This talk presents contributions to understanding complex flow systems across scales, focusing on three areas of environmental fluid and granular mechanics through innovative experimental techniques and theoretical frameworks. (1) Turbulent streams over permeable beds and mesoscale modelling – In steep streams research, shallow turbulent flows over permeable beds were studied using a mesoscopic approach with double-averaged Navier-Stokes equations. Coupling particle velocimetry and refractive index matched scanning enabled reconstruction of double-averaged flow quantities (velocity, turbulent and dispersive stress), leading to algebraic closure equations depending on a continuous porosity profile that accurately captures flow profiles. This alternative mesoscale modeling approach offers potential for improved flow resistance modeling in low submergence streams and could also be useful in other domains dealing with turbulent boundary layers at rough/permeable interfaces (e.g. fluid-porous structure interaction, canopy flows). (2) Continuum based non-smooth granular avalanches – For granular flows, a continuous three-dimensional model employing the Drucker-Prager yield criterion and Material Point Method (MPM) was validated against experimental granular collapses over inclined erodible beds. The non-smooth model with constant friction coefficient set to the avalanche angle produced accurate results, with potential extensions to hysteretic modeling for improved early-stage dynamics prediction. (3) Phosphor-based temperature, oxygen or pH imaging –  Lastly, an affordable time-gated imaging platform was developed using CMOS cameras in multi-gate accumulation mode for visualizing temperature, oxygen or pH distributions with different possible pulsed illumination (LED lights on foils coated with sensor materials or LASER sheet illuminating phosphor particles in suspension). This system enables rapid lifetime determination for luminescent indicators with decay times exceeding 10 microseconds. We demonstrate applications including oxygen mapping, temperature imaging, and pH detection through dual lifetime referencing. With a total setup cost below €5,000, this accessible methodology offers high-resolution chemical mapping capabilities for various thermal fluid applications while enabling broader adoption of luminescence imaging techniques.