An assessor from Syracuse in the US noticed my discussion paper on bottomless sand filters (BSFs) and was asking about filter sand specifications and maximum slope angles for such installations. Richard Mason from Sorell Council has done more work on BSFs than anyone in Tasmania and I asked him to join the on-line discussion.
The filter sand specification was not an issue, but the slope angle side of things got me thinking – I’ve not seen any guidance on slope of a site and BSF suitability. Richard made the point that “the NYS regulations seem to be saying that maximum permissible slope for raised beds and mounds are 15% (8.5⁰) and 12% (just under 7⁰) respectively. (See “Alternative Systems – clauses (b)(2)(111) and (c)(2)(iv)”. I’m happy, for example, to place my nonconventional beds on almost any slope angle. The basic module for all system needs to be built level, and for the BSF this can easily be achieved by starting with a level base of filter sand on any slope. Treated effluent from a BSF moving through soil downslope and away from a basic module does three things:
- it evapotranspires vertically upwards,
- it infiltrates vertically downwards, and
- it moves parallel to the slope in the apron soil.
The first two ought to dominate over the third if the system has been sized and designed properly. But the steeper the slope below the module, the faster the treated effluent moves parallel to the slope through the apron soil, and the less chance it has to evapotranspire and infiltrate. So, there is more chance of leakage from the lower, outer limit of the apron.
So, we should perhaps lengthen the flow path. But then, we are dealing with highly treated effluent so downslope leakage should not be too much of a problem. Perhaps one answer is to install a cut-off drain along the lower edge of the apron.
Read what the US EPA has to say about BSFs and intermittent sand filters.
Equally interesting is the US EPA Fact Sheet on Mound Systems (http://water.epa.gov/infrastructure/septic/upload/mound.pdf) which states that,
“Mounds can be constructed on sites with slopes up
to 25%. The slope limitation is primarily for
construction safety, because it is difficult to operate
equipment on steep slopes, and they pose a
construction hazard. From a hydraulic perspective,
mounds can be positioned on steep slopes.”
Converse & Tyler in their 2000 version of the Wisconsin Mound design & construction manual also state:
“Site slopes are not a limitation for on-site soil units. Slope limitations are primarily for construction safety concern. Systems on steep slopes with slowly permeable soils should be long and narrow to reduce the probability of toe leakage.
A 25% limit is recommended which is based on construction concerns and not soil hydraulic properties.”
I’m not sure that a downslope cut-off drain is the answer though; I tend to favour construction a low bund downslope of the land application area (there’s usually plenty of soil on a construction site to do this) in order to catch any toe leakage and protect downslope env/public health values.
Evergreen vegetation plantings on both sides of the bund will also help to mop up any excess moisture too.
Ooops. Pressed submit too soon.
Developing on the theme of “Systems on steep slopes with slowly permeable soils should be long and narrow to reduce the probability of toe leakage”, going back to AS1547.2012, this simply means using a smaller linear loading rate.