{"id":2850,"date":"2024-02-06T15:39:27","date_gmt":"2024-02-06T15:39:27","guid":{"rendered":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding\/research-overview\/unconfined-major-aquifers\/"},"modified":"2025-11-13T10:58:00","modified_gmt":"2025-11-13T10:58:00","slug":"unconfined-major-aquifers","status":"publish","type":"page","link":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/","title":{"rendered":"Unconfined, major aquifers"},"content":{"rendered":"\t\t\t<section class=\"hero-section hero-section-with-social\">\n\t\t\t\t\t\t\t\t<div class=\"hero-info-wrap\">\n\t\t\t\t\t<div class=\"container\">\n\t\t\t\t\t\t<div class=\"row\">\n\t\t\t\t\t\t\t<div class=\"col-12\">\n\t\t\t\t\t\t\t\t<div class=\"hero-info\">\n\t\t\t\t\t\t\t\t\t<h1>Unconfined, major aquifers<\/h1>\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t\t\t\n\t\t\t<div class=\"social-wrap social-wrap-inline-them\">\n\t\t\t\t<div class=\"sea-background-f\">\n\t\t\t\t\t<span>Share this article<\/span><a target=\"_blank\"  href=\"https:\/\/www.facebook.com\/sharer\/sharer.php?u=https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\"><span class=\"icon-facebook\"><span class=\"hidden\">Facebook<\/span><\/span><\/a><a target=\"_blank\"  href=\"https:\/\/twitter.com\/intent\/tweet?text=Unconfined,%20major%20aquifers&#038;url=https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\"><span class=\"icon-twitter\"><span class=\"hidden\">Twitter<\/span><\/span><\/a><a   onclick=\"javascript:void( (function() {var e=document.createElement('script' );e.setAttribute('type','text\/javascript' );e.setAttribute('charset','UTF-8' );e.setAttribute('src','\/\/assets.pinterest.com\/js\/pinmarklet.js?r='+Math.random()*99999999);document.body.appendChild(e)})());\"  href=\"javascript:void(0);\"><span class=\"icon-pinterest\"><span class=\"hidden\">Pinterest<\/span><\/span><\/a><a target=\"_blank\"  href=\"https:\/\/wa.me?text=https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\"><span class=\"icon-whatsapp\"><span class=\"hidden\">WhatsApp<\/span><\/span><\/a><a   href=\"mailto:?subject=Unconfined,%20major%20aquifers&#038;body=%20(https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/)\"><span class=\"icon-mail\"><span class=\"hidden\">Email<\/span><\/span><\/a><a href=\"#\" data-copy=\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\" class=\"copy-url-on-click\"><span class=\"icon-link\"><span class=\"hidden\">Copy Link<\/span><\/span><\/a><\/div><\/div>\t\t\t\t\t\t\t<\/section>\n\t\t\t\n\n\n<section class=\"wp-block-bgs-blocks-content-container content-section\"><div class=\"container\"><div class=\"row\"><div class=\"col-12\">\n<p>Long-lasting, often regionally extensive, groundwater flooding can be caused by the water table in an&nbsp;unconfined aquifer&nbsp;rising above the land surface in response to extreme rainfall. This is sometimes referred to as&nbsp;clearwater flooding and is this form of flooding that has caused significant damage to properties on the Chalk outcrop of southern England in recent years.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">When does groundwater flooding occur?<\/h2>\n\n\n\n<p>Groundwater flooding occurs when existing high groundwater levels and high unsaturated zone moisture content combine with intense rainfall.<\/p>\n\n\n\n<p>In the Chalk aquifer, this can cause groundwater levels to fluctuate over several tens of metres. For example, in the winter of 2000 to 2001, groundwater flooding was associated with unusually high levels of rainfall: for an eight-month period starting in September 2000, rainfall in south-east England was 183 per cent of the long-term average, equivalent to a greater than 100-year return period.<\/p>\n\n\n<figure  class=\"img-wrap\"><img decoding=\"async\" src=\"https:\/\/www.bgs.ac.uk\/groundwater\/wp-content\/uploads\/sites\/4\/2025\/10\/P641021.jpg\" class=\"lazy-blur\" alt=\"A flooded road with &#039;Road Closed&#039; signs in the water\" data-src=\"https:\/\/www.bgs.ac.uk\/groundwater\/wp-content\/uploads\/sites\/4\/2025\/10\/P641021.jpg\" \/><figcaption class=\"caption \">\n\t\t\t\t\t<div class=\"caption-icon\">\n\t\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.bgs.ac.uk\/groundwater\/wp-content\/themes\/bgs\/img\/info-caption.svg\" alt=\"Information icon\">\n\t\t\t\t\t<\/div>\n\t\t\t\t\t<div class=\"caption-text\"><p>Roads in Berkshire had to be closed during the exceptionally wet winter of 2000 to 2001 due to groundwater flooding. BGS \u00a9 UKRI.<\/p>\n<\/div>\n\t\t\t\t\t\n\t\t\t\t<\/figcaption><\/figure>\n\n\n<p>Groundwater levels were already high as there had been several years of above-average recharge before 2000. Significant groundwater flows occurred in dry valleys in localities far removed from floodplains. <\/p>\n\n\n\n<p>River flows also reflected the high rainfall as well as the contribution from groundwater discharge. Flows at most river gauging stations greatly exceeded the long-term averages for extended periods and, in some catchments, sustained groundwater flooding occurred for several months.<\/p>\n<\/div><\/div><\/div><\/section>\n\n\n<section class=\"cta-section  sea-background-e \">\n\t<div class=\"container\">\n\t\t<div class=\"row md-row\">\n\t\t\t<div class=\"col-12\">\n\t\t\t\t<h2>Need more information?<\/h2>\t\t\t\t\t\t\t\t<div class=\"btn-wrap\"><a href=\"https:\/\/www.bgs.ac.uk\/people\/macdonald-alan\/\"  class=\"button primary-background   \">Contact Alan MacDonald<\/a><\/div>\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n<\/section>\n\t","protected":false},"excerpt":{"rendered":"","protected":false},"author":6,"featured_media":0,"parent":78,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_acf_changed":false,"_relevanssi_hide_post":"","_relevanssi_hide_content":"","_relevanssi_pin_for_all":"","_relevanssi_pin_keywords":"","_relevanssi_unpin_keywords":"","_relevanssi_related_keywords":"","_relevanssi_related_include_ids":"","_relevanssi_related_exclude_ids":"","_relevanssi_related_no_append":"","_relevanssi_related_not_related":"","_relevanssi_related_posts":"1","_relevanssi_noindex_reason":"","footnotes":""},"tags":[],"class_list":["post-2850","page","type-page","status-publish","hentry"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.2 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Unconfined, major aquifers - BGS Groundwater<\/title>\n<meta name=\"description\" content=\"Long-lasting, often regionally extensive, groundwater flooding can be caused by the water table in an\u00a0unconfined aquifer.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Unconfined, major aquifers - BGS Groundwater\" \/>\n<meta property=\"og:description\" content=\"Long-lasting, often regionally extensive, groundwater flooding can be caused by the water table in an\u00a0unconfined aquifer.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\" \/>\n<meta property=\"og:site_name\" content=\"BGS Groundwater\" \/>\n<meta property=\"article:modified_time\" content=\"2025-11-13T10:58:00+00:00\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data1\" content=\"1 minute\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\",\"url\":\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\",\"name\":\"Unconfined, major aquifers - BGS Groundwater\",\"isPartOf\":{\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/#website\"},\"datePublished\":\"2024-02-06T15:39:27+00:00\",\"dateModified\":\"2025-11-13T10:58:00+00:00\",\"description\":\"Long-lasting, often regionally extensive, groundwater flooding can be caused by the water table in an\u00a0unconfined aquifer.\",\"breadcrumb\":{\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/#breadcrumb\"},\"inLanguage\":\"en-GB\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/\"]}]},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Home\",\"item\":\"https:\/\/www.bgs.ac.uk\/groundwater\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"Extremes\",\"item\":\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/\"},{\"@type\":\"ListItem\",\"position\":3,\"name\":\"Flooding\",\"item\":\"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/\"},{\"@type\":\"ListItem\",\"position\":4,\"name\":\"Unconfined, major aquifers\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/#website\",\"url\":\"https:\/\/www.bgs.ac.uk\/groundwater\/\",\"name\":\"BGS Groundwater\",\"description\":\"\",\"publisher\":{\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/#organization\"},\"potentialAction\":[{\"@type\":\"SearchAction\",\"target\":{\"@type\":\"EntryPoint\",\"urlTemplate\":\"https:\/\/www.bgs.ac.uk\/groundwater\/?s={search_term_string}\"},\"query-input\":{\"@type\":\"PropertyValueSpecification\",\"valueRequired\":true,\"valueName\":\"search_term_string\"}}],\"inLanguage\":\"en-GB\"},{\"@type\":\"Organization\",\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/#organization\",\"name\":\"BGS Groundwater\",\"url\":\"https:\/\/www.bgs.ac.uk\/groundwater\/\",\"logo\":{\"@type\":\"ImageObject\",\"inLanguage\":\"en-GB\",\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/#\/schema\/logo\/image\/\",\"url\":\"https:\/\/www.bgs.ac.uk\/groundwater\/wp-content\/uploads\/sites\/4\/2023\/11\/BGS_Groundwater_pos.svg\",\"contentUrl\":\"https:\/\/www.bgs.ac.uk\/groundwater\/wp-content\/uploads\/sites\/4\/2023\/11\/BGS_Groundwater_pos.svg\",\"width\":547,\"height\":136,\"caption\":\"BGS Groundwater\"},\"image\":{\"@id\":\"https:\/\/www.bgs.ac.uk\/groundwater\/#\/schema\/logo\/image\/\"}}]}<\/script>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Unconfined, major aquifers - BGS Groundwater","description":"Long-lasting, often regionally extensive, groundwater flooding can be caused by the water table in an\u00a0unconfined aquifer.","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/","og_locale":"en_GB","og_type":"article","og_title":"Unconfined, major aquifers - BGS Groundwater","og_description":"Long-lasting, often regionally extensive, groundwater flooding can be caused by the water table in an\u00a0unconfined aquifer.","og_url":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/","og_site_name":"BGS Groundwater","article_modified_time":"2025-11-13T10:58:00+00:00","twitter_card":"summary_large_image","twitter_misc":{"Estimated reading time":"1 minute"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"WebPage","@id":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/","url":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/","name":"Unconfined, major aquifers - BGS Groundwater","isPartOf":{"@id":"https:\/\/www.bgs.ac.uk\/groundwater\/#website"},"datePublished":"2024-02-06T15:39:27+00:00","dateModified":"2025-11-13T10:58:00+00:00","description":"Long-lasting, often regionally extensive, groundwater flooding can be caused by the water table in an\u00a0unconfined aquifer.","breadcrumb":{"@id":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/#breadcrumb"},"inLanguage":"en-GB","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/"]}]},{"@type":"BreadcrumbList","@id":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/unconfined-major-aquifers\/#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.bgs.ac.uk\/groundwater\/"},{"@type":"ListItem","position":2,"name":"Extremes","item":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/"},{"@type":"ListItem","position":3,"name":"Flooding","item":"https:\/\/www.bgs.ac.uk\/groundwater\/flooding-and-drought\/flooding\/"},{"@type":"ListItem","position":4,"name":"Unconfined, major aquifers"}]},{"@type":"WebSite","@id":"https:\/\/www.bgs.ac.uk\/groundwater\/#website","url":"https:\/\/www.bgs.ac.uk\/groundwater\/","name":"BGS Groundwater","description":"","publisher":{"@id":"https:\/\/www.bgs.ac.uk\/groundwater\/#organization"},"potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/www.bgs.ac.uk\/groundwater\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-GB"},{"@type":"Organization","@id":"https:\/\/www.bgs.ac.uk\/groundwater\/#organization","name":"BGS Groundwater","url":"https:\/\/www.bgs.ac.uk\/groundwater\/","logo":{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/www.bgs.ac.uk\/groundwater\/#\/schema\/logo\/image\/","url":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-content\/uploads\/sites\/4\/2023\/11\/BGS_Groundwater_pos.svg","contentUrl":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-content\/uploads\/sites\/4\/2023\/11\/BGS_Groundwater_pos.svg","width":547,"height":136,"caption":"BGS Groundwater"},"image":{"@id":"https:\/\/www.bgs.ac.uk\/groundwater\/#\/schema\/logo\/image\/"}}]}},"_links":{"self":[{"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/pages\/2850","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/comments?post=2850"}],"version-history":[{"count":29,"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/pages\/2850\/revisions"}],"predecessor-version":[{"id":7701,"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/pages\/2850\/revisions\/7701"}],"up":[{"embeddable":true,"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/pages\/78"}],"wp:attachment":[{"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/media?parent=2850"}],"wp:term":[{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bgs.ac.uk\/groundwater\/wp-json\/wp\/v2\/tags?post=2850"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}