Terrain Underlain Research Articles

Multi-stage metal enrichment and formation of gold mineralization in black shales: the role of high heat flow in a rift setting

Black shales may serve as an important source of metals such as Co, Ni, or As, largely due to anoxic to euxinic conditions in association with high concentrations of sulfur leading to efficient scavenging and transport of metals from seawater into the seafloor sediment. We report on an unusual type of Au mineralization newly discovered in Ediacaran trench-slope black shales in the Bohemian Massif, Czech Republic. The Au enrichment is related to the formation of a quartz–sulfide vein system and a progressive evolution of ore-forming fluids with decreasing temperature, from Sb- to As-rich to final precipitation of native gold from silica and Au-bearing low-temperature hydrothermal colloidal solutions. The hydrothermal nature of these solutions is also documented by Li contents and isotope compositions which differ markedly between barren black shales and those carrying significant late-stage quartz-rich veins. The structural relationships and orientation of the associated quartz veins point to a close connection between vein emplacement and high heat flow in response to Ordovician rifting, and breakup of the northern margin of Gondwana, and opening of the Rheic Ocean. This triggered metal and sulfur remobilization, including Au, from the associated Neoproterozoic–Cambrian volcanosedimentary successions. The documented Au mineralization and its association with the Ordovician rift-related magmatic activity is different from the widespread Variscan Au occurrences in the Bohemian Massif. Our study thus provides a new genetic model potentially important for future exploration of Au also in other terrains underlain by a rifted Cadomian basement.

Engineering geomorphological and InSAR investigation of an urban landslide, Gisborne, New Zealand

The East Coast region of New Zealand has some of the highest erosion rates in the world, due to its proximity to an active plate boundary, susceptibility to high-intensity storms and steep terrain underlain by young soft sedimentary rock and soil. In the city of Gisborne, expansion of residential blocks into steeper terrain in peri-urban areas has required improved capacity for the characterisation and monitoring of slope stability. Landslides have affected several properties and have destroyed infrastructure. Slope failure commonly occurs during heavy rainfall events when slow-moving retrogressive slides transition into earthflows and mudflows. In this study, we used in situ sampling and testing methods combined with remote sensing techniques to provide an understanding of the pre-failure and post-failure behaviour of an urban landslide in Gisborne. High-resolution aerial imagery, unmanned aerial vehicle imagery and light detection and ranging data revealed slope morphology and contours of prehistoric failures in the area, and highlighted the more recent impacts of deforestation on slope stability. Furthermore, Sentinel-1 InSAR analysis determined that gradual deformation began in 2017, following two ex-tropical cyclone events. Deformation downslope continued until an initial failure in July 2020. Following that event, some parts of the slope proceeded to accelerate, leading to a further reactivation event in November 2021, following heavy rainfall. During this November 2021 event, average line of sight velocities ranged from −7.9 to −11.2 mm/year, and deformation rates in the vertical direction (related to rotational slumping) averaged −11.2 to −11.9 mm/year, consistent with field observations.

The transition of very coarse-grained meandering to straight fluvial drainage systems in a tectonized foreland-basement landscape during the Holocene (SE Germany) – A joint geomorphological-geological study

Very coarse-grained meandering to straight fluvial drainage systems evolved during the Holocene (<7950 BP) in a terrain underlain by crystalline basement rocks of Precambrian to Paleozoic age and Mesozoic sedimentary rocks which are separated from each other by an area intersected by fault zones of Cenozoic age. Faulting provoked ridges in the foreland, dykes in the basement and it is accountable for a significant uplift and stream strong gradient. All structural elements show the same NW-SE strike as a consequence of the modern fault tectonic pattern. The Holocene fluvial drainage system under study evolved homoclinal with a presumed paleosurface (relic landscape/peneplain) and represents the upper reaches of the Main River Drainage System in SE Germany. During the Neogene, chemical weathering created a regolith while uplift in the hinterland and subsidence in the foreland provoked tilting of the paleosurface. The coarse-grained meandering drainage system is described by a characteristic trilogy that encompasses (1) intermediate depocenters/sediment traps, (2) subsequent strike streams, and (3) consequent dip streams that upstream grade into straight streams with no braided streams to be. The drainage system developed under temperate humid climates and can be described by five lithofacies types which were investigated with regard to the origin of the individual landform types, their sedimentological, hydrographic, mineralogical and petrographic features. The hydrographic parameters sinuosity, channel density and channel-floodplain ratio are markers for the paleo-gradient and tectonic activity and can also be used as flow rate and fluvial confinement indices. Mass wasting and fluvial processes both shape the morphology of the valleys. Autochthonous and par-autochthonous minerals and organic matters constitute the basis for the physico-chemical facies analysis and for constraining the climatic and early diagenetic regimes. For correlation and chronology of the lithofacies types radiocarbon dating and optically stimulated luminescence proved to be successful.

Geology and Geochemistry of Iroko-Granites Southwestern Nigeria: implication on provenance

The geology, geochemistry of Iroko-granites have been studied and reported in this paper. The study area has been described as of Archaean –Early Proterozoic terrain underlain by migmatite-gneiss-quartzite complex with supracustal rocks. Large number of granites which outcrops in Iroko-Ekiti represent a typical occurrence of granitoids sporadically distributed in the basement and are known to belong to the Older Granite suites, which are attributable to the Pan-African Orogeny(750± 150Ma). The rocks occur, mostly as flat and low lying within sparse vegetation. Structures common on and around the outcrops include quartz vein, veinlets, pegmatite dykes which trend North-south, discrete exfoliated surfaces and xenoliths of older rocks. This study reveals that the granites belong to calc-alkalic suites, demonstrate metaluminous nature, and exhibit characteristics of I-type granites. The granite is a distinctive type in that it is relatively highly potassic, has high FeO/(FeO + MgO) ratio, and high average Zr (299.75ppm) concentration with other high field strength elements. The trace elements study implicates pronounced fractional crystallization during evolution of the granites and thus petrogenetically discriminates as Syn-collision provenance.

Groundwater exploration in limestone–shale–quartzite terrain through 2D electrical resistivity tomography in Tadipatri, Anantapur district, Andhra Pradesh

Two-Dimensional (2D) Electrical Resistivity Tomography (ERT) survey was carried out at 11 sites within an area of 10 km2 to delineate deeper potential groundwater zones in a complex geological terrain underlain by quartzite, shale and limestone formations with varied resistivity characteristics. The area is in medium rainfall zone in Tadipatri mandal of Anantapur district, Andhra Pradesh state, India. The investigation was carried out to meet the growing demands of water supply. Interpretation of the high-density 2D resistivity dataset results revealed potential zones at only three sites in Tummalapenta, Ayyavaripalle and Guruvanipalle villages within the depth zone of 24–124 m. A major fault zone oriented in EW direction is mapped at Tummalapenta site. Based on high resolution geophysical data interpretation and significant anomalies, four boreholes were drilled in complex, viz., limestone, shale and quartzite formations up to a maximum depth of 192 m in the area with the yield ranging from 300 to ~5000 liter per hour (lph). These four anomalous drilled borehole sites corroborates with the aquifer zone delineated through ERT technique. The aquifer parameters estimated from pumping tests show that the transmissivity varies between ~0.3 and 179.5 m2/day while the storage coefficient ranges from 0.137 to 0.5 indicating large variation in aquifer characteristics of the system in a smaller area. Suitable water conservation measures were suggested for improving the groundwater condition and yield of the pumping wells.

Sustainability and Conceptual Groundwater Hydraulic Models of Basement Aquifers

Abstract Groundwater flow of the basement terrains of the Ibarapa region was studied by carrying out pumping test and measurement of borehole inventory. The view was to identify the associated aquifer systems from the time-drawdown curves, quantify the estimable hydraulic properties and develop hypothetical models for the understanding of the groundwater flow in the area underlain by diverse crystalline bedrocks. Three aquifer types were identified namely, dual, leaky and regolith. The yield of groundwater in dual and leaky aquifers that dominated terrains underlain by amphibolite and gneisses was sustainable, but the discharge of regolith aquifers mainly associated with migmatite and granite terrains declined at late pumping stage. The transmissivities of the dual and leaky aquifers were between 2.02 and 11.65 m2/day, while those of regolith aquifers were mostly less than 1.00 m2/day. The average aquifer transmissivities in m2/day by bedrocks were: 6.85, 2.57, 0.76 and 1.72, correspondingly. The inter-relationships between transmissivities and groundwater discharge showed diverse aquifer representations, from sustainable high-yielding to unsustainable low-yielding types. Conscientious effort is, therefore, required for well construction in the area.

Groundwater Occurrence from Hydrogeomorphological Study of Hard Rock Terrain of Part of Southwestern Nigeria

Abstract Studies of structural and hydrogeomorphological units (HGU) that are indicators of groundwater occurrence were carried out across an area extent of more than 700 km2 within the hard rock terrain of southwestern Nigeria. These studies integrated geological remote sensing techniques (RST) and geographical information system (GIS) methods to generate thematic maps that included elevation, drainage, lineaments and vegetation index for characterising the attributes of groundwater occurrence across the area. The results revealed that the lineament system is mainly rectilinear with major trends of NNW-SSE and NE-SW on the gneiss, NW-SE and NE-SW on porphyritic granite and NNE-SSW, NW-SE and E-W on migmatite. The discharge zones in the area are the lowland terrains underlain by gneiss and amphibolite. Similarly, variably directional discontinuities that are related to rock contacts are equally laden with groundwater. Conversely, the recharge areas are the high-lying terrains characterised by higher fracture density and underlain by porphyritic granite and migmatite. Additionally, there are evidences of groundwater seepage along the major river channels. Therefore, besides the rock structures, landform is another crucial factor that guides groundwater distribution in the study area.

Infiltration on sloping terrain and its role on runoff generation and slope stability

A modified Green-and-Ampt model is formulated to quantify infiltration on sloping terrain underlain by homogeneous soil wetted by surficial water application. This paper’s theory for quantifying infiltration relies on the mathematical statement of the coupled partial differential equations (pdes) governing infiltration and runoff. These pdes are solved by employing an explicit finite-difference numerical method that yields the infiltration, the infiltration rate, the depth to the wetting front, the rate of runoff, and the depth of runoff everywhere on the slope during external wetting. Data inputs consist of a water application rate or the rainfall hyetograph of a storm of arbitrary duration, soil hydraulic characteristics and antecedent moisture, and the slope’s hydraulic and geometric characteristics. The presented theory predicts the effect an advancing wetting front has on slope stability with respect to translational sliding. This paper’s theory also develops the 1D pde governing suspended sediment transport and slope degradation caused by runoff influenced by infiltration. Three examples illustrate the application of the developed theory to calculate infiltration and runoff on a slope and their role on the stability of cohesive and cohesionless soils forming sloping terrain.

Lithologic characterisation of the basement aquifers of Awe and Akinmorin areas, Southwestern Nigeria

Just like many communities in Nigeria, people living in Awe and Akinmorin areas rely on groundwater supply for their domestic water need. However, these areas are underlain by impermeable crystalline rocks that included migmatite, schist and quartzite that can only transmit water when fractured and where there is sufficient weathered regolith overlying the bedrocks. Therefore, a total of twenty-five vertical electrical soundings (VES) was conducted across the study area with the aim of investigating the lithological characteristics of the regolith and the underlying bedrocks for groundwater prospect. An aggregate of nine soundings were conducted on quartzite terrains and eight each across terrains underlain by migmatite and schist. From the VES results, the dominant curves across the study areas were the H and KH types. These geo-electric curves indicate that the middle (saprolite) layers are more conductive and the bedrocks are more resistive. The total regolith thickness ranged from 11.9 to 50.9m with an average of 26.4m on migmatite terrains, 11.7 to 28.8m and average of 19.4m on schist terrain and 8.4 to 41m with an average of 22.3m on quartzite terrain. From the lithologic characterisation, 70% of the saprolite layers are interpreted as fine grained with resistivities of less than 100Ωm, which is an indication of deep and intense weathering across the study area. The occurrences of fractured and fresh bedrocks are almost of the same proportions. However, there is more dominant of partially weathered bedrocks than those that are either fresh or fractured. Altogether, bedrocks resistivities of more than half of the surveyed area are <1500Ωm and were either fractured or slightly weathered. This increased groundwater prospect across the study area, particularly within terrains underlain by fractured bedrocks. Generally, the potential for groundwater development in the study area is reasonably credible. Although, the saprolite units are mostly fine grained and may not be the best groundwater bearing zone, they may act as confining units for areas underlain by fractured or partially weathered bedrocks. In addition, the fairly thick fine grained saprolite layer may reduce the risk of groundwater contamination from direct recharge.

Integrating Geoscience Data for Delineating Potential Gold Targets-a Case Study from Sian Goldfields Limited, Ghana

Sian Goldfields Limited (SGL) recently re-examined its concession by integrating data in a GIS environment for the improvement of the mapping of the local geology as well as for delineating potential gold targets. Data employed included geochemical, topographic, geological and aerial photographic information. The geochemical data included 5660 soil samples collected over a terrain underlain by six different lithologies namely, rhyolites, greenstones, sheared greenstones, phyllites, sandstones and granites. The various lithologies and their boundaries were obtained from the shaded relief map derived from the topographic information whilst “ground truthing” confirmed and established the other geological contacts. The updated geology map generated from the geosciences data was superimposed on the soil gold geochemical plot. The integration of these datasets in the form of maps resulted in the easy definition of anomalous gold targets. The targets were delineated using background and threshold values of 30 ppb and 50 ppb respectively. The integration model approach portrayed many of the geological boundaries to have relations with mineralization. The use of the MapInfo-Discover GIS markedly aided to ascertain the geological contacts in areas where outcrops were uncommon. The re-analysis of the remote-sensed data and the ground truthing showed the clear contact between the Birimian system and the Voltaian units at the north eastern side of the concession. The integration of this information with soil geochemical gold results highlighted the most prospective gold targets. It is recommended that the granite-sheared greenstones/sheared greenstone-massive greenstones contacts and areas with defined soil anomalies should be subjected to further exploration works.

Earth Hummocks in West Dartmoor, SW England: Characteristics, Age and Origin

ABSTRACTExtensive fields of vegetated, dome‐shaped earth hummocks 5–35 cm high and typically 80–200 cm in diameter occur on slopes of up to 15° at elevations of 320–440 m asl on Dartmoor, SW England, but are limited to terrain underlain by metasedimentary rocks and dolerite, and absent from granite areas. Hummocks occur within archaeological sites, implying formation within the last ~ 3000 years and ruling out development over permafrost. The hummocks are composed of frost‐susceptible silty soil (modal grain size 10–100 µm) with occasional clasts, but exhibit no evidence for cryoturbation or diapirism. Hummock size and spacing are fairly consistent at particular sites but vary between sites. Hummock age, distribution and characteristics are incompatible with non‐frost action origins and most proposed frost action origins (cryostatic pressure, soil circulation, permafrost aggradation, soil injection), but regular spacing and granulometry favour initiation by differential frost heave under conditions of periodic shallow (0.2–0.4 m) seasonal ground freezing. We suggest that hummock growth may reflect migration of silt in advance of inclined freezing planes until equilibrium is achieved with soil loss down hummock sides. Our results confirm that hummock formation by frost action occurs on silty soils in humid cool temperate climates with only limited seasonal frost penetration. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Effects of freezing on soil temperature, freezing front propagation and moisture redistribution in peat: laboratory investigations

Abstract. There are not many studies that report water movement in freezing peat. Soil column studies under controlled laboratory settings can help isolate and understand the effects of different factors controlling freezing of the active layer in organic covered permafrost terrain. In this study, four peat Mesocosms were subjected to temperature gradients by bringing the Mesocosm tops in contact with sub-zero air temperature while maintaining a continuously frozen layer at the bottom (proxy permafrost). Soil water movement towards the freezing front (from warmer to colder regions) was inferred from soil freezing curves, liquid water content time series and from the total water content of frozen core samples collected at the end of freezing cycle. A substantial amount of water, enough to raise the upper surface of frozen saturated soil within 15 cm of the soil surface at the end of freezing period appeared to have moved upwards during freezing. Diffusion under moisture gradients and effects of temperature on soil matric potential, at least in the initial period, appear to drive such movement as seen from analysis of freezing curves. Freezing front (separation front between soil zones containing and free of ice) propagation is controlled by latent heat for a long time during freezing. A simple conceptual model describing freezing of an organic active layer initially resembling a variable moisture landscape is proposed based upon the results of this study. The results of this study will help in understanding, and ultimately forecasting, the hydrologic response of wetland-dominated terrain underlain by discontinuous permafrost.

The influence of upper-crust lithology on topographic development in the central Coast Ranges of California

A fundamental geological tenet is that as landscapes evolve over graded to geologic time, geologic structures control patterns of topographic distribution in mountainous areas such that terrain underlain by competent rock will be higher than terrain underlain by incompetent rock. This paper shows that in active orogens where markedly weak and markedly strong rocks are juxtaposed along contacts that parallel regional structures, relatively high topography can form where strain is localized in the weak rock. Such a relationship is illustrated by the topography of the central Coast Ranges between the Pacific coastline and the San Andreas fault zone (SAFZ), and along the length of the Gabilan Mesa (the “Gabilan Mesa segment” of the central Coast Ranges). Within the Gabilan Mesa segment, the granitic upper crust of the Salinian terrane is in contact with the accretionary-prism mélange upper crust of the Nacimiento terrane along the inactive Nacimiento fault zone. A prominent topographic lineament is present along most of this lithologic boundary, approximately 50 to 65 km southwest of the SAFZ, with the higher topography formed in the mélange on the southwest side of the Nacimiento fault. This paper investigates factors influencing the pattern of topographic development in the Gabilan Mesa segment of the central Coast Ranges by correlating shortening magnitude with the upper-crust compositions of the Salinian and Nacimiento terranes. The fluvial geomorphology of two valleys in the Gabilan Mesa, which is within the Salinian terrane, and alluvial geochronology based on optically-stimulated luminescence (OSL) age estimates, reveal that the magnitude of shortening accommodated by down-to-the-southwest tilting of the mesa since 400 ka is less than 1 to 2 m. Our results, combined with those of previous studies, indicate that at least 63% to 78% of late-Cenozoic, northeast–southwest directed, upper-crustal shortening across the Gabilan Mesa segment has been accommodated within the Nacimiento terrane. This is significant because perpendicular to orogenic strike the Nacimiento terrane constitutes less than ¼ of the distance between the coast and the SAFZ, and the other ¾ (or greater) of the distance between the coast and the SAFZ is underlain by the granitic upper crust of the Salinian terrane. We propose that strain and mountain building are localized within the Nacimiento terrane because it consists predominantly of the relatively weak Franciscan Complex mélange, and because the upper crust of the Salinian terrane is composed of relatively strong granitic rocks. Our hypothesis is supported by the distribution of post-seismic surface uplift associated with the 2003, 6.5 M W San Simeon earthquake, which mimics the topography of the southwestern part of the Gabilan Mesa segment of the central Coast Ranges.

A method for groundwater prospect zonation in data poor areas using remote sensing and GIS: a case study in Kalikavu Panchayath of Malappuram district, Kerala, India

The present study was an attempt to delineate potential groundwater zones in Kalikavu Panchayat of Malappuram district, Kerala, India. The geo-spatial database on geomorphology, landuse, geology, slope and drainage network was generated in a geographic information system (GIS) environment from satellite data, Survey of India topographic sheets and field observations. To understand the movement and occurrence of groundwater, the geology, geomorphology, structural set-up and recharging conditions have to be well understood. In the present study, the potential recharge areas are delineated in terms of geology, geomorphology, land use, slope, drainage pattern, etc. Various thematic data generated were integrated using a heuristic method in the GIS domain to generate maps showing potential groundwater zones. The composite output map scores were reclassified into different zones using a decision rule. The final output map shows different zones of groundwater prospect, viz., very good (15.57% of the area), good (43.74%), moderate (28.38%) and poor (12.31%). Geomorphic units such as valley plains, valley fills and alluvial terraces were identified as good to excellent prospect zones, while the gently sloping lateritic uplands were identified as good to moderate zones. Steeply sloping hilly terrains underlain by hard rocks were identified as poor groundwater prospect zones.

Hydrogeochemistry of the groundwater flow system in a crystalline terrain: a study from the Kurunegala district, Sri Lanka

Water samples collected from dug wells and tube wells from the Kurunegala District of Sri Lanka have been studied for their major hydrogeochemical parameters to understand the chemical quality of water in the terrain. The region is composed of Precambrian metamorphic rocks where groundwater is only available in the regolith and along weak structural discontinuities. The study of the major chemical constituents of groundwater revealed sev- eral relationships with the aquifer lithology. Groundwater from mafic rocks have high dissolved solids, while quartzose metaclastic rocks yield water with low dissolved solids. The study area displays very low SO4 contents of the groundwater. The chloride content is higher in the dry regions and in terrains underlain by pink granite and marble/calc gneiss while areas with marble, as expected, show high concentrations of Ca and Mg ions. The waters in the region can be classified into non-dominant cations to Na + K dominant and Cl - and HCO3 dominant types. Water from charnockite-bearing areas tends to have non- dominant cations and more CO3 + HCO3 types. Effects such as soluble salts in the regolith, fracture intensity and climatic variations play a significant role in the behavior of the hydrogeochemistry in the area.

Runoff generation mechanisms in high-relief mountainous watersheds with different underlying geology

Summary In order to examine the runoff generation processes in an extremely steep terrain underlain by relatively unweathered granite and shale, four small (0.78–7.1 ha) watersheds were monitored – two in shale and two in granite. The two shale watersheds exhibited distinctly different runoff responses to identical storm events, whereas the two granite watersheds exhibited similar responses in which the maximum runoff peak coincided with the rainfall peak. The majority of storm water runoff events in the shale watersheds and more than half of those in the granite watersheds was “old water”. Based on soil water percolated into the bedrock in the shale watersheds even during storm events, whereas subsurface storm flow parallel to the slope was observed in the granite watersheds. These data and observation suggest that the subsurface storm flow was dominant even in these extremely steep mountains. Furthermore, subsurface stormflow through the soil mantle was dominant in the granite watersheds, whereas bedrock flow was dominant in the shale watersheds.

Early African diaspora in colonial Campeche, Mexico: Strontium isotopic evidence

Construction activities around Campeche's central park led to the discovery of an early colonial church and an associated burial ground, in use from the mid-16th century AD to the late 17th century. Remains of some individuals revealed dental mutilations characteristic of West Africa. Analyses of strontium isotopes of dental enamel from these individuals yielded unusually high (87)Sr/(86)Sr ratios, inconsistent with an origin in Mesoamerica, but consistent with an origin in West Africa in terrain underlain by the West Africa Craton, perhaps near the port of Elmina, a principal source of slaves for the New World during the 16th century. These individuals likely represent some of the earliest representatives of the African Diaspora in the Americas.

The Jurassic of Warwickshire: perspectives on collecting

Southern and eastern parts of Warwickshire, central England, are a dominantly lowland terrain underlain by highly fossiliferous Lower and Middle Jurassic strata. Historically, these beds were revealed in numerous quarries and cuttings, but are now relatively poorly exposed. The Warwickshire Museum continues to collect Jurassic rocks and fossils as well as site records. Recently acquired specimens have been used for displays, outreach and research projects. Additionally, Jurassic sites continue to provide geological materials for student projects, and attract limited numbers of amateur collectors.

Pre‐ and post‐reforestation gully development in Mangatu Forest, East Coast, North Island, New Zealand

AbstractFollowing clearance of the indigenous forest and conversion of the land to pasture early in the 20th century, gully erosion became a pervasive feature in the headwaters of the Waipaoa River basin, and was notably problematic in the 140‐km2 area now covered by the Mangatu Forest. In this area, before reforestation in 1961, gully erosion affected c. 4% of the terrain. After a 24‐year exotic reforestation programme the area affected by gullies was reduced to 1.5%, but of the eight gullies larger than 10 ha in 1960 none had stabilized by 1988, although four had at least halved in size. Estimates that a gully will stabilize or increase in size under a range of conditions suggest that in the case of gullies &lt; 1 ha in area, formed in terrain underlain by Cretaceous rock, there is a &gt; 80% probability of stabilization after one forest rotation (c. 24 years). For gullies between 1 and 5 ha in area the probability of stabilization is c. 60%. Gullies of 5 ha have an even chance of stabilizing over the time frame of a single rotation. The key determinant is gully size and shape at the time of planting and, within this size range, these relationships were stronger for linear than for amphitheatre‐shaped gullies. Between 1939 and 1988 sediment production from gullies in the portion (76%) of the Mangatu Forest underlain by Cretaceous‐aged rock was c. 22 000 t km−2 yr−1, and during the period of maximum sediment production (1939–1960) they may have accounted for c. 17% of the Waipaoa River's average annual suspended sediment load. Reforestation reduced the contribution to c. 8% in the period between 1970 and 1988. However, the off‐site (downstream) impact of sediment generated by the remaining 420 active gullies in the Waipaoa River catchment is significant, not least on the capacity of the scheme that protects high‐value agricultural land on the Poverty Bay Flats from flooding. A targeted reforestation programme may be an alternative to raising the height of the existing artificial levees. It is estimated that additional exotic plantings totalling c. 15 400 ha (c. 7% of the Waipaoa River basin area) would produce a &gt; 64% reduction in sediment production from gullies on pastoral hillslopes within one forest rotation (c. 24 yr). Copyright © 2005 John Wiley &amp; Sons, Ltd.

Debris-flow generation from recently burned watersheds

Abstract Evaluation of the erosional response of 95 recently burned drainage basins in Colorado, New Mexico and southern California to storm rainfall provides information on the conditions that result in fire-related debris flows. Debris flows were produced from only 37 of 95 ( approximately 40 percent) basins examined; the remaining basins produced either sediment-laden streamflow or no discernable response. Debris flows were thus not the prevalent response of the burned basins. The debris flows that did occur were most frequently the initial response to significant rainfall events. Although some hillslopes continued to erode and supply material to channels in response to subsequent rainfall events, debris flows were produced from only one burned basin following the initial erosive event. Within individual basin, debris flows initiated through both runoff and infiltration-triggered processes. The fact that not all burned basins produced debris flows suggests that specific geologic and geomorphic conditions may control the generation of fire-related debris flows. The factors that best distinguish between debris-flow producing drainages and those that produced sediment-laden streamflow are drainage-basin morphology and lithology, and the presence or absence of water-repellent soils. Basins underlain by sedimentary rocks were most likely to produce debris flows that contain large material, and sand- and gravel-dominated flows were generated primarily from terrain underlain by decomposed granite. Basin-area and relief thresholds define the morphologic conditions under which both types of debris flows occur. Debris flows containing large material are more likely to be produced from basins without water-repellent soils than from basins with water repellency. The occurrence of sand-and gravel-dominated debris flows depends on the presence of water-repellent soils.