Gridded Surface Subsurface Hydrologic Analysis

Published Nov. 21, 2012
Updated: Oct. 21, 2022
A map of the cell by cell flood depth differences from the existing flood depth to the flood depth for the proposed changes for the Picayune Strand study. The team recommended shortening a proposed levee. All of the proposed changes saved $40M for the project.

A map of the cell by cell flood depth differences from the existing flood depth to the flood depth for the proposed changes for the Picayune Strand study. The team recommended shortening a proposed levee. All of the proposed changes saved $40M for the project.

Simulations for Successful Watershed Management

Weather is not the only cause of flooding, stream erosion, and pollution. These problems usually occur from human impacts to watersheds, including urban development, construction activities, hydrologic modifications, and forestry, mining, and agricultural practices.

Predicts and Mitigates Watershed Management Problems

To help control the impact of these serious economic and environmental issues, experts in watershed engineering at the ERDC Coastal and Hydraulics Laboratory (CHL) developed Gridded Surface Subsurface Hydrologic Analysis (GSSHA) to conduct studies in watershed modeling.

With GSSHA, engineers can perform complex studies of atmospheric, land-based, wetland, riverine, and coastal systems to help predict and mitigate watershed management problems. CHL studies in watershed modeling and analysis integrate hydrology, hydraulics, and water quality. This helps city or military installation planners, civil engineers, and others involved in planning and resource management make informed decisions about watershed management.

Adapts to Different Environments with Flawless Performance

GSSHA is a multidimensional modeling technology that uniformly couples overland, surface, and subsurface flow for accurate watershed simulation. It is a physics-based, distributed, hydrologic, sediment and constituent fate and transport model that features the following:

  • Two-dimensional (2D) overland flow and groundwater and one-dimensional (1D) stream flow and soil moisture
  • Fully dynamic pipe networks for urban and agricultural drainage systems
  • Wetland peat layer hydrodynamics and several in-stream weir and culvert models
  • Lakes, detention basins, levees, rating and rule curve releases
  • Permafrost and soil thermal profile
  • Boundary conditions for hurricane storm surge or levee breach inundation modeling
  • Full coupling among groundwater, vadose zone, streams, and overland flow
  • Fully-coupled groundwater to surface-water interaction to model Hortonian and non-Hortonian basins

GSSHA can be used as an episodic or continuous model where soil surface moisture, groundwater levels, stream interactions, and constituent fate are continuously simulated. The fully coupled groundwater to surface-water interaction allows GSSHA to model basins in both arid and humid environments. The model simulates sediment and constituent fate and transport in shallow soils, overland flow planes, streams, and channels.

Success Stories

Hurricane Inundation Depth at Landfall – New York City, August 25, 2011

Effective forecast is vital in emergency flooding risks assessment. On August 25, 2011, the U.S. Army Corps of Engineers (USACE) commander in charge of coastal defenses in the northeast region of the United States instructed ERDC CHL to forecast inland flooding effect.

GSSHA had been in use for several years, helping CHL researchers study the effect of storm surge and sea-level change on coastal military facilities and cities. Answering the commander’s call, USACE and ERDC CHL used GSSHA to predict the inland effects of predicted storm surge from Hurricane Irene on New York City and Long Island. Engineers used storm surge forecast data with GSSHA to simulate the flow of sea-water over New York City and Long Island. GSSHA model outputs included maps showing location and maximum depth of flood water. The maps were animated using Google Earth and shown to the leadership of New York City. Mayor Michael Bloomberg used this information to evacuate coastal and low-lying areas in New York City and plan for the coming disaster.

Picayune Strand Restoration Project – 2016

The Picayune Strand Restoration Project is one of many components of the Comprehensive Everglades Restoration Project (CERP) intended to restore nearly 700 hectares of a failed residential development in southwestern Collier County, FL, to its predevelopment wetland conditions. A detailed analysis was performed to derive a restoration plan to achieve this goal. As required by the Water Resources Development Act (WRDA) 2000, the U.S. Army Corps of Engineers (USACE) is required to ensure that no component of CERP results in an effective taking of land by adversely impacting the level of flood protection of adjacent landowners. To ensure the current level of flood protection is maintained, a hydrologic model was developed to assess the potential for flooding and to refine the proposed flood mitigation features. The GSSHA model was selected for this effort because GSSHA is able to simulate fully coupled rainfall distribution, extraction, retention, overland flow, and one-dimensional channel flow. Models of varying resolution were developed from existing and proposed design data and were initially populated with parameter values from a previous hydrodynamic modeling effort. Parameters were then tuned to observed stage and flow data using the Secant Levenberg-Marquardt method, a nonlinear least squares minimization computer-based local search method. The calibrated model is capable of reproducing canal flows, canal stages, and overland stages with very high Nash Sutcliffe Forecast Efficiencies, generally 0.9 or higher. Subsequent uncertainty analysis allowed water stages to be estimated with 95% certainty. Modeling and uncertainty analysis results allowed for refinement of the proposed flood mitigation features. The hydrologic models and analysis demonstrated that some of the features in the original plan were either unnecessary or overdesigned and could be modified or eliminated, resulting in $40M in flood control feature construction cost savings.


The scalability of GSSHA is a key component of its robust watershed modeling power. CHL engineers use it for large studies, such as the management of military training lands, and for smaller projects where detail at the street level is critical, such as urban flooding. GSSHA also does the following:

  • Tracks the fate of associated pollutants through the coupled system
  • Provides soil moisture, runoff, and flooding predictions that can be used to asses fire threat, irrigation needs, and effects on natural systems
  • Analyzes future conditions and management scenarios—such as land use changes and wetland restoration
  • Helps develop Best Management Practices (BMP) and Total Maximum Daily Load (TMDL) values for flood control, sediment transport, and pollutant transport
  • Utilizes unique boundary conditions to simulate coastal flooding due to storm surge


New features of version 7.1 includes support for gridded evapotranspiration and permafrost.


  • Spatially and temporally varying precipitation
  • Snowfall accumulation and melting
  • Precipitation interception, infiltration, evapotranspiration, surface runoff routing
  • Simple lake storage and routing, unsaturated zone soil moisture accounting
  • Saturated groundwater flow, wetland peat layer hydraulics, overland sediment erosion
  • Transport and deposition, in-stream sediment transport
  • Overland contaminant transport and uptake

New Formulations and Processes:

  • New stream formulations, improved evapotranspiration and frozen soil, and improved overland roughness model
  • Wider range of weirs, culverts, in-stream hydraulic structures, lakes, and detention basins
  • Improved sediment uptake, geomorphology, and stability when digital dams are present
  • Permafrost and soil thermal profile
  • Robust simulations of almost any locale
  • Flexible ability to vary parameters from cell to cell
  • Thorough defining of spatial distributions of parameters by land use, soil type, and vegetation cover maps

For more information about GSSHA, visit the GSSHA Wiki.

Charles Downer, P.E
(561) 682-2922

Intro to the OHWM Manual

Video by Jared Eastman
Introduction to the Interim Draft of the National Ordinary High Water Mark (OHWM) Manual
U.S. Army Corps of Engineers, Engineer Research and Development Center
Jan. 30, 2023 | 24:23
Introduction to the Interim Draft of the National Ordinary High Water Mark (OHWM) Manual for Rivers and Streams.