Stepping up

IN RECENT years, the design floods of a number of dams have been re-evaluated and the revised flows were often found to be larger than those used for the original designs. In many cases, if the revised design floods occurred, the result would actually be dam overtopping because the existing reservoir has insufficient storage and spillway capacity.

A number of overtopping protection systems have been developed for embankments and earthfill dams. These include concrete overtopping protection systems, timber cribs, sheet-piles, riprap and gabions, reinforced earth, minimum energy loss weirs, and the precast concrete block protection systems developed by Russian engineers.

Soviet engineers, under the leadership of P I Gordienko, were among the first to propose a stepped concrete chute design on the downstream face of embankment dams. The choice of a stepped structure makes it possible to use individual blocks interlocked with adjacent elements and the design assists in the energy dissipation. For new dams, a stepped spillway made of concrete blocks may be the primary flood release structure of the embankment.

The design concept was tested more recently in the US and the UK. However, it did not prove cost-effective there, and in developed countries concrete-stepped overtopping protection systems are preferred.

An interesting feature of the concrete block system is the flexibility of the stepped channel bed allowing differential settlements of the embankment. Individual blocks do not need to be connected to adjacent blocks. Another advantage is the short construction time on site. In a typical design, the blocks lie on a filter and erosion protection layer. The layer has two main functions: to filter the seepage flow out of the subsoil and to protect the subsoil layer from erosion by flow in the drainage layer. In addition the protection layer reduces or eliminates the uplift pressures acting on the concrete blocks.

Usually, a geotextile membrane is laid on the embankment before the layer is placed and another membrane covers the protection layer before the installation of the blocks. The pressure differential created by the high velocity flow over the vertical face of the step creates suction that abstracts the fluid from underneath the concrete steps. Drains placed in areas of sub-atmospheric pressure act to to relieve uplift pressures.

Hydraulic calculations

The overflow embankment protection system is designed to operate in a skimming flow regime. The steps contribute to a substantial flow resistance and most of the energy dissipation takes place as a form drag process. On the stepped chute, both the flow acceleration and boundary layer development affect the flow properties significantly. The complete flow calculations can be tedious and most backwater calculations are not suitable. In 1999 the author proposed a pre-design calculation method which provides a general trend to be used for a preliminary design (see Dam Engineering, Vol. 11, No. 4). Ideally, the maximum velocity at the downstream chute end is Vmax = 2*g*H1. In practice the downstream flow velocity V is smaller than the theoretical velocity Vmax because of friction losses. A design chart was developed giving V/Vmax versus H1/dc where H1 is the upstream total head and dc is the critical depth.

In skimming flow, free-surface aeration is always significant.

It occurs downstream of the inception point of air entrainment, defined as the point where whitewater appears. It is generally accepted that the inception point occurs when the outer edge of the turbulent boundary layer reaches the surface. Downstream of the point of inception, a layer containing a mixture of air and water extends gradually through the fluid. The rate of growth of the layer is small and the air concentration distribution varies gradually with distance.

Design considerations

There are two fundamental design rules for precast concrete block systems: skimming flow, or a straight prismatic chute. The step block system was developed for a skimming flow regime so maximum block stability can only be achieved in skimming flows. All but one of the Russian applications were designed for relatively small discharge capacity (q ~ 3m2/sec). Pravdivets said 'the alignment of the spillway should be straight from the crest to the toe. Any curvature of the spillway in plan, or change in cross-section, will cause an uneven distribution of flows within the spillway which, in general, should be avoided'.

Usually the channel side walls are flat inclined slopes (ie. the spillway cross-section is trapezoidal). The slopes of the side walls can be designed as inclined stepped surfaces (in the flow direction) and may use the same concrete block system as the main channel. Typical side wall slopes are about 1V:3H (18º). A known construction weakness is the joint between the chute invert and the side walls. At Brushes Clough, two longitudinal concrete guides were built to make it easier to install the blocks and the connection with the stone-pitched sidewalls. At the downstream end, the residual energy of the flow must be dissipated with a small flip bucket arrangement and a conventional concrete pool. Laboratory tests showed high risks of block uplift and failure under a hydraulic jump.

The Russian experience with overflow earth dam spillways (chute slope 9.4º to 26.6º) showed the potential of the concept and highlighted that the quality of the drainage layer is most important. Failures were caused by improper drainage revetment. If the drainage requirements are fulfilled, the stepped spillway can sustain large floods, discharges and even ice debris. In Siberia, the Magadan experimental dam has been in operation for over 15 years without accident

Embankment overflow stepped spillways have common features with stepped stormwater systems and sabo/debris channel systems, which differ significantly from concrete dam stepped spillways. The channel often has a trapezoidal cross-section, the bed slope is moderate (less than 30º) and the step height ranges from about 0.05m to 0.3m. Further strong interactions may occur between seepage and so a stepped configuration cannot be used on a steep chute.

At the Brushes Clough dam spillway numerous acts of vandalism were reported, including destruction of concrete blocks. In practice it is recommended that the concrete blocks used are heavy enough to avoid them being moved by individuals and to make them more resistant to acts of vandalism. In the worst case if man-made destruction cannot be prevented another construction technique (eg RCC overlays) should be selected.

In Russia, the cheapest construction method uses precast concrete slabs made for the building industry. The slabs are rectangular (typically 3m long, 1.5m wide and 0.160m thick) and they are installed in an overlapping arrangement with mild steel spacers. A long-term issue would be the corrosion of the spacers.