Peter W. Downs

How integrated is river basin management

Land and water management is increasingly focused upon the drainage basin. Thirty-six terms recently used for schemes of “integrated basin management” include reference to the subject or area and to the aims of integrated river basin management, often without allusion to the multiobjective nature. Diversity in usage of terms has occurred because of the involvement of different disciplines, of the increasing coherence of the drainage basin approach, and the problems posed in particular parts of the world. The components included in 21 different approaches are analyzed, and, in addition to showing that components related broadly to water supply, river channel, land, and leisure aspects, it is concluded that there are essentially five interrelated facets of integrated basin management that involved water, channel, land, ecology, and human activity. Two aspects not fully included in many previous schemes concern river channel changes and the dynamic integrity of the fluvial system. To clarify the terminology used, it is suggested that the termcomprehensive river basin management should be used where a wide range of components is involved, whereasintegrated basin management can signify the interactions of components and the dominance of certain components in the particular area.Holistic river basin management is advocated as a term representing an approach that is both fully comprehensive and integrated but also embraces the energetics of the river system and consideration of changes of river channels and of human impacts throughout the river system. The paradigm of working with the river can be extended to one of working with the river in the holistic basin context.

An interdisciplinary approach to evaluation of potential instability in alluvial channels

Abstract A modular procedure to assess the magnitude, distribution, and potential for channel instabilities at a large number of sites has been designed and implemented. The procedure, based on diagnostic interdisciplinary criteria of alluvial channel morphology and associated riparian vegetation, is presented. The modules include (1) initial site evaluations, (2) GIS-based data input and management, (3) ranking of relative channel stability, (4) identification of spatial trends, (5) ranking of socio-economic impacts and identification of most “critical” sites, and (6) collection of additional field data for more detailed evaluation of the magnitude and type of future instabilities and the effects of proposed mitigation measures. The procedure, using site evaluation forms as the fundamental means of data collection, takes a trained person 1 to 1.5 hours to complete. Site evaluation forms can be altered according to the specific environment being studied and the objectives of the study. An objective ranking scheme based on physical attributes extracted from the GIS data base permits the identification of the most unstable channel sites and, thereby, focuses attention on potentially “critical” sites. If a significant concern about a bridge or adjacent lands arises, or if mitigation measures are considered, various methods to estimate future channel changes are proposed. These include (1) numerical alluvial channel modeling, (2) empirical models of channel evolution, (3) regime equations, and (4) empirical relations based on process dominance in different fluvial environments. The methods require differing amounts of additional field data and provide results of varying detail and accuracy. The decision on which method to use must be based on the objectives and resources of the agency involved in the evaluation study.

Fluvial geomorphological analysis of the recruitment of large woody debris in the Yalobusha River network, Central Mississippi, USA

Abstract The management of large woody debris (LWD) should be based on a rational assessment of its recruitment rate relative to its natural decay and removal. LWD recruitment may be controlled by ‘natural’ episodic terrestrial factors or by in-channel geomorphological controls related to the rate of bank erosion. The geomorphological controls are hard to quantify in laterally migrating channels, but in incising channels, a conceptual model may be developed based on the density of riparian trees relative to the knickpoint migration rate and bank stability analyses that predict the post-knickpoint width of the channel. The Yalobusha river network in Central Mississippi, USA, has twice been destabilised by channel straightening for flood defence and land drainage, most recently in 1967. System-wide rejuvenation has followed through a series of upstream migrating knickpoints several metres high that have caused mass failure of streambanks and the recruitment of large volumes of trees to the channel. LWD recruitment is maximised at the transition between stage III and stage IV channels, focusing attention on 11 sites in the network. The sites are upstream of knickzones ranging between 2.2 and 5.4 m high and migrating at rates of 0–13.8 m year−1, based on 23–30 months of monitoring. Riparian conditions in 500 m2 plots on each bank upstream of the knickpoints range from treeless to forested, containing 0–98 trees with an average diameter at breast height of 0.18 m and average maximum height of 14.0 m. The average volume of wood on each bank is 0.02 m3 m−2. Under rapid drawdown conditions, bank stability analyses suggest that the channels will widen in amounts ranging from 1.8 to 31.5 m. Combined with the knickpoint migration rates, riparian land losses are estimated to range from 8.0 to 433.8 m year−1, resulting in the recruitment of almost 28 m3 of wood (or 100 trees) annually from the 11 sites. Assuming this LWD recruitment rate, a model is developed for the in situ potential for debris dam initiation and growth, based on the ratio of tree height to channel width under current and post-knickpoint conditions, the annual delivery of ‘large’ trees and the annual total of LWD recruitment by volume. A longer-term model is also developed, based on ‘knickpoint severity’ and vegetation density in upstream and headwater riparian zones of each tributary. The 11 study sites are classified into groups with similar LWD management concerns based on these analyses. The models developed in this research provide the first precise quantification of LWD recruitment according to geomorphological controls and standing vegetation, and a rational assessment of its meaning, but further research is required to improve the accuracy of such estimates.

Managing reservoir sediment release in dam removal projects: An approach informed by physical and numerical modelling of non‐cohesive sediment

Abstract Sediment management is frequently the most challenging concern in dam removal but there is as yet little guidance available to resource managers. For those rivers with beds composed primarily of non‐cohesive sediments, we document recent numerical and physical modelling of two processes critical to evaluating the effects of dam removal: the morphologic response to a sediment pulse, and the infiltration of fine sediment into coarser bed material. We demonstrate that (1) one‐dimensional numerical modelling of sediment pulses can simulate reach‐averaged transport and deposition over tens of kilometres, with sufficient certainty for managers to make informed decisions; (2) physical modelling of a coarse sediment pulse moving through an armoured pool‐bar complex shows deposition in pool tails and along bar margins while maintaining channel complexity and pool depth similar to pre‐pulse conditions; (3) physical modelling and theoretical analysis show that fine sediment will infiltrate into an immobile coarse channel bed to only a few median bed material particle diameters. We develop a generic approach to sediment management during dam removal using our experimental understanding to guide baseline data requirements, likely environmental constraints, and alternative removal strategies. In uncontaminated, non‐cohesive reservoir sediments we conclude that the management impacts of rapid sediment release may be of limited magnitude in many situations, and so the choice of dam removal strategy merits site‐specific evaluation of the environmental impacts associated with a full range of alternatives.

A Geomorphological Justification of River Channel Reconnaissance Surveys

Geomorphologists involved in river management in many countries have shown an increasing interest in developing schemes for evaluating river channel geomorphology based on a single reconnaissance survey which uses skills of observation and interpretation. However, many questions surrounding this approach have yet to be resolved. This paper presents arguments to demonstrate first that reconnaissance surveys are the only viable source of relevant geomorphological data for many projects, secondly, that reconnaissance complements other components of integrated and multidisciplinary approaches to river management and, thirdly, that, although experience suggests that reconnaissance survey data can never be a substitute for detailed, repeat data collection, this limitation is more than offset by the external visibility and demystification afforded to the discipline of geomorphology by the surveys. Finally, comments are made on the potential for developing standardized approaches.

Restoring Ecological Integrity in Highly Regulated Rivers: The Role of Baseline Data and Analytical References

The goal of restoring ecological integrity in rivers is frequently accompanied by an assumption that a comparative reference reach can be identified to represent minimally impaired conditions. However, in many regulated rivers, no credible historical, morphological or process-based reference reach exists. Resilient restoration designs should instead be framed around naturalization, using multiple analytical references derived from empirically-calibrated field- and model-based techniques to develop an integrated ecological reference condition. This requires baseline data which are rarely collected despite increasing evidence for systematic deficiencies in restoration practice. We illustrate the utility of baseline data collection in restoration planning for the highly fragmented and regulated lower Merced River, California, USA. The restoration design was developed using various baseline data surveys, monitoring, and modeling within an adaptive management framework. Baseline data assisted in transforming conceptual models of ecosystem function into specific restoration challenges, defining analytical references of the expected relationships among ecological parameters required for restoration, and specifying performance criteria for post-project monitoring and evaluation. In this way the study is an example of process-based morphological restoration designed to prompt recovery of ecosystem processes and resilience. For the Merced River, we illustrate that project-specific baseline data collection is a necessary precursor in developing performance-based restoration designs and addressing scale-related uncertainties, such as whether periodic gravel augmentation will sustain bed recovery and whether piecemeal efforts will improve ecological integrity. Given the numerous impediments to full, historical, restoration in many river systems, it seems apparent that projects of naturalization are a critical step in reducing the deleterious impacts of fragmented rivers worldwide.

Estimating bedload transport rates in a gravel-bed river using seismic impact plates: Model development and application

Abstract A data-driven, uncertainty-bound estimation technique for bedload transport rates is developed based on passive sensing devices. The model converts sediment samples to a mass in transit for each instantaneous discharge according to impacts detected and a Monte Carlo simulation of the load determined at random from the particle size distribution. Using impact count data autogenically produces a supply-limited, location-specific and high-resolution time-series of bedload rates, while the probabilistic approach inherently accommodates the stochastic nature of bedload transport. Application to the River Avon (Devon, U.K.) provides cross-sectional bedload rate estimates within the bounds of experimental data and calibrated to observed field behaviour. This new procedure offers an alternative ‘class’ of bedload estimation to existing approaches and has the potential for wide-ranging applications in river management and restoration, while contributing to the integration of ‘big data’ into a progressive agenda for hydrogeomorphology research.

Learning from River Restoration Projects

In funding and planning river restoration projects, agencies tend to focus on physical implementation, and include monitoring only as an afterthought. The situation is improving, with more funding agencies requiring monitoring of projects to assess the effectiveness of their investments, but most river restoration projects constructed today are still not monitored well enough to learn effectively from them. In the relatively rare cases where a large number of projects have been systematically monitored, typically half or more of the projects are shown to be failing or not meeting design objectives. Because of the hydraulics of alluvial channels are indeterminate, we must live with uncertainty in the outcome of our interventions. With an adaptive management approach, each restoration project can be treated as an experiment, from which we can learn, but only provided we undertake adequate post-project appraisal based on good baseline data and a sound study plan. Examining a range of channel reconstruction projects, we can often find commonalities that can inform future restoration efforts.