This post is dominated by my experience supporting regulatory review, including a failed attempt to revise waterway policy - largely due to challenges of inter-agency jurisdictions.
Setting a target for flushing rates, particularly when modelling can be easily manipulated, gives a tendency towards poor water quality outcomes. An objective to maximise flushing, while admittedly rather vague, should be the target.
Evaluating flushing time is useful, in that it is something which can be physically measured (e.g. rhodamine dye testing) and it isn’t subject to as much conjecture as WQ modelling.
Hard flushing time criteria (e.g. 4-day, 7-day or 10-day e-folding times) are appallingly abused. They are equivalent to saying ‘about head height’.
Development of modelling techniques for evaluating flushing occurred through the 1980s and 1990s, converging towards some relatively standard techniques (e.g. e-folding time). In Western Australia a shift to unofficial acceptance criteria occurred through the early 2000s. For small craft harbours, a 10-day e-folding time for ‘worst case’ (tide only) was typically used, although this remained highly susceptible to manipulation of model parameters - the same waterway, modelled two different ways (i.e. ¼ grid scale, but otherwise identical) will give a different answer. Sometimes not much different…
Following investigation of a couple of harbours with groundwater-related WQ issues, someone decided to get smart and incorporate all sorts of other mixing parameters (not me, but we all make mistakes!) but retain the SAME 10-day e-folding criteria. This was inappropriate, as the ability to make a teensy adjustment (driving time series, boundary conditions, dispersion coefficient) to get different outcomes makes using any criteria very challenging.
Once a modelled flushing time became an excuse for plain old poor layout, things had ‘crossed over’.
I don’t think that numerical modelling shouldn’t be done… it’s just that it’s so horribly wobbly as a tool, that it needs to be very carefully used and interpreted. Following international practice, and Western Australian EPA guidance:
Make sure that the water body is designed with good flushing in mind (there’s the old USEPA 1985 guidance, as well as more recent work by PIANC
Get an understanding of WQ pressures
Undertake numerical modelling, to demonstrate flushing performance across a range of conditions. There’s difficulty getting hard criteria here, but something where there was Tf<4 days for >90% of the year, and Tf>15 days for <1% of the year would be much better than a waterway with 8<Tf<10 days for 100% of the year. The bigger problem is understanding how Tf varies with the modelling approach.
Have a WQ Management Plan that incorporates monitoring, tolerance criteria and the ability to detect trends.
So... some take-home messages:
A 10-day flushing time is not a suitable target
Modelled flushing times are almost meaningless (comparative only in themselves)
Water quality is not just about algal blooms
For those of you who aren’t aware… we attempted to develop a major revision to WAPC DC1.8 about 15 years ago, incorporating EPA’s Water Quality Management Framework. It had a lot of effort involved, but ultimately failed due to the difficulty of getting cross-jurisdictional agreement.
Note 1: Canal Water and Sediment Quality Management
Canals and coastal marinas are acknowledged to provide conditions under which reduced water exchange and increased potential for contamination provides pressure on water and sediment quality. This commonly involves a deterioration of water colour or odour, and under more severe pressure may create conditions under which algal blooms (sometimes toxic) are frequent, can cause deoxygenation leading to fish-kills, or can result in development of longer-term eutrophic conditions.
Development of these issues is often progressive, particularly when it involves accumulation and reactivation of nutrients trapped in sediments. However, detection using water quality measures can be substantially obscured by the episodic nature of poor water quality events. In many cases these develop due to a short-term coincidence of high nutrient input, small tides, still conditions and warm temperatures, which may only last a few days, and usually is focused on particular seasons. A consequence is that untargeted, discrete sampling has a limited chance of detecting issues, and requires careful evaluation to ascertain trends. For example, if a typical event lasts 5 days in 30, then the chance of identifying it with monthly sampling is 16%, and the chance of capturing two events in two months (if they occurred) is approximately 2%.
Water quality is developed through combined influence of multiple factors, some of which may be external to the waterbody (i.e. possibly beyond direct control of a waterway manager). Performance is ultimately measured by a set of receptors, including waterway users, and environmental values, both inside and adjacent to the waterway. A framework for developing a water quality management plan is outlined in EPA (2016) Technical Guidance: Protecting the Quality of Western Australia’s Marine Environment.
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