profile which best fits the observed geometries. Neglecting possible
external inputs, we have estimated this transport capacity by
considering a relatively uniform river reach and relating the sediment
transport rate to the flowing discharge through the total load
predictor of [12]. The mean annual total load discharge associated
with the best fit discharge (2300 m3/s) is 4.7 Mm3/yr (12.5 Mt/
yr). Using the methodology proposed by van Rijn [29] to estimate
the ratio of suspended load to total load, it results that the mean
annual bedload discharge is 1.45 Mm3/yr (3.8 Mt/yr i.e., about
31% of the total load). Fig. 9 compares this value with the mean
annual bedload discharge estimated by ADBPO [2] in the period
1982–2005 by considering a balance between the erosion/deposition
occurring in the active bed, on bars and on the floodplains
according to the topographic surveys carried out in the various
years, the sedimentation associated with finer suspended load
fractions, the external volume of sediment provided by tributaries
and the deficit caused by sediment mining. The annual bedload
discharge predicted by our model is very close to that experienced
by the river in the upstream portion (H) of the investigated reach
(Fig. 9). This result is reasonable, given the localized deliver of sediment
ensured by the Mincio (0.026 Mm3/yr), Secchia (0.17 Mm3/
yr) and Panaro (0.10 Mm3/yr) tributaries. These injections of sediment
are driven by the floods taking place in these three streams
and determine sediment pulses in the Po River likely analogues
to that associated with sediment waves triggered in it by larger
floods. As a consequence, tributary floods induce a temporary
departure of the Po bed from its equilibrium configuration which,
however, is progressively smoothed by the continuous reworking
ensured by dominant flows.
Knowledge of the equilibrium configuration of a river reach is of
great importance not only for interpreting field observations but
also for design and management purposes. The present modeling
approach can be used to easily assess the morphological scenarios
resulting from changes in hydrological conditions (e.g., due to climate
change) or regulation works, such as narrowing or widening
of the river width, constructions of mobile barrages for hydropower
or navigation reasons, dredging activities, etc. For example,
in the presence of a reduced sediment supply coming from
upstream (e.g., due to the interruption of sediment transport
induced by the construction of a barrage) the present model can
provide a quick estimate of the new long term equilibrium configuration
that is going to be attained and, hence, of the overall erosion
that is expected in the river bed.
Clearly, the proposed modeling framework has some limitations.
The procedure for determining the formative discharge, in
fact, requires that the river reach has been monitored for a long
enough period, such that the departures of the river bed from its
equilibrium configuration can be smoothed out by the passage of
a statistically significant number of relatively weak floods. In the