Nab's Head Storm Inlet Channel and Storm Pumping Station

Client: LiMA
Scale: 1:6

Nab's Head Inlet Channel

Testing determined that the model flume was compliant with the theoretical values.

On confirmation that the flume was compliant, determination of the influence of the two modulating penstocks was assessed.  To ensure the maximum pass forward flow of 2700 l/s towards the flume and downstream works was maintained, two modulating penstocks would restrict the flow accordingly.  Under the maximum permissible pass forward flow of 2700 l/s with the maximum total inflow rate of 7940 l/s introduced into the system, the twin penstocks were closed with a common opening of 0.550m.

A result of the penstocks impinging on the flow was a localised area of turbulent reverse flow tracking the water surface local to the downstream side of the second penstock, in addition, an extremely high velocity jet tracked the invert of the channel.  The flow regime expanded approximately 2-3D downstream of the second penstock with mass oscillation of the water surface evident.

The inlet channel operated as required with no alterations to the design.

Nab's Head Storm Pumping Station

Testing of the model had determined that the conceptual design of the pumping station was essentially acceptable.  The initial testing programme however, concluded that pump intake conditions were far from ideal and would, in our considered opinion, be a main contributory factor to likely operational difficulties that could be experienced on site.  Testing identified the presence of multiple coherent Type 2 sub-surface vortices with the additional of surface vorticity on all pump units regardless of selection.
Pre-swirl values were abstracted from the model, each pump regardless of pump combination or output experienced moderate to excessively high pre-swirl values.

Following review of the arrangement, remedial options were required to correct the adverse flows within the vicinity of the pump intakes, these included:

Profiled vane:- The proposal of a profiled vane is primarily to deny the previously observed sub-surface vortex from forming off the rear pump plinth.

Additional rear benching:- Additional rear benching along the pump plinth is required to prevent the rear sub-surface vortex from forming off the pump plinth.

Suspended baffle and single element vane:- In order to prevent the flow re-circulation and subsequent formation of surface vorticity a suspended baffle spanning the width of the pumps was installed.

Confirmatory testing
With the adopted modifications installed after the hydraulic assessment, a satisfactory pumping operating environment was achieved within the Nab’s Head Storm Pumping Station.  Acceptable pump intake conditions were maintained throughout the operating range and various pump combinations.  The pump suctions operated in an environment free from submerged and free surface vorticity with pre-swirl rotation maintained well within acceptable limits.  At no stage was air entrained to the pump suctions on the proviso the operating levels defined from testing are adhered to.
The study of grits and floating solids determined that, provided the recommended pump stop levels are adopted and the proposed modifications are installed, the station will remain materially self-cleansing over a storm and normal operating range.