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Acid sulfate soil

Mapping of drainage networks

Most drainage works on the coastal floodplains of northern NSW were constructed around the turn of the 20th century. Another period of drain expansion followed, after a decade of major flooding in the 1950s. With the availability of heavy earthmoving equipment, drain construction and expansion for flood mitigation accelerated during the 1960s and 1970s.

The drains were intended primarily for flood mitigation, but were also used for land reclamation, and drained coastal land generally. Drains were extended, widened and deepened; and hundreds of floodgates and other drain control structures were installed.

With advances in scientific knowledge about acid sulfate soil, it is now recognised that drainage lowers local watertables and exposes pyritic sediments to oxidation, as well as forming a conduit that exports acid, low dissolved oxygen and other acid sulfate products to estuaries.

The location and dimensions of all drains and engineered waterways within the coastal floodplains of northern NSW have been mapped from Tweed Heads to Nabiac. Information available from NSW government departments, local councils and other local reports on the drain control structures (such as weirs, culverts and other structures that significantly inhibit tidal flow or fish passage) have been collated in a comprehensive database that includes digital images, which accompany each map.

The maps and data allow determination of drainage density, intensity, origin and degree of connectivity; location, number, size and condition of floodgates and other structures; floodgate and drain ownership; and the extent of drainage networks associated with known acid discharges.

Part of the map for the Clarence drainage network.

The data is being used, together with hydrological modelling, to predict a range of outcomes and to develop strategies for managing acid sulfate soils.

Mapping methodology and results

All constructed drains and modified natural water courses were mapped by the interpretation of stereo-aerial photographs at 1:25 000 scale. They were then coded for depth, width and height of spoil mound to the nearest 1 m. Natural watercourses were not coded or mapped, unless sections of the natural watercourse had been widened, deepened or straightened. Drains were divided into segments. Each segment was separately coded, and new segments were created where a drain changed direction or dimension, or connected to another segment of the drain. The smallest mappable drain segment was 250 m. Draft mapping was field-checked before digitising.

A total of 61 1:25000 topographic sheets were covered in this mapping from the Manning to the Tweed, inclusively.

Each map is accompanied by an inventory of drain control structures. Details of floodgates, weirs, culverts and other structures that significantly inhibit tidal flow or fish passage are recorded on an accompanying database. Additional structures were visited, photographed and attributes recorded.

Results were compiled for 63 catchment management units and 28 hotspots.

In summary, there are 5,039 km of drains mapped over 744,000 ha. The average drain density for a catchment management unit is 8.5 m/ha with a maximum of 31.7 m/ha. Within the hotspot areas, the average density rises to 73.3 m/ha with a maximum of 285 m/ha. Areas with the highest density of drains are the cane lands of the north.