Reducing Flood Risk with Natural Water Retention Measures and Drain - PowerPoint PPT Presentation

Reducing Flood Risk with Natural Water Retention Measures and Drain Blocking in Drained Blanket Bogs Geography Symposium 2020 Rob Halpin

Human Activities and Flooding Land Use Changes Impacts • Increases in stocking densities • Higher crop yields • Intensive cultivation • Larger commercial herds • Removal of hedgerows • Decreased lag times • Construction of under-drainage and • Loss of permeability ditching works • Reduction in flood plain area • Drainage/infilling of wetlands • Removal of natural storage basins • Increased urbanisation = = Incre reas ased flo lood peak aks

What is Natural Water Retention? 1. 1. Aims to reduce the fre requency and severi rity of flo loodin ing g through gh land management strategies and ‘Catchment - based flood management’. 2. NWR 2. R activ ivit itie ies also hav ave the over veral all l aim of reducin ing g flo lood haz azar ard, whil ile also sustain inin ing or enhancin ing other r potentia iall lly sign gnif ific icant co-benefit its: • Greater biodiversity • Improved soil • Increased water quality • Carbon sequestration • Reduced soil erosion • Greater agricultural productivity • Improved public health and well-being.

Research Question Do drain blocks and permeable barriers on blanket bog reduce time-to- peak/duration of flood flows and decrease DOC levels of flows downstream?

Blanket Bog Characteristics and Formation The term bla lank nket bog og was first used by Tans nsle ley(1939) to describe ibe peat terrain n • whic ich h conf onfor orms to the he unde nderly lying ing topo opography, except pt on n very steep p slope lopes. Bogs are ombr brot otroph ophic ic peatla land nds whic hich h are depe pende ndent on n precipi ipitatio ion n fo for • water and nd nut utrie ient supp pply ly. (Fens are reli liant on n grou oundw ndwater fo for water and nd nutrie ients) Blank nket peat is also usua uall lly cons nsid idered d to o be hy hydr drolog ologic icall lly discon onne nected from om • the unde nderly lying ng mine neral l layer making ng bog ogs hig ighly hly acidi idic (pH pH <4). ). (Fens tend nd to o be less acidi idic and nd base rich) h) Develo lopm pment of f bla lank nket bog og is con ontrolle olled d by clim imatic ic fa factor ors such h as coo ool l • summers and nd hig igh h rain infall ll (> 1,250 mm)

Location of Bogs in Ireland Heavy vy ra rain infal all l + cool l summers rs are re charac acteri ristic ic feature in many areas reas alo long g the western rn seab aboar ard and in hig igher r hil ill l and mountai ainous areas as in Ire rela land whic ich receiv ive high gher r ra rain infal all l Re Red - Rais ised Bog Blue – Lowla land At Atla lantic ic bog Green – High gh leve vel l Montan ane Bog ~66% of this is peatla land area rea in Irelan reland is blanket bog, g, makin ing g it the most common form rm of peat in Irela land. Source: Connolly et al, 2007

Anthropogenic Interference of Bogs

Impacts of Drainage on Peat – Vegetation and Hydrology Lowering of water tabl ble • Increased d cracki king ng and nd the he formation on of soi oil pipe pes, below ow the peat sur urface • Reduc uced d cove over of spe pecies which h are depe pende ndent on n a high h water tabl ble. • Vegetation on cove over is reduc duced d and nd sur urface roug oughne hness decreases, leadi ding ng to o inc ncreased d • leve vels of ove verland nd flow ow veloc ocities and nd faster delive very y of water int nto o cha hanne nnels Bare peat sur urfaces becom ome expos posed d to com ompaction on by y raindr ndrop op action. on. • Reduc uction on in n inf nfiltration on rates and nd inc ncreases overland nd flow ow prod oduction on dur uring ng • signi nificant nt rainfall event nts. As a resul ult, the formation on of gul ully y networ orks ks can n inc ncrease draina nage dens nsity, and nd • channe nnel conn onnectivi vity, leading ng to highe her catchm hment dr draina nage efficienc ncy, flashi hier hydr drog ographs phs and nd highe her stor orm flow ow peaks ks

Impacts of Drainage on Peat – Dissolved Organic Carbon • 72-84% of peatlands to be drained to increase the area of land suitable to forestry, agriculture, or the extraction of peat for fuel • Estimated that approximately 25% of global soil organic carbon is stored in peatlands, despite peat only covering ~3% of earth’s terrestrial surface. • Drainage lowers the water table, enabling aerobic decomposition of peat, and therefore, increases DOC release. • Drained catchments export ~15-33% more DOC than pristine catchment, with the final figure depending on the spacing between drains ( Worrall et al, 2007 )

Dissolved Organic Carbon in Water • DOC in water decreases aesthetic quality • DOC also consumes free residual chlorine, which can result in the formation of tri-halomethanes (carcinogens) • Concentrations of such chemicals are limited by law in drinking water and, therefore, DOC removal, represents one of the major costs in relation to water treatment in Northern Europe

Why Restore Bogs? GHGs Peatlands are listed for protection in Annex 1 of • the EU Habitats Directive: Those that are damaged and suitable for restoration, should be restored (92/43/EEC) Important for meeting current climate • agreements Irish government aiming to restore 50% of • peatlands within the next 11 years New EU ‘Green Deal’ aims to make the EU • carbon neutral by 2050

Why Restore Bogs? • Majority of studies of the last 15-20 years have focussed on GHG releases or vegetation recovery • More recently, growing interest in applying similar restoration practices for the purposes of delivering wider ecosystem services, such as: 1. Reduced soil erosion rates 2. Improving water quality 3. Stabilising water tables 4. Stabilising discharge

Research Aims Research Question: Do drain blocks and permeable barriers on blanket bog reduce time-to-peak/duration of flood flows and decrease DOC levels of flows downstream? 1. Undertake experiments using drain blocks and barriers at two Irish blanket bog sites. 2. Monitor the response of drain block/barrier designs under a range of flow conditions. 3. Undertake hydrographical and water quality analysis on these data to assess the efficacy of drain blocks/barriers. 4. Deliver a range of recommendations to stakeholders for future national implementation.

Study Site 1: Wicklow Mts. Blanket Bog • Highly drained montane blanket bog • Approx 50% site used for industrial turf cutting • 8 main drains onsite • Steep slopes (3-22 degrees) • Drain depths vary between 0.05-2.5m • Largely bare peat surrounding drains

Study Site 2: Near Oweninny Bog, Co. Mayo. • Drained low-level Atlantic blanket bog (70 m asl) • Some of the site has been industrially cut beginning in 1950s, but all has been drained • Mixture of low slopes • Peat depths vary between 0.04-0.58 m • Site selection of site 2 will begin once COVID restrictions are lifted.

Peatland Restoration Upland Drain Blocking • Can be constructed from range of cheap materials (wood, plastic, compacted peat) • Encourages excess flow to be distributed back onto the peatland surface • Decrease runoff velocity • Aid in restoring degraded peatlands and increase biodiversity • Decreases (and eventually stops) GHG emissions from peatlands

Engineered Log Jams (Leaky Dams) • In stream structures constructed from locally-sourced timber (or other materials e.g. stone) • Can be simple, small structures or dozens of logs secured to the river bank • Causes the roughness coefficient to become abnormally high (Manning’s n>1) when in -stream debris is greater than the flow depth • Provide multiple benefits for nutrient cycling, sediment transport and micro environments for terrestrial and aquatic organisms

Experimental Design

Experimental Design

Experimental Design

Experimental Design

Experimental Design

Experimental Design

Some Preliminary Results Topographic Survey Flume me installed Discharge (m 3 /s) Location Upper Stream 0.00803678 Lower Stream 0.009079433 Storage areas calculated Peat depths Slope Map

Questions?