Hi Graeme

Presumably your objective is minimise erosion in this sensitive and precious ecological environment. Concern about flood and ebb priority should be subordinate to maintaining the natural flow of tide water in each random channel that intersects your planned excavation. To minimise erosion at ebb tide, each channel would require a barrier adjacent to the excavation. Each simple barrier gryke would be equipped with a vertical series of non-return valves arranged to impound the neap to spring tidal range. The lowest valve would allow for slow release of the impounded water.

M.

Even in salt marsh, there are channel or flow depressions for return flow that may be difficult to see. These might be more evident with LiDAR coverage. Avoid disturbing, connecting to or excavating in flow channels. Excavating on contour may help. Leave undesturbed salt marsh buffer to maintain flow roughness to encourage sediment deposition. If flow paths develop, dikes might work to contain flow and sediment, but you might try temporary to semi permanent structures made of steel fence posts, wire fencing and filter fabric as described in our researchgate publication on Sediment from a Small Ephemeral Gully. In your instance, you might want to use posts that eventually biodegrade, rope or hemp fencing for support. The filter fabric we used probably would last several decades, so you might choose materials that break down quicker, but remain until sediment and marsh recovers on excavated site.

One added point, installing dikes, diversions or filter silt fencing can after flow pathways. Under normal conditions, undesturbed salt marsh should hold up to small to moderate changes in flow stress. But monitoring is advisable until there is assurance of recovery, especially after major storms, winds that might alter forces.