‘Natural’ rivers, ‘hydromorphological quality’ and river restoration: a challenging new agenda for applied fluvial geomorphology

Fluvial geomorphology is rapidly becoming centrally involved in practical applications to support the agenda of sustainable river basin management. In the UK its principal contributions to date have primarily been in flood risk management and river restoration. There is a new impetus: the European Union's Water Framework and Habitats Directives require all rivers to be considered in terms of their ecological quality, defined partly in terms of ‘hydromorphology’. This paper focuses on the problematic definition of ‘natural’ hydromorphological quality for rivers, the assessment of departures from it, and the ecologically driven strategies for restoration that must be delivered by regulators under the EU Water Framework Directive (WFD). The Habitats Directive contains similar concepts under different labels. Currently available definitions of ‘natural’ or ‘reference’ conditions derive largely from a concept of ‘damage’, principally to channel morphology. Such definitions may, however, be too static to form sustainable strategies for management and regulation, but attract public support. Interdisciplinary knowledge remains scant; yet such knowledge is needed at a range of scales from catchment to microhabitat. The most important contribution of the interdisciplinary R&D effort needed to supply management tools to regulators of the WFD and Habitats regulations is to interpret the physical habitat contribution to biodiversity conservation, in terms of ‘good ecological quality’ in rivers, and the ‘hydromorphological’ component of this quality. Contributions from ‘indigenous knowledge’, through public participation, are important but often understated in this effort to drive the ‘fluvial hydrosystem’ back to spontaneous, affordable, sustainable self‐regulation. Copyright © 2006 John Wiley & Sons, Ltd.     

The use of ‘rock hard’-sodium sulphate tablets for quickly determining preferential water flow routes in soils

Subsurface water flow routes have been inferred from gypsum sphere weight loss with minimal disturbance to soil structure or flow nets (Crabtree and Trudgill, 1984). This technique is limited by the relatively slow, fixed dissolution of gypsum. A solution is to use tablets consisting of Durham's ‘Rock Hard Water Putty’ and sodium sulphate. Laboratory tests indicate that tablet dissolution is controlled by the ratio of Rock Hard to sodium sulphate and is proportional to net throughflow yet independent of throughflow pH (between 3·0 and 9·0) and throughflow rate (at rates < 0·04 1 hr^?1). The tablets are also mechanically stable, economical, and environmentally safe. Field study results using the Rock Hard-sodium sulphate tablets are consistent with both the laboratory findings and the results of previous investigators.     

‘Bedload’ dynamics: Grain-grain interactions in water flows

Owing to experimental difficulties, the transport stage at which collisions between moving ‘bedload’ grains might become significant has never been investigated, yet the existence or otherwise of such collisions is of some importance in the understanding of the mechanics of sediment transport, in particular the theory developed by Bagnold. Application of the basic principles of gaseous kinetic theory to ‘bedload’ grains moving in saltant trajectories and the adoption of a ‘characteristic’ saltation path leads to the prediction that grain-grain collisions should dominate in the transport of coarse sands over plane beds in water flows above a transport stage of about 2, i.e. when the mean boundary fluid shear stress exceeds the critical boundary shear stress for grain motion by about 4 times. Above this stage interrupted saltations should always occur, with the ‘bedload’ grains held above the stationary bed by a combination of fluid and solid momentum transfer mechanisms. A classification of the types of grain motions is given and evidence is presented for the existence of an upward decrease in grain collision frequency and of grain concentration at the top of the ‘bedload’ zone.     

Investigation of ‘equal displacement’ rule for bridges subjected to differential support motions

The ‘equal displacement’ rule is employed in seismic design practice to predict inelastic displacements from analyses of the corresponding linear elastic structural models. The accuracy and limitations of this rule have been investigated for ordinary structures but not for bridges subjected to spatially varying ground motions. The present study investigates this rule for moderate levels of inelastic behavior for four highway bridges in California accounting for the effects of spatial variability of the support motions due to incoherence, wave passage and differential site response. The bridge models vary significantly as to their fundamental periods and their overall configurations. Statistical analyses of pier‐drift responses are performed using as input simulated arrays of nonstationary ground motions in accordance with prescribed coherency models. It is found that the ‘equal displacement’ rule is fairly accurate for cases when the fundamental period of the bridge is longer than the transition period between the acceleration‐controlled and velocity‐controlled ranges of the response spectrum. Otherwise, the rule is non‐conservative for cases with large ductility factors and conservative for cases with small ductility factors. Wave passage and incoherence tend to reduce ratios of mean peak inelastic to elastic pier drifts, whereas incorporation of the differential site‐response effect by locating piers on softer soils tends to increase the same ratios. Mild or moderate positive correlation between these ratios and ductility demands is observed in most cases. Effects of spatial variability are more pronounced for longer and stiffer bridges. Copyright © 2013 John Wiley & Sons, Ltd.     

‘frost and salt’ weathering: Further experimental results

A laboratory experiment has been conducted to examine the effects of ‘frost and salt’ weathering (i.e. physical breakdown by the freezing of salt solutions) on a limestone. Results show that the presence of certain salts in solution can inhibit frost damage. These findings are in direct conflict with those presented by Goudie (1974) and, more recently, Williams and Robinson (1981). Comparison of the experimental methods used in each of these three studies suggests that opposing results can be explained in terms of the different experimental procedures which were employed. If salt supply is frequent and plentiful then it seems likely that rock breakdown will be enhanced-this is the case represented by the experiment of Williams and Robinson. Conversely if the salt supply is limited and the amounts of salt remain more or less constant then rock breakdown will be inhibited-the case of the present experimental study. Caution is therefore advocated when attempting to extrapolate laboratory-derived results to infer on the behaviour of rocks under natural conditions. Several environmental situations in which ‘frost and salt’ weathering may be a possibility are dsiscussed, but it is concluded that further field data, especially concerning temperature regimes and salt availability at and below rock surfaces in cold regions, would be necessary before more definite statements could be made about the efficacy of this process.     

The ‘humped’ soil production function: eroding Arnhem Land,Australia

We report erosion rates and processes, determined from in situ‐produced beryllium‐10 (¹⁰Be) and aluminum‐26 (²⁶Al), across a soil‐mantled landscape of Arnhem Land, northern Australia. Soil production rates peak under a soil thickness of about 35 cm and we observe no soil thicknesses between exposed bedrock and this thickness. These results thus quantify a well‐defined ‘humped’ soil‐production function, in contrast to functions reported for other landscapes. We compare this function to a previously reported exponential decline of soil production rates with increasing soil thickness across the passive margin exposed in the Bega Valley, south‐eastern Australia, and found remarkable similarities in rates. The critical difference in this work was that the Arnhem Land landscapes were either bedrock or mantled with soils greater than about 35 cm deep, with peak soil production rates of about 20 m/Ma under 35–40 cm of soil, thus supporting previous theory and modeling results for a humped soil production function. We also show how coupling point‐specific with catchment‐averaged erosion rate measurements lead to a better understanding of landscape denudation. Specifically, we report a nested sampling scheme where we quantify average erosion rates from the first‐order, upland catchments to the main, sixth‐order channel of Tin Camp Creek. The low (～5 m/Ma) rates from the main channel sediments reflect contributions from the slowly eroding stony highlands, while the channels draining our study area reflect local soil production rates (～10 m/Ma off the rocky ridge; ～20 m/Ma from the soil mantled regions). Quantifying such rates and processes help determine spatial variations of soil thickness as well as helping to predict the sustainability of the Earth's soil resource under different erosional regimes. Copyright © 2009 John Wiley & Sons, Ltd.     

Hydrogeochemical balance of forest umbrisol profiles (‘Sierra de Gata’, central western Spain)

Three techniques for obtaining soil water solutions (gravitational and matrical waters extracted using both in situ tension lysimeters and in vitro pressure chambers) and their later chemical analysis were performed in order to know the evolution of the soil‐solution composition when water moves down through the soil, from the Ah soil horizon to the BwC‐ or C‐horizons of forest soils located in western Spain. Additionally, ion concentrations and water volumes of input waters to soil (canopy washout) and exported waters (drainage solutions from C‐horizons) were determined to establish the net balance of solutes in order to determine the rates of leaching or retention of ions. A generalized process of sorption or retention of most components (even Cl^?) was observed, from the soil surface to the C‐horizon, in both gravitational and matrical waters, with H₄SiO₄, Mn²⁺, Na⁺, and SO₄^2? being the net exported components from the soil through the groundwater. These results enhance the role of the recycling effect in these forest soils. The net percentages of elements retained in these forest soils, considering the inputs and the outputs balance, were 68% K⁺, 85% Ca²⁺, 58% Mg²⁺, 7% Al³⁺, 5% Fe³⁺, 34% Zn²⁺, 57% Cl^?, and 20% NO₃^?, and about 75% of dissolved organic carbon was mineralized. Copyright © 2007 John Wiley & Sons, Ltd.     

The contribution of the built environment to the ‘free-field’ ground motion in Mexico City

Soil–structure interaction (SSI) effects on building dynamic behaviour have been studied extensively. In comparison, the radiation of waves away from the soil–foundation interface has received little attention. Recent studies point out that SSI in an urban environment can modify the ground motion recorded in the free-field. These modifications will be important when two conditions are met: structures founded on soft soils and coincidence between the vibration periods of the structure and those of the superficial layers. Both conditions are met in Mexico City lake zone. In this study, we investigate SSI effects on ‘free-field’ motion. The data we use consist of microtremors recorded on soft soils in Mexico City, a densely built environment. Our objective was to identify the modifications to free-field ground motion caused by neighbouring structures. Data were analysed using H/V spectral ratios. Large variations in the level of amplification and resonant frequency were determined from microtremors in very closely spaced stations. Our results suggest consistently that free-field ground motion is significantly affected by the presence of neighbouring structures.     

A bootstrap calculation of confidence regions for proportions of sediment contributed by different source areas in a ‘fingerprinting’ model

When ‘fingerprinting’ is used to identify what proportions P_s (s = 1, …, g) of suspended sediment come from g different source areas, measures of the uncertainties in estimates of the P_s are also required. These uncertainties are influenced by two kinds of correlation whose effects are rarely recognized in the literature. These are (i) correlation between the estimated P_s because they must add to 1 and (ii) correlation between the geochemical tracers measured in sediment samples. This paper uses bootstrap procedures to identify joint confidence regions for the estimated proportions [responding to correlation of type (i)] and to explore alternatives to the ‘standard’ least‐squares criterion used to estimate the proportions when tracer measurements are correlated [correlation of type (ii)]. Using a limited dataset with three sediment source areas for illustration (g = 3), results were obtained from 5000 bootstrap samples, using two criteria (standard and generalized least squares, GLS) with two inequality constraints: (a) 0 ≤ P_s ≤ 1, where P_s is the fraction of suspended sediment contributed by the s‐th source area (s = 1, 2, 3) and (b) 0 < P_s < 1, which, the paper argues, better represents reality. Approximate 95% confidence regions for the P_s, given by the two criteria and two inequality constraints, were compared. Using inequality constraint (a), the confidence region given by the GLS criterion was slightly smaller than that given by the standard; using constraint (b), the two confidence regions' boundaries were almost identical, suggesting that the effects of correlations between tracers were not large for the dataset used. For both criteria, the scatter amongst estimated proportions P_s obtained by bootstrapping was large, raising issues concerning the efficiency of sampling and the allocation of sampling effort, both in source areas and in transported suspended sediment. The results suggest that apparently small differences in the constraints applied to the proportions P_s can give quite different numerical results. Copyright © 2014 John Wiley & Sons, Ltd.     

The Icelandic ‘rofabard’ soil erosion features

Soil erosion and desertification are severe problems in Iceland. Erosion processes are numerous, and more than one can occur at each site, resulting in many erosional forms. Erosion forms and an erosion severity scale are the basis for a recent national survey of erosion in Iceland. One of the most distinctive erosion forms in Iceland is an erosion escarpment, termed ‘rofabard’ in Icelandic. Rofabards are formed in thick but non‐cohesive Andosols that overlie more cohesive materials such as glacial till or lava. The relatively loose Andosols beneath the root mat are undermined, creating escarpments, or rofabards. The rofabards retreat as a unit, with a fully vegetated and rich ecosystem on top but leaving barren desert in their place. Rofabards are common within a 20 000 km² area. The Agricultural Research Institute and Soil Conservation Service erosion database suggests that erosion associated with rofabards has denuded 15 000 –30 000 km² of land that was previously fully vegetated and had fertile Andosols, but is now mostly desert. Erosion rates associated with rofabards are reported as the loss of vegetated land with Andosol mantle, measured as hectares per square kilometre per year. This measure of erosion has more meaning for Icelandic landscapes than the traditional tonnes per hectare per year. Estimated losses of Andosol cover in rofabard areas for the whole country are currently about 230 ha a⁻¹. This rate is about 10 times lower than the rate needed to cause estimated losses of Andosol mantle in rofabard areas since settlement, 1125 years ago. During peak years of soil erosion, losses were probably several thousand hectares per year, but the erosion rates slowed down as extensive Andosol areas have become barren deserts. Copyright © 2000 John Wiley & Sons, Ltd.     

Assessing the ‘two water worlds’ hypothesis and water sources for native and exotic evergreen species in south‐central Chile

Recent studies using water‐stable isotopes (δ¹⁸O and δ²H) have suggested an ecohydrological separation of water flowing to streams or recharging groundwater and water used by trees, known as the ‘two water worlds’ (TWW) hypothesis. In this study, we measured water isotopic composition in precipitation [open field and throughfall, i.e. local meteoric water line (LMWL)] and the mobile water compartment (i.e. stream and soil solution), bulk soil water and xylem water over a period of 1.5 years in two headwater catchments: NF, covered with old growth native evergreen forest (Aetoxicon punctatum, Laureliopsis philippiana and Eucriphya cordifolia), and EP, covered with 4 and 16‐year‐old Eucalyptus nitens stands. Our results show that precipitation, stream and soil solution plot approximately along the LMWL, while xylem waters from all studied tree species plot below the LMWL, supporting the TWW hypothesis. However, we also found evidence of ecohydrological connectivity during the wet season, likely controlled by the amount of antecedent precipitation. These observations hold for all investigated tree species. On both sites, a different precipitation source for stream and xylem water was observed. However, in EP, bulk soil showed a similar precipitation source as xylem water from both E. nitens stands. This suggests that E. nitens may use water that is recharging the bulk soil compartment. We conclude that under a rainy temperate climate, the TWW hypothesis is temporal and does not apply during wet seasons. Copyright © 2016 John Wiley & Sons, Ltd.     

Origin and karst geomorphological significance of the enigmatic Australian Nullarbor Plain ‘blowholes’

The Australian Nullarbor Plain, one of the world's largest limestone platforms (~200 000 km²), has few distinctive surface karst features for its size, but is known for its enigmatic ‘blowholes’, which can display strong barometric draughts. Thousands of these vertical tubes with decimetre–metre (dm–m) scale diameter puncture the largely featureless terrain. The cause and distribution of these has remained unclear, but they have been thought to originate from downward dissolution and/or salt weathering. To elucidate blowhole distribution and mode of formation we (i) correlated existing location data with Shuttle Radar Topography Mission (SRTM) data, which distinguishes the subtle undulations (< 10 m per km) of the landscape, (ii) surveyed blowhole morphology and (iii) determined their rock surface hardness. Over a sampled area of 4200 km², the distribution of 615 known blowholes is not correlated with present topography. Blowholes are often connected to small or, in some cases extensive, but typically shallow cavities, which exhibit numerous ‘cupolas’ (dome‐shaped pockets) in their ceilings. Statistical arguments suggest that cavities with cupolas are common, but in only a few cases do these puncture the surface. Hardness measurements indicate that salt weathering is not their main cause. Our observations suggest that blowholes do not develop downwards, but occur where a cupola breaks through the surface. Lowering of the land surface is suggested to be the main cause for this breakthrough. Although cupolas may undergo some modification under the current climate, they, as well as the shallow caves they are formed in, are likely to be palaeokarst features formed under a shallower water table and wetter conditions in the past. The findings presented have implications for theories of dissolutional forms development in caves worldwide. The environmental history of the Nullarbor platform allows testing of such theories, because many other factors, which complicate karst evolution elsewhere, have not interfered with landform evolution here. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation of predictors of non‐linear seismic demands using ‘fishbone’ models of SMRF buildings

Predictors (or estimates) of seismic structural demands that are less computationally time‐consuming than non‐linear dynamic analysis can be useful for structural performance assessment and for design. In this paper, we evaluate the bias and precision of predictors that make use of, at most, (i) elastic modal vibration properties of the given structure, (ii) the results of a non‐linear static pushover analysis of the structure, and (iii) elastic and inelastic single‐degree‐of‐freedom time‐history analyses for the specified ground motion record. The main predictor of interest is an extension of first‐mode elastic spectral acceleration that additionally takes into account both the second‐mode contribution to (elastic) structural response and the effects of inelasticity. This predictor is evaluated with respect to non‐linear dynamic analysis results for ‘fishbone’ models of steel moment‐resisting frame (SMRF) buildings. The relatively small number of degrees of freedom for each fishbone model allows us to consider several short‐to‐long period buildings and numerous near‐ and far‐field earthquake ground motions of interest in both Japan and the U.S. Before doing so, though, we verify that estimates of the bias and precision of the predictor obtained using fishbone models are effectively equivalent to those based on typical ‘full‐frame’ models of the same buildings. Copyright © 2003 John Wiley & Sons, Ltd.     

Development of a new rubber seismic isolator: ‘Ball Rubber Bearing (BRB)’

This paper presents an experimental research aimed at developing a new rubber‐based seismic isolator called ‘Ball Rubber Bearing (BRB)’. The BRB is composed of a conventional steel‐reinforced multi‐layered rubber bearing with its central hole filled with small diameter steel balls that are used to provide energy dissipation capacity through friction. A large set of BRBs with different geometrical and material properties are manufactured and tested under reversed cyclic horizontal loading at different vertical compressive load levels. Extensive test results indicate that steel balls do not only increase the energy dissipation capacity of the elastomeric bearing (EB), but also increase its horizontal and vertical stiffness. It is also observed that the energy dissipation capacity of a BRB does not degrade as the number of loading cycles increases. Copyright © 2011 John Wiley & Sons, Ltd.     

Historical discharge measurements on the Middle Mississippi River,USA: no basis for ‘changing history’

This study tests the hypothesis that historical float‐based discharge measurements on the Mississippi River systematically over‐stated actual flood flows by 10% to > 30% relative to measurements using current meters. This assertion has been repeated over the past 25 years and recently has been used to adjust historical discharges used for flood‐frequency analysis. This study tests the hypothesis above using 2150 historical discharge measurements digitized from the three principal gauging stations on the Middle Mississippi River (MMR): data that include 626 float‐based discharges and 1516 meter‐based discharges, including 122 paired measurements. Multiple comparative tests show that the hypothesis above cannot be supported; if anything, the float‐based measurements slightly underestimate flows (not over‐estimate) over a broad range of discharges up to large floods. In response to the purported data bias above (‘changing history’; Dieckmann RJ, Dyhouse GR. 1998. Changing history at St. Louis—adjusting historic flows for frequency analysis. First Federal Inter‐Agency Hydrologic Modeling Conference, April 20–22, 1998. Las Vegas, NV; 4·31–4·36), historical flood discharges on the MMR have been modified, most by 10–20% and several by > 30%. These altered discharges are now being promulgated, in particular, through the Upper Mississippi River System Flow Frequency Study (UMRSFFS). New flow frequencies, flood profiles, and new flood maps from the UMRSFFS may significantly underestimate the actual flood hazard on the MMR if the original hydrologic data have been erroneously altered on the basis of an assumption of data bias. Copyright © 2010 John Wiley & Sons, Ltd.     

‘Bedload’ dynamics: Grain impacts,momentum transfer and derivation of a grain froude number

A test is proposed for Bagnold's postulate that the normal weight stress due to moving ‘bedload’ is balanced by a solid transmitted stress due to the rate of change of momentum produced by grain impacts on unit area of the stationary bed. The test involves derivation of an expression for normal momentum transfer due to saltating grains at moderate transport stages when grain-to-grain collisions and partial suspension during saltation may be ignored. A dimensionless number, Φ, (a grain Froude Number) is derived, given by ū²/g where ū is the mean grain forward velocity, g is the gravitational acceleration and L? is the length of a single saltation ‘jump’. Equilibrium demands that Φ be unity during bedload transport involving saltating grains if Bagnold's postulate is correct. Experimental data shows Φ < 1, the discrepancy between theory and experiment being due to the existence of lift forces acting upon bedload grains. Bagnold's postulate is correct for concentrated dispersions of grains, as in grain flows, when fluid lift forces may be neglected due to high particle concentration.     

Laboratory simulation of ‘the wick effect’ in salt weathering of rock

‘The wick effect’ is the upward migration of saline solutions into rocks and their subsequent crystallization. Lower Carboniferous sandstone blocks of rectangular shape have been subjected to this process in the laboratory using a range of salt types, a range of salt concentrations, and various mixtures of salts. Some treatments produced severe disintegration, notably sodium carbonate and a mixture of sodium carbonate with magnesium sulphate, whereas other salts (including sodium chloride and gypsum) were much less effective. The debris produced by this experimental salt weathering included appreciable quantities of silt-sized material, which were analysed with a laser granulometer. Such material could provide a source for desert loess.     

Subsurface storm flow formation at different hillslopes and implications for the ‘old water paradox’

Although many studies over the past several decades have documented the importance of subsurface stormflow (SSF) in hillslopes, its formation is still not well understood. Therefore, we studied SSF formation in the vadose soil zone at four different hillslopes during controlled sprinkling experiments and natural rainfall events. Event and pre‐event water fractions were determined using artificially traced sprinkling water and ²²²Rn as natural tracer. SSF formation and the fraction of pre‐event water varied substantially at different hillslopes. Both intensity of SSF and fraction of pre‐event water depended on whether SSF in preferential flow paths was fed directly from precipitation or was fed indirectly from saturated parts of the soil. Soil water was rapidly mobilized from saturated patches in the soil matrix and was subsequently released into larger pores, where it mixed with event water. Substantial amounts of pre‐event water, therefore, were contained in fast flow components like subsurface storm flow and also in overland flow. This finding has consequences for commonly used hydrograph separation methods and might explain part of the ‘old water paradox’. Copyright © 2007 John Wiley & Sons, Ltd.     

14C dating and palaeoenvironment of the historic ‘little ice age’ glacier advance of Nigardsbreen Southwest Norway

Moss and grass remains associated with a well-developed in situ palaeosol buried beneath a moraine ridge in front of Nigardsbreen (Jostedalsbreen ice cap, southern Norway) have been ¹⁴C-dated. Pollen preserved with the plant remains suggests the existence of an agricultural landscape prior to the deposition of the moraine. The calibrated dates and the pollen spectra are in close accord with historically-documented evidence for the timing and palaeoenvironment of the ‘Little Ice Age’ advance of the glacier. Considerable potential is indicated for estimating the maximum ages of moraine ridges and for palaeoenvironmental reconstruction where such documentary evidence does not exist.