GIA image technology helps in stormwater planning

The primary components of a comprehensive stormwater management program are: establishing a stormwater utility, managing stormwater infrastructure and conducting basin master planning.

Accomplishing these tasks is not always easy, but with raster and vector stormwater GIS data as an information management and planning tool, the job can be done.

The city of Portsmouth, Va., recently began combining raster and vector stormwater GIS data to determine impervious areas for commercial and industrial properties. By using the technology, the city has set stormwater fees for nonresidential parcels.

City and county governments can use GIS image technology to monitor drains, floodplains and basins to prevent disasters before they happen — or at least for notification as soon as overflow begins.

Raster and vector GIS data can be used to identify and calculate impervious areas of non-residential parcels so that appropriate utility service charges can be assessed.

Airborne digital imagery and spectral analysis — red, green and near-infrared color bands — allow users to distinguish between pervious and impervious areas. Polygons (raster data) of impervious areas are then automatically generated using image processing — spectral analysis software.

Once impervious-area polygons are generated, parcel lines (vector data) can be input into the GIS as an overlay.

The square footage of impervious areas in each non-residential parcel can be calculated and appropriate fees determined. Raster and vector GIS data can also be used to identify utility billing credits for stormwater Best Management Practices.

For example, the surface square footage of a pond or determanition structure, which reduces discharge peak, and the surface square footage of wet detention structures, which increase water quality, can be calculated using digital orthophotos.

French drains and other structures that reduce the discharged volume to the surface drainage system can be identified during data collection and input as vector data.

A community can effectively combine raster and vector GIS data to establish a spatial inventory of stormwater features and structures for better infrastructure management digital orthophotos have been created.

Communities with accurate and current stormwater may depicting fine details about the system can scan and incorporate these detail maps into the GIS as raster images.

After the scanning process a basic map of major stormwater structures and features can be digitized as vector data. Thus, if detailed locational data was needed on a particular stormwater structure, the detail map/raster image that it linked to the structure via an ID number can be accessed via a point-and-click operation.

A city or county also could establish a stormwater inventory by using either GPS methods and hand-held data collectors in the field or heads,up digitized source documents and hand-held data collectors instead of relying exclusively on existing stormwater maps.

Once the inventory is established, raster and vector data can be used with a work-order processing system to ensure that stormwater complaints from customers are addressed and resolved quickly.

For example, a dispatcher receiving a complaint via telephone would input a customer’s address into the GIS, and the address would be linked to its appropriate pipe ID numbers, which would automatically tell city crews where to begin investigating the drainage problem.

A vector map of the affected area and detail maps on associated storm structures could plotted, and the raster and vector stormwater GIS data collected for utility development and infrastructure management can also be used for modeling and basin master planning.

By linking the stormwater GIS with HEC-1 (hydrologic) and HEC-2 (hydraulic) models, modeling capabilities can be enhanced. And by modeling certain “what if” scenarios, users will be able to determine what impact — positive or negative — proposed developments will have on the downstream structures and will ultimately allow for development of effective stormwater master plans.

This article was written by Stephen Phipps, GIS project manager, Woolpert, Dayton, Ohio.