High-impact strategic river corridors

Methodology

This document describes how we identified potential corridors that we believe can yield high social and environmental impacts. The following general concepts lie behind these corridors:

A river or canal forms the backbone of the potential corridors. By following a river, there is an opportunity to use an existing ecological corridor.
Rivers were chosen that have poor water quality. These rivers are often a liability in social and environmental terms. By working on restoring the vegetation around the river, the actual river receives attention and this rehabilitation project can have additional environmental and social impacts.
The selected areas are currently not close to high-quality nature, based on the Bionet network. By working on gardens in this area, there is an opportunity to reduce the distance to nature for people living here.
We prioritised areas that are either generally less affluent (and where fynbos rehabilitation is often most needed), or where the rivers cross both affluent and low-income areas and where there is thus an opportunity to cross these socio-economic divides and roll out a project where people work together across these boundaries.

The maps below demonstrate the steps that were taken to identify the high-impact strategic river corridors. We welcome your input and we hope to further refine this project by receiving feedback. If you have any suggestions or comments, please get in touch.

Poor quality rivers

In order to assess which rivers have poor water quality - and are thus most in need of our attention - we relied on information from a great river study published by the City of Cape Town: Water quality of rivers and open waterbodies in the City of Cape Town: Status and historical trends, with a focus on the period April 2015 to March 2020. An important quote from this document is the following:

The quality of water in urban watercourses is perhaps one of the best indicators of the efficacy of a range of management activities in a city, reflecting the degree of containment of sources of contamination from both widespread land use and specific activities generating point source pollution streams. The implications of poor water quality can also be profound, cutting across a broad range of user sectors, including human health, sewer and stormwater infrastructure, tourism, recreation and biodiversity. (p3)

We have taken these ideas to heart and therefore start with the selection of priority areas by looking at river quality data. For this purpose, we primarily used figure 4.7.3 which included E.coli levels per subcatchment area (which generally encompasses individual rivers). We did not find a general river quality indicator, but felt that the E.coli levels are a suitable alternative for now.

Comparison of current (April 2019 to March 2020) E. coli data with data from the previous 4 year record per subcatchment, plotted against the thresholds for different levels of risk.

The next step for us was to identify these rivers in our Open Water Courses shapefile. In this shapefile rivers are sometimes split up in several segments, so we did our best to pick up all the relevant segments. The list below shows the identified rivers:

In the list above, we noticed that the Lotus Canal was missing. However, from experience working along this canal, we feel that the Lotus Canal should also be included. It is not of a high quality and has a lot of potential. We therefore added it to the list.

The map below shows the baseline for our low-quality river network.

Adding a buffer

In order to find suitable areas, we added a 400m buffer on both sides of each river. We consider this is suitable distance, as it will generally be within an easy walking distance for most people, and it is furthermore a distance that most birds and insects can cross - especially if you find multiple stepping stone gardens within this bandwidth.

The resulting map is displayed below:

Bionet

The next step was to pull up the Bionet map. This gives us insight into the accessibility of nature in different areas of the city. The original shapefile is shown below.

Buffering Bionet

We are wanting to exclude areas that already have access to high-quality nature by using the Bionet map. However, in order to do that we need to figure out who lives close to these areas within the shapefile. We do that by again buffering the outline, and we add 400 meters around the bionet network. The result can be seen below.

Overlaying the rivers onto that map

We can now start combining the two maps. Here we show what the buffered Bionet and the buffered river network look like...

Cropping the river network

The last step is to crop the river network, and remove those segments that are already covered by Bionet. This results in the map that is displayed below.

Based on this map, we then identified five different river segments that we consider the most strategic elements. We excluded the northern river system from the list because this is much less urbanised than the other river catchment areas. There might very well be a lot of value in focusing on the northern part, where agricultural impacts certainly threaten Fynbos just as urbanisation does in the south. However, our initial focus is on stepping-stone corridor creation in an urban setting, and we thus focused on the other river systems.

Of the remaining six larger river systems that are identified, we eliminated the eastern-most river. This too is in a much less urbanised area than the other rivers, and it is for that reason less interesting as a starting point from our perspective. What remains are the five corridors that are listed on our overview page.

Room for improvement

We don't consider this a final verdict, but instead hope for this map to start a more in-depth conversation. There are already a few elements which we hope to refine as more data and information become available. These include:

Instead of basing the river quality on E.coli measurements, we hope that a more general river quality assessment might help us to better pinpoint the rivers to target. Ideally that includes a longer list of rivers than the ones currently reported.
The vleis and other open water bodies such as wetlands are also very interesting to take into account. However, we still need to cross-link the data from the Cape Town study with the shapefiles that we have. Any help here is most welcome.
We are aware that taking Bionet as a basic layer to identify access to nature might not be the most accurate assessment. There might be potential or future natural elements in Bionet which are not actually representative of 'access to high-quality nature'. We might need to refine this.
When the green infrastructure plan from the city is released, we might be able to use some of their insights to further improve our mapping.

Suggestions or comments? Get in touch