Application of a novel automatic erosion and deposition monitoring system at a channel bank site on the tidal river Trent, UK

There is a well-defined need to improve understanding of the dynamics of se diment erosion and deposition on intertidal channel banks, given their impo rtance to channel stability, sediment budgets, depth maintenance, pollutant and nutrient transport, and ecological processes in estuarine systems. Con ventional, manual methods for field monitoring of erosion and deposition, h owever, normally deliver information of low temporal resolution conditioned by infrequent field resurveys. To address this problem, this paper discuss es a recently developed and improved automatic erosion and deposition monit oring technique, the Photo-Electronic Erosion Pin (PEEP) system, and its ap plication to a tidal channel bank site at Burringham on the River Trent, U. K. The PEEP system allows the magnitude, frequency and timing of individual erosion and deposition events to be monitored much more precisely than wit h conventional manual methods. PEEP sensors also monitor light intensity an d sediment temperature, variables which can influence bank stabilizing and destabilizing processes. Example results at both the event and spring-neap timescales are presented from a short PEEP system deployment between March and May 1997 at Burringham. These establish that discrete erosion events of > 60 mm and 100 mm can occur in response to individual tidal cycles, event s which are readily monitored automatically and quasicontinuously by the PE EP system. The capability of the PEEP approach to enhance temporal resoluti on of monitoring is demonstrated by the determination of the timing of the 100-mm bank erosion incident to an 'event window' of 2.75 h: this converts to mean bank erosion rate of 36 mm h(-1) over the period of inundation. In addition, the PEEP system defines the magnitude and date of two example dep osition events of 47 and 92 mm on the lower bank during a sequence of risin g spring tides. These represent mean deposition rates of 4.5 and 8.4 mm h(- 1) respectively over the periods of inundation. The Burringham site is show n to be highly active, with regular and dynamic erosion and deposition cycl es. Upper bank surface elevation oscillations, driven by this sediment cycl ing, were characterized by a strong 14-day cycle which clearly reflected th e spring-neap cycling of tidal range. Sediment was deposited on the bank re latively quickly, but removed by erosion rather slowly, giving an asymmetri c sediment cycling profile. Higher bank elevations were strongly correlated with high tidal ranges, and especially to water level peaks 2 days previou sly. Incorporation of a simple Wind Stress Index farther improved the stati stical explanation of tidal bank elevation, and suggested that high on-shor e wind speeds were associated with increased bank erosion. Such vigorous se diment cycling means that many erosion-deposition sequences on tidal banks can be self-concealing and therefore may not be recorded by infrequent manu al resurveys which will inevitably underestimate total activity. This reinf orces the need for an automated method such as the PEEP system to determine these typically cyclic sequences of self-cancelling accretion and removal activity if site dynamism is to be correctly quantified. The ability of the PEEP approach to generate such detailed and high-resolution information on the temporal distribution of erosion and deposition events in tidal enviro nments should significantly enhance future process and applied studies, esp ecially of entrainment thresholds, sediment recycling, estuarine sediment s upply and sediment fluxes, the operation of biomediation mechanisms, bank e rodibility changes, storage and residence times of contaminants and site ma nagement options. (C) 2001 Academic Press.