Kromme River Valley
A case study of the wetland rehabilitation project in the Kromme River Valley floor marshes, Eastern Cape
Sponsors: Water Research Commission, Working for Wetlands (SANBI)
Leader: Ms L Haigh
Staff: Mr P Illgner (Consultant), Mr J Wilmot (Consultant), Mr J Buckle (EC Working for Wetlands)
Duration: 2006 - 2007
The aim of the project was to:
Investigate and detail the land use changes and the environmental deterioration of the Kromme River valley and its peat marshes, the site of the Churchill Dam (34°00'S 24°29'E capacity 35.710 106 m3) and the Mpofu Dam (34 °05'S 24°42'E capacity 107.06 106 m3) which together deliver about 34% of the water requirements for Port Elizabeth, the main city of the Nelson Mandela Metropole
Analyse the costs and benefits of the extensive wetland rehabilitation measures that had been undertaken since 2000, not taking the alien plant clearing that had also been undertaken, into consideration.
Asses the health of the wetland before and after the flood of 2006 in the light of the rehabilitation measures that had been taken and project a scenario of the consequences of no rehabilitation in the valley.
The land-use in the catchment is predominantly private farming (orchards, some cultivated crops and pasture for sheep, goats and cattle) with small areas of nature reserve in the upper catchment of K90A. This deterioration in the sandstone dominated catchment was posing an increasing threat to the water security of this important city so that rehabilitation was proposed in the mid 1990s, commencing with clearing of the alien trees through the Working for Water Programme. The clearing along the river course revealed the extent of the deterioration of the rivers and marshes. A wetland rehabilitation project was initiated through Working for Water and the Department of Water Affairs and Forestry in the Eastern Cape in 1999.
The upper valley (K90A&B) was the focus of the in depth investigation and for descriptive purposes was divided into basins 1, 2 & 3. Portions of aerial photos from the 1942, 1954, 1969, 1986, and 2000 were geo-rectified by and heads-up digitizing to quantify the changes on the marsh, riparian zone and floodplains (Basin 1 only). The 2003 colour aerial photographs of the entire valley are geo-referenced and available from Working for Water. A more detailed study of the deterioration of a high impact zone basin 1 area was conducted in the seasonal and peat permanent zones. To establish the present state and degree of deterioration assessment tools such as WETecoservices and WETassess and a Rapid Geomorphological Assessment of flood damage to the erosion control structures was undertaken during a site visits in 2006 and 2007.
By 2005 the catchment condition had improved slightly due to the exotic plant clearing done by Working for Water and attention had been paid to rehabilitation of the valley floor marshes. A large flood estimated to be a 1:70 to 1:100 year return interval between 2-4 August 2006 caused extensive damage on the valley floor and damaged a number of the erosion control structures that had been constructed
Percentage transformed areas in basin 1. This state can be extrapolated to basins 2 and 3 "Good" = marsh vegetation in place. "Poor" = wetland extensively sedimented or invaded by exotic terrestrial vegetation.
|Date||Valley Floor Marsh Area (Ha)||% Marsh Invaded State||% Riparian banks invaded||% Terrestrial area invaded|
|1954||23||26 sparse good||5||15|
|1969||23||48 dense poor||14||20|
|2003||1.2 good||0 cleared poor||3.7||48|
Rehabilitation of the wetland which started in 2000 was designed to prevent further destruction of the peat basins through erosion by stabilising the headcuts in the gullies, ensure a sustained supply of good quality water by decreasing sedimentation rates and ensuring flood retention and baseflow support to the storage dams. Over the seven year period 13 large structures were built at a total cost of R6.5 mill. mostly by hand labour, under the supervision of the Gamtoos irrigation board.
Efficacy of the structures
Until the flood the structures appeared to be protecting the wetlands effectively. However all the structures were damaged to some extent in the flood, the further downstream in general, the more severe the damage. The present state and reasons for the level of damage of each structure was assessed after detailed inspection March 2007. Four structures were rated 1 or 2 = poor and needed urgent attention although they were still standing in part; Five structures were rated 3 to 3.5 = reasonable condition needing few small repairs and 2 were rated 4 = good condition needing only minor repairs. None were intact.
The problems recorded sorted into two main categories namely construction problems or incorrect designs. Their failure could usually be attributed to a combination of both.
Inadequate foundations were built on consolidated sandstone not bedrock. Gabion construction - baskets were not packed densely enough with too few small stones in the voids. More angular, flatter, stones should be selected to pack the outer skin. Impervious membrane for water proofing on the upstream side of the gabion weir needs to be at least 500µm (such orange Hyperlastic from Gundle Plastics), and should be installed below the foundation into an impervious layer such as clay to prevent seepage beneath the structure and the consequent sagging of the structure.
Poor design options:
The spillway & freeboard should be designed for at least 1:50 year return period floods (in this case a 1:20 years was selected) given the size and shape of the catchment. Vegetation type and size was not taken into consideration in freeboard designs. Reeds are more flexible than palmiet which requires a higher freeboard. Key-walls and shoulder wing walls should be longer and better protected to protect the investment in the structure.
An assessment of the state of the valley floor in 2007 after the flood is presented in Box 1 and in Box 2 a scenario is extrapolated, should the funds not be forthcoming to rehabilitate and rebuild the damaged structures. The greatest threat to a sandstone dominated valley floor peat basin is erosion gullies as the soils are friable.
It was concluded that the project was well managed given the available funding despite the use of inexperienced labour in construction of very large structures in a problematic environment.
Last Modified: Fri, 14 Aug 2015 11:03:48 SAST