Road design

Table of Contents

3.11 Blockage of Culverts

The likelihood of blockage should be considered for all culverts. Blockage can occur through siltation or by debris (such as vegetation). To assist in preventing and/or minimising blockage, the culvert grades should match the stream grade.

During the times of flood, the water contains silt, vegetation and other debris from the catchment. The designer should therefore consider the impacts on the effects of the culvert becoming blocked. Allowance for blockage is commonly provided in the sizing of the culvert (see Section 3.7.4 – Siltation and Blockage).

Blockage reduces the waterway area of the culvert and therefore adversely affects the capacity/performance of the culvert. The result of blockage is typically:

  • an increase in upstream peak water levels/flooding
  • an increased potential for water to overtop the road
  • an increased risk of failure to the road embankment/culvert.

Silt deposits and some debris can be detected and removed during normal maintenance processes at times outside of rainfall events and therefore these deposits/debris would not impede any flows; however the effectiveness of this measure is highly dependent on the efficiency of maintenance in the area.

Where debris blockage during an event is considered likely (typical in catchments that contain significant woody riparian vegetation), larger culvert sizes may be required, in accordance with the extent of adverse impacts that could occur to the roadway or to surrounding properties.

Where large or long branches and/or tree trunks are a possibility, sloped extensions to piers, as shown in Figure 3.14 can be used to ‘turn’ long objects into the culvert barrel.

Figure 3.14: Flood water flowing into box culverts

Source: DTMR (2010).

Designers must consider the potential for and impacts of blockage for each catchment/culvert installation and, where impacts are considered unacceptable, include appropriate mitigating treatments in the design.