Peter Minting looks at the design and performance of long sea outfalls
For the purposes of scientific study the effects of an outfall discharge can be divided into two types, near and far field. In the near field, intense mixing quickly results in dilutions of the order of hundreds or more. This mixing is caused by turbulence generated by the discharge itself. It ends when the turbulence collapses, marking the end of the near field. The wastefield at this point is said to be established. Beyond the near field the established wastefield is advected by ocean currents and diffused by oceanic turbulence in a region known as the far field. Here the rate of mixing is much less than in the near field, with additional dilutions of usually less than ten. But eventually the plume is dispersed by large-scale current motions and flushed away in the direction of the mean drift.
Density stratification, current speed and direction have major effects on plume rise height and dilution in the near field and in the far field currents can result in intermittent transport of the plume to distant locations. Acoustic doppler current profilers (ADCPs) and thermistor strings can be used to continuously measure the velocity and density structure of the water column. In the Mamala Bay outfall studies Professor Roberts used 300kHz ADCPs set to record current speed and direction at half-hour intervals.
A computer model designed to analyze multiport diffuser performance, the Roberts-Snyder-Baumgartner (RSB) model, was applied to the results of the Mamala studies. Roberts claims: "RSB has a solid physical basis, as it is derived directly from realistic experiments." It has been used to analyze outfall performance from a number of cities in the US, including Los Angeles, San Francisco and Boston. It has also been used in a successful bid for a secondary treatment waiver for San Diego's outfalls.
With the Sand Island outfall the plume was normally submerged at an average depth of around 40m. However plume rise height was very seasonal with the model predicting it would have reached the surface 11% of the time during summer and 28% in winter. Professor Roberts thinks this may be an overestimate as the thermistor closest to the surface was at 13m. The model assumed water above this depth was of equal temperature but in reality density stratification may have prevented the plume reaching the surface. He added: "It would be desirable in future studies to place the top thermistor as close to the water surface as possible." Surfacing is more frequent in winter as stratification is not so pronounced. Results for the Honoliuli outfall were similar but with increased submergence and higher dilutions due to the lower output per unit diffuser length.
Although sea temperatures in Hawaii are somewhat higher than in the UK many of the findings of these studies can be applied here. The fact that stratification assists plume submergence is important because in the UK the bathing water season is generally when stratification is at its most pronounced, i.e the summer months of April to September. By monitoring the currents and sea temperatures around a proposed site using programs such as the RSB model, designers of long sea outfalls could provide a robust prediction of environmental impacts.
Tides and currents in the UK are similarly strong to those in Hawaii and stratification also breaks down in winter as the surface temperature decreases and vertical mixing occurs. In relatively non-tidal water bodies such as the Mediterranean density stratification is likely to be an even more important factor in terms of stopping effluent reaching the surface.
In 1979 the original designers of the Sand Island outfall in Hawaii realised continuous submergence of the waste plume could not be achieved at realistic depths and reached a compromise where near-field dilution was in the order of 100 during stratification and 300-1,000 during surfacing. Professor Roberts' results show surfacing dilutions actually range from 600-5,400. The 1979 model did not include complex mixing effects of currents in the spreading zone, a likely cause of the discrepancy. This design therefore appears to have exceeded its objectives.
The most important points to consider at the design stage are;
- effluent quantity and quality,
- outfall site in relation to prevailing currents and local beaches,
- outfall depth and risk of plume surfacing,
- diffuser head design in terms of outlet size and distribution.
In the UK bathing water quality failures in terms of microbiological standards
are primarily caused by raw sewage spills from combined sewer outfalls (CSOs).
But environmental groups such as Surfers Against Sewage (SAS) have questioned
the performance of long sea outfalls. The ability of pathogens such as enteroviruses
to survive in the sea is difficult to determine and so SAS has argued for UV
treatment of all discharges. Other issues of concern include the the long-term
effects of nutrient-rich waste entering the marine environment, especially where
tidal currents create 'dead zones' with no dispersal to the open ocean.