By Deirdre Hart
Hapua are a type of predominantly freshwater river mouth lagoon that occurs on high energy temperate coasts. In New Zealand, they comprise a group highly-dynamic and socio-culturally important environments (Figure 1). Hapua behave differently to lagoons with tidal prisms. This means that classic estuary models cannot be applied to their understanding or management. Over the last three decades there have been significant advances in our understanding of hapua systems, with new concerns arising regarding the effectiveness of recent management regimes. This article briefly highlights key advances and gaps in our knowledge of these significant rivermouth lagoon systems.
Hapua comprise coastal conveyor belts for freshwaters and sediments on their passage from river catchments to the marine environment. Heavy catchment modification and water resource use intensification necessitates accurate understandings of their functioning under different freshwater resource scenarios in order to manage and sustain the habit quality and hazards associated with hapua. The diverse group of lagoons classified under the umbrella of ‘hapua’ share a common set of drivers, behaviours and morphologies (Figure 2) but variable behaviour frequencies and scales. Two key starting points for understanding how hapua respond to freshwater regimes are Kirk’s (1991) ‘geomorphic model for water resource planning’ and Kirk and Lauder’s (2000) exploration of the different freshwater lagoons in New Zealand. They show that hapua are wave-dominated at all but the largest river flow stages, with dynamics driven by the balance of marine and fluvial processes affecting each particular system (Figure 3). They also highlight that (with the exception of the Hurunui hapua) these lagoons typically exist on chronically eroding coasts, managing to persist in some form over geological time by retreating with the long-term erosion of surrounding shorelines.
Another important determinant of hapua behaviour is ‘antecedent geomorphology’: that is, the pre-existing state of the lagoon or barrier beach affects how hapua respond to daily river flow fluctuations and longer-term shifts in river regimes (Hart 2007, 2009; Hicks 2012; Patterson et al. 2001). Significantly, for hapua the frequency and duration of low-to-medium river flows are important drivers of hapua closure and migration events as well as of the length and duration of lagoon outlet offsets away from the main river channel. The tendency for antecedent periods of low river flow to determine hapua response to flood flows is a characteristic that sets them apart from many estuarine models and is a factor that needs to be considered in freshwater resource use plans for their hinterland catchments.
Prolonged lagoon closures (Figure 2) have long been an issue associated with hapua due to concerns around lagoon water quality, accelerated erosion and anadromous fish passage. While considering this issue it is important to acknowledge that these lagoons are significantly affected by the quality of river water inputs and, as mentioned above, erosion is typically a chronic and natural process on most of the coasts where hapua occur, while effective fish passage tends to result from larger and cooler freshwater flows than those just above closure thresholds.
Closure was previously believed to only occur in small to medium sized hapua such as in the Pareora and Ashburton. Recent observations such as those by local Rakaia Huts resident Bill Southward have shown that, under current conditions, a hapua as large as the Rakaia close for at least periods of a few hours. Southward has also documented a significant shift in the Rakaia lagoon over recent decades, including reduced spring recharge, macro-algae growth and geomorphic dynamics, pointing to possible causative changes in Rakaia River and plains water resources (Biswell 2013). With the assistance of ECAN, NIWA and University of Canterbury scientists, Southward has also highlighted the mismatch between the current river flow monitoring regimes which measure upstream flows to determine water availability versus the need for accurate coast-proximal estimates of the amount of flow remaining in rivers post channel and ground water resource use for effective hapua management.
Upcoming developments in hapua science
Two other developments in understanding hapua worth mentioning are the case of ‘small hapua’ and work towards a comprehensive classification of New Zealand hydrosystems that includes a broad range of hapua. Systems in the ‘small hapua’ category include lagoons at the mouths of Canterbury’s Kowhai, Conway, Orari, Pareora, Otaio, Kahutara, Kowai and Makikihi Rivers and Saltwater Creek, plus numerous possible examples in the West Coast and other New Zealand regions. Previously these systems have received scant scientific or statutory attention but there is increasing interest in using ‘global’ coastal plan changes to open them under prescribed ponding and/or low freshwater flow conditions. An ECan supported, University of Canterbury honours dissertation by Sam Creed due for publication next month will provide the first catalogue and description of a set of ‘small hapua’, including recommendations to aid in their sustainable management.
Secondly, a team including University of Canterbury (Deirdre Hart), NIWA (Terry Hume) and DOC (Philippe Gerbeaux, Don Neale, Helen Kettles, John Benn) scientists are currently working on a classification of the hydrosystems of New Zealand’s coasts. Initiated by Hume and Gerbeaux, the project has a specific focus on ‘classification for resource management purposes’. Unlike previous NZ estuarine classifications, this system will effectively compare and contrast hapua characteristics with those of other key hydrosystems.
In conclusion, while the international literature on hapua is not as extensive as that on tidal prism estuaries, a significant body of scientific observations and analyses have been produced over recent decades towards understanding hapua characteristics, functioning and management. This work highlights the incredible dynamics of hapua systems, their distinctive behaviours and sensitive responses to changes in their adjoining catchment, river and marine environments, ki uta ki tai.
Biswell, S., 2013. At the mouth of the Rakaia River. Coastal News 53, 1-2. http://www.coastalsociety.org.nz/Newsletter/pdfs/CN53%20EMAIL.pdf
Kirk, R.M., 1991. River-beach interaction on mixed sand and gravel coasts: a geomorphic model for water resource planning. Applied Geography, 11, 267-287.
Kirk, R.M. and Lauder, G.A., 2000. Significant Coastal Lagoon Systems in the South Island, New Zealand: Coastal Processes and Lagoon Mouth Closure, Wellington, New Zealand: Department of Conservation, 47p.
Hart, D.E., 2007. River-mouth lagoon dynamics on mixed sand and gravel barrier coasts. Journal of Coastal Research SI50, 927-931.
Hart, D.E., 2009. Morphodynamics of non-estuarine rivermouth lagoons on high-energy coasts. Journal of Coastal Research, SI 56, 1355-1359. doi: 10.2307/25738010
Hicks, D.M., 2012. Sediment transport and geomorphology. Evidence provided for CRC regarding the Proposed Hurunui and Waiau River Regional Plan and Proposed Plan Change 3 to the Canterbury Natural Resources Regional Plan. Section 42A Report September 2012, 33p, http://www.crc.govt.nz/publications/Plans/hurunui-waiau-plan-012-s42a-report-hicks.pdf
Paterson, A., Hume, T., & Healy, T., 2001. River Mouth Morphodynamics on a Mixed Sand-Gravel Coast. Journal of Coastal Research SI34, 288-294. doi: 10.2307/25736295
Dr Deirdre Hart is a Senior Lecturer at the Department of Geography, University of Canterbury.