Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/547956
The existing inlet grille was replaced with a new FRP version. It features doors to allow access for divers in the future A new outlet headwall and channel was installed (All photographs are courtesy of the Canal and River Trust) The auger bore was the most effective way to install the new outlet pipe 10 WET NEWS AUGUST 2015 Reinforced Polymer (FRP) grille, installed at in front of the culvert, with doors to allow access for divers in the future. The grille bars had a clear open- ing of 75mm, designed to allow a maximum flow rate of 400l/s. FRP was used due to the benefits of being a lightweight and easy-to-install structure compared to conventional steel. The grille would be submerged, so UV degradation would be minimal. UV degradation The diameter of the Neptune sha„ makes man entry to the sha„ and culvert for any work, or future maintenance, very restricted. To reduce the fre- quency of maintenance visits, the existing timber system was replaced with a new wall- mounted cast iron penstock with a clear opening of 300mm. The penstock is located within, and just above, the floor slab of the Neptune sha„ with a vertical rising spindle within the sha„ (approximately 5m) and a bevel gearing system within the existing kiosk oper- ated by a hand wheel. An auger was proposed as the most effective way to install a new outlet pipe with minimal disruption to the embankment. The auger pipe consisted of 355mm diameter steel pipe sec- tions, with a total length of 18m. A launch pit was required to accommodate the installa- tion equipment. In order to minimise temporary works, the new energy dissipation cham- ber was designed to be used as the auger launch pit. The auger level was set to minimise the potential for a clash with the existing timber and stone culvert, whilst main- taining a suitable invert level. The auger pipe was extended sufficiently into the Neptune embankment and discharges via a brick headwall into a feeder channel. The reservoir had been drawn down in 2009 to repair the timber plug and mechanism associated with the Neptune sha„, and again in November 2011 when a CCTV survey of the outlet was under- taken. The condition of the culvert was significant as its primary importance, apart from acting as a feeder to the canal, was as an Emergency Draw- down facility. The reservoir has a specific onsite Emergency Drawdown Plan (EDP) and within this there is a require- ment to drawdown water levels by 50% within five days. Access to the site was from the adjacent main road with the contractors' compound (40m x 40m) located alongside the reservoir. This served a dual purpose – acting as both the temporary compound area, and subsequent use as a car park for the reservoir fishing club a„er completion of the works. In order to access the main inlet and brick culvert the res- ervoir level was drawn down to 10%. If the reservoir had been drawn down lower than 10% aeration would have been required to ensure existing fish stocks were not affected, or a fish rescue undertaken. The contractor entered the reservoir from the compound area via the reservoir bank (not part of the dam structure) and a coffer dam was installed around the existing headwall and brick culvert to allow inspection. The inlet culvert was inspected and a decision was made to provide a more robust solution by insertion of a 610mm ID pipe within the cul- vert and grouting of the annu- lus. The existing grille was replaced with a new Fibre ONSITE ASSeT mAnAGemenT sha„ to allow installation of the new penstock, and a struc- tural liner installed to give a continuous smooth lined pipe surface. In order to connect with the Neptune Sha„ the auger would have to go through the clay core within the embankment. The contractor monitored arisings from installation of the steel pipe to verify the presence and location of the core to improve existing records of the dam's construction. The discharge chamber was constructed of pre cast man- hole rings, incorporating the inlet pipe (auger pipe) from the Neptune sha„, an energy dissi- pation wall (cross wall), and a larger discharge pipe at approx- imately 90deg to the inlet pipe, at a lower level. The chamber was designed in accordance with the impact stilling basins methodology of the United States Bureau of Reclamation "Design of Small Dams". The manhole chamber is relatively deep considering its location at the base of the embankment and care was therefore taken to provide suf- ficient support for all the tem- porary works. From the discharge chamber an outlet pipe transfers flow a short distance, including a change in the direction of flow, and discharges into a lined out- let channel. This arrangement accommodates the maximum discharge flow (400l/s) at a velocity of <1.5ms, to minimise erosion to the existing bank. Historic records indicated that the existing box culvert was of timber and stone con- struction 0.3m by 0.46m, and approximately 18m long. Some voids had been identified along the line of the culvert. The scheme was completed at a capital cost of £350,000.