By-passing sediments

Introduction

Figure 1: Sediment pass-by tunnel with two different locations of the intake: a) intake located at the reservoir head, b) intake located inside the reservoir. Longitudinal sections (top) and plain views (bottom) are provide (click to expand).

Dams act as a barrier for sediment transport in river systems. Sediment-laden inflows transport sediments from the 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 concentrations, reservoirs can be filled after some years, rendering the infrastructure useless. Consequently, sediments are not transported past the dam, resulting in sediment starvation in the downstream river reach. Lack of sediments can induce severe morphological and ecological impacts.

Sediment by-passing is a measure which aims at routing bed load and part of the suspended sediment load through or around the reservoir (Morris et al. 1998). The objective is to maintain the storage capacity of the reservoir in addition to insure sediment continuity in the river and avoid morphological and ecological impacts (Hauer et al. 2018; Boes et al. 2019).

Sediment by-pass consists in diverting bed load and part of the suspended load around the reservoir to prevent them from entering the reservoir. The sediment-laden inflows are diverted through a tunnel at the entrance of the reservoir and conveyed to the river reach downstream of the dam. A weir or a guide wall located at the upstream head of the reservoir re-directs the water to the tunnel during periods of high flow and high sediments loads, and allows water entering the reservoir during periods with low sediment loads (Fig. 1a). Alternatively, the intake structure can be located inside the reservoir, leading to some deposition in the upstream part of the reservoir (Fig. 1b).

An alternative to bypass sediments through a tunnel is in transporting them with trucks or boats. Accumulated sediments are excavated from the reservoir, bypassed around the reservoir via trucks or boats, dumped downstream of the dam and transported downstream by flood flows.

Methods, tools, and devices

During planning

The design of bypass tunnels depends on catchment characteristics like topography, geology, hydrology and the reservoir's shape and size (Tiger et al. 2011, Hauer et al. 2018, Boes et al. 2019). They function well in small reservoir with steep slopes as the gradient of the diversion channel needs to be sufficiently large to insure the transport of sediments. The use of bypass tunnels with intake located at the reservoir head is a measure that does not interfere with hydropower operations since it does not require a drawdown of the reservoir. In contrast, if the tunnel intake is located in the reservoir, a partial reservoir drawdown is required to transport incoming sediment to the tunnel guiding structure (Fig. 1). In addition, sediment routing through bypass tunnels induces less impacts on the downstream ecosystems than reservoir flushing or sluicing. However, bypass structures are not well adapted to flood control reservoirs as they undercut the main role of these reservoirs (Kondolf et al. 2014).

During implementation

Construction of bypass structures (canals or tunnels) have relatively high investments costs. They should be built at the time of reservoir construction to minimize technical efforts.

During operation

The main challenge of bypass tunnels is abrasion. The intense bed load transport degrades the invert of the tunnels and can induce deep abrasion of the material. High-strength concrete or hard natural stone materials such a sgranit are recommended for the invert protection of the sediment bypass tunnels (Müller-Hagmann et al., 2020).

Relevant MTDs and test cases

Relevant MTDs
BASEMENT
Bedload monitoring system
HEC-RAS
Shaft hydropower plant
Relevant test cases	Applied in test case?
Gotein test case	-
Guma and Vadocondes test cases	-
Las Rives test case	-
Trois Villes test case	-

Classification Table

Classification	Selection
Fish species for the measure	Gravel spawners
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
	Juveniles
	Adult fish
	Movements of migration of fish
Which physical parameter is addressed	-
	-
	-
	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	-
	-
	-
	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
Cost of solution	See cost table

Relevant Literature

Boes, R.M., Müller-Hagmann, M., Albayrak, I. (2019). Design, operation and morphological effects of bypass tunnels as a sediment routing technique. Proc. 3rd Intl. Workshop on Sediment Bypass Tunnels, pp. 40-50, National Taiwan University, Taipei, Taiwan.

Müller-Hagmann, Albayrak, M., Auel, C., I. Boes, R.M. (2020). Field Investigation on hydroabrasion in high-speed sediment-laden flows at sediment bypass tunnels. Water 12(2), 469, https://www.mdpi.com/2073-4441/12/2/469.