GIS & Mapping Components

GIS-Based Floodplain Submittal

Request in writing that a floodplain study be conducted by the county using GIS-based hydrology and hydraulic models with the property boundary (lot/parcel) highlighted on a tax map which clearly indicates the area of interest. The request should be sent to:

Department of the Environment
Sustainability Division - Program Support Section
1801 McCormick Drive
Suite 500
Largo, MD 20774

Once a letter is received, along with the floodplain study fee (see Chapter 2.8.3 of the County’s Stormwater Management Design Manual for Fee Schedules), the request will be logged in and a 2-week turnaround time is anticipated unless there are extenuating circumstances or information is missing which precludes the study from being completed.

Request Review

A county engineer will review the original request. In some cases, additional information may be required for the study to be completed. The county engineer will notify the applicant of any missing information. The existing or proposed stream crossing information necessary to complete the study is:

  • Federal Highway Administration chart number for the culvert
  • Federal Highway Administration scale number for the culvert
  • Headwall or wingwall type
  • Length of pipe/culvert
  • Minimum road elevations
  • Number of piers
  • Number of pipes
  • Pier shape
  • Pipe material and type
  • Pipe/culvert dimensions
  • Road profile
  • Upstream and downstream maximum low chord elevations of each pipe
  • Upstream and downstream stream invert elevations of each pipe
  • Width of piers

Other Requirements/Steps

  • The information provided must be surveyed and tied into existing county topography. In addition, if there is a proposed stream crossing, the location should be clearly shown in the plan view.
  • If the study involves a balance of cut and fill, the area of cut as well as fill must be clearly defined.
  • Once a study is complete, a letter and map will be sent to the applicant with cross-section locations and 100-year floodplain elevations.
  • If the floodplain elevations are determined based on the County’s GIS topography, a freeboard will be added. However, if the applicant provides more detailed topography, no freeboard will be added.
  • The floodplain will need to be delineated (by the applicant) on a detailed site plan and submitted to the Department of Public Works and Transportation (DPW&T) for review and approval.

Note: Both FEMA and MDE will accept the County approved GIS-based floodplain.