Causes of River Nyamambwa floods

By Alex Kwatampora Binego

trueKasese District experiences heavy rains in the Rwenzori Mountains in the months of April, May, September and October every year and thereafter, some disastrous floods stream in the low lands through the incised gorges, valleys and the vast Nyamwamba valley.

Nyamwamba valley also known as Kilembe valley houses Kilembe Mines housing estates, workshops, schools, the hospital, commercial centres and the underground mine entrances and workings.

Kilembe valley last experienced the worst floods on May 1, 2013 following that of April 7, 1966. The floods destroyed the nursing quarters, part of Kanyaruboga, part of Katiri and similar floods that were quickly contained and averted in 1980. So Kilembe had not experienced such a disaster for about 47 years.

Kilembe Mines valley now known as Bulembia Division is part of Kasese Municipality which is located in Kasese District and Kasese town is one of the fastest growing urban areas with a vibrant population of about 126,000 people.

It is estimated that the population of Bulembia Division and adjacent parts of Kilembe Sub County is about 20,000 people.

Kilembe Mines is associated with the presence of economic copper and cobalt mineralisation deposits adjacent to the Nyamwamba fault. Because of copper, the Canadian companies, Frobisher and Falconbridge of Africa decided to construct offices, workshops, housing estates and other infrastructure within this vast Nyamwamba valley while they carried out economic mining at Kilembe Mines Ltd.
 
Unlike other major mountains associated with the East African Rift Valley system, which are of volcanic origin, the Rwenzori Mountain is an uplifted tilted block which lies in the centre of the Western branch of the East African rift valley system.

Whereas a number of urban structures are mushrooming in Kasese, there is a very high risk of continued floods, landslide, massive soil erosions and high seismicity or Earthquakes in this area that may continue to cause devastating catastrophes, if no stringent measures are taken to mitigate these catastrophes.
 
Historically, before the major rift movements, the main rivers in Uganda had an Atlantic drainage. Due to the tilting of the Rwenzori blocks in the southerly direction and an area of weakness at the confluence of the L. George and Semuliki faults (i.e. Kazinga channel), a north-south drainage pattern resulted.

This caused the Chako, Kanyampara, Dwimbi, Nyamugasani and Dunglea rivers with the Nyamugasani -Dunglea complex extending to the Peak areas.

Due the above factors, a number of streams formed along the Kasese fault scarp at the time of rifting and the drainage pattern including rock hardness, traverse shears, etc which led to the formation of River Nyamwamba.

Whereas the R. Nyamwamba encroached westwards followed by down cutting to achieve the L. George base level, it was undoubtedly very rapid in the youthful landscape.

With time and associated with the above, the river capture with its vastly increased catchment area substantially increased the river Nyamwamba load which effected the increased river load carrying materials up to big boulder size which eventually cascaded from higher areas during floods into more mature Nyamwamba Valley where the reduction of the river velocity culminated into coarser material being deposited in the valley.

The load and rate of deposition in the Nyamwamba valley follows the high rain fall in the mountains, steep slopes and the rapid weathering rate of the Rwenzori metamorphic rocks that yield the boulders brought down by the river.

The rate of denudation in the river Nyamwamba catchment area is very high as this area covers about 72.4 sq km and the material in terms of boulders, cobbles and others transported by the river is over 1,000,000 tonnes per year.

The devastating floods that destroyed part of the Nyamwamba valley that houses Kilembe Mines Ltd and the entire infrastructure started on May 1, 2013 following heavy rains in the high Rwenzori Mountains.

The floods swept all the bridges over river Nyamwamba in Kilembe valley save for Katiri main Bridge and Concentrator Bridge.

These floods destroyed part of Bulembia School, the Chairlift area, the old fuel KML fuel station, the drawing office and others. All the other offices were affected by Nyamwamba water and the associated alluvial sand and silt depositions.

Downstream, floods destroyed almost all the wooden houses including the small commercial centre belonging to Mzee Andrea Baziga in Kanyaruboga, part of Katiri and eroded the new tarmac main road connecting to the offices.

Four people lost their lives and these are Karusu Paddy (former KDLG Procurement officer), Gad Biryomumaisho (Miner), Kabugho Agnes from Nyakabingo and Kibusu Raphael a teacher at Buwata Primary school. Their dead bodies were all recovered from the river banks scattered up to Base camp area.

Further downstream, part of Katiri main bridge was destroyed; the Nursing housing estate and part of Kilembe Mines Hospital were completely destroyed. In Kasese, it cut off the Kasese-Fort Portal road at a place known as Nyakasanga near the Airfield and flooded the area nearby.
 
POSSIBLE CAUSES OF THIS FLOODING AND DEVASTATING DESTRUCTION

In view of the above geographical and geological factors, there are a number of causes of floods in the Nyamwamba valley and below are some of them;

1.    Heavy rains or “cloud bursts” in the catchment area. The term “cloud bursts” simply means heavy, localised down pours common in the tropical and temperate climates.
 

2.    Damming up of outlet of high attitude swamps with vegetation and sediments followed by bursting. Land slide damming up valleys in the R. Nyamwamba catchment area usually form a lake and eventually burst the walls during heavy sustained rainfall.
 

3.    Historically, in the past 40 years, Nyamwamba floods about three to four times a year but serous catastrophic floods have been about four with the worst being the most recent one which occurred on May 1, 2013 while the earlier known catastrophic one was on April 7, 1966.
 

4.    Taking into consideration the high level swamps which occur in the high catchment areas that were formed by terminal moraine when the permanent snow line was lower and the fact that they were lakes but have since filled with sediments and are now organic bogs, these burst following heavy rains causing floods.

5.    These lakes are about eight in number and the largest of these lakes has a surface area of about 762,000 sq m and in view of this, the blockages at the outlets of these swamps and lakes by organic material and silt, followed by bursting have caused unprecedented catastrophic floods.
 

6.    The direct effects of heavy rains and “cloud burst” within the River Nyamwamba catchment areas have caused a permanent run-off with steep slopes and permanently saturated conditions of over 70-80% within the catchment area.
 

7.    The high rate of weathering of metamorphic rocks in the Rwenzoris have led to big boulders being eroded downstream during floods narrowing the river channel exacerbating the flooding. 
 

8.    Depreciation of the bridges foundations and cumulative damage to the pillars with no repairs. Apart from Katiri main bridge that was rebuilt about seven years ago, all the other bridges over River Nyamwamba had depreciated over time and their foundations were in appalling conditions. The embankments of these bridges had been eroded over a period of time and when the river swept the bridges, it took a new course and burst its banks.
 

9.    Historical old river channels. It is on record that River Nyamwamba used to flow along the main road from Bulembia, through Kanyaruboga, via the Nursing quarters, down to road barrier till Kasese. This disaster is not the first of its kind in Kilembe though the May 2013 incident had far reaching effects and was very fatal.

In April, 1966, the River Nyamwamba floods affected Bulembia School, washed off the whole bus shelter below Katiri bridge, the Police, destroyed all the Nursing quarters and other houses downstream.  Nyamwamba taking its original course destroying part of the office in the presence of the writer
                              
10.    Old wooden houses at Kanyaruboga had out lived their usefulness and were vulnerable to any agent of destruction like the Nyamwamba floods. The foundation here is a backfill of stones and sand-fill, though it was strong but was susceptible to erosion over time.
 

11.    Cultivation near the river banks has caused and is still causing a number of river and stream bank failures and when the streams and river swell, the banks are easily washed off. 
 

12.    Poorly constructed houses due to low costs involved like the use of murram. This is very common especially in Nyamwamba Division where it is well known that it’s a flood zone and residents have defied council directives and have constructed houses in gazetted river channels.
 

13.    Inappropriate Institutional capacity for flood and other hazards predictions risk assessment and risk mitigation. This is coupled with inadequate training and information flow in regard to Meteorological factors, geological situational analysis in the country including the Rwenzori region.
 

14.    Poor environmental management practices in urban areas that end up exposing people to geological and seismic related hazards. People constructing houses in wet lands, gazetted river bank and flood areas. A case in mind is that one near the Red Cross area in lower Base camp.
 

15.    Population growth resulting into high population densities with marginal land for developmental use. People have ended up encroaching on land near the river banks or have decided to block some flood channels and have constructed houses in flood zones.
 

16.    Lack of funds by the responsible institutions to carry out preventive and mitigation measures have also exacerbated such hazards.

Urgent mitigation measures

1.    We urgently need Bulldozers D-8 to D-14 models to clear and divert the river back to its original channel starting from Bulembia School, behind the wooden houses, at the nursing quarter’s area, road barrier and at Nyakasanga.
 

2.    Construction of bridges over River Nyamwamba in Kilembe and Kasese-Base camp area for vehicle and pedestrians.
 

3.    Serious restriction of cultivation or degrading activities near the river and stream banks should be enforced.
 

4.    Capacity building in regard to floods, seismic hazard assessment and risk mitigation and mass public sensitisation of the above dangers should be put in place.
 

5.    Proper environmental management practices in urban areas and proper planning should be enforced.
 

6.    Government or Kilembe Mines should provide machinery and other logistics in order to clear river Nyamwamba channel. The local govt and Kilembe Mines as of now cannot handle this problem given their meagre resources.
 

7.    Regular monitoring of the rain pattern by re establishing a weather station.
 

8.    Re establishment of flood control points
 

9.    Regular monitoring and constant engineering inspection of Bridge foundations masonry work.
 

10.    Increase Geo-Environmental effects and management sensitisation of the public living near river banks.

However, considering the high population density that is mushrooming in Kasese and given the fact that development has to take place, such hazard assessment and risk mitigation should be every body’s concern and should not be left to the Government or Kilembe mines alone.

The urban authorities, where I also belong, should emphasise sensitisation of the masses in order for them to construct good and durable houses on gazetted plots that have structural and building plans approved by the municipal engineer, stop degrading activities near streams, rivers and roads and above all, should not construct houses along river channels.

Good Geo-environmental management practices must be effected and the local governments should not underestimate the devastating effects that may arise due to further floods, excessive soil erosions, earthquake occurrences and landslides.

Lastly, contrary to what most people living in Kasese Town think regarding the flooding of the Kilembe Mines underground tunnels, the impacts of the floods underground can never be the same as those of River Nyamwamba.

Whereas it is true that there is a lot of water underground, it comes gradually and does not flow like a river. In fact, in future when copper/cobalt will be mined out completely, while decommissioning the mine, the mine tunnels will deliberately be flooded and the water will flow out at 4300level and join River Nyamwamba.

Due regard to environmental concerns will be put in place to neutralise all the acidity and minerals that may contaminate the River Nyamwamba.                                                                     

The writer is a mining engineering geologist