Stream bank sensitivity to physical and anthropogenic factors: an analysis of the Waikato River

Abstract

River channels are inherently unstable, dynamic, and variable. They adjust their morphology, sediment load, bed material size, and hydraulic characteristics in response to changes in a range of properties including discharge, sediment availability, and valley slope. This creates problems when attempting to characterise the sensitivity of the banks to physical and anthropogenic factors. An investigation was undertaken along the Waikato River: to identify the geomorphic processes and mechanisms of bank erosion operating; to assess whether the river could be classified into geomorphically distinct reaches; and to determine the sensitivity of the banks in these reaches to physical and anthropogenic influences. Channel width adjustments occur within a wide range of geomorphic, hydraulic, and anthropogenic contexts. Therefore, the rates of bank retreat and channel change vary at all spatial and temporal scales. Analysis of change along the Waikato River was consequently carried out: spatially at the catchment, reach, and site-specific scales; and temporally over long, intermediate, and short timescales. Daily flows downstream from Lake Taupo are now dependent on regulated lake outflows. However, the discharge regime within the Waikato catchment is ultimately controlled by the spatial and temporal distribution of rainfall, moderated by the influence of evapotranspiration, geology, and land use. Unconsolidated pumice, volcanic ash, and ignimbrite (i.e., the middle Waikato) absorb the majority of rainfall resulting in steady, spring-fed base flows. These contribute a larger fraction of median and low flows in the Waikato River. Catchments of impermeable sedimentary rock, like the upper Waipa, typically have a high proportion of storm runoff and fast-forming floods. These areas therefore have a significant influence on high flow conditions. The Waikato River was initially divided into 30 geomorphically consistent reaches. These were determined on the basis of field investigations; topography; and significant changes in channel gradient, geology, land cover, and hydraulic properties. Statistical analysis of the characteristics of these reaches showed that 12 distinctive clusters exist. Reaches within each cluster are expected to behave similarly under the same hydrologic or climatic conditions, or in response to a change in the factors influencing erosion. As a result, the classification scheme provides a reliable predictive and analytical tool for further investigation of the river system. Given the unconsolidated nature of much of the material forming the banks of the Waikato River, fluvial entrainment is the dominant bank erosion process. Field reconnaisance and aerial photographic analysis of erosion sites along the river suggests that erosion occurs where meander bends are tightest, channel gradients are steep, and bank material is weak. Such sites reflect higher energy (high flows and/or intense rainfall) conditions. However, rates of bank retreat within the system are not extreme when compared to other rivers. Even between Aratiatia Dam and Lake Ohakuri, where the proportion of eroded bank is greatest, 94% of the banks are stable and covered with vegetation. Present day retreat rates at several actively eroding sites are comparable to the average rate over the past 2000 years within the same river section. Anthropogenic activities, such as riparian management and landscape change, have been associated with relatively short-term increases in erosion. At specific sites along the river and hydro lakes the effects of changes in flow; near-bank velocity; bank geology, vegetation, geometry, and material properties; wind-generated waves; and vessel-generated waves on bank stability were measured and analysed. These results show that the extent of erosion at each site is controlled by a unique combination of factors relating to material and energy distribution. Maximum near-bank velocity, within the range of flows regulated for electricity generation, is not a good indicator of sediment entrainment or transport along the river. Wind-generated waves are an important cause of erosion along the banks of the hydro lakes, particularly where maximum fetch lengths coincide with maximum wind speed and/or the prevailing wind direction. Where powered craft travel close to the shore e.g., in the narrow riverine reaches or near jetties and boat ramps, the higher energy waves they create also cause erosion. The magnitude of wave erosion depends on the resistance of the bed and banks (controlled by sediment, vegetation, and profile characteristics), the conditions under which the waves were generated (water depth; channel width; vessel size, displacement, and speed; the distance from sailing line to shore; and frequency of vessel traffic), and the characteristics of the resulting waves (wave energy and frequency of wave impact). The Waikato River is a complex and dynamic system within which channel adjustment is a natural and ongoing process. The stability of the riverbanks, both temporally and spatially depends on the balance between the forces motivating (associated with the fluid flow) and those resisting (associated with the channel boundary) erosion. Anthropogenic activities have, and continue to, affect the natural balance between these forces. Therefore, understanding fluvial and "coastal" processes, their interactions, and the geomorphic context within which they occur, are essential for the effective management of the riparian zone of the Waikato River.