Critical erosion profiles in macro-tidal estuary sediments: Implications for the stability of intertidal mud and the slope of mud banks

Vertical profiles of the critical erosion threshold (iota(crit)) in sediment have been measured at 11 stations along the axis of the Tamar Estuary and at a single station in a tributary of the Tamar at St. John's Ford. The iota(crit) of surface sediment increased from 0.04 Pa in the upper, brackish estuary to 0.09 Pa in the lower estuary. In the upper estuary iota(crit) only increased slightly with depth whereas in the marine estuary iota(crit) increased rapidly from 0.09 Pa at the surface to 0.25 Pa at 15 cm below the sediment surface. The results showed that the relationship between iota(crit) and bulk density (rho(b)) obtained previously for surface sediment was also applicable to sediments from depths of 10-15cm and probably deeper. Profiles of rho(b) were measured to depths of 70cm using a corer. In the lower (marine) estuary rho(b) increased with depth in the sediment from 1580kgm(-3) at the surface to 1720 kg m(-3) at 70 cm. In the upper estuary rho(b) values were lower at 1170-1200 kg m(-3) and profiles were almost homogeneous indicating that consolidation was not occurring. The mid-estuary was transitional between these two situations. These results are consistent with the seasonal accumulation and loss of 'mobile' sediment observed previously in the upper estuary with changes in river flow, and with the apparent stability of intertidal mud in the lower marine estuary deduced from historical bathymetric survey records. The slopes of the intertidal mud banks ranged from 1-2% in the lower estuary to 20-25% in mid-estuary but, instead of continuing to increase in steepness towards the head as the estuary became narrower, the measured slopes actually decreased. It is speculated that the lack of consolidation through continual mobilisation and settlement cycles combined with an increase in silt content in the upper estuary resulted in sediment that lacked the mechanical strength to maintain steep slopes. (C) 2007 Elsevier Ltd. All rights reserved.