Who would have thought? Construction has begun on the Cashmere Valley Dam. Where and why is a dam required so near Christchurch.

As you pedal your bike up the Cashmere Valley towards the Christchurch Adventure Park (CAP), earthmoving machinery can be seen at work laying down the bed and building up the beginnings of a low earth dam about 4m high, with a base width of about 50m, that stretches from the roundabout on McVicar Drive across the valley to the slopes below Shalamar Drive. This is the final part of the Cashmere Flood Detention Facility designed to minimise flooding in the lower reaches of the Ōpāwaho Heathcote River. Modelling by the Christchurch City Council (CCC) indicates that in a 1-in-50year rain event, the whole finished facility will reduce the number of houses flooded above floor level by 113.

Will this dam create a new permanent lake in the valley? The dam will only retain water in the form a shallow lake during, and for up to 36 hours after, a significant rain event. In normal times, the natural low flow of the Cashmere Valley Stream will pass through the normally-open dam control mechanism. In smaller rain events, ponds will form in the area behind the dam but the flow through the culvert under the dam will be unimpeded. As a rain event becomes more significant, the control mechanism in the culvert will be closed automatically by the flood control system run by computers at Bromley and the dam will fill. Once the rain has stopped falling and the rest of the Cashmere Valley detention system has begun to empty, the dam control system will gradually release the stored water behind the dam into the wetland detention basin below which in turn will be emptied by the Cashmere Valley Drain that ultimately leads into the Ōpāwaho Heathcote River.
.

What happens when the dam is full but it keeps raining? The dam will form a storage basin that holds approximately 213,000m3 of water and it is anticipated that filling will occur once or twice per year. If the storage capacity of the basin is exceeded by continuous heavy rain falling, water will flow over a concrete spillway on the dam crest and flow down a specially prepared slope into the wetland detention basin below.

Has anyone thought about what will be the consequence of the dam bursting? Actually, they have; it is a requirement for any dam design process to model the consequences of the dam failing. The scenario chosen was failure caused by blockage of the spillway and overtopping of the dam during a 1-in-50-year, 27-hour flood event; failure would therefore occur at the fullest possible storage level. The modelling showed that flooding above floor level would occur for five additional properties compared to a scenario with the intact dam in place. However, all these properties, and an additional nine properties, would be flooded above floor level if the lower valley storage basin was in place, but there was no dam. In a dam failure, the increase in water level experienced downstream as a result would be gradual, with a rise of slightly less than 200mm over about two hours.

What about the fish and tuna? The movement of fish and eels throughout a catchment is very important and has been taken into account in the dam design. Since the culvert under the dam is normally open and only closed for a few days at a time in periods of heavy rain, the dam itself does not pose an issue to migrating fish. The culvert through the dam has been designed to enable the passage of fish upstream.

Will the dam reduce the amount of sediment in the water? Short answer: not appreciably although there will be some sedimentation in the wetlands and ponds behind the dam. Unfortunately, the dominant soil in the Cashmere Valley is loess which has a very fine grain and which when mixed with water forms an almost permanent suspension. While some will deposit in the basin behind the dam, once the water is released and starts to flow out of the dam, much of the silt will be re-agitated and transported with the flow. We can expect that the Ōpāwaho Heathcote River will flow at a high level for longer after a significant rain event as the detention basins will take an appreciable period to empty: the river will be coloured brown during this high flow, caused by loess from the Cashmere and Hoon Hay Valleys.

When will the dam be completed? In perhaps an excess of optimism, the CCC anticipates that, depending on the weather, construction will be completed by September 2023. There will be substantial planting of the wetlands below and above the dam in the next couple of seasons after construction is completed, and continuous maintenance of the plantings will be necessary after that.

You can download a set of plans here and keep up-to-date with progress on the CCC website.