The study was approached with the following strategy:
- Understand how the catchment works
- Appreciate the context of historic land management and the requirements of existing land managers; rural land managers are the key to the success of sustainable catchment management
- Target management to the appropriate place: the source of the problem rather than the impact zone
- Strategic and coordinated planning ensures that changes in one part of the catchment don’t counteract works carried out elsewhere.
The catchment assessment reveal revealed that past and present land and channel management activities may increase flood risk and decrease river channel stability, for example:
Previous flood events
Gravel bed rivers typically have a coarser surface layer (armouring or pavement) which offers a level of protection to the finer subsurface layer. Once this layer is lost the bed is exposed to erosion. Scouring of steep tributaries, especially headwater gullies and the College Burn introduce a surplus of sediments to the river which will persist for a long time.
Deforestation & reduced vegetation cover
Changing land use is likely to be the most significant factor in altering catchment response to rainfall. Historic conversion of forest to agricultural land may have resulted in increased peak flows and a flashier response, by reducing the capacity of the catchment to retain water and snow retention. It is also likely to have increased soil erosion rates
Grazing patterns
Intensive grazing reduces vegetation cover. Heavily grazed grassy slopes are exposed to soil erosion & landslides in headwaters, also rapid runoff and snow melt from steep, bare hillslopes. Poaching (trampling) by cattle destabilises banks and increases erosion rates.
Artificial drainage
Funnelling of floodwater through artificial drainage networks, and tillage lines. Access tracks and associated drainage also interrupt water movement through the soil profile and create surface runoff, which is rapidly channelled into nearby streams.
Loss of natural flood storage
Land use changes, such as loss of large woody debris in headwater channels and drainage of wetlands reduces the catchment’s natural ‘flood buffering’ capacity.
Loss of riparian vegetation & reduced bank stability
Removal of riparian (streamside) vegetation or disturbance of the stream banks reduces the stability of channel and banks. More sediments build up in the channel, reducing water capacity and increasing the frequency of floodplain inundation. Channels become wider and shallower, increasing the likelihood of braiding. River cliffs are destabilised by the loss of vegetation cover and by water flowing over the crest from drains or tillage lines
Increased sediment supply and reduced sediment removal
Channel change can occur in response to changes in both flow and sediment regimes. Reduced channel management in recent years, combined with increased sediment in the system means that channels are shifting laterally at a greater rate than in recent decades. The abundant sediment supply reduces capacity to convey floodwaters and sediments. This increases flood risk and tendency to braid.
Channel management and flood embankments
Increased likelihood of breaching and depositing large quantities of bedload – straightened sections are steeper than natural, making them susceptible to incision and mass bank failure.
Development
Developed and compacted areas such as roads and yards can reduce infiltration and increase overland flow. Roads can intercept subsurface flow and route it more rapidly along drains. Construction projects within or adjacent to streams can also dislodge or expose soils and sediments.
Unsuitable structures
Localised scouring and deposition caused by bridges, crossings & structural failures.
