The stability of tidal flats in Venice Lagoon—the results of in-situ measurements using two benthic, annular flumes

摘要: Seabed properties in Venice Lagoon were examined situ two multidisciplinary field campaigns. The purpose of this study was to understand the mechanisms controlling stability bed types. Two benthic annular flumes (Sea Carousel and Mini Flume) deployed simultaneously from a floating pontoon at 24 sites during summer (1998), which considered representative range bed/habitat As well, bottom sampling coring, water-column monitoring habitat analyses carried out. All but three on cohesive sediments. Bed types included bare shelly mudflats regions colonised by seagrasses Cymodocea nodosa Zostera noltii, filamentous cyanobacteria, patches macrophytes Ulva rigida Chaetomorpha sp. A subset (13) these visited subsequent winter evaluate seasonal changes. Six intertidal, remainder sublittoral zone. Water temperature varied between 5 30 °C, salinity 20 38 psu. In absence waves, turbidity generally low (<10 mg/l) composed high amounts organic matter (25–50%). This indicates that tidal flows not competent support estuarine sediment. Higher levels measured wind events or boat passage as result resuspension bed. (saturated) density was, average, 1770 kg/m3, extremely for sediments. Sea Flume provided comparable results, despite large differences instrument footprints. Trends instruments similar showed summertime strength exceeded up five times. Mean erosion thresholds Sea 1.10 0.82 Pa, respectively, whereas winter, they 0.69 0.74 Pa. northern lagoon had most resistant flats due stabilizing effect cyanobacteria (Biostabilization Index: BI=244%), microphytobenthos (BI=153%) Z. noltii (BI=206%). effects C. (BI=74%), U. (BI=115%) shell debris intermediate, while mud beds least (BI=58%). Summer rates (as function applied stress) lower than ones, probably water algorithm E=χτsβ, yielded good results indicated high, notwithstanding thresholds. mean friction coefficient φ=62° highest central lagoon. wintertime φ=69° there no fluctuation stability. Mass settling strong suspended sediment concentration (S) decay constant (k) exponential S(t)=Soe−kt; it found be continuity with examples other locations worldwide (and therefore normal).