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The Hydrology of Sherborne


Dr Paul Webster

March 2022

Introduction

Dr Paul Webster, a hydrologist, discussed the challenges of natural water systems and water resource management, highlighting features of particular interest within the Sherborne area. Paul’s expertise is within the fields of Physical Geography and Engineering Hydrology. He studied at Bristol, Imperial and Birmingham Universities and has extensive professional experience overseas, including in Africa, Pakistan, China, and Singapore. In addition, more recently, he has been a volunteer hydrologist for The Dorset Wildlife Trust and The National Flood Forum. He has written numerous articles and published many papers on water resources and flood management and is Principal Hydrologist at Corylus (planning environmental consultancy).

Water Cycling

Hydrology centres around the hydrological cycle, whereby water follows a generally predictable trajectory through various ‘reservoirs’ in the Earth system (see Fig. 1).


Fig.1 Hydrological cycle

Within this somewhat simplified water cycle, there are many sub-cycles of interest to both scientists and hydrologists.

Whilst 70% of the Earth’s surface is ocean, it is over the continents, a much smaller proportion of the Earth’s surface, where the interaction of land and water are particularly important and relevant to human life and therefore to hydrologists.

As an academic subject with strong practicalities, hydrology encompasses several activities:

  1. Hydrometry- that is data, rainfall, flow, evaporation etc, etc. Monitoring is done mainly by the Environment Agency and data is available in real time, for example river flows. The Met Office is another significant source, as are many individuals with private weather stations. The explosion in freely available data needs careful assessment; not all data is of high quality and care must be taken when considering its use in investigations. Some data is of considerable historical interest. A recent article in The Times noted that UK rain records back to the 1800’s had been consolidated by a group of volunteers during the COVID lockdown to better inform of past rainfall. Hydrometric data also includes chemical analysis of water sources. Sherborne water, for example, is very hard (360.73 CaCO3 mg/litre). Wessex Water looks at various solutes, such as Al and Cl etc, in its water and publishes solute concentrations for various sites on its web pages.

  1. River basin management- ensuring water of sufficient quality and quantity. Such activity includes contingency planning such as being prepared to manage supplies during drought conditions. This is done mainly by water companies working with the Environment Agency. The year of the London Olympics, 2012, was a particularly interesting year. Until mid-March there was a severe drought. Thames Water considered cutting back water supplies (which would have been highly unpopular in Olympic year with the country showcased to the world). Then, from mid-April onwards, whilst the Olympic torch was carried around the UK, storm after storm not only drenched the torch carriers but threatened to extinguish the flame itself. The sheer variability of the weather can make planning quite challenging. Whilst difficult in the UK, consider Australia, where the climate can flip between drought and deluge within a few months, the most recent manifestation being in Brisbane this year.


  1. Flood Risk Management- involves assessing flood risks from all sources and flood protection over a range of scales, for both large areas down to individual small properties. Models are extensively used and provide the flood zoning used by planners.

Concern was expressed at the quality of river water. In certain cases, agricultural development adjacent to river courses could mitigate against clean river water. The River Wye, in Wales, for example, has many recently established chicken farms which leach effluent into the river badly affecting water quality. Some wild swimmers make the serious claim that the river, with its once extensive ecosystem, is largely dead. Also, mismanaged pasture can lead to fine sediments remaining suspended in the water. Another detrimental aspect is the design of many older and still operating sewage systems. In these systems (from the 19th/20th century) sewage and runoff are combined (in modern units, they are separated), with the result that during a heavy downpour, the system may be overwhelmed, and some raw, untreated sewage discharged into the river. Whilst all sewage release is logged there are probably also questions of suitable maintenance in older systems.

A Local Perspective

As noted earlier, the water cycle can be considered at different scales with Fig. 1 indicating continental scale water cycling. At a more local level, a sub-cycle of the global water cycle can be found, the so-called urban water cycle (Fig 2).


Fig. 2 Urban water cycle-main components and pathways

Paul, with a series of photographs demonstrated how this sub-cycle fitted together in Sherborne, and how it developed from historical roots. For example, the Waterworks Museum, was and still is the origin of much drinking water for Sherborne. Originally, a 26-foot waterwheel, one of the largest in Dorset, delivered drinking water to a reservoir on the Bristol road. The same source of water is still used, only this time extracted from a borehole by electric pump. On a map of Sherborne, important sites were indicated, such as boreholes, service reservoirs, including one halfway up Bristol Road at its junction with St Aldhelm’s Road. This service reservoir is supplied by pumping from a borehole and allows demand for water, with its predictable swings in demand throughout the day, to be seamlessly met, the early morning being the biggest draw on the system. In a different part of town, adjacent to the recycling centre and the River Yeo, is the sewerage treatment centre which discharges into the River Yeo.

This local cycle is very relevant to the residents of Sherborne and local water systems should ideally run by gravity. In areas where surface relief mitigates against it, sewerage pumps can be used. They are not ideal because, if they fail, things can get very messy.

Managing Fresh Water Resources

A first requirement for managing water resource is to know what falls from the sky and the atmospheric variables that can influence this. Data is available from the Met Office. Usefully, there are many private weather stations linked by the internet (e.g. Davis Weather Network) which can supply data to the public domain. With no universal standards enforced to the siting of such stations, the quality of data can vary but where constraints are known, quality, usable information is available. One such station Paul had found particularly useful is situated next to the school in Milborne Port. Data for such stations includes temperature, dew point, gust velocity and wind direction, rainfall, all plotted against time. Paul indicated that meteorological data generally has shown the last two winters to be somewhat dry, with only 50% of expected rainfall. Whilst unlikely to impact later this year, it may do so next year if the expected precipitation fails to materialise. Conversely, the last 20 years have been a flood risk prone period, most recently in 2020 with Storm Clara and (one week later) Storm Dennis depositing heavy rain on already saturated catchment areas.

Another important element to understanding water resources is to identify catchment areas and how these combine into progressively larger areas as meteoric (rain) water which, once deposited on the ground, returns by gravity to the ocean (thus fulfilling the return part of the water cycle- see Fig.1).

To the north of Sherborne is the Coombe catchment area, drained by Coombe stream originating near to the golf course, which passes beneath the road at Newell. It once visibly flowed along the western side of Sherborne School and Abbey with sufficient flow to power a water mill (see Fig. 3 below). The Coombe catchment occupies an area of 3.7km2.











Fig. 3 Sherborne Abbey circa 1530 with water mill to the right

Adjacent is another catchment area draining into lake at Sherborne Castle, representing a catchment area of 40km2 which flows into the River Yeo. Lake levels are monitored by the Environment Agency and provide another source of data which is accessible in (almost) real time. The Agency maintains a large number of river level monitoring stations throughout the country (see https://riverlevels.uk/map#.Yk4TIS1Q1QI) providing 5 days of data and, in some cases a prediction of future flow.

As the Yeo flows downstream, contributary catchments become progressively larger. At Yeovil Pen Mill the catchment is 216km2, the next, the Yeovil/Ilchester catchment is 319km2, the following Langport catchment is 764km2, and finally the Parrett (Bridgwater) catchment, being the largest, has an area of 1,200km2.

A comparison of catchment areas shows an upward progression in area looking downstream. From a resource management point of view the connectivity between land and sea provides not only an indication of water flow but a hydrological pathway for transmission of pollutants. Phosphate pollution is of particular concern at the moment with planning requirements for new developments to counter this to slow contamination of the Somerset Levels.

Flooding

Husbandry of fresh water is only part of the job of a hydrologist. Also of importance is flooding, of which there are several categories. Surface water flooding caused by a sudden downpour can be concentrated in particular areas, for example, near to the old Antelope pub in Cold Harbour where surface water, during a heavy downpour, can deluge down Bristol Road. The good news is that generalised modelling is very good at capturing such flooding events therefore permitting better management of the problem. There was significant flooding in the town and its environs in 1979. A response to this can be seen in the Sherborne flood scheme by the railway close to the station. River flooding is another category of flooding which can be severe when a catchment area is primed by previous rainfall.


Fig. 4. Flood level marker in Sherborne Abbey- an item of useful historical data

In May 1709 a prolonged, very heavy hail shower, blocked and Coombe adjacent to the Abbey causing not inconsequential flooding in the building itself (see Fig. 4). Monuments and flood levels placed on buildings after extreme events provide items of useful historical data.

The Future

Horizons regarding the future are focussed on the next international climate conference, COP27 scheduled for Sharm-El-Sheikh, Egypt, in November of this year. For Paul, the future of water resources is closely linked to climate change. He regards greenhouse warming due to CO2 release of particular concern together with ocean acidification due to the dissolution of vast amount of CO2 into the ocean conveyor system. The prognosis for the UK is rising sea levels (a ‘rock solid’ trend), hotter drier summers, longer more intense droughts, warmer winter months with an increase in both flooding and storm intensity. The changes are, as they are presently manifesting themselves, happening astonishingly rapidly compared to previous episodes of climate change found in the geological record over the past tens of millions of years (see previous presentation report for February 2022: ‘Hot Air- inside story of battle against climate change denial’, by Professor Peter Stott).

Concern was raised from the audience about the quality of land management. Soil is rightly regarded as our future and has been much abused in the name of agriculture. More hedges and schemes to protect against erosion and compaction were suggested as being desirable. Also, opportunities to not only mitigate flooding but increase biodiversity had been tragically missed, namely the flood attenuation pond on the new Bradfords estate. Rivers, generally, are increasingly channelled and making space for water will help mitigate flooding episodes.

It was noted that there were episodes of flooding in properties where for over 200 years there had been none recorded. The potential for increased flooding with increasing climate change is taken into account in the planning process. In planning, design life is 100 years, and in accounting for possible future storms, a margin of 40% is usually allowed for a 1% flood risk. Another requirement of planning is to provide a net gain of biodiversity of 10% within the site or adjacent to it. While developers are reluctant to provide for an increase in biodiversity on site (they want to ‘plant’ as many houses as possible), they are increasingly turning to adjacent low grade agricultural land and farmers are offering such lots at 10-15k per acre. Beavers, it was suggested, could possibly help, and evidence from Devon suggests by building dams they do attenuate floods and filter sediment. However, in doing so they create wetlands and land is generally sacrificed for these wetland areas which might otherwise be usefully available for other purposes.

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