Linking environmental flows to sediment dynamics

We are living in the Anthropocene Era and we are becoming increasingly aware of the large body of evidence showing that human interactions with the hydrological cycle have serious consequences for rivers and ecosystems (Vörösmarty et al., 2003). Gerten et al. (2013) pointed out the existence of a ‘planetary boundary’ for fresh water used by humans, and proposed ways forward to refine and reassess it. They suggested that a key element involves quantifying local water availabilities taking account of environmental flow (e-Flow) requirements.

Human populations have water demands that are prioritized according to various needs: (1) vital water (drinking water, hygiene, sanitation), (2) social water (gardens, swimming pools) and especially (3) commercial water (required for hydropower, intensive agriculture, industrial processes, tourism infrastructure). In warmer climates all these demands are greater than in colder climates. Typically, commercial water use represents more than 80% of all demand, which is often even greater than water availability.

Of all European regions, Mediterranean countries also use the most water stored in reservoirs for irrigation. In addition, historical land overexploitation and today's intensive agriculture on slopes cause high catchment erosion, sediment yield, and transport. This latter problem is widespread and shared by continental lowland basins too.

This paper draws on the Water Framework Directive (WFD) common implementation strategy (CIS) Guidance Document ‘Ecological flows in the implementation of the Water Framework Directive’ (European Commission, 2015) as a starting point. Our main objective is to emphasize the importance of HYMO, especially for rivers that are heavily regulated by large dams. Thus, we adopt the perspective of the Guidance Document that environmental flows (e-Flows) are more than just minimum flows, and have to include all the components of the hydrological regime.

E-Flows play different roles in different fluvial settings. Ideally we can view e-Flows as restoration measures since their aim is to support the achievement of good ecological status in rivers subject to hydrological pressures. When these pressures are exerted by major infrastructures such as large dams, however, the changes caused in the river ecosystem can be so profound that e-Flows can only be considered as mitigation measures. In addition, River Basin Management Plans (RBMPs) often consider e-Flows as preventive measures for many river sections that are not regulated or affected by water abstraction. Furthermore, for fluvial segments under some form of protection, e-Flows represent conservation measures. With such different objectives of e-Flows, the question arises: should all types of e-Flows be quantified in a single manner?

In the context of the WFD, ecological flows are defined as a flow regime consistent with the achievement of the environmental objectives of a water body (i.e. good ecological status – for natural water bodies; good ecological potential – for heavily modified – HMWB – and artificial water bodies; and good quantitative and chemical status for groundwater bodies). Ecological flows represent therefore a ‘potential’ measure to reach the objectives, as the real measure will derive from the evaluation of all the physical, legal and socio-economic constraints related to the water body.

As a potential measure, ecological flows come into play when the results of the WFD Art.5, risk analysis on a catchment, show that some water bodies are at risk of failing their objectives due to an inadequate (in terms of magnitude and timing) flow regime (e.g. a reach downstream of a reservoir). Identifying whether it is possible to manage such a regime to make it consistent with the environmental objectives set, requires determining the current natural or anthropic constraints on the catchment (HYMO economic, social, etc.) through analysis of scenarios. Such scenarios need to evaluate remedial measures not only in terms of their impacts on the status of water bodies, but also on the uses of water in the actual system. This is crucial when addressing HMWBs, as they are designated on the basis of their legitimate use.

The CIS Guidance Document ‘Ecological flows in the implementation of the Water Framework Directive’ (European Commission, 2015) presents an overview of methodologies for e-Flows implementation. However, it does not address in depth certain crucial aspects, among which is the definition of a HYMO regime consistent with a desired ecological state and relevant scales to be used in the assessment.

Rivers and their ecosystems reflect hierarchies of control. We can consider rivers as complex organisms, whose functioning needs both water flowing and its particular metabolites (sediments; woody debris; particulate organic matter; dissolved solids and gases). However, a large dam on a river disturbs not only the natural water flow regime, but often to a greater extent, the natural fluxes of these metabolites. Therefore, when we use environmental flows as an instrument to improve the ecological status of water bodies, we should also consider the fluxes of all ‘metabolites’ that allow the existence of biological communities.

Such holistic methodologies considering the many interacting components of aquatic systems, including sediments, are increasingly recommended although in many cases assessment of e-Flows is mainly based on hydrological and hydraulic assessment (Anderson et al., 2006; Meitzen et al., 2013). But, other approaches defining e-Flows integrating diverse disciplines should be mentioned, like the Holistic Approach (Arthington et al., 1992). The Building Block method developed in South Africa (King & Louw, 1998) considers geomorphology as the physical template for biological processes. Sediment assessments for e-Flows have been carried out by others, such as King et al. (2003) who developed the Downstream Response to Imposed Flow Transformation methodology. However, the sediment-flow component is not included in e-Flows: they remain e-Flows confined to only water and incorporate geomorphology as an effect of flow (‘large floods mobilize coarse sediments’). The Building Block method has been widely applied in Southern and Eastern Africa and in many other parts of the world, and is suggested as the basic methodology to be applied in the UK (Acreman & Dunbar, 2004).

In the context of the WFD, e-Flows represent a possible measure to reach the objectives of good ecological status or potential. There is still too little experience with the implementation of e-Flow based measures; a review of the hydrological measures applied at the EU level, based on the information derived from the RBMPs showed that they have been established according to the ‘minimum flow’ concept (Sánchez-Navarro & Schmidt, 2012). As such, no consideration has been given to the morphological evolution of the affected reaches or channels, which could have caused a consistent channel conveyance change. Beyond the flow regime, sediment transport plays a fundamental role in determining and maintaining channel morphology and related habitats.

A river habitat is the result of a balance between interacting geomorphological forces: water, sediments and riparian vegetation in a spatial template (fluvial reach). Water flow has the hydraulic energy able to erode, transport and deposit sediments and riparian vegetation growth is able to consolidate deposited sediments, but old vegetation stands may reduce water erosion capacity. Thus, habitat morphology depends also on the structure and composition of the riparian stand and on its present interaction with the hydro-geomorphic pattern of the river.

The importance of sediment transport and related geomorphic processes as key components to evaluate has only recently begun to be acknowledged. Meitzen et al. (2013) emphasized how fluvial geomorphology and riverine ecology represents an ideal confluence to examine the contribution of the geomorphic field tradition to environmental flows. They developed a question-based framework that will facilitate holistic and interdisciplinary environmental flow assessments.

According to the WFD, the environmental objective of a river water body coincides with its ecological status, mainly given by the combination of the status of the relevant biological quality elements, each assessed through indicators. However, the majority of these commonly used indicators do not respond, with a necessary degree of sensitivity, to hydrological and morphological pressures or to multi-stressor systems, as acknowledged by the scientific community (Friberg et al., 2011, 2013). Therefore, objectives can hardly be defined and/or measured in terms of current biological indicators. Among biological quality elements, fish is the most reactive one to HYMO pressures, but no efficient/official method to assess their status is currently available for use in Mediterranean countries like Spain or Italy.

The notions of ‘ecological status’ and ‘ecological potential’ are highly dependent upon generally negotiated choices of metrics and thresholds, the definitions of which are constrained by the limits of assessment methods, their interpretability and ability to accurately assign a given system to a particular class (Friberg et al., 2011). Therefore, there is a need to develop (HYMO) pressures – specific indicators based on HYMO responsive biological elements, including alternative sampling strategies. Moreover, because of the strong nexus between HYMO and biology, where hydrology is the main pressure affecting the status of water bodies, it is suggested to define objectives also in the context of a HYMO restoration action and measure it through hydrological and morphological parameters.

Water releases downstream of a dam entrain sediments through a size selective process that causes river substrate evolution with different stages (Collier et al., 2000):

• • Over many years there is a ‘wave’ of sediment deficit that moves downstream along the river, changing its substrate traits: sediment calibre increases, as does armouring.

• • Later, substrate comes to equilibrium between the regulated flow regime and sediment input by tributaries.

• • The effects on the biota vary in space and time according to these stages of substrate change.

Therefore, setting e-Flows (including water and sediments) must take into account this substrate evolution for each reach of the river. In order to clarify concepts, we introduce an alternative term, Environmental Water & Sediment flows (EWS-Flows), leaving traditional term e-Flows for only water environmental flows.

We should remark on the great influence of riparian vegetation dynamics (Hupp, 1999; Corenblit et al., 2007) that must be considered in specifying e-Flows below large dams, especially in warm climates that promote intensive growth and recruitment. This vegetation encroachment stabilizes the new channel, even within extraordinary floods.

Prevention of vegetation encroachment could be a basic objective of effective e-Flows, particularly in Mediterranean streams, where common irrigation reservoirs release high summer flows, thus supporting maximum plant growth potential as the normal summer drought is eliminated (Magdaleno & Fernández, 2011; Stella et al., 2013; González del Tánago et al., 2015; Lobera et al., 2015). Ultimately, regulated river dimensions are so reduced that they develop into a small remnant: a ‘Bonsai river’.

Water and sediment transport in rivers are intrinsically linked and actions on one component will interact with the other. Therefore, managing environmental flows without considering sediment dynamics will not yield the desired positive effects. By contrast, the combined management of the two components may have more cost-benefit impacts, from reduced water releases to temperature mitigation or pollutant abatement.

We propose a policy where water and sediments are considered together when dealing with the impacts of reduced flows and the use of e-Flows as a possible mitigation action, expanding the present definition of e-Flows and coupling flows with sediment dynamics. Benefits from this policy proposal come from both the ecological perspective as well as from reservoir management as sediments cause problems through siltation of reservoirs and loss of their functionality and ability to regulate, and by silting up river beds.

With this objective, we define a HYMO framework to assess the status and impacts of sediment and flow management and an e-Flows toolbox adapted to couple flows to sediment.

Although the importance of sediment and geomorphological processes has been acknowledged in the CIS Guidance Document ‘Ecological flows in the implementation of the Water Framework Directive’ (European Commission, 2015), the links between hydrology and channel morphology are still only marginally considered in the evaluation of e-Flows.

Within the context of the REFORM project (http://www.reformrivers.eu/), a multi-scale, spatial-temporal geomorphological framework has been developed. This framework can be used to assess HYMO conditions and to identify suitable restoration measures. In this section, we will set e-Flows within the context of the HYMO assessment framework. The aim of this section is to provide a ‘road map’ on possible future developments with wider inclusion of geomorphological processes. The basic hypothesis (paradigm) is that enhancing morphological conditions will promote a positive ecological response.

The current approach to setting e-Flows is to focus on the hydrological regime in anticipation of promoting some ecological response. However, two other types of actions are also possible: focusing on the sediment transport regime (e.g. releasing sediments downstream of dams or other obstructions) or directly manipulating channel morphology. Any of these actions may induce morphological channel changes, therefore promoting habitat recovery and diversity. The choice of the best option to be considered in combination with changes in the hydrological regime (i.e. sediment transport versus morphological enhancement) depends on the specific context, for example the reach sensitivity and morphological potential (see below). Therefore, selecting the appropriate measures requires setting the river reach within a wider spatial-temporal framework.

We make use of a HYMO assessment framework to provide a stronger foundation for determining e-Flows. The spatial and temporal contexts are based on the multi-scale, process-based, hierarchical framework developed in the REFORM project (Gurnell et al., 2014). The framework is structured into a sequence of procedural stages and steps to assess river conditions and to support the selection of appropriate management actions (REFORM Deliverable 6.2; Rinaldi et al., 2015).

Stage I aims to provide a catchment-wide delineation, characterization and analysis of the river system. This is fundamental to properly set the existing HYMO pressures (dams, weirs, water abstraction, etc.) within a catchment-wide context, and to better understand the factors controlling channel morphology and processes in the current condition.

Relevant aspects for e-Flows include: identification of main sediment sources, delivery processes, and sediment transport along the river network to set the existing alteration (e.g. dam) in the catchment context; evaluation of effective discharge and of the specific flow needed to initiate sediment transport; and evaluation of impacts of existing alterations on sediment budget.

After setting the stream and causes of alteration in an appropriate spatial context, it is fundamental to investigate past conditions and factors influencing changes. A first step is to identify the major changes in controlling variables (e.g. factors influencing flow and sediment transport) that may have determined changes in the channel and river corridor conditions over the last decades or centuries. These steps aim to reconstruct trajectories of morphological changes of the potentially impacted reaches. Relevant aspects for e-Flows include understanding how HYMO alterations (e.g. dams, weirs, water abstraction) have impacted channel morphology, and the spatial and temporal extent of any alteration.

Stage III applies methods and procedures to assess river conditions and the degree of HYMO alteration related to existing pressures. This type of assessment requires knowledge of past and current conditions. Three types of assessment are carried out:

• (1) Hydrological assessment: pre-impact and post-impact periods are analysed and the deviation of the hydrological regime from unaltered conditions quantified.

• (2) Sediment budget assessment: pre-impact and post-impact periods are analysed and the deviation of the sediment regime from unaltered conditions quantified.

• (3) Morphological assessment: consists of a geomorphological evaluation of river conditions including assessment of channel forms and processes, geomorphological adjustments, and human alterations.

The assessments enable classification of the state (e.g. good, poor) of each investigated river reach to identify portions of the river system that potentially require different types of management actions (e.g. preservation or enhancement). Relevant aspects for e-Flows include: hydro peaking, modification of effective discharge, impacts on sediment budgets downstream of barriers to sediment transport.

Stage IV includes an assessment of potential morphological changes, identification of potential restoration measures, and evaluation of their impacts on future morphological trends. The first step diagnoses the condition and sensitivity of specific reaches to changes in hydrological and sediment conditions that can be associated with e-Flows. Adoption of some restoration action requires an evaluation of the likelihood that river change will take place, and of the morphological potential that could be achieved in response to a given modification of flows. This assessment is based on the knowledge gained during the previous stages, i.e. on current conditions and past changes. Based on the assessment of sensitivity and of morphological potential, target reaches and possible morphological conditions that can be achieved are identified.

The next steps are aimed at identifying possible restoration actions, and assessing scenario-based possible future trends related to selected actions. Relevant aspects for e-Flows include: identification of flows needed to initiate transport, coupling peak flows with sediment availability, determining and maintaining channel morphology and related habitats, quantification of sediment deficit or surplus, release of sediments downstream of barriers, removal of barriers and evaluation of effectiveness of different measures.

Managing hydrological and sediment regimes together to meet geo-ecological objectives in dynamic riverscapes deals with measures such as: (a) to modify flow regime; (b) to modify sediment transport regime; (c) to modify sediment supply; and (d) to engineer channels and habitat. Any decision making on the measure(s) to be used (single or a combination of them) needs to diagnose the state of the channel and to predict its response to such measures. These diagnoses and predictions are based on the comparative assessment of upstream sediment supply to channel transport capacity. Suitable and detailed methods for this assessment are shown in Grant et al. (2003) and Schmidt & Wilcox (2008).

Too much sediment in the channel must be managed primarily by reducing the production at source or intercepting it before it reaches the channel. The lack of sediment in a river reach is a more common problem than excess sediment. The reintroduction of sediments in a reach with sediment deficit can be carried out by means of upstream dam removal, or by mitigating a dam's trapping effects, or by adding sediments directly to the river.

Sediment replenishment basically consists of dredging or excavating the accumulation of sediments in a dam's reservoir and transporting them to the reach just below the dam, where natural or artificial floods will distribute them along the riverbed. In order to improve downstream ecological status, optimal sediments for replenishment must be selected, as coarser substrates are more beneficial for benthic communities than silt, which may impact interstitial habitats by clogging bed sediments, and causing high turbidity (Ock et al., 2013). The construction of check-dams, located upstream of reservoirs, where coarse particles settle, may trap larger sediments before they enter the functional reservoir and facilitate their removal by land-based excavation, and do not require any water level modification in the larger reservoir (Okano et al., 2004).

In order to relocate sediments in the riverbed efficiently, we need to know the effects of grain size, the amount of sediments replenished, the frequency of operations and when the sediment should be deposited (Cajot et al., 2012).

Another effective measure to limit sediment trapping by reservoirs and to decrease the reservoir sedimentation involves constructing sediment bypass tunnels. These tunnels route sediments (both bed load and suspended load) around the reservoir into the tail water during flood events, thereby reducing sediment accumulation. The number of actual sediment bypass tunnels globally is, however, limited (six in Switzerland and five in Japan) due to high capital and maintenance costs. The design of a bypass tunnel consists of a guiding structure in the reservoir, an intake structure with a gate, a short and steep acceleration section, a long and smooth bypass tunnel section, and an outlet structure (Auel & Boes, 2011; Figure 7). Fukuda et al. (2012) demonstrated the recovery of riffles and pools and the grain size distribution in the downstream reaches below Asahi Dam reservoir, after the construction of a sediment flushing tunnel.

Estimation of e-Flows that are necessary to maintain the desired river ecological state is not straightforward, as the quantitative links between HYMO and biology are not yet well known, due to the insufficient number of consistent data and to the weak response of current biological metrics to HYMO pressures.

Geomorphic dynamics of a river and the functioning of natural physical processes are essential to create and maintain habitats and ensure ecosystem integrity and the links between hydrology and geomorphology are generally well known. Therefore, one approach to estimating e-Flows is to identify those flows required to maintain certain geomorphic processes and forms that directly contribute to aquatic habitat and ecosystem functioning. Such an approach would broaden the current strategy for setting e-Flows, which is to focus on the hydrologic regime in anticipation of promoting some ecological response.

Elements of this broadened approach include other types of actions beyond specifying flows alone, such as focusing on the sediment transport regime (e.g. releasing sediments downstream of dams or other obstructions), or directly manipulating channel morphology (i.e. morphological reconstruction). Any of these actions (HYMO-based measures) may induce morphological channel changes, therefore promoting habitat recovery and diversity.

The choice of the best option to be considered in combination with changes in the hydrologic regime (i.e. sediment transport vs. morphological reconstruction) depends on the specific context, for example the reach sensitivity and morphological potential. Therefore, selecting the appropriate measures requires setting the river reach within a wider spatial-temporal framework. It is also important to state that sediment releases should be timed with natural sediment fluxes – just as is the case for water releases.

Within the context of the project REFORM, such a multi-scale framework has been developed and can be used as a strong methodological foundation for determining e-Flows, dealing with hydrological, morphological and ecological processes in concert.

The implementation of e-Flows is constrained by our understanding of the ecological processes, of the services they provide, and by the socio-economic requirements on water resources.

Whilst the latter issue is clearly recognized, ecosystem services related to natural processes and to e-Flow releases are yet not sufficiently acknowledged by managers and stakeholders. Therefore, ecological benefits that can be achieved by e-Flows, and in particular the added value of considering sediment related e-Flows, should be clearly justified, both from an ecological perspective (maintenance and development of habitats) and from an economic one (e.g. less water discharge if combined with sediment delivering strategy, mitigate incision problems, etc.).

Water and sediments are intrinsically interconnected in natural river systems. Fluvial communities have evolved to adapt to this interaction, and thus many of their habitat requirements depend on HYMO dynamics. Setting e-Flows (EWS-Flows including water and sediments) must take into account past morphological evolution and trajectories as well as the current status of the river system, and the water and sediment fluxes in the network, to inform the possible scenarios, prior to implementation of measures in each targeted reach of the river. Sediment management therefore needs to be built into the analytical and decision-making framework.

Environmental flows, including sediment EWS-Flows, should be implemented and monitored within an adaptive management framework. Monitoring the outcomes of e-Flows is needed because our understanding of water and sediment requirements by key aquatic biota and ecosystem functions is not precise and often critical decisions are made with relatively weak ecological evidence to support them. E-Flow monitoring programmes should have a practical approach to ensure that e-Flow implementation achieves its objectives and, in any case, identifies gaps and provides recommendations for relevant improvements.

The management of intensive flow regulated water bodies must be framed in a policy that includes sediment management strategies into e-Flows, which are what we have called EWS-Flows. We recommend starting with demonstration or pilot projects in large reservoirs where sediment release mechanisms (e.g. in association with high flow events) can be planned and implemented. These experimental EWS-Flow releases should be assessed evaluating the ecological recovery of downstream reaches together with the benefits of reservoir operation (avoiding dams silting up and maintaining reservoir capacity). The basic issue to which a dam manager must respond is how much sediment needs to be transferred downstream and how frequently during a certain period. Once cases have been implemented and practical experience has been developed it will possible to generate a plan for a widespread application of EWS-Flows.

However, we must mention that e-Flow and EWS-Flow implementation (via RBMP linked to relicensing dams) implies changes in the conditions of water concessions and previously acquired rights, making it necessary to consider the legal constraints and possible socio-economic compensation.

There is a need for long-term research (that should incorporate existent experiences, including the outcomes from the REFORM project), based on the following specific critical points:

• • Ecological benefits of e-Flows, although acknowledged, are not well supported by quantitative evidence and too few well-documented cases exist.

• • As the majority of current biological methods do not detect the impact of HYMO pressures or the effects of HYMO-based measures, including e-Flows, with a necessary degree of precision, revision of such methods should be promoted.

• • Alternative biological methods should be developed, accounting for HYMO functionality measurement, riparian zones and stressor-specific deviation estimation.

• • Riparian vegetation should be considered as a quality element per se as well as HYMO traits.

• • Long-term experiments are needed to implement and validate the revised/new approaches. In the meantime, as process-based HYMO assessment methods can easily and directly assess HYMO alteration, they should be used along the whole gradient to support ecological assessment. Moreover, assessment of spatio-temporal alteration of local HYMO (physical habitat) could be used as a proxy for ecological status.

• • Experimental use of reservoirs for research could provide empirical data that link instream flows with biological elements and their ecological status. Also, these experiments could provide valuable data on how coupling flow and sediments create adequate habitats to be colonized by aquatic biota.

The research agenda should include, as a priority, all the necessary steps to develop new alternative, multi-scale approaches to ecological monitoring and assessment, so that the WFD can be better implemented and possible enhancement proposed for its coherent implementation in time for the next revision of the RBMPs and of the Directive.

This paper has been produced under the REFORM project (REstoring rivers FOR effective catchment Management), which has received funding from the European Union's Seventh Programme for research, technological development and demonstration under Grant Agreement No. 282656.