title: On the stabilizing influence of silt on sand beds

authors: Bartzke, Gerhard

subjects: sediment mixtures

abstract: In marine environments, sediments from different sources are stirred and dispersed, generating beds that are composed of mixed and layered sediments of differing grain sizes. Traditional engineering formulations used to predict erosion thresholds are however, generally for unimodal sediment distributions, and so may be inadequate for commonly occurring coastal sediments. We tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two-grain fraction (silt (D50  =  55 µm) and sand (D50 =  300 µm)) in a laboratory-based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed. To mimic recent deposition of particles following large storm events and the longer-term result of the incorporation of fines in coarse sediment, we designed two suites of experiments: (1) “the layering experiment”: in which a sandy bed was covered by a thin layer of silt of varying thickness (0.2–3 mm; 0.5–3.7 wt %, dry weight in a layer 10 cm deep); and (2) “the mixing experiment” where the bed was composed of sand homogeneously mixed with small amounts of silt (0.07–0.7 wt %, dry weight). To initiate erosion and to detect a possible stabilizing effect in both settings, we increased the flow speeds in increments up to 0.30 m/s. Results showed that the sediment bed (or the underlying sand bed in the case of the layering experiment) stabilized with increasing silt composition. The increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds, combined with, in the case of the mixed bed, decreasing erosion rates. Our results show that even extremely low concentrations of silt play a stabilizing role (1.4% silt (wt %) on a layered sediment bed of 10 cm thickness). In the case of a mixed sediment bed, 0.18% silt (wt %, in a sample of 10 cm depth) stabilized the bed. Both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed. These observations are summarized in a conceptual model that suggests that, in addition to the effect on surface roughness, silt stabilizes the sand bed by pore-space plugging and reducing the inflow in the bed, and hence increases the bed stability. Measurements of hydraulic conductivity on similar bed assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22% in the case of a layered bed and by up to 70% in the case of a mixed bed.

text: On the stabilizing influence of silt on sand beds In marine environments, sediments from different sources are stirred and dispersed, generating beds that are composed of mixed and layered sediments of differing grain sizes. Traditional engineering formulations used to predict erosion thresholds are however, generally for unimodal sediment distributions, and so may be inadequate for commonly occurring coastal sediments. We tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two-grain fraction (silt (D50  =  55 µm) and sand (D50 =  300 µm)) in a laboratory-based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed. To mimic recent deposition of particles following large storm events and the longer-term result of the incorporation of fines in coarse sediment, we designed two suites of experiments: (1) “the layering experiment”: in which a sandy bed was covered by a thin layer of silt of varying thickness (0.2–3 mm; 0.5–3.7 wt %, dry weight in a layer 10 cm deep); and (2) “the mixing experiment” where the bed was composed of sand homogeneously mixed with small amounts of silt (0.07–0.7 wt %, dry weight). To initiate erosion and to detect a possible stabilizing effect in both settings, we increased the flow speeds in increments up to 0.30 m/s. Results showed that the sediment bed (or the underlying sand bed in the case of the layering experiment) stabilized with increasing silt composition. The increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds, combined with, in the case of the mixed bed, decreasing erosion rates. Our results show that even extremely low concentrations of silt play a stabilizing role (1.4% silt (wt %) on a layered sediment bed of 10 cm thickness). In the case of a mixed sediment bed, 0.18% silt (wt %, in a sample of 10 cm depth) stabilized the bed. Both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed. These observations are summarized in a conceptual model that suggests that, in addition to the effect on surface roughness, silt stabilizes the sand bed by pore-space plugging and reducing the inflow in the bed, and hence increases the bed stability. Measurements of hydraulic conductivity on similar bed assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22% in the case of a layered bed and by up to 70% in the case of a mixed bed.

year: 2013

title: Betterment taxes, capital gains and benefit cost ratios

authors: Coleman, Andrew

subjects: betterment tax

abstract: ‘Betterment’ taxes can be used to fund infrastructure investments. We relate betterment taxes to the benefit: cost ratio, deriving conditions under which a project can be funded by such taxes, and relate betterment taxes also to a capital gains tax.

text: Betterment taxes, capital gains and benefit cost ratios ‘Betterment’ taxes can be used to fund infrastructure investments. We relate betterment taxes to the benefit: cost ratio, deriving conditions under which a project can be funded by such taxes, and relate betterment taxes also to a capital gains tax.

year: 2010

title: How many deaths were caused by the Tarawera eruption?

authors: Lowe, David J.

abstract: Most texts and brochures describing the Tarawera eruption of 10 June, l 886, record that it resulted in about 150 fatalities (e.g., Scott & Nairn, 1989; Ansell & Taber, 1996). Indeed, a figure of 153 deaths is recorded at the excellent new visitors' centre at the Buried Village, Te Wairoa, and in a number of other contexts. Elsewhere, numbers ranging from as few as about one hundred to possibly 'thousands' of deaths have been suggested (e.g., Houghton & Johnston, 1999).

text: How many deaths were caused by the Tarawera eruption? Most texts and brochures describing the Tarawera eruption of 10 June, l 886, record that it resulted in about 150 fatalities (e.g., Scott & Nairn, 1989; Ansell & Taber, 1996). Indeed, a figure of 153 deaths is recorded at the excellent new visitors' centre at the Buried Village, Te Wairoa, and in a number of other contexts. Elsewhere, numbers ranging from as few as about one hundred to possibly 'thousands' of deaths have been suggested (e.g., Houghton & Johnston, 1999).

year: 2001