IAHR2015 - Natural foreshores as an alternative to traditional dike re-enforecements, Penning, Deltares, 20150629

Ellis Penning, Henk Steetzel, Robbin van Santen, Jasper Fiselier,

Marieke de Lange, Vincent Vuik, Sonja Ouwerkerk, Jaap van Thiel de

Vries

NATURAL FORESHORES AS AN ALTERNATIVE

TO TRADITIONAL DIKE RE-ENFORECEMENTS:

A FIELD PILOT IN THE LARGE SHALLOW LAKE MARKERMEER,

THE NETHERLANDS

What are natural foreshores?

Long term safety:

Marshes grow

with SLR

reefs

Marshes, mangroves, reedbeds

Foreshore & beach sand nourishments

Use & produce

ecosystem services

Limited wave reduction

Enhanced wave reduction

Maximized wave reduction

What are natural foreshores?

Long term safety:

Marshes grow

with SLR

reefs

Marshes, mangroves, reedbeds

Foreshore & beach sand nourishments

Use & produce

ecosystem services

Limited wave reduction

Enhanced wave reduction

Maximized wave reduction

What are natural foreshores?

Long term safety:

Marshes grow

with SLR

reefs

Marshes, mangroves, reedbeds

Foreshore & beach sand nourishments

Use & produce

ecosystem services

Limited wave reduction

Enhanced wave reduction

Maximized wave reduction

From coastal to inland applications

Markermeer: 700 km2 Fetch: up to 40 km Avg. Depth: 2-4 meters No tides, substantial waves and water level fluctuations due to wind Managed unnatural water level: -0,4 NAP winter -0,2 NAP summer Houtridbijk splits lake Markermeer from lake IJsselmeer Houtribdijk needs maintenance (26 km)

Considerations during design phase Pilot under the National High water protection programme – innovation funds:

• Only sand nurishments applied from water - cheaper and faster

• Stable slope – steeper is cheaper

• Account for sediment consolidation of the underground

• Can vegetation aid the stability of

the construction during normal

and extreme situations

• Legal requirements

Details design

Section 1 en 4 : sand

Section 2 en 3: top layer mixed with clay

Vak 1 Vak 2 Vak 4 RestvakVak 3

zand

zand

zand

zand

holoceen+zand holoceen + zand

zand

zand

• Each section half planted, half empty

• Willow mats with reeds (section 3 and 4)

• Exclosures to define effect of grazing

Role of vegetation

Section 1 Section 2 Section 4 Section 3

zand

zand

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Monitoring

4 years

• Hydrodynamics and meteorology (continuous)

wave- and flow patterns as forcing factor

• Morfodynamics (monthly)

consolidation, erosion/sedimentation due to hydrodynamics and vegetation

• Vegetation development (2x year)

safety and biodiversity

Hs = 0.9 m

Water levels under influence of wind

Morfological developments - september

Morfological developments - october

Morfological developments - november

Morfological developments - december

Morfological developments - january

Morfological developments - februari

Morfological developments - march

Morfological developments- april

Morfological developments sept-march

Wind screen

Effect interpolation

Interaction with dike

Slope development

Analyses slope development

Analyses morphological developments

First impressions

• A stable, yet dynamic foreshore

• Steep slope (1 : 10) along the waterline

• Swash-berm up to NAP+0,5 m

• Underwater plateau nearly flat at -1m NAP

• Effect of coarse sand?

Sediment grain size

Role between mean grain size and slope

Design:

D50 = 200/250 µm slope 1:30/40

Present:

D50 = 350/400 µm slope 1:10

Next steps

• Monitoring until 2018

• Use of data for improvement and validation of models

• Extrapolation to other locations

• Need for testing of extreme conditions in controlled

flume experiment