Journal of Hydrology (NZ) 49 (2): 79-98 2010 © Hydrological Society (2010)

The interaction of river engineering and geomorphology in the Lower , Marlborough, New Zealand

Kyle Christensen1 and Crile Doscher2 1 River Engineering Sector Leader, Pattle Delamore Partners, Wellington, New Zealand 2 Faculty of Environment, Society and Design, Lincoln University, Canterbury, New Zealand. Corresponding author: [email protected]

Abstract the different regimes are also examined, The Wairau River is one of the most along with their implications for sustainable engineered river systems in the Marlborough management decisions. region. Historical river control works, particularly the blocking of the Opawa Keywords Breach and the subsequent construction of Geomorphology; river engineering; suspended the Wairau Diversion, have had a significant sediment transport capacity; aggradation. impact on flow and sediment transport in the Lower Wairau. Currently, the Lower Wairau Introduction is aggrading as a result of an increase in An understanding of the geomorphic setting sediment concentration and a decrease in the of rivers and the effects of past river control duration for which flows exceed the critical works on their current morphology is needed threshold to keep sediment entrained. These to anticipate how proposed management changes are attributable to the substantial policies or engineering works will affect reduction in flow in the Lower Wairau that is current processes and the rivers’ sediment a result of over half the total Wairau flow now transport capacity and future morphology. passing down the Wairau Diversion. During Such an understanding can inform the choice the previous period, following the blocking of of management options to minimise the the Opawa Breach up until the construction of need for future remedial works. Both water the Wairau Diversion, the opposite occurred, and sediment need to be considered, as their with significant increases in flow down the interaction determines the morphology of Lower Wairau and widespread erosion. the river channel, a key factor in terms of This paper reviews the geomorphic flood conveyance, land drainage, recreation setting of the Wairau deposition zone and and habitat. the impacts of the Opawa Breach and the The current form of the Wairau River Wairau Diversion on the Lower Wairau by system in Marlborough is a result of the examining historical cross-sectional data interaction between catchment geological (1927 to present) and assessing the distinct processes and the large-scale river engineering flow and sediment transport regimes driving works undertaken over the past 150 years. The the aggradation and erosion. The effects on geologically active catchment and active slope channel slope, width and bed level through processes, combined with significant rainfall

79 in the headwaters, produce a large volume of This paper reviews the impact of historical sediment which is transported through the flood control measures on the Lower Wairau system. The northwest tilting of the geologic River and their effects on channel morphology. block underlying the Wairau River and the Historical cross-sectional data will be used to deposition of erosion-resistant beach ridges illustrate the changes to bed levels, channel 6000 years ago determined the river setting widths, and bed volumes, and the impacts on that European settlers encountered during sediment transport capacity will be discussed. development of the and Results from this study have been used in Blenheim Township in the mid 1800s. the design and evaluation of a flow control While there has been wide-scale structure to address aggradation in the Lower modification of many rivers throughout the Wairau. Marlborough region by channel diversions, detention dams, stopbanking, and erosion Wairau River catchment control works, the Wairau River, which is the The 3825 km2 Wairau catchment is located largest in terms of flood discharge, is the most at the northeastern end of the highly modified. of New Zealand (Fig. 1). The 170 km long Historically, competing River Boards built Wairau River is the main channel that control structures that effectively transferred transports water and sediment through the flood hazard to other Boards’ districts. The system. two most significant river control works that The headwaters of the Wairau catchment have affected the Lower Wairau River are are in the Spenser Mountains, which are the the blocking of the Opawa Breach in 1917 northernmost extent of the Southern Alps, and the construction of the Wairau Diversion and are bounded by the Richmond Ranges to in 1963. the north, the Bounds and Raglan Ranges to The blocking of the Opawa Breach, which the south and by the sea to the east. was effectively the southern channel of the Strong topographically-controlled rainfall Wairau River, was attempted many times, gradients characterise the Wairau River but in 1917 there was success and the flood catchment (Rae et al., 1988). The east-to- west annual rainfall variation ranges from hazard was transferred from the main town 600 mm at the coast up to 1800 mm in the of Blenheim to the outlying settlements of upper reaches of the middle , , Spring Creek and Grovetown. increasing to 2400 mm to the southwest in Approximately 50 years later the flood hazard the headwaters. High-intensity thunderstorms created by the blocking of the Opawa Breach are also a feature of the Wairau catchment, was mitigated with a large diversion channel with an average of 5 to 10 thunderstorms per from Tuamarina to the sea (the Wairau year (Simpson et al., 1980). Figure 2 presents Diversion). This temporarily mitigated the a long profile of the river. flood risk, but the subsequent redistribution Within the mid-Wairau Valley, the Wairau of water and sediment has led to a 1.5 m River flows on a wide braided gravel bed over aggradation of the Lower Wairau, which has the 75 km from where it emerges from the again led to an increased flood risk in the Raglan and St Arnaud Range to the confluence Spring Creek, Grovetown, Wairau Pa and of the Waihopai. This reach of the river has a Lower Wairau districts. As well as increasing slope of 1:200 and flows along the fault-angle flood risk, aggradation has caused other depression between the Marlborough Sounds adverse impacts, including reduced drainage and Awatere geologic blocks. Recent evidence and a reduction in recreation, habitat and suggests that bed levels are stable in this reach environmental quality. (Christensen, 2005a).

80 – The Wairau Catchment and Lower Wairau River (after Rae et al ., 1988). River Wairau Catchment and Lower Wairau The 1 – Figure

81 Figure 2 – Long profile of the Wairau River.

Below the Waihopai confluence the river during the post-glacial and Holocene Wairau River flows on the northern flank of a period (14,000 yrs BP to present) (Rae 170 km2 alluvial fan, originally formed by the et al., 1988; Basher et al., 1995). The towns of Blenheim, Grovetown and Spring Creek are located on a mixture of swamp, lagoon and beach deposits (Basher et al., 1995). The slope of the river decreases from 1:200 to 1:700 from the head of the fan to the bifurcation

Figure 3 – Detailed map of the Lower Wairau River and the Wairau Diversion, including locations of cross sections.

82 of the Lower Wairau and Wairau Diversion Geomorphic setting of the at Tuamarina. The active, semi-braided, Lower Wairau River gravel bed channel is generally 400 m wide The Alpine (Wairau) Fault is the dominant and is now flanked by vegetated berms and tectonic feature of the Wairau catchment and stopbanks, providing a confined floodway separates the southern and northern sub- width of 800 m. The upstream supply of catchments, which have distinctive differences gravel bedload to the Wairau River deposition in underlying geology. The fault extends zone has been estimated at 70,000 m3/yr, through virtually the whole catchment and based on the mapped volume of the Raupara has experienced 70 m of horizontal and 1.2 m Formation created over the current post- of vertical movement over the past 20,000 glacial/Holocene period (Christensen, 2007). years (Simpson et al., 1980). To the south At the bifurcation of the Wairau River at the bedrock is greywacke and argillite, to the Tuamarina, the man-made Wairau Diversion north it is schist. The ranges of the southern cuts a direct path to the sea. It has a flood slope tributaries, the Waihopai in particular, are of 1:1000 and a rock-lined active channel of extremely steep, with exposed, eroding 150 m flanked by 75-m flood berms to give a greywacke surfaces. Because of the active floodway width of 300 m. From this point, the tectonic landscape and substantial rainfall, Lower Wairau River flows in a southeasterly the river transports a significant volume direction for approximately 12 km from the of sediment. The majority of the sediment bifurcation at Tuamarina before discharging (95%) is silt and fine sand transported in into Cloudy Bay through the . suspension. The Lower Wairau has a flood slope of Over the current post-glacial/Holocene 1:2000 and a confined floodway width of period the Wairau River deposition zone 350 m. The permanently wetted, tidal main has received approximately 28 × 109 tons of channel is typically 120 m wide. fine-grain sand and silt and 1.4 × 109 tons of The average annual flood in the Wairau gravel (Christensen, 2005a). All of the gravel River at Tuamarina is 2100 m3/sec and the has been deposited within the landward 1% annual exceedance probability (AEP) boundary of Wairau River deposition zone, flood peak is 5500 m3/sec (Williman, 1995). while most of the fine-grained sand and silt Based on analysis of suspended sediment have been deposited out in Cloudy Bay. The gaugings, Christensen (2007) estimated gravel-sand transition is located at Dicks the annual suspended sediment load at Road in the Lower Wairau, and alluvial gravel Tuamarina to be 1.8 × 106 t/year. Another is present down to the Thomas Road pump published estimate of the suspended load station on the Wairau Diversion (Fig. 3). based on geological and hydrological Over time, if the river aggraded sufficiently it mapping calibrated on catchments with could potentially avulse to the southern side similar characteristics and gauged sediment of the fan. Presently this is not an issue, as fluxes is given as 0.84 × 106 t/year (Hicks and gravel extraction currently exceeds the natural Shankar, 2003). However, there is a significant supply rate and the river control works are error in this estimate due to underestimation well maintained. of the Upper Waihopai sub-catchment, where Two key factors have affected the present measured dam sedimentation data provides morphology of the Lower Wairau River – the an annual suspended sediment load of first is the northwest tilting of the geologic 0.83 × 106 t/yr for this sub catchment alone block on which the Wairau Fan is located. (Christensen, 2005b). This has encouraged the main Wairau River

83 to flow on the northern side of the fan. The effects on the Lower Wairau River, were the second factor is the creation of erosion- blocking of the Opawa Breach in 1917 and resistant beach ridges 6000 years ago when the construction of the Wairau Diversion in sea level was higher than at present. The 1963 (see Fig. 1). beach ridges are formed of coarse marine sediments and are of sufficient height and The Opawa Breach resistance to erosion that the Lower Wairau Through geological time the Wairau River has River has maintained its position without occupied portions of the whole flood plain. overtopping them for at least the past 2000 At the time of European settlement the main years (Pickrill, 1976). The beach ridges are channel was located at the northern edge of approximately 3 km inland of the present- the floodplain, with overflows occurring into day coast and were deposited as the sea the Opawa River. In April 1861 the Wairau receded from its maximum level 6000 years was in significant flood and the flow was ago (Pickrill, 1976). divided between its own channel and that As a result of geological tilting and the of the Opawa (Davidson et al., 1959). This erosion-resistant beach ridges, the Wairau at event showed the settlers that the Opawa Tu a marina makes an abrupt 90 degree turn River was a significant distributary channel to flow in a southeasterly direction for 12 km of the Wairau River and posed a serious flood as the Lower Wairau River before flowing out threat to Blenheim. to sea at the Wairau Bar, which is also the In October of 1861 the Marlborough outlet for the two main lowland rivers (Lower Provincial Council made their first Opawa and Upper Opawa/Roses Overflow) unsuccessful attempt to block the Opawa as well as the Vernon Lagoons and Wairau Breach with a timber-based groyne. Flooding Estuary. This is a key feature of the Lower continued to be a significant problem for the Wairau system; in the absence of the beach Lower Wairau and Spring Creek Boards, with ridges and the geological tilting, it is likely Blenheim reported to have had nine floods in that the Wairau would have flowed directly 11 weeks in 1893, and a large flood in 1904 to the sea. with an estimated 2000 m3/s flowing down the Upper Opawa – at least half the Wairau Historical river control works flow (Williman, 1994). There is evidence of river control and drainage In 1914 the Lower Wairau River Board works on the Wairau Plain dating back to its began a clandestine operation to close the first occupants back in the 1100s (Williman, Opawa Breach with a groyne of stone and 1994). However, it wasn’t until the mid netting. The groyne was nearly completed 1800s when the Wairau/Opawa River trade before it was discovered by a member of became established and development of the the Spring Creek Board and construction floodplain intensified that large-scale river ceased. Despite this, by 1917, the Opawa control works were undertaken. was successfully blocked. Subsequent public Flooding was recognised as a significant unrest led to the creation of the Wairau River problem by the early European settlers and Board. the name given to the first trading post, ‘The On 9 May 1923 the board faced a major Beaver’, was in recognition of the frequent flood when one of the greatest inundations flooding. The desire to protect property of the Wairau Plains occurred (Thomson, from flooding led to large-scale modification 2002). The flood was more remembered for of almost every watercourse on the Wairau the serious flooding in the Taylor, Fairhall Plain. The two key works, in terms of their and Doctors Creek catchments, but it was a

84 significant event in the Wairau, especially as especially of the erosion-resistant beach the main Wairau channel now had to take all ridges near the coast, and only reached design the floodwater, with little or no overflow down capacity in 1998. As the Wairau Diversion the Opawa. This event caused significant developed, flood flows were greatly reduced overtopping and failure of stopbanks in the in the Lower Wairau. The Wairau Diversion Wairau and Lower Wairau channels. has effectively replaced the flow capacity that The Wairau River Board addressed many was previously provided by the Opawa Breach of the issues concerning flooding on the before it was blocked off. Wairau Plains, but the critical flood threat from the Wairau River remained unresolved. Impact of the Wairau Diversion The two large floods in June 1954 (5% While the Wairau Diversion was enlarging AEP) and February 1955 (10% AEP) caused and capturing an increasing proportion substantial damage and turned the public of flood flows, the Lower Wairau bed was of Marlborough towards replacing the aggrading with fine sand and silt and being Wairau River Board with the Marlborough reduced in capacity. Up until 1998 the Catchment Board (Williman, 1994). increasing capacity of the Wairau Diversion was greater than the decreasing capacity of The Wairau Diversion the Lower Wairau such that there was a net In January 1956, the Marlborough increase in the combined system capacity. Catchment Board was formed under the With the Wairau Diversion reaching its provisions of the Soil Conservation and design capacity, and further enlargement Rivers Control Act 1941. The Catchment limited by erosion-control structures, it has Board was made responsible for the whole remained stable since 1998. However, the Wairau Catchment from ‘the source to the Lower Wairau has continued to aggrade at sea’ and its activities included soil conser­ an increasing rate and the system is now not vation works as well as river control. The able to pass the 1% AEP flood flow without Wairau Valley Scheme outlined a 15-year stopbanks being overtopped. The 1% AEP programme with significant capital works, flood is the specified standard of protection including the construction of the Wairau in the Wairau River Floodways Management Diversion to provide an alternative flood path Plan, a regional plan prepared under section to the sea in Tuamarina. 65 of the Resource Management Act (1991). The construction of the Wairau Diversion As well as affecting the flood flow capacity began in 1963 with a 10-m-wide (bottom of the channel, aggradation is also adversely width) pilot channel excavated from affecting recreation, drainage and water Tuamarina directly to the sea. The material quality. The deposited sediment has reduced excavated was used for stopbank construction. the water depth during periods of normal Trenched rocklines were also installed to flow and has significantly reduced the volume limit the channel development to the desired of the tidal prism. The reduced tidal prism design width. The concept of the pilot cut was means that contaminants from the numerous that natural erosive forces would enlarge the rural drains and the Spring Creek waste channel from its initial capacity of 700 m3/s water plant, which discharge into the Lower to the full design capacity of 3000 m3/s, Wairau, are more concentrated than they while enlarging the channel width to 150 m. otherwise would have been. The reduced tidal The natural enlargement occurred much volume has also affected the functioning of more slowly than anticipated and had to be the Wairau Bar river mouth, with longshore supplemented by mechanical excavation, sediment transport more readily blocking the

85 mouth, resulting in perched water levels in the Lower Wairau now had to pass 100% of the Lower Wairau at low tide. The loss of the the Wairau River flood flow. This significant lower end of the tidal cycle has a significant increase in flows resulted in widespread impact on the effectiveness of gravity drainage erosion in the Lower Wairau. This regime for the whole Lower Wairau Plain. remained until the construction of the Wairau The reduced depth and water quality Diversion in 1963. When first constructed, also adversely affects recreational activities, the Wairau Diversion took only 20% of flood including rowing, kayaking and water skiing, flows, but it gradually enlarged due to natural which require minimum water depths. All erosion and mechanical excavation to now of these activities, as well as swimming and take up to 60% of the Wairau flood flows. fishing, are affected by reduced water quality. Since its construction, aggradation has been There are also strong Iwi links with the observed in the Lower Wairau. The low flow Lower Wairau, particularly around the split between the Lower Wairau and Wairau Grovetown Lagoon and the Wairau Bar. Diversion is dictated by the morphology of After 100 years of channel diversions and the gravel bar around the bifurcation, but this stopbanking, it is now the management of has in the past been mechanically adjusted the sediment build-up resulting from these to ensure that at least 50% of the discharge works that is critical to achieve a suitable flows down the Lower Wairau to maintain level of flood protection while providing for water quality. ecological, drainage, recreational, social and cultural values in the Lower Wairau River. Changes to sediment transport The sediment of interest in this study is the Lower Wairau flow and sediment fine sand-sized particles transported as both regimes suspended load and bedload (depending The flow characteristics of the Lower Wairau on the shear stress conditions) before being River have changed significantly over the past deposited in the Lower Wairau beyond 100 years. Flood flows and flow exceedance the downstream limit of gravel deposition values for the Lower Wairau increased for the (Fig. 3). Samples of sediment deposits in the 1920-1963 period due to the blocking of the Lower Wairau River have been analysed to Opawa Breach. Since the construction of the determine a D50 of 0.126 mm and a D75 of Wairau Diversion in 1963, however, they have 0.165 mm. Sediment in this size range will be been gradually reverting to the approximately transported through the system in suspension pre-1920s state (Christensen, 2007). Changes during periods of high flow, but as the flow in flood flows and flow exceedance have a and velocity diminish particles will fall significant effect on sediment concentration, towards the bed where they may continue supply and transport capacity. to be transported further downstream as There have been three distinct flow regime bedload before being deposited on the bed phases in the Lower Wairau River in the (Hicks et al., 2004). recent past (1850–present). At the time of The sediment transport capacity of the European settlement the Opawa Breach was a Lower Wairau depends on the rate and significant distributary of the Wairau, flowing frequency of flow and the sediment entering on the southern side of the fan and swamp the river. These characteristics have changed deposits. During floods it typically carried significantly as a result of channel diversions, 30–50% of the total Wairau discharge. With firstly increasing then decreasing the flow the blockage of the Opawa Breach in 1917, into the Lower Wairau River.

86 An increase in flow will increase the depth profile develops across the bend at the head of and, in tidal reaches, also the slope of the the Lower Wairau. As the total Wairau flow water surface, with corresponding increases in increased above 1500 m3/s (Lower Wairau shear stress and sediment transport capacity. flow ~ 650 m 3/s), the transverse pressure Decreases in flow will have the opposite gradient supplied by the water surface sloping effect, resulting in decreases in shear stress. downwards towards the inside of the bend In New Zealand rivers the suspended load became significant, with higher sediment is largely dependent on supply, not capacity concentrations measured on the inside of the (Hicks et al., 2004). However the transport bend. However, it has been determined that capacity of most rivers diminishes downstream at Lower Wairau flows greater than 300 m3/s, due to the reduction in slope and resultant there will be sufficient shear stress to keep fine- reduction in shear stress. In the Wairau River grained sediment in suspension so that it is the slope decreases from 1:200 in the upper transported through the system (Christensen, reaches to 1:400 in the middle reaches of the 2007). This determination was made by fan and floodplain. In the lower reaches the extracting shear stress data from a calibrated flood slope is reduced to between 1:900 and Mike 11 hydraulic model and comparing it to 1:1400 for the Wairau Diversion and 1:2000 the critical threshold required for suspension to 1:3400 for the Lower Wairau River. of the D75 particle size entering the system. The suspended transport rate is likely to Based on a critical Shields Factor of FS = 0.6 be limited by supply for all the reaches of for suspension (FS = 0.1 for in­itiation of the Wairau except for the Lower Wairau, motion), it was determined that there would where transport capacity is the likely to be be sufficient shear stress (1.8 n/m2) during a 3 the limiting factor, particularly during the 300 m /s flood for a 0.165 mm grain (D75) recession of flood flows. This conclusion to be transported in suspension through the is made based on the fact that the Lower Lower Wairau and out to Cloudy Bay. Wairau is the only reach where there has It is therefore concluded that the increase been long-term deposition of fine-grained in sediment concentration that occurs at sediment in the active channel. However, this Wairau flows greater than 1500 m3/s, due to has not always been the case, as evidenced by the bend at the head of the Lower Wairau, is analysis of the Lower Wairau cross sections not a factor that contributes to the deposition from the erosional regime that occurred from that is occurring. This consideration, as well the 1920s until 1963. as the lack of data available to quantify the The suspended sediment concentration sediment split, provides justification for in the Lower Wairau has been estimated using the simplification of assuming that the using the sediment rating curve developed sediment splits in proportion to flow. at Tuamarina (Christensen, 2007) and is It is likely that the total sediment supply shown in Figure 4 for flow at Tuamarina up from the catchment has remained relatively to 2500 m3/s, along with sediment gauging constant over this period, although soil points from a November 2006 flood event. conservation plantings in the 1960s and It is then assumed that sediment splits sediment trapped in the Benhopai Dam in proportion to the flow split at the between 1926 and 1948 may have had a bifurcation with the Wairau Diversion. This minor impact. is a simplification of what actually occurs, as The significant change that has affected it has been observed during a sediment and sediment transport capacity in the Lower flow gauging that a transverse concentration Wairau is that of flow regime.

87 Figure 4 – Estimated suspended sediment concentration rating curve at Tuamarina.

With the Wairau Diversion now capturing 600 m3/s, the sediment concentration is over over 50% of flood flows, there has been 300% higher, at 850 mg/l. a significant reduction in flows down the Lower Wairau River. The number of days that there is sufficient flow (> 300 m3/s) to Morphological changes in the keep the D75 sediment in suspension and Lower Wairau transported through the system has decreased Changes in the Lower Wairau will be from 18 to 5 (Christensen, 2007). More examined through the lens of cross-sectional significantly, the sediment concentration data, focusing on channel slope, channel during these periods of likely deposition width and bed level changes over the three (flow < 300 m3/s) has increased substantially. flow and sediment regimes. Following the blocking of the Opawa Breach and prior to the Wairau Diversion being Overview of available cross-sectional data constructed, the Lower Wairau carried the Historical cross-sectional data were used to total river flow, such that a Lower Wairau gain a better understanding of the impacts flow of 300 3m /s had an estimated sediment of the river control works on channel concentration of 250 mg/l (see Fig. 4). With morphology, particularly channel slope, main the Wairau Diversion capturing over 50% of channel width and bed volume. the total Wairau flow, a Lower Wairau flow of The earliest cross section survey data 300 m3/s now requires a flow at Tuamarina in available for the Lower Wairau River is from excess of 600 m3/s. For a total Wairau flow of a survey in 1927. This survey, however, is

88 not considered particularly accurate due to still provide a useful measure of how much uncertainty in the actual locations of the material is actually there, but caution is surveyed sections. The next comprehensive needed in using these data for estimating survey was in 1957, with surveys also carried sediment supply. out in 1963 and 1967. There is then a Also, the distance between each of the significant break in the data due to the focus nine cross sections is typically 800 m, so the on monitoring the natural enlargement of the calculations of bed level changes are assumed Wairau Diversion. The 1989 survey revealed to be indicative of that reach of the river, significant changes, and the Lower Wairau has although there may be variation between been surveyed every five years since. The most cross sections due to localised effects. Reach recent cross section survey was undertaken characteristics may be misrepresented when in October 2004, with sixteen cross sections the cross section is on a bend. Scour holes surveyed over almost the whole of the Lower often develop on the outside of bends and Wairau. Of these sixteen cross sections, nine beaches can build up on the inside of a bend. could be used for comparative analysis with Also propagation of meander patterns up or historical surveys back to 1957 (see Figure 3 downstream can affect cross section profiles. for the cross section locations on the Lower This is not such a problem with the post- Wairau.) 1957 data, but it is one of the reasons that the earliest survey (1927) cannot be used for Method of analysis and limitations comprehensive comparative analysis. The The method of calculating bed level changes timing of the survey, whether before or after is based on determining the mean bed level a significant flood, can also have a significant defined between an active channel width at effect on the rates of change calculated from each cross section. This information can then deposited or eroded material. be used to determine the change of area at a Due to these factors, the changes in bed cross section between two survey dates. Using level and volumes of material should be the average change of area between adjacent considered only as estimates. This is, however, cross sections, the volume of deposited or the best information available and the method eroded material between cross sections over a used to analyse this data is considered the survey period can be calculated. most accurate at this time. A significant point to note on using The slope of the river channel has been discrete survey data to determine bed volume determined by linear regression of mean bed changes is that none of the changes that levels within the active channel over the tidal occurred between the survey periods are reach of the Lower Wairau. picked up; it is simply the difference in bed The river cross sections at the downstream volumes at two points in time. Care must end of the reach have not been included due be taken in interpreting results from this to the influence of coastal processes. method, as sediment inflows and outflows are not necessarily reflected in the cross sections Erosional Phase – 1927–1963 (Fuller et al., 2003). Lindsay and Ashmore From around the early 1920s until 1963, the (2002) note that calculated volumes may Lower Wairau was in an erosional regime, be negatively biased when bed changes have as 100% of the Wairau flow was entering occurred between surveys. Though sparsely the Lower Wairau River during this period. spaced cross sections provide few insights Quantifying the erosional rate during this into the bed level changes (Brasington et period provides very good information on al., 2003), the volumes calculated here can what can be expected when investigating the

89 design of flow control structures to erode approximately 385,000 m3 (65,000 m3/yr) sediment. was eroded from the Lower Wairau channel. The only cross sections from the 1927 This lowered the mean bed level by 0.271 m survey that are suitable for quantitative (0.045 m/yr). Changes in bedslope for this comparison with the more recent datasets are period are shown in Figure 5 and a typical cross section 16 and cross section 14. These cross section is shown in Figure 6. sections are located in the straight reach The less certain value of 0.039 m/yr from between the Ferry Bridge and the Grovetown the analysis of the 1927-1963 period and Lagoon, which has maintained a relatively the more accurately determined value of constant width and alignment over the survey 0.045 m/yr from the 1957-1963 period give a good indication of the rate of erosion that period. Based on the average changes at cross would be expected if a similar flow regime sections 16 and 14, the mean bed level in was created using a flow control structure. this reach decreased by 1.404 m (0.039 m/yr) Using the mean bed level data from the between 1927 and 1963. This depth of accurately surveyed and located 1957 and erosion is possibly similar to the current 1963 surveys to determine the river slope depth of deposition, suggesting that the river shows a flattening of the slope from 1:13,000 may be at a level similar to that prior to the to 1:50,000. The approximate slope in 1927 beginning of the 1920s erosional cycle. was 1:3,000. The flattening of bed slope is The 1957 cross section survey is the earliest a typical response in an erosional channel full and reliable dataset to quantify bed level regime where sediment transport capacity changes. Between the 1957 and 1963 surveys, exceeds supply and the bed is erodible.

Figure 5 – Change in Lower Wairau bedslope – erosional phase (1927-1963).

90 Figure 6 – Typical Lower Wairau cross section 1927-1963 (erosional phase).

Aggradational Phase – 1963-2004 shows a typical cross section in the aggrading The Lower Wairau River has been in an reach. aggradational phase since 1963, which The rates of change in terms of bed level coincides with the opening of the Wairau over all phases are graphically presented in Diversion. Since the beginning of this phase Figure 9. the total volume of material deposited The aggradational phase has seen the slope downstream of the Ferry Bridge is estimated of the channel increase from 1:50,000 to at 1.9 × 106 m3 (50,000 m3/yr). Assuming a 1:10,000 in 1989 and 1:5000 since 1999. density of 1.2 t/m3, this deposition accounts The steepening of the channel slope through for 8% of the total sediment supplied aggradation is a typical response where (800,000 t/yr) to the Lower Wairau, with the the sediment supply exceeds the sediment remaining 92% passing through the system transport capacity of a river reach. and being deposited into Cloudy Bay. The As well as the changes in bed levels and deposition in the Lower Wairau has resulted slope, the width of the channel has also in the mean bed levels in this reach increasing changed. In the lower reaches of the Lower by 1.5 m (0.032 m/yr). Wairau, the channel width has reduced The whole reach of the Lower Wairau by an average of 40 m since 1963. In the from the Ferry Bridge to the Roses Overflow upper reaches the width has reduced by confluence has aggraded over this period, approximately 10 m over the same period. with slightly greater deposition occurring The width of the channel is also important upstream of the Blenheim Rowing Club at in determining sediment transport capacity, cross section 8 (Fig. 3). Figure 7 shows the with narrower channels in the sand/silt phase bedslope changes in this phase, while Figure 8 typically providing greater sediment trans­port

91 Figure 7 – Changes in Lower Wairau bedslope – aggradational phase (1963-2004).

Figure 8 – Typical Lower Wairau cross section 1963 – 2004 (aggrading phase).

92 Figure 9 – Cumulative linear changes in the Lower Wairau river bed. capacity. The opposite is generally true in the The erosional phase is less accurately gravel phase of a river, but there are arguments quantified due to the lack of survey data and examples for both sides. The reductions and uncertainty over survey locations, but in width should theoretically increase the erosion, possibly of a similar extent and at average velocity and therefore shear stress, but a similar rate, occurred from 1917 through the reduction in width could also amplify any to 1963. Within the limitations of the data, deposition that is occurring. The deposition particularly pre-1927, the best estimate that has occurred over the period where of the rate of erosion during this period is the channel has narrowed suggests that the 0.035 myr. channel narrowing has not been sufficient It is also interesting to note the significant to compensate for the reduction in sediment variation in the rates of aggradation over transport capacity resulting from the Wairau the past 15 years, with the highest rate of Diversion. aggradation during the recent survey period (1999-2004), but the preceding period Summary of bed level changes (1994-1999) showing the lowest rate. The The present well quantified aggradational reasons for this are related to sediment supply phase in the Lower Wairau River has seen and/or flow regime. nearly 2 × 106 m 3 of sediment deposited over The river flow over the two periods also the 10 km tidal reach of the channel. This differed significantly, with the 1994 to 1999 has raised the average bed level by 1.5 m, or period (Fig. 10) having 12 freshes over on average 0.32 m/year, since 1963. Changes 1500 m3/s and the significant July 1998 over the 1927=2004 period are summarised flood of 3800 m3/s, compared with the in Table 1. 1999-2004 period (see Fig. 11) with only

93 Table 1 – Summary of bed level changes and rates of erosion or deposition

Annual rate of Total bed level Annual rate of Total volume deposition (+) change change deposited (+) or erosion (-) Period (m) (m/yr) or eroded (-) (m3) (m3/yr) 1927-1957 – 1.002* – 0.033* N/A N/A 1957-1963 – 0.271 – 0.045 – 385,000 – 65,000 1963-1967 + 0.056 + 0.015 + 4,000 + 1,000 1963-1989 + 0.782 + 0.035 + 1,000,000 + 45,000 1989-1994 + 0.173 + 0.036 + 290,000 + 60,000 1994-1999 + 0.048 + 0.010 + 20,000 + 5,000 1999-2004 + 0.428 + 0.090 + 615,000 + 130,000 Erosional phase – 1.610** – 0.035** N/A N/A (1917-1963) Depositional phase + 1.500 + 0.032 + 1,900,000 + 50,000 (1963-2004)

* Based on average measured changes at cross sections 16 and 14. ** Based on average of 1927 – 1963 annual rate

Figure 10 – Wairau River flow at Tuamarina 1994-1999.

94 Figure 11 – Wairau River flow at Tuamarina 1999-2004. three events greater than 1500 m3/s and a Looking at the longer term change in bed maximum of just over 2000 m3/s. Another levels, it could be suggested that the Lower significant statistic is the percentage of time Wairau is returning towards the level it was the river flow was greater than 600 m3/s prior to the Opawa Breach being closed (see (shown as a horizontal line in Figures 10 and Figs. 5 and 6), although it is likely that the 11). This is the flow that would be required Wairau Diversion takes a greater proportion to have approximately 300 m3/s in the of Wairau flow than the Opawa Breach Lower Wairau, which is estimated to be the typically did. However, it is recent changes flow required to keep suspended sediment (in the past 10-15 years) that set the standards entrained (Christensen, 2007). for evaluating recreational, ecological and During the 1994 to 1999 period, the flow flood hazard effects. in the Wairau was greater than 600 m3/s It is also likely that the present aggradational (~300 m3/s in the Lower Wairau) for cycle will continue into the future, resulting approximately 42 days, compared with only in further reductions in flow and further 20 days for the 1999 to 2004 period. The reinforcing the aggradational cycle. The rate period of time the critical flow for sediment of future aggradation will depend on the transport is exceeded gives a good indication sediment supply and the flow regime, but of the total sediment transported over that based on the past 40 years of data, it is likely period. Conversely, the time that the critical that bed levels will aggrade by approximately flow is not exceeded gives a good indication 0.032 m per year. of the likely deposition that could occur over The 1927 and 2004 bed levels appear to that period. be very similar, as could be expected with the

95 flow regime being about the same as it was Lower Wairau and Wairau Diversion to be before the Opawa Breach was blocked off. fully utilised for carrying flood flows. The design of the erodible gravel Design and construction of an erodible flow embankment was analysed in detail using a control structure numerical sediment transport model linked The current aggradational cycle is causing a to a hydraulic model (Christensen, 2007). number of problems, including increasing A range of predicted erosion rates were flood hazard, reducing water quality, determined from the modelling, but limiting recreational opportunities and comparison with the estimated rate between impairing drainage. A number of options 1927 to 1963, when there were higher were considered, including doing nothing, flows in the Lower Wairau, suggested that raising the stopbank, mechanical control predicted erosion rates with the erodible gates, dredging, or the use of an erodible gravel embankment in place would likely gravel embankment to control flows. These be at the lower end of the range. Based on options were assessed and an erodible a lower range estimate of the rate of erosion, gravel embankment to control flows and and combined with flood berm and stopbank induce erosion (Fig. 12) was selected as the improvement works, it was estimated that optimum solution, based on its economic, it would take at least 20 years for the Lower environmental and social attributes, as well as Wairau channel to enlarge sufficiently to future sustainability. achieve the required flood control standard The erodible gravel embankment has been and meet the recreational, ecological and designed to divert a greater proportion of drainage objectives. flow down the Lower Wairau during small The results of the analysis and design work floods, with the aim of flushing out deposited were sufficiently positive and conclusive sediment. During larger floods it could be to warrant a trial period of embankment overtopped and eroded to allow both the operation. Resource consent was granted in

Figure 12 – Location of flow control embankment at the bifurcation of the Lower Wairau and the Wairau Diversion.

96 July 2009 and construction was completed years have fallen short of providing a long- soon after in August. On 26 August 2009 term solution that meets the communities’ a small fresh of 690 m3/s occurred and requirements. The changes in flow regime the embankment performed as intended, resulting from the blocking of the Opawa diverting 71% of the total flow down the Breach and the subsequent construction of Lower Wairau, with the remaining 29% the Wairau Diversion have had a significant flowing down the Diversion. The peak water effect on sediment supply, sediment transport level was approximately 1 m below the crest capacity and the channel morphology of the of the embankment. Lower Wairau River. The blocking of the On 31 August another small flood Opawa Breach increased flows, increased occurred, causing the embankment to overtop sediment transport capacity, decreased and erode as intended. The erosion occurred sediment supply from minor floods and very quickly and the within 30 minutes resulted in channel erosion and flattening a substantial flow was occurring through of the channel slope. The construction of the breach and into the Wairau Diversion the Wairau Diversion had the opposite (Williman pers com., 2010). The small effect, with reduced flows, reduced sediment flood peaked at 1130 m3/s and the design transport capacity and increased sediment levels were shown to be overly conservative. supply for minor floods, and it resulted in The embankment was reconstructed with a channel aggradation and narrowing, with a crest 350 mm higher than the initial design. steepening of the slope. Another small flood occurred on 7 June 2010, An erodible gravel embankment has now which peaked at 1300 m3/s and came within been constructed which diverts a greater 50 mm of overtopping the embankment proportion of flow down the Lower Wairau without causing the berms of the Lower during small floods in an attempt to flush Wairau to flood. This event confirmed that out the deposited sediment and enlarge the the embankment height is optimised for the channel. It will be a number of years before present state of the Lower Wairau. it will be possible to determine whether this It will take a number of years of operating solution has been effective. in this manner, with reconstruction following each failure, to determine whether the Acknowledgements Lower Wairau is enlarging as predicted by The work reported here was carried out the numerical modelling and morphological while one of the authors (Christensen) was analysis. The next bed level survey of the employed by the Marlborough District Lower Wairau is scheduled for 2014-15. If Council and also undertaking postgraduate this survey shows that erosion of the deposited study. The authors wish to acknowledge sediment has occurred, then the height of the contributions of Mr Brin Williman the embankment can be further increased to (Rivers and Drainage Engineer) at the MDC provide greater sediment transport capacity to this paper. in the Lower Wairau. References Conclusions Basher, L.R. et al. 1995: Geomorphology of the The geomorphic setting of the Wairau River Wairau Plains: Implications for floodplain made it difficult to provide flood protection management planning. Landcare Research for the early European settlements. Large- Science Series No. 11, Manaaki Whenua scale river engineering works of the past 150 Press.

97 Brasington, J.; Langham, J.; Rumsby, B. 2003: Lindsay, J.B.; Ashmore, P.E. 2002: The effects of Methodological sensitivity of morphometric survey frequency on estimates of scour and fill estimates of coarse fluvial sediment transport. in a braided river model. Earth Surface Processes Geomorphology 53: 299-316. and Landforms 27: 27-43. Christensen, K.J. 2005a: Wairau River Pickrill, R.A. 1976: The Evolution of Coastal Gravel Extraction Policies, technical report, Landforms of the Wairau Valley. New Zealand Marlborough District Council. Geographer 32(1):17-29. Christensen, K.J. 2005b: Deposition Upstream Rae, S.N. et al. (eds.) 1988: Water & Soil Resources of the Benhopai Dam, technical file note, of the Wairau – Volume 1 Water Resources. Marlborough District Council. Marlborough Catchment Board and Regional Christensen, K.J. 2007: The use of a flow control Water Board. structure to erode deposited sediment from the Simpson, P.G.; Bargh B.J.; Brown I.; Hayward Lower Wairau River (Marlborough). Unpublished J. D.; Wisheart C.J. 1980: Wairau Mountain master’s thesis, Lincoln University, Lincoln, Lands: a study of the catchment environment New Zealand with particular emphasis on erosion, National Davidson, C.C. et al. 1959: Wairau Valley Scheme. Water & Soil Conservation Organisation, Report to the Marlborough Catchment Board. Marlborough Catchment Board and Regional Fuller, I.C.; Large, A.R.G.; Charlton, M.E.; Water Board. Heritage, G.L.; Milan, D.J. 2003: Reach- Thomson P.A. 2002: The History of the Grovetown scale sediment transfers: an evaluation of two Lagoon and the Grovetown Drainage Area. morphological budgeting approaches. Earth Report to the Marlborough District Council. Surface Processes and Landforms 28: 889-903. Williman, E.B. 1994: Wairau River Floodways Hicks, D.M.; Quinn, J.; Trustrum, N. 2004: Management Plan, Marlborough District Chapter 12: Stream sediment load and organic Council document. matter. In Harding, J.S., Mosley, M.P., Pearson, Williman, E.B. 1995: Flood frequency analysis C.P.and Sorrell, B.K. (eds.), Freshwaters of New for the Wairau River, Marlborough. Journal of Zealand. New Zealand Hydrological Society Hydrology (NZ) 33(2): 71-93. Inc. and New Zealand Limnological Society Inc., Christchurch, New Zealand, 12.1-12.16. Hicks, D.M.; Shankar, U. 2003: Sediment from New Zealand Rivers. NIWA Chart, Misc. Series No.79. National Institute of Water and Atmospheric Research, Wellington, New Zealand.

Manuscript received 27 April 2010; accepted for publication 18 November 2010

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