## 1 Introduction  
Environmental Science and Engineering dealing with riverine and coastal areas [1] are in need of reliable hydrodynamic models [2; 3; 4]. Such numerical models are of fundamental importance in order to provide some vital information. Currents, water levels, turbulence intensity and bottom stresses are only some of the variables required in order to carry out technical and scientific analyses that meet the stringent requirements that either public stakeholders or the scientific community require in order to accept the results of a study [5; 6; 7].  
Several open-source and community based suites are now available in order to carry out such analyses. Among the vast options it is worth mentioning Delft3D-Flow [8], regional oceanic modelling system (ROMS), Princeton ocean model (POM) [9], shallow water hydrodynamic finite element model (SHYFEM) [10] and nucleus for European modelling of the ocean (NEMO) [11]. In particular, the present study is focused on the analysis of the performance of two models of the latter list, i.e., Delft3D-Flow and ROMS, them being two widely used models, particularly the former in the engineering community, and the latter in oceanography. However, several conclusions drawn in the present work might be applied to models based on similar approaches, especially regarding the turbulence closures.  
During the last few decades, great efforts have been spent to provide such numerical models with turbulence closures [12] and grid generators [13] that allow for customising the numerical models for the various needs.  
Both models have been extensively used in ocean and coastal areas. A vast literature is available on the implementation, validation and use of such numerical models. As far as Delft3D is concerned, it is possible to cite [14; 15] as seminal works that established the reliability of this code. Applications of Delft3D can be found, for example, in [16], wherein tidal dynamics in a schematised mangrove Creek catchment were modelled, and in [17], wherein water renewal was studied inside a Spanish estuary. Several other implementation can be found in literature; for example, in the open sea [18] or in estuaries [19]. Analogously, ROMS was effectively validated, for example, in [20], wherein the three dimensional terrain following grid was applied; in [21], where turbulence models were developed; and [22], wherein a high-resolution numerical model of the circulation around the Corsica Channel was implemented. Other applications of ROMS can be found, for example, in [23], in which the numerical simulations of the circulation in the Hudson River estuary are compared with an extensive set of time series and spatially resolved measurements, and in [24], in which the model was implemented to evaluate the near-shore circulation in Monterey Bay, California.  
This work aims at critically comparing the results of Delft3D-Flow and ROMS over two well-known benchmarks. The first one is an open channel flow; its velocity profiles are carefully compared with the logarithmic theoretical one. Such a case can well model a tidal channel or an estuarine area. Theoretical bed shear stresses and model outputs are compared evaluating the relative error in order to assess the reliability of turbulence models adopted. The second benchmark considers the circulation inside a wind driven closed basin whose velocity profiles are analogous to Couette-Poiseuille turbulent profiles. Such velocity profiles are critically analysed considering the ratio between surface and bottom stresses and depth-averaged velocity.  
The paper proceeds with Section 2 where the numerical models are introduced and the benchmarks described. Then, Section 3 describes the results obtained by comparing Delft3D-Flow, ROMS and their turbulence models over the chosen benchmarks. Eventually, the conclusions are drawn in Section 4.