Off-channel reservoir storage

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.

Dams act as a barrier for sediment transport in river systems. Sediments-laden inflows bring sediments from upstream catchment that will be trapped when reaching the reservoir. Sediments deposit in the bottom of the reservoir and reduce its storage capacity. In geographical areas with very high sediment concentration, reservoirs can be filled after some years, rendering useless the infrastructure. Consequently, sediments are not transported downstream the dam, resulting in sediment starvation in the downstream river. Lack of sediments can induce severe morphological and ecological impacts.

Off-channel reservoirs, like sediment bypasses, are measures which aim at routing bed-load and part of the suspended sediment load through or around the reservoir (Morris et al. 1998, Kondolf et al. 2014). The objective is to maintain the storage capacity of the reservoir in addition to ensure sediment continuity in the river and avoid morphological and ecological impacts.

A diversion dam or weir is located in the river, allowing diverting of clear water to the reservoir at period of low flow and low sediment loads only, and leaving sediment-laden waters in the river.

Methods, tools, and devices

During planning

Similar to bypass tunnels, off-channel reservoir requires a sufficient gradient in the diversion tunnel to insure flow transport to the reservoir. The design of diversion tunnels to the off-channel reservoir depends on catchment characteristics like topography, geology, hydrology.

The advantage of off-channel reservoirs is that they prevent almost all bed load to enter the reservoir, and do not interfere with sediment continuity required to achieve good ecological status in rivers. However, the amount of water that can be diverted from the river and stored is limited to the flow capacity of the diversion channel. So, this measure is less suited to semi-arid and arid zones with short periods of very intense flows.

During implementation

Building of off-channel reservoirs have relatively high investments costs. They are part of the large structural measures to insure sediment continuity, requiring months to years for implementation.

During operation

Water discharge and sediment concentration in the river need to be permanently monitored so that clear water is diverted to the off-channel reservoir and sediment-laden waters flows through the river. The sediment concentration threshold for diverting or not waters to the reservoir will depend on the characteristics of the reservoir (shape, size) and the diversion structure, as well as the discharge.

Classification table

Classification	Selection
Fish species for the measure	All
Does the measure require loss of power production	Operational (requires flow release outside turbine)
	-
	-
Recurrence of maintenance	Irregular at events
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
	-
	-
	-
	-
Hydropower type the measure is suitable for	Plant in dam
	Plant with bypass section
Dam height (m) the measure is suitable for	Higher than 10
Section in the regulated system measure is designed for	-
	Bypass section
	Downstream outlet
River type implemented	Steep gradient (up to 0.4 %)
	-
	-
Level of certainty in effect	Moderately certain
Technology readiness level	TRL 9: actual system proven in operational environment