Geohazards Impacting Transportation in the Appalachian Region – 11th Annual Technical Forum

Chattanooga, TN – August 4, 2011

Using GIS Techniques to Identify and Delineate Karst Features in Tennessee

Background

Karst features (such as sinkholes) present challenges to the planning and construction of roads and

other public works natural-resources management topographic/hydrologic analysis

Approximately 2/3 of Tennessee is in karst terrain

Problem

Hand-delineation from maps is inefficient for large areas

Automated techniques using digital elevation data can be used, but it is not straightforward Digital elevation data are never “perfect” Digital filters must be used to control the detection of

false positives and false negatives

GIS Karst Delineation

Needs: Digital Elevation Model (DEM)

GRID format – array of cells with elevation values

Hydrography vector format – like a polyline shapefile

Types of elevation data

National Elevation Dataset (NED) Derived from USGS 1:24k topos State-wide

State base-map data Mass points and breaklines

1:4800 rural, 1:1200 in some urban areas Needs thorough QAQC Inconsistent quality across TN

Best data are in West TN (no karst) LiDAR

Best data available Very limited availability

Hydrography Data

High-resolution National Hydrography Dataset (NHD) Primary data source is blue lines from 1:24k

topo maps Proposed local-resolution (TN State base

map) NHD Primary data source is 1:1,200-4,800 imagery

(same as State base map elevation data)

GIS definitions – depressions vs. sinks

GIS Sink – area where surficial flow direction cannot be resolved

GIS Depression – area that must be filled in order to contribute flow to runoff

sinks

depression

GIS Definitions – Depressions vs. Sinks

Depressions and sinkholes

For this analysis and this talk, sinkholes and depressions are synonymous

This terrain analysis method will delineate depressions - areas with some amount of internal drainage

Delineation of depressions - hurdles

All digital data have limitations DEMs

Have some amount of error associated with elevations Elevations are averaged for each cell Tend to have some erroneous small depressions Roads and bridges tend to build artificial dams Discrepancies in depression density between datasets

We must accept a reasonable definition of a “true” depression to weed out “false” depressions

Delineation of depressions - hurdles

Hydrography Huge datasets such as NHD have line work errors

and connectivity errors

Proposed local-res NHD - highly and inconsistently dense Needed to weed out drainage ditches, etc.

Chose only streams that existed in high-res NHD Result was local-res (1:1,200-4,800) line work at high-

res (1:24k) density

Delineation of depressions – Process

Goal is to create a “hydrologically correct” DEM Must have a process that leaves “true” depressions

in DEM and eliminates “false” depressions Steps

1. DEM manipulation and delineation of preliminary depressions

2. Evaluation of depressions3. “True” depression enforcement/delineation and

delineation of watersheds4. Calculation of depression characteristics

DEM Manipulation and preliminary depression delineation

• Hydrography enforcement (“burning”)• Allows for more accurate basin delineation

and correctly routes flow across the landscape

• Fill DEM enforced with hydrography• Gives us a DEM with no depressions

• Subtracting “burned” from “filled” DEM delineates preliminary (first cut) depressions

Hydrography Enforcement

Hydrography Enforcement

Unfilled DEM

Filled DEM

filled area

Evaluation of depressions

Identify depressions with “true” depression characteristics

• Must...• Be > 60 ft (2 cells) away from a connected stream• Be larger than 1 cell (900 sq ft)• Contain a 10-ft contour with an area > 1 cellOr• Include or comprise a disconnected stream

Hydrography Enforcement

60-foot buffer around stream

Tiny depressions must go

Must contain a 10-ft contour

sinking streams must stay

Evaluation of depressions

3. Identify depressions with “true” depression characteristics

• Must...• Be > 60 ft (2 cells) away from a connected stream• Be larger than 1 cell (900 sq ft)• Contain a 10-ft contour with an area > 1 cell,Or• Include or comprise a disconnected stream

The depressions that meet these criteria are our “true”depressions.

“True” depression enforcement and delineation

Fill DEM except areas where true depressions exist

Create grid of “true” depressions

Create watershed for each new depression

Calculation of depression characteristics

Depth characteristics Fill elevation Area characteristics Storage characteristics

raw DEM

depression fill elevation grid

deptharea

volume(storage)

Outcomes

State-wide (eastern 2/3) data set of karst features derived from best elevation data available

Statistical analysis of karst features to understand differences in different physiographic/geologic settings

A tool to aid in the design of roads and other structures built in karst areas of TN

Questions and Comments