Water Flow & Dilution
The RCFMG aim is to increase the flow in the river back to what it was in the 1980’s.
The mean flow of the river is currently 3.48 cubic metres per second measured by SEPA at the Headswood Level Monitoring Station in Denny. In 1984 the rivers mean flow was much higher at 4.70 cubic metres per second at this location.
The headwaters of the Carron are effectively compensation waters from a series of reservoirs which have been built on the system to harness water for the population of much of the eastern central belt of Scotland. The largest of these is Carron Valley Reservoir.
Currently up to 75% of the catchment's water is abstracted out of the system for drinking water usage. Before the reservoir dams were built and the major water abstractions took place, the River Carron was 4 times its current flow rate, which made migratory fish passage easier over the natural waterfalls such as Lady's Leap, Fankerton and the Auchinlillylinn Spout, Carron Glen. Before the dams were built and pre-industrial pollution on the river, salmon and sea trout migrated easily over the Lady's Leap waterfall and Auchinlillylinn Spout waterfall in Carron Glen due to the vast amount of water flow in the river at that time. Salmon migrated right up into Carron Valley and up to the base of Meikle Bin where the source of the River Carron starts.
SEPA have stated that Scottish Water should provide a constant compensation flow from Carron Valley Reservoir of 8.2 million litres/day. This "Q95 flow" is calculated by SEPA and is based on the land area above the Carron Dam. The Carron Dam slipway outlet flow is supplemented with additional water when the dam overflows during prolonged wet weather. This compensation flow requirement will be raised to 9.2 million litres/day in 2010 in order to make this compliant with national SEPA guidance.
The SEPA CAR Licence No. CAR/L/1014292 ((DENNY WTW) states that Scottish Water have to maintain a minimum flow of 41.8 million litres/day at Langhill Farm Weir at OS grid reference NS 773 844. This higher flow is made up by additional releases of water from other reservoirs.
Every reservoir dam has a compensation flow valve. Each valve should ideally be set to give a SEPA "Q95 flow", which is calculated by SEPA and should be a value that is exceeded for 95% of the year (this value is approximately equal to the normal low summer flow flowing into each reservoir). Ideally, the operator should provide a Q95 compensation water flow at all reservoir dams.
Furthermore, the CAR licence states a compensation flow from Loch Coulter of 1,659,323 m3/year, with a maximum daily rate of 11,463 m3/day is to be released for the River Carron system (RCFMG understand this water is for the purposes of maintaining a specific flow at the Headswood Weir, Denny). Unfortunately currently 100% of this flow is abstracted from the Loch Coulter Burn by Howietoun Fishery and is not returned back into the Carron system. The biodiversity of the Auchenbowie and Avon Burn is therefore severely restricted at present because of the very low flows in it due to this abstraction on the Loch Coulter Burn.
There are 8 reservoirs in the catchment that affect runoff (namely Carron Valley Reservoir, Loch Coulter Reservoir, Earls Burn 1 Reservoir, Earls Burn 2 Reservoir, Buckie Burn Reservoir, Faughlin Reservoir, Little Denny Reservoir and Drumbowie Reservoir.
The SEPA CAR licence states a maximum daily combined abstraction to Denny WTW (Broadside Filter Station) and Gartcarron WTW for potable drinking water is 152.5 million litres/day from the catchment. It also states the minimum flow in the river at Langhill Weir should be 41.8 million litres/day. This indicates that only one quarter of the catchments water is allowed into the river when reservoirs are below their full level (when no dams are overflowing).
This is a significant problem for the river system as the reservoirs rarely overflow. This massive reduction in water flow in the river which has occurred as a direct result of the Carron Dam and other dams being built has significantly reduced the ecological potential of the river system and has made it very difficult (almost impossible) for migratory fish to ascend Fankerton gorge and the Auchenlillylinn Spout, two natural falls that were easily ascended prior to the dams being built. We believe this negative impact should be taken into account by the agencies and stakeholders to allow modifications to be made at these two falls to allow migratory fish access into the areas they accessed easily before the dams were built and before heavy industry and pollution occurred on the river.
The runoff is increased by Sewage Plant effluent returns from the 4 Sewage Treatment Plants in the system (namely Denny Sewage Works – Denny, Bonnybridge Sewage Works - Roughmute, Dunswood Sewage Works - Wardpark and Dalderse Sewage Works – Falkirk). Dilution water from the Reservoir Dams Compensation valves is paramount to dilute the treated water.