Ch 12 River Training and Bank Protection.pdf

Feasibility Study for the Stabilisation of the Course of Songwe River

Feasibility Study Report Volume 1, Page 12-1

12

RIVER TRAINING AND BANK PROTECTION

12.1

Introduction

Terms of Reference

The feasibility of river training works is an essential part of the study. The TOR specifies the scope of work in item 4.1 (vi) “Determine river training, dams and other measures necessary to contain floods within the channel”. Item 4.1 (viii) specifies “Identify critical sections along the river course of the lower basin zone that require stabilisation and carry out analyses of stabilisation procedures”. The present chapter presents solutions for river training works to enhance river stability in the lower Songwe River Basin.

Objective

Bank protection implies restoring and protecting banks of streams, lakes, estuaries and excavated channels against scour and erosion by using vegetation-oriented soil bioengineering and structural systems. Bioengineering has some important drawbacks. Bank protection is not immediate and substantial maintenance is required. Protection during establishment of the plan-based river training will be substantial. Finally, the efficiency of protection against undercutting is uncertain. For these reasons the bioengineering measures are ruled out and only structural systems are considered. Revetment designs do not reduce the energy of the flow significantly, thus using revetments for spot protection may just move erosion problems downstream or across the river channel. Therefore, revetments must be planned for whole reaches and not just for single locations.

Four options

Different options for river stabilisation are investigated: 1.

Dams that change the hydraulic conditions

The design of the first option is discussed in the previous chapter. The effect of different dam operation strategies in terms of river stabilisation is analysed in this chapter. The use of dams combined with bank protection works is addressed as the Combined Option in the TOR (section 3.1). 2.

Bank protection using revetments

This option maintains the river course at its present location and secures critical sections against erosion by protecting the bank lines. 3.

Bend protection using loop cut-off spillways

This option is considered as a way to maintain the thalweg of the existing river course and at the same time secure bends with a critical curvature against cut-offs in the near future. Without full bank protection (option 2), this option will only reduce the bank erosion in the critical bends. 4.

Dredging of abandoned channels in the delta

The river morphology near the outlet to Lake Malawi is inherently unstable, because the river mouth is gradually moving towards the

in association with COWI, DHI and W&PES



lake through deposition of sediment. This will eventually lead to development of alternative flow routes. Instead of forcing the river to stay in the same channel, where it will become increasingly unstable, dredging of the present main channel is proposed as an option to maintain the channel should a new main channel develop at the outlet to the lake. 12.2

Basic assumptions and design guide lines

12.2.1

Flow and flood conditions

River discharge

The discharge time series at Mwandenga is a reference for all river stabilisation measures. The hydrological background is documented in Chapter 4. The change of the hydrograph through dam operation was described in Chapter 11.

Two types of erosion

Throughout the report, a distinction is made between two different types of erosion: The gradual bank erosion causing slowly increasing loss of land (and gain in the opposite side) and the sudden loop cutoff causing instantaneous loss of land. Bank protection works must address both types of erosion.

12.2.2

Criteria for bank protection

Time scales

In the lower Songwe River Basin, certain areas exhibit higher degree of erosion than other areas in recent years (since 1990s). However, when a longer time scale is considered (since 1969), the erosion appears to be nearly uniformly distributed along the Songwe River. Time scales (i.e. the time over which erosion at a particular location is critical) are important to consider as they determine the feasibility of a phased bank protection plan stretched over several years.

Cut-off within 10 years It is assumed that all the critical bends along the river, which are likely to be cut off within a time frame of 10 years, should be protected against this. Erosion rate

Furthermore, it is assumed that the gradual bank erosion should not necessarily be arrested completely. However, bank erosion should be substantially reduced. Thus, bank protection, which only leads to reduced bank erosion, is also an acceptable solution.

Existing thalweg

Finally, it is specified in the Terms of Reference, that the existing thalweg must be maintained (Option 2: Construct river bank protection works along the reaches where erosion is most active, TOR section 2.9), i.e. no re-alignment is allowed in the proposed river training design.

Land use types

In the evaluation of the sites to be protected, different land use types are considered: • Settlements and infrastructure • Fields with crops • No land use (natural vegetation) The evaluation is very preliminary and serves as guidelines only, for where and how to protect the riverbanks.




12.3

Design of bank protection works

12.3.1

Revetments

Slopes

The natural slopes of the banks of the Songwe River in the lower reaches can be extracted from the measured cross-sections, see Appendix HH. The average slope is 1:1.63.

Design data

The discharge in the Songwe River is controlled by the elevation of the natural levees that divide the river from the adjacent floodplain. The bankfull discharge is 250 m3/s. Any discharges above this threshold will spill out to the floodplain. See Appendix N for more discussions. Bank

1:1.6

Max.depth=11.6m River bed during medium/low flow

Scoured river bed during extreme high flow

Figure 12.1: Sketch of a riverbank along an outer bend

The maximum depth along the outer bends varies significantly with the discharge as described in Appendix Q. The reason is both changing water levels and not least changing bed levels. The predicted maximum depth from the top of the bank to the bottom of the river is 11.6m. Maximum flow velocities as predicted in the mathematical model are 4.0 m/s, which can occur locally along outer bends. Design

A sketch of the revetment design is shown below. Detailed drawings are provided in Volume 4. It is proposed to apply a revetment with a falling apron to avoid costly work with excavation of the river bends during construction. The protection at the toe of the revetment will then automatically be lowered down to the maximum scour depth the next time river bend scour is taking place.




Bank

Max.depth=11.6m

1:1.6–1:2

Figure 12.2: Sketch of a riverbank along an outer bend

Gabion mattress

Using loose stone material would require large stone sizes to ensure slope stability during flood events with high flow velocities. In order to reduce construction costs, the use of gabion mattress is recommended. Stone material (blasted rock) from the upstream part of Songwe River can be applied as fill material. Underneath, a filter of geotextile is placed. See the drawings in Volume 4 for further details.

Construction

The gabion mattresses will be constructed on site and placed using floaters, which allow for accurate lowering of the mattresses to the riverbed.

Location and extent

The lower Songwe River Basin is divided into 15 separate locations (shown in 12 sites in the drawings). The reason for distinguishing between different locations is that within each location, the bank protection should preferably be completed at one time. This is necessary in order to avoid that the erosion is simply shifted further downstream or upstream, or to the opposite river bank. The extent of the bank protection works is displayed in the drawings. The table below describes each location in terms of land use percentage between 1) settlements, 2) fields with crops and 3) no land use (including forest). (In this connection “no land use” means areas in which vegetation appears to be natural and land is not used for agriculture, houses, etc.) In addition, the number of years until a major cut-off will occur is estimated. Finally, the required length of revetments as well as the required number of flood spillways is estimated.



5

6

6

6

7

7

8

8

9

9

10

10

11

10

12

11

13

11

14

12

15

Total

Land loss

Number of spillways (#)

5

Length of revetments (m)

4

1.4

0.3

-

20

1800

-

0.2

1.6

0.2

-

20

2300

-

0.3

1.3

0.2

-

20

2400

-

0.2

1.2

0.2

-

10

2100

-

0.5

1.5

0.3

2

8

3100

2

0.8

1.6

0.3

2

4

3300

1

0.2

0.6

0.1

-

30

1200

-

0.6

1.6

0.1

1

5

2200

1

0.7

1.4

0.1

0.5

2

4100

1

1.0

2.2

0.1

3

7

3300

1

0.7

1.4

0.1

1.5

6

2500

2

0.3

0.7

0.1

1.5

6

1800

2

0.2

0.6

0.1

0.5

3

1500

1

0.7

2.0

0.1

4

7

3200

2

0.6

1.1

0.1

-

-

2800

-

6.8

20.1

2.4

16

39600

13

(h ) Years before cutoff

4

0.1

(ha/yr)

3

Cut-off loss

3

No land use

2

588, 8937 589, 8936 590, 8935 591, 8934 592, 8934 593, 8932 594, 8932 595, 8932 596, 8931 597, 8930 598, 8930 598, 8929 599, 8929 600, 8928 600, 8927

(h / ) Crops

2

586, 8938 588, 8937 589, 8936 590, 8935 591, 8934 592, 8934 593, 8932 594, 8932 595, 8932 596, 8931 597, 8930 598, 8930 598, 8929 599, 8929 600, 8928

Erosion characterisation of each location for bank

Settlements

From approx.

1

(utm-km)

Location

1

( t k ) To approx.

Site (drawing)

Table 12.1: protection.


The land loss through gradual erosion is 6.8+20.1+2.4 = 29.3 ha/year. This corresponds to 1 ha per river kilometre per year. Most of this is agriculture land. Within a time horizon of 10 years the total loss of land through sudden cut-offs is 16 ha. The average land loss per cut-off is approximately 1-4 ha.

Implementation

It is possible to implement bank protection works along the various locations at different points of time as long as the bank protection is complete within each location. The entire implementation time could be for instance 10 years, taking into consideration financial constraints, etc.

12.3.2

Loop cut-off spillways

Design

During the course of the study it became evident that the costs of bank protection works would be substantial. Consequently, during the




feasibility study another design option for protecting river bends has been developed. A detailed description of the hydraulic calculations is provided in Appendix HH. During low flow, the river will follow the original river course, whereas during high flow, part of the water will pass over artificial loop cut-offs designed as flood spillways, see Figure 12.3.

Figure 12.3: Layout of the flood cut-off spillway Loop Cut-off Location A typical location is displayed in Figure 12.4 below. The identified critical loops are shown on the drawings in Volume 4. The loops are identified based on the criteria that there is a significant risk that the loop will be cut off within a time horizon of 10 years.

Figure 12.4: Example of location of two sets of loop cut-off spillways Discharge Control

The capacity of the spillway is important as it determines how much of the flow will be diverted over the spillways during high flow. This is



Feasibility Study Report Volume 1, errata Page 12-7

controlled through the design of the side weir at the inlet from the channel to the spillway. An example of the side weir is shown below. 488.0m

Intake structure

487.3m 487.0m

Cross-section of flood spillway

10m 20m 50m

Figure 12.5: Example of the dimensions of the side weir at the entrance to the loop cut-off spillway. Riverbed protection

The inlet (side weir) and outlet to the spillway need to be well protected. It is proposed to use gabion mattresses as for conventional bank protection (previous section). The riverbed of the flood spillway also needs to be protected against scouring, which may eventually cause damage to the inlet and outlet structures. The flow velocities in the spillway are designed (side weir control and width of spillway) to be maximum 1.5 m/s. For bed material, it is therefore possible to apply natural graded stone material (d50 is 50mm) excavated from the Upper Songwe River basin.

Drawings

Detailed drawings of the spillway are provided in the separate Volume 4.

12.3.3

Maintenance dredging of delta channels

Scenarios

Shore accretion is a natural process, where sediment from upstream is deposited in the lake near the river mouth. The development of a spit at the river mouth continues until the river course becomes too long compared to alternative short-cut routes. When the main channel takes such a new river course, it starts to build up a new spit and eventually shifts once again. The identified solution to avoid sudden shifts in the channels is to carry out dredging at or near the river mouth:

Implementation

•

Dredge the sand bar in the mouth of the aging main river course to minimise the risk that the flow will take another course.

•

Drege in the river downstream of the location where it is expected that the flow would tend to take another course and thereby maintain it as the main Songwe River (the international border).

Dredging of the sand bar and/or the original river course can be done independently from river training works further upstream.




12.4

Future stability of River course

12.4.1

Operation of dams

Approach

Quantification of the effect of dam operation on the stability of the river course is examined by means of the mathematical models, see Appendix N to Appendix Q. The rainfall-run off model predicts the runoff from adjacent sub-catchments. The one-dimensional (quasi2D) model predicts the discharges along the main Songwe river course. Finally, the fully 2-D morphological model takes the boundary condition from the 1-D model and simulates the bank erosion rate for different discharge time series reflecting different modes of dam operation. The 2-D model is further applied to establish a function that relates the average bank erosion with the river discharge at Mwandenga.

Simulation

Two hydrographs are considered in the 2-D simulations: The 20012002 hydrograph with high discharges as well as long duration of the flood peak, and the 1988-89 hydrograph. The 1988-89 simulation is for illustration purposes. For 2001-2002 the hydrographs were extracted from the 1-D floodplain model, whereas for 1988-89 a schematic (idealised) reservoir operation hydrograph was created. For 2001-2003 the following scenarios are considered: Existing conditions, Power generation priority operation and Flood control priority operation.

Impact on erosion

The simulated erosion after one year is shown in Figure 12.6. For the 2001-2002 scenarios it is seen that the reservoir reduces the erosion for both operation modes, the reduction being slightly larger with flood control operation. In summary, the simulated reduction of gradual bank erosion rates is 29% (operation based on hydropower option) and 33% (operation based on flood control). The flood control scenario produces the largest reduction in bank erosion despite the fact that it has the largest average channel discharge.

Two erosion formulae Simulation of bank erosion is inherently uncertain. The detailed models applied in the present study are state-of-the-art and very little documentation exists from other projects. The basic uncertainty is related to a theoretical discussion whether bank erosion is depending mostly on the absolute discharge (i.e., the higher discharge, the more erosion) or whether it is depending mostly on the frequency of bed level changes (i.e., the more peaks in river discharges, the more erosion). The calibration appears to be best for the latter formulation, which is therefore applied for the basic computations. However, to address the overall reduction in erosion rates, both formulations are considered to provide an idea of the range of uncertainties in the predictions.




10 Existing Pow er

8

Flood Control

Bank erosion (m)

6

4

2

0 0

500

1000

1500

2000

2500

3000

2500

3000

-2

-4

-6 Distance along river axis (m)

12 Existing Pow er

10

Flood Control 8

Bank erosion (m)

6 4 2 0 0

500

1000

1500

2000

-2 -4 -6 Distance along river axis (m)

Figure 12.6: Simulated bank erosion for the 2001-2002 scenarios (formula 1). Top: Right bank, Bottom: Left bank. Erosion formula 1

A correlation between river discharge and bank erosion is established as part of the mathematical modelling, see Appendix Q. The formula reads




E=1.21⋅10-6⋅Q1.7652. E has the unit (m2/day/m) and denotes the eroded land per day per meter of the river. Q is the discharge at the location. Erosion formula 2

Another correlation between the changes in the river discharge and the bank erosion is established as part of the mathematical modelling. The formula reads E=3.29⋅10-3⋅|dQ/dt| E has the unit (m2/day/m) and denotes the eroded land per day per meter of the river. |dQ/dt| is the increase in the discharge at the location in the last three preceding days.

Table 12.2: Table of simulated bank erosion effect of reservoirs in the floodplain of the lower Songwe River with different dam operation scenarios: Flood priority option and Hydropower priority option. Considered period Dec 16, 1964 – Oct 31, 2002 (38 years) Reservoir operation

Base line Flood mitigation option Design flow 150 * Design flow 200 * Design flow 150, +10m ** 2- casc., design flow 150 * Hydropower option 3 dams in cascade 2 dams in cascade

ERODED AREA (overtopping of Songwe River) Formula 1 Formula 2 Average Reduction

ha

ha

ha

%

796

532

664

0.0%

814 797 752 831

167 363 318 298

490 580 535 565

26.1% 12.7% 19.4% 15.0%

555 584

186 282

370 433

44.2% 34.8%

* Flow at Mwandenga ** HRWL reduced, FWL=HRWL+10 m

In general, the bank erosion can be reduced by 13-44% depending on the mode of operation of the dams. Reservoir operation based on flood mitigation option reduces the bank erosion less than an operation based on the hydropower option. A substantial difference between the two formulas is noted. The uncertainty should be kept in mind in the design work. Sedimentation rate

The sediment trap efficiency of the upper, middle and lower reservoirs has been investigated in the previous chapter.

Impact on delta

The morphological development of the delta area is a function of the amount of sediment imported to the lower river basin at Mwandenga and the split between sediment being exported to the floodplain and sediment being transported all the way to the lake. If all discharge is contained in the main river course, all the sediment will eventually be transported to the lake. Thus the share of 71% of all sediment as reported in the base line study is increased to 100%. It means that the shoreline accretion rate is increased from 2.7 ha/year to 3.8




ha/year. The implication is that the frequency of channel shifts in the delta area is likely to increase (higher morphological activity). On the other hand, as some of the sediment is trapped in the reservoirs (no sediment management scenarios such as flushing implemented) and the sediment passing Mwandenga is reduced, the export of sediment to the lake will be correspondingly reduced. 12.4.2

Bank protection

Approach

The effect of bank protection on the stability of the river course is fairly simple to analyse, as bank erosion is completely arrested in the protected area. The estimated land loss without bank protection is calculated from model simulations, GIS analyses and cross-section analyses.

Assumed erosion rate As described above, the existing annual land loss through gradual erosion is estimated at 29.3 ha/year in the lower reach of Songwe River, see Table 12.1. The land loss through sudden cut-offs within a time horizon of 10 years is 16 ha. Solution number 1

If all river bends are protected, there is in principle no need for flood spillways to protect against loop cut-offs. Length of revetments = 39.6 km

Solution number 2

Protected area

= 29.3 ha/year

Eroded area

= 0 ha/year

It is assumed that only the critical river bends are protected via the flood spillway cut-offs. Assuming that revetments of the flood spillway cut-off constructions protect approximately 1 km of the river per spillway, the total length of river revetments is: Length of revetments = 1 km x 13 = 13 km

Solution number 3

Protected area

= 13 ha/year

Eroded area

= 16.3 ha/year

It is assumed that no protection is provided: Length of revetments =

0 km

Protected area

=

0 ha/year

Eroded area

=

29.3 ha/year+16ha/10yr=30.9 ha/year.

12.4.3

Flood cut-off spillway

Approach

Quantification of the effect of flood cut-off spillways on the stability of the river course is examined by means of the mathematical models. The fully 2-D morphological model takes the boundary condition from the 1-D model and simulates the bank erosion rate for different




design layouts. The following scenarios are simulated in the model of the detailed study reach (see Appendix Q): • Loop cut version 1 (small portion cut), morphological and hydrodynamic simulations with and without loop cut channel • Loop cut version 2 (large portion cut), morphological simulations with and without loop cut channel The elevation of the weir at the inlet to the loop cut channel has been set to 479 m in all cases. This elevation gives roughly a 50/50% distribution of flow in the main channel and the loop cut channel at high discharges. See the second version in the Figure 12.7 below.

Figure 12.7: Bathymetry, loop cut geometry version 2. Morphology

During flood peaks the increased conveyance in the loop cut area will induce sedimentation in the old river channel. Where the artificial cutoff channel meets the old main channel, there is substantial scour and bank erosion (unless the bank is protected). When the water level decreases again after the flood peak and the loop cut channel is deactivated, scouring of the sediment in the old loop will take place, and deposition of the scour hole in the confluence between the cut-off channel and the old channel will occur.




Figure 12.8: Simulated bathymetry development without (left) and with (right) loop cut. Top: Initial bathymetries, Middle: Flood peak 15 April 1988, Bottom: End of simulation. Loop cut geometry version 2. Impact on erosion

Bank erosion in the old river bend will be reduced, but not completely arrested. Substantial bank erosion will take place just after the confluence between the cut-off spillway channel and the old river channel. Thus, protection of the riverbanks downstream of the outlet from the flood spillway is imperative.

Impact on cut-offs

By fixing an old river bend, the cut-off of this particular loop is avoided. However, the risk for other loop cut-offs further downstream or upstream will not change substantially as the hydraulic flow conditions have not changed considerably. On the other hand, reduction of the overall channel length during floods increases the slope and thereby the flow velocities. Therefore, higher flow velocities outside the area of loop protection (artificial flood spillway channels) may be anticipated.

Impact on delta

The total amount of sediment being transported to the lake is not changing in the long term due to artificial flood spillway channels.

12.4.4

Maintenance of delta channels by dredging

Approach

The requirements for maintenance dredging is estimated based on the sediment transport calculations. The total amount of sediment per year transported to the lake is 1.0 million m3.

Calculation

The average accretion rate at the shoreline is 2.7 ha/year. Thus, if dredging of for instance half the total amount of transported sediment is carried out, the accretion rate is tentatively reduced by 50%.

Impact

Dredging has an impact on the stability of the delta channels




12.5

Conclusions River Training

Erosion

The estimated erosion along the lower Songwe River is 29 ha/year. Loop cut-offs amount to approximately loss of land corresponding to 16 ha within the next 10 years, i.e. in average 1.6 ha per year.

Solutions

Two other solutions for active river training are considered besides reservoir operation and downstream dredging: revetments and loop cut-off spillways.

Revetments

Bank protection using revetments is based on a design with gabion mattresses. This will arrest the bank erosion completely in the protected area. The lower reach is divided into 15 locations. Each location should be protected completely in order to avoid that the bank erosion is simply shifted a few hundred metres downstream or to the other side. The bank protection works can be implemented at different steps between the different locations.

Loop cut-off spillways A total of 13 possible loop cut-offs have been identified, which may potentially develop within the next 10 years. Construction of an artificial cut-off spillway can arrest further development of these loops and at the same time maintain the existing thalweg of the river. Bank erosion will be arrested in the specific sites where an artificial cut-off is constructed. This corresponds to approximately 13 ha/year. Between these sites, bank erosion is unchanged (16 ha/year). Protection works are based on gabion mattresses as well as natural gravel and cubble material from the Songwe River basin. Impact on delta

With reservoirs, the sedimentation in the delta area will increase (short-term, a few years) as more flow is contained in average in the main channel. On the other hand, sedimentation in the delta area will decrease with reservoirs (long-term, decades), as the total import of sediment to the lower river basin will be reduced because some of the sediment will be trapped in the reservoirs.



12.6


Costs The costs of river training as calculated is shown in Table 12-3. The cost of the access road could depend on the work schedule, as limiting the work to the dry season may require less costly access roads.

Table 12-3: Cost of river stabilisation work

Excavation for gabions with spillways Excavation for gabions without spillways Cost of gabions with spillway Cost of gabions without spillway Access roads Subtotal Misc. Compensation and mitigation cost Engineering supervision Administration Continuency GRAND TOTAL

With Spillway 992,850

Without spillway 980,400

8,337,470 65 km 10% 8% 4% 15%

9,900,000 19,230,320 1,923,032 63,600 1,538,426 769,213 2,884,548

8,987,000 9,900,000 19,867,400 1,986,740 55,000 1,589,392 794,696 2,980,110

26,409,138

27,273,338


Ch 12 River Training and Bank Protection.pdf

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