Flooding and erosion after filling and compacting of previously permeable roadside

Update July 2020:
Our reporting of this issue to Auckland Transport was evidently likely to result in, if anything, curbing and drainage of the roadside, so we withdrew the case via the Auckland Transport call centre.

Summary: Since reporting this issue we have observed no further flooding and at least some run off feeding the uphill margins of Gahnia Grove. We are unable to say whether the run off feeding the forest margin below has been reduced or not, since soil moisture deficit was extreme when winter rains began.

As we perceive the situation, the greatest threat to the maintenance of vegetation in the Reserve at this part of the ridge is any loss of road run-off to the adjacent soil and forest margin below. If in fact the filling and compacting of the roadside has reduced widespread filtration and absorption by soil, we hope for a simple and economical remediation through restoration of run off directly from paved road to permeable surface. Especially in the present drought and expected repetitions due to climate change, the outcome of our restoration trial and the health of the forest and stream below are entirely dependent on winter rain.

Attempts to remediate present flooding and erosion by interference with the soil and vegetation on the steep bank of the current runoff channel would result in the destruction commonly observed to streams, estuaries and steep banks throughout the catchment as a result of channeling (earthen or concrete), piping, rock baskets or rip rap. Hopefully such solutions will not be considered.

UPDATE FRIDAY July 17
Several heavy rains this week, and light rains during which we were present, and no further flooding has been observed.

Runoff in the uphill parts of Gahnia Grove appears to be as expected for this time of year, so hopefully there will not be a deficit and the forest will receive adequate hydration to see it through the summer.

Perhaps the flooding and erosion was due to the blocked drain on the corner of Speedy Crescent?

Nonetheless, it seems likely that a series of small drainage channels from the roadside directly into the Reserve would further increase the absorption and filtration of stormwater, and assist revegetation in the drier areas of forest margin, especially South, ie uphill of Gahnia Grove, which does not appear to be reached by much stormwater at all.

A well-developed ephemeral stream has been disclosed under dense weed. The stream emerges from a steep bank about 30m from the roadside withtout evidence of piping or channelling, meanders fro several metres across the bank, then appears to pursue a steep course in a deep channel through the forest, probably contributing to the Kaipatiki Stream below the boardwalk of the forest path.

UPDATE FRIDAY July 10
The Stormwater Engineers have been in liaison with Parks and the issue has now been escalated to Auckland Transport.

Despite ongoing light rains, there has been no further flooding or erosion observed.

UPDATE WEDNESDAY JULY 1
The meeting with Stormwater engineers is scheduled for 10am Tuesday 7th July.

UPDATE TUESDAY 30 JUNE
We were delighted to receive a call today from Healthy Waters (the new name for the Stormwater department of the Auckland Council), and be updated on current Council policies and procedures in relation to stormwater, suggesting solutions along the lines proposed below are likely to be considered. A site meeting with members of that team is expected tomorrow.

UPDATE MONDAY 29 June
After a period of steady rain this morning we observed the area again, and the flow pattern seen yesterday was being additionally fed by a steady stream of water down along about 100m of roadside edge uphill, mostly channelled in the unfilled section of road edge adjacent a roughly grassed berm, which is slightly inundated, with water pooled in some deep ruts and holes made by vehicles in the grassed area last year.

Following this roadside stream as far as the bus stop opposite Speedy Crescent, where the kerbside is paved, we saw a gutter drain with no apparent movement of water into. We eventually reached Call Centre and asked them to advise Stormwater North, to whom the issue has been apparently escalated as we were advised yesterday would happen, of this additional observation.

Video of roadedge/berm flow, viewed from CHF Bank uphill to the bus stop and gutter drain opposite Speedy Cres
https://youtu.be/wmKqUeM_H7M

SUNDAY 28 June
Recent erosion, and damage to vegetation within the forest, has been seen only in the last few weeks, since recent first rains after drought and after filling and compacting of previously permeable roadside. Close observation throughout this roadside edge at least weekly since June 2018 has revealed no flooding or visible erosion by runoff during that period, although there is some channelling presumably due to run off patterns at earlier stages of road development. Eg We note that this section of road was raised and levelled during widening approximately 15-20 years ago.

New erosion channels have been formed in recent weeks, down the bank and through the forest below, washing away native seedlings, burying juvenile treeferns in piles of debris and topsoil up to 50cmH, and exposing roots of mature trees on the steep bank of the canopy margin, including kanuka 10mH, pigeonwood, ti kouka, mapou and treeferns to about 8mH.

Adjacent to this new run off channel down through the forest are several mature nikau, the only mature nikau in Gahnia Grove (with a few present on Rimu Ridge and Tanekaha Ridge).

The new runoff channel through the forest was observed when it was not raining, and no running water was seen. The recently dumped soil and debris ended some metres above the boardwalk of the forest path, presumably running underground or through deep debris from that point to the one-metre-wide, several-metre-deep chasm, or stream tributary, below the boardwalk.

Our first consideration was whether the eradication of shrub and vine weeds in June-December 2018, or of kikuyu, the eradication of which was extended a metre or so in this area during 2019, had contributed to increased runoff. However:

1. Though the water is more evident where there is no kikuyu, there is also current flooding of the mown kikuyu slightly uphill, particularly in a well-trodden line evident through the centre of the mown swathe since the summer drought when even the kikuyu became sparse.
2. There is no indication of increased runoff - or indeed any - in the adjacent areas of Arena and Apron, where kikuyu eradication was similarly extended.
3. The major removal of shrub and vine weeds was completed by January 2019, and there was no evidence of flooding or erosion after the heavy rains of April 2019 or thereafter.

We note also that the Gahnia Grove zones encompassing the twenty metres of roadside uphill of here, ie the Annexe, Arena and Apron, remain dry, the soil only superficially moist and crumbly, despite similar weed eradication (which is always accompanied in our Methodology by at least superficial replacement of ground cover and soil retention, immediately by plant material placed across the slope in such a way as to retain debris and slow run off, and as soon as possible thereafter by growing plants, eg exotic herbs to begin with).

The volume of water now seen running down the roadside surface, (presumably evident during heavy rain continuing down the steep bank and forest floor below), instead of reaching the mown grass adjoining these areas of slightly higher elevation than the flooded area, suggests those higher areas are currently missing out on much of the gentle widespread winter soil seepage observed in 2018 and 2019, until the apparently impermeable recent roadside surfacing is remediated.

There is no stormwater piping mapped on this side of the road, and we have observed no need for reticulation or detention of stormwater to date, with the vast expanse of mown grass and the forest below to absorb runoff which we presumed was, until the recent surfacing, running through the soil, and being filtered of at least some of the roadside contaminants, and providing much-needed water for the dry forest ridge margin.

We have been pleased to see nearly all trees, juveniles and seedlings in this area survive the present drought so far, and hope to see them develop to maturity, providing shade and lowering the ground temperature in summer both for the comfort of pedestrians and the health of plant life.

We hope for a simple and economical remediation plan by the restoration of widespread filtration and absorption by soil, by returning run off channels directly from paved road to permeable surface. Especially in the present drought, our restoration and the health of the forest and stream below are entirely dependent on winter rain.

Attempts to remediate present flooding and erosion by interference with the soil and vegetation on the steep bank of the current runoff channel would result in the destruction commonly observed to streams, estuaries and steep banks throughout the catchment as a result of channeling (earthen or concrete), piping, rock baskets or rip rap. Hopefully such solutions will not be considered.

This regular volunteer and Reserve visitor is particularly grateful for this recent filling of the deep rut along this part of the road edge, as the level surface between road and roadside berm now allows vehicle access to the roadside when approached from the North, by turning from the median strip instead of having to drive several kilometres via Beachhaven to arrive facing North to park and access the Reserve safely.

A speedy remediation might be channels in the roadside surfacing and if necessary immediate earthen channeling across the grass berm, to reclaim the winter rain for the upper part of the site, to avoid significant ongoing destruction of the restoration regrowth and planting, native forest below, and the Kaipatiki Stream and estuary.

This could be an excellent opportunity for low-cost community-supported vegetative filtration of roadside stormwater run off. We look forward to being further educated about the engineering possibilities for this situation, and sharing our detailed site-specific knowledge of vegetation, soil stability and run off patterns, towards the restoration of roadside permeability or channeling to the abundant vegetative filtration available directly from where this water leaves the road from about 20-50 metres further up the hill.

UPDATE After witnessing the current storm drain overflow from 100m uphill, we believe that with appropriate planting and wild revegetation, Gahnia Grove and adjacent forest margin could, without flooding or erosion, absorb and filter much water currently polluting and eroding the banks of Kaipatiki Stream within the forest.

We have seen many beautiful raingardens have been planted, but all that I have seen have been subject to non-selective herbicides before becoming obliterated by invasive weeds, the wet environment being conducive to vegetation of whatever sort survives the herbicides, often the most aggressive weeds. Gahnia Grove is a site-specific weeding project based on ongoing monitorihng, and could perhaps, with comprehensive planning, implementation and ongoing flow assessment in collaboration with Healthy Waters, provide an example of a successful stormwater absorption.

Current status of the situation should be available from Council using the Reference number 8160034765, for my call, which used the switchboard menu option Water Pollution. A contractor inspected the site with me shortly after, on Saturday 28 Jun, during intermittent light rain following steady rain which totaled 9mm for the day, had totaled 25mm in the previous 24 hours, and had not exceeded 32mm in 24 hours*. A light rain occurred during our inspection, and we then witnessed the roadside runoff flowing in meandering channels through and across the mown kikuyu pedestrian strip, and via numerous channels to 10mm deep through the recent planted native herbs, leaving many plants with all roots exposed and one or two floating.

An unsuccessful attempt was made to advise Parks, via Call Centre, of the current flooding of the roadside path, hoping to get permission and perhaps help to use a spade to channel at least some of the escaping precious water back towards the dry parts of the site, but after reaching Call Centre the first time we were accidentally cut off, and after waiting five minutes to get through again we abandoned the attempt, since there was no evidence of danger to people.

A short video made at the top of the bank shows water flowing through the native and benign exotic herbs currently replacing the kikuyu in Gahnia Grove's "kikuyu margin": https://www.youtube.com/watch?v=tsr0Rtql85w

Some preliminary observations have been made of the effects of the runoff flow through the mown kikuyu and ex-kikuyu margin of CHF Bank , down the lower half of CHF Bank, through the forest below CHF Bank , and on Towai Bank, down to the boardwalk and stream below.

*Based on a domestic rain gauge 500m away. Metservice data for “North Shore” reflects its closest weather station at Whangaparaoa

Julkaistu kesäkuu 28, 2020 12:22 AP. käyttäjältä kaipatiki_naturewatch