SECTION III - GEOGRAPHIC INFORMATION SYSTEM

A web-based Geographic Information System (GIS) was developed for the City of Houston’s Comprehensive Drainage Plan project. The GIS was developed to reflect storm sewer systems and other drainage-related features. The GIS was created using ArcGIS and ArcIMS computer programs developed by Environmental Systems Research Institute, Inc. (ESRI).

A. GIS COVERAGES

A summary of the GIS coverages used for development of the CIP follows:

1. Storm Sewer Pipes

A line coverage (file) was developed to include information for all existing storm sewer pipe segments. The line coverage was developed based on either a coverage previously developed for the City or on information obtained as part of this project. A line coverage reflects items that can be shown as a line, such as storm sewer pipes, waterways, etc. A pipe segment is defined as a storm sewer pipe having a manhole (node) located at each end of the pipe, having one pipe diameter, and a flowline slope that does not change. In some areas where a series of pipes is connected by manholes, and all have the same pipe diameter and flowline slope, only one pipe segment is entered into the GIS instead of all the pipes having the same characteristics. As a result, some manholes are not entered into the GIS. Some of the storm sewer pipe coverage attributes include pipe diameter for existing and proposed pipes, construction material type, upstream and downstream flowline elevations, and pipe identification number.

Each storm sewer pipe system outfall is assigned a unique system outfall identification (ID) number. The system outfall ID number consists of a letter followed by a four-digit number. The system outfall ID letter designated is consistent with the letter used by HCFCD to designate major watersheds in Harris County, except for the Ship Channel watershed, which has the same ID number as the Hunting Bayou watershed. System outfall ID letters and numbers for each watershed are as follows:

2. Storm Sewer Manholes

A point coverage was developed to include data for manholes (nodes) located in the storm sewer system. As with the storm sewer pipe coverage, the source for the manhole coverage was developed using either a coverage previously developed for the City or information obtained as part of this project. A point coverage reflects items that can be shown as points, such as manholes. As explained in the storm sewer pipe coverage, not all manholes have been included, since some of the manholes were eliminated when several pipe segments were entered as one. Manhole attributes include rim and flowline elevation, manhole type, and system ID number.

3. Drainage Areas

The drainage area coverage is a polygon coverage developed to reflect drainage areas for the storm sewer systems. A polygon coverage reflects items that can be shown as a closed shape, such as drainage areas. This coverage also contains the system ID number. Drainage areas were delineated using topographic maps, generated as part of the City’s Monumentation and Mapping Program, and existing street patterns.

4. Land-Use Categories

The land use coverage reflects the different land use categories within the City of Houston in or about 1992. The land use coverage contains two formats, a Vector format, which is more defined and displayed at a scale of 1:20,000 or less, and a Raster format, which is more of an approximation that is displayed at a scale of 1:20,000 or greater.

5. Flooding Complaints and Repetitive Claims

As described in Section II - Background Considerations, three sources were used to identify previous reported flooding. The flood complaint coverage presents data from two of the sources, which includes records obtained from the City’s Maintenance and Right-of-Way Division and from FEMA’s files for repetitive insurance claims which were obtained through the City. This data was then entered into a database and previous reported flooding coverage developed. The location of the flooding complaint data was approximately located based on a linear interpolation of address ranges.

6. Drainage Survey Results

As part of this project, a drainage survey questionnaire was sent out with the City’s water bills in July 1996 to collect information regarding flooding. The drainage survey questionnaire was also sent to residents in the Kingwood area in October 1998. More than 51,600 questionnaires were returned by the public to the City for evaluation. The flooding complaint data was approximately located, based on a linear interpolation of address ranges. The coverage includes information such as street location, whether the street has experienced flooding, whether the house has ever flooded, and whether the problem has been corrected.

7. Floodplain Boundaries

The floodplain coverage includes floodplain and floodway boundaries as defined by HCFCD and FEMA as shown on the draft Tropical Storm Allison Flood Insurance Rate Maps (FIRMs). This coverage includes the boundaries for the floodway, 100- and 500-year floodplains, and base flood elevations.

8. Street Patterns

The street coverage includes streets, major highways, and railroads located in the City of Houston. Sources for the street coverage included information from TIGER 94 files, developed by the U.S. Census Bureau, and from TxDOT.

9. Drainage Ditches and Detention Basin Alignments

The drainage ditch coverage includes the alignment of major drainage channels, ditches, and detention basins located in the City of Houston. This coverage was developed using information obtained from TxDOT and from HCFCD. The drainage ditch coverage does not include all major channels, roadside ditches, and detention basins. Some of the drainage ditch alignments were modified to match apparent location as shown on the digital ortho photographs.

10. Digital Ortho Photographs

The Digital Ortho Photographs coverage includes digital aerial photos of the City of Houston obtained by HGAC. The digital photos reflect conditions in the City as of January 2002.