Mitigating reduced flood peaks, magnitudes, and frequency

Introduction

Figure 1: The disappearance of floods in Orkla in mid-Norway due to the river regulation. The blue graph is the observed low before regulation, while the orange is the flow after regulation. Note that the data series are shown for different time periods.

Reservoir-based hydropower will normally lead to a dramatic reduction in floods, unless the flood occurs when the reservoir is more or less filled, or the reservoir is small compared to the inflow. Some reservoirs are built for this purpose, i.e. flood protection. The ecological function of floods to the river system will then also disappear. The reduced frequency of flooding events may result in a deterioration of habitat quality, both by the silting of spawning habitats and the clogging of sheltered habitats. Floods act as 'habitat fresheners' as they remobilise the river bed material, if the floods are sufficiently large and flush out finer sediment fractions. In later sections of this document, habitat measures to refresh the substrate have been described, which will reinstate the natural substrate conditions and improve the habitat. Reinstating floods is one way of refreshing the substrate with use of the flow. Floods can also have other ecological functions, for instance to facilitate migration and to attract fish for spawning. These floods (freshets) are typically smaller than the natural, annual flood, but the river regulation can also reduce these floods. Such freshets are usually named attraction flow to initiate upstream migration and trigger flow to initiate downstream migration. These floods can also be reinstated, and similar to floods to remobilise the substrate, the magnitude, timing and frequency of these should be when the ecological function is maximised, and the losses in power production minimised.

Methods, tools, and devices

During planning

A first step in considering placement of spawning gravel as a measure would be to assess if the total and distribution of spawning areas are limiting the development of fish population, i.e. diagnosis in the environmental design terminology. The spawning areas are often assessed by visual inspection of the river, i.e. by foot beside the river, by wading or from boat. Aerial photos can also in some cases assist this step. When the spawning areas are identified, they can be mapped in a GIS and the total area and their distribution assessed. For Atlantic salmon, spawning areas are considered being large if more than 10% of the total river has suitable spawning conditions, moderate if between 1-10% and small if less than 1% (Forseth & Harby, 2013). The distribution/spread is considered large if more than 500 meters between identified spawning areas, medium if between 200-500 meters, and small if less than 200 meters. These threshold values are considered indicative for Atlantic salmon, and will be different for other fish species.

		Extent of spawning habitat as a percentage of river area
		Small (<1%)	Moderate (1-10%)	Large (>10%)
Distance between spawning habitats (across all segments)	Large (>500m)	Small	Small	Moderate
	Medium (200-500m)	Small	Moderate	Large
	Small (<200m)	Moderate	Large	Large

Hydraulic analysis can support the identification of the best location to place the spawning gravel, in order to meet the preferences of the fish of concern, avoid locations with too slow-flowing water and areas exposed to flushing during high flow conditions. A high number of hydro-dynamic tools are available for such analysis with different functionality and data needs, ranging from more simplistic 1-dimensional (1D) hydraulic tools, to highly advanced 3-dimensional (3D) tools solving a range of partial differential equations (Navier-Stokes) in all directions. The all require details description of the bottom topography of the areas the gravel might be placed, and a flow regime the river will undergo. As average flow velocities will not be sufficiently detailed to identify the best locations, 2D- or 3D models will be required. Examples of such models are River2D, HEC-RAS 2D, Flow2D/3D, Mike21 and OpenFoam.

During implementation

The implementation of the measure would require access to substrate of suitable grain size distribution, shape and of proper mineral composition, preferably similar to the substrate naturally present in spawning areas. In order to transport and place the new material at the right locations in the river, heavy machinery such as dumpers and tractors would be needed. In case where the site is difficult to access, use of helicopters can be the best option. The dumping of the substrate in the river would normally require supervision of a biologist, hydraulician or another experienced person in order to secure the right positioning of the substrate, proper thickness of substrate layer and finish of the surface preparation.

The construction work will often require use of heavy machinery. Depending on the location of the river and how accessible it is and the costs, the transport of gravel will typically be made by dumper or by helicopter. If helicopter is used, the gravel can normally be dumped directly into the river, under the supervision of a biologist or another experienced person. If the new material is transported into the site by dumpers, an excavator will normally be needed at the site. This will also require the presence of an experienced person in order to ensure the correct placement and thickness of the gravel.

The implementation of the measures, i.e. dumping of the gravel, should be made in a time of the year where the fish population of the concern, and the rest of the ecosystem in the river will be least affected by the construction work. This will be individual to the different rivers and species. In addition, the hydrology of the river must be taken into consideration as it is clearly easier to carry out the work during low-flow conditions than during floods. For Scandinavia and species like salmon and trout, the period from July to September is probably the least problematic.

During operation

Habitat measures in regulated rivers must often be maintained unless the natural functions related to flow and sediments are restored, such as flood events and connectivity of the sediments. How often the maintenance must be made will differ from river to river and can vary from for instance every 5 years to every 20 years. Rivers with intense growth of moss, algae and macrophytes would need more frequent maintenance than rivers with cold water and low nutrient concentrations (less growth).

Classification Table

Assessment criteria	Assessment
Fish species measure designed for	Atlantic salmon (salmo salar) Trout (salmo trutta)
Which life-stage of fish is measure aimed at?	Spawning
Which physical parameter mitigated?	Substrate
Section in the regulated system measure designed for	Bypass section Upstream of hydropower plant Downstream outlet
River type implemented in	Gravel-bed rivers
Climatic region suitable for	C: Temperate (mesothermal) climates D: Continental (microthermal) climates E: Polar and alpine (montane) climates
Level of certainty in effect	Very certain
Technology readiness level (maturity)	TRL9: actual system proven in operational environment (competitive manufacturing in the case of key enabling technologies; or in space)