TMDL Non-Rooftop Disconnection Analysis

As part of the MDOT State Highway Administration (MDOT SHA) Watershed Implementation Plan (WIP) Action Plan, SHA anticipated that its future NPDES Municipal Separate Storm Sewer System (MS4) permit would require additional water quality treatment of its legacy impervious areas.  To proactively meet this requirement, based on the Maryland Department of the Environment’s (MDE) Accounting for Stormwater Wasteload Allocations and Impervious Acres Treated guidance document, non-rooftop runoff disconnection areas can be claimed as treated impervious surfaces.  These areas, once identified, could be applied against SHA baseline impervious surfaces.  The Non-Rooftop Disconnect analysis focused primarily on roads, buildings, and driveways. A proprietary Non-Rooftop Impervious Disconnection protocol has been developed, which leverages GIS technology, to identify these treated areas. 

In order to claim credit for this analysis, MDE has requested that field verification be conducted for 20% of the evaluated impervious surface area.  The Non-Rooftop Disconnect analysis focused primarily on verification of roads, buildings, and driveways.

Field Investigations: A mobile field data collection platform was implemented using an ArcCollector application and ArcGIS Online web maps.  These web maps displayed the results of the non-rooftop disconnect desktop analysis, highlighted areas that field crews were expected to review, identify areas where the application over and/or under credited, and allowed field crews to take notes and photos of field conditions. The goal of the field investigations was to determine the accuracy of the GIS model by verifying 20% of the credited impervious developed from the model.  

MDOT SHA provided detailed documentation covering the exact field data review processes to be used. Once the ArcCollector platform was developed and the field review areas identified, CRI and other consultant field crews attended a field training to ensure that all field crews were consistently conducting field verifications as defined in the process developed by MDOT SHA.

CRI performed field verifications at the selected sites in order to determine whether the model accurately reflected field conditions related to the disconnection of the impervious surfaces. If areas were identified where the impervious areas treated were either over-estimated, or under-estimated, field crews took notes and photographs of the discrepancies within the ArcCollector application.

Once the field verification was completed, the team examined any errors found during the field review of the final 50% of the field review areas and determined if those errors affect the overall model and any previously reviewed areas. 


Baltimore and Harford Counties, MD


MDOT State Highway Administration – Environmental Programs Division

Key Services:

  • Project Management
  • Field Reviews
  • ArcGIS Analysis and Data Management