cl-17 dem and hydro logical modelling

8/3/2019 Cl-17 DEM and Hydro Logical Modelling

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Spatial InformaticsSpatial Informatics

Lecture 17: DEM and Hydrological ModellingLecture 17: DEM and Hydrological Modelling

Dr. K. S. RajanLab for Spatial Informatics, IIIT Hyderabad

Oct 31st, 2011

DEMs are important for modelling

Data Model Based on BehaviourFollow a drop of water from where it falls on theland, to the stream, and all the way to the ocean.

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Integrating Data Inventory using a

Behavioural Model

Relationships between

objects linked by tracing path

of water movement

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Digital Elevation Grid

a grid of cells (square or

rectangular) in some

coordinate system having

land surface elevation as

the value stored in each

cell.

the ground surface elevation at each point


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Flow Direction Grid

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Contributing Area Grid

The output of FLOWACCUMULATION would then represent the

amount of rain that would flow through each cell, assuming that all rain

became runoff and there was no interception, evapotranspiration, or loss

to groundwater. This could also be viewed as the amount of rain that fell

0 0 0 0 0 0 0

0 1 2 1 2 1 0

0 3 3 5 0 2 1

0 5 4 7 3 0 2

0 1 19 4 0 4 0

0 4 0 25 6 0 1

0 1 5 1 34 2 0

0 1 4 13 14 55 0

2 2 4 4 4 8 8

2 4 4 4 8 4 4

2 4 4 4 8 8 4

2 2 4 8 8 4 8

2 4 2 4 4 8 4

1 2 2 2 4 8 8

1 2 2 4 2 4 8

1 1 1 1 1 4 16

It creates a grid of accumulated flow to each cell, by accumulating the

weight for all cells that flow into each down slope cell.

Flow Direction Flow Accumulation

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Flow accumulation Algorithms D8:

uses 3x3 filter to calculate direction of flow as steepest

downhill slope

Rho8:

statistical version of D8 adding a uniformly distributed

stochastic element

Monte Carlo simulation of D8:

repeat D8 x 100 and use most probable result

FD8 and FRho8:

modifications allowing flow dispersion to more than one

downhill cell on a slope weighted basis

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Flow accumulation gridsFlow accumulation (upslope area > 100)

Flow accumulation (upslope area > 1000)


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DEM

FLOWDIRECTION

SINK

FILL

Yes

Are there anysinks?

No

WATERSHED BASIN FLOWACCUMULATION

Threshold FLOWACCUMULATION output

streamnet = con (flowacc > 100, 1)

STREAMLINE STREAMLINK STREAMORDER

Delineate watersheds Delineate stream network

Creating a hydrologically correct DEMCreating a hydrologically correct DEM

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Generated RiversActual Rivers

DEM

from 1:50,000 Scale

Contour Map

DEM

from 1:50,000 Scale

Contour Map

+ Spot elevations

in lower flat areas

Agno River Basin, Philippines

In medium river basin: From DEM of 100m resolution

Drainage Network Generated from DEM using D-8 Technique

Further improvement of DEM

But, improvement up to what level can be done?

Is it always possible to obtain high precision topography data for model

application?

If not, is there any alternative way?

-Yes, there are a few techniques to improve topographical data for

generating hydrologically correct DEM (from manual to

automatic)

- One of such methods is drainage-enforced algorithm

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River network from drainage-enforced DEM

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Simulated flood inundation


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mulated flood inundation with drainage-enforced DEM

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Algorithmic issues of Flow Accumulation

Paper that will be discussed (READ it) is -

Simple I/O-effcient ow accumulation on grid

terrainshttp://www.win.tue.nl/~hermanh/stack/hj-flowongrids-massive2009-paper.pdf

Issues -

1. Spatial Data are Large

2. Data Access and its effect on the Naive Algorithm

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