Background Studies

The Lower Willamette River has been studied extensively by several agencies.  Some of the prior studies include:

· A water quality model of the Willamette River mainstem (RM 0 to 187) was developed by Tetra Tech, Inc. (Tetra Tech, Inc., 1995) using QUAL2EU for the Oregon Department of Environmental Quality.  QUAL2EU (Brown and Barnwell, 1987) is a one-dimensional, steady state, hydraulic and water quality model.
· The Army Corps of Engineers (USACOE), Portland District, developed a flow routing model for the Willamette River and Columbia River using UNET (HEC, 1997). This model is being used to predict water levels and flow characteristics in the Columbia and Willamette system. The UNET model is a hydraulic, one-dimensional, unsteady flow model. The UNET cross-sectional data for both rivers were used in the current modeling effort with CE-QUAL-W2.
· An investigation of the Lower Willamette and the tidal influence on the combined sewer overflow (CSO) area was conducted by Limno-Tech, Inc. using DYNHYD for the City of Portland, Bureau of Environmental Services (Limno-Tech, Inc., 1997).  DYNHYD (Ambrose et al. 1988) is a one-dimensional, unsteady hydraulic model with no water quality modeling capabilities.  This study also investigated the magnitude of flows through Multnomah Channel. Unfortunately, in order to calibrate the flow model, the location of the Oregon City Falls was moved 75 miles upstream and the location of the Bonneville Dam was also moved 39 miles upstream. These unrealistic approximations show that the DYNHYD bathymetry was incorrect.
· Montgomery Watson, Inc. conducted two water quality studies for the City of Tigard (Montgomery Watson, 1999) and the Tualatin Valley Water District (Montgomery Watson, 1997) to investigate the feasibility of using the Willamette River as a drinking water source.  Data collected from these studies were valuable in establishing the upstream boundary condition for the Willamette River.
· A bathymetric review was conducted of the Willamette River upstream of the Oregon City Falls for Portland General Electric as part of their re-licensing effort for facilities at the falls.  This review provided bathymetric data for developing the model grid above the Oregon City Falls.

The Oregon Department of Environmental Quality (DEQ) is concerned about Oregon water quality standards being exceeded in the Willamette River as a result of untreated sewage and stormwater discharges into the river (Bloom, 1997). The Federal Clean Water Act and the Endangered Species Act require DEQ to develop Total Maximum Daily Loads (TMDLs) for the Willamette River Basin in order to meet water quality standards during all seasons. There are 1,436 miles of streams in the Willamette River Basin listed in the Oregon 303(d) list that do not meet water quality standards. Most of the river miles listed are due to temperature and bacteria violations. DEQ has to develop TMDLs for the Willamette River mainstem by the end of 2003 (Bloom, 2000).

The initial process for the development of TMDLs in the Willamette River Basin included the analysis of five water quality modeling options proposed by DEQ. These modeling options have been analyzed according to the time and resources necessary to develop them. The options proposed by DEQ included available models, enhancement of available models, and development of new models for the Willamette River Basin. DEQ is considering using a dynamic water quality model for the Willamette River because it is tidally influenced up to the Willamette Falls (RM 26.5). The water quality model selected should also be capable of modeling other water quality constituents in the future. Two phases are considered during the development of the TMDLs with the intention of meeting the 2003 TMDLs deadline. Phase I will consider modeling existing temperature and bacteria listings by the end of 2002. This phase is also considering modeling the major reservoirs in order to reduce the margins of safety for the TMDL development. Phase II will address other water quality constituents that are not listed in the 303(d) list such as dissolved oxygen, pH, and algae.

Modeling Approach

A previous report by Wells (2000) discussed the background of various water quality and hydrodynamic models and why the CE-QUAL-W2 Version 3 model was chosen for the Willamette-Columbia system. CE-QUAL-W2 Version 3 was proposed as the most appropriate model for the Lower Willamette system primarily because it contained the following elements:

· Two-dimensional, dynamic hydrodynamics and water quality capable of replicating the density stratified environment of the tidally influenced river sections as well as the sloping river channel sections. This is especially important when there are deep “holes” where a 1-D model would predict erroneously flow through the entire cross-section of the “hole”.
· River-estuary and hydrodynamic-water quality linkage is transparent for the Model User.
· The model can handle two-dimensional branches added on to the main stem of the Willamette River such as the lower reach of the Clackamas River as well as flow around islands.
· The CE-QUAL-W2 Version 3 code has many state-of-the-art model refinements that reduce numerical errors and improve the accuracy of model simulations.

The model time period consists of simulating the summers, May 1st to October 1st, for the years, 1993, 1994, 1997, 1998 and 1999.   Model results from 1993 will be compared with results obtained from the DYNHYD model developed by Limno-Tech, Inc. for the City of Portland.  Model results from 1994 will be compared with the QUAL2E model results produced by Tetra Tech, Inc. In addition to modeling the hydrodynamics and temperature, the Willamette River model will also simulate: dissolved and particulate non-living organic matter (both refractory and labile components), ammonia, nitrate, dissolved PO4, algae, TDS, pH, dissolved oxygen, and bacteria.

Conceptually the model elements, shown in Figure 2, are:

· Willamette River above the Oregon City Falls
· Willamette River below the Oregon City Falls
· Columbia River with side channels
· Multnomah Channel
· Lower Clackamas River and adjacent gravel pit

Figure 2.  Conceptual Layout of the Lower Willamette River System Model (Points Represent Model Segments)