Nicaragua Overview: Hydrologic Analysis

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Nicaragua Overview Hydrologic Analysis

• By Ketan Shah
• CEVE 499
• Final Individual Report

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Hydrologic and Hydraulic Analysis

• The hydrologic model takes many different factors
  such as rainfall, soils (infiltration),
  elevation, drainage area, and watershed shape
  into account to create an outflow hydrograph
  (flow vs. time).
• The hydraulic model uses this data and calculates
  the height to which the water will rise according
  to the geometric properties of the stream.
  
• The ultimate goal of the analysis is to find the
  height of the water at the site to ensure that it
  will never reach the height of the bridge.

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The Analysis Model
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Data Acquisition

• Digital Elevation Model (DEM)
• Elevation data of land every 90 meters
• VMAP International Vector Data
• Infrastructure
• Geographic information
• Topographic Map of Site Quad
• 150,000 scale topographic map
• Contour interval of 20 meters
• Also displayed roads and other structures
• All data was bought online from www.mapmart.com

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Nicaragua Overview
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Nicaragua Overview

• The coordinates of Jinotepe are 11.85 N and
  -86.20 W.
  
• The site is located in the Department of Carazo.
• The major river in the Department of Carazo is
  the Rio Grande.
  

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DEM and Site Location
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DEM and Site Location

• The exact site (El Panama) is located about 10 km
  southwest of Jinotepe.
  
• The coordinates of Jinotepe are 11.76 N and
  -86.24 W.
  
• The site is located near the foot of a small
  mountain (1000 m).
  
• The steep slopes of the mountainside greatly
  influence the watersheds and increase the
  response time and peak flow of watersheds on the
  mountainside.

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What is a watershed?

• A watershed is the entire drainage area of a
  certain stream or river.
  
• Any drop of water placed in a certain rivers
  watershed will eventually flow into that river.
  
• Large watersheds can be split up into smaller
  watersheds, which in turn can be split into even
  smaller watersheds.

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Using ArcView GIS 3.2and HEC-GeoHMS 1.0

• Using GeoHMS, elevation data can be used to
  create stream definitions and their watershed
  boundaries.
  
• The following steps were taken

• Fill Sinks
• Flow Direction Grid
• Flow Accumulation Grid
• Stream Definition (stream threshold)
• Stream Segmentation
• Watershed Delineation
• Watershed Polygon Processing
• Stream Segment Processing
• Watershed Aggregation

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HEC-GeoHMS
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Watersheds and Rivers
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Watersheds and Rivers

• As the figure shows each larger watershed (green)
  encompasses several smaller watersheds (red)
  the size and quantity of watersheds depends on
  the definition of a river.
• In this area, all the watersheds are long, thin
  slivers of land extending radially outward from
  the peak of the mountain.

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Why?

• The horizontal elevation around the mountain is
  almost the same. Small changes in horizontal
  elevation define the horizontal boundaries of the
  watersheds
• Due to the horizontal flatness, the steep
  downhill slope dominates the flow direction,
  leading to long and narrow watersheds.

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Rio Grande Watershed
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Rio Grande Watershed

• The Aragon watershed is a sub-watershed of the
  Rio Grande watershed. Thus, the Aragon river is
  a tributary of the Rio Grande.
  
• Narrow watersheds dominate on the steeper slope
  of the mountain, but as the terrain flattens near
  the foot of the mountain, the watersheds become
  wider. The flow of water in the lower watersheds
  is defined more by the horizontal or overland
  slope rather than the steep downhill slope.
• The bulge at the right of the Rio Grande
  watershed is a result of the divide along the
  side of the mountain. All the water on the other
  side flows down the other side of the mountain.

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Topographic Map and Site
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Topographic Map and Site

• The watershed area is only 2.81 sq. miles.
• The site location is near the outlet of the
  watershed
  
• The Aragon watershed has no tributaries, and is
  thus independent from the watersheds surrounding
  it.
  
• This simplifies the model because the site is not
  affected by rainfall anywhere but in this small
  area.

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Aragon Watershed and Slope of Site

• Because the elevation ranges are relatively
  equal, we can assume an almost constant slope
  throughout the watershed.
  
• The slope is about 176 ft/mi (in comparison,
  Houston is 1 ft/mi).
  
• The slope and narrowness of the watershed cause a
  quick response time after rainfall.
  
• If rain is falling over the entire span of the
  watershed, there is a large risk of high water
  levels at the site.
  
• This problem is intensified by the fact that the
  site is located near the outlet of the watershed
  thus, all of the water from the watershed must
  pass through the site.

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What is next?

• Next, rainfall data need to be obtained for the
  watershed for the hydrologic model. HEC-HMS will
  be used to create this model, and then the survey
  data from Nicaragua will be used to create the
  hydraulic model for the final analysis of the
  peak height of the water flow.