Assessment of river flow with significant lateral inflow through reverse routing modeling

Abstract

The discharge hydrograph estimation in rivers based on reverse routing modeling and using only water level data at two gauged sections is here extended to the most general case of significant lateral flow contribution, without needing to deploy rainfall–runoff procedures. The proposed methodology solves the Saint‐Venant equations in diffusive form also involving the lateral contribution using a “head‐driven” modeling approach where lateral inflow is assumed to be function of the water level at the tributary junction. The procedure allows to assess the discharge hydrograph at ends of a selected river reach with significant lateral inflow, starting from the stage recorded there and without needing rainfall data. Specifically, the MAST 1D hydraulic model is applied to solve the diffusive wave equation using the observed stage hydrograph at the upstream section as upstream boundary condition. The other required data are (a) the observed stage hydrograph at the downstream section, as benchmark for the parameter calibration, and (b) the bathymetry of the river reach, from the upstream section to a short distance after the downstream gauged section. The method is validated with different flood events observed in two river reaches with a significant intermediate basin, where reliable rating curves were available, selected along the Tiber River, in central Italy, and the Alzette River, in Luxembourg. Very good performance indices are found for the computed discharge hydrographs at both the channel ends and along the tributaries. The mean Nash‐Sutcliffe value (NSq) at the channel ends of two rivers is found equal to 0.99 and 0.86 for the upstream and downstream sites, respectively. The procedure is also validated on a longer stretch of the Tiber River including three tributaries for which appreciable results are obtained in terms of NSq for the computed discharge hydrographs at both the channel ends for three investigated flood events.

Unsteady State Water Level Analysis for Discharge Hydrograph Estimation in Rivers with Torrential Regime: The Case Study of the February 2016 Flood Event in the Crati River, South Italy

“…We observe first that Model 2 (Diffusive, 1D) and Model 3 (Diffusive, 2D) provide the best NS h value, almost equal to 0.88 and 0.85, which is much better than the value 0.71 obtained using Model 1 (Complete, 1D). This is consistent with previous results documented, for example in [7], and can be explained with the motivation given in Section 3.2. A sensitivity analysis is also carried out to test the robustness of the NS h measure with respect to small (five percent) parameter changes.…”

“…Specifically, the increment of the Manning coefficient, n, leads to a reduction of both the peak time and the stage hydrograph volume; on the opposite, the value of the tributary constants C 1 and C 2 mainly affects the volume of the computed hydrographs. In spite of this, equifinality of the parameters with respect to the sought after measure clearly plagues the problem [7]. See in Table 2 that similar values of the sought after NS h measure are obtained with a five percent variation of all the parameters.…”

“…In the methodology proposed by [7], the discharge hydrographs are computed as solution of a flow routing problem, solved along a domain of some kilometers, where the upstream boundary condition is given by the stage hydrograph measured in the reach upstream section. The zero diffusion downstream boundary condition is set in a river section located some hundred meters downstream of the second gauged section, far enough to avoid any significant inference with the stages here computed.…”

“…In the last years, some researchers have developed indirect methods for the discharge hydrograph estimation, based on the measure of two stage hydrographs at the ends of a selected reach of the river and on the use of unsteady-state hydraulic modeling [4][5][6][7]. The main advantage of the method is that it allows the simultaneous estimation of the unknown river bed parameters and discharge hydrographs.…”

Floodplain Terrain Analysis for Coarse Resolution 2D Flood Modeling

“…The inundation domain goes along the river channel for about 120 km between Orte (12 • 23 39.52" E, 42 • 27 27.08" N) and Castel Giubileo (12 • 29 59.34" E; 41 • 59 40.29" N), just upstream of the city of Rome, before the river flows out into the Tyrrhenian Sea (Figure 1). The Tiber River-the third longest in Italy-is the subject of frequent overflow, considering that the conveyance of the incised channel in the selected reach is approximately 900 m 3 /s, as stated in the flood risk management plan issued by the Tiber River Basin Authority (TRBA) [23], and that floods with peak discharges greater than 2000 m 3 /s occurred frequently in the last decade [24]. The river floodplain, which is predominantly developed, but mainly characterized by agricultural activities, is often the subject of channel overbank flows inundating the floodplain, with damages and service interruption to economic activities and minor urban settlements [25,26].…”