Environmental design of embankments and erosion protection

Introduction

Figure 1: Construction of embankments with stones at the inter-section of Otta River and Gudbrandsdalslågen in Norway, for the purpose of protecting the nearby houses and infrastructure

Embankments and erosion protections are put in place in many rivers to reduce the erosion that ultimately might threat infrastructure. In highly urbanised areas or areas of critical infrastructure, the protection is so extensive that rivers are completely channelized. In many cases this protection is made of concrete blocks with close to smooth surfaces, providing no or little hydraulic variation and shelter for fish along the shoreline. Sometimes, the man-made embankment is also placed a bit into the river, reducing the total available habitat.

Environmental designed embankments could be introduced by replacing smooth-surface concrete with stones of rougher surfaces providing more hydraulic diversity and shelter. The embankments should also be placed into the very shore of the river to maximize the areas available for the aquatic environment.

It can be unrealistic to re-build existing embankment for the purpose of improving the conditions for fish, but if the embankments for other reasons should be re-built or new embankments should be put in place, they should be designed in an environmentally friendly way.

A variant of embankment is called 'rip-raps', which are basically rough stones dumped by the shore of the river with the primary purpose to reduce erosion. If rip-raps are designed with care, they can also sustain or improve the ecological conditions along the shore. An alternative is to use anchored dead trees or other types of current deflectors for bank protection. Ultimately, the removal of bank protection is of course a way of improving the ecological function, if possible weighed against other impacts.

Methods, tools, and devices

During planning

Normally no sophisticated tools are needed to design embankments in a way that they sustain the fish population. There are, however, a set of engineering guidelines that are useful when embankments are designed. The principles of standard hydraulic engineering handbooks should be followed to guarantee the stability of the river, which will be based on the discharge coming out of a selected return period, the lateral and longitudinal gradient/slope and the river type.

The stones should also have a certain composition and shape to improve the hydraulic variation and provide shelter. Larger stones will directly provide shelter and will also anchor the substrate of smaller size. In some cases, also the bottom of the river channel needs to be protected, as there is a tendency that rivers with shoreline protection tend to start eroding vertically. A guiding principle is that the substrate should have high roughness and be as nature-like as possible.

During implementation

Similar to several other types of habitat modification, construction of environmentally designed embankment would require use of heavy machinery. In order to transport and place the new material at the right locations in the river, dumpers and tractors would be needed. In case where the site is difficult to access, use of helicopters can be the best option. The placement of the stones/substrate in the river would normally require supervision of a biologist, hydraulic engineer or another experienced person in order to secure the right positioning of the substrate, proper composition of stones and finish of the surface preparation. Preferably, local stones and gravel should be used.

During operation

If the embankment is correctly designed and properly built, very limited maintenance should be needed. In areas with ice breaks and jamming, the embankment might be damaged, and repairs needed. The ecological functioning can be evaluated by carrying out measurements of juvenile fish densities, for instance by use of electro-fishing equipment.

Classification Table

Classification	Selection
Fish species for the measure	All
Does the measure require loss of power production	-
	-
	Structural (requires no additional flow release)
Recurrence of maintenance	Less often than yearly
Which life-stage of fish is measure aimed at	Spawning / Recruitment
	Juvenile habitat (0+)
	Juvenile habitat (1+)
	Juvenile habitat (older than 1+)
	Adult fish
	Movements of migration of fish
Which physical parameter is mitigated	-
	-
	Substrate and hyporheic zone
	-
	-
	Water velocity
	Water depth
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	-
	Bypass section
	Downstream outlet
River type implemented	Steep gradient (up to 0.4 %)
	Fairly steep with rocks, boulders (from 0.4 to 0.05 %)
	Slow flowing, lowland, sandy (less than 0.05 %)
Level of certainty in effect	Moderately certain
Technology readiness level	TRL 9: actual system proven in operational environment