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Flood Mitigation by Forestry
Prof. Dr. Gebhard SchülerResearch Institute for Forest Ecology and Forestry Rhineland-Palatinate, Germany
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Flood generation
Identification of runoff processes
Precautionary measures for flood mitigation
Efficiency of decentralized retention measures
Conclusions
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Runoff and flood are natural processes …
… water needs enough space to spread …
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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.
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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.
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It is important to know how and where runoff is generated (hot spots of flood generation) before implementing water retention measures
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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)
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Result of this assessment is a map about runoff sensitivity in headwater catchments
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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
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… and also an assessment ofthe runoff along forest roads !
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Inventory map about runoff along forest roads
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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
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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
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ForestForest Clear cutClear cut
Forest and water budget (one year)
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Supporting silvicultural measures
Early regeneration decades before harvesting
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fast reafforestation after storm damages …. or clear cuts
Supporting silvicultural measures
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Reafforestation of runoff hot spots
Supporting silvicultural measures
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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
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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.
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Driving with heavy machinery and soil compaction increase the danger of fast surface runoff and flashflood generation
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… 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
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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
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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)
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Disturbed wetlands should be renaturated.
Drainage ditches should be dammed.
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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
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The natural structure of forest rivers should be protected or renaturated …
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Renaturating of river structure includes their environment
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The „natural“ development can be accelerated by „non-natural“ management …
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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
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Retention basins are efficient measures for flood mitigation. They can be „naturally“ integrated behind crossing forest roads
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Not longer used fishing ponds are also effective retention basins, …
but they should not be filled with water ….
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How to close small retention basins …
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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
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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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