Water Dispersal - From Drain to Culvert Outlet & Race

In the section on drains and culverts, we have already seen how the work of clever surveyors and engineers, highly skilled stonemasons, overseers and convicts were able to make allowance for the quantities of water which would come down in a rainstorm. Instead of flowing over the roadway, the water was diverted into the carved drains, through the box culverts to spill out in cascades from the culvert outlet over its spillstone. From there, it was channelled by the stone races and out into the valley where it would do no harm to roadway or walls. John Rigg was able to photograph such circumstances with delightful effect. It is now time to look at the races which conducted the water away from the road and the walls of structures supporting it.

In the photograph, the spillstone extends beyond the line of the wall to ensure that the water falls in a cascade onto other stonework instead of running down the wall itself causing erosion. One of the design factors in setting the spillstone would have been to estimate the extension beyond the wall to produce a good clearance. The water still had to be conducted away in some sort of spillway or gutter and that is where the assistant surveyor had to take into account the way the land fell away from the structure through which the culvert had been placed. It is worth starting with places where the land fell away sharply from the prepared road, as on the Devines Hill section.

It is interesting to start with complex culvert exits and spillways. Those would have to be the spillways which were built into Devines Hill buttresses to let the water cascade down the spillway and into a stone-walled race at its base. The buttress with its culvert exit has been shown in the earlier culvert exit section. In the photograph shown, taken from the valley end of the race with the buttress in the distance, the assistant surveyor has realised that the flow of water in a rainstorm would be great so that the channel is wide and well constructed with a stone base and stone side walls.

The picture immediately left is the spillway from a culvert exit on the Mt Baxter section of the Great North Road between Clare's Bridge and Hungry Flat. There are many examples of spillways from culvert exits along this part of the road where the land fell away from the roadway.

Where the water falls less height from the culvert exit than on the Devines Hill buttresses, there are, as it were, steps over which the water would cascade down. That is the case in the Ramseys Leap spillway shown in this picture. Although rather obscured by greenery, the way down from the culvert exit is evident. The stonework in the wall structure, the culvert exit and that forming the spillway are clearly of a high standard. This work would have come under the supervision of Heneage Finch who, by that time, was the assistant surveyor in charge of the work around the Wollombi area.

As the road proceeded north, the countryside was less mountainous along the actual route and there are further examples of culverts where it was only necessary to conduct the water away on what was essentially a creek bed. The pictures above, left to right, have been taken on the culvert exit sides for the spillways at Bucketty, Fernances Crossing and Murrays Run. Stones along the bed ensured that the water did not spread out in all directions around the culvert exit.

Before passing on to other topics, there is an interesting double culvert, well on the way to Maitland, at Sawyers Gully. It had been somewhat overgrown with weeds and small shrubs. Maintenance work was carried out, under the guidance of the Convict Trail Project Director, by a group about which there will more information given in another section. The result is that the double culvert is visible and the bed along which the water runs has been cleared.