Report analyses Motueka's stormwater handling

May 13th, 2013

Motueka's infrastructure "coped adequately" to the April 21st torrential downpour (see our story here), but there are "some areas of concern that need attention", says Community Board chairman David Ogilvie.

David has provided some detailed analysis of successes and failures during the extreme event, in a report to the board to be presented at tomorrow's monthly meeting. Following is the text of David's report.

Showers on Friday 19 April 2013 were followed by steady rain on Saturday, becoming progressively heavier overnight and on Sunday morning. From approximately 3.00pm on Sunday for over an hour, the district experienced a torrential downpour. Over the 48 hour period over 200mm fell with approximately 100mm in the hour or so from 2.00pm Sunday.

The drains were unable to cope with kerb to kerb water (and worse in some places) in Ledger Avenue, Totara Park Avenue, Moffatt Street, Kingstan Place, Greenwood Street and Goodman Drive.

There were particular blockages or drainage issues at Eginton Street and Vosper Street Community Cottages, as well as the Riwaka Valley Road and the Little Sydney/Riwaka area. The fire brigade were busy pumping water after being called out.

The Thorp Drain and the Woodlands Canal were full to capacity, but the Inglis, Lammas, Kumeras Drain had significant problems.

On Sunday, high tide was at 6.00pm (3.2 metres), which held back some drainage. Once the rain eased and the tidal flow reversed, the problems diminished. The drainage system took over.

There remain some areas of concern:

Eginton Street, where it adjoins the developing Te Maatu subdivision. (This may be resolved when the stormwater infrastructure is in place.)
Rear of 79 High Street, adjacent to Primmer Lane and the walkway to the Council’s community cottages. The drain on the rear boundary seems to be flowing in the wrong direction, away from the Primmer Lane. This needs attention.
Availability of sand bags. Where can these be stored? How can they be allocated?
Floodgates at Staples Street, Old Wharf Road, Wharf Road. These do an excellent job in keeping the tide out, but they do not open quickly or sufficiently wide to release stormwater into the estuaries. Can this be looked at? Is it time for the renewal programme to be instigated?
Generally, the drainage infrastructure coped adequately but if another 50mm had fallen, the situation could have been much worse.
The town was fortunate that the tidal flows were relatively low. Initially, the stormwater backed up against the floodgates, but after high tide (6.00pm) the release was reasonably quick. (A higher tide would have had serious repercussions.)
The Motueka Urban Drainage Area raises approximately $500,000 in stormwater rating annually. (The proposed increase for 2013-14 is 6.9%). During the next two years the LTP notes only that a discharge consent is to be applied for. There is no sizeable capital or renewal item mentioned. The current debt for Motueka stormwater remains at $1m.

Overall, the period from Friday 19 April to Sunday 21 April (inclusive) demonstrated a need to improve the stormwater systems to meet the current growth rates within Motueka.

Summarising, David recommended that his report be referred to the Council’s Engineers (Stormwater) for their comment, report and proposed actions (to June 2015); that the board highlights for consideration the capacity of the town’s open drains and operation of the floodgates; and requests that the availability and location of sandbags be publicised.

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