Ayres Knowledge Center The Power of Association

Projects The power of association The concerns of dam owners extend decades beyond the careful design and construction of their facilities. Operation, maintenance, inspection, and emergency action plans all address long-term safety. Here Ayres Associates in the US discuss the role of floating bulkheads in gate testing and repair, the proper use of rope access and underwater diving in gate inspection, and the use of 1D and 2D modelling in dam-break inundation studies DAM OWNERS WHO NEED to complete maintenance or testing on dam gates often face steep challenges when determining how to accomplish these tasks without either draining the reservoir or building an expensive cofferdam each time rehabilitation or operational checks are needed. The floating bulkhead is a practical answer – even when the spillway does not have an adequate sealing surface, such as a pier nose with a flush spillway crest, upon which to seat a bulkhead. Most dams have some type of gate(s) to pass water either for normal river flows or flood flows. Many of these gates are over 50 years old and either need maintenance or periodic operation to verify they can be opened during a high-flow event. If the dam spillway was not constructed with a stop log system or other mechanism to separate the gate from the impoundment, the owner’s choices include constructing a costly cofferdam or bulkhead system to dewater the gate, or dewatering the entire reservoir, which is costly to any power-generation role the dam may play, and may pose environmental and recreational concerns. Traditional cofferdam construction is costly, time- consuming, and not always feasible. Conventional bulkheads can be massive, requiring significant cranage positioned above each spillway opening. A floating bulkhead is a system of individual caissons that can be placed in the reservoir, pinned together, and floated into position upstream of a spillway that is to be dewatered. Once assembled on the reservoir, the floating bulkhead can be installed and the spillway dewatered in as little as one day. The crane to place each caisson on the reservoir does not need to be positioned above the bulkhead as required for a non-floating bulkhead but can be located basically anywhere on the reservoir. Often a larger crane can be located off the dam and can be used to place the caissons onto the reservoir, where they can then be floated to the spillway that is to be dewatered. Once the bulkhead is in position upstream of a spillway bay, the lower caissons are flooded with water, causing the completed bulkhead system to rotate to the vertical position while remaining buoyant for final positioning. Most dams have multiple spillway bays with similar geometry. Once the gate test or repairs are done on a particular bay, the bulkhead is raised to the surface by displacing water in lower caissons with air. It can then be floated in its horizontal position either to the next spillway bay or to the offloading area and removed from the reservoir for storage. Floating bulkheads are designed to either be hinged and installed like a garage door or fixed so that the entire unit, once the caissons are pinned together, rotates to the vertical position for placement. Many shapes and sizes An important characteristic of floating bulkheads is that their installations can be customised to be effective even on spillway bays that do not have a flat, flush surface for placing the bulkhead, including bays where: ● No sill is available for the bulkhead to rest on. The floating bulkhead is held in place by the pressure of the water against the reservoir side of the bulkhead. ● The pier noses are sloped, tucking into the spillway, requiring lower caissons to be pulled up to seal the bay. ● Extensions are required on the bulkhead to seal gates with complex shapes. ● The spillway bays at the right and left end of a dam often do not have a pier nose but an abutment that extends into the reservoir. In these situations, an abutment seat is fabricated to position the bulkhead. Ayres Associates has designed floating bulkheads throughout the US, solving many of these challenges. Floating bulkheads have spanned openings measuring 6.7-18m wide and 4-14m high. Not only can one floating bulkhead system be used at multiple spillway gates; in certain applications, the same bulkhead can be used on multiple dams. Sometimes this requires simple modifications to the design to facilitate use at other structures. The hollow steal floating bulkhead caissons are fitted with valves to allow flooding when sinking them into place and valves through which compressed air is blown into the caissons to empty the water when it is time to float them away from the spillway bay. The hinged floating bulkhead technology was developed in 1987 by Xcel Energy and Ayres Associates engineers. That first hinged bulkhead is still in service today. From top to bottom: Time progession on installing Floating Bulkheads Recent floating bulkhead projects The Conowingo Dam on the Susquehanna River in Maryland presented owner/operator Exelon Generation Company a tall order when Exelon was determining what dewatering system to use to service 52 spillway gates measuring 11.5m wide by 6.7m high. The cofferdam method of dewatering f www.waterpowermagazine.com | Yearbook 2018 | 25