Respecting waterways and land.

On the 27th of April 2011 the Press carried an article featuring landscape architect Di Lucas (page A5). Entitled Rebuild must ‘respect’ waterways, it discussed how parts of the inner city had been built on top of tributaries to the Avon, creating hidden vulnerabilities. To quote from the article: “When transposed on the present-day central city, the waterways mirrored some of the areas of worst earthquake damage.”

The article lists a few of the inner city streets where there had once been streams, and which have now experienced significant land and building damage. A map showing the inner city in the 1850s, and the tributary streams, accompanies the article. An on-line version without the map is here.

Focused as it was on the inner city, the article did not mention the situation in the suburbs. We are most likely to find the same situation, and consequences, occurring there. For those who want to look at this on a broader scale, while waiting for the geotech reports in June or later, there are two items worth tracking down.

Black Maps

The first to look at is a copy of the original “black maps”, or early maps derived from them (such as the one used in the Press article). The black maps were published in 1850s, based on the work of early surveyors. Unlike today’s surveys, these maps do not contain micro levels of detail. What they do provide is a good description of the nature of the various areas within the city. Some areas have more detail than others, which are only broadly described (e.g. between Travis Road and Lower Styx Road the whole area is described as “swamp, Manuka scrub fern, swamp hills, manuka scrub and fern, sand hills”. The Avonside part of the map was published on the blog in March, here.

The maps cover everything inside the area starting at the mouth of the Waimakariri, over to the back of the airport, across to Hornby, Halswell and the bottom of the Port Hills, through Cashmere, then follows the course of the Heathcote River and the land on either side, to Opawa, Ferrymead, the Estuary, Redcliffs and the Sumner bar.
The early maps don’t give us a means for DIY geotech analysis, but do show what the land was originally like. When coupled with EQC’s geotech results we can deal knowledgably with a few basic issues:

  • why some people do have to go
  • why some people should consider going instead of staying (or vice versa)

And ask the questions:

  • why were some developments or constructions allowed to proceed?
  • what developments and construction can be done safely throughout the city and the vulnerable suburbs?

A PDF map, based upon the black maps, is available from the CCC website here.  The map can be enlarged quite significantly to examine just one small part of the city.

Swamp City

Christchurch – Swamp to City, published by the Christchurch Drainage Board in 1989 (author John Wilson, ISBN 0908714041, copies at most libraries) is a history of the Drainage Board from 1875 to 1989.

Accompanying the history of turning Christchurch from swamps to dry land are interesting photographs of flooding in Christchurch at various times, and some useful maps. The map facing page 16 shows the height of the city using 1 metre contour lines. Its amazing just how low lying the eastern suburbs were back then, and now even lower. The fold-out map inside the back cover is a compilation from the original black maps overlaid with streets. Identical, or very similar, to the PDF version mentioned above.

NOTE: Di Lucas has a website

Earth flood barriers along Avonside Drive

Flood protection work has been extended further up Avonside Drive. An earth barrier now runs from just below Retreat Road, at the Dallington Bridge end, around the loop stopping just short of the western end of Morris Street.

At the eastern (downstream) end of Robson Ave the barrier is about 1m above the level of the road.

The first two photographs show the area as it was a few days ago. The trees are being pruned to avoid damage from the earthmoving equipment.

Tree trimming about 150m upstream from Robson Ave.

Tree trimming at Robson Ave (looking upstream)

These are the barriers as at the 23rd of April.

Upstream from Robson Ave., Banks Ave top right.

Upstream from Robson Ave. (taken a little below the first image).

Downstream from Robson Ave.

Downstream from Robson Ave, Morris Street, middle right.

For the photo buffs who noticed the difference: the top two images were taken with a 20+ year old compact film camera, the rest with a compact digital.

Dallington Terrace – flood protection update.

Work has been done to increase the height of the flood protection on the river bank.

As the photographs show the height of the bank has been noticeably increased, and is at the the point where it matches the height of the bank upstream.

Dallington Terrace, just upstream from Dallington Bridge

Dallington Terrace, just upstream from Dallington Bridge

Dallington Terrace, just upstream from Dallington Bridge

Flood Protection – Dallington Terrace

Parts of the city adjacent to the river have sunk, creating a number of places likely to flood when there are extremely high tides and/or heavy and prolonged rain. Higher than normal tides are expected on Sunday and Monday.

These photographs show the preparations being put in place along a small part of Dallington Terrace, just above the Dallington Bridge. Similar preparations are going on in other parts of the Eastern suburbs, as a precaution.

River bank protection, Dallington Terrace.

Flood protection sand bags, just in case.

House on Dallington Terrace near the flood protection works.