OpenStreetMap

Vaal River tracing

Posted by Gerhardus Geldenhuis on 18 December 2012 in English (English)

I have been creating river banks for the Vaal river that runs through Parys. The bedrock over which the river flows is rather solid and has not eroded much over the past few thousand years. This makes the river littered with small islands and deep fast flowing streams. There is lots of sections where you can see water does not flow during normal winter flow. I had to make a choice as to where I draw the river boundary, in the end I decided to take into consideration summer flow where you can see rocks have been scoured and there is not plant growth and included that in the trace. I have also tried to add as much islands as possible but that is work in progress.

I have been using Google Maps as a benchmark for my mapping and am glad to say that I believe that with regards to accuracy, OSM now exceeds Google maps with regards to river bank mapping in the Parys area.

Roads are also improving and in some cases OSM is now more complete than Google Maps, especially with regard to smaller dirt roads and new developments on islands in the river.

The twisting river and large amount of islands has meant that I had to redraw the river path to not run over any of the islands.

With regards to river flow: The river has a lot less flow during winter. The first summer rains if significant will mean that the Vaal Dam will open sluice gates in anticipation of inflow of rain water. This water will reach the Barrage a few hours/days later and then on towards Parys. The river is overgrown with Willow trees which impedes flow and can cause the river to dam up significantly. A major flood was experienced in '97 which cleared away lots of old trees. The bedrock lying underneath the river has also prevented deeper riverbeds being eroded. The golf island in Parys becomes completely submerged in 50 and 100 year flood events. If you ever visit the town you will see stone walls build along the edge of the river about 0.5 meter high from the road side that marks the 100 year flood mark. However it is important to take into consideration that this flood mark was established before the building of the Vaal Dam. This means that floods reaching this high mark is actually now more often because the flood waters are less spread out.

Its called the Vaal river because of the large amount of clay silt that it contains giving it a grey/brown look. Vaal means 'ashen' in Afrikaans.

Location: Ngwathe Ward 13, Ngwathe Local Municipality, Fezile Dabi District Municipality, Free State, RSA

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