Rugged Plateaus and Extensive Floodplains — Modelling Landform Evolution in a Northern Australian Catchment

Medium to large natural catchments are often more spatially heterogeneous than small catchments or single landforms. Attempting to model landform evolution of large areas is consequently more complex. This paper demonstrates that modelling landform evolution in medium to large catchments can be improved by calibrating the model to smaller, more geomorphologically homogenous sub‐catchments. The paper investigates landform evolution in the Ngarradj catchment in the Northern Territory of Australia (a medium scale catchment of approximately 67 km²). The catchment is complex and contains two distinct landform regions; an upland plateau region with highly dissected sandstone and shallow, sandy soils, and a lowlands region with gentle, wooded slopes and floodplains with deep, sandy soils. The SIBERIA landform evolution model is calibrated and applied to the Ngarradj catchment. The complexity of the Ngarradj catchment is incorporated into the modelling by dividing the catchment into three sub‐catchments (Swift Creek (SC), Upper Main (UM) and East Tributary (ET)) which are relatively homogeneous and for which hydrology and sediment transport data are available. A discharge‐area relationship and long‐term, sediment loss rates for the catchment are derived based on an annual series flood frequency analysis of a 20 year runoff record predicted in a previous study. Sediment transport modelling incorporates both suspended and bedload sediment loss. The denudation rates derived using these data are 37, 63 and 77 mm kyr^?1 for the SC, UM and ET sub‐catchments, respectively. Model predictions indicate that the UM sub‐catchment will have the greatest mean erosion. This is balanced by the large amount of deposition that will occur in the upper Ngarradj valley of the UM sub‐catchment. Further deposition occurs on the floodplain of Ngarradj, with the area between the SC and ET/UM (up‐stream) sub‐catchments experiencing a small net accretion of sediment (15 mm kyr^?1).     

Fluvial Sediment Supply to the North-East Queensland Coast,Australia

Rivers play a dominant role in supplying sediment to the mainly siliceous sandy north-east Queensland coast. The interaction of geology and the seasonally wet tropical climate in the coastal catchments results under natural conditions in a high, but seasonally variable river discharge and sediment yield. These are examined in the sample catchments of the Barron, Mulgrave-Russell and Burdekin Rivers Peak sediment delivery occurs with major river floods produced by tropical cyclones. The initial erosive impact of these floods and subsequent constructional effects are examined along the Burdekin and Barron delta coasts. Human interference in the coastal catchments has caused increased sediment yield especially with extensive clearance of tropical rainforest and with some agricultural practices; but when dams and weirs trap sediment and sand is extracted from river beds a decrease occurs.     

‘Torrents of Terror’: the August 1998 Storm and the Magnitude,Frequency and Impact of Major Floods in the Illawarra Region of New South Wales

The Illawarra Region some 80 kilometres south of Sydney is characterised by a prominent coastal escarpment that rises to 700 m within 12 km of the coast and forms a locus for frequent, high intensity rainfall events. One of the most recent recorded events occurred on 17 August 1998 with rainfall intensities at several pluviometers exceeding 120 mm hr^‐1 over a duration of one hour, with up to 249 mm falling in 3.5 hours during the main storm burst. Detailed pluviometer data indicate that the storm was non‐stationary and moved down catchment producing a widespread zone of 120 mm hr^‐1 intensity rainfall over a 30 minute duration across mid‐lower catchment areas after similar intensity but longer duration rainfall in catchment headwaters. Slope‐area reconstructions of peak discharge indicate that small catchments on the escarpment within the zone of maximum intensity experienced close to 100% rainfall‐runoff relationships, with peak discharges correlated to short duration (<1 hr) peak rainfall intensities. Widespread erosion occurred particularly where urban development had encroached on natural water courses. Debris/hyperconcentrated flows originating from both anthropogenic and natural sediment sources caused damage to urban areas. This paper provides an overview of the spatial and temporal characteristics of the 17 August 1998 storm, the hydrologic and geomorphic response of the streams, and the nature of damage to urban areas. It reassesses the frequency of recent high‐magnitude rainfall/flood events in the region, discussing the relationships between rainfall intensities, estimates of flood magnitudes and stormwater channel capacities.     

The deserts of the Kashmir‐Tibetan frontier

Abstract

Semi-arid landscapes are vulnerable to cultivation, overgrazing and climate variability, although it is difficult to identify the relative significance of these three factors. In the South African Karoo, the ‘desertification debate' seeks to explain a change to more shrubby vegetation in heavily grazed areas. We examine these issues in catchments where farm reservoirs provide sediment stores with 137Cs, 210Pb, geochemical and mineral magnetic signatures. Rainfall data and stocking numbers are reported and current erosion rates are estimated. Sediment accumulation in the reservoirs increases between 1935 and 1940, probably due to rises in the frequency of rainfall events of >25mm day. Significant increases in sedimentation rates (×8) occur during rain-fed wheat cultivation. In an uncultivated catchment, sediment yields remain relatively high and reflect increases in erosion from hillslopes, colluvial storage, and cultivated land sources. Gully systems have acted as transport routes rather than sediment sources over the last ～70 years. Badland erosion rates average ca. 50t ha^?1 yr^?1. At Ganora, their development in the 1920s strongly influences peak sedimentation between 1970 and 1980. Any delay between badland initiation and increased sediment yield appears to be a function of landscape connectivity. Recovery following disturbance is occurring slowly and is likely to take >100 years. De-stocking and better management systems are reducing erosion rates but may be offset by increases in rainfall intensity.     

A Role for Geomorphology in Integrated Catchment Management

Catchments (watersheds) are integrated through water movements. Water moves sediments; and the translocation of materials has geomorphic and pedological responses. Fertiliser and pesticide residues and other toxic substances are also transported by water, in solution and attached to sediment particles, affecting various ecological components of catchments. The insertion of pollutants is directly caused by land‐use practices. In turn, land uses are influenced by the pedo‐geomorphology of the catchments (uplands, valley sides, flood plains and terraces, spurs and hollows, convexities and concavities; position in the upper, middle or lower catchment; and by catenary relationships). Therefore catchment management must deal with interrelationships amongst human activities and various geomorphic elements. Several case study examples from southwestern Australia demonstrate the importance of geomorphology in catchment management. They include: rates of hillslope erosion and translocation of P in micro‐catchments within a drainage basin context; stream salinisation; accumulation of heavy metals in stream sediments; geomorphic elements influencing the spread of weeds and the erosion of access tracks; and some relationships between valley‐side land uses and stream morphology. The incorporation of these and other issues in a management strategy is then illustrated. Complex skills are involved in producing and implementing a management strategy, including but extending well beyond those of the geomorphologist.     

Sediment Loss from a Waste Rock Dump,ERA Ranger Mine,Northern Australia

During the 1994/95 wet season, runoff, suspended load and bedload loss from large scale erosion plots under natural rainfall events were measured at three sites with different treatments on the Energy Resources of Australia Ranger waste rock dump (cap, soil, fire). The fire site has well established trees, the soil site smaller shrubs and the cap site minimal vegetation. All three sites are located on the flat areas of the waste rock dump with similar slopes and cap, fire and soil are local names for the sites. The quantity of bedload eroded from the soil and fire sites decreased during the monitoring period, however, the same trend was not observed on the cap site. Bedload loss from the fire and soil sites is significantly log‐linear with time and reduces at a decreasing rate until becoming statistically constant toward the later part of the first half of the wet season. For corresponding storms at each site, bedload erosion was highest from the unvegetated and unripped cap site and lowest from the well vegetated fire site. Site specific linear relationships between bedload and total sediment load have been derived and can be used to derive total sediment load in the absence of measured suspended load.     

History,Geomorphological Problems and Effects of Dredging in Cleveland Bay,Queensland

The history of dredging is examined for Cleveland Bay, 1883–1988, as the port of Townsville developed. Between 1883 and 1964 only intermittent data exist on localities, depths and quantities dredged but later much more detailed records are available for analysis. Geomorphological problems emerged and are considered with particular reference to material forming the seabed, the sediment yield from the coastal catchments and its seasonally and annually irregular delivery to the coast by the rivers, and sediment movement within the bay in response to wind, wave and tidal processes. Analysis of aerial photographs, 1941–1988, has enabled some dredging effects to be studied, especially relating to localised land reclamation and more widespread probable effects on seagrass beds and mangroves.     

Antarctic history: A reply to professor W. H. Hobbs

On July 25th 1983 a severe thunderstorm brought torrential rain and flooding to a small valley near Newcastleton in Roxburghshire. At one site within the valley, around 65 mm of rainfall was recorded in 75 minutes, though this was probably 2 km from the centre of the storm. The resulting peak flow was estimated at almost 170 m³s^‐1 from a drainage area of 36.9 km². Lightning killed some livestock whilst many more were drowned. The storm triggered peat slides on several hillsides and caused extensive erosion of river banks and damage to buildings and bridges. The associated high sediment load killed fish stocks in the receiving waters. The flood was a significant event in terms of its human disruption and its impact on the natural environment.     

Basin‐Wide Sediment Grain‐Size Numerical Analysis and Paleo‐Climate Interpretation in the Shiyang River Drainage Basin

Basin‐wide sediment transport affects estimates of basin sediment yield, which is a fundamental scientific issue in drainage basin studies. Many studies have been conducted to examine erosion and deposition rates in drainage networks. In this study, we proposed a new approach using grain‐size standard deviation model of sedimentary samples from different geomorphological units for numerical analysis and paleo‐climate interpretation in the Shiyang River drainage basin, arid China. 1043 sedimentary samples were obtained from the upper reaches, the midstream alluvial plain and the terminal lake area; chronological frames were established based on 58 radiocarbon ages. Grain‐size standard deviation model was introduced to examine sediment components according to grain‐size and transport forces. In addition, transient paleo‐climate simulations, including the Community Climate System Model version 3 and the Kiel models, were synthesized, as well as the results from PMIP 3.0 project, to detect the long‐term climate backgrounds. Totally, we found four major common components, including fine particulates (<2 μm), fine silt (2–20 μm), sandy silt (20–200 μm), coarse sand (>200 μm), from basin‐wide sedimentary samples. The fine particulates and fine silt components exist in all the sedimentary facies, showing long‐term airborne aerosol changes and its transport by suspended load. There are some differences in ranges of sandy silt and coarse sand components, due to lake and river hydrodynamics, as well as the distance with the Gobi Desert. Paleo‐climate simulations have shown that the strong Asian summer monsoon during the transition of the Last Deglaciation and Holocene was conducive to erosion and transport of basin‐wide suspended load, also enhancing sediment sorting effects due to strong lake hydrodynamics. Our findings provide a new approach in research of long‐term basin‐wide sediment transport processes.     

Sediment and solute transport in a representative basin

Suspended sediment and total dissolved material concentrations in stream flow were studied in Deep Creek Representative Basin near Cessnock, N.S.W. between April, 1976 and June, 1977. Sediment and solute transport rates were calculated at two gauging stations for individual flood events and low flow periods using the hourly hydrograph record, stage/season concentration-discharge relationships and calculated rating curves. Limitations in the use of the rating curve suggest that during flood hydrographs of long return period, calculated transport rates may be in error. A continuous record of concentration variation was sampled during one runoff event enabling comparisons to be made between methods of load calculation. For the satisfactory calculation of transport rates during extreme flood hydrographs, it is essential that concentration variation be accurately defined.     

Fire,Humans and Denudation at Wangrah Creek,Southern Tablelands,N.S.W.

The influence of altered fire regimes on the denudation of a catchment is determined from alluvial deposits of the last 10,000 yrs and by monitoring runoff and erosion before and after a wildfire. An increase in fire frequency beginning at 3,000–4,000 yrs BP, as a result of intensified Aboriginal burning, did not change the mechanisms or rates of denudation nor did it cause widespread alluviation as suggested by others. The results of monitoring show that before and after mild fires there is insufficient runoff on most slopes to entrain sediment. Only after intense fires are runoff and erodibility increased enough to significantly accelerate erosion. Conditions are then identified which are most likely to lead to accelerated erosion from altered fire regimes in other catchments.     

Estuarine Beach Morphology in Microtidal Middle Harbour,Sydney

Microtidal Middle Harbour, Sydney, contains a range of estuarine beach morphologies from steep reflective beachfaces to wide low‐tide terraces. The most exposed beaches are morphologically similar to lowest‐energy open‐ocean beaches; however, the remainder of the profiles cannot be adequately explained by established beach morphodynamic models. Estuarine beach morphology in Middle Harbour appears to be primarily influenced by storm events. Deep‐water ocean waves rework profiles in the lower part of estuary. For beaches protected from these waves, tidal fluctuations are the main determinant of beach profile. Locally‐generated wind waves are only a significant energy contributor where the fetch is large enough; however, this only occurs on two beaches where low‐tide terraces are formed.     

Paleoenvironmental catastrophies on the Peruvian coast revealed in lagoon sediment cores from Pachacamac

Paleoenvironmental reconstruction combining data from diatom, pollen, macrophytic, and archaeological analyses had not been attempted previously for coastal Peru. We analyzed two radiocarbon-dated sediment cores extracted from the “sacred” Urpi Kocha Lagoon at the pre-Hispanic religious center of Pachacamac (ca. AD 350-1533), near Lima, to characterize the paleoenvironment of the site. The data reveal an environmental history characterized by alternating periods of severe drought and catastrophic flood. Recurrent pulses with distinctive sediment, diatom, pollen, and macrophytic characteristics reflecting variations in water depth, substrate, turbidity and salinity, usually contain a thick basal sandy storm deposit overlain by thin organic accumulations of peat, gyttja, and snails. The most intense of these disturbance pulses, when correlated with pollen, plant macrofossils, radiocarbon dates, and regional proxy data, provide local evidence of four major floods associated with sustained, catastrophic El Niño events, tsunamis and/or severe storms in the Andean highlands over the last 2000 years.     

MAN-INDUCED CHANGES IN OVERALL RUNOFF IN THE ARAL SEA BASIN

The article examines the prospects of using runoff characteristics and changes in runoff composition as a basis for predicting environmental modification. The technique is employed to assess changes in the circulation of matter that are taking place in the drainage basin of the Aral Sea as a result of the intensification of irrigation. Overall runoff, including its liquid, solid (suspended sediment) and chemical phases, is analyzed in connection with a shift in the deposition of material from the Aral Sea to the irrigated plains.     

HIGH‐RESOLUTION (PIXE) ANALYSES OF CARBONATE DEPOSITS IN A ROMAN AQUEDUCT (FRÉJUS,SE FRANCE): IMPLICATIONS FOR THE STUDY OF PALAEOHYDROLOGICAL VARIABILITY AND WATER RESOURCES MANAGEMENT IN SOUTHERN GAUL DURING THE ROMAN PERIOD*

Urban water was supplied to the Roman city of Forum Julii (Fréjus, southeastern France) for at least 200 years by a 39.4 km long aqueduct, operating in ad 50. Two perennial springs were successively collected, the Foux and the Siagnole, located at the outlet of Triassic and Jurassic karstic reservoirs, respectively. In this study, we performed high‐resolution PIXE (Particle‐Induced X‐ray Emission) measurements of Ca, Si, Fe and Sr concentrations on selected cross‐sections of laminated carbonate sampled along the sidewalls, before and after the connection of the two collection channels. Seasonal variations of water composition, suspended sediment load and discharge are recorded by alternating clear and dark layers, and can be traced by their Sr and Fe contents. On the basis of an annual bimodal high‐discharge regime for the two karstic reservoirs, the concentration measurements allow the derivation of a chronological record (117.5 years) of water supply and maintenance activities for a part of the operational period of the aqueduct. The water level in the channel was mainly controlled by the extent of carbonate deposition and by dredging, repair and maintenance operations rather than by the past hydrological regime of the two springs.     

A comparison between the Mediterranean climates of Eurasia and the Americas

On the 19th and 20th September 1981 heavy rain fell throughout Scotland. Almost 140mm was recorded for the 24 hours up to 9.00 am on the 20th at Dundonnell in Wester Ross, an event with a recurrence interval of approximately 120 years. The storm generated a peak discharge of c.60m³ s‐¹ on the Ardessie Burn which drains a 13.3km² catchment on the northwestern slopes of An Teallach. The resulting flood caused severe localised bank erosion and partially destroyed a fish farm. The total volume of measured flood deposits exceeded 1800 tonnes, approximately equal to 14 years normal average annual non‐dissolved sediment output. The identification of specific sediment source areas and deposition sites allows an assessment of geomorphic work in terms of direct landscape adjustment in response to extreme rainfall. The flood stresses the importance of rare events in the landscape evolution of upland environments and the influence of antecedent catchment conditions.     

The Impact of Urbanisation on the Bed Sediments of South Creek,New South Wales

The textural and geochemical characteristics of the bed sediments in South Creek are examined. The bed substratum in the urbanised reaches differs from that of the rural reaches, in terms of the volume and size of the fine matrix sediment and the concentration of heavy metals. The impact of urbanisation on the river-bed sediment is variable, with less matrix than ‘expected’ within the substratum of the urban reach and more than ‘expected’ in the below urban reach. The urban matrix sediment is also significantly finer in comparison to the rural reach matrix sediment. Heavy metal concentrations within the urbanised matrix sediment are up to 800 times greater than background levels and are considered to be indicative of extreme contamination despite only SO years of urbanisation.     

Thunderstorm charging: calculations of the effect of ice crystal size and graupel velocity

A one-dimensional model of a developing thunderstorm has been adapted in order to test the importance of various charge transfer parameters to the electrical development of the storm. The charge transfer data are taken from laboratory measurements obtained when ice crystals rebound off soft-hail particles in the presence of supercooled water droplets. The experiments showed that the charge transfer depends on ice crystal size and impact velocity, so a range of size and velocity dependences have been tested in the model. The calculations have been performed using initial conditions from three storm cases having a range of storm severities. The charging rates calculated were found to be affected more by the initial storm conditions than by the specific size and velocity dependencies chosen.     

Forests for cotton: Institutions and organizations in Brazil's mid-twentieth-century cotton boom

The western region of São Paulo state, Brazil, became one of several sites of global cotton production during the first half of the twentieth century in response to increased global demand and fears of cotton shortages. The cotton boom tapped a ‘forest rent’ that helped Brazil rise to become the largest producer in Latin America, providing both export revenue and critical raw material to a growing industrial economy that would become the largest in South America. This paper uses an organizational and institutional perspective to analyze causes and effects of the mid-century cotton boom that centered on São Paulo state. Organizations and institutions relating to cotton production are considered using oral histories, judicial documents, agronomic texts, and the sediment record in small catchments as empirical evidence. The state dramatically reformed some organizations to provide the key inputs to cotton production, while most contemporary observers ignored institutions, such as sharecropping and tenant farming, that supported cotton. São Paulo's institutions and organizations were characterized by the borrowing and adaptation of existing labor institutions, the creation of new state organizations that subsidized elite producers, and the weak development of institutions or organizations in response to soil fertility issues and labor supply.     

Agricultural Transformation and the Politics of Hydrology in Northern Thailand

Tensions over water resources in upland areas of northern Thailand are often attributed to reductions in water supply caused by forest clearing. This article argues that the hydrological evidence for such reductions in supply is very weak and that, rather, the key hydrological issue in upland catchments is a significant increase in water demand, especially during the dry season. The arguments are illustrated with a detailed examination of the Mae Uam catchment, located in Chiang Mai province, where the development of dry–season soybean cultivation appears to have tested the hydrological limit of the catchment, and even exceeded this limit in drier years. The author argues that a shift in focus from water supply to water demand has fundamentally important political implications. As long as the focus of public debate is on water supply, the regulatory focus will be on those resident in the forested upland areas that are seen as being crucial in securing downstream flows. But if the water management focus is shifted to water demand, then regulatory attention must shift to the diverse sources of demand that exist throughout the hydrological system.