Hydrology and Floodplain Analysis, Chapter 10

Hydrology and Floodplain Analysis, Chapter 10.1

Introduction to GIS �  GIS – Geographical

Information System �  Spatial Data

�  Data linked with geographical location

�  Observations in GIS: �  Features ○  Point, line, and area

features �  Human Activities ○  Population maps,

census maps, land use

�  Event ○  Occurs in point of

time, mapped over time

○  Time series of rainfall data at point in watershed

GIS Hydrologic Adoption Accelerants

�  Availability of worldwide geospatial data

�  Inexpensive, powerful computer hardware

�  Inclusion in engineering curriculums

�  Hydrologic GIS extensions

Hydrology and Floodplain Analysis, Chapter 10.2

Raster and Vector Data �  Vector

�  Points, lines, or polygons �  Maps a geographic feature with lines drawn

between coordinate pairs �  Can be associated with multiple attributes

�  Raster �  Rectangular array of grid cells ○  Digital elevation models (DEMs), gridded rainfall data,

satellite imagery

�  Most hydrologic applications use combination of both �  e.g. vector watershed delineation, NEXRAD

raster rainfall data

Left: Vector delineation of Brays Bayou watershed!

Right: Raster data of gridded array of rainfall totals in southern Texas!

Map Scale and Spatial Detail �  Native Scale

�  The scale and resolution at which data are collected or measured of a map

�  Easily manipulated within a GIS system

�  Out-of-scale analysis can skew results

�  Small scale map �  Features appear small, less

detail (1:1,000,000)

�  Large scale map �  Features appear large, more

detail (1:500)

Datum and Spheroid �  For cartography, Earth treated as

spheroid �  Ellipse rotated around minor or major axis

�  Horizontal datum �  Reference frame on Earth’s Surface �  Orients lines of latitude and longitude �  Geocentric or Local

�  Local Examples: �  1927 North American Datum ○  Uses the Clarke 1866 spheroid

�  Passes through Meades Ranch, Kansas

�  Vertical Datum �  Measured from mean sea level �  NGVD29 and NAVD88 are North American

standards

Geographic and Cartesian Coordinate Systems �  Projection necessary to transform from

geographic coordinate system to Cartesian �  Different projections maintain various properties

from geographic system ○  Conformal – maintain angles and shapes ○  Equal area – maintain area ○  Equidistance – maintain distance ○  True-direction – maintain direction with respect to

central point

�  Universal Transverse Mercator (UTM) �  Conformal and equal area �  Earth divided into 60 UTM zones

The UTM projection of the Earth’s surface, w/ central meridian through Dodge City, Kansas"

Two streams shown on a digital orthophoto showing hydrographic features with mismatched datum (Black Stream, NAD27; White Stream, NAD83) "

Hydrology and Floodplain Analysis, Chapter 10.3

Parameters Direct Measures �  Rainfall �  Infiltration �  Evapotranspiration �  Hydraulic Roughness

Indirect Measure �  Soil Type

�  Used to estimate infiltration characteristics

�  Land Use/Cover �  Used to estimate hydraulic

roughness and evapotranspiration

�  NEXRAD Radar Reflectivity �  Used to estimate rainfall

Hydrology and Floodplain Analysis, Chapter 10.4

Digital Terrain Model

A DTM is a representation of the terrain � Gridded elevations (DEM) �  Triangular facets (TIN) �  Series of points (x,y,z) � Contour lines

Example of a TIN"

Hydrology and Floodplain Analysis, Chapter 10.5

Watershed Delineation

� Watershed boundaries are delineated by using a DEM.

� Assume that water flows from each cell to its lowest neighbor.

Fixing Errors Problem: Existing streams do not line up with generated stream paths in flat areas."

Problem: Stream paths exit known watershed not at outlet"

Solution: Burn in the stream lines to the DEM with a stream vector map. "

Solution: Establish a watershed boundary with a watershed vector map."

Hydrology and Floodplain Analysis, Chapter 10.6

Software Description Modules ArcGIS Desktop Collection of software to

create, import, edit, query, map, analyze and publish.

Includes ArcView and other products that provide mapping data use and analysis.

ERDAS Distributed by Earth Resource Data Analysis System (ERDAS).

Support for remotely sensed data includes unsupervised classification, geometric correction and map production tools.

IDRISI A popular software package for teaching and research. Distributed by Clark Labs.

An integrated GIS and image processing software solution for the analysis and display of digital spatial information.

Bentley Software dedicated to architects, engineers, constructors and owner-operators.

Consists of the Bentley geospatial desktop and server products. Full featured 2-D and 3-D GIS.