By late July this year we’ve already exceeded our entire rainfall for 2015, and for now, the rain shows no signs of abating. This is fantastic news for our revegetation efforts, and our dam is now almost full for the first time in two years. With heavy rains – we managed to top 100mm (4 inches) in a single day – it’s also a chance to test the effectiveness of the erosion control strategies we’ve employed.
With significant erosion in some key areas of the property, we’ve worked to adapt erosion control strategies such as those practised by Bill Zeedyk and Craig Sponholtz (see April’s Waterway Restoration workshop/working bee and our Resources page for more information). In particular, we’ve constructed Zuni bowls, for arresting headcuts, and One Rock Dams, to slow water flow, catch sediment and gradually lift the floor of erosion gullies. After the recent deluge, we toured the works to see how we went. The Zuni bowls have had mixed success: those in relative stable locations have been effective, those in dispersive soils have been unpredictable. The One Rock Dams (ORDs) have been generally successful, if swamped by sediment!
The impact of 100mm of rain in one day is significant: exposed areas lose significant amounts of soil (some areas of gully floor had almost 30cm of freshly deposited sediment), and areas of dispersive soil go berserk, collapsing in all directions. For some of these areas, we’re continually seeking further advice, but for those we can manage, we monitor and tweak over time, and try to “let the water do the work” in healing the landscape.
One Rock Dam #1: Dam Creek
Our first One Rock Dam was constructed below a bare, erosive gully wall in January 2015, and after the dry winter of that year it was showing promise at holding moisture and catching sediment. After the 100mm-in-a-day experience of this winter, it’s totally swamped by sediment and ready for it’s second layer of stone to continue to raise the gully floor. On reflection, this dam should have been about three times wider (aiming for about 1.5m) to more effectively hold sediment.
One Rock Dam #2: Kalta Creek boundary
While ideally erosion control and water harvesting efforts should begin at the top of a watershed, in addition to our other strategies, we thought it useful to construct an ORD at the bottom of our watershed in an effort to limit the amount of sediment carried off our property. This ORD was constructed from rubble gathered from nearby historic dumping, and went through a couple of tweaks to increase its effectiveness. Following winter rains, it’s effective at holding sediment.
One Rock Dam #3: Kalta Creek, clean-up zone
This dam was constructed by our amazing community of volunteers below the main clean-up area at 2016’s Waterway Restoration workshop/working bee. It utilised dumped construction rubble from nearby, and once again it’s working well holding sediment from upstream and slowly raising the gully floor.