An Evaluation of a Small Water Catchment - Essay Example

An Evaluation of a Small Water Catchment Meaning and Understanding of Catchment: A catchment is an area of land that collects water and channels itinto a waterway .To understand what this means, one can imagine a funnel that collects water the entire area before channeling it. Hence it is the area of land from which the surface water flow is drained by a receiving water body. In a natural catchment, rain or snow that falls on land will find a way to a lowest point in that area which may be a lake, dam or sea through large and small creeks, streams, tributaries and rivers. In agricultural and urban areas the water-related infrastructure is altered by man and made to flow through gutters, drains, ditches and other storm water systems. To put it in a nutshell, the term, 'catchment' includes the sum of the area, drainage patterns and the environment of a waterway. Other terms used interchangeably are watershed, drainage basin, water basin and drainage area. This drainage basin is separated from adjacent basins by a drainage divide. ("Drainage Basin") Importance of catchment: They are important as geopolitical boundaries. Trade is carried out between regions by water. They are important in hydrology (study of water).In the hydrological cycle, movement of water is studied by focusing on catchment. Rain gauge data is used to measure the precipitation or rain over an area. Stream gauge located at the outlet of the catchment is used to measure the water discharge from the catchment. It is important in ecology. Flowing water picks up sediments, nutrients and pollutants. Minerals and artificial fertilizers are added by man and these are deposited near the mouth of the catchment. Hence, the natural mineral balance is disturbed. They are important in resource management. Since they are distinct bodies in hydrology, water resources are managed on the basis of individual catchments. (Drainage Basin) Flow of the River Itchen towards sea: The 3 tributaries, the Candover Stream, the River Alre, and the Cheriton Stream join together just west of New Alresford and give rise to River Itchen in Hampshire, England. The river flows west to Winchester and southwards through Eastleigh collecting a small tributary, the Bow Lake. It enters the tidal estuary at Woodmill where it joins the Monks Brook. It joins the River Test at 'Dock Head' to become Southampton Water. The world's premier chalk stream - River Itchen Total length - 28 miles (45 kms) Catchment area - 400 sq.kms. The size of the catchment does not really matter. For eg. : The size of the camel catchment is 210 sq.kms. but there was extensive damage of life and property in this catastrophic flood. Factors that affect catchment: They can be divided into two categories: 1. The time taken for rainwater to reach the river. This is dependent on two factors. Topography: In the steep mountain areas, water will reach the river faster than in the lesser sloping areas. River Itchen: The land drained by the river consists of a gravelly loam in the upper chalk. There are many water channels that drain the grasslands near the river. At Itchen Abbas the water level is higher than the surrounding marshes. The numerous channels make is very difficult to carry out any survey here. Shape: Water from a circular catchment reaches the river faster than the long, thin catchment. The landscape of the area was shaped by stream erosion when the ground was frozen, causing rainfall to run over the surface rather than soaking into the chalk. 2. Amount of water that reaches the river. This is dependent on three factors. Size: If the catchment is larger, the potential for flooding is also high. Type of soil: Rainfall on the clayey soil is not absorbed. So it runs off and there is greater chance for flooding. Rainfall on the sandy soil is absorbed by the ground and only a part of it reaches the river. This is worsened only when there is continuous rainfall and the soil becomes saturated. As far as river Itchen's is concerned, its rocks slope gently, or dip, from north to south. The underlying rock contains Cretaceous Chalk, a porous, finegrained limestone which has a natural storage capacity because the rain soaks in to the chalk rock rather t than running off. From Eastleigh to the sea the river flows over sands and clays which are less permeable than the Chalk and hence the river has more tributaries in this area. Land use: Rainfall on heavily engineered roads, pavements, roofs reaches the rivers with no ground water absorption.("Catchment") ("Fact Files-The River Itchen") 'River Itchen Sustainability Study' findings: The purpose of this study was to compare a range of environmental parameters (such as water quality, flow rates etc) and a key range of habitats and species in the Itchen catchment. The study judges the impact of various land use and water use (abstractions and discharges) on the river catchment and to what extent each is accountable in the observed differences between optimum conditions and current status. In the context of sustainability social, economic and environmental needs have to be balanced. Studies indicate that water quality and water quantity of the river Itchen are deteriorating. Factors responsible for deterioration of water quality: Sewage disposal - Chalk streams of river Itchen have crystal clear water quality because they are fed by springs from the aquifer and filtered through chalk. They are also alkaline by nature so they can cope with the discharge of effluents. But today, licensed discharges to the river system total 800.though most are small, they come from septic tanks. The major discharge is from Chickenhall Lane Sewage Treatment Plant in Eastleigh. So, many water quality parameters are at their poorest in the Eastleigh area. Water quality is also tested on the basis of their oxygen, nitrogen and phosphorous levels and when there are high volumes of these, it can lead to excessive growth of aquatic plants. Soluble phosphorous levels are 4 times higher than agreed levels in Eastleigh. In-river silt derived from decaying in-river plant, is encouraged by high phosphorous levels leading to poor salmon egg survival. Fish farming - After river water is used, it contains high level of chemicals. The return of this water affects fish farming. So, it is in unfavourable condition for various species and habitats. Land management / Agriculture - In the flood plain, grasslands are converted to arable land. So there are more silt loads in the river. ("River Itchen Sustainability Study") Higher flow rates (water quantity) generally mean higher water quality. Factors responsible for water quantity: Water abstraction for domestic and commercial use The amount of water falling on the catchment and entering the river affects the depth of the river , the flow rate of the river and the water table levels in the flood plain. Low flow rates mean reduced food availability and increased pollutants so, salmons migrate upriver. Low flow also means more silt and that reduced the survival rate of salmon eggs. Demand on River water resources (Abstractions and Discharge): ("Demand on River Water Resources") These are the interactions, both physical and environmental between water catchments, coastal areas and coastal waters. One of the important features of the Itchen's habitat is the diverse plant community, especially the water crowfoot. This influences the water levels and the flow of the river. It also provides habitat for the river farmer. In 1998, under the European Habitat's Directive, the Itchen was designated as a Special Area of Conservation (cSAC). The salmon, bullhead, brook lamprey, crayfish, trout, southern damselfly, mayfly, otter, water vole and shrimps are all, the creatures which were to be conserved and River Itchen was designated to support it. Downstream of Easton, the river has been maintained to provide optimum conditions for trout and fly-fishing. The river has been heavily engineered for milling, navigation and to support the water meadow systems. This has influenced the flow control and channel shape. Downstream of Bishopstoke, salmon and sea trout fishery is carried out. In recent times, there has been poor salmon egg survival. So, the agency annually removes silt from the gravels. Crayfish populations are also under threat because of a fungal disease which has spread there. Suitable habitat management is required for preserving the special southern damselfly. River Itchen can support a large number of invertebrates because of high water quality and also, water quantity. Poor quality of water and drought conditions can reduce their abundance. Two endangered mammal species in Britain, otter and water vole have been in decline of late. So effort is being made to increase water quality of River Itchen tributaries and provide safe passage through urban areas so that otter and water vole populations can be promoted. The agency promotes water-based recreation of water and associated land by permitting the harmonious enjoyment of the environment. There are many long-distance footpaths close to River Itchen like Itchen Valley Walk, Clarendon Way, Wayfarers Walk and the Pilgrims Way. ("Fact Files-The River Itchen") FLOOD: There are flood risks to people, property and the environment. The historic records for the floods that occurred in the catchment show the Sources of flooding as: 1) Groundwater 2) Rivers 3) Seas Groundwater- Sewer flooding of drainage systems-Downstream of the few major built-up areas, surface water flooding and its impact on inadequate road drainage and sewerage systems has become a real threat. High groundwater levels-This is caused by prolonged periods of rainfall, especially during winter. The elevated groundwater levels sustain high base flows that exceed channel capacities and this leads to flooding, thereby affecting properties. High groundwater levels -They rise above the ground surface locally. They affect properties directly, through flooding of cellars, basements and ground floors. Localised surface water runoff/over land flow-In wet winters, the groundwater levels can reach the surface of the chalk and result in springheads migrating upstream into previously dry areas of the Itchen catchment. These conditions worsen the localized surface water flooding. Surface water runoff exceeding river system capacities-In their downstream reaches, the Itchen retains it's groundwater influence but also receives significant surface water contributions that drive more distinct food peaks. Localised storms are a more important concern around the Southampton and Totton areas.( A frozen catchment and snow melt are also aspects that are important in chalk areas. River flooding- Rivers are a source of flooding and it can happen due to the following reasons: During continuous wet winters, there is gradual but continuous seepage of groundwater to the rivers that causes flooding. Some tributaries have clayey deposits. There is rapid runoff of rainwater during short but intense storms causing floods. Within the catchment at Winchester, there are restrictions and deficiencies in the capacity of the channel which results in greater flooding in the town. The Itchen river has a complex network of hydraulic structures and watercourses. Flooding from the Sea- Flooding of low-lying land from the sea and tidal estuaries is caused by storm surges and high tides. The onset of flooding from the sea can be extremely rapid with little warning. The severity of such flooding will depend on a number of factors, often in combination. Floods can occur due to any of the above reasons. The Itchen catchment area can be divided into four areas with differing key issues for flood risk management as described below. Environment Agency Test and Itchen Catchment Flood Management Plan - Consultation Draft Plan, June 2007 This table summarizes the main sources that have historically been seen in the Itchen area. It provides an indication of the scale of flooding of the area for notable events in the recent past, together with a list of further significant events from available records, the areas in the catchment they affected and the main source of flooding. Environment Agency Test and Itchen Catchment Flood Management Plan - Scoping Report, Jan 2006. Data required to identify areas at most risk from flooding, depths of flooding. This data has been obtained from Environment Agency. Test and Itchen Catchment Flood Management Plan - Consultation Draft Plan, June 2007. Existing river models of Andover, Romsey, Tadburn Lake Stream, Tidal Test, Monks Brook through Eastleigh and Bishopstoke, Lower / Middle Itchen have been used to understand flood risk in the Itchen area. In Winchester flood predictions from current mapping work on the River Itchen have been used. Flood records from the 2000 / 2001 extreme flood event have also been used for the same purpose. Flood extents and approximate flood depth for Winchester Note: key shows increasing flood depths; yellow areas are ground within 0.5m of flooding. Area Estimated Depth in metres Depth near property(m) Andover 2 Read More