Using the Rank Option pages ​

General comments ​

Rank is a number between 1 (best) and 5 (worst). Corresponding to these ranks are five site or environmental limitations between Very low' (rank of 1; best) and 'Very high' (rank of 5; worst) which describe the decreasing degree of suitability of the factor for on-site disposal. A rank and its corresponding limitation are guidelines (or trigger values) only, and you may elect to change the Trench defaults. A 'high' or 'very high' rank or limitation in Trench should alert you to possible constraining features of a site which may make the site unsuitable for disposal, or may require design modifications to make it suitable.

Some Australian assessment guidelines automatically call a site unsuitable if high ranks and limitations arise during assessment. This may or may not be appropriate or defensible. For example, a low soil permeability may be a highly limiting factor if the area available for disposal is small, but it may not be a limiting factor if the area available for disposal is large. In a similar fashion, a high rainfall area may be highly limiting in low permeability soils and/or small disposal areas, but rainfall effects may be lessened if evapotranspiration rates are high and/or the wastewater volume is small.

The Confidence level box (Recording the quality of entered data) ​

When you enter values in Trench, you should record the degree of confidence you have in the data, on the Rank Option page. This will be important when you start to make recommendations about system design and location, and it will help others assess your work. Your confidence levels will be entered in the assessment report.

Trench uses a four-tiered classification system for data quality, as follows:

1. Measured
   

   Data have been measured, tested or analysed by you or others. Site-specific data have been collected at, not near, the site, and enough information is available to be able to demonstrate an acceptably low variability across the site. Field, laboratory or other records are available to substantiate your use of the data.
   
   Possible examples: slope angle, area of the site, mean monthly rainfall (but not adopted rainfall), uniform soil thickness, water table depth (but not necessarily seasonal depth), SAR, PAC, CEC and permeability if the soil is uniformly-textured and enough tests are done.

2. Informed estimate
   

   Data which may have been measured at or near the site by you or others, but not in sufficient detail to show acceptably low variability across the site. Such data, like variable soil thickness or a range of permeability values, are often averaged. Also included in this category are all inputs which are not measured, but are reasonably estimated from direct observation or testing.
   
   Possible examples: adopted monthly rain, crop factor and evapotranspiration rate, soil thickness, permeability and chemistry if observed to be variable, depth to seasonal water table.

3. Estimate
   

   Data which cannot be supported by direct observation or records, but are nonetheless reasonable and generally accepted by others.
   
   Possible examples: water table depth from an adjacent site, permeability obtained from soil texture, chemical tests in 'similar' soils elsewhere, wastewater volumes in the absence of metered records, wastewater quality

4. Guess
   

   In the absence of data, guesses must be made, at the lowest level of confidence. This situation is common.
   
   

Changing Trench's default limitations ​

Leave the Amended box blank if you have not assessed the factor at all. If you have assessed the factor, and you agree with its default limitation, you may either leave the Amended box empty, or insert the same rank as the default. Otherwise, change the default by entering 1, 2, 3, 4 or 5 in the Amended box. If you do this, you should explain why.

Explaining why a default limitation has been changed ​

Explain why a default limitation has been changed by entering 1, 2, 3 or 4 in the box at the bottom of the Rank Option page. This reason is recorded as a brief comment or remark in the assessment reports, and if it is a significant factor in wastewater disposal, you may need to separately elaborate on it. Site or design modifications might be implicated.

What if a factor is not assessed or is not applicable to a site? ​

If (for any reason) you do not enter data in the grey input pages for a factor, the Rank Option pages will show no defaults, the summary pages will show a blank entry, and the assessment reports will show blank spaces (except for the 'Remarks' column which will show 'Factor not assessed').

INFO: Using the Rank Option pages (4 of 5) What do the limitations mean? Limitations of 'High' and 'Very high' for any factor are automatically entered in the 'Alert' column of the assessment reports, to flag one or more of the following possible actions:

• site modification
• system modification
• considering another site Site or system modification should be directed at reducing, to acceptable levels, the potential impacts of factors with high or very high limitations. Scores for sites (optional) By separately adding all the Site Capability ranks and all Environmental Sensitivity ranks for a single site you can obtain a pair of scores for the site which plot as a single point on a graph. By plotting several points, you can show the similarities of, or differences between, a group of lots on a subdivision, the effects of (say) similar soils in an area, a set of assessments in a particular area or municipality, or a group of possible disposal sites on a large lot. Click More to see a sample graph. It is good practice to set up a separate worksheet or database to record the scores for each full assessment you do in Trench.

Graphing Site Capability and Environmental Sensitivity Scores (optional) Plot Environmental Sensitivity Score on the vertical axis, and Site Capability Score on the horizontal axis. Each site assessment plots as a single point. Assign acceptability areas to the graph. Site modification or appropriate system design should have the effect of moving a point towards the more acceptable region of the graph.