NIWA were commissioned by Auckland Regional Council (ARC), North Shore City Council, Rodney District Council, Waitakere City Council, and Transit NZ to: 1) predict the impact of proposed rural residential developments upon sediment loss in the Waiarohia catchment, and to determine whether the proposed earthworks are to result in a significant change in sediment load compared with the existing land use. Additional aims of the study were: 2) to assess the effectiveness of specified sediment control measures associated with the proposed earthworks; 3) generate information for feeding into the wider Upper Waitemata Harbour (UWH) contamination accumulation study; 4) provide validation (or not) of the need for modelling sediment yield in the UWH study; 5) provide an assessment of the risk of excessive sedimentation occurring in the Upper Harbour as a result of the proposed developed in the Waiarohia catchment
The study focuses upon a 201-hectare parcel of land that lies to the south of the Whenuapai airbase. A physically based field scale computer model GLEAMS (Knisel 1993) is used to predict sediment generation for the existing pastoral landuse and the earthworks phase of the proposed development, both with and without sediment control measures. Three control measures are simulated, both singularly and in combination. These are; sediment retention ponds, a restriction of the earthworks season with earthworks stabilisation in the off-season, and riparian grass buffer strips.
The results are derived in a 2-stage process. The predictions from stage 1 enable a direct comparison of the impact of control measures, whilst the results from stage 2, weighted to account for the relative proportions of pasture and earthworks, enable impact assessment over the development area as a whole. Results from both stages are expressed as kg/ha of sediment loss but it is important to note the difference in their derivation since they yield markedly different values.
Simulations are conducted at a 1-hectare scale for the dominant combination of existing landuse and soil, and mean slope angle (pasture, silt loam and 3°, respectively) within the Waiarohia development area. GLEAMS is then re-run using the same soil type (but with the topsoil removed reflecting earthworks practice) and slope angle, with i) bare earth and ii) bare earth with sediment control measures.
The effectiveness of three sediment control measures is assessed both individually and in combination. This assessment is made with respect to both individual storm events and annual averages. These are:
Predicted sediment yields from stage 1 are combined based on the relative proportions of pasture and bare earth (97.7% and 2.3% respectively) across the Waiarohia development area.
In stage 1, eight scenario’s were developed and the predicted mean annual sediment loss predicted (kg/ha/year). The scenarios are:
The proposed rural residential developments in the Waiarohia are predicted to increase sediment loss relative to that under the existing pastoral landuse. A 72% increase in average annual sediment load is predicted across the development area under the worst-case (no controls) scenario. This figure is lowered, however, through the incorporation of sediment control measures, and simulation of all three controls in combination is predicted to increase sediment loss across the area by 12%.
Under only one scenario incorporating a control measure (grass buffer) is an increased sediment loss of > 50% simulated. Retention ponds are predicted to be the most effective single control measure (67% efficiency), whilst seasonal restrictions to earthworks (31%) and a grass buffer (20%) have less impact.
The results of this study are dependent upon the density of ongoing earthworks: During the worst-case stage, ongoing development of 7 houses and 1.65 km of roads is expected for the 201-hectare area. The Waiarohia development area is characterised by a mean slope angle of about 3°, and is therefore relatively flat compared to some areas in the wider UWH catchment. However, the modelling results are broadly applicable to areas of proposed rural residential development across the UWH catchment, particularly those upon alluvial soil and flat or gently sloping land. The sensitivity of some of the predictions to slope angle is illustrated in Table 4, whereby sediment losses upon a 6° slope are substantially higher than those upon 3°.
Predicting sediment loss under proposed development in the Waiarohia catchment, Auckland Regional Council, Technical Publication 218, August 2003. Prepared for Auckland Regional Council by Rob Collins, NIWA