(from the Introduction in the User's Manual)

DYNHYD5 solves the one-dimensional equations of continuity and momentum for a branching or channel-junction (link-node) computational network. Driven by variable upstream flows and donstream heads, simulations typically proceed at 1- to 5-minute intervals. The resulting unsteady hydrodynamics are averaged over larger time intervals and stored for later use by WASP5.

The hydrodynamic model solves one-dimensional equations describing the propagation of a long wave through a shallow water system while conserving both momentum (energy) and volume (mass). The equation of motion, based on the conservation of momentum, predicts water velocities and flows. The equation of continuity, based on the conservation of volume, predicts water heights (heads) and volumes. This approach assumes that flow is predominantly one-dimensional, that Coriolis and other accelerations normal to the direction of flow are negligible, that channels can be adequately represented by a constant top width with a variable hydraulic depth (i.e. "rectangular"), that the wave length is significantly greater than the depth, and that bottom slopes are moderate. Although no strict criteria are available for the latter two assumptions, most natural flow conditions in large rivers and estuaries would be acceptable. Dam-break situations could not be simulated with DYNHYD5, nor could small mountain streams.

A flexible, computationally efficient type of network has been developed for the equations. The "link-node" network solves the equations of motion and continuity at alternating grid points. At each time step, the equation of motion is solved at the links, giving velocities for mass transport calculations, and the equation of continuity is solved at the nodes, giving heads for pollutant concentration calculations.

A physical interpretation of this computational network can be developed by picturing the links as channels conveying water and the nodes as junctions storing water. Each junction is a volumetric unit that acts as a receptacle for the water transported through its connecting channels. Taken together, the junctions account for all the water volume in the river or estuary. Parameters influencing the storage of water are defined within this junction network. Each channel is an idealized rectangular conveyor that transports water between two junctions, whose midpoints are at each end. Taken together, the channels account for all the water movement in the river or estuary. Parameters influencing the motion of water are defined within this channel network. The link-node computational network, then, can be viewed as the overlapping of two closely related physical networks of channels and junctions.

Junctions are equivalent to segments in the water quality model, whereas channels correspond to segment interfaces. Channel flows are used to calculate mass transport between segments in the water quality model. Junction volumes are used to calculate pollutant concentrations within water quality segments.

Link-node networks can treat fairly complex branching flow patterns and irregular shorelines with acceptable accuracy for many studies. They cannot handle stratified water bodies, small streams, or rivers with a large bottom slope. Link-node networkds can be set up for wide, shallow water bodies if primary flow directions are well defined. Results of these simulations should be considered descriptive only.

(Summary from TMDL Case Study, USEPA, 1993, Case Study No. 9, EPA841-F-94-004)

The Appoquinimink River watershed is located in eastern Delaware. The TMDL for phosporus was developed by the Delaware Department of Natural Resources and Environmental Control (DNREC) for the Appoquinimink River using the phased approach to TMDL development. The objectives of the TMDL included characterization of the nonpoint source nutrient loads and their impact on water quality and description of further modeling studies necessary to refine the TMDL. DNREC used available ambient water quality data and existing point and nonpoint source loading data to conduct the initial assessment and characterize the Appoquinimink's water quality problems. In addition, the EUTRO4 version of EPA's Water Qulaity Analysis Simulation Program (WASP4), a water quality model, was used to analyze the dissolved oxygen (DO) and nutrient economy of the river. Phosphorus overenrichment was determined to be the ultimate cause of excursions of applicable DO criteria. A phosphorus TMDL of 18,947 lb/yr was calculated as the sum of the point source allocation (6,862 lb/yr) and the background/nonpoint source allocation (12,085 lb/yr). These allocations reflect a reasonable margin of safety and will prevent further water quality degradation.

This case study describes the specific modeling efforts in more detail. The WASP4 model was used to predict the water quality impacts of various point and nonpoint source loading scenarios. With the additional information about nonpoint source loads collected as part of the initial phase of TMDL development, the modeling study found that even the most aggressive pollution control scenario- which consisted of total removal of point source loads, 50 percent removal of nonpoint source phosphorus and nitrogen loads, and 50 percent removal of the oxygen demand (SOD), ammonia, and phosphorus flux of sediments- provided only a marginal difference in DO levels. These results indicated that the system is driven by SOD. The TMDL has included a schedule for continued monitoring and modeling to address the SOD issue.

Cheng, C., Atkinson, J.F., and DePinto, J.V., 1994: A coupled GIS-water quality modeling study. In Proceedings of the 1994 Hydraulic Engineering Conference, American Society of Civil Engineers, Buffalo, NY, 1994, pp. 247-251.

Cockrum, D.K., and Warwick, J.J., 1994: Assessing the impact of agricultural activities on water quality in a periphyton-dominated stream using the Water Quality Analysis Program (WASP). In Proceedings of the Symposium on the Effects of Human-Induced Changes on Hydrologic Systems, American Water Resources Association, Jackson Hole, WY, June 26-29, 1994, p. 1157.

Lang, G.A., and Fontaine, T.D., 1990: Modeling the fate and transport of organic contaminants in Lake St. Clair, Journal of Great Lakes Research, vol. 16(2), pp. 216-232.

Lu, Z., April, G.C., Raney, D.C., and Schroeder, W.W., 1994: DO, BOD, and organic nitrogen transport in Weeks Bay, Alabama, In Proceedings of the National Symposium on Water Quality, American Water Resources Association, Chicago, IL, November 6-10, 1994, pp. 191-200.

Lung, W., and Larson, C.E., 1995: Water quality modeling of the upper Mississippi River and Lake Pepin, Journal of Environmental Engineering, vol. 121(10), pp. 691-699.

Mohr, M., and Sandstroem, S., 1996: Comparison between measurements and simulations with WASP and the MIUU model, Swedish National Board for Industrial and Technical Development, Technical Report NUTEK-VIND-96-5, Stockholm, 53 p (NTIS Order number DE97706031INZ).

Tetra Tech, 1995: Hydrodynamic and water quality mathematical modeling study of Norwalk Harbor, Connecticut: Final Report, Tetra Tech, Inc., Fairfax, VA.

Vuksanovic, V., De Smelt, R., and Van Meerbeeck, S., 1996: Transport of polychlorinated biphenyls (PCB) in the Scheldt Estuary simulated with the water quality model WASP, Journal of Hydrology, vol. 174, no. 1/2, p. 1.

Warwick, J.J., and Heim, K.J., 1995: Hydrodynamic modeling of the Carson River and Lahontan Reservoir, Nevada, Water Resources Bulletin, vol. 31(1), p. 67-77.

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