Effects of discharge reductions on salt wedge dynamics of the Ebro River

Abstract

The Ebro River is the largest freshwater course of Spain with a discharge (in the lower course) that has been reduced in the last few decades due to the construction of several dams and the corresponding increase of water demand and river regulation. The lowest stretch of this river (about 42 km) behaves most of the time as a highly stratified estuary with a salt wedge. The dynamics of this salt wedge are important from the ecologic and economic points of view and depend mainly on the river discharge, although other factors (like bathymetry and climatology) play a non-negligible secondary role. The wedge dynamics would also be affected by the planned river diversion scheme towards other areas of Spain. This diversion plan would mean a reduction of about 15% of the annual discharge (in the Ebro lower course) and the corresponding impact should be carefully assessed.With that aim a two-layer hydrodynamic numerical model has been employed to evaluate the role of flow regulation in the Ebro salt wedge dynamics, with particular emphasis on the potential discharge reduction due to the planned water diversion. The model has been calibrated with data acquired during four field campaigns (one per season) carried out during 1999 and 2000. Different scenarios of discharge reduction have been studied. Model results suggest a significant increase of salt wedge extent and permanence associated with the planned diversion scheme. This would entail an increase of saltwater intrusion problems both for aquifers and crops.

Hydrodynamic modeling of stratified estuary: case study of the Jadro River (Croatia)

“…In addition, wind stress at the water surface has been ignored. Sierra et al (2004) in fact showed that wind action is not a dominant factor for salt wedge dynamics, even though it can play non-negligible secondary role.…”

“…Interaction of fresh and salt water in an estuary is generally a very complex process, as explained in many research works (Arita and Jirka, 1987;Hodgins et al, 1977;Ralston et al, 2010;Schijf and Schonfeld, 1953;Sierra et al, 2004). In the case of strongly stratified estuaries, the salt circulation can be described with a relatively simple model, since salt transport results from a balance between horizontal and vertical advection of salt.…”

“…This condition typically occurs in seas with relatively small tides, such as the Mediterranean Sea (Ibanez et al, 1997;Kurup et al, 1998;Sierra et al, 2004). The most comprehensive measurement of this phenomenon on the east Adriatic coast have been conducted on the Neretva estuary, due to great economic and agricultural importance of that region for the Republic of Croatia.…”

“…For example, Sierra et al (2004) simulated the salt wedge dynamics in the Ebro River estuary by means of the MIKE 21 package, developed by the Danish Hydraulic Institute (DHI, 2011). The estuary was treated as a water body with two layers of different characteristics.…”

Numerical modelling of two-layer shallow water flow in microtidal salt-wedge estuaries: Finite volume solver and field validation

“…On the other hand, two-layer numerical models can adequately describe all relevant processes in salt-wedge estuaries if the stratification is strong enough to suppress the interfacial mixing, and the thickness of the interfacial layer is much smaller than the thickness of the upper and lower layer. In the past few years, several time-dependant two-layer numerical models (Liu et al, 2015;Ljubenkov, 2015;Sierra et al, 2004) were developed to describe the two-layer flow in salt-wedge estuaries. Although the main features of the stratified flow were captured by these models, they are not shock-capturing, and thus cannot correctly describe the internally transcritical flow which may occur under highly dynamical conditions due to sills or lateral contractions (Armi, 1986, Farmer andArmi, 1986).…”