In DRAINS you can specify a factor at each pit to represent the reduction in capacity due to blockage by debris that may be washed into a pit during a storm, or be present beforehand because of poor maintenance.

This is an uncertain factor and little research has been conducted on this matter. In the past, it was fairly common to assume 50% blockage for sag pits and a lesser amount, usually 20%, for on-grade pits. In Queensland, the widely-used Brisbane City Council inlet capacity relationships assume 10% blockage for both on-grade and sag pits. This allowance is incorporated into the BCC relationships, so zero blockage factors should be specified in DRAINS when these pit types are used.

The factors used in DRAINS are given as a number between 0.0 (no blockage) and 1.0 (complete blockage). Thus a value of 0.2 would result in a 20% reduction of the inlet capacity calculated from the factors provided for the given pit type. You can set default blocking factors for sag and on-grade pits in the Project Options property sheet opened from the Project menu.

The blockage calculation process used in DRAINS until 2005 simply multiplied the inflow capacities for an on-grade pit by a blockage factor. This assumed that the same percentage reduction occurs for low and for high approach flows, which is likely to be incorrect. A new blocking assumption was developed that results in a lower reduction at low approach flows. A given approach flow is divided by the blocking factor, the inflow corresponding to the resulting number is found from the inflow capacity table and this is reduced by multiplying it by the factor. An example is shown below for an old type of Queensland pit with 50% assumed blockage.

This procedure is now the only one that can be used, but the obsolete multiplication procedure may be encountered in older DRAINS models where this was chosen. For these files, the following buttons may appear in the Project Options property sheet opened from the File menu.

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These can be used to make a choice.