Steep Slopes and Sandy Soil: Drainage on Bellevue Hill

There’s a part of Bellevue Hill history that most residents are not aware of, but still impacts their homes in the present day. The preparation of the Cooper Estate land on the north-eastern slopes of the suburb in the early 1900s was an actual engineering problem because of the sandy soils and the steep topography. Woollahra Council did not permit the blocks to be sold until they were convinced that careful work had been carried out to ensure the stability and good drainage of the blocks. That is, the drainage issues that were known from the outset at Bellevue Hill remain the same today, and the landscape that caused them in the past continues to influence drainage patterns today.

The terrain: steep, sandy and rocky

As the name suggests, Bellevue Hill is a hill: undulating, steep in places and formed on the Hawkesbury sandstone which underlies much of the Eastern Suburbs, with shallow soils and rocky outcrops. The hills slope towards the harbour. The steep slopes, sandy soil over sandstone and the rocky ground create a unique character in the drainage systems of the suburb, and a few common problems.

Slope and drainage fall

The correct fall in drains is used to remove the waste. Drains on the steep blocks of Bellevue Hill can move water at very high speeds, depositing solids behind them that form blockages, and drains across a steep, terraced site may experience slow-downs (or sags) and level-offs (or flat spots) down the hill, which are more likely to block repeatedly.

Sandy soil movement

Sandy soil is more prone to shifting, washing and settling than stable clay or solid rock. Over the decades, movement stresses rigid old pipe joints, placing them under tension and cracking them, allowing roots to enter and groundwater to flow in. Cracked pipes can also wash in fine sand and cause a build-up.

Stormwater on a hillside

A steep suburb will have water running rapidly downhill when there is a lot of rain, building up volume as it goes. The increased runoff on properties at lower elevations may overwhelm storm water drains, particularly if the drains are partially clogged. Water harvesting and distribution of roof and surface water is important, especially on steep slopes to stop water entering the house, but also to ensure ground stability around the foundations.

What slope does to water pressure

The other side of all that elevation is water pressure, it changes with elevation. Lower properties on the hill may be able to achieve a higher mains pressure, while higher properties on the ridges and at the higher elevations around Bellevue Park may have lower mains pressure. Neither extreme is a desirable option. Constant high pressure can cause pipes, fittings and flexi hoses to become stressed, and is a contributing factor to the likelihood of bursts. A pressure limiting valve may be the solution, but many older homes are not fitted with such a safety device. Low pressure up high means weak flow and underwhelming hot water. If you feel like you are low or high on pressure, it’s good to check.

Getting drainage right on a Bellevue Hill block

It is a lesson learned by the original engineers, that “On this soil drainage must be done properly”. In cases of ongoing blockage, storm water accumulation in places where it shouldn’t, or water intruding into a lower level, the steep, sandy soil is typically a part of the problem. A camera inspection of drains and evaluating the management of stormwater on your block will demonstrate what is occurring and what is in need of attention. A plumber who knows Bellevue Hill’s slopes can determine the correct repair for the situation, so that the sorting process can be carried out properly.

The bottom line

The steep, sandy and sandstone topography of Bellevue Hill is what makes it a favourite place to live for its residents and its views, and was also a problem that the suburb’s builders had to solve as they began constructing the streets. That challenge hasn’t gone away. Where the drains continually clog, pressure is typically not normal or storm water is causing issues, the terrain is likely at play. The only way a hillside house will be trouble-free is to have drainage and pressure evaluated and corrected as the original engineers would have.