Testing a theoretical resistance law for overland flow on a stony hillslope

Overland flow, sediments, and nutrients transported in runoff are important processes involved in soil erosion and water pollution. Modelling transport of sediments and chemicals requires accurate estimates of hydraulic resistance, which is one of the key variables characterizing runoff water depth and velocity. In this paper, a new theoretical power–velocity profile, originally deduced neglecting the impact effect of rainfall, was initially modified for taking into account the effect of rainfall intensity. Then a theoretical flow resistance law was obtained by integration of the new flow velocity distribution. This flow resistance law was tested using field measurements by Nearing for the condition of overland flow under simulated rainfall. Measurements of the Darcy–Weisbach friction factor, corresponding to flow Reynolds number ranging from 48 to 194, were obtained for simulated rainfall with two different rainfall intensity values (59 and 178 mm hr⁻¹). The database, including measurements of flow velocity, water depth, cross-sectional area, wetted perimeter, and bed slope, allowed for calibration of the relationship between the velocity profile parameter Γ, the slope steepness s, and the flow Froude number F, taking also into account the influence of rainfall intensity i. Results yielded the following conclusions: (a) The proposed theoretical flow resistance equation accurately estimated the Darcy–Weisbach friction factor for overland flow under simulated rainfall, (b) the flow resistance increased with rainfall intensity for laminar overland flow, and (c) the mean flow velocity was quasi-independent of the slope gradient.     

Similarity between morphological characteristics of rills and ephemeral gullies in Sicily,Italy

This paper reports the results of a field investigation aimed to establish morphological similarity between rills and ephemeral gullies. Rill measurements were made on 14 plots having a surface area of 22–352 m² located on a 14·9% slope and on a plot 6·0 m wide and 22·0 m long having a uniform 22·0% slope. The plots are located on the experimental station for soil erosion measurements, ‘Sparacia’, of the Agricultural Faculty of Palermo University, in Sicily, Italy. All plots are subjected to natural rainfall. The measurements were made immediately following five events between November 2004 and December 2005. The ephemeral gully measurements were made on a cultivated area of about 120 ha, located in Central Sicily, which is representative of many soil‐crop conditions in the Mediterranean basin. The morphological similarity between rill and ephemeral gully was first tested. Then a power relationship between rill or gully volume and length, theoretically deduced by dimensional analysis and self‐similarity theory, was applied. This power relationship needs a different scale factor for rill and gully measurements. Finally, using two dimensionless groups representative of the channel morphology variables, the analysis showed that a single relationship can be applied to rill and gully measurements. Copyright © 2009 John Wiley & Sons, Ltd.     

Mount Etna eruptions of the last 2,750 years: revised chronology and location through archeomagnetic and <Superscript>226</Superscript>Ra-<Superscript>230</Superscript>Th dating

A careful re-examination of the well-known written documents pertaining to the 2,750-year-long historical period of Mount Etna was carried out and their interpretation checked through the high-accuracy archeomagnetic method (>1,200 large samples), combined with the ²²⁶Ra-²³⁰Th radiochronology. The magnetic dating is based upon secular variation of the direction of the geomagnetic field (DGF) and estimated to reach a precision of  ±40 years for the last 1,200 years, and ±100 to 200 years up to circa 150 B.C. Although less precise, the ²²⁶Ra-²³⁰Th method provides a unique tool for distinguishing between historic and prehistoric lavas, which in some cases might have similar DGFs. We show that despite the abundance of details on ancient historical eruptions, the primary sources of information are often too imprecise to identify their lava flows and eruptive systems. Most of the ages of these lavas, which are today accepted on the geological maps and catalogues, were attributed in the 1800s on the basis of their morphology and without any stratigraphical control. In fact, we found that 80% of the “historically dated” flows and cones prior to the 1700s are usually several hundreds of years older than recorded, the discrepancies sometimes exceeding a millennium. This is proper the case for volcanics presumed of the “1651 east” (actually ∼1020), “1595” (actually two distinct flows, respectively, ∼1200 and ∼1060), “1566” (∼1180), “1536” (two branches dated ∼1250 and ∼950), “1444” (a branch dated ∼1270), “1408” (lower branches dated ∼450 and ∼350), “1381” (∼1160), “1329” (∼1030), “1284” (∼1450 and ∼700), “1169 or 812” (∼1000) eruptions. Conversely, well-preserved cones and flows that are undated on the maps were produced by recent eruptions that went unnoticed in historical accounts, especially during the Middle Ages. For the few eruptions that are recorded between A.D. 252 and 750 B.C., none of their presumed lava flows shows a DGF in agreement with that existing at their respective dates of occurrence, most of these flows being in fact prehistoric. The cinder cones of Monpeloso (presumed “A.D. 252”) and Mt. Gorna (“394 B.C.”), although roughly consistent magnetically and radiochronologically with their respective epochs, remain of unspecified age because of a lack of precision of the DGF reference curve at the time. It is concluded that at the time scale of the last millennia, Mount Etna does not provide evidence of a steady-state behavior. Periods of voluminous eruptions lasting 50 to 150 years (e.g., A.D. 300–450, 950–1060, 1607–1669) are followed by centuries of less productive activity, although at any time a violent outburst may occur. Such a revised history should be taken into account for eruptive models, magma output, internal plumbing of the volcano, petrological evolution, volcano mapping and civil protection.     

Flow resistance in gravel‐bed channels with large‐scale roughness

A previously published mixing length (ML) model for evaluating the Darcy–Weisbach friction factor for a large‐scale roughness condition (depth to sediment height ratio ranging from 1 to 4) is brie?y reviewed and modi?ed (MML). Then the MML model and a modi?ed drag (MD) model are experimentally tested using laboratory measurements carried out for gravel‐bed channels and large‐scale roughness condition. This analysis showed that the MML gives accurate estimates of the Darcy–Weisbach coef?cient and for Froude number values greater than 0·5 the MML model coincides with the ML one. Testing of the MD model shows limited accuracy in estimating ?ow resistance. Finally, the MML and MD models are compared with the performance of a quasi‐theoretical (QT) model deduced applying the P‐theorem of the dimensional analysis and the incomplete self‐similarity condition for the depth/sediment ratio and the Froude number. Using the experimental gravel‐bed data to calibrate the QT model, a constant value of the exponent of the Froude number is determined while two relationships are proposed for estimating the scale factor and the exponent of the depth/sediment ratio. This indirect estimate procedure of the coef?cients (b₀, b₁ and b₂) of the QT model can produce a negligible overestimation or underestimation of the friction factor. Copyright © 2003 John Wiley & Sons, Ltd.     

The “Pomici di mercato” Plinian eruption of Somma-Vesuvius: magma chamber processes and eruption dynamics

The Pomici di Mercato (PdM, 8,010 ± 40 a), also known in the literature as Pomici Gemelle or Pomici di Ottaviano, is one of the oldest Plinian eruptions of Somma-Vesuvius. This eruption occurred after the longest (7 ka) quiescence period of the volcano and was followed by more than 4 ka of repose. The erupted magma is phonolitic in composition. All the products have very low phenocrysts content (less than 3%) and show evidence of mineralogical disequilibria. They contain K-feldspar ± clinopyroxene (salite and diopside) ± plagioclase ± garnet ± biotite ± amphibole ± apatite ± Fe-Ti oxides. Pumice fragments collected at different stratigraphic heights are slightly less evolved and more enriched in radiogenic Sr composition upsection. The glass composition is fairly homogeneous in single pumice fragment and among pumice fragments from different layers. Glass separated from pumice fragments collected at different stratigraphic heights is homogeneous in the Sr-isotope composition (around a value of 0.70717). Glass is in isotopic equilibrium with salite throughout the entire sequence and with diopside at the base of the sequence. Diopside becomes more radiogenic upsection, reaching a value of 0.707458 ± 7, whereas feldspar is consistently slightly less radiogenic than glass. Nd-isotope composition is fairly uniform (ca. 0.51247) through the whole sequence. The isotopic disequilibria among glass, feldspar and diopside, together with the homogeneous isotopic composition of pumice glass in equilibrium with salite, and the mineralogical disequilibria between plagioclase and K-feldspar, imply that most of the diopside and plagioclase crystals are xenocrysts incorporated into the phonolitic magma during residence in a magma chamber and/or during ascent towards the surface. The PdM Tephra are compositionally and isotopically similar to the phonolitic, first-erupted products of the subsequent Pomici di Avellino Plinian eruption. On the basis of this similarity, we suggest that the magma feeding both eruptions resulted from the tapping of a unique magma chamber. Prior to the PdM eruption, this chamber was formed by a large and homogeneous phonolitic magma body. After the PdM eruption, as a consequence of new arrivals of more radiogenic in Sr, less-differentiated magma batches, the magma chamber progressively developed a slightly stratified phonolitic uppermost portion, capping a tephriphonolitic layer, both emitted during the subsequent Pomici di Avellino eruption.     

A comparison of surface and underground array measurements of ambient noise recorded in Naples (Italy)

In this study, we describe two experiments of seismic noise measurements carried out in Naples, Italy. The site allowed measurements to be obtained both at the surface and in a tunnel that is 120-m-deep. The main goal was to compare the seismic response evaluated at the surface to the in-tunnel response, through spectral, polarization, and resonance directivity analyses. In the 1 to 20 Hz frequency band, the noise level was up to 15 dB higher at the surface than in the tunnel. The polarization properties and horizontal-to-vertical spectral ratios appear not to be influenced by the tunnel geometry or by the topography. Some preferential alignments were observed in the polarization azimuths computed at the surface, which are likely to be due to local sources, rather than morphological features. The absence of directivity effects and the low noise levels in the tunnel make this site suitable for installing seismic stations. We also studied how the subsoil structure affects the seismic motion at the surface. The dispersive properties of the Rayleigh waves were investigated using the spatial autocorrelation method. A joint inversion of the dispersion data and the horizontal-to-vertical spectral ratios provided the subsurface Vs profile. The derived model has a low velocity contrast at depth, such as to generate moderate and broad H/V spectral ratio peak amplitude. The normalized spectral ratio appears more appropriate to identify the soil-resonance frequencies.     

Ability of soil bacterial composition as an indicator of levels of soil erosion in a badland

Calanchi(plural of calanco) are typical Italian badlands created by a combination of morphogenetic processes(rill and interrill erosion, gullying, piping, and mass movements) mainly originated by the effect of water. Calanchi are characterized by the sparse and patchy distribution of vegetation, and, in interplant areas, the soil surface is colonized by an association of organisms known as biological soil crust(BSC). A morphometric analysis of 45 basins in the studied calanchi area, based on a h...