This second article examines the principal aims of the Bowmont-Glen Catchment Initiative, which included:
- Investigating the dynamics of water and the changing nature of the gravel bed channels in the Bowmont-Glen catchment
- Interpreting the relationships between land use, river management, flooding & sediment dynamics
- Deducing the root cause of recent changes in the catchment, particularly reasons for perceived increase in flooding and channel movement
- Proposing a range of sustainable land and river management strategies that could be implemented by land managers, with a flexibility that allowed them to adapt to a changing catchment, climate and regulatory framework
- Producing detailed designs for specific ‘hotspot’ areas where impacts have been particularly severe
- Assessing the resources required to carry out the proposals, identify potential sources of financial assistance and provide advice on regulatory requirements
- Considering the socio-economic as well as environmental impacts of the recent flood events and the proposed management measures, and potential benefits
- Making recommendations on how the initiative should be implemented to optimise the benefits
Main issues
In 2008 and 2009 exceptional flood events led to major channel changes, with severe impacts throughout the catchment, particularly extensive flooding and associated sediment deposits and damage to infrastructure. The following impacts have been established through a combination of detailed site assessment, land manager workshops, and 1:1 discussions with land managers between February and June 2010:
- Large areas of valuable haugh land lost due to erosion, channel switching, and massive gravel deposits
- Increased flood risk to rural & urban properties
- Livestock drowned, fences & gates washed out
- Rural residents isolated by flooded access roads
- Threat to amenity and vital services, including water and electricity supplies
- Damage to structures e.g. bridges & embankments
- Fish & habitat disturbance & obstructions to fish passage
- Increased soil erosion & landslides on the hills
- Channel switching moving field boundaries & threatening structures
- Bypassing and erosion of poorly maintained flood defences with disastrous potential consequences
In more detail:
- Two very severe floods were experienced within a 12 month period the first in Sept 2008 and the second in July 2009.
- The Environment Agency gauging station at Kirknewton estimated the 2008 flood peak to reach around 121 cubic metres per second. A severely breached flood embankment during the 2009 event meant that the gauging station could not record the full magnitude of the flood, however witness accounts place this event in the same order of magnitude as the 2008 flood.
- The last time a flood around this scale occurred in the catchment was in 1948; analysis of flow records suggests that the 1948, 2008 and 2009 flood events were all in the range of between 1 in 50 to 1 in 100 year floods.
- The College burn was identified as being by far the largest single contribution to flooding in the main river, and this has been confirmed by river gauging data from Kirknewton. The College burn’s contribution can typically be seen as a second peak in the flood hydrograph, as exemplified below:
(click on hydrograph to enlarge)
- The 2008 and 2009 floods were different in character and impact;
- The September flood occurred during the day; the July flood during the night, catching many people unawares
- Both events had serious land management impacts but on the whole the September flood had a more significant impact in the Glen as it destroyed crops due to be harvested, while the July flood impacted more in the Bowmont with in-bye land and property/infrastructure damaged and individuals isolated
- Interestingly the locations of critical impacts were different for each event and in different parts of the catchment, perhaps due to different directions of weather patterns between the two events
- Disparate parts of the catchment responded in different ways to the flooding:
- The upper catchment witnessed unprecedented levels of erosion
- The middle catchment experienced large areas of sediment deposition and channel switching, which resulted in loss of valuable in-bye haughland
- The main concerns in the lower catchment involved the reliability of flood embankments and the persistence of standing water on the floodplain
- The threats floods of this scale pose to humans also vary through the catchment:
- The most common threats in the upper catchment related to infrastructure and human isolation as access routes become flooded out, and vital bridges are washed away
- In general the main threats in the middle catchment related to livestock and property
- The threats in the lower catchment related to crops and livestock, and for the latter, safe and timely evacuation
8. The approach of land managers to dealing with the aftermath of the flooding was very different in the 2008 /09 floods than it would have been in 1948. After the 1940’s flood the river was immediately cleared of sediments and flood embankments were constructed to protect people and property.
Some examples of the ‘traditional’ approaches to catchment flood management are given below:
- Creation of artificial drainage channel networks
- Engineered bank protection
- Restraining of braided channel into single channel with bank protection / embankments
- Straightening a meandering channel, re-grading pool-riffle sequences, & reprofiling e.g. excavation of a trapezoidal cross-section
- Sediment management (dredging)
- Construction of flood banks.
- Construction of structures e.g. bridges, weirs, mill lades
