The Importance of Spatiotemporal Fish Population Dynamics in Barrier Mitigation Planning

In this study, we propose a novel framework combining spatially explicit population viability analysis and optimization for prioritizing fish passage barrier mitigation decisions. Our model aims to maximize the equilibrium population size, or alternatively minimize the extinction risk, of a target fish species subject to a budget on the total cost of barrier mitigation. A case study involving a wild coho salmon (Oncorhynchus kisutch) population from the Tillamook basin, Oregon, USA is used to illustrate the benefits of our approach. We consider two different spawning adult dispersal patterns, river and reach level homing, as well as straying. Under density dependent population growth, we find that homing behavior type has a significant effect on barrier mitigation decisions. In particular, with reach homing, our model produces virtually the same population sizes as a more traditional barrier prioritization procedure designed to maximize accessible habitat. With river homing, however, we find that it is not necessary to remove all barriers in order to maximize equilibrium population size. Indeed, a stochastic version of our model reveals that removing all barriers actually results in a marginal increase in quasi-extinction risk. We hypothesize that this is due to a population thinning effect of barriers, resulting in a surplus of recruits in areas of low spawner density. Our findings highlights the importance of considering spatiotemporal fish population dynamics in river connectivity restoration planning. By adding greater biological realism, models such as ours can help conservation managers to more strategically allocate limited resources, resulting in both cost savings and improved population status for a focal species

How to Choose? A Bioeconomic Model for Optimizing River Barrier Mitigation Actions

River infrastructure can cause adverse impacts on fish populations, which, in turn, compromises the ability of river ecosystems to provide a range of ecosystem services. In this paper, we present a methodological approach to assess the potential economics costs and benefits of river connectivity enhancement achieved through removal and mitigation of fish dispersal barriers. Our approach combines the results of a stated preference study for nonuse values of rivers and statistical models of fish population responses to barrier mitigation actions within an integrated bioeconomic optimization framework. We demonstrate the utility of our methodology using a case study of the River Wey catchment in southeast England, which contains over 650 artificial barriers. Our results reveal the presence of benefit-cost trade-offs which can form the basis for river barrier mitigation policy development. In particular, we find that benefits exceed costs in the River Wey for all levels of investment in barrier mitigation considered (£2.5 to 53.4M). Furthermore, from an economic efficiency standpoint, a total budget of approximately £22.5M allocated to barrier mitigation would maximize net societal benefits derived from anticipated increases fish species richness and abundance

Engaging Stakeholders To Extend The Lifecycle Of Hybrid Simulation Models

Developing a simulation model of a complex system requires a significant investment of time, expertise and expense. In order to realize the greatest return on such an investment, it is desirable to extend the lifecycle of the simulation model as much as possible. Existing studies typically end after the `first loop' of the lifecycle, with the computer model suitable for addressing the initial requirements of the stakeholders. We explore extending the modeling lifecycle to a `second loop' by introducing an existing hybrid simulation model to a new group of stakeholders and further developing it to capture new requirements. With the aid of an example application, we explain how the hybrid model facilitated stakeholder engagement by closely reflecting the real world and how the model lifecycle has been successfully extended to maximize the benefit to Eurostar International Limited

A dynamic model for road protection against flooding

This paper focuses on the problem of identifying optimal protection strategies to reduce the impact of flooding on a road network. We propose a dynamic mixed-integer programming model that extends the classic concept of road network protection by shifting away from single-arc fortifications to a more general and realistic approach involving protection plans that cover multiple components. We also consider multiple disruption scenarios of varying magnitude. To efficiently solve large problem instances, we introduce a customised GRASP heuristic. Finally, we provide some analysis and insights from a case study of the Hertfordshire road network in the East of England. Results show that optimal protection strategies mainly involve safeguarding against flooding events that are small and likely to occur, whereas implementing higher protection standards are not considered cost-effective

Best practices for selecting barriers within European catchments

With over 1.2 million dams and other instream barriers, Europe has possibly the most fragmented rivers in the world, but also the opportunity to benefit enormously from barrier removal. Resources available for barrier removal, however, are limited and some form of prioritization strategy is thus required to select barriers for removal that will provide the greatest gains from restoring river connectivity in the most efficient possible way. To properly restore river function, barrier removal programs need to consider all types of artificial instream barriers that cause river fragmentation, not just those that impede fish movements. Opportunities for barrier removal depend to a large extent on barrier typology, as this dictates not only where barriers are typically located, but also their size, age, condition, and impacts. Crucially, the extent of river fragmentation depends chiefly on the number and location of barriers, not on barrier size. However, because barrier removal costs typically increase with barrier height, acting on many small barriers may be more cost-efficient than acting on fewer larger structures. Here we review the main strategies available to prioritize barriers for removal and mitigation, with special emphasis on removing non-ponding, low-head (<3 m) barriers, as these are the most abundant across Europe and other regions. To increase the success of barrier removal programs, we recommend that barriers considered for removal fulfill four essential conditions: (1) they would bring about a meaningful gain in connectivity; (2) are cost-effective to remove; (3) will not cause significant or lasting environmental damage, and (4) are obsolete structures. There are dozens of prioritization methods in use. These can be broadly grouped into six main types depending on whether they are reactive or proactive, whether they are typically applied at local or larger spatial scales, and whether they employ an informal or a formal approach. These include, in increasing order of complexity: (1) opportunistic response; (2) use of local knowledge and expert opinion; (3) scoring and ranking; (4) geographic information system (GIS) scenario analysis; (5) graph theory; and (6) mathematical optimization. We review their strengths and weaknesses and provide examples of their use. Overall, mathematical optimization sets the gold standard for effective and robust barrier mitigation planning, but to be practical, it needs to factor in the constraints imposed by uncertainties and opportunities. Accordingly, a hybrid approach that considers uncertainty, the presence of natural barriers, the importance of future-proofing, and opportunities provided by local knowledge is likely to be the best overall approach to adopt. Various studies have shown that a small proportion of barriers is typically responsible for the majority of river fragmentation. These ‘fragmentizers’ can be identified and located using the prioritization methods discussed herein and a targeted approach can produce substantial gains in connectivity by acting on a relatively small number of structures. Unfortunately, many of these ‘fragmentizers’ cannot be easily removed. Removal, therefore, is constrained by opportunities and what is practically feasible. Mapping of barrier removal projects according to the three axes of opportunities, costs, and gains can help locate the ‘low hanging fruits.’ Opportunities normally develop over time as infrastructure ages, so acting on some barriers now will likely open opportunities for acting on others later on to create a snowballing effect. The ability to simulate benefits and costs of barrier removal and select barriers for removal is critically dependent the quality of the data at hand, particularly with respect to the number of barriers, which can be grossly underrepresented. Uncertainty caused by incomplete barrier records diminishes the effectiveness of barrier mitigation actions but these can be overcome to some extent by (1) ground truthing via river walkovers or (2) predictive modelling. Other critical sources of uncertainly include those caused by inaccurate stream networks and spatial errors regarding the exact locations of barriers. Although uncertainties can be reduced by collecting more information, it needs to be weighed against the cost of waiting. Waiting to collect more data to reduce uncertainties tied to barrier removal may lead to ‘paralysis by analysis,’ while species and ecosystems continue to decline due to stream fragmentation. To better understand how barrier prioritization is implemented in the real world, we sent out an online questionnaire to river restoration practitioners located in Europe and North America. Results show that most organizations (~60%) have a plan to achieve free-flowing rivers. Most respondents (34%) use expert judgment, followed by consultation with stakeholders (17%) and a combination of methods (28%) to prioritize barriers for mitigation. Only 12% used specialized software or algorithms. Attributes most frequently considered by practitioners in barrier prioritization were barrier ownership and rights, results of field surveys, and the obsolescence and conservation status of barriers. The most important rational flagged by practitioners to prioritize barriers for removal was to improve fish passage. Our study suggests that no matter what prioritization approach is ultimately adopted, decision makers need to be mindful that no priorities should be set in stone. Planning needs to be agile and flexible enough to adapt to changes and react to opportunities

River infrastructure planning decision support tool

A decision support tool has been developed to optimise environmental and socioeconomic trade-offs associated with river infrastructure (i.e. barriers), including river connectivity, hydropower generation, and shipping and other costs. The model extends previous work by considering (i) multithreaded river sections, essential for estimating river connectivity in the presence of diverted channels for hydropower and shipping, (ii) backwater effects caused by the lowering or raising of artificial in-stream structures, which , in turn, affect hydropower generation potential and transport capacity of shipping vessels, and (iii) the integration of realistic cost functions for estimating the cost of hydropower installation/retrofitting and cross-port shipping of goods by a heterogeneous fleet of vessels. To demonstrate the applicability of the planning tool, a database for the Neckar River in Germany was created. The database contains more than 1000 existing river barriers and more than 4,000km of river and includes detailed information on river flow, hydropower, and waterborne traffic. The planning tool is designed to find the best combination of river infrastructure modification, mitigation, and removal actions in order to optimise three key performance indicators (KPIs): 1) river connectivity based on a generalisation of the well-known Dendritic Connectivity Index, 2) hydropower potential/revenue and 3) total cost, broken down by the cost of structural engineering works, installation and retrofitting of hydropower, and annual shipping. The model focuses specifically on the socioeconomic benefits of hydropower and shipping, as these are the two main human uses of rivers within the Neckar catchment. The model could be readily modified to consider other environmental and socioeconomic factors, which may be important in other planning areas, such as irrigation, water supply, fishing, recreation, and water purification. The planning model has been specially configured to examine ten scenarios for adaptive barrier management. The results show which river infrastructure modification, mitigation, and removal actions could be undertaken to maximise connectivity, maximise hydropower, or minimise total cost subject to defined river connectivity and energy production targets

Developing a Hybrid Simulation Model using both Parsimonious and Highly Descriptive Approaches: A Case Study from the Transport Industry

We put forward some initial thoughts about using both parsimonious and highly descriptive approaches to engage stakeholders during the development of a hybrid simulation study in the transport industry. The hybridisation we discuss involved combining discrete-event and agent-based simulation. We discuss how both parsimonious and highly descriptive modelling approaches, which are seemingly incompatible, were used in the development of a hybrid model to help facilitate stakeholder engagement. In our experience stakeholders with limited understanding of the system being modelled engaged with more ease when presented with highly descriptive approaches. When working with stakeholders with a better understanding, parsimonious approaches can be beneficial. We also discuss potential techniques for managing the complexity of large simulation projects by adapting ideas from software development to help modellers work with stakeholders

Reliable hub network design: Formulation and solution techniques

In this paper, we investigate the issue of unreliability in hub location planning. A mixed integer nonlinear programming model is formulated for optimally locating p uncapacitated hubs, each of which can fail with a site-specific probability. The objective is to determine the location of hubs and the assignment of demand nodes to hubs in order to minimize expected demand weighted travel cost plus a penalty if all hubs fail. A linear version of the model is developed employing a specialized flow network called a probability lattice to evaluate compound probability terms. A Tabu search algorithm is proposed to find optimal to near optimal solutions for large problem instances. A parallel computing strategy is integrated into the Tabu search process to improve performance. Experimental results carried out on several benchmark instances show the efficiency of our linearized model and heuristic algorithm. Compared to a standard hub median model that disregards the potential for hub failures, our model produces solutions that serve larger numbers of customers and at lower cost per customer

A Hypergraph Multi-Exchange Heuristic for the Single-Source Capacitated Facility Location Problem

In this paper, we introduce a large-scale neighborhood search procedure for solving the single-source capacitated facility location problem (SSCFLP). The neighborhood structures are induced by innovative split multi-customer multi-exchanges, where clusters of customers assigned to one facility can be moved simultaneously to multiple destination facilities and vice versa. To represent these exchanges, we use two types of improvement hypergraphs. The improvement hypergraphs are built dynamically and the moving customers associated with each hyperedge are selected by solving heuristically a suitably defined mixed-integer program. We develop a hypergraph search framework, including forward and backward procedures, to identify improving solutions efficiently. Our proposed algorithm can obtain improving moves more quickly and even find better solutions than a traditional multi-exchange heuristic (Ahuja et al., 2004). In addition, when compared with the Kernel Search algorithm (Guastaroba and Speranza, 2014), which at present is the most effective for solving SSCFLP, our algorithm is not only competitive but can find better solutions or even the best known solution to some very large scale benchmark instances from the literature

A toolkit for optimizing fish passage barrier mitigation actions

1. The presence of dams, stream–road crossings and other infrastructure often compromises the connectivity of rivers, leading to reduced fish abundance and diversity. The assessment and mitigation of river barriers is critical to the success of restoration efforts aimed at restoring river integrity. 2. In this study, we present a combined modelling approach involving statistical regression methods and mixed integer linear programming to maximize resident fish species richness within a catchment through targeted barrier mitigation. Compared to existing approaches, our proposed method provides enhanced biological realism while avoiding the use of complex and computationally intensive population/ecosystem models. 3. To estimate barrier passability quickly and at low cost, we further outline a rapid barrier assessment methodology. The methodology is used to characterize potential passage barriers for various fish species common to the UK but can be readily adapted to different planning areas and other species of interest. 4. We demonstrate the applicability of our barrier assessment and prioritization approach based on a case study of the River Wey, located in south-east England. We find that significant increases in species richness can be achieved for modest investment in barrier mitigation. In particular, dams and weirs with low passability located on mid- to high-order streams are identified as top priorities for mitigation. 5. Synthesis and applications. Our study shows the benefits of combining a coarse resolution barrier assessment methodology with state-of-the-art optimization modelling to cost-effectively plan fish passage barrier mitigation actions. The modelling approach can help inform on-the-ground river restoration decision-making by providing a recommended course of action that best allocates limited resources in order to restore longitudinal connectivity and maximize ecological gains