope, the equilibrium width thus obtained oscillates with a frequency twice the frequency of channel curvature and experiences the maximum width close to inflection points. This pattern is typically observed in canaliform rivers. We then show that a similar pattern is observed in sinuous point bar rivers, provided the hydrodynamic width (width of the free surface) is replaced by the active width, namely the width of the portion of the cross section where transport occurs at formative conditions. Theoretical results are substantiated by a satisfactory comparison with field observations referring to the Mississippi River (United States) and to the Bollin River (United Kingdom)
Modeling the morphodynamic response to width variations with and application to the Magra River (Italy)
No full textCan tide dominance be inferred from the point bar pattern of tidal meandering channels?
No full textWe performed 2-D numerical simulations of flow and bed topography in a channel consisting of a sequence of tidal meanders connected to a tidal sea at one end and closed at the other end. Our main goal was to investigate whether the location of point bars relative to the bend apex is correlated with the character of the local flow field, i.e., its flood or ebb dominance. Validation of the model was achieved performing a comparison with results of laboratory observations. Simulations did reproduce the observed evolution of the laterally averaged bed profile toward an equilibrium configuration characterized by the classical landward aggrading trend typical of straight tidal channels with the formation of a shore at the landward end. The presence of meanders led to small amplitude spatial oscillations of the profile on the meander scale. The bar pattern developed when the morphology was far from equilibrium, such that the sediment transport was sufficiently intense to drive significant morphodynamic perturbations. Numerical results did show conclusively that the key factor controlling the phase of the point bar pattern relative to curvature is the flood- or ebb-dominant character of the basic flow field. More precisely, ebb/flood dominance led to point bars located seaward/landward relative to the bend apex. Moreover, two almost symmetrical long lobes that trailed away from the meander apex in both the ebb and flood directions formed in the transition region where the flow field shifts from flood into ebb dominant
On the equilibrium profile of river beds
No full textDespite the wide spectrum of perturbations of flow and sediment transport experienced by rivers as a result of hydrologic variations, the paradigm of morphodynamic equilibrium has long been present in the geomorphological literature where it is traditionally associated with the semiempirical notion of formative discharge, whereby the unsteady forcing is taken as morphologically equivalent to some effective steady forcing. Here we investigate the mechanisms responsible for maintaining a quasi-equilibrium bed profile of a river reach sufficiently short to have no significant tributary inputs. More importantly, we assume the channel banks to be fixed, hence, the case we have in mind is that of rivers protected by levees which cannot respond to hydrologic forcing by changing their width like natural rivers. Employing a 1-D model of river morphodynamics, we first determine the equilibrium profile of the river reach for given steady forcing conditions and discuss the capability of this approach for interpreting bed profiles observed in the field by applying it to the terminal reach of the Magra River, Italy. Field observations turn out to be reasonably well fitted by the equilibrium profile associated with a steady effective discharge, which however differs from the typical formative discharge (mean annual flood) for natural channels with erodible banks. Finally, we clarify how fluctuations of the hydrodynamic forcing associated with the recorded historical sequence of hydrologic events of variable intensities have acted to maintain the river equilibrium
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