This should have been yesterday’s story but I as unable to make time for my blog. The sketch of the spillway was repeated in the studio using a pen with non-waterproof ink and a pentel water brush to wash out the pen lines, with this result:
It works better and much quicker than what I sketched at the site with a waterproof pen, as it feels more like painting; beats cross-hatching anytime. It was copied from a black & white photograph taken from the same spot. The photo will also sort out the tonal values for you. Would that be called cheating?