numerical simulation of stream channels in floodplains

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numerical simulation of stream channels in
floodplains
PUERTO RICO WATER RESOURCES AND ENVIRONMENTAL
RESEARCH INSTITUTE UNIVERSITY OF PUERTO RICO
MAYAGUEZ CAMPUS

• Walter F. Silva Araya and Alejandra Rojas
  González

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Presentation Program

• Introduction
• Objectives
• Previous Works
• Hydraulic Model
• Methodology
• Results
• Conclusions and Recommendations
• Questions

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INTRODUCTION

• Floods are a major cause of losses in the
  caribbean
• HH studies are requested in Puerto Rico for new
  developments in flood zones
• The Planning Board adopted FEMA FIRM maps as
  regulatory maps for flood studies in Puerto Rico.
  
• Water levels are computed using one dimensional
  models
• 2-D and 3-D models are better choice for
  simulations in alluvial valleys

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OBJETIVES

• To study the river response to a change in the
  alluvial valley geometry in terms of the water
  levels . Two effects were studied
• The meanders geometry
• The angle of aperture of the alluvial valley

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PREVIOUS WORKS
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HYDRAULIC MODELS

• Multi-dimensional hydraulic models are available
  since the 70s.
• In the 90s models became more efficient and able
  to solve mixed flow situations (subcritical and
  supercritical regimes)
• Models based on the Finite Element method have
  become powerful however, special conditions
  could be required for transition regimes.
• Finite Element Surface Water Modeling System
  (FESWMS-2D) developed for the U.S. Federal
  Highway Administration represents
  state-of-the-art techniques to solve practical
  problems in two dimensions

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HYDRAULIC MODEL
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SMS (SURFACE WATER MODELING SYSTEM)

• Graphics interphase developed by Bringham Young
  University, in collaboration with the USACE and
  FHWA.
• The interphase allows data management for
  several two-dimensional models
• RMA2 water levels and velocities
• SED2D-WES sediment transport
• HIVEL-2D supercritical flow
• FESWMS similar to RMA2 but includes structures
  and mixed flow regimes

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METHODOLOGY

• Select geomorphic parameters
• Create hydraulic models for different sets of
  selected parameters
• Run simulations
• Compare results to get possible generalizations

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Selection of geomorphic parametersSinuosity

• Describes the degree of meandering
• Defined as K L/Lm
• Straight channel K 1
• Channel with prominent meanders K gt 3

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Selection of geomorphic parametersRatio of the
meander amplitude (Am) to the wave length (Lm)

• Biederharn (1999)
• 0.5 lt Am/Lm lt 1.5
• Lm and Am depend on
• 1) Flow discharge
• 2) Sediment discharge
• 3) Materials forming the channel (affects the
  irregularities in the river alignment)

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Selection of geomorphic parametersRatio of flood
plain width to meander amplitude

• Encroachment ratio Valley width/Full bank
  channel width (A/w)
• Ratio Meander amplitude/Full bank channel width
  (Am/w)
• Were combined as A/Am

A
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Selection of geomorphic parametersRadius of
curvature

• Radius of curvature (r)
• Radius of a circle that defines the curvature
  between two adyacent inflection points
• Curvature Ratio (r/w)
• Describe and compare meanders
• Depend on the same factors as Lm and w.
• 1.5 lt r/w lt 4.5
• (mostly between 2 and 3)

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Geomorphic parameters for numerical experiments
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River and valley parameters for all cases

• Flood plain width (A) 1040 meters
• Slope of the alluvial valley 0.0727
• Length of the alluvial valley 800 meters.
• Width of full bank channel 40 meters
• Channel bottom width 30 meters
• Channel lateral slope 11
• Channel bottom slope 0.038
• Maximum depth of main channel 5 meters
• Roughnesses
• Main channel 0.015
• Banks 0.035
• Flood Valley 0.05

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Sinusoidal meanders of group 1 (Am/Lm 0.5)
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Sinusoidal meanders of group 2 (Am/Lm 1)
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Sinusoidal meanders of group 3 (Am/Lm 1.5)
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Hydraulic model Boundary conditions

• Discharge 1000 m³/s

Hydrograph with Tp 3hr
4000 m³/s was also tested
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Hydraulic model

• FE grid

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RESULTS
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Effects of meanders in water levels

• Am/Lm 1
• A/Am 3.1
• K 2.3

An irregular pattern is observed, most evident
near the meanders zone. The 2D model predicts
drastic changes in the water elevations near the
river banks.
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Effects of meanders in water levels
Am/Lm 1.5 A/Am 3.1 K 3.22
High sinuosity
Moderate to low amplitude compared with the
floodplain width
Contours are not straight lines near the meander
or far from it. Higher meander density creates
larger changes in water elevations across the
floodplain.
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Effects of meanders in water levels

• Velocity vectors for same floodplain with
  different sinuosity

Moderate sinuosity High sinuosity (K
2.3, Am/Lm 1.0, A/Am 3.1) (K 3.22, Am/Lm
1.5, A/Am 3.1)
Large sinuosity creates smaller velocities behind
the meanders due to the impact of water against
the main river channel. Smaller sinuosity allows
the establishment of velocity vectors in the main
flood direction along the floodplain creating
higher velocities There are two currents with
different magnitude and direction near the
meanders, creating an intense interchange of
momentum and local water level changes
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Angle of the Flood Valley

• Grid and contours for 20º angle

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Angle of the Flood Valley

• Grid and contours for 40º angle

Section 4
Section 5
Section 6
Section 7
Change in water level occur near the meanders The
level could change depending on the meander
location and the cross section location Constant
water level cross sections in alluvial fan-shaped
valleys are not easy to determine without
previous knowledge of the multi-dimensional
nature of the flow patterns
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CONCLUSIONS
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CONCLUSIONS

• Constant water level contours are not straight
  and do not follow the common practice of
  selecting the cross sections perpendicular to the
  main flood direction
• The constant water elevation lines change with
  the river and the valley geometry particularly
  the meanders and the degree of interaction with
  the floodplain.
• The real constant-depth cross section is a smooth
  curve which extends toward the floodplain lateral
  boundaries. This effect is more significant when
  the ratio between the floodplain width and the
  meander amplitude (A/Am) decreases
• Two-dimensional hydraulic models for floodplains
  could be used to improve the parameters used by
  simpler one-dimensional models
• It is recommended that 2D models be constructed
  for alluvial flood plains and used as a tool for
  creating regulatory one-dimensional models

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Questions ???