Diversion channels

Category: MODIFYING THE SURFACE WATER REGIME – Surface drainage

Description

Diversion channels are mostly artificial channels designed to divert excess amount of water to prevent flooding, erosion and landsliding. On the basis of the purpose of use, diversion channels can be grouped into:

  • river diversion channels;

  • runoff diversion channels.

Runoff diversion channels are describe in fact sheet 3.1. River diversion channels are discussed here.

River diversion channels are artificial channels built or used to divert all or part of the river flow from the toe of a slope/landslide, either to prevent or remediate toe erosion, or to make space for the implementation of other mitigation measures, as was carried out for example on the Taren landslide (Figure 1, Kelly and Martin, 1985); it may be temporary or permanent based on the duration of use.

Figure 1: River diverted to allow construction of toe berm, Taren landslide, UK (source: Kelly and Martin, 1985)
Figure 1: River diverted to allow construction of toe berm, Taren landslide, UK (source: Kelly and Martin, 1985)

Diversion channels, often in tunnel, are also used to divert water from landslide dams to protect the areas below and around the landslides dams; they are used either after the event, as for the Val Pola, Italy 1987 landslide and for the landslides reported in Table 1, or as a preventive measure, as carried out for the Séchilienne Landslide in France (Durville et al., 2004).

 

Table 1: Examples of landslide dam break and flood prevention through diversion channel (Schuster, 2006; Liu et al., 2010)

N°

Case/year

Problem

Material in dam

Mitigation

Consequences

1

Madison River, Montana, USA/1959

21x106 m3 landslide, triggered by earthquake, created 70m high dam

Rocks, gravels

75 m wide open channel spillway designed for a discharge of 280 m3/s

Prevented dam failure

2

Pisque River, Northern Ecuador/1990

3.6x106 m3 landslide, triggered by irrigation wastewater, created 58m high dam

Silty sands from volcanic tuff, fragments and blocks of soft tuff, sandstone, breccia

100m x11m x 9m open channel constructed in 7 days to reduce the severity of expected flood by limiting the depth of the lake

Dam failed due to erosion at channel, but reduced 40% of the lake discharge

3

Yingong River, Eastern Tibet, China/2000

300x106 m3 debris avalanche dammed the river; the dam was 60 to 100m high, 2.5 km long, 2.5 m wide

-

Open channel spillway

Dam failed by overtopping and eroding  the discharge channel; severe property and life loss downstream

4

Tongkpo River, Sichuan, China

Earthquake triggered landslide, creating Tanjiashan barrier lake with storage capacity of 3.2x108 m3

Sandstone

Discharge channel 890 m long, 13 m deep and 8 m wide

The lake water was drained, reducing the risk of flooding upon dam breakage

 

Landslides dams (Figure 2) cause mainly two types of floods: 1) upstream flooding as in the impoundment fills (Figure 3); or 2) downstream flooding resulting from failure of the dam (Figure 4). A landslide dam and its impounded lake may last from a few hours to thousand of years, depending on:

  • Rate of inflow to the lake, which is based on the size of the drainage basin upstream of the dam and on the amount and rate of precipitation into the basin.

  • Size and shape of the dam. High dams will need a longer time to fill than low dams and wide dams will be more resistant than narrow dams to failure upon overtopping.

  • Rate of seepage through the dam.

  • Resistance to erosion at the dam surface and subsurface.

Figure 2: Mechanism and consequences of landslide dams (source: www.kingston.ac.uk)
Figure 2: Mechanism and consequences of landslide dams (source: www.kingston.ac.uk)
Figure 3: Flooding induced by landslide dam, Randa, Switzerland, 9 May 1991 (source: www.crealp.ch)
Figure 3: Flooding induced by landslide dam, Randa, Switzerland, 9 May 1991 (source: www.crealp.ch)
Figure 4: Flooding induced by breeching of landslide dam (source: http://yeehowcentral.blogspot.com/2008_06_01_archive.html Posted by Dr. Jerque)
Figure 4: Flooding induced by breeching of landslide dam (source:  http://yeehowcentral.blogspot.com/2008_06_01_archive.html Posted by Dr. Jerque)

Istantaneous lake depletion can be caused by rapid erosion of the landslide dam on overtopping or dam collapse caused by piping and internal erosion.

Significant technical and financial resources are necessary to design and construct diversion channels for channelling the huge volumes of water involved. Diversion channels are expensive and they take a long time to build.

The cases presented in Table 1 show the difficulty of dealing with landslide dams. In cases 2 and 3 the discharged channels constructed across the landslide dam were not successful because of retrogressive erosion of the channels. In cases 1 and 4 the surface geology was mainly composed of weathered rock materials and erosion of these channels was minimal, so they were successful. In order to minimize the risk of erosion, the diversion channel should be designed and constructed with all the necessary precautions typical of major hydraulic works. However, this is seldom possible in an emergency. To minimize this risk, significant temporary pumping was carried out at the Val Pola landslide dam to allow sufficient time to construct erosion protection works in the emergency spillway channel. A more radical solution is to place the diversion channel in tunnel, but this requires a much longer time and can hardly be considered in an emergency. 


Design methods

Diversion channels are complex hydraulic structures that need to be designed accordingly. Critical aspects are the design of headworks and outlet, cross section, horizontal and vertical alignment, flow speed and profile, bank stability, lining of banks and base. All design calculations are based on design flows derived from full hydrological analysis of the catchment area. 

To design and construct a diversion channel across a landslide dam, it is necessary to estimate the amount of discharge from the dam. The accuracy of dam-break flood routing is affected by many hydrological and topographical factors; the calculated results may be quite different from the real situation. An example of a dam breaking flood analysis is represented by case 4 of Table 1; the equations used to calculate the dam-break flood are summarized below :

  • The maximum flood discharge at the entrance (Qmax) has been calculated according to the formula of broad-crest weirs:

where: b = width of the weir; Ho = effective water head during the maximum flood; d = coefficient of lateral contraction; m = coefficient of discharge, g = acceleration of gravity.

  • The maximum flooding discharge at a distance L from the lower reaches of the landslide dam (Q, in m3/s) has been calculated on the basis of the following equation:

        

where: L = distance downstream the landslide dam in meters; W = total storage capacity of the reservoir in m3;
Vmax = velocity of the maximum flood discharge im m/d; K = empirical coefficient (1.1÷1.5 in mountain areas; 1 in hilly areas; 0.8÷0.9 in plain areas).

 

 

 


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 6 Appropriate for any type of landslide, in so far as it may form a landslide dam. Diversion to prevent toe and/or basal erosion typically relevant to all types of slides.
Topple 6
Slide 8
Spread 6
Flow 6

Material type

Descriptor Rating Notes
Earth 8 Appropriate for landslide in any type of material, in so far as it may form a landslide dam.
Debris 8
Rock 8

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 0 Typically applicable and justified only to very deep landslides.
Shallow (0.5 to 3 m) 0
Medium (3 to 8 m) 4
Deep (8 to 15 m) 6
Very deep (> 15 m) 10

Rate of movement

Descriptor Rating Notes
Moderate to fast 8 The works are carried out outside the landslide body. However, they may be located in the run-out area of larger slides than considered in design, thus special care is required in areas susceptible to run out of fast to very fast landslides.
Slow 8
Very slow 8
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 8 Applicable to all landslide groundwater conditions. Adequate drainage must provided at the interface between impervious channel linings and natural soil.
High 8
Low 8
Absent 8

Surface water

Descriptor Rating Notes
Rain 6 Applicable to water courses. Most useful in high energy environments. Unaffected by and ineffectual with respect to rain and snowmelt.
Snowmelt 6
Localized 6
Stream 8
Torrent 8
River 8

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 6 In emergency works, the reliability depends on the possibility of implementing appropriate erosion control. Otherwise depends on the hazard study.
Feasibility and Manageability 6 Simple technique. Potential benefits and limits of applicability are well established.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 6 Major erthworks or even tunneling works. Time consuming. Compex to implement in emergencies.
Environmental suitability 2 will be updated
Economic suitability (cost) 2 Very high. Only justified for major risk situations.

References

  • Durville J.L., Effendiantz L., Pothérat P., Marchesini P. (2004). “The Séchilienne landslide”. In: Identification and mitigation of large landslide risk in Europe. Advances in risk assessment. Ch. Bonnard, F. Forlati, C. Scavia (eds.), European commission Fifth Framework Programme, IMIRILAND Project, A.A. Balkema, Leiden, 251-269.
  • Kelly J.M.H., Martin P.L. (1985). “Construction works on or near landslides”. In: Landslides in the South Wales Coal Field, Polythecnic of Wales, C.S. Morgan (eds.), 85-106.
  • Liu N., Chen, Z., Zhang J., Lin W., Chen W., Xu W. (2010). “Draining the Tanijashan barrier lake”. Journal of Hydraulic Engineering, Vol. 136, 914p.
  • Schuster R.L. (2006). “Risk-reduction measures for landslides dams”. Italian Journal of Engineering Geology and Environment, Special Issue 1, 9-13.

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