Irrigation is being blamed for intensifying flooding during heavy rain, raising issues about the long-term effects on roads, Waitaki District Council roading manager Michael Voss says in a report on the district's August floods.
The council is facing spending about $664,000 on repairs to 141 roads and two bridges damaged in four separate heavy-rain events that started on July 30 and finished on August 23.
Mr Voss said the floods were managed by the council and its contractor, SouthRoads Limited, in an "effective and cohesive manner", with few complaints.
However, the floods highlighted several issues the council needed to consider for the future to cope better with more events.
One was saturation of irrigated farm land which intensified the effects on river flows and roading networks from heavy rain, the ground lacking the ability to soak up any excess stormwater.
This raised the issue of mitigation, including raising roads, developing lifeline alternative routes and seeking restorative money to spread costs.
The council also had problems with culverts, run-off from fields now making them irrelevant or too small.
Heavy, wide agricultural vehicles such as large tractors or machinery running down the edge of roads and water tables caused blockages and affected drainage, he said.
Waitaki's narrow roads exacerbated the problem.
Culverts had been inspected and works proposed.
Mr Voss also raised the issue of who was responsible for maintaining riverbeds so they were not blocked by vegetation, forcing water on to roads.
One example was Gards Rd, next to the Otiake River, which was badly damaged because willows and scrub blocked the riverbed.
This also affected SH83, the river flowing over the highway east of the bridge.
Options were being discussed with Environment Canterbury and the Department of Conservation, but a solution had been found by having gravel removed and stockpiled by a contractor at no cost.
