Geomorphic Effectiveness Research Articles

The Geomorphic Effectiveness of Landslides

AbstractLandslides are widely recognized as key components of landscape evolution in areas of steep topography. Here, we present a new framework for examining landslide inventories in the context of the volume‐based impact that different landslide sizes have on shaping the landscape, that is, their geomorphic effectiveness (GE). Focusing on an actively retreating coastal cliff in the Eastern Mediterranean and utilizing a LiDAR‐derived inventory of over 1,100 cliff landslides that occurred between 2014 and 2019, we show that segments of the cliff are characterized by two principal types of GE distributions: (a) A “humped” GE distribution where the accumulated erosion volume of the largest and rarest collapses in the inventory is similar or lower than that of more frequent, mid‐range collapses and (b) Nearly monotonically increasing GE distribution where the cumulative volume of larger collapses consistently surpasses that of smaller magnitude collapses. Regardless of the GE distribution type, we found that the cumulative geomorphic impact of the small and most probable collapses was negligible. Extending this new GE framework to 9 other previously published landslide inventories (coastal and mountainous), we demonstrate that precipitation and seepage‐induced landslide inventories are commonly characterized by monotonic‐type GE distributions, dominated by large landslides (>10−1 of the volume of the largest landslide), and that hump‐shaped GE distributions, dominated by more frequent mid‐size landslides, commonly occur under “dry” triggers (e.g., earthquakes). We propose that the humped GE distribution could reflect the lack of deep mechanical weakening, which exerts a higher probability of the largest landslides in the inventories triggered by “wet” factor.

Little tools, big job: The periglacial conveyor system in cryoplanated uplands

AbstractThe mechanisms of sediment removal associated with the nivation process suite are underinvestigated, a situation that hinders development of a unified, coherent theory of cryoplanation terrace formation. This study links sorted stripes—a type of periglacial patterned ground frequently encountered on cryoplanation terrace treads—to active hydrologic networks capable of transporting large quantities of fine sediments on periglacial hillslopes. Traditional interpretations hold that the presence of sorted patterned ground indicates geomorphic quiescence, a view that has contributed to the dismissal of these features as a factor in the formation of erosional periglacial topography. We address the geomorphic role of sorted stripes as fluvial features by investigating their hydrologic potential for transporting weathered material across and off developing cryoplanation terraces. Flow modeling and watershed geomorphometric analyses were conducted using a high‐resolution digital elevation model of a large cryoplanation terrace in a geomorphically active periglacial upland near Atlin, British Columbia, Canada. Results demonstrate the landscape‐scale spatial organization and geomorphic effectiveness of sorted‐stripe networks—“little tools”—for transporting water and suspended sediment across large cryoplanated surfaces. We present a qualitative model of sediment production and transportation, “the periglacial conveyor system,” that outlines erosional processes responsible for cryoplanation terrace formation and defines the distinctive hydrologic–geomorphic imprint imparted by sorted stripes on periglacial hillslopes.

Restoring geomorphic integrity in urban streams via mechanistically-based storm water management: minimizing excess sediment transport capacity

Stream channel erosion, enlargement, and habitat degradation are ubiquitous in urban watersheds with conventional stormwater management that increase channel-eroding flows relative to undeveloped watersheds. Hydrologic-based restoration aims to discharge a more natural flow regime via stormwater management interventions. Whether such interventions facilitate geomorphic recovery depends, in part, on the degree to which they restrict discharges that would otherwise contribute to channel erosion. Erosion potential (E), the ratio of post-developed to predeveloped sediment transport capacity, provides a simplified, mechanistic framework to quantify the relative influence of stormwater interventions on the geomorphic effectiveness of the flow regime. This paper compiles ca. five years of data following stormwater-based interventions in three distinct settings in the United States and Australia to demonstrate how the E framework can elucidate the role of hydrologic restoration interventions in facilitating trajectories of geomorphic recovery (or lack thereof). In a previously developed watershed with unstable streams, substantial reductions in E in one stream coincided with a trajectory of geomorphic recovery, whereas the control stream without E-reducing interventions exhibited continued instability. Furthermore, a stream downstream of a greenfield development that optimized their stormwater control measures to match the sediment transport capacity of the predeveloped regime (E = 1) was able to maintain a recovery trajectory in a legacy-impacted setting that is otherwise highly susceptible to hydromodification. Streambed material size, channel evolution stage, and the hydrogeomorphic setting also likely affect the level of E reduction necessary to promote geomorphic recovery, with coarser-grained and over-widened streams potentially needing less reduction than finer-grained and more entrenched channels. Although available space and funding will limit the ability to fully reduce E in previously developed watersheds, these case studies underscore the value of using stormwater control measures to maximize reductions in E if geomorphic stability is a goal of stormwater interventions.

Modelling differential geomorphic effectiveness in neighbouring upland catchments: implications for sediment and flood risk management in a wetter world

In July 2007 an intense summer storm resulted in significant activation of the sediment system in the Thinhope Burn, UK. Catchment- and reach-scale morphodynamic modelling is used to investigate the geomorphic work undertaken by Thinhope Burn; comparing this with the more subdued responses shown by its neighbours. Total sediment efflux for Thinhope Burn over the 10 yr period 1998-2007 was 18, 801 m3 four times that of the larger Knar Burn catchment and fifty-four times that of the smaller Glendue Burn catchment. For a discharge of 60 m3s-1, equivalent to the July 2007 Thinhope flood, sediment efflux was 575 m3, 76 m3, and 67 m3 for Thinhope, Glendue and Knar Burns respectively. It is clear that Thinhope Burn undertook significantly more geomorphic work compared to its neighbours. Analysis of the population of shear stress for reach-scale simulations on Thinhope Burn highlighted that the final three simulations (flood peaks of 60, 90, 236 m3s-1) all produced very similar distributions, with no marked increase in the modal shear stress (∼250 Nm-2). This possibly suggests that flows >60 m3s-1 are not able to exert significantly greater energy on the channel boundary, indicating that flows in the region of 60 m3s-1 attain ‘peak’ geomorphic work. It is argued that factors such as strength resistance of the key sediment sources (e.g. paleoberms perched on terraces), structural resistance to flood waves imposed by valley form resistance, location sensitivity and transmission resistance, may all offer explanations for increased geomorphic effectiveness compared with its neighbours. With the expectation of greater rainfall totals in the winter and more extreme summer events in upland areas of the UK, it is clear that attention needs to focus upon the implications of this upon the morphological stability of these areas not least to aid future sustainable flood risk management.

Flooding duration and volume more important than peak discharge in explaining 18 years of gravel–cobble river change

AbstractFloods play a critical role in geomorphic change, but whether peak magnitude, duration, volume, or frequency determines the resulting magnitude of erosion and deposition is a question often proposed in geomorphic effectiveness studies. This study investigated that question using digital elevation model differencing to compare and contrast three hydrologically distinct epochs of topographic change spanning 18 years in the 37‐km gravel–cobble lower Yuba River in northern California, USA. Scour and fill were analysed by volume at segment and geomorphic reach scales. Each epoch's hydrology was characterized using 15‐min and daily averaged flow to obtain distinct peak and recurrence, duration, and volume metrics. Epochs 1 (1999–2008) and 3 (2014–2017) were wetter than average with large floods reaching 3206 and 2466 m3/s, respectively, though of different flood durations. Epoch 2 (2008–2014) was a drought period with only four brief moderate floods (peak of 1245 m3/s). Total volumetric changes showed that major geomorphic response occurred primarily during large flood events; however, total scour and net export of sediment varied greatly, with 20 times more export in epoch 3 compared to epoch 1. The key finding was that greater peak discharge was not correlated with greater net and total erosion; differences were better explained by duration and volume above floodway‐filling stage. This finding highlights the importance of considering flood duration and volume, along with peak, to assess flood magnitude in the context of flood management, frequency analysis, and resulting geomorphic changes.

Vegetation disturbances and flood energy during an extreme flood on a sub‐tropical river

AbstractThe advent of 2D hydraulic modelling has improved our understanding of flood hydraulics, thresholds, and dynamic effects on floodplain geomorphology and riparian vegetation at the morphological‐unit scale. Hydraulic concepts of bed shear stress, stream power maxima, and energy (cumulative stream power) have been used to characterize floods and define their geomorphic effectiveness. These hydraulic concepts were developed in the context of reach‐averaged, 1D hydraulic analyses, but their application to 2D model results is problematic due to differences in the treatment of energy losses in 1D and 2D analyses. Here we present methods for estimating total and boundary resistance from 2D modelling of an extreme flood on a subtropical river. Hydraulic model results are correlated with observations of the flood impacts on floodplain geomorphology and the riparian vegetation to identify thresholds and compute variants of flood energy. Comparison of LiDAR data in 2011 and 2014 shows that the 2011 flood produced 2–4 m of erosion on floodplain bars that were previously forested or grass‐covered. Deposition on flood levees, dunes, and chute bars was up to 3.4 m thick. Various hydraulic metrics were trialled as candidates for thresholds of vegetation disturbance. The accuracy of thresholds using metrics extracted at the flood peak (i.e. boundary resistance and stream power maxima) was similar to that using energy as a threshold. Disturbance to forest and grass on vegetated bars was associated with stream powers of >834 W/m2and unit flows of >26 m2/s, respectively. Correlation of the hydraulic metrics with erosion and deposition depths showed no substantial improvement in using flood energy compared to metrics extracted at the flood peak for describing erosion and deposition. The extent of vegetation disturbances and morphological adjustments was limited for this extreme flood, and further 2D studies are needed to compare disturbance thresholds across different environments.

Geomorphological impact, hydraulics and watershed- lake connectivity during extreme floods in mountain areas: The 1959 Vega de Tera dam failure, NW Spain

Dam-failure floods typically involve greater peak discharge than the largest meteorological flood at a basin. Determining the geomorphic effectiveness of extreme flooding caused by a breach mechanism provides insight into the role of flood scale on the resulting processes and landforms. Here, we present a geomorphological and hydraulic analysis of the 1959 Vega de Tera (NW Spain) dam-break flood, a worldwide notable dam-failure incident that released a flow of 7.8 106 m3 that caused the death of 144 people at Ribadelago before reaching Lake Sanabria (9 km down valley). This watershed-lake connection provides a comprehensive analysis of an extreme sediment delivery event in the context of a millennial long lake depositional record. One-dimensional unsteady flow computation shows a peak flow hydrograph attenuating from 13,000 m3 s−1 to 5150 m3 s−1, that reached a maximum flow depth of 34 m and velocity of 30 ms−1. Spatial variation of erosional and depositional landforms are related with local flow hydraulics: i) in steep sectors flow regime was supercritical (shear value up to 11,200 Pa) and produced up to 30 m deep bedrock channel erosion; ii) at the boundary of steep and flatten sectors, transition to subcritical regime generated large plunge pools (up to 6000 m2 and 15.2 m in depth); iii) in low-gradient sectors low shear stress gave rise to depositional landforms, namely gravel bars with dam boulders up to 3 m long, and a debris cone with coarse gravel and expansion sand bars. The depositional landforms amount for a total volume of ca 2.11 · 106 m3 in the Tera valley (37% in the gorge and 63% in the floodplain). The dense, energetic sediment-laden flow reached Lake Sanabria forming a debris cone close to the mouth and caused an underwater hyperpycnal current, depositing a ~10 cm-thick sandy-silt layer all over the two distal subbasins. The estimated volume of the deposited fine sediments in the lake ranges between 200,000 and 368,000 m3. The lake record shows that this was the largest flood in the basin during the Holocene. Previous to the dam break, the sediment connectivity between the Sanabria watershed and the lake was limited because of the “staircase” topography and the presence of small glacial depressions filled with sediments since deglaciation. Even during the flood, the great majority of the sediments were deposited along the flood pathway, and only a small percentage (10–20%) reached the lake. Although the hydraulics of the Tera River were not changed after the flood, the newly formed pools in the watershed could diminish the connectivity between the river and the lake in the future, as some new sedimentation areas (pools) were generated acting as natural dams and thus decreasing sediment input to the lake.

Sediment flux during flood events along the Trotuș River channel: hydrogeomorphological approach

The degree of the river channel change, and particularly the amount of transported sediment, defines the geomorphic effectiveness of a flood event. This study aims to improve the level of understanding of the control exerted by the streamflow and the flow energy on the sediment flux regimes during flood events, which are ranked depending on their geomorphic effectiveness. The study focused on the Trotuș River, Eastern Carpathians, Romania. Two data sources were used: gauging stations (discharge and suspended sediment load) and field observations (grain size, channel bed slope and channel width corresponding to bankfull discharge). The bedload sediment transport was estimated using the Bedload Assessment for Gravel-bed Streams (BAGS) program. The parameters and variables taken into consideration (magnitude, duration, stream power, total energy expenditure, total sediment load) were statistically analysed for each flood event. Just 15% of the total 76 investigated flood events ranked in the class which includes the flood events with the highest geomorphic effectiveness. Of the 41 × 106 t of sediment carried through the four gauging stations during flood events, approximately 74% was transported at stream power values above 300 W m−2. In the years with type B flood events, the sediment flux was much higher than the multiannual average value. Changes of the sediment rating curves were documented at all gauging stations subsequent to major flood events. Clockwise loops accounted for the highest percentages in all types of flood events. Major flood events generate significant changes in river channels, which are subsequently reflected in the sediment regime. The main cause of the changes occurring in the sediment regime likely lies in the channel adjustments subsequent to flood events. This investigation introduces a methodology for ranking flood events depending on their magnitude, duration, stream power and transported sediment.

Effective discharge variability for suspended sediment transport in the middle Yangtze River

Effective discharge is the flow rate corresponding to the maximum suspended sediment transport within a certain period in a natural river, which can reflect the medium- to short-term bed-forming effect. The middle Yangtze River has been in a long-term unsaturated state since 2003 (the impoundment of the Three Gorges Dam (TGD)) due to altered water and sediment regimes. Based on the prototype data from 1991 to 2016 on the sandy bed (from Shashi to Hankou), the magnitude and duration of the effective discharge (Qe) before and after impoundment of the Three Gorges Dam (TGD) corresponding to the suspended sediment grouped transport are investigated via a magnitude-frequency approach, and then, its geomorphic effectiveness is discussed. The results indicate the following: (1) The magnitude of the effective discharge corresponding to particle-size group (> 0.062 mm) increases, but the frequency of it decreases after the TGD. (2) The Qe magnitude is 1.3–1.4 times and 1.6–2.0 times of the annual average discharge before and after the TGD. The effective discharge (Qe) is smaller than the bankfull discharges (Qb), while the values of Qb/Qe become smaller after TGD and gradually decrease from Shashi to Hankou. (3) The geomorphic effectiveness at each hydrological station after the TGD is generally less than that before the TGD. The effective discharge corresponding to particle-size group (0.125–0.25 mm) has the largest geomorphic effectiveness after the TGD. It is also found that the geomorphic effectiveness of Qe corresponding to the particle-size group (0.062–0.125 mm) increases along the distance from Shashi to Hankou, but that to the group (0.125–0.25 mm) remains basically unchanged after the TGD. It highlighted the increase of effective discharge after the TGD essentially reflects the decrease of sediment carrying capacity, which is mainly affected by the coarse particle-size groups (> 0.125 mm).

Assessment of the geomorphic effectiveness of controlled floods in a braided river using a reduced-complexity numerical model

Abstract. Most Alpine rivers have undergone significant alterations in flow and sediment regimes. These alterations have notable effects on river morphology and ecology. One option to mitigate such effects is flow regime management, specifically through the reintroduction of channel-forming discharges. The aim of this work is to assess the morphological changes induced in the Piave River (Italy) by two distinct controlled-flood strategies, the first characterized by a single artificial flood per year and the second by higher-magnitude but less frequent floods. This work involved applying a two-dimensional reduced-complexity morphodynamic model (CAESAR-LISFLOOD) to a 7 km long reach, characterized by a braided pattern and highly regulated discharges. Numerical modelling allowed the assessment of morphological changes for four long-term scenarios (2009–2034). The scenarios were defined considering the current flow regime and the natural regime, which was estimated by a stochastic physically based hydrologic model. Changes in channel morphology were assessed by measuring active-channel width and braiding intensity. A comparison of controlled-flood scenarios to a baseline scenario (i.e. no controlled floods) showed that artificial floods had little effect on channel morphology. More channel widening (13.5 %) resulted from the high-magnitude flood strategy than from the application of the other strategy (8.6 %). Negligible change was observed in terms of braiding intensity. The results indicate that controlled floods do not represent an effective solution for morphological recovery in braided rivers with strongly impacted flow and sediment regimes.

A customised approach to determining the geomorphic effectiveness of small flood events in a regulated river

AbstractThe construction of dams significantly alters flow and sediment regimes with subsequent deleterious effects on the morphological and ecological character of rivers. Effective experimental floods can ameliorate the downstream geomorphic impacts of dams. The traditional view is that large floods are required to perform effective geomorphic work, and the geomorphic outcomes of small floods are often overlooked. Many river restoration frameworks do not consider small floods. Yet, there is evidence that the hydrological characteristics that ameliorate specific geomorphic impacts in a river are unique to each river, and a customised approach to setting the right mix of floods (including small experimental floods) is needed. In this study, we modify an existing flood effectiveness model developed for large floods, for determining the geomorphic effectiveness of small floods in a highly regulated Australian river. Two flood classes were added to the model (medium peak stream power and moderate total energy expenditure), and the flood power characteristics were rescaled to reflect the relative difference in the magnitude of the small floods and the magnitude of the geomorphic work performed. Using a step‐wise approach, this customised model determined the geomorphic effectiveness of small floods. The best flood for ameliorating the geomorphic impacts of flow regulation had medium to long duration (10 to 51 days), high peak unit stream power (77 to 123 Wm−2) and moderate to large total energy expenditure (78,600 to 342,320 × 103 J). This approach to determining flood effectiveness for small floods is applicable to other geomorphically impacted river channels downstream of dams and can be used to inform experimental flood releases for geomorphic outcomes.

Geomorphic effectiveness of check dams in a debris-flow catchment using multi-temporal topographic surveys

Torrent control works, such as grade control dams and sediment retention dams, are structural measures for controlling debris flows and debris floods. In spite of the widespread presence of such hydraulic structures in steep mountain streams worldwide, there are still few studies monitoring the effects of check dams on sediment dynamics.The use of repeated topographic surveys allows us to characterize debris-flow activity at multiple temporal and spatial scales and its interaction with torrent control works. Structure from Motion (SfM) technique paired with Multi-View Stereo (MVS) algorithms represents a low-cost opportunity to conduct such multi-temporal surveys. This enables us to better study the effects of individual debris flows, track geomorphic changes and evaluate the effectiveness of torrent control works (e.g. check dams).The effect of check dams on sediment dynamics is investigated by means of multi-temporal topographic surveys before and after debris-flow events in the Moscardo torrent (eastern Italian Alps) where two check dams have recently been built. Multi-temporal SfM based on images taken from the ground in combination with imagery taken by an Unmanned Aerial Vehicle (UAV) is used to obtain DEMs and to study topographic changes through the comparison of repeat DEMs (DEM of Difference, i.e. DoD).The results show that the new check dams considerably modified sediment dynamics in the studied channel but their performance cannot be considered satisfactory. The sediment flowed around the upstream check dam on the right wing, while deep erosion observed downstream of the check dam threatened to undermine the foundation's stability. Moreover, debris-flow lobes deposited upstream of the check dams could act as sources of sediment further increasing downstream debris-flow volume. The analysis proposed in this work could help improve design strategies and permitting the identification of a proper site for check dam building.

Basin-scale analysis of the geomorphic effectiveness of flash floods: A study in the northern Apennines (Italy)

Large floods may produce remarkable channel changes, which determine damages and casualties in inhabited areas. However, our knowledge of such processes remains poor, as is our capability to predict them. This study analyses the geomorphic response of the Nure River (northern Italy) and nine tributaries to a high-magnitude flood that occurred in September 2015. The adopted multi-disciplinary approach encompassed: (i) hydrological and hydraulic analysis; (ii) analysis of sediment delivery to the stream network by means of landslides mapping; (iii) assessment of morphological modifications of the channels, including both channel width and bed elevation changes.The spatial distribution of rainfall showed that the largest rainfall amounts occur in the upper portions of the catchment, with cumulative rainfall reaching 300 mm in 12 h, and recurrence intervals exceeding 100–150 years. The unit peak discharge ranged between 5.2 and 25 m3 s−1 km−2. Channel widening was the most evident effect. In the tributaries, the ratio between post-flood and pre-flood channel width averaged 3.3, with a maximum approaching 20. Widening was associated with channel aggradation up to 1.5 m and removal of riparian vegetation. New islands formed due to the fragmentation of the former floodplain. In the Nure River, the average width ratio was 1.7, and here widening occurred mainly at the expenses of islands. Bed level dynamics in the Nure were varied, including aggradation, incision, and overall stability. The flood geomorphic effectiveness was more pronounced in the middle-higher portions of the basin. Planimetric and elevation changes were well correlated. Regression analysis of the relationship between widening and morphological/ hydraulic controlling factors indicated that unit stream power and confinement index were the most relevant variables.The study provides useful insights for river management, especially with regard to the proportion of the valley floor subject to erosion and/or deposition during large events.

Morphological study of Upper Tapi river using remote sensing and GIS techniques

Present study is an effort to quantify the bank erosion and bankline migration in the Upper Tapi River, India. The migration of banklines have been quantified during period 1977–2010 using satellite imagery, SOI topo-sheets and GIS tools at every 20 km chainages of the river. The bank erosion/deposition areas (km2) are also computed by dividing the river in 50 km stretches. The study indicated that, significant bank migration has occurred between CH: 300 km and CH: 350 km. Field investigations suggest that this phenomenon is due to the infilling of Hatnur Reservoir and its post-reservoir sedimentation. The right bank line has been observed to be more erosion prone than left bank line of the river due to presence of relatively soft geological formation in the right bank. The developed hydraulic geometry equations clearly indicate that flow depth increases rapidly with increase in discharge without a significant increase in width at Dedtalai stream gauging station. The analysis of the geomorphic effect of extreme flood events of the years 1994, 1998 and 2006 showed that the unit stream power, for same order of discharge of year 1994, at Dedtalai stream gauging station is higher than Burhanpur stream gauging station. Apart from the magnitude of unit stream power, the duration of flood event has been found to be significant parameter in geomorphic effectiveness of the flood events. The critical erosion/shifting prone locations are also supported with field visits undertaken in the study reach.

Geomorphic effectiveness of a long profile shape and the role of inherent geological controls in the Himalayan hinterland area of the Ganga River basin, India

Long profiles of rivers provide a platform to analyse interaction between geological and geomorphic processes operating at different time scales. Identification of an appropriate model for river long profile becomes important in order to establish a quantitative relationship between the profile shape, its geomorphic effectiveness, and inherent geological characteristics. This work highlights the variability in the long profile shape of the Ganga River and its major tributaries, its impact on stream power distribution pattern, and role of the geological controls on it. Long profile shapes are represented by the sum of two exponential functions through the curve fitting method. We have shown that coefficients of river long profile equations are governed by the geological characteristics of subbasins. These equations further define the spatial distribution pattern of stream power and help to understand stream power variability in different geological terrains. Spatial distribution of stream power in different geological terrains successfully explains spatial variability in geomorphic processes within the Himalayan hinterland area. In general, the stream power peaks of larger rivers lie in the Higher Himalaya, and rivers in the eastern hinterland area are characterised by the highest magnitude of stream power.

Geomorphic effectiveness: a linear concept in a non‐linear world

AbstractGeomorphic effectiveness has been an influential concept in geomorphology since its introduction by Reds Wolman and John Miller in 1960. It provided a much needed framework to assess the significance of an event by comparing event magnitude to the resultant geomorphic effects. Initially, this concept was applied primarily in river channels, under the linear assumption that geomorphic responses to similarly sized flood events will be consistent. Numerous authors have since attempted to quantify a direct, proportional relationship between event magnitude and different forms of geomorphic response in a variety of geomorphic settings. In doing so, these investigations applied an array of metrics that were difficult to compare across different spatiotemporal scales, and physiographic and geomorphic environments. Critically, the emergence of other geomorphic concepts such as sensitivity, connectivity, thresholds, and recovery has shown that relationships between causes (events) and geomorphic effects (responses) are often complex and non‐linear.This paper disentangles the complex historical development of the geomorphic effectiveness concept and reviews the utility of various metrics for quantifying effectiveness. We propose that total energy (joules) is the most appropriate metric to use for quantifying the magnitude of disturbance events (cause) and volumetric sediment flux associated with landform modification is the most appropriate metric for quantifying geomorphic effects. While both metrics are difficult to quantify, they are the only ones which facilitate comparison across a range of spatiotemporal scales (comparability) in a variety of geomorphic environments (flexibility). The geomorphic effectiveness concept can continue to be useful provided that geomorphologists use flexible and comparable metrics. Today, geomorphologists are better prepared to consider the influence of non‐linear processes on determinations of geomorphic effectiveness, allowing investigators to not only determine if a disturbance event was effective but also to explain why or why not. Copyright © 2016 John Wiley & Sons, Ltd.

Hydrologic and Geomorphic Aspects of High-magnitude Floods on the Par River in Western India

The geomorphic effectiveness of floods is evaluated in terms of unit stream power (ω) and boundary shear stress (τ) for floods on the Par River. The highest values for ω and τ for a flood on the river are 52125 W/m2 and 3320 N/m2, respectively. The estimated Froude numbers are <1 indicating subcritical flows. It is >1 for a few constricted reaches showing supercritical or shooting flows. High values of Reynolds number reveal that the flood discharges were extremely turbulent. Values of critical velocity for the inception of cavitation (Vc) show that none of the powerful floods on the river, except two, exceed the conditions. Estimates of ω, τ and velocity associated with transported boulders indicate that all floods were competent to move large boulders of more than 5.5 m in diameter. The efficiency of high-magnitude flood events is evident from the presence of a variety of geomorphic features.

An approach for measuring confinement and assessing the influence of valley setting on river forms and processes

AbstractValley setting and confinement (or lack thereof) are primary controls on river character and behaviour. Although there are various proxies for valley confinement, direct measures that quantify the nature and extent of confinement are generally lacking and/or inconsistently described. As such they do not lend themselves to consistent analysis over large spatial scales. Here we clearly define forms of confinement to aid in quantification of degrees of confinement. Types of margin that can induce confinement are differentiated as a valley margin, valley bottom margin, and/or anthropogenic margin. Such margins sometimes overlap and share the same location, and in other situations are separated, giving immediate clues as to the valley setting. We apply this framework to examples from Australia, United States and New Zealand, showing how this framework can be applied across the spectrum of river diversity. This method can help to inform interpretations of reach‐scale river behaviour, highlighting the role of antecedent controls on contemporary forms and processes. Clear definitions of confinement are shown to support catchment‐scale analysis of river patterns along longitudinal profiles, and appraisals of the geomorphic effectiveness of floods and sediment flux in catchments (e.g. process zone distribution, lateral sediment inputs and (dis)connectivity). Copyright © 2015 John Wiley & Sons, Ltd.

Geomorphic response to catastrophic flooding in north-central Pennsylvania from Tropical Storm Lee (September 2011): Intersection of fluvial disequilibrium and the legacy of logging

More than 25 cm of rainfall from Tropical Storm Lee (TS Lee) over 2 days in September 2011 resulted in catastrophic flooding (U.S. Geological Survey estimated recurrence interval >100 yr) on several Susquehanna River tributaries emanating from the Appalachian Plateau in north-central Pennsylvania (USA). Helicopter photography and field work were used to prepare a detailed geographic information system database of geomorphic response to the flood along ∼250 km of Loyalsock, Muncy, Lycoming, and Fishing Creeks. Unlike the response of many streams to previously described Appalachian floods, fluvial response to the TS Lee flood was extensive in these gravel bed streams, characterized by (1) large-scale avulsions and chute development on the insides of meanders, (2) erosion of gravel from channel margins and transport downstream in large pulses, (3) headwater landslides and alluvial fan activation, (4) major floodplain erosion and deposition, and (5) breaching of anthropogenic berms and reconnection of the main channel to prehistoric floodplain anabranches. Geomorphic work, expressed both as bedload sediment transport and landform change (geomorphic effectiveness) was significant: as much as 55,000 m 3 /km of gravel was transported within a single watershed. Landform changes included erosion of chutes (to 500 m long), gravel bars (point bars and mid-channel bars), channel widening (in places >100%), and reoccupation of former multithread channels previously cut off from the mainstem by historic channel straightening, berming, and dredging. Streams in this region appear to be in a phase of disequilibrium largely in response to major shifts in sediment delivery from their watersheds caused by historic logging and a series of floods ∼100 yr ago. Widespread clearcutting (A.D. 1850–1920) contributed large volumes of sediment to these streams. Dendrogeomorphic data bracket a period of aggradation of these logging legacy sediments between the 1870s and 1930s, creating a significant low terrace inset into Pleistocene outwash and glacial sediments. Recent floods in 1972, 1996, 2004, and especially TS Lee in 2011 initiated an enhanced phase of disequilibrium as a geomorphic threshold was crossed, resulting in widespread erosion of logging legacy sediments deposited nearly 100 yr ago. The change in sediment load (increased coarse bedload) as a result of widespread bed and bank erosion caused a change in channel pattern from single thread to multithread. Pattern change was facilitated by aggradation of gravel bars above floodplain elevations which promoted avulsion and chute formation. Based on preflood and postflood geomorphic mapping, >6,700,000 m 3 of gravel were mobilized during the flood across 4 watersheds. Mobilization of logging legacy sediment is occurring as pulses of gravel move downstream episodically. This paper demonstrates the important influences of drainage basin morphometry (e.g., ruggedness number) and fluvial history (land use and geomorphic) in understanding current channel dynamics and basin response to heavy precipitation and flooding. The findings presented herein have significant implications for watershed management and planning. Streams in this region are likely to remain in a protracted phase of readjustment for many decades as complex response to historical land-use change continues. In this disequilibrium phase most significant rainfall events will likely trigger additional readjustment and channel change.

Upper limits of flash flood stream power in Europe

Flash floods are characterized by strong spatial gradients of rainfall inputs that hit different parts of a river basin with different intensity. Stream power values associated with flash floods therefore show spatial variations that depend on geological controls on channel geometry and sediment characteristics, as well as on the variations of flood intensity: this stresses the need for a field approach that takes into account the variability of the controlling factors. Post-flood assessment of peak discharge after major floods makes it possible to analyse stream power in fluvial systems affected by flash floods. This study analyses the stream power of seven intense (return period of rainfall >100years at least in some sectors of the river basin) flash floods that occurred in mountainous basins of central and southern Europe from 2007 to 2014. In most of the analysed cross sections, high values of unit stream power were observed; this is consistent with the high severity of the studied floods. The highest values of cross-sectional stream power and unit stream power usually occur in Mediterranean regions. This is mainly ascribed to the larger peak discharges that characterize flash floods in these regions. The variability of unit stream power with catchment area is clearly nonlinear and has been represented by log-quadratic relations. The values of catchment area at which maximum values of unit stream power occur show relevant differences among the studied floods and are linked to the spatial scale of the events. Values of stream power are generally consistent with observed geomorphic changes in the studied cross sections: bedrock channels show the highest values of unit stream power but no visible erosion, whereas major erosion has been observed in alluvial channels. Exceptions to this general pattern, which mostly occur in semi-alluvial cross sections, urge the recognition of local or event-specific conditions that increase the resistance of channel bed and banks to erosion or, like short flow duration, reduce the geomorphic effectiveness of the flood.