Development of a Set of Nutrient Simulation Sub-Models within HEC-RAS to Quantify the
Transport and Fate of Nutrients throughout the Channel System
Zhonglong Zhang1 and Billy Johnson2
1BTS,
Environmental Laboratory, Engineer Research and Development Center, 3909 Halls Ferry
Rd, Vicksburg, MS 39180
2Environmental Laboratory, Engineer Research and Development Center, 3909 Halls Ferry Rd,
Vicksburg, MS 39180
Abstract
Nutrient pollution is a leading cause of water quality impairment in lakes and estuaries and is
also a significant issue in rivers (USEPA, 2007). Of particular concern to human health and
ecosystem function are elevated concentrations of nitrogen (N), phosphorus (P), pesticides and
their degradates. HEC-RAS is a one-dimensional (1D) hydraulic model that can be used to
analyze river flows and sediment transport. It is widely used in the United States and around the
world. By including nutrient simulation capabilities in HEC-RAS, an existing riverine hydraulics
model can quickly be adapted to model water quality. A set of Nutrient Simulation Sub-Models
(NSM) has been developed within HEC-RAS. Riverine water quality simulation can be conducted
with the chosen complexity level using NSM. NSM I computes riverine algal biomass, nitrogen
and phosphorus cycling, and DO. NSM I has the capability of simulating water quality variations
up to the 10 state variables including algae, organic nitrogen, ammonia, nitrate, nitrite, organic
phosphorus, dissolved inorganic phosphorus, carbonaceous BOD, dissolved oxygen and benthic
algae.
NSM II has the capability of simulating up to the twenty-four state water quality variables and
computes multiple algal biomass, nitrogen, phosphorus, and carbon cycling, DO, pH and
pathogen, as well as numerous additional constituents and processes. NSM II water quality
state variables include Inorganic Suspended Solids, Phytoplankton group, Nitrate-Nitrite, Total
Ammonium, Dissolved Organic Nitrogen, Labile Particulate Organic Nitrogen, Refractory
Particulate Organic Nitrogen, Total Inorganic Phosphorus, Dissolved Organic Phosphorus, Labile
Particulate Organic Phosphorus, Refractory Particulate Organic Phosphorus, Dissolved Inorganic
Carbon, Labile Dissolved Organic Carbon, Refractory Dissolved Organic Carbon, Labile
Particulate Organic Carbon, Refractory Particulate Organic Carbon, Dissolved Oxygen, Benthic
Algae, Chemical Oxygen Demand, Pathogen, Alkalinity, and pH. Additionally, derived water
quality variables including chlorophyll-a, DIN, TON, TKN, TN, PIP, TDP, TOP, TP, TOC, TC, and
CBOD are computed in NSM II internally from the state variables and could be used for direct
comparison to measured data. NSM II is designed to model intermediate aquatic
eutrophication. It only handles the water column nutrient processes.
In addition to NSM II water column simulation, NSM III incorporates a dynamic bed sediment
diagenesis component, which simulates the chemical and biological processes undergone at the
sediment-water interface after sediment are deposited.
All levels of NSM libraries are compiled as dependent dynamic libraries (DLLs) and then
incorporated into the HEC-RAS model. The integrated system has been tested and validated
using analytical solutions. The NSM algorithms and integration within HEC-RAS will be further
tested and validated directly against field data and existing water quality models such as CEQUAL-W2. The incorporation of NSM water quality capabilities in HEC-RAS will provide public a
fully integrated riverine hydraulic, sediment and water quality model that encompasses
diagnostic, predictive, and operational applications and will greatly aid in Total Maximum Daily
Load (TMDL) development and implementation required by the Clear Water Act.