Field Trip 3 - Loyalty Road Flood Basin

·         Darling Mills Creek, North Rocks

·         UBD Map 191 D1

·         Need key for access (from Trust)

·         Good senior site, not suitable for junior classes

·         Sewer overflows and weed invasion

·         Coachwood and Blue Gum vegetation communities at site (endangered)

·         Good transect site (shale – sandstone – rainforest)

·         Good macro-invertebrates

Site Information

The basin was constructed by the UPRCT in 1995/6 at a total cost of $9 million. It is the largest flood-retarding basin in the NSW. The 30 meter high wall has significantly reduced major flooding in North rocks, Northmead, parts of North Parramatta, Parramatta CBD and Rosehill. During a 1/100 year flood event, the basin floods Darling Mills Creek for 2km upstream. This work has lowered flooding in central Parramatta by 1metre. Developers are quickly moving into this newly available flood free land. Households that used to get flooded twice a year are no longer experiencing flooding since the flood retarding basin was installed.

The site has significant Blue Gum Forest Community and Shale Sandstone Transition Forest vegetation communities (endangered ecological community). There are also important stands of Coachwood Rainforest community. (Coachwood is the same species as Christmas Bush).

The construction project included extensive bush regeneration, which continues to be maintained by the Trust’s bush regeneration contractor and periodic detainees under qualified supervision. During construction topsoil was removed and stored. On completion of the wall the topsoil was replaced and bush was replanted with indigenous species.

Problems

There are problems with sediment build up and weed invasion upstream of the basin. Accumulating sediment has been identified in the Environmental Impact Statement (EIS) as Hawkesbury sand from building sites in the catchment. Gabion baskets have been installed to stabilise streambank erosion. There are problems with erosion and slip along the path upstream of the wall. There are many sewer overflows at the bottom of the creek, below the wall. Mosquito fish (Gambusia) have invaded the lower creek. There is a stormwater basin at the entry to the area. The impact of weed invasion on natural bushland is clearly visible.

Site Walk

Walk down the tarred track to the top of the retarding basin wall.

1. Walk onto the top of the wall. Look upstream.

§         Give at least two reasons why engineers may have chosen this site for a retarding wall.

§         Name the suburbs that are upstream of this wall.

§         What is happening in the suburbs upstream of the wall which has increased the likelihood of downstream flooding?

§         Name the suburbs that are downstream of the wall.

2. Draw a cross section of the wall. Why is it much wider at the base?

Continue walking down the track to the base of the wall. Be careful if there has been recent heavy rain.

3. Describe the main vegetation types you see. (The forest communities in the Darling Mills Creek are Blue Gum High Forest and  Sydney Sandstone Gully Forest.)

§         Do you think this would make a good habitat for native animals?

§         Is there any indication of human impact on the forest community?

4. Draw a transect at right angles to the creek.

§         Use the height of the wall to estimate the height of the cliffs.

§         The creek bed can be paced out to estimate distance.

§         Mark the creek bed, rock exposures and main vegetation types.

§         What is the main rock type exposed here?

5. On the western bank, in the rock wall there are two sewage overflow points.

§         Why does the sewerage system need overflow points?

§         What impact would an overflow have on water quality in the creek?

6. Darling Mills Creek is one of the few bushland corridors in the Upper Parramatta River catchment which is almost ecologically intact.

§         Why has this sub catchment fared better than other sub catchments in the district?

§         Do you think it should be preserved in this natural state?

Walk through the tunnel at the base of the wall. Take heed of the safety warning signs.

7. Even though this sub catchment is in relatively good condition it still shows signs of human impact.

§         Look in the creek bed. Where do you think all the sand in the creek has come from?

§         Note the weeds along the creek bank. Identify some of them. How have these weed species found their way into the creek corridor?

§         Why are weeds a problem?

§         Further upstream there are wire baskets of stones (gabions) along the creek bank. What purpose do these serve?

Site Activities

o        Water quality monitoring,

o        Habitat assessment,

o        Water bugs

o        Transects, quadrats

o        Mapping