Reservoir improvement works: overflow and outlet screens

Overview

Overflow and outlet screens

Overflow/outlet screens are often fitted to reservoirs to prevent human exposure to hazards, to catch large debris, or to prevent fish and mammals being washed downstream.

Whatever its primary purpose, a screen will collect debris and block temporarily. This blockage can lead to an increase in reservoir water level and could alter the stage-discharge relationship of the overflow or outlet. Furthermore, blocked screens will reduce the available freeboard and overflow capacity. Their impact must therefore be considered in reservoir flood studies and the design of outlet structures. This is particularly important for flood storage reservoirs (FSRs) that operate infrequently and rely on maintaining the design stage/discharge relationship to achieve the required flood attenuation.

Case studies are presented concerning two FSRs fitted with self-activating flow-control devices on their outlets that failed to operate as anticipated on first filling. In both cases the unexpected operation was attributed to downstream screens fitted to mitigate perceived hazards. A third case study concerns the impact of a ‘fish’ screen placed in the overflow spillway of an amenity lake.

This presentation summarises research on the impact of screen size on fish and mammal passage, and on debris movement, in particular the relationship between debris volume and bar spacing. It looks at some alternative screen design and management measures to reduce the impact on reservoir water level and overflow capacity.

Bwlch Nant yr Arian Reservoir Improvement Works

The change requiring formal registration of reservoirs >10,000m3 capacity, implemented within Wales from 2016, led to a large number of reservoirs being reassessed and registered with Natural Resources Wales (NRW). This included sites within the forest estate, such as Bwlch Nant Yr Arian, situated about 14km due east of Aberystwyth.

A Section 10 Inspection was undertaken in November 2014 and subsequently October 2017. Completion of measures from the 2014 inspection clarified the capacity of the reservoir was 40,000m3, with no overflow, controllable drawdown facility and with insufficient freeboard. The construction works undertaken in 2020/21 consisted of removal of the existing outlet pipe (a corroding corrugated Armco pipe), installation of a new Reinforced Concrete (RC) spillway with a pedestrian footbridge over the top and an integrated low level drawdown facility, and rip rap placed along the upstream face.

This paper discusses the particular construction challenges faced, during a project to address safety measures applying to the reservoir, such as the very steep sided valley limiting construction access, the visitor centre remaining open during the works, working limitations due to red kite feeding times, adverse weather experienced, emergency planning and the impact of Covid.

No registration is required to attend this event in person. Please register if you wish to attend online.