River floodplain regime and stratigraphy. Drs. Nanette C. Kingma.

Transcription

1 River floodplain regime and stratigraphy. Drs. Nanette C. Kingma.

2 Formation of floodplain. Formation of floodplains: lateral channel migration in channel deposition suspended-load fall out during higher than normal stage bank overtopping flow width >> v< sediments in suspension increased roughness sediment load Independent processes, may occur simultaneous. 2

3 Sedimentary facies. Two sedimentary facies:. Channel bed deposits Bottom stratum relative coarse sedimentary deposits on the channel- bed; preserved by burial during the lateral migration. 2. Bank and overbank deposits Top stratum relative fine sedimentary deposits on the upper channel bank: levee, terrace surfaces etc formed during infrequent high streams. 3

4 Sedimentary facies. Channel bed deposits: channel lag deposits; pointbar deposits; low stage slackwater deposits. 4

5 Sedimentary facies. Bank and overbank deposits: levee and bank deposits; floodbasin or backswamp deposits; crevasse splay deposits; terrace veneer. 5

6 Sedimentary facies. Crevassse splay: also relative coarse, due to breach of the levee. Low stage slack water deposits; fine grained deposits that accumulate on the channel bed in local slack water areas during low discharges. Often at downstream parts of pointbars. thin fine layers interbedded by coarser layers. 6

7 Bottom- versus top stratum. 7

8 Alluvial channel adjustments to infrequent higher than normal discharges. Quick response Large time basis; decades - centuries. 8

9 Adjustment of floodplain facies geometry to higher than normal discharges. 9 Four different river flow regimes (situation: a,b,c,d) ( plotted as frequency of various daily mean river stages) compared to inferred thickness of the local floodplains top stratum and bottom stratum facies. Capacity of river channels is commonly set by higher than normal discharges but not necessarily rare high magnitude events. System at equilibrium: local river stage associated with (this) bankfull discharge determines by definition the max. elevation reached by the top stratum facies. equilibrium thickness!

10 Adjustment of floodplain facies geometry to higher than normal discharges: situation a. Constant flow regime:a river without flood events. No top stratum; only bottom stratum, since no floods occur. such rivers are uncommon in nature. 0

11 Adjustment of floodplain facies geometry to higher than normal discharges: situation b. Modal flow slightly below bankful stage; Q modal < Q bankfull development of a thin top stratum.

12 Adjustment of floodplain facies geometry to higher than normal discharges-2. The bankfull stage may,in turn, be either much higher than,or slightly higher than the most frequent stage which defines the channel bed environment (compare b and c).; In the zone between the most frequent river stage and the top of the channel bank, rooted vegetation strongly retards the flow competence. 2

13 Adjustment of floodplain facies geometry to higher than normal discharges: situation c. Bankfull flow much larger than modal flow. Q modal << Q bankfull development of a thick top stratum. 3

14 Top stratum Top stratum controlled by: Flood regime; Lateral migration: for rapidly meandering rivers, pointbar sedimentation is the most important factor in local floodplain development. 4

15 Situation d. Channel is comparatively large in relation to mean daily discharge; common in semi-arid regions; top stratum is absent. Channel is shaped by rare high discharge events. 5 Overbank floods essentially absent due to quick channel enlargements in response to high discharges.

16 Situation d. Favorable conditions: Occurrence of high discharge events; they are large in relation to normal discharges; Low local water tables deeply cut channels preserved when created. In up- and or downstream reaches the channel is usually broad and shallow; Abrupt geomorphic thresholds (headcuts) separate such aggrading from degrading, entrenched reaches. 6

17 Situation d. Include figure 0 pp 47 Fl Geomorphology. 7

18 Floodplain stratigraphy Floodplain stratigraphy is the science of mapping and interpreting floodplain deposits of different age. Basic goal:reconstruction of the sequence of depositional events. 8

19 Techniques. Absolute dating methods; soil- stratigraphy: using buried soils as stratigraphic markers; morpho-stratigraphy: using surface topography to define terraces, etc. litho-stratigraphy; defining sedimentary units on the basis of physical characteristics and stratigraphic relations. 9

20 Non-continuous sedimentation. The existence of floodplain stratigraphy itself indicates that non-continous sedimentation occurred e.g. Laterally migrating stream channels bankerosion (uncomformity) & slow accretion on pointbars & channel banks (conformable). As a the channels move, they maintain equilibrium. 2 0

21 States of channel floodplain morphologies (for meandering rivers).. Stable floodplain configuration: adjusted situation; net top stratum deposition slow. 2. Active floodplain configuration: floodplain sedimentation & bank erosion may occur rapid. Alternation of () and (2) is an important cause of floodplain stratigraphy. 2

22 Accumulation of a new floodplain. New top stratum is accumulated, during a period of fluvial activity. Slow channel migration, relatively slow deposition rates and soil profiles can develop 2 2

23 Morpho-stratigraphy. Episodicity of floodplain sedimentation leads to the development of floodplain and river terraces. Not always a - relation between surfaces and lithostratigraphic units or soil stratigraphic units. 2 3

24 Litho-stratigraphy. Non-continuous floodplain sedimentation also results in recognizable lithologic contrasts between alluvia of different age. Contrasts may develop from: effects of in-situ weathering; changes from sediment source; changes in dominant fluvial processes. 2 4