Tiemens has recently redone his web site at watersketch.com with an emphasis on his gouache and watercolor paintings and sketches. There are also galleries of oil paintings, drawings and photography.
Tiemens’ gouache paintings, though sometimes combined with watercolor or pastel, often take advantage of the unique qualities of gouache that allow it to be both painterly and linear at the same time.
Gouache often gets short shrift as a medium for gallery art, perhaps because it is often associated with design and illustration, or simply seems like the poor bastard child of transparent watercolor.
Tiemens uses it to great effect, contrasting slabs of flat color with drybrush passages, linear hatching and wonderfully loose and suggestive washes of wetter blending. His paintings and drawings sometimes have a feeling of Dutch masters, and at other times reflect his fondness for Corot and the painters of the Barbizon school.
The galleries on his site give an overview of his gallery work, but you will find larger versions of may of the images on his blog.
There is also a brief video demo of gouache sketching in a blog post form 2008.