Minimizing sediment arrival to reservoir

Introduction

Figure 1: a) Conventional reservoirs. Sediments are trapped at the dam. b) Off-channel reservoir: clear water is diverted to a reservoir located offside of the river, while sediment-laden water is flowing through the river. c) Sediment by-pass: sediment-laden waters are diverted through a bypass tunnel and released downstream the dam. From Kondolf et al. (2014).

Natural variations in sediment concentration occur over days, weeks, seasons and years. They are induced by land erosion resulting from high flows from rain fall or snow melting. These variations are generally handled by ecosystems if they do not change dramatically hydro-morphological conditions in rivers over long periods. However extreme concentrations of sediments related to anthropogenic causes (mining, building of roads, spilling water from dams, permafrost melting due to climate change, use of pesticides in agriculture, economy of cultures as plants for biofuel, etc.) or natural events (flood, volcanic eruptions) might affect significantly physical conditions and habitats for fish. Extreme sediments levels can also occur in rivers where hydropower regulation has introduced dramatic reduction of discharge levels compare to the natural flow conditions.

High levels of fine sediments induce a lack of water clarity, called turbidity, which might impact fish's feeding and growth, cover and risk of predation, displacements, egg survivals, or food abundance. Additionally, changes in water quality and chemistry might even more degrade habitat conditions as chemicals adhere to fine sediments. In some cases, mitigations measures to reduce the amount of sediments in water can be implemented to improve fish habitat quality. Measures consisting in reducing the amount of sediments entering the reservoir have been primarily developed to address loss of reservoir capacity and avoid the flow of fine sediments through turbines. If carefully applied, these

Methods, tools, and devices

During planning

Erosion control in catchment aims at reducing sediment yield to rivers and reservoirs affected by land-use change in the surrounding catchment. It can be realized through reforestation, contour farming in agriculture (planting across slope and following elevation lines), and land stripe planting along rivers. The aim is to induce sediment deposition where vegetation is planted.

It requires agronomist knowledge to develop a design plan (selection the most suitable plants, determination of the best combination of vegetation, assess vegetation planting density, etc.)

Check dams are small dams constructed across waterways that locally slow down the flow velocity and reduce the channel gradient. Consequently, they reduce erosion in the river channel and induce both debris and sediment deposits. The design of check dams requires a good knowledge of both the spatial and temporal variability of river hydraulics and the catchment sediment yield to the river.

Sediments traps are low dams constructed in watercourse where sediments settle. They can be installed where sediments tend to accumulate. The design of the sediment trap depends on the flow velocity and the particles sizes. Sediments traps should be designed for an easy removal of accumulated sediments.

During the planning phase, scenarios of implementation of the different measures can be simulated to estimate the impacts of the measures on sediment yield reduction. Numerical model, integrating land-use and structure can be applied to the selected catchment. RUSLE and SWAT models (see Chapter 9.1 for reference) are among them.

During implementation

Planting of vegetation should follow a specific planning and be followed up by agronomists. Reforestation is a mitigation measure with mid and long-term effect as soil strength and vegetation cover can require months to years to develop the required condition for controlling erosion. It might require regular watering in semi-arid and arid areas during the implementation phase.

During operation

Check dams require maintenance and surveillance. In river basins with high sediment yield, check dams can be quickly filled with sediments. Sediments needs to be regularly excavated to preserve its functionality and avoid risk of instability.

Sediment traps require regular maintenance to preserve their functionality. Heavy equipment is necessary to excavated sediments out of the traps.

Control of the areas where erosion control has been introduced should be carried out regularly to check the effective percentage cover and assess the effectiveness of the measure against erosion.

Classification table

Classification	Selection (multiple)
Fish species measure designed for	All
Does the measure require loss of power production?	Structural (requires no additional flow release)
Recurrence of maintenance	irregular at events
Which life-stage of fish is measure aimed at?	All
Which physical parameter mitigated?	Substrate and hyporheic zone
Hydropower type the measure is suitable for	Plant in dam Plant with bypass section
Dam height [m] the measure is suitable for	All
Section in the regulated system measure is designed for	All
River type implemented	All
Level of certainty in effect	Very certain
Technology readiness level	TRL 9	actual system proven in operational environment