Consequences of river regulation works: altered fluvial processes + increasing flood hazard

Transcription

1 Consequences of river regulation works: altered fluvial processes + increasing flood hazard Dr. Tímea Kiss assistant professor Department of Physical Geography and Geoinformatics, University of Szeged, Hungary WVU, Morgantown Map of Hungary

2 What is the problem? Problem: Increasing flood level hazard Increase in flood levels between 1901 and 2006 Yearly highest water levels on the fluviometer at Mindszent

3 What can be the possible causesc of the increasing flood levels? Changes on the catchment Climate (storminess) Land-use (canopy) Changes on the floodplain Roughness (vegetation) Aggradation Buildings, new constructions Changes on the channel Horizontal (migration) Vertical (narrowing) Increasing height and frequency of floods Artificial vs. natural Content 1. Natural and economic conditions before regulation works (mid-19 th c.) 2. Engineering works (late 19 th c., early 20 th c.) 4. Present-day situation

4 1. Conditions before regulations Natural: -Huge floodplain (swamps) -Long-lasting floods Social-economic: -increasing population -Opened W-Eu market -Eneneering -Financial background + PEACE! 2. History of river management The great (19 th c.) river regulations From 1846 until the 1890 s Uniform ideas on the whole catchment

5 2. History of river management The great (19th c.) river regulations 1. Cut-offs (Tisza isza: 102 places) decreased length ( km) increased gradient (3,7 7cm/km) 7cm/km) Semi-natural channel development 2. Levee construction flood protected area 26,500 km 2 length: 2940 km Semi natural floodplain development 3. Drainage system (40,000 km) 2. History of river management River management in the 20 th century Floodplain: hardly maintained (dense vegetation, aggradation, (il)legal constructions) Levees: continuous heightening (local,, not enough) Channel: bank stabilisation following local ideas and design 1. Revetments 260 km on the 590 km long section 2. Groynes 189

6 Shortened Increased gradient Cut-offs Fish population Fast, GREAT floods Land-use changes Climate change Pattern change INCISION widening Extra sediment load Floodplain aggradation Lowering the water table Land slides : Cut-offs Channel slope changes Present slope conditions

7 : Cut-offs incision : Cut-offs incision Area (m 2 ) width depth Max (m) mean (m) max (m) mean (m) shape index

8 : Cut-offs pattern change levee constructions Artificial (active) floodplain Protected (inactive) floodplain Large sediment imput High floods (5 m waterdepth) narrow Intensive aggradation High friction Altered land-use, Invasive species GREAT, large floods Soil alterations (salinization, dry-land solis) Decreased area of wetlands Micro-climate change (dryer, warmer) Need of irrigation Lowered water table Inland water problems (active subsidence, no accumulation)

9 levee construction Intensive accumulation levee construction Intensive accumulation

10 (m) 3,0 2,5 2,0 1,5 1,0 levee construction Intensive accumulation 0,5 0, szelvények Amount of aggradation (m) hullámtéri akkumuláció mértéke I. Military survey (late 18 th c.) levee construction Wet-land reductions II. Military survey (late 19 th c.)

11 I. Military survey (late 18 th c.) levee construction Wet-land reductions Topographical map (1980) levee construction Salinization

12 Slight incision revetment constructions No lateral erosion Smaller cross-section Smaller flood conductivity Active point-bar formation GREATer, large floods Sharper bends Large sediment imput Narrowing due to dense riparian vegetation Slope decrease Something wrong!! Discharge Stage (water level) Falling stage Rising stage Same discharge increasing flood height Tisza, Szolnok

13 SIXTH INTERNATIONAL CONFERENCE In case of freely developing meanders revetment construction revetment construction Revetment construction (1940), upper section straightened (1886)

14 Cross-sectional sectional changes Decrease of cross-s. area 2-3% at inflexion point Decrease of cross-s. area 4-6 % in axis of meander Decrease of cross-s. area 6-19 % at revetment Flood conductivity?? Flood conductivity chages due to altered cross- sectional parameters

15 Conclusions River management vs. channel development Planimetry Cross-sections sections Flood conductivity Natural state high sinuosity narrow bends wide channel large area normal 19 th c. regulations ( ) cut-offs 20 th c. regulations (since 1930 s) revetments lower sinuosity large bends low sinuosity narrowing bends wide, deeper channel area increase narrower, deep channel area decrease higher % declining % (worse than in natural state) What to do now? To do nothing = heroic work, spending money during flood (on nothing)

16 What to do now? River restoration? New ways of river and watershed management? Changes on the catchment Climate (storminess) Land-use (canopy) Changes on the floodplain Roughness (vegetation) Aggradation Buildings, new constructions Changes on the channel Horizontal (migration) Vertical (narrowing) Decrease run-off: Stream restoration? Land-use optimalization? Storage lakes? Decrease roughness: Floodplain restoration? Land-use optimalization?? Increase flood conductivity: Channel restoration? Removal of levees? Land-use optimalization?? Learn from our mistakes! -Long river regulation history -To monitor changes: long hydrological data set: water level data (since the 1860 s) spatial data: precise Q data (since 1960 s) map series (first survey 1790 s) cross sections (since 1890) -Easy to start the devil s circle (continuous maintenance should be needed)

17 Thank you for your attention!