The concept of flooding has a multi-disciplinary definition based on the interest of the defining discipline. However, flood is generally taken to include ‘any case where land not normally covered by water becomes covered by water’ (FWMA, 2010: Pt 1). In recent decades there have been raging scientific and media debates on likely changes in flood regimes generated by land-use changes and climate change (Ranzi et al, 2002). The crux of most of these debates is centred on the simulated risk from such flood events. These risks are related to human health, infrastructure, socio-economic well-being of affected individuals and damage also to archaeological relics. Methods of evaluating and assessing flood risk have been developed in the field of insurance, technological and environmental fields (Molak, 1997; Jones, 2001). Although river flooding is often related to natural disasters, the impacts of human activities such as urbanization have been observed by many scholars (Sala and Inbar, 1992; Kang et al, 1998; Ranzi et al, 2002). Flood risk is expressed in terms of the probability of occurrence of adverse effects of flood related hazards and vulnerability with potential consequences (Mileti, 1999; Merz, et al, 2007; FWMA, 2010). Although a number of approaches have been tested for flood events prediction, hydraulic models have been specifically designed to predict flood inundation (Horritt and Bates, 2002). Consequently, in the development of an effective and efficient flood risk management strategy hydraulic, hydrologic and socio-economic factors must be taken into consideration (Merz et al, 2007).
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2.2 Urbanisation and Flooding
Urbanization and flooding are intricately linked in both developed and developing countries. Increasing population growth and continued urban expansion has led to a reduction in surface permeability which invariably increases surface runoff in the absence of alleviating urban drainage design (Kang et al, 1998; Parker, 1999; Ranzi et al, 2002). Although the UK has only small rivers by world standards, with the tendency for smaller-scale floods to occur (Wheater, 2006), considerable economic and infrastructural losses arise from urban flooding (Mark et al, 2004). This loss is significantly higher in smaller river bank communities. For instance, Wheater (2006) notes that the 24hr rainfall in Carlisle on the 8th and 9th of January 2005 resulted in the loss of two lives, an estimated damage of £450 million and flooding of over 2000 properties when the flood defences were over-topped. The engineering and design of flood defences are based on hydrological and hydraulic models of river catchments. Hydrological models simulate surface runoff from rainfall while the hydraulic model describes structural controls of the river system (Kite, 2001; Mark et al, 2004; Kidson et al, 2006; Heatlie et al, 2007).
“We conclude that urbanization can represent a very significant increase in flood risk at small catchment scale, but that the effects are commonly mitigated, to a greater or lesser extent, by design measures. The impacts of effects at larger scales are complex and depend on the relative magnitude and timing of sub-catchment responses and the performance of mitigation strategies. Relative effects of urbanization on flooding are expected to decrease with increasing storm return period, but the performance of mitigation strategies for events rarer than the design criteria adopted is largely unexplored”.
2.3 Global warming and Flooding
Though it is still difficult to attribute global warming recorded this century to the enhanced greenhouse effect and the resultant increase in observed rainfall (Reynard et al, 2001; Robson et al, 1998), the recurrent incidence of floods and their magnitude in the UK in recent times have raised major concerns that the effect of climate change is already being felt across the country (Robson, 2002). Milly et al, (2002) produced the theory Global Climate Models have been used to determine the likelihood of increased flood risk from global warming. Reynard et al (2001) used the CLASSIC (Climate and LAnd use Scenario Simulation In Catchments model) continuous flow simulation model to assess the potential impact of climate and changes in land use on the flood regimes of the Severn and Thames rivers. They found that for the 2050s, the climate change scenarios results in an increase in both the frequency and magnitude of flooding events in both rivers. Similarly, Milly et al (2002) observed that the frequency of great floods increased significantly during the twentieth century. Accordingly, the statistically significant positive trend in the risk of floods was consistent with the results from the climate model (Milly et al, 2002). Roy et al (2001) investigated the impact of climate change on summer and autumn flooding on the Chateauguay river basin. Their study reveals serious potential increases in the volume of runoff, maximum discharge and water level with future climate change scenarios for a three 20-year periods spanning 1975 – 1995, 2020 – 2040, 2080 – 2100.
2.4 Modelling of Flood events
The simulation of extraordinary flow events characterized by high hydraulic risk has posed serious problems for policy makers, engineers and environmentalists around the world. The use of 1-D modelling for predicting flood risk generated by events of different return period or multiple land use and climate change scenarios is widespread (Lin et al., 2005; Mark et al., 2004; Horritt and Bates, 2002; Mark et al., 2004; Lin et al., 2005; Hall et al, 2005). In their study, Bates and De Roo (2000) demonstrated the use of a 1-D model type storage cell called LISFLOOD-FP to produce designated channel cells for channel routing and uniform flow formulae for floodplain routing, through the process of discrete raster-based analysis derived from a DEM at 100, 50 and 25m resolutions respectively and applied to a major flood on a 35km reach of River Meuse. Syme (2001) notes that in addition to rapid wetting and drying, the strength of TUFLOW is its powerful 1D linking options, modelling of hydraulic structures, treatment of levees and embankments, effective data handling and quality control outputs.
Horritt and Bates (2002) conclude that HEC-RAS models calibrated against discharge gave good flood predictions of inundated area on a 60 km reach of the river Severn, UK.
Reed and Robson, (1999, cited in Dawson et al., 2006) stressed that many flood estimation problems were likely to arise at ungauged sites due to the unavailability of flood peak data recorded in the UK Flood Estimation Handbook (FEH). The FEH is produced by the Centre for Ecology and Hydrology with information about River catchments in the UK such as, rainfall frequency estimation, statistical procedures for flood frequency estimation, rainfall-runoff and catchment descriptors. Dawson et al., (2006) used the Artificial Neural Networks (ANN) technique to estimate flood statistics for un-gauged catchments (for most of the River catchments in the UK). The index flood analysis from the ANN results produced a comparable accuracy to that obtained from the Flood Estimation Handbook (FEH), but the flood estimation for each catchment was carried out for only a 10, 20 and 30 year flood event period giving room for short term flood defence preparation thereby incurring future expense on what can be predicted for up to 1000 years.
A study by Yang et al (2002) on prediction of flood inundation and risk, using GIS and Hydrodynamic model showed the ability to use a DEM manipulated in GIS and translated into MIKE21 (a modelling environment). In the study, different scenarios were evaluated and results translated to the GIS environment for visualization and analysis on flood events for an estimated 100-year flood return period. However, Yang et al, stressed that there were no real means to calibrate the simulations from the modelling output, as flow and stage data were rarely recorded for flood events and also, compare between outputs from MIKE21 and MIKE1, the former, being an upgrade of the latter.
2.5 One-Dimension ISIS flood Modelling
The ISIS model has been used extensively in modelling inundated flow regimes of rivers across the UK (Heatlie, et al. 2007). The Manchester Ship Canal, a 58 km long river located in North West England and constructed in 1894 to include the navigable part of River Irwell (including River Irwell at Radcliffe, Bury) was one of the last major watercourses in the United Kingdom to be analysed with hydraulic modelling techniques (Heatlie, et al. 2007). In preparation of an indicative flood mapping (IFM), the EA used an unsteady ISIS 1-D hydraulic model for the mapping of a 47km length of the Upstream Bristol Forme catchment to define areas at flood risk in 2002 (Syme et al, 2004).
According to past studies (Costa-Cabral and Burges, 1994; Bodis, 2007; Rees, 2000) it is evident that the use of Digital Elevation Model (DEM) in flood model creation have played a big role in the successful presentation of hydrological and topographical drainage basin data analysis (Peckham, 1998) because it depicts an array of elevations across the basin at regularly spaced intervals (Cunha, 2009). This eliminates the assumption that the catchment or area is a flat surface without contours.
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In research carried out by Sansena & Bhaktikul (2006) on the integration of hydraulic modelling and GIS towards the study of river the Mae Klong (Bangkok, Thailand). The runoff frequency analysis was used in the creation of a flood risk map. The study also showed that the results from the simulation carried out, was properly presented in GIS and DTM format, by making use of the contour and river spot height data. Sansena & Bhaktikul (2006) conclude their study by suggesting that further studies be done on larger basins by dividing them into sub-basins and the network link to integrate them should be introduced to have an overview of the basin. The runoff flow in flood plains, river channels and man-made structures are important factors in the study of runoff flow behaviour prediction of flood areas, they added, and thus further studies are therefore recommended to include rainfall runoff models in upstream and unsteady areas.
2.6 Aim
To develop an appropriate one-dimensional ISIS hydraulic model of flood events that includes upstream catchments of the River Irwell and produce a flood map to predict flood extents an extreme flood event period.
2.7 Research Question
Does the inclusion of upstream catchments improve the net flood prediction model of the river Irwell?
What areas are most vulnerable to flood risk in Radcliffe, Bury?
To what level should the flood defences be built around the Radcliffe area?
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