Recent diary entries
Imagery will reveal all.
I just realized that the Vaal river in South Africa, which has been mapped by different people has been mapped using Bing and CD:NGI imagery. This is not necessarily a problem in itself apart from the fact that in places the Bing imagery were taken during flooding and does not represent the normal flow of the river. There is thus some work required to differentiate this. In a previous diary entry people have suggested using a flood plain tag. It might be appropriate but only if it is a regular occurrence which I am not convinced it is. I will need to spend some time comparing exceptional flood event dates with the imagery dates and then make a decision on the frequency of occurrence to guide the choice in tag being used. Since I mapped a large part of the Vaal river, or at least it's river banks, this diary entry is more of a note to self, to do the work at some stage.
The most used tag seems to be: flood_prone
I was using CD:NGI imagery to trace the Vaal River borders and thought I would double check an inlet using Bing, was amazed to see the difference. The CD:NGI imagery is much closer to normal probably winter flow if I were to hazard a guest with the Bing imagery taken during a severe flooding event by the looks of it. Interesting.
It does raise another point though. Where is a river's boundaries? What is normal flow? Should we in an ideal world map drought, "normal/average" and flood boundaries if we had the data? I think so but it would be a massive project and accuracy would certainly need to be stepped up a bit. It also becomes a 3D problem because what would normally be an island in a river now suddenly disappears beneath the water.
I have started using JOSM, the initial learning curve is going to be a bit steep to get to the same speed and efficiency I were at in Potlatch but I am fairly certain that I won't be seeing a web editor as often any more. The web editor is still unbeatable for quick and dirty edits and I am really looking forward to iD as the beta looks very cool, albeit a bit buggy and VERY resource hungry.
Found what looks like a dyke in the river...
Noting it down for future interest.
After seeing much more of these "cracks" it looks more like a change in underlying geology from hard rock to softer rock which means the water has a more severe effect and possible the river banks are steeper. All speculation though having not visited the site.
Saw a really interesting set of "cracks" looks like tension cracks in a anticline or maybe an underlying dome. Having not seen the Geology first hand it would be difficult to judge but worthy noting down for later inspection.
The Vaal river has filled in these cracks and it does not look man made. Most of these inflows have a stream connected to them and there is no man made structures or other indicators that would point to a more human origin.
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.