Complex sentence: The Sea Cut was primarily to stop flooding in West and East Ayton further downstream this means that water flowing in the River Derwent takes before reaching the sea via the Humber Estuary as opposed to the relatively short distance via the Sea Cut.
Split into simpler sentences: The Sea Cut was primarily to stop flooding in West and East Ayton further downstream. This means that water flowing in the River Derwent takes before reaching the sea via the Humber Estuary as opposed to the relatively short distance via the Sea Cut.