Geomorphic impacts of a 100-year flood: Kiwitea Stream, Manawatu catchment, New Zealand
Introduction
The impact of floods on channel morphology is highly variable. Some major floods produce catastrophic change (e.g. Schumm and Lichty, 1963, Baker, 1977, Lisle, 1981, Gupta, 1983, Miller, 1995), while others have little effect (e.g. Costa, 1974, Costa and O'Connor, 1995, Magilligan et al., 1998). Floods of similar magnitude and frequency may therefore produce dissimilar morphological response, even within the same catchment (Costa, 1974, Nolan and Marron, 1985, Nanson, 1986, Miller, 1990, Magilligan, 1992, Butler and Malanson, 1993, Pitlick, 1993, Costa and O'Connor, 1995). Wolman and Gerson (1978) suggest the geomorphic importance of an event is a product of an array of factors, including magnitude, recurrence interval, processes occurring during the interval between recurrence and work performed during this intervening period. Therefore, given the variety of processes and boundary conditions, a spectrum of impacts for a given magnitude event in any one catchment is to be expected.
The role of flooding in fluvial geomorphology has been persistently controversial (Lewin, 1989) and much debated since Wolman and Miller (1960) advocated the view that channels were broadly adjusted to frequent events. Thus, for example, Harvey et al. (1979) suggest systems adjust to major flood events (recurrence interval (RI) up to 2 years), which control channel morphology, while moderate events (RI 14–30 times a year) influence adjustments within the overall morphology created by the major events. However, increasingly, the role of the extreme event has been recognized as significant in conditioning channel form (e.g. Reid and Frostick, 1994). Erskine (1994) suggested catastrophic floods (> 10 times the magnitude of the mean annual flood) determine channel capacity, while smaller floods control the form of the channel bed. Similar conclusions were drawn by Pickup and Warner (1976), and Hack and Goodlett (1960) suggested that rare, large magnitude floods could have a dominant impact on some (mountain) landscapes (Miller, 1995). Erskine (1986) also described wholesale river metamorphosis during a series of large floods between 1949 and 1955 in the lower Macdonald River in NSW, Australia, which persisted more than 30 years. Such a response was also observed in the Cimarron River (Kansas) (Schumm and Lichty, 1963). Thus, large floods may either initiate long periods of river instability and give rise to a flood-dominated channel morphology (Hickin, 1983), or they may have little impact on a channel (Miller, 1990).
Richards (1999) suggests the morphological context in which the flood takes place is critical to conditioning the scale of its impacts (cf. Wolman and Gerson, 1978). Baker (1977) argued that there is a high potential for catastrophic channel response in small catchments with highly variable flood magnitudes. Within a broader context, catchment-scale boundary conditions may condition the geomorphic effectiveness of floods (Brooks and Brierley, 1997). Vegetation cover exerts a fundamental control on hydrology and sediment supply and may determine the sensitivity of a landscape (or channel) to flood-induced change, with the possibility of extreme impacts increasing in cleared catchments (Erskine and Bell, 1982, Erskine and Warner, 1988).
This paper seeks to quantify the impacts of a single flood event generated by a “150-year” storm in the Kiwitea Stream, Manawatu catchment, lower North Island of New Zealand, which occurred on 15–16 February 2004. The Kiwitea catchment within the western Manawatu drainage basin has been cleared of native forest cover within the last 150 years. The effectiveness of flooding in this system is thus likely to have increased. The ARI of the flood exceeded 100 years (Fuller and Heerdegen, 2005) and may thus be classified as large using Kochel's (1988) definition (ARI > 50 years). This work examines the extent to which the channel system was modified by this rare event.
The western Manawatu catchment drains the southwestern flanks of the Ruahine Ranges, which here rise to 1643 m, in the southern North Island of New Zealand (Fig. 1). The Range comprises highly fractured greywacke (alternating siltstone and sandstone) and forms part of the North Island's axial ranges (Fig. 1). Uplift and erosion rates are high: up to 3 mm year− 1 and 0.7 mm year− 1, respectively (Whitehouse and Pearce, 1992). Dissected hill country immediately to the west of the ranges forms much of the 254-km2 Kiwitea catchment. This is located in the eastern margins of the Wanganui Basin, a major structural depression where up to 4000 m of marine sediment accumulated above the greywacke basement during the Plio-Pleistocene (Heerdegen and Shepherd, 1992). Poorly consolidated sands and gravels underlie the Kiwitea catchment. In terms of specific sediment yield, steepland grazed hill country in this area yields up to 2000–5000 t km2 year− 1 (Hicks and Shankar, 2003). Land use includes plantation forestry (pines), varying grades of pasture and scrub.
This physiographic setting places the long (48 km), narrow (average width 6.5 km) Kiwitea catchment at the upland fringe of the axial ranges, with a relatively steep gradient (0.005), gravely bed and highly erodible boundary conditions. The Kiwitea planform is best defined as wandering, using Neill's (1973) and Ferguson and Werritty's (1983) term. This represents a transitional pattern between multi-thread braided and single-thread meandering channels; lacking the sinuosity to be classified as meandering (1.44), or the degree of flow division to be braided, but combining both mid-channel bars and some well-developed bends, with extensive lateral bar forms often present. Wandering rivers are by nature dynamic (e.g. Ferguson and Werritty, 1983, Fuller et al., 2003a), although the active channel of the Kiwitea prior to the flood on 15–16 February 2004 was between 10 and 15 m wide and tree-lined for much of its length (Philpott, 2005), which compares with the 23-m-wide meandering channel of 1877 (Anon, 1980). Such channel constriction increases frequency of sediment transport (Laronne and Duncan, 1992). The result of this increased movement of sediment is bed degradation of the Kiwitea such that the 10-year flood would not overtop its banks (Anon, 1980). Prior to the February 2004 flood, the Kiwitea was therefore over-narrow and over-deep, largely due to riparian plantings in a narrow riparian strip.
There is a steep rainfall gradient moving up the catchment towards the ranges. Mean annual rainfall varies from 958 mm at Feilding to 1267 mm at Rangiwahia (locations shown in Fig. 1). Annual, seasonal and monthly rainfalls throughout the catchment are subject to variability of up to ± 20%, with slightly more rainfall occurring in the winter–spring than summer–autumn (Anon, 1980).
Flooding was caused by a storm on 15–16 February 2004, which was one in a sequence of depressions to affect the North Island. Heavy rain also fell on 1–3, 4–5 and 10–12 February, saturating soils in the region (Parfitt, personal communication). On 15 February a cold-pool low became stationary just to the east of the North Island and intensified. Persistent heavy rain fell over most of the lower North Island, with rainfall in the Ruahine Ranges exceeding 200 mm in 24 h (Meteorological Society, 2004). The resulting large area flood was associated with a long duration rainfall event. Rainfall intensities did not generally exceed 10 mm h− 1 (Fuller and Heerdegen, 2005), but much of the upper catchment had more than 20 h of rainfall at fairly constant intensities (Fuller and Heerdegen, 2005). Gauges at lower elevations recorded lesser totals and lower intensities (e.g. a rain gauge near Feilding at Halcombe Road (100 m) adjacent to the lower Kiwitea and Oroua recorded 115 mm over 19 h at an average intensity of 6.1 mm h− 1). The recurrence interval for the quantities of rainfall recorded in this region over a 24-h period is > 150 years (Fuller and Heerdegen, 2005). Continuous rainfall records at Feilding extend from 1890 (Anon, 1980). Event magnitude in terms of discharge is more than twice any previous recorded flood event in the Kiwitea and more than five times the magnitude of the mean annual flood in this catchment (Fig. 2, Table 1) and in the adjacent Oroua and Pohangina catchments (Fuller and Heerdegen, 2005).
Section snippets
Impact assessment
To assess the geomorphic impacts of the floods on the Kiwitea channel and floodplain, aerial photographs of a 30-km-long reach of the Kiwitea (cf. Fig. 1) were acquired in February 2004 in the immediate aftermath of the flooding. These were orthorectified and georeferenced before being overlaid on February 1999 orthophotos (2.5 m resolution) using ArcMap™ GIS. The positional accuracy of these orthophotos is given as ± 12.5 m (LINZ, 2005), with the 2.5-m photograph resolution setting the limit of
Channel response
The geomorphic impact of large (sensu; Kochel, 1988) floods is variable. Sometimes impacts are major, while at other times only minor changes may occur (e.g. Magilligan, 1992, Costa and O'Connor, 1995). The changes observed along the lower 30 km of the Kiwitea are summarized in Table 2, and the impact of this 100 year event was categorized as severe (Miller, 1990) to catastrophic (Magilligan, 1992). The dimensions of the wetted channel enlarged by 171% and in some reaches active channel width
Conclusions
The Kiwitea Stream eroded ∼ 1.4 million m3 of valley floor as the narrow and over-deepened channel responded to the largest recorded flood, which was five times bigger than the mean annual flood with an ARI of 100 years. Geomorphic impacts were, however, spatially discontinuous and highly reach specific. In some reaches, channel change was catastrophic while in others minimal changes occurred. The variability was conditioned by thresholds of flood power, in conjunction with the local channel
Acknowledgments
I would like to thank Erin Hutchinson for provision of cross-section data. Horizons Regional Council staff (Palmerston North), especially Marianne Watson and Joseph McGehan, are thanked for contributing hydrological data and towards Fig. 1. Richard Heerdegen is thanked for facilitating communication with Horizons and hydrological analyses. David Livingston (Research Volunteer, Geography Programme, Massey University, spring 2004) is thanked for processing aerial photograph overlays, made
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