Navbar is hidden behind carousel when using wp-boostrap-navwalker

Well figured it out by myself. z-index was set on the wrong element. Add a new class to the container_class property:

<?php wp_nav_menu(
                array(
                    'theme_location'    => 'header-menu-left',
                    'dept'              => 2,
                    'menu_class'        => 'navbar-nav justify-content-end',
                    'container_class'  => 'collapse navbar-collapse YOURCLASS',
                    'container_id'    => 'navbarNav',
                    'fallback_cb'     => 'WP_Bootstrap_Navwalker::fallback',
                     'walker'         => new WP_Bootstrap_Navwalker(), 

                )
            ); ?>

and change the css:

.YOURCLASS {
  padding: 0px;
  top: 0;
  z-index: 10;
}

Note: Before those changes the z-index was set on the navbar itself. To get it work properly you have to set z-index on the container within the navbar. Just to clarify this.