![]() ![]() ![]() Weirs, dams and rural water storages (farm dams) also modify natural water flow patterns, by holding water that would otherwise flow straight into the stream network. Increases in the volume and speed of runoff can increase gully and streambank erosion in the landscape and the stream channels, resulting in sediment being transported downstream and adversely affecting water quality. Modifications to channels, such as straightening and diversions, can also increase flow rates. This increases the rate of flow and the potential for erosion. Modificationsīuildings and infrastructure such as roads, railways and creek crossings create impermeable surfaces that stop the ingress of water into soils and can create barriers that redirect water through single points or culverts, leading to channelling of water. Simple habitats such as bare ground, or open forests with cleared understorey will provide little resistance to flows. Complex habitats such as forested wetlands, with high density of trees and a developed understorey will significantly reduce flow speed. The roughness of the landscape also influences the speed of water movement across it. Rain that is intercepted by, or held on, the vegetation canopy often evaporates to the atmosphere. Some of the water that reaches the ground infiltrates into the soil and may be taken up by plant roots. Vegetation affects water runoff in a number of ways. Soil porosity affects the movement and retention of water through the soil profile. As a result, water (and dissolved nutrients) can move faster through coarse-textured soils than fine-textured soils, conversely fine textured soils will tend to generate more surface runoff. The porosity is larger in coarse-textured soils (sands) than in fine-textured soils (clays). The rate of this movement is expressed as its hydraulic conductivity. The ease with which water can move through the soil profile depends on the porosity of the soils or in other words the amount of “free spaces” (voids) through which water can flow. Soil texture affects the movement and retention of water. Topography varies over several orders of magnitude, from centimetres (roughness) to hundreds of kilometres (mountain ranges). Topographic features such as slope and curvature can serve as indirect measures of many hydrological processes. The topography (shape of an area) has a direct impact on the hydrology. Permeable rocks, such as basalts and unconsolidated sediments such sands and loams, can contain one or more aquifers, where groundwater is stored and transmitted through intergranular pore space, fractures, vesicles and/or weathered zone of the rock. Some rocks (such as some fine-grained sedimentary rock, metamorphic rock and igneous rock) and some unconsolidated sediments (such as clay deposits) have low/limited permeability and water falling on these surfaces can generate surface water run-off. The underlying geology of catchments in a region strongly influences the hydrology. Physical characteristics Geology and topography Solar radiation, temperature, wind and vegetation cover influence these processes. The processes of evaporation and transpiration (collectively ‘evapotranspiration’) cause the loss of water from the land to the atmosphere at a more local scale. Rainfall, and other forms of precipitation, provide the source of surface water flows such as waterways and other, non-channelised overland flow, as well as groundwater. It should be noted that even non-structural measures require some construction work and that every measure or a combination of measures should be evaluated from the cost-benefit point of view.The climate at the regional and subregional scales determines the overall water in the system. Therefore, a holistic catchment-scale approach is needed. ![]() The importance of hydrologic and hydraulic feedback between river reaches should be noted. Numerous examples are presented showing positive and negative effects of different measures. This chapter presents basic information about main hydrological, hydraulic, flood protection, and flood risk concepts. Each measure should be thoroughly evaluated using a combination of hydrological and hydraulic models with the consideration of their uncertainty. Numerous options are available for flood protection. Therefore, flood protection is needed in order to reduce flood risk. Additionally, climate change could affect the frequency and magnitudes of floods around the globe. Floods are one of the natural hazards that can cause large economic damage and endanger human lives. ![]()
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