US Army Corps of Engineers
Engineer Research and Development Center Website


Published Oct. 29, 2014
Linked SWAT and HEC‐RAS modeling system

Linked SWAT and HEC‐RAS modeling system

A Model for Water Resources Management and Ecosystem Restoration

The Hydrologic Engineering Center – River Analysis System (HEC‐RAS) is an industry standard hydraulic model used widely in the U.S. and around the world. It can be used to analyze river flows and sediment transport. The Nutrient Simulation Modules (NSM), developed by the ERDC Environmental Laboratory, is a set of water quality dynamic-linked libraries (DLLs) within HEC‐RAS. The NSM is able to simulate carbon, nitrogen, phosphorus, silica cycles, algae, benthic algae, dissolved oxygen, carbonaceous biological oxygen demand and pathogen and sediment diagenesis processes. The HEC‐RAS‐NSM provides the public with a fully integrated riverine hydraulic, sediment and water quality model and will greatly aid users with determining total maximum daily loads (TMDLs).


NSM Libraries Can Be Added Quickly and Efficiently

HEC-RAS-NSM allows users to simulate the fate and transport of nutrients in a 1-D dendritic channel system. Before HEC-RAS-NSM, no 1-D model existed that allowed detailed simulation of nutrient fate and transport at this level of detail. Since most large river systems — as well as many smaller rivers and streams in the U.S. — have already been modeled with HEC-RAS, the addition of the NSM to the HEC-RAS modeling system allows users to quickly extend the existing models to simulate water quality, thus saving time and money.

Success Stories

HEC‐RAS‐NSM I was applied to the lower Minnesota River (LMNR).  The model results were closely aligned with observed water quality concentrations along the LMNR; moreover, the model reproduced temporal and spatial distributions of nitrogen species, phosphorus species, algae and dissolved oxygen against results created from the CE-QUAL-W2 model. The HEC-RAS-NSM I model allowed realistic predictions based on the combined effects of hydraulic, biological and chemical processes on longitudinal variations in water quality. A greater degree of spatial resolution of water quality was obtained than would otherwise have been possible, due to the relatively complex hydraulics of HEC-RAS. This shows that the HEC-RAS-NSM I model can provide a real predictive capability and aid in assessing riverine water quality.

The ERDC Environmental Laboratory is currently developing several HEC-RAS-NSM I models for the lower Yellowstone and Missouri River system as part of the development of the Missouri River Recovery Management Plan and Programmatic Environmental Impact Statement. The HEC‐RAS‐NSM II has been selected for Wisconsin River TMDL study and will serve as an assessment and management tool for the Wisconsin River basin. The Wisconsin River basin-wide soil and water assessment tool (SWAT) model will provide discharge and nutrient loads from major drainage tributaries to the HEC‐RAS model.  A linked SWAT and HEC-RAS-NSM will be applied in this study.


Water quality simulation can be conducted using the chosen complexity level of NSM through HEC‐RAS‐NSM.

  • NSM I simulates algal biomass, simple nitrogen and phosphorus cycles, organic carbon, carbonaceous biological oxygen demand, dissolved oxygen and pathogen. There are two options for each state variable: simulated and bypassed. Any combination of water quality constituents can be included or excluded from the simulation.
  • NSM II simulates multiple algal biomass, complete nitrogen, phosphorus, carbon and silica cycles, dissolved oxygen, carbonaceous biological oxygen demand, pathogen, alkalinity and pH. In addition, NSM II couples the water column simulation with a benthic sediment diagenesis module. Sediment-water fluxes of dissolved oxygen and nutrients are simulated internally rather than being prescribed.
  • Any number of generic constituents, suspended solids groups and carbonaceous biological oxygen demand groups can be simulated from NSM I and NSM II.
  • The HEC-RAS graphic user interface (GUI) allows for the rapid development of NSM input datasets. Under the GUI, the preprocessor provides detailed descriptions of all water quality parameters and kinetic coefficients. The postprocessor provides an efficient method for reviewing model results and comparing them with observed data for calibration. The primary output of water quality simulation is concentrations of water quality constituents reported for each cell as well as by each time step.
  • A working HEC-RAS hydraulic model can be quickly adapted to model water quality.


The NSM has been integrated with the HEC‐RAS model. The HEC‐RAS is a state‐of‐the art riverine model that can be used to analyze flow, sediment transport and water quality. A linked SWAT and HEC‐RAS modeling system is also available.

Cost, Distribution

There is no cost to download this product.  It can be found at:

Documentation, Training & Support

Zhang, Z. and B.E. Johnson (in review). “Aquatic Nutrient Simulation Modules (NSM) developed for hydrologic and hydraulic Models.” ERDC/EL TR‐14‐X, U.S. Army Engineer Research and Development Center, Vicksburg, MS.

Zhang, Z., B.E. Johnson, 2014. “Application and Evaluation of the HEC‐RAS-Nutrient Simulation Module (NSM I).” ERDC TN‐EMRRP‐SR-47, U.S. Army Engineer Research and Development Center, Vicksburg, MS.


ERDC Points of Contact

Questions about HEC-RAS-NSM?
Contact: Todd Steissberg
Phone: 530-574-5572
Update 25 August 2020