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

• Increases in stocking densities
• Intensive cultivation
• Removal of hedgerows
• Construction of under-drainage and

ditching works

• Drainage/infilling of wetlands
• Increased urbanisation

Impacts

• Higher crop yields
• Larger commercial herds
• Decreased lag times
• Loss of permeability
• Reduction in flood plain area
• Removal of natural storage basins

= = 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.

• 2. NWR

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

• g was first used by Tans

nsle ley(1939) to describe ibe peat terrain n whic ich h conf

• nfor
• rms to the

he unde nderly lying ing topo

• pography, except

pt on n very steep p slope lopes.

• Bogs are ombr

brot

• troph
• phic

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

• undw

ndwater fo for water and nd nutrie ients)

• Blank

nket peat is also usua uall lly cons nsid idered d to

• be hy

hydr drolog

• logic

icall lly discon

• nne

nected from

• m

the unde nderly lying ng mine neral l layer making ng bog

• gs hig

ighly hly acidi idic (pH pH <4). ). (Fens tend nd to

• be

less acidi idic and nd base rich) h)

• Develo

lopm pment of f bla lank nket bog

• g is con
• ntrolle
• lled

d by clim imatic ic fa factor

• rs such

h as coo

• ol

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

• n of soi
• il pipe

pes, below

• w the peat sur

urface

• Reduc

uced d cove

• ver of spe

pecies which h are depe pende ndent on n a high h water tabl ble.

• Vegetation
• n cove
• ver is reduc

duced d and nd sur urface roug

• ughne

hness decreases, leadi ding ng to

• inc

ncreased d leve vels of ove verland nd flow

• w veloc
• cities and

nd faster delive very y of water int nto

• cha

hanne nnels

• Bare peat sur

urfaces becom

• me expos

posed d to com

• mpaction
• n by

y raindr ndrop

• p action.
• n.
• Reduc

uction

• n in

n inf nfiltration

• n rates and

nd inc ncreases overland nd flow

• w prod
• duction
• n dur

uring ng signi nificant nt rainfall event nts.

• As a resul

ult, the formation

• n of gul

ully y networ

• rks

ks can n inc ncrease draina nage dens nsity, and nd channe nnel conn

• nnectivi

vity, leading ng to highe her catchm hment dr draina nage efficienc ncy, flashi hier hydr drog

• graphs

phs and nd highe her stor

• rm flow
• w 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
• f 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

Location Discharge (m3/s) Upper Stream 0.00803678 Lower Stream 0.009079433

Topographic Survey Flume me installed Storage areas calculated Peat depths Slope Map

Questions?