Tidal channel networks: field observations, mathematical modeling & laboratory experiments

Andrea D’Alpaos

Tidal channel networks: field observations, mathematical modeling & laboratory experiments

Andrea D’Alpaos Andrea Rinaldo, Stefano Lanzoni, Marco Marani

Enrica Belluco, Luana Stefanon and Luca Carniello

San Felice salt marsh – Venice Lagoon

TIDAL CHANNEL NETWORKS: UBIQUITOUS FEATURES of the TIDAL LANDSCAPE

TIDAL CHANNEL NETWORKS: UBIQUITOUS FEATURES of the TIDAL LANDSCAPE

Lago dei Teneri – Venice Lagoon

Tidal networks have received less attention than their fluvial counterparts [e.g., Rodríguez-Iturbe & Rinaldo, 1997]

particularly in terms of the chief processes governing their initiation and evolution, and their response to variations in external forcings

[e.g., Rigon et al., 1994; Rinaldo et al., 1995].

Mathematical models developed to describe tidal network morphogenesis and evolution [e.g., D’Alpaos et al., 2005; Marciano et al., 2005; Kirwan & Murray, 2007; Temmerman et al., 2007].

Field observations and related conceptual models [e.g., Redfield, 1965; Allen, 1997; Perillo et al., 2005; D’Alpaos

et al., 2007; Hughes et al., 2009].

Laboratory experiments [Stefanon et al., 2010; Vlaswinkel & Cantelli, 2011;

Kleinhans et al., 2012].

INDICATORS of DYNAMICS and MORPHOLOGY: TIDAL PRISM and DRAINAGE DENSITY

S: area of the basin, H: local maximum high water level (HWL); h: local minimum low water level (LWL); z: is the local bed elevation.

TIDAL PRISM: the total volume of water exchanged through a given section between low water slack and the following high water slack

SIMPLIFIED HYDRODYNAMIC MODEL (Rinaldo et al., WRR 1999a,b; Marani et al., WRR 2003)

S: area of the basin, H: local maximum high water level (HWL); h: local minimum low water level (LWL); z: is the local bed elevation.

S

S S

INDICATORS of DYNAMICS AND MORPHOLOGY: TIDAL PRISM and DRAINAGE DENSITY

DRAINAGE DENSITY: ratio of network total length to its watershed area [Horton, 1945], measure of the degree of channelization of a given catchment. D = Σ L

A

INDICATORS of DYNAMICS AND MORPHOLOGY: TIDAL PRISM and DRAINAGE DENSITY

(Marani et al., 2003; Tucker et al., 2001)

DRAINAGE DENSITY: ratio of network total length to its watershed area [Horton, 1945], measure of the degree of channelization of a given catchment. D = Σ L

ADRAINAGE DENSITY: inverse of the mean flow distance from any point on the marsh to the nearest channel, ℓmean, indicates how efficiently the network serves its catchment

MODELLING TIDAL NETWORK INITIATION & EARLY DEVELOPMENT: (D’Alpaos et al., 2005, 2007)

Headward channel growth driven by exceedences of a critical shear stress

Instantaneous adaptation of the channel cross sections to the local tidal prism

A constant width-to-depth ratio characterizes channel cross sections

WHETHER or NOT the MORPHOLOGICAL FEATURES of a GIVEN LANDSCAPE RETAIN SIGNATURES of PAST CLIMATES

is a CLASSICAL and FASCINATING QUESTION in GEOMORPHOLOGY…

CAN ONE READ the GEOMORPHOLOGICAL SIGNATURES of PAST CLIMATE in the CHARACTERISTICS of TIDAL NETWORKS?

!HOW do TIDAL NETWORKS RESPOND to

CHANGES in RELATIVE MEAN SEA LEVEL?

Pagliaga salt marsh– Venice Lagoon

MATHEMATICAL MODELLING and LABORATORY EXPERIMENTS

IntroduzioneIn the FLUVIAL LANDSCAPE………

RESPONSE TO CHANGES IN RELATIVE MEAN SEA LEVEL

NUMERICAL EXPERIMENTS to ANALYZE TIDAL NETWORK RESPONSE

to Relative Mean Sea Level CHANGES over TIMESCALES which WOULD PRECLUDE

NETWORK MONITORING through FIELD OBSERVATIONS.

NETWORK RESPONSE to CHANGES in RELATIVE MEAN SEA LEVELa) INITIAL CONFIGURATION: Constant marsh elevation (zm=-0.30m), network structure reminiscent of the San Felice channel network

d) INCREASE in RMSL 1st case: 50cm-increase in MSL, re-expansion of the network through the carving of the non-uniform topography: hysteretic behavior, with signatures of past RMSLs.

e) INCREASE in RMSL 2nd case: Network re-expansion over a uniform topography: contractions and expansions tend to occur within the same planar blueprint: cyclic regressions/transgressions(D’Alpaos et al., JGR-ES 2007; D’Alpaos, Geomorphology 2011)

b-c) MARSH ACCRETION: Depositional period: non-uniform vertical accretion of the marsh platform (zm=0.40m) →shrinking of channel cross sections, contraction and & retreat of the network.

RESPONSE TO CHANGES IN RELATIVE MEAN SEA LEVEL

LABORATORY EXPERIMENTS to ANALYZE TIDAL NETWORK RESPONSE to CHANGES in the TIDAL PRISM, TRIGGERED by RMSL CHANGES

over TIMESCALES which WOULD PRECLUDE NETWORK MONITORING

through FIELD OBSERVATIONS.

• 2 adjoining basins: • sea (1.6m x 4. 0m); • lagoon (5.3m x 4.0m);

• lagoon bottom covered with sediments; • inlet (variable shape and width); • beach; • tide generated by a vertically oscillating steel sharp-edge weir; • water flowing over the weir collected to a separate tank where a set of pumps recirculates the flow (feedback process to obtain a prescribed sinusoidal tide);

• computer driven pantograph to survey bottom elevations:

✓ laser system (300 µm resolution) ✓ ultrasound probe

Correct laser measurements from refraction

EXPERIMENTAL APPARATUS

Φ (°) c (kPa)

dry sediment 25 0saturated sediment 25 0

wet sediment 23 1,68

- Median grain size: d50= 0.8 mm

Grain size distribution

cohesion

- Additive used to reduce the surface tension

- density: γs = 1041 kg/m3

d50 = 0,80 mm

SEDIMENTS: cohesionless plastic grains

EXPERIMENTAL RESULTS Stefanon et al., CSR 2010

Regressioni e trasgressioni marineAnalisi degli effetti prodotti da brusche variazioni del MSL

RESPONSE TO CHANGES IN RELATIVE MEAN SEA LEVEL

(Stefanon, Carniello, D’Alpaos & Rinaldo, GRL, 2012)

Regressioni e trasgressioni marineNETWORK INCISION & RETREAT TRIGGERED by RMSL VARIATIONS

Is the process of network contraction/expansion cyclic? !• 9351 cycles: network structure before the reduction in MSL • 9631 cycles: contracted network • 11355 cycles: re-expansion of network structure after 2000 cycles

(Stefanon, Carniello, D’Alpaos & Rinaldo, GRL, 2012)

CHANGES in RMSL, TIDAL PRISM, DRAINAGE DENSITY

CHANGES in RELATIVE MEAN SEA LEVEL IMMEDIATELY AFFECT the TIDAL PRISM. !!CHANGES in the TIDAL PRISM RAPIDLY & STRONGLY INFLUENCE: - CHANNEL CROSS-SECTIONAL AREAS, - NETWORK STRUCTURE, & - DRAINAGE DENSITY

The DRAINAGE DENSITY - is a MEASURE of NETWORK EFFICIENCY in

DRAINING the LANSCAPE; !- CONTROLS the TRANSPORT of WATER,

SEDIMENTS, NUTRIENTS &POLLUTANTS.

(Stefanon, Carniello, D’Alpaos & Rinaldo, GRL, 2012)

DRAINAGE DENSITY vs TIDAL PRIISM

CONCLUSIONS

CHANGES in RMSL IMMEDIATELY AFFECT the TIDAL PRISM, and the TIDAL PRISM RAPIDLY & STRONGLY INFLUENCES CHANNEL CROSS-SECTIONAL AREAS, NETWORK STRUCTURE, & DRAINAGE DENSITY…. NETWORK EFFICIENCY in DRAINING the LANSCAPE and the TRANSPORT of WATER, SEDIMENTS, NUTRIENTS &POLLUTANTS.

THE DRAINAGE DENSITY of TIDAL CHANNELS is LINEARLY RELATED to the LANDSCAPE-FORMING PRISM….

A DECREASE (INCREASE) in the TIDAL PRISM LEADS to NETWORK RETREAT (REINCISION) & CONTRACTION (RE-EXPANSION) of CHANNEL CROSS SECTIONS.

SUBSTANTIALLY REVERSIBLE PATTERNS of NETWORK CONTRACTION and RE-EXPANSION OCCUR: this CAN RESULT in the DISAPPEARANCE of the SIGNATURES of PAST CLIMATES.

Higher values of the bottom shear stress are located at channel tips or near channel bends

LOCAL VALUE OF THE BOTTOM SHEAR STRESS

D’Alpaos et al., JGR-ES, 2005; Feola et al., WRR, 2005

HEADWARD GROWTH CHARACTER of NETWORK DEVELOPMENT (Hughes et al,. GRL 2009)

(e.g. Steers, 1960; Pestrong, 1965; French and Stoddart, 1992; Collins et al. 1987; Wallace et al. 2005; D’Alpaos et al., 2007)

O’BRIEN-JARRETT-MARCHI LAW(O’Brien, 1969; Jarrett, 1976; Marchi 1990;

D’Alpaos et al., 2009)

(D’Alpaos et al., JGR 2010)

(Lanzoni & D’Alpaos, submitted)

WIDTH-to-DEPTH RATIO