Figure 1: Map showing fieldwork sites
(Scale = 1:10,000) www.multimap.com)
Method
At each of the sites shown above the following measurements were taken:
- Width (from wetted perimeter and bank-full)
- Depth (at meter intervals across the channel)
- Velocity (at meter intervals across the channel)
- Clast volume (15 stones sampled at random from either side of the river channel)
The data gathered at the restored site was compared with the data gathered at the un-restored section. Statistical tests (t-tests) were used to make sure that any differences seen were large enough to draw valid conclusions from.
The data gathered was then collated and entered into Microsoft Excel. The data will be presented:
- Histograms- Clast volume
- Scattergraph- Velocity against depth
- Column chart- dishcharge
Results
Clast volume:
The above graph shows that the restoration has had an effect on clast volumes. The mean clast volume in the managed river is substantially smaller than the mean clast volume in the restored site. The graph also shows that the variation in clast size is much larger in the restored river. The Standard deviations also appears to support this. The spread of volume is similar at both sites, with more larger clasts and fewer small ones.
Discharge:
The above chart shows that at two of the sites the discharge was greater in the restored flow, however at the third site the opposite was true. The variation in discharge was far larger in the restored section of the river than it was in the managed.
Velocity and depth
The graphs above show the relationship between depth and velocity in both the restored and managed river sections. To decide whether any correlation between the two variables exists a statistical test known as the Pearson’s test will be used (appendix 1)
The graph above shows the relationship between depth and velocity in both stretch’s of the river. Using Pearson’s test it has been concluded that a correlation does exist. The points outside of the red circle are outliers and are not to be considered. It shows a positive correlation; the deeper the river, the higher the velocity.
Discussion
All the results
In recent decades it has been acknowledged that river training (e.g. channelisation) has detrimental effects on the geomorphology and ecology of rivers. In recent years, there has been a shift towards the rehabilitation and restoration of rivers to a semi-natural state and to improve ecological habitats by working in combination with natural processes. One such river restoration scheme has been undertaken recently along the River Crane between Hounslow and Twickenham in west London. The placement of arcuate boulder berms at angles on alternate banks was designed to promote meandering within the confines of a channelised reach. This project maps the changes in erosion and deposition along the reach produced by “soft” river engineering measures. Particular attention is paid to the rates and distribution of sedimentation behind the berms, and also the invertebrates and wildlife along the reach, in order to evaluate the success of the scheme.
BGRG
In London more than 90 per cent of rivers have suffered from land drainage works for navigation, flood defence work and building development which has resulted in widespread environmental degradation of the capital’s rivers. Many residential and business communities near London’s rivers also suffer from a lack of green space and many areas are in need of regeneration.
River restoration is not possible along the entire lengths of some of North London rivers but areas that could see improvements include parts of the upper and lower Colne, the Lower Lee, the Brent, Crane and parts of the Roding, Beam and Ingrebourne rivers.
These improvements could include removing concrete banks and riverbeds, putting natural meanders back into a straightened river channel, adding fish passes at weirs, creating floodplains, encouraging sustainable urban drainage systems and reducing water pollution, and improving the natural environment for different plant and animal species.
Human activity has profoundly affected rivers and streams
in all parts of the world, to such an extent that it is now
extremely difficult to find any stream which has not been in
some way altered, and probably quite impossible to find
any such river.
- H.B.N. Hynes 1979
The first and most critical step in implementing
restoration is to, where possible,
halt disturbance activities causing degradation
or preventing recovery of the
ecosystem (Kauffman et al. 1993,
Kauffman et al. 1995)
