BGRG held its usual three meetings this year. Papers at the Annual General Meeting were on the th... more BGRG held its usual three meetings this year. Papers at the Annual General Meeting were on the theme of'Fluvial dynamics of lowland river channel and floodplain systems' and ranged from aspects of morphology to ecology and river management. The conference ...
ABSTRACT Flood flows disturb vegetation growing in ephemeral channels but more information is nee... more ABSTRACT Flood flows disturb vegetation growing in ephemeral channels but more information is needed on the thresholds for damage and removal and on the regrowth processes and timescales after floods. Once vegetation is re-established then it has feedback effects on processes and may raise thresholds. Several sites in SE Spain have been monitored for the effects of flows and for the growth and responses of plants over a period of >15 years. Two major floods and many minor flows have occurred. Measurements on quadrats and in different zones of the valley floor have allowed quantification of the thresholds for damage of different species of plant. Position of the plants in the channel also has a marked influence on effect of flows; velocities and flow forces for different parts have been calculated. The threshold for removal or mortality of certain plants in these Mediterranean valleys is very high. Types and species of plants regrowing in different zones have been identified and rates of growth measured. The relationship to climatic and weather conditions between channel flows is analysed. Growth rates between floods are closely related to moisture availability, mainly influenced by inter-annual variability of rainfall but also varying with location in the channel. One site in which hydrological regime was altered by human actions has shown marked change in vegetation cover and in channel response. Feedback effects reduce erosion and increase sedimentation and these effects have been measured directly and by calculation of roughness and resistance effects. The results demonstrate the different degrees of adaptation of plants to disturbance, natural vegetation such as phreatophytes showing high resilience but crop trees such as olives and almonds on floodplains being vulnerable to high flows.
Summary River restoration is a key-issue for European hydrosystems that were modified over centur... more Summary River restoration is a key-issue for European hydrosystems that were modified over centuries by human activities. This is particularly true for numerous low energy rivers flowing in the plateaus of Western Europe. Because of this limited energy, their potential in terms of autogenic restoration a priori appears strongly restricted. This study examines the conditions under which the morphogenesis on one of these systems occurs in relation to various hydrologic conditions. Two complementary approaches are combined on three reaches of the meandering Cher River (France). Firstly, we examine at a pluri-decadal scale the control of duration, frequency and intensity of floods on the planimetric erosion (bank retreat mainly). Secondly, we estimate the range of effective discharge for bedload transport. The results show that the morphogenesis is controlled by low magnitude hydrological events. Two major controlling factors are suggested: low differential of energy between small and large floods, peculiar to low energy rivers of mid-latitude, and low critical discharges for lateral erosion and bedload mobilization. For these reasons, the ability of the alluvial Cher River to self-restore its fluvial dynamics seems to be relatively high.
The past year has been one of reflection by the BGRG on its own organisation as well as continuat... more The past year has been one of reflection by the BGRG on its own organisation as well as continuation of a vibrant programme of meetings and publications. External relations have been fostered through a series of joint conferences with other bodies and representations have been made to ...
ABSTRACT To understand river channel changes and fluvial activity it is necessary to know the mag... more ABSTRACT To understand river channel changes and fluvial activity it is necessary to know the magnitude, frequency, and sequence of flows and the controls upon them and compare the channel responses in periods with known conditions. This paper identifies the characteristics of mean and maximum flows of the Gila River in southeast Arizona in the twentieth century and examines their relationship to precipitation, to the Palmer Hydrological Index, and to land and water management. Flow records show very high total flows in the period 1905–1916 and very low flows in the 1940s and 1950s. Eleven of the 20 highest annual peaks since 1914 have occurred since 1965. The 1983 flood was nearly comparable with the flood of 1905, which is associated with the destabilization of the Gila River. Increased flooding is found to be associated primarily with increased winter precipitation and the evidence available indicates that water usage and land management have much less influence on flows than has climate. Major variations, on the scale of a few decades, are similar to those identified in Australia, which have had profound effects on river channels. Widespread recognition of the magnitude and persistence of the flow variations has important implications for water resources and flood management. [Key words: hydrology, climatic variation, precipitation, river discharge, floods, Arizona.]
ABSTRACT Accurate records of flash flood magnitudes are required to inform flood forecasting and ... more ABSTRACT Accurate records of flash flood magnitudes are required to inform flood forecasting and planning. However, whilst a distributed flood survey is desirable to capture spatial heterogeneity in peak water surface elevation, the field time required for a distributed survey often limits the spatial coverage of such reconstructions. For the first time, we demonstrate the application of Structure-from-Motion (SfM) with Multi-View Stereo (MVS) to reconstruct the magnitude of a flash flood. This approach required only standard digital photographs and ground control points, took only similar to 30 min in the field, and can be embedded within existing protocols easily. We validated the method against a conventional dGPS survey in three stages: (i) comparison of topographic data revealed that SfM was accurate to within 0.1 m; (ii) high water marks extracted from the SfM model were within 0.25 m of those surveyed in the field with no consistent over or under-estimate; (iii) peak discharge reconstructed from a two-dimensional hydraulic model was within the range of more conventional estimates. With low uncertainty in our terrain model and our reconstructed flood water surface, we highlight the added value of the SfM approach for incorporating reach scale spatial variability into hydraulic reconstructions.
BGRG held its usual three meetings this year. Papers at the Annual General Meeting were on the th... more BGRG held its usual three meetings this year. Papers at the Annual General Meeting were on the theme of'Fluvial dynamics of lowland river channel and floodplain systems' and ranged from aspects of morphology to ecology and river management. The conference ...
ABSTRACT Flood flows disturb vegetation growing in ephemeral channels but more information is nee... more ABSTRACT Flood flows disturb vegetation growing in ephemeral channels but more information is needed on the thresholds for damage and removal and on the regrowth processes and timescales after floods. Once vegetation is re-established then it has feedback effects on processes and may raise thresholds. Several sites in SE Spain have been monitored for the effects of flows and for the growth and responses of plants over a period of >15 years. Two major floods and many minor flows have occurred. Measurements on quadrats and in different zones of the valley floor have allowed quantification of the thresholds for damage of different species of plant. Position of the plants in the channel also has a marked influence on effect of flows; velocities and flow forces for different parts have been calculated. The threshold for removal or mortality of certain plants in these Mediterranean valleys is very high. Types and species of plants regrowing in different zones have been identified and rates of growth measured. The relationship to climatic and weather conditions between channel flows is analysed. Growth rates between floods are closely related to moisture availability, mainly influenced by inter-annual variability of rainfall but also varying with location in the channel. One site in which hydrological regime was altered by human actions has shown marked change in vegetation cover and in channel response. Feedback effects reduce erosion and increase sedimentation and these effects have been measured directly and by calculation of roughness and resistance effects. The results demonstrate the different degrees of adaptation of plants to disturbance, natural vegetation such as phreatophytes showing high resilience but crop trees such as olives and almonds on floodplains being vulnerable to high flows.
Summary River restoration is a key-issue for European hydrosystems that were modified over centur... more Summary River restoration is a key-issue for European hydrosystems that were modified over centuries by human activities. This is particularly true for numerous low energy rivers flowing in the plateaus of Western Europe. Because of this limited energy, their potential in terms of autogenic restoration a priori appears strongly restricted. This study examines the conditions under which the morphogenesis on one of these systems occurs in relation to various hydrologic conditions. Two complementary approaches are combined on three reaches of the meandering Cher River (France). Firstly, we examine at a pluri-decadal scale the control of duration, frequency and intensity of floods on the planimetric erosion (bank retreat mainly). Secondly, we estimate the range of effective discharge for bedload transport. The results show that the morphogenesis is controlled by low magnitude hydrological events. Two major controlling factors are suggested: low differential of energy between small and large floods, peculiar to low energy rivers of mid-latitude, and low critical discharges for lateral erosion and bedload mobilization. For these reasons, the ability of the alluvial Cher River to self-restore its fluvial dynamics seems to be relatively high.
The past year has been one of reflection by the BGRG on its own organisation as well as continuat... more The past year has been one of reflection by the BGRG on its own organisation as well as continuation of a vibrant programme of meetings and publications. External relations have been fostered through a series of joint conferences with other bodies and representations have been made to ...
ABSTRACT To understand river channel changes and fluvial activity it is necessary to know the mag... more ABSTRACT To understand river channel changes and fluvial activity it is necessary to know the magnitude, frequency, and sequence of flows and the controls upon them and compare the channel responses in periods with known conditions. This paper identifies the characteristics of mean and maximum flows of the Gila River in southeast Arizona in the twentieth century and examines their relationship to precipitation, to the Palmer Hydrological Index, and to land and water management. Flow records show very high total flows in the period 1905–1916 and very low flows in the 1940s and 1950s. Eleven of the 20 highest annual peaks since 1914 have occurred since 1965. The 1983 flood was nearly comparable with the flood of 1905, which is associated with the destabilization of the Gila River. Increased flooding is found to be associated primarily with increased winter precipitation and the evidence available indicates that water usage and land management have much less influence on flows than has climate. Major variations, on the scale of a few decades, are similar to those identified in Australia, which have had profound effects on river channels. Widespread recognition of the magnitude and persistence of the flow variations has important implications for water resources and flood management. [Key words: hydrology, climatic variation, precipitation, river discharge, floods, Arizona.]
ABSTRACT Accurate records of flash flood magnitudes are required to inform flood forecasting and ... more ABSTRACT Accurate records of flash flood magnitudes are required to inform flood forecasting and planning. However, whilst a distributed flood survey is desirable to capture spatial heterogeneity in peak water surface elevation, the field time required for a distributed survey often limits the spatial coverage of such reconstructions. For the first time, we demonstrate the application of Structure-from-Motion (SfM) with Multi-View Stereo (MVS) to reconstruct the magnitude of a flash flood. This approach required only standard digital photographs and ground control points, took only similar to 30 min in the field, and can be embedded within existing protocols easily. We validated the method against a conventional dGPS survey in three stages: (i) comparison of topographic data revealed that SfM was accurate to within 0.1 m; (ii) high water marks extracted from the SfM model were within 0.25 m of those surveyed in the field with no consistent over or under-estimate; (iii) peak discharge reconstructed from a two-dimensional hydraulic model was within the range of more conventional estimates. With low uncertainty in our terrain model and our reconstructed flood water surface, we highlight the added value of the SfM approach for incorporating reach scale spatial variability into hydraulic reconstructions.
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Papers by J. Hooke