Flood Mitigation by Forestry

Prof. Dr. Gebhard SchülerResearch Institute for Forest Ecology and Forestry Rhineland-Palatinate, Germany

Flood generation

Identification of runoff processes

Precautionary measures for flood mitigation

Efficiency of decentralized retention measures

Conclusions

Runoff and flood are natural processes …

… water needs enough space to spread …

Flash floods can become very dangerous, and it is expected that they will happen in a higher frequency in future with a changing climate.

They occur in shortest time – and efficient protection measures are rarely to find.

A changing landuse with decreasing forest areas, shortening river courses, increasing intensified agriculture and settlement areas accelerates and increases the runoff and overland flow.

In consequence protection against floods and flash floods should start when runoff occurs.

It is important to know how and where runoff is generated (hot spots of flood generation) before implementing water retention measures

Hot spots of flood generation…

… can be derived from digital information about forest sites with a GIS-based expert system

Infiltrationsvermögen / Wasserleitfähigkeit im Oberbodenmittel – hoch (> 10 cm/d)

Infiltrationsvermögen / Wasserleitfähigkeit im Oberbodenmittel – hoch (> 5 cm/d)

Standorte mit dichten Zwischenlagen (Wasserleitfähigkeit gering, < 10 cm/d)

ohne Klüfte

janein

nein ja

gering (< 50 l/m2)Hang- Abfluss-

neigung prozess< 2° SOF32-20° SOF2

20-40° HOF2> 40° HOF1

mittel, hoch, sehr hoch (> 50 l/m2)

Hang- Abfluss-neigung prozess

< 2° SSF32-20° SSF2

20-40° HOF2> 40° HOF1

nWSK(Grob- und Mittelporen

0,2-50 µm)

gering (< 50 l/m2)Hang- Abfluss-neigung prozess

< 2° SOF32-20° SSF220-40° SSF1> 40° HOF1

mittel, hoch (50-200 l/m2)

Hang- Abfluss-neigung prozess< 2° SSF32-20° SSF220-40° SSF1> 40° HOF1

nWSK(Grob- und Mittelporen

0,2-50 µm)

Geologie durchlässig

janein

(Sande, Lehmsande, Sandlehme, Schluffe)(Tone, Tonlehme,

Lehmtone, Felsflächen -ohne Humusauflage,

ohne Vegetationsbedeckung)

Felsflächen

porös

nein ja

Standorte mit hydromorphen Merkmalen

janein

(Sande, Lehmsande, Sandlehme, Schluffe)

(Tone, Tonlehme,Lehmtone -

ohne Humusauflage,ohne Vegetationsbedeckung)

Stau-/Grundwasser im Oberboden

> 7 Monate/Jahr

Staunässestufe s 3/4Grundnässestufe g3/4Grundform VIII/ IXHang- Abfluss-neigung prozess< 2° SOF32-5° SOF2

5-40° SOF1> 40° HOF1

Staunässestufe s5Grundnässestufe g5Grundform IX/XHang- Abfluss-neigung prozess

< 5° SOF25-40° SOF1

> 40° HOF1

janein

Stau-/Grundwasser im Oberboden

> 10 Monate/Jahr

Staunässestufe s6Grundnässestufe g6Grundform XHang- Abfluss-neigung prozess

< 2° SOF22-40° SOF1> 40° HOF1

janein

HOF1 HOF2

nein ja

DSSF DP

nein ja

Geologie durchlässig

nein ja

sehr hoch (> 200 l/m2)Hang- Abfluss-neigung prozess

< 2° SSF32-40° SSF2> 40° HOF1

gering (< 50 l/m2)Hang- Abfluss-neigung prozess

< 2° SOF32-20° SOF2

20-40° HOF2> 40° HOF1

gering (< 50 l/m2)Hang- Abfluss-neigung prozess

< 2° SOF32-20° SOF2

20-40° HOF2> 40° HOF1

mittel, hoch, sehr hoch (> 50 l/m2) : Hang- Abfluss-neigung prozess

< 20° DSSF20-40° HOF2> 40° HOF1

mittel, hoch, sehr hoch (> 50 l/m2) : Hang- Abfluss-neigung prozess

< 20° DP20-40° HOF2> 40° HOF1

nWSK(Grob- und Mittelporen

0,2-50 µm)

nWSK(Grob- und Mittelporen

0,2-50 µm)

nWSK = nutzbare Wasserspeicherkapazität (= nFK We)

Result of this assessment is a map about runoff sensitivity in headwater catchments

Besides hot spots linear structures (e.g. tracs of heavy machines, forest roads, drainage ditches) accelarate the discharge

So, an inventory of discharge lines is also necessary

… and also an assessment ofthe runoff along forest roads !

Inventory map about runoff along forest roads

Runoff management in forests

Forest road networkand logging tracks

Forest road networkand logging tracks

WetlandWetland

Renaturating retention areasRenaturating retention areas

Forest brooks and flood plainsForest brooks and flood plains

SilvicultureSilviculture

Retention basinsRetention basins

Forest and water retention / flash flood generation (after heavy rain event

Bott & Schenk 2000)

0

1

2

3

4

1 2 3 4 5 6 7 8 9 10 1112 1314 1516 17 [h]

Frei

land

-NS

[mm

/h]

0

5

10

15

20

25

Abfluss [m

m/h]

37 %

Clear cut

0

1

2

3

4

1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17[h]

Frei

land

-NS

[mm

/h]

0

5

10

15

20

25

Abfluss [m

m/h]

25 %

Deciduous forest

ForestForest Clear cutClear cut

Forest and water budget (one year)

Supporting silvicultural measures

Early regeneration decades before harvesting

fast reafforestation after storm damages …. or clear cuts

Supporting silvicultural measures

Reafforestation of runoff hot spots

Supporting silvicultural measures

Runoff management in forests

Forest road network and logging tracks

Forest road network and logging tracks

WetlandWetland

Renaturating retention areasRenaturating retention areas

Forest brooks and flood plainsForest brooks and flood plains

SilvicultureSilviculture

Retention basinsRetention basins

On forest sites with a reduced infiltration capacity the road surface water should be collected in small decentralized artificial hollows with a capacity of about 50 m3 to 2000 m3, where it can evaporate or seep away.

Forest roads are only stable, if water saturation of the road

substructure can be avoided.

Hence forest roads need a transverse profile to drain the

water from the road surface not into ditches but into the adjacent

forests.

Driving with heavy machinery and soil compaction increase the danger of fast surface runoff and flashflood generation

… Forest soils should not be compacted by heavy machineries. If harvester and forwarder have to work in the forests, they should only drive on a permanent network on logging trails or forest roads … and use the best technical solutions to prevent soil damages.

… alternative felling and logging systems can prevent soil compaction

and linear surface runoff

Runoff management in forests

Forest road networkForest road network

WetlandWetland

Renaturating retention areasRenaturating retention areas

Forest brooks and flood plainsForest brooks and flood plains

SilvicultureSilviculture

Retention basinsRetention basins

In wetlands with a big organic layer peak flows will be minimized

(„Flood planner, a manual for the natural management of riverfloods“;

WWF Scotland 2006)

Disturbed wetlands should be renaturated.

Drainage ditches should be dammed.

Runoff management in forests

Forest road networkForest road network

WetlandWetland

Renaturating retention areasRenaturating retention areas

Forest brooks and flood plainsForest brooks and flood plains

SilvicultureSilviculture

Retention basinsRetention basins

The natural structure of forest rivers should be protected or renaturated …

Renaturating of river structure includes their environment

The „natural“ development can be accelerated by „non-natural“ management …

Runoff management in forests

Forest road networkForest road network

WetlandWetland

Renaturating retention areasRenaturating retention areas

Forest brooks and flood plainsForest brooks and flood plains

SilvicultureSilviculture

Retention basinsRetention basins

Retention basins are efficient measures for flood mitigation. They can be „naturally“ integrated behind crossing forest roads

Not longer used fishing ponds are also effective retention basins, …

but they should not be filled with water ….

How to close small retention basins …

Decentralized retention measures have only a chance to be efficient in headwater catchments, if all possibilities in these catchment areas are achieved, always beginning close to the spots of run-off generation

Above the point of intersection, when the reduced discharge curves meet the “normal” floodfrequency curve, technical flood-protection measures are necessary, …

Efficiency of precautionary forestry water retention measures

In the microscale each additional flood precautionary measure makes the discharge curves diverge from their starting points and meet on the flood frequency curve at a higher flood return period.

. flood return period

discharge

dischargeprecautionary silvicultureprecautionary silviculture + soil protectionprecautionary silviculture + soil protection + runoff reduction from forest roads

technical flood protection measures are necessary

… particularly if large scale climatic situations are responsible for the generation of floods.

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