New Visual Tools for Load Allocation Analyses: Using STELLA Software for Water Quality Accounting

Transcription

1 New Visual Tools for Load Analyses: Using STELLA Software for Water Quality Accounting Kimberly O. Siemens, Kirk S. Westphal, P.E., Daniel G. Bounds, P.E., D.WRE Kimberly O. Siemens, CDM Plaza East Office Center 330 East Kilbourn Ave, Suite 1219 Milwaukee, WI ABSTRACT STELLA is a visual platform for customized modeling of flow networks and tracking multiple constituents through a water system. STELLA has recently been used as a conservative water quality accounting tool to support total phosphorus (TP) and total suspended solids (TSS) TMDL allocation analyses for a large watershed with multiple reaches and discharge inputs. STELLA was chosen as the tool for this purpose due to its visual presentation of reaches, inputs, and variables, as well as its flexibility. STELLA is essentially an open worksheet in which a user builds a network of flow elements, storage elements, and variables to represent any water system. For this TMDL analysis, the mainstem and tributaries were set up in a model within STELLA, and nonpoint source flows and loads from a previously developed model were incorporated, along with point source flows and loads compiled from existing data. MS4 flows and loads were also entered. The baseline pollutant loads were reduced through a series of reduction factors until concentrations met proposed water quality objectives. As needed, STELLA has also been used to simulate first-order decay or attenuation of total phosphorus in impounded flow networks. STELLA provides a visual feature not available with other modeling platforms. The network constructed for the load allocation analyses has a similar on-screen visual structure to the actual river network and interdependencies between system elements are clearly represented. The visual aspect of STELLA makes for a straightforward explanation of how the model works to public stakeholders. It is easy to visualize how the entire water network is constructed conceptually, how reaches and discharges connect to one another, and how pollutants flow through the system and are mathematically reduced. STELLA was chosen for these analyses because the platform can be used to quickly screen through reduction percentages and identify effective allocations reach by reach by examining resulting in-stream concentrations. The model was set up to include both global and local reduction factors. Global reduction factors are applied universally throughout the entire watershed, while local reduction factors can be applied to each individual reach. For each model run, these reduction factors are easily edited on 466

2 a control interface screen and results can be exported to a spreadsheet for further analysis and load allocation calculations. KEYWORDS TMDL, STELLA, allocation, model, presentation INTRODUCTION STELLA is a visual platform that can be used for customized modeling of flow networks and tracking multiple water quality constituents through a water system. STELLA has recently been used as a conservative water quality accounting tool to support total phosphorus (TP) and total suspended solids (TSS) TMDL allocation analyses for a large watershed with numerous reaches and discharge inputs. In addition to requiring a tool that could aggregate information from multiple sources (water quality models, permit databases, etc.), the analyses required a method to rapidly screen alternative load reductions and estimate appropriate load allocations to meet in-stream TP and TSS targets. STELLA was chosen as the tool for this purpose due to its ability to produce a visual presentation of reaches, inputs, and variables, as well as its flexibility to quickly evaluate the effects of a broad range of management alternatives. STELLA is essentially an open worksheet in which a user builds a network of flow elements, storage elements, and variables to represent a water system. For a TMDL allocation analysis, the mainstem and tributaries can be set up in a model within STELLA. Point and non-point source flows and loads can be entered from raw data or from a previously developed model. MS4 flows and loads can also be entered. Using STELLA, the baseline pollutant loads can be reduced through a series of reduction factors for each load category in each reach, until concentrations meet proposed water quality objectives. STELLA can also been used to simulate first-order decay or attenuation of water quality constituents in impounded flow networks. STELLA provides a visual feature not available with other modeling platforms. A model network constructed a load allocation analyses can have a similar on-screen visual structure to the actual river network, and interdependencies between system elements are clearly represented. The visual presentation aspect of STELLA provides for a straightforward explanation to public stakeholders, including how upstream loads and management decisions propagate downstream. It is easy to visualize how the entire water network is constructed conceptually, how reaches and discharges connect to one another, and how pollutants flow through the system and are mathematically analyzed. STELLA was chosen for these analyses because the platform can be used to quickly screen through reduction percentages and identify effective allocations reach by reach by examining resulting in-stream concentrations, both near-field and far-field. 467

3 The model can be set up to include both global and local reduction factors. Global reduction factors are applied universally throughout the entire watershed, while local reduction factors can be applied to each individual reach. For each model run, these reduction factors are easily edited on a control interface screen and results can be exported to a spreadsheet for further analysis and load allocation calculations. The allocation model is not used to generate new information on pollutant loads or streamflows. Rather, it can be developed as a way to synthesize the routing of existing information through a system and apply simulated load reductions on a reach-by-reach basis. The model can be used to identify load allocations for each reach of a water system. It can also be used to determine load allocations at a downstream endpoint or outlet. MODEL STRUCTURE AND USE A river and its tributaries can be divided into reaches based on the segmentation of a 303(d) list, and additional reaches can be added for river segments not listed on the 303(d) list to represent the entire system. The STELLA structure allows a basic flow network to be developed schematically, and this central network is used to route water through the system of tributaries and mainstem reaches of a river. Parallel networks are then constructed to route pollutants through the system from their point of introduction all the way downstream, or until they are fully attenuated. STELLA is an ideal tool for simulating first order decay dynamics, and can also simulate more complex dynamics if programmed by the user (as one might do with similar programming syntax in a spreadsheet). Even in a conservative allocation analysis, there are many combinations of load allocations that will yield results achieving a water quality target, since most reaches are affected by numerous loads from various sources. Typical allocation methods can be used with STELLA to provide for a sound approach. Figure 1 illustrates the overall model construct for an example water system, containing zoomed in figures to illustrate how pollutants enter each reach and are mathematically reduced, how one reach connects to another, and how the entire water network is constructed conceptually. Total Phosphorus (TP) and Total Suspended Solids (TSS) are shown as example constituents for illustrative purposes. 468

4 Red PS Tot TP PS Red Load 2 NPS Red Load 2 Tot TP Tot Remain Tot Remain TP TP 2 Hydrograph 2 TP 2 Flow 2 Flows 2 Phosphorus Into 1 Reach TSS Red PS PS Red Load 2 NPS Red Load 2 Local TSS TP TSS Flows 2 Hydrograph 2 TSS Red PS TSS Input Flow 2 TP 2 TSS 2 TSS TP 2 TSS 2 TSS TSS TSS TSS TSS TSS TSS TSS TP TSS 2 Representative Reaches Lbs Remov ed TSS TP Red PS TP N N TP TP TP PS Red Load 3 NPS Red Load 3 N TP N N TP N TP N Local TSS TP MS4 3 TSS MS4 3 MS4 Red 3 MS4 Red 3 Flows 3 Hydrograph 3 TP MS4 3 MS4 3 TSS Red PS TSS Input Flow 3 TP 3 TSS 3 TP N TSS N TP 3 TSS 3 N TSS TSS TSS N TSS N N TSS N TSS N TSS TSS N TP N TSS N Figure 1: Example Model Structure Note: Red arrows indicate transfer of information, blue arrows symbolize actual flows of water, or water quality constituents. 469

5 CONCLUSION STELLA is a tool that can be used for a variety of TMDL development and water quality analysis where a visual platform adds value for analysis and presentation purposes. Key benefits of using STELLA are the ability to aggregate information from a variety of sources into a dynamic analytical platform, the functionality to quickly screen through load reduction percentages to identify load allocations, and the ability to help explain how upstream decisions affect the entire flow network. 470