Projects » Stream Sediment Study

Influence of Land Use on Sediment Production in Wharawhara and Boyd Streams, Katikati – Bay of Plenty Polytechnic/Auckland University of Technology – Barbara Risi, 2008

There is increasing evidence from monitoring that link sediment runoff with ecological effects in estuaries.  Two streams were selected near Katikati as suggested by Andrew Jenks. The state of this catchment has been of concern, and this research was carried out to identify the effect of different land use practices on the effect they have on sediment production. The Uretara Estuary Managers are concerned with the sediment in the harbour, and this study helped to quantify their observations.

Aims and Objectives

 This report investigates sediment source within Wharawhara and Boyd Streams, Katikati, as a factor in sediment loading into Tauranga Harbour via the Uretara River. 

  • A comparison was made between sediment production from traditional land use (horticulture and agriculture) in comparison to that of an operational quarry.
  • The objective was to collect water samples from 3 aligning sites on both streams, on 15 occasions (representing 3 different water volume rates) and measure the quantity of suspended solids within the 1 litre samples collected, to enable comparison of the sediment inputs between these sites.
  •  The relationship between water level and sediment inputs was examined to determine the effects of rainfall on movement of suspended solids along the stream network.
  • The average volume of water in both catchments was measured to evaluate the approximate amount of sediment transferring out of these two streams and filling the downstream catchment and estuary.

Results:

sediment results 2

  

Figure 1 Effect of rainfall on the quantity of suspended solids in the stream (CI± 95% n=15)

 

sediment 2

  

Figure 2  Kilograms of suspended solids transported through water column, potentially delivered to estuary, over 1 day (24 hrs). Quarry Stream > 800kg, Boyd Stream <200kg.

Discussion

Boyd Stream showed no significant rise in suspended solids along the length of the channel, with the recorded varience of mean weight of suspended solids increasing only 0.0025 grams per litre over the entire 15 samples.  Riparian margins assist with reducing sediment inputs from bank erosion (Reeves, et.al. 2004), as well as prevent livestock from entering the stream, causing local disturbances. There is a definate correlation between lowered turbidity in the presence of riparian margins over the catchment area (Allan, 1995)The sample sites with the least amount of suspended bedload (B1, 2, 3 & Q1) were relatively unchanged by the increase in water (Figure 1). High rainfall and increased water flow causes the quarry sediment pond to overflow not allowing the wastewater to be retained for the sufficient period of time required to settle out the suspended solids (Caldwell, 2006). The contribution of suspended solids potentially entering Tauranga Harbour via the Boyd and Quarry Stream is shown in Figure 2, and demonstrates the greater degree of sediment production and environmental effects of an operational quarry, in comparison to agriculture and horticulture.

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