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Hydrological and hydraulic modelling of the Nyl River floodplain Part 2: Modelling hydraulic behaviour AL Birkhead1*, CS James2 and MT Kleynhans2 1Streamflow Solutions cc, East London, South Africa 2Centre for Water in the Environment, School of Civil & Environmental Engineering, University of the Witwatersrand, Johannesburg, South Africa Abstract Catchment land-use and water resource developments may threaten the ecological integrity of the Nyl River floodplain, a world-renowned conservation area. The effect of developments on the water supply regime to the floodplain can be pre- dicted by hydrological modelling, but assessing their ecological consequences requires transformation of discharge into eco- logically-relevant hydraulic characteristics. Studies of Wild Rice (Oryza longistaminata) have indicated that the important variables are the extent, depth, duration and timing of flood inundations. One-dimensional hydraulic modelling has been successfully applied to predict these flood characteristics, using a suite of models comprising Quicksurf, RiverCAD, HEC- RAS and HEC-DSSVue. Separate but linked models were developed for three contiguous portions of the large floodplain. The topography was surveyed by airborne laser mapping, and extensive discharge, water level, rainfall, infiltration and eva- potranspiration data were collected for model calibration and verification. The models were successfully calibrated through adjustment of Manning’s n for the main channel and floodplain in each portion, and are able to predict flooding characteristics at a resolution appropriate for ecological interpretation. Keywords: Nyl River floodplain, Nylsvlei, Nylsvley Nature Reserve, modelling wetland hydraulics, wetland evapotranspiration, floodplain water balance, LIDAR. Introduction The Nyl River is situated in the Limpopo Province of South Africa and represents the southernmost tributary of the Mogalakwena River. Along its course, the Nyl River passes through the Nyl River floodplain, known as ‘Nylsvlei’ (Fig. 1), flanked to the north and west by the Waterberg Mountains, and to the south and east by the Springbok Flats (Fig. 2). The flood- plain is a unique and highly biologically productive ecological system (Higgins et al., 1996). Because of its large size and the variety of wildlife it supports, including several Red Data bird species, the area is recognised internationally as an important wetland. The ecological status of the floodplain is under threat from increasing water resource developments in the upstream catch- ments, which affect the amount and timing of water delivered to Figure 1 the floodplain. To assist with future water resources planning, Photograph of the upper floodplain (between Middelfontein and the Department of Water Affairs and Forestry (DWAF) commis- the Nylsvley Nature Reserve) looking downstream, with the sioned this study in 1996 to develop models for predicting the Deelkraal gauge (A6H002) in the distance (Photo: K Rogers, impacts of upstream development on the flooding regime of the WITS University) floodplain. The Hadejia-Nguru wetlands in northern Nigeria were to predict water depths, inundation areas and outflows from the modelled by Thompson and Hollis (1995) using a water balance Okavango Delta in Botswana. The cell-based model was cali- to predict the effects of upstream dam developments on inun- brated using observed water levels and used to predict changes dation. A hydrological monthly mass-balance was calibrated in inundated areas and outflows due to changes in inflows and using flood extents and applied to determine the economic channels of the delta. impact of the dam construction on wetland productivity. Dincer In the Nylsvlei study, temporal and spatial modelling et al. (1987) also used a water balance at a monthly time-scale scales were determined by the nature of the ecological inter- pretations required from the output, as defined in the next sec- tion: ’Model scale and resolution‘. Modelling was carried out * To whom all correspondence should be addressed. in two stages. First, hydrological models were used to predict +27 (82) 853-5779; fax: +27 43 740-5313; the supply regime to the floodplain (accounting for the effects e-mail: streamfl[email protected] of land use and water exploitation in the catchments), and then Received 16 May 2006; accepted in revised form 22 November 2006. this was transformed into flooding characteristics by hydraulic Available on website http://www.wrc.org.za 9 ISSN 0378-4738 = Water SA Vol. 33 No. 1 January 2007 ISSN 1816-7950 = Water SA (on-line) Modimolle A6H010 Bad se Loop Nyl River Study area Middelfonteinspruit Boekenhoutfontein 0590486 A6H021 A6H020 0590307 Nyl River Mokgoopong N 1 N 0 10km Figure 2 Location of the study area for hydraulic modelling Figure 2 Location of the study area for hydraulic modelling modelling. The hydrological modelling is described in Part 1 flooding and drought fluctuations. Modelling of flooding fre- 16 (Havenga et al., 2007) of this paper in 3 parts. quency, depth and duration, with respect to these three spatial This paper describes the development of a computational directions at appropriate spatial and temporal scales provides model for simulating the hydraulic behaviour of the flood- the data required to link Wild Rice response to changes in the plain. The input to the model is provided by output from flooding regime. Specification of appropriate scales for hydrau- the hydrological models as discharge time series at differ- lic modelling was derived from three key findings from research ent floodplain entry points. The hydraulic model output is undertaken at the Nature Reserve, known as “Nylsvley”: a corresponding set of time series of stage, discharge and • At a broad scale, three vegetation communities on the flood- inundation areas for the floodplain, including their spatial dis- plain can be recognised along the vertical axis: the sub- tributions along its length. This information is provided at merged and floating leaved aquatic species in the channel, appropriate scales for determining ecological impacts associ- the Wild Rice (Oryza longistaminata) community at mid- ated with an altered flow regime. In this study, attention is elevations, and the Rooigras (Themeda triandra) zone at the focused on floodplain vegetation as an indicator of ecologi- highest elevations (Higgins et al., 1997). The elevation dif- cal impact because the response of a key species (Wild Rice, ference between these communities is 0.5 to 1.0 m. Oryza longistaminata) to the occurrence of water has been • A statistically significant change in plant species composi- well described (Marneweck, 1988). Scenario applications of tion along a transect can be detected for a 90 mm change the hydrological and hydraulic models to describe impacts of in elevation, indicating that this amount of change in water development on the habitat requirements of Wild Rice are pre- depth during flooding and associated water deficit during sented in Part 3 of this paper by Kleynhans et al. (2007). dry periods, can alter conditions sufficiently to affect the persistence of some species (Coetzee and Rogers, 1991). Model scale and resolution • The species with the most diagnostic potential in terms of response to flooding is the Wild Rice, which has been exten- Appropriate modelling scale and resolution are determined sively studied on the Nyl River floodplain (Marneweck, by the nature of the interpretations to be made from the out- 1988). Studies have shown that rice grows best in water put. Because Wild Rice is used as the ecological indicator, its depths between 100 and 500 mm. Depths of 500 to 750 mm characteristics define the model requirements. Vegetation along result in reduced vigour of underground rhizome develop- the Nyl River floodplain is differentially distributed along three ment with consequences for persistence during droughts. spatial directions: laterally away from the channel, vertically Above 750 mm, flowering is markedly reduced and the plants above the channel, and downstream along the channel (Hig- become too spindly to remain upright when levels drop. At gins et al., 1997). Flooding frequency, duration and depth all least 25 days of flooding is needed for rice to flower and set decrease in these directions, especially the last two, and plant seed if flooding occurs in January/February, but a longer species distribution is a manifestation of species tolerance to inundation period is needed if flooding occurs earlier or later 10 Available on website http://www.wrc.org.za ISSN 0378-4738 = Water SA Vol. 33 No. 1 January 2007 ISSN 1816-7950 = Water SA (on-line) Olifantspruit Bad se Loop A6H012 Middelfonteinspruit Boekenhoutfontein A6H021 Naboomspruit A6H020 A6H010 Road A6H019 Rainfall Rainfall Rainfall Losses Nature Losses Losses Reserve A6H006 A6H039 A6H002 GP2 GP4 GP6 A6H037 A6H038 GP7 GP8 Nyl I O I GP3 GP5 O I O Mosdene Middelfontein Deelkraal Vogelfontein Groot A6H011 Nyl Blindefontein A6H018 N1 Deelkraal Road Vogelfontein Road Roedtan Road Hydraulic models I Inflow O Outflow DWAF gauges CWE gauge plates Figure 3 Schematic diagram of the hydraulic modelling area showing its division into three smaller regions. Significant inflows (Nyl River inflow, tributaries and rainfall) and outflows (Nyl River outflow and losses), and the location of water level gauges along the floodplain are indicated. in the season. The growth pattern of rice during this 25-day Study area and model structure period shows shorter response periods as floods rise and fall, suggesting that modelling water distribution at 5-day inter- The

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