In my opinion, a runnel is a low area where the tidal flow, currents and wind erodes an area to the point where a "creek" forms on a tidal flat. This creek can run parallel to shoreline, diagonally towards the water, or straight off the beach. A true runnel washes away enough sand to allow hunters to reach the layer of strata that holds the heavier targets. Larger rocks, shells or clay are a good indicator that what you see is a true runnel. Some only last for a day or two, but the best ones will shift and migrate across the flats. Working the edge in the direction the runnel is shifting can be extremely productive. Jim's book is one of my favorites, his methods are proven winners. Best of luck out there.