Model l i ng bi furcati ons i n the Tabasco coastal pl ai n, Mexi co
I . R. Lokhorst
1, C. A. M. Nooren, M. G. Kl ei nhans, W. Z. Hoek
i . r. l okhorst@uu. nl
1, Utrecht Uni versi ty, Dept. of Physi cal Geography, Utrecht, Netherl ands
Acknowledgements:
Thanks to H.Middelkoop,K.D.Smits and B postfortheirhelp with the data acquisition in Mexico
This study is a contribution to the NWO-ALW project:‘Five millennia ofsea-level,palaeostorm,and Maya land use variability reconstructed from the world’s largest beach ridge plain’
Fi el d si te:
-The Tabasco coastalplain contains severalbeachridge dominated deltas and two majorrivers,the Grijalva and Usumacinta
-Sedimentologicalresearch points to multiple avulsions during the late Holocene -Currently instable bifurcations threaten the city ofVillahermosa
-The Grijalva (orMezcalapa)riverhad a much higheravulsion frequency
-The sedimentload ofthe Grijalva riveris relatively high due to the large inputof -The sedimentload ofthe Grijalva riveris relatively high due to the large inputof
easily eroded and transported volcaniclastic sediments from ElChichonalvolcano
The Usumacinta rivershows thatitavulses after 5 km delta progradation.A confluence with the Grijalva riverstrongly decreases the time in- volved in the avulsion process.
The Grijalva riverdid notavulse unless sig- nificanttectonics were applied in the new branch only.The figure above experienced 1 cm/y subsidence.This is an unlikely
setting and thus uncertainty aboutthe cause remains.
Questi ons:
-Which rivercreated the currently active Grijalva delta?
-Whatdrove the recentavulsions ofthe Grijalva and Usumacinta?
-Whatcauses the higheravulsion frequency ofthe Grijalva?
-Does the ElChichonalvolcono affectthe Grijalva (orMezcalapa’s)behavior? -The Grijalva’s sedimenthas shown to be fining downstream,does this
signicantly a
signicantly affectthe river’s behavior?
Scenari os:
Usumacinta:
-0 km,5 km and 10 km delta progradation -2000 m3/s discharge and 3700 m3/s (flood) -0.1 cm/y and 0.5 cm/y tectonics
-0.5 mm/y sealevelrise
-With and withoutconfluence with the Grijalva -0.5 mm and 0.1 mm D50grainsize
-300 m and 400 m upstream width
Grijalva
-0 km,and 2 km delta progradation
-700 m3/s discharge and 1700 m3/s (flood) -5 mm/y sealevelrise
-With 0,1,or3 concluences
-Tectonics up to 1 cm/y in eitherboth or one ofthe brances
Methods:
-1D bifurcation modelling,the modelincludes:
-Transverse bedslope effects -Hellicalflow
-Adjustable channelwidths based on hydraulic geometry
Ai ms:
-To find outwhy the Grijalva and Usumacinta avulsed in its recenthistory -To determine the cause forthe Grijalva’s higheravulsion frequency
Modelled Grijalva river Modelled Usumacinta river -Schematization ofthe riverslopes
-Red:initialslope
-Black:slope after100 years -Blue:slope after200 years
-Pink:slope after100 years with suddenly fining grainsize on the profile knick
-Grijalva riverstarts aggrading quickly -Grijalva riverstarts aggrading quickly -Usumacinta riverbarely aggrades
-Suddenly fining grainsize counteracts accumulation
measured riverslopes
Resul ts: Avul si on frequency
Resul ts: Cause of avul si on
Concl usi ons:
-The Usumacinta avulsed due to its own delta progradation -The confluence with the Grijalva accelerates the avulsion
process
-Higheravulsion frequency ofthe Grijalva likely to be caused by its strongerdecrease in gradient
by its strongerdecrease in gradient
-Gradientmightbe caused by sudden decrease in grainsize -Reason forthe Grijalva’s avulsion stillunclear
Overview ofthe positioning ofthe ElChichonalvolcano and the two avulsions ofinterest
Di scussi on:
-Influence ofsea levelrise is very minor,because the rivers are able to easily compensate with aggradation ofthe channelbed
-Tectonics has shown to be a majorfactorin many avulsions butis ofminorinfluence in these scenarios.
-The Usumacinta has shown to be strongly affected by smallchanges in gradient -
-The presence ofa confluence strongly affects the system,justas was found in for example the Columbia river.
-The ElChichon volcano mighthave a significantinfluence on the behaviorofthe Grijalva river,butthis was notinvestigated
Digitalelevation modelofthe area.The western riversystem (Grijalva)has expe- rienced much more avulsions overits history
