Sediment is not a problem.

M. Detering and D. Bartelt

Abstract:

In many reservoirs and hydropower installations sediment is seen as a problem which should be avoided, because a high load of sediment in the outflow is causing wear on machinery. Furthermore some ecologists fear clogging of interstitials when passing on sediment. By seeing sediment as a problem in such a way and blocking sediment transport, sediment accumulation in reservoirs in the long run leads to even larger problems in both cost and ecology. The only sustainable and environmentally solution in the end is not to see sediment as a problem, but to enable a cost efficient near-nature sediment transport.

1 Introduction

In nature like rivers, passage of water, species and sediment was granted for millions

of years. Upon the construction of dams, this situation changed profoundly. Every

dam and reservoir - built for a reasonable and well considered technical use – also

implies an interruption for water flow, fish passage and foremost of sediment transfer.

Hydrology is well considered in reservoir planning. For low level dams fish passage

techniques are well established, for high dams in most cases it is no issue. But

sediment transfer in many cases is still neglected, despite the large problems

evolving from this.

2 The two sides of sediment

2.1 Sedimentation of reservoirs

By this, sooner or later every reservoir operator is faced with siltation in the reservoir.

As long as this sedimentation takes place within the dead storage, the operator might

not feel the need for action. But even then, the downstream river stretch is affected

as described in 2.2.

As soon as the sedimentation is reducing the active storage, also the reservoirs use

and thus the economics are affected. Because sedimentation is a sneaking process,

the problem of reduced storage is often transferred from generation to generation of

responsible staff, always considered ‘normal’. Psychologists refer to this development

as ‘shifting baseline’.

However, even under these circumstances operational and contractual obligations

have to be met, e.g. sufficient storage to ensure reserves, black start capability etc.

As conventional dredging is extremely costly and landfill is – if at any possible – a

further consumption of space, operators often accept sedimentation as long as

possible. The consequence of this development is shown in Figure 1.

Fig 1: Worldwide development of reservoirs and siltation (Source: DB Sediments)

Worldwide hydro storage volume is increases around 1 % per year by new build of

dams. At the same time 2 % of reservoir volume is lost due to sedimentation. As Fig.

1 clearly shows, this development is progressing. Without further action one quarter

of all dams in the next 25 to 50 years will lose their storage function by sedimentation

(WCD 2000). Sedimentation is probably the most serious technical problem faced by

the hydropower business.

As an assumed solution or compensation, many silted reservoirs are replaced by

new build of neighboring reservoirs. But this is clearly not sustainable as the

replacements will suffer the same problems only few decades later. Then at the

latest, room for even more reservoirs will not be available one day.

So far, operators tend to flush out sediment. For some installations, this might be

adequate if the downstream river does not bear any significant aquatic life. For the

most rivers, reservoir flushing causes the temporary coating of downstream river

stretches with often sediment of anaerobic quality. During flushing, hydraulics usually

allow only sediment close to the outlet or in a narrow ditch to be mobilized. The

sediment volume being mobilized is thus only a small fraction of the overall siltation.

The focus on flushing therefore is not on re-gaining storage volume, but to keep the

outlets operational before blocking becomes an irreversible problem.

As the flushing is being conducted during just a short time period of usually several

hours and the sediment transfer during this period is massive, the sediment transport

capacity of the downstream river as well as the rivers ability to handle a massive load

of anaerobic sediment is overstrained. Flushing therefore in many cases is to be

avoided from an ecological standpoint and unlawful in most European Countries.

2.2 Downstream river erosion

Sedimentation in reservoirs does not lead only to problems within the reservoir. Due

to the retention of sediment the important equilibrium within the river system is

disturbed. The lack of sediment in the downstream river causes a change of the bed

structure and massive erosion of the riverbed. This erosion can lead to immense

scale. The River Rhine e.g. faces a sediment deficit of 2.5 Million tons per year due

to sediment retention in its tributaries. One consequence is an annual erosion of 3 to

30 mm. Within only a few decades this will lead to required measures on harbors

along the Rhine and major construction works to ensure structural safety of hydro

and waterway installations.

To compensate for the most urgent damages on the riverbed the authorities actually

dump several hundred thousand tons of substitute material which is excavated on

land and which leads to further consume of space.

For these reasons the European Water Framework Directive (WFD) has identified not

only fish passage and hydrology, but also sediment transfer as a major aim, being

emphasized within the second WFD planning period. Unlike other elements of the

WFD, operators should understand that sediment transfer does bear long term cost

benefits, if applied in the right way.

3 Solutions

It could all be so easy. And in fact, it is. It begins with changing the view from

considering sediment a problem towards simply dealing with sediment as a standard

issue for reservoir operators. If reservoirs cause an imbalance in the sediment

transport, combined with problems upstream and/or downstream, why not set this

balance back where it belongs, especially if this can be done in a very cost efficient

way?

3.1 Analysis

As the hydrological and sediment situation varies from reservoir to reservoir, the first

step to develop a cost efficient sediment transfer should be to conduct a study. If

required, an actual bathymetry is performed. Most important part of the analysis is to

identify technical and environmental constraints with the core element being to

assess the transport capability of the downstream river stretch. The analysis also

gives information about the kind of applicable equipment and dimensioning.

3.2 Implementation

Given this information, the practical implementation can be prepared and installed. A

main difference to formerly dredging and landfill is that not disposal cost occur,

transportation is limited and the overall installation and application usually can be

performed without lowering the reservoir or even interrupting power generation.

Mechanical dewatering of the sediment is of course unnecessary, too (sample see

Fig. 2).

Fig. 2: Continuous Sediment Transfer at HPP Rodund/Austria (image source: DB Sediments)

In difference to dredging campaigns or flushing, the sediment outflow is not massive

during a short time, but smaller over a longer time period. This allows for smaller

equipment compared to conventional approaches (sample see Fig. 3).

Fig. 3: Sample for Sediment Transfer Equipment

The sediment transfer is performed according to the actual transport capability of the

downstream river. Thus, blocking of the riverbed system is prevented. If required, the

transferred sediment can also be vented to prevent a lack of oxygen in case of

massively anaerobe sediments. The sediment is given back to the river system in a

near-nature-way, allowing for erosion compensation.

4 Technical issues

Sediment transfer to the downstream river section can take place in different ways:

through the turbines/power station

through the base outlet or similar outlets

over the dam

One of the first glance, many mechanical engineers fear the effects on sediment on

turbine abrasion if sediment is transferred across the power station. Damage on

turbine equipment can amount to a hundred thousand Euros and more. On the

second glance a transfer across the turbine for many applications still is the favorable

one for several reasons:

additional abrasion usually is no or limited concern for heads below 200 m

expenditures for turbine rehabs or even runner replacements sum up to just a

fraction of the cost saved in difference to conventional dredging, so in the end

this method still is very cost effective

modern coating gives protection to runner surfaces (see Fig. 4)

no water is wasted for sediment transfer

not additional infrastructure is required

Fig. 4: WC-coated Francis Runner (left) and its labyrinth seal (right) after passage of 159,000 t of abrasive sediment without significant abrasion effects (image source: University of Katmandu)

A recent development for pressure pipe desilting [4] could potentially accompany this

approach and make it applicable even for high head Pelton systems. By this, the

penstock would be used for sediment transfer and the majority of the sediment

removed before the turbine.

In case there are still concerns on a turbine passage, the sediment flow can also be

performed via other reservoir outlets or over the dam. However, in these cases often

additional equipment will be required which in the end still gives economic benefit.

5 Recommendations

With a different view on sedimentation issues, hydropower operators on the one hand

will find more economic solutions on reservoir maintenance and provide real

sustainability for their installations. By this, sediment transfer might be the only part of

the European Water Framework directive, granting operators a direct economic

benefit. On the other hand sediment transfer re-establishes an urgently needed

compensation for ongoing erosion for downstream river stretches, caused by a lack

of sediment in theses river sections.

Ongoing projects will ease the way to establish this ecological benefit as a standard

solution, also due to the lack of alternatives on many sites, according to: “If you think

sediment transfer is costly - try siltation.”

References

[1] Hr. Schüttrumpf and M. Detering: Innovative sediment handling to restore reservoir capacity, Dams and reservoirs under changing challenges / Ed. by Anton J. Schleiss, ISBN: 978-0-415-68267-1, pp 345- 352, 2011, CRC Press

[2] WCD: The Report of the World Commission on Dams, London/Sterling, Earthscan Publications (2000)

[3] E. Doujak: Development of an Axial Hydro Cyclone for Pressure Pipe Desilting; HydroVision 2011, 6 S.

Authors

Dr. Michael Detering DB Sediments GmbH Bismarckstr. 142, 47057 Duisburg, Germany Phone +49 203 306-3625, Fax +49 203 306-3629, Mobile +49 177 8531100 E-mail: m.detering@db-sediments.com Dr. Dietrich Bartelt DB Sediments GmbH Bismarckstr. 142, 47057 Duisburg, Germany Phone +49 203 306-3626, Fax +49 203 306-3629, Mobile +49 152 29888321 E-mail: d.bartelt@db-sediments.com

ABOUT THE AUTHORS

Michael Detering graduated from German RWTH Aachen University of Technology

in 1995 with a master in mechanical engineering and also a commercial background. Later he added a Doctors degree in civil & hydraulic engineering. Since then he has been involved in almost any type of renewable generation but always with a strong focus on hydropower. With on-site experience he conducted a number of projects, rehabs and assessments. Being responsible for a major European Power Producer in Hydro Asset Management for 82 plants in six European countries he took the entire plant fleet back on a sustainably operated course whilst significantly increasing profits. Michael Detering is co-founder and associate of DB Sediments. Besides his business function he teaches at Aachen University of Technology and contributes to several technical associations, including ICOLD and its TC Environment.

Dietrich Bartelt studied civil engineering and business administration at Aachen

University of Technology (RWTH Aachen), Germany. He specialized in hydraulic engineering, water resources management, soil mechanics and environmental management. Since 1992 he has been working for a major European Utility and held various technical and management positions. As Senior Manager he was responsible for the coordination of environmental issues for several hundred companies of the Group. In 2012 he published his doctor thesis on “trust of employees’ in their management”. In the fields of renewable power generation, he has been active in setting up multi-national projects, project management structures, site development, authorization procedures, plant operations, and joint implementation projects with a focus on hydropower. He is associate of DB Sediments.