Optimising Reservoir Safety for Loch Finlas, Scotland
- oliverashton3
- Jan 12
- 4 min read
“ Flood Modeller has played a key role in our work on the flood study for Loch Finlas reservoir. The complex hydraulic behaviour at the reservoir overflow was represented using the features available in the software, with model validation carried out using CRM physical modelling results to enhance the representation of the overflow arrangement.”
Louise Connolly, Engineer, Fairhurst
Key Facts
Loch Finlas reservoir acts as a major water resource for Scottish Water’s public supply, feeding two Water Treatment Works (WTWs) that serve the Dumbarton area in the West of Scotland.
Fairhurst conducted a flood study to assess the reservoir under conditions corresponding to a 1-in-1,000-year event and a 1-in-10,000-year event, in accordance with industry guidance.
Flood Modeller’s 1D solver with double precision engine and Transcritical solver were utilised to represent the complex hydraulics of the overflow arrangement & spillway chute.
Calibration of the 1D model using the results of physical modelling conducted by CRM Rainwater Drainage Consultancy improved the model's representation of the overflow arrangement.
The calibrated model was used to optimise the design of subsequent remedial works, embedding constructability and cost-effectiveness into the proposed solution.
Project Overview
Scottish Water commissioned Fairhurst to carry out a flood study for Loch Finlas reservoir, which is located in the hills to the west of Loch Lomond in the west of Scotland. This is a large reservoir that acts as a major water resource for public supply, with Loch Finlas feeding two water treatment works (WTWs) that serve the Dumbarton area downstream.
The flood study was undertaken in response to measures to be taken in the interests of safety (MIOS recommendations) for the reservoir, which were identified by the Inspecting Engineer during the latest statutory inspection carried out under the Reservoirs (Scotland) Act 2011.
The flood study aimed to assess peak water levels in the reservoir and wave overtopping rates at the dam, to determine the need for any remedial works, and to address uncertainty in the performance of the stepped spillweir structure in comparison to previous studies. Spillway containment was also assessed under the MIOS recommendation.
The embankment dam at Loch Finlas is currently classed as a Category B dam, and so the flood study targeted the 1 in 1,000 year return period event (the ‘Design Event’) and the 1 in 10,000 year return period event (the ‘Safety Check Event’).
Modelling Approach
Flood studies are typically informed by simple 1D reservoir routing models; however, the complex hydraulic behaviour at the Loch Finlas overflow arrangement did not lend itself well to this approach. Rather, a more involved approach was required, with a 1D reservoir routing model and a separate Transcritical spillway model used to represent the overflow arrangement.
Capturing Hydraulic Behaviour
By utilising various features within Flood Modeller - such as spill units and a critical depth downstream boundary - Fairhurst were able to take the key features of the hydraulic behaviour of the overflow into account within our model, including progressive drowning of the overflow spillweir, and the influence of the downstream spillway chute and side channel capacity on outflows from the reservoir.
The performance of the stepped spillweir structure was also represented in the model by adjusting the weir coefficient in the spill units. Flood Modeller’s Transcritical solver was adopted within the separate spillway model to better represent the transition to supercritical flow conditions in the steep spillway chute downstream of the side channel when assessing spillway containment.
Physical Modelling of the Overflow
Initial 1D reservoir routing model results indicated that remedial works may be required to address predicted wave overtopping rates, but the outcome was marginal. Given the uncertainty surrounding the weir's performance and the potential costs and risks associated with remedial works at the reservoir, CRM Rainwater Drainage Consultancy was commissioned to conduct physical modelling of the overflow to improve understanding of its behaviour and inform the design of subsequent remedial works.
Improving the Efficiency of Reservoir Flood Studies
CRM simulated the hydraulic behaviour of the spillweir across a range of flow conditions, providing a rating curve that was implemented in the 1D reservoir routing model using a flow-head control unit. Based on the results derived from the validated model, the preferred remedial solution was identified as the construction of a notch in the top step of the spillweir. The design of the notch was optimised through iterative modelling using Flood Modeller, and supported by CRM rating curves.
The batch runner functionality within Flood Modeller is vital to our work on reservoir flood studies, which must identify the critical storm duration for a given return period event, and has played a key role in improving the efficiency of the notch design optimisation. Ongoing development of the software has the opportunity to further improve the efficiency of reservoir flood studies, with tools such as the simulation builder offering potential.
Louise Connolly
Louise is an engineer on the Water Services team in Fairhurst's Glasgow office. Her experience extends to working on a range of projects across the flooding and reservoir sectors, developing 1D, 2D and linked 1D-2D hydraulic modelling capabilities using industry-standard software packages, including Flood Modeller and TUFLOW.
Within these sectors, she has contributed to various stages of project development, including hydrology and hydraulic modelling, as well as subsequent engineering design.
