A 60-metre-high slip is threatening the Whang\u0101rei Whau Valley Dam’s emergency spillway.<\/p>\n
The hillside at the top of the slip is falling away with a three-metre deep and half-metre wide crack at its top, to the north-east of the dam accessway and recreational carparking area.<\/p>\n
The crack is about 30 metres long. A further secondary “relaxation zone” above the slip is also moving.<\/p>\n
Whang\u0101rei District Council (WDC) infrastructure committee chairman Simon Reid said the situation at the dam was “quite a concern”.<\/p>\n
The dam and surrounds have been shut to the public since 28 March due to public access safety concerns.<\/p>\n
WDC water services manager Andrew Venmore said the slip had been kicked off with Cyclone Gabrielle’s huge rains and then ongoing wet weather which meant groundwater levels were at a record high.<\/p>\n
The one hectare slip was heading down the hillside towards the spillway at its base.<\/p>\n
The slip covers the equivalent of about 1.4 rugby fields.<\/p>\n
Venmore said there would be significant consequences for the dam if the whole slip failed catastrophically and came down blocking the emergency spillway, but there was a very low chance of this. It would have to happen in conjunction with a major one-in-500-year-plus weather event, which would potentially create problems with the amount of water in the dam building but having no emergency spillway to overflow.<\/p>\n
He said the threatened emergency spillway was designed to cope with a one in 10,000-year weather event and had never been used since the dam was built in 1969.<\/p>\n
The emergency or auxiliary spillway is Whau Valley Dam’s overflow backup. The dam’s primary is its giant plughole-like bellmouth spillway, designed to cope with a one-in-500-year weather event and in the dam at the western end of its front wall.<\/p>\n
Venmore said over time the slip’s soil would dry and stop moving, but it was currently very wet and still moving inches a week.<\/p>\n
“It’s slowed down a bit but is still moving,” he said.<\/p>\n
Venmore said the slip did not mean the whole dam was about to fail with people downstream at risk as a result.<\/p>\n
The more than six-decades-old dam is Whang\u0101rei’s main drinking water source, storing 1.8 million cubic metres of water, weighing 1.8 million tonnes.<\/p>\n
Venmore updated councillors on the slip at a briefing meeting on Thursday.<\/p>\n
WDC has put 14 metre deep boreholes into the slip to monitor action, using a helicopter to lift the rig into place.<\/p>\n
There have been delays getting contractors on site to do the bore drilling, due to major demands on their time from around the country due to Cyclone Gabrielle damage.<\/p>\n
Data is being collected from the boreholes and the slip monitored weekly.<\/p>\n
Venmore said he was aware some visitors were frustrated about not being able to access the dam and surrounds for recreation.<\/p>\n
He said the council was working as quickly as possible to remedy the situation.<\/p>\n
Forestry on the slipping hill was harvested by the council about five years ago.<\/p>\n
Reid said the hill’s current pampas grass vegetation covering sent warning signals.<\/p>\n
“Pampas grass doesn’t let water get away,” Reid said.<\/p>\n
Venmore said the hill’s profile was lower than it was originally due to a lot of its soil being used to build the earth dam.<\/p>\n
Meanwhile, a second 2009 slip in the dam’s south western end is causing issues, its sediment bringing extra water quality processing issues for the council.<\/p>\n
Venmore said dam monitoring included regular drone flyovers.<\/p>\n
Source RNZ<\/p>\n","protected":false},"excerpt":{"rendered":"
A 60-metre-high slip is threatening the Whang\u0101rei Whau Valley Dam’s emergency spillway. The hillside at the top of the slip is falling away with a three-metre deep and half-metre wide crack at its top, to the north-east of the dam accessway and recreational carparking area. The crack is about 30 metres long. A further secondary […]<\/p>\n","protected":false},"author":6,"featured_media":5415,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[174],"tags":[],"class_list":["post-5414","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-nz-news"],"amp_enabled":true,"_links":{"self":[{"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=\/wp\/v2\/posts\/5414","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5414"}],"version-history":[{"count":1,"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=\/wp\/v2\/posts\/5414\/revisions"}],"predecessor-version":[{"id":5416,"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=\/wp\/v2\/posts\/5414\/revisions\/5416"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=\/wp\/v2\/media\/5415"}],"wp:attachment":[{"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5414"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5414"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/thebuzz.apnanetwork.co.nz\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5414"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}