D ESCRIPTORES DE COMPETENCIAS

Generally, floods have naturally inundated and altered floodplains throughout time and space to create a state of dynamic equilibrium. In other words, inundations of floodwaters have generated erosion and deposition of sediments in a period of time to form natural floodplains. Over the years, these floodplains have developed their own ways of responding to floodwaters and sediments, with their vital roles being to provide storage and conveyance. Floodplains are continuously shaped by the forces of water and the transport of sediment, which may vary in their characteristics, depending mainly on the associated environments. As an example, the coastline in the UK is dissected by over 170 estuarine floodplains that differ markedly in their physical characteristics (DEFRA, 2001).

Throughout time, developments on floodplains have contributed to changes in their overall physical characteristics, in particular as shown in Figure 1.1. Both natural and urban floodplain developments and encroachment activities have resulted in the removal of existing flood storage and conveyance, and creating a variety of hydraulic obstruction types (i.e. vegetation, agricultural plants, hydraulic structures, bridge piers, buildings, vehicles on roads, etc.) to floodwater flows. Often the effects of flooding are made worse due to these obstructions encroaching on floodplains and blocking floodwater flows. Consequently, these have caused significant impacts on the hydrodynamic interactions with a wide range of obstructions, which is the subject of this thesis. Both of these development types on floodplains are briefly discussed as follows, to provide the basic concepts for further investigations in the next few chapters.

2.3.2.1 Natural Development of Floodplains

Natural floodplains provide ideal lands for habitat, where flora and fauna mix in a myriad of diversity forms. This part of a river system supports a great variety of fishes, gastropods (i.e. crabs, prawns, etc.), bivalves, and other aquatic animals, and there are often other rich riparian vegetation, forests and swamps on floodplains, including mangroves. All of these habitats form an interaction and they interlink with one another in a complex web of life that either directly or indirectly contribute to the natural developments of floodplains.

These floodplains are regularly beneficial with rich resources of water and nutrients from the natural processes of flooding through the features of irrigation and soil fertilisation. Accordingly, floodplains encourage the establishment of natural

development in particular riparian vegetation, forests and swamps, which contribute a lot to alter their physical characteristics (see Figure 1.5). Most of these vegetations have survived well along natural floodplains, which are frequently inundated and generated hydraulic obstructions to floodwater flow. For example, mangroves play a significant role in the more affluent nations along the floodplains, as noted by Wolanski and Ridd (1986). They cover a significant fraction of the estuary for floodwater storage and conveyance, help to stabilise the banks of tidal rivers and creeks, help to maintain deep tidal channels and control sediments, and are also well known for natural coastal protection and flood defence.

Even though floods and natural floodplains are beneficial to each other in many cases, uncontrolled natural development with massive hydraulic obstructions, may make flood situations worse and can cause subsequent damage to adjacent areas in certain conditions. In such cases, floods constitute a risk of hazard to humans where their encroachment activities into these natural floodplains have occurred. Further investigation studies into this subject of natural development of floodplains will be discussed in Chapter 4.

2 3.2.2 Urban Development of Floodplains

Over the past few hundred years, many of the world’s natural floodplains have been transformed, mainly by humans for rural and urban development particularly for agriculture, development including: buildings, roads, pavements, parking lots for vehicles, etc. (see Figure 1.1). As an example, Wheater (2006) reviewed flooding for UK practice and found that rural land use had intensified significantly over the past 30 years, where this potentially had lead to an increase of flood risk at the local scale.

Indeed urban development in floodplains, particularly the continuing increase of urbanisation, coupled with associated land encroachment and in filling activities along the river banks or coastal areas, has drastically distorted the natural form of floodplains and created a new variety of hydraulic obstructions to floodwater flows.

It also reduces the floodplain’s ability to store excess water, sending more water downstream and causing higher flood peaks and increased floodwater velocities. Continuing urban developments in floodplains are also completely altering the surface profiles by increasing the impermeable urban areas. Before urbanisation, rainwater gets intercepted by the vegetation, infiltrates into the ground and takes time to travel to the river, but now it is quickly collected from the roofs and other paved ground and drained efficiently to public drains, which in turn rapidly delivers the water to the nearest river. Hence, flash floods are now more frequent than ever, where the rising water occurs within a matter of a few hours after a storm (DID, 2000). In addition, as infiltration has decreased, the water table has dropped and, consequently this has reduced the groundwater resource for wetlands, riparian vegetation, wells and other uses.

Due to the rapid growth in urbanisation, high intensity of rainfall and in addition the uncontrolled development over the past years within floodplains have caused a gradual deterioration in the efficiency of rivers to act as conduits of water and sediment, which has resulted in an increase in the frequency and magnitude of flooding. Consequently, incidences of floods in urban areas are now on the rise. These developments have led to the construction of extensive flood protection systems by building embankments along the rivers, enlarging and deepening the river channels, etc., which have restricted the natural morphological evolution of floodplains and

eliminated flood attenuation properties. Natural waterways have ended up being used as drainage channels, and are frequently lined with rocks or concrete to move water more quickly and prevent erosion (OEHHA, 2006). Further studies into this subject of urban development of floodplains will be discussed in Chapter 5.