INFO: Adjusting the wetted area for cobbles and boulders in the soil profile The wetted area of a trench or bed required to accept the wastewater is obtained from an adopted TAR, which is in turn derived from a measured or estimated permeability. There is no accurate way of knowing the extent to which the measured permeability reflects the nearby presence of cobbles and boulders, and there is no method yet available which allows an assessor to reliably adjust for cobbles and boulders when estimating permeability from a textural examination. Trench assumes that cobbles and boulders in the soil profile reduce the permeable volume available for wastewater absorption. It makes an adjustment (an increase) to your adopted wetted area,

For example, for 10% cobbles in the soil, and an adopted wetted area of (say) 300 square metres,

If site conditions indicate that this approach is not appropriate, you can negate the adjustment by entering a smaller adopted wetted area, and on the Rank Option page for cobbles alter the limitation by changing the default rank.