A Flow-Simulation Model of the Tidal Potomac River
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A Flow-Simulation Model of the Tidal Potomac River A Water-Quality Study of the Tidal Potomac River and Estuary United States Geological Survey Water-Supply Paper 2234-D Chapter D A Flow-Simulation Model of the Tidal Potomac River By RAYMOND W. SCHAFFRANEK U.S. GEOLOGICAL SURVEY WATER-SUPPLY PAPER 2234 A WATER-QUALITY STUDY OF THE TIDAL POTOMAC RIVER AND ESTUARY DEPARTMENT OF THE INTERIOR DONALD PAUL MODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director UNITED STATES GOVERNMENT PRINTING OFFICE: 1987 For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 Library of Congress Cataloging in Publication Data Schaffranek, Raymond W. A flow-simulation model of the tidal Potomac River. (A water-quality study of the tidal Potomac River and Estuary) (U.S. Geological Survey water-supply paper; 2234) Bibliography; p. 24. Supt. of Docs, no.: I. 19.13:2234-0 1. Streamflow Potomac River Data processing. 2. Streamflow Potomac River Mathematical models. I. Title. II. Series. III. Series: U.S. Geological Survey water-supply paper; 2234. GB1207.S33 1987 551.48'3'09752 85-600354 Any use of trade names and trademarks in this publication is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey. FOREWORD a rational and well-documented general approach for the study of tidal rivers and estuaries. This interdisciplinary effort emphasized studies of the Tidal rivers and estuaries are very important features transport of the major nutrient species and of suspended of the Coastal Zone because of their immense biological sediment. The movement of these substances through five productivity and their proximity to centers of commerce and major reaches or control volumes of the tidal Potomac River population. Most of the shellfish and much of the local and Estuary was determined during 1980 and 1981. This finfish consumed by man are harvested from estuaries and effort provided a framework on which to assemble a variety tidal rivers. Many of the world's largest shipping ports are of investigations: located within estuaries. Many estuaries originate as river (1) The generation and deposition of sediments, nu valleys drowned by rising sea level and are geologically trients, and trace metals from the Holocene to the present ephemeral features, destined eventually to fill with sedi was determined by sampling surficial bottom sediments and ments. Nutrients, heavy metals, and organic chemicals are analyzing their characteristics and distributions. often associated with the sediments trapped in estuaries. (2) Bottom-sediment geochemistry was studied and Part of the trapped nutrients may be recycled to the water the effects of benthic exchange processes on water-column nutrient concentrations ascertained. column, exacerbating nutrient-enrichment problems caused (3) Current-velocity and water-surface-elevation data by local sewage treatment plants, and promoting undesir were collected to calibrate and verify a series of one- and able algae growth. The metals and organics may be concen two-dimensional hydrodynamic flow and transport models. trated in the food chain, further upsetting the ecology and (4) Measurements from typical urban and rural threatening the shell and finfish harvests. Our knowledge of watersheds were extrapolated to provide estimates of the the processes governing these phenomena is limited, and the nonpoint sources of sediments, nutrients, and biochemical measurements needed to improve our understanding are oxygen demand during 1980 and 1981. scarce. (5) Intensive summertime studies were conducted to In response to an increasing awareness of the impor determine the effects of local sewage-treatment-plant efflu tance and delicate ecological balance of tidal rivers and ents on dissolved-oxygen levels in the tidal Potomac River. estuaries, the U.S. Geological Survey began a 5-year inter (6) Species, numbers, and net productivity of phyto- disciplinary study of the tidal Potomac River and Estuary in plankton were determined to evaluate their effect on nutri October of 1977. The study encompassed elements of both ents and dissolved oxygen. the Water Resources Division's ongoing Research and River (7) Wetland studies were conducted to determine the Quality Assessment Programs. The Division has been con present-day distribution and abundance of submersed aquat ducting research on various elements of the hydrologic cycle ic vegetation, and to ascertain the important water-quality since 1894 and began intense investigation of estuarine and sediment parameters influencing this distribution. processes in San Francisco Bay in 1968. The River Quality (8) Repetitive samples were collected to document Assessment program began in 1973 at the suggestion of the the distribution and abundance of the macrobenthic infaunal Advisory Committee on Water Data for Public Use, which species of the tidal river and estuary and to determine the saw a special need to develop suitable information for effects of changes in environmental conditions on this distri river-basin planning and water-quality management. The bution and abundance. Potomac assessment was the first to focus on a tidal river The reports in this Water-Supply Paper series docu and estuary. In addition to conducting research into the ment the technical aspects of the above investigations. The processes governing water-quality conditions in tidal rivers series also contains an overall introduction to the study, an and estuaries, the ultimate goals of the Potomac Estuary integrated technical summary of the results, and an execu Study were to aid water-quality management decision- tive summary, which links the results with aspects of concern making for the Potomac, and to provide other groups with to water-quality managers. Philip Cohen Chief Hydrologist James P. Bennett Potomac Study Coordinator A Flow-Simulation Model of the Tidal Potomac River III CONTENTS Foreword III Abstract 1 Introduction 1 The tidal Potomac River 1 The branch-network flow model 4 Model development 5 Network schematization 5 Channel geometry 5 Boundary conditions 6 Initial conditions 7 Model calibration and verification 7 Source and accuracy of measured data 7 Computation-control parameters 8 Flow-conveyance parameters 8 Simulation results 9 Sensitivity analyses 12 Computation control 12 Boundary-condition errors 15 Initial-condition errors 15 Operational guidelines 16 Effects of wind 17 Model applications 19 Particle transport 20 Flow-volume assessment 21 Summary and conclusions 23 References cited 24 Conversion factors 25 Appendix I, Program-control record format 29 Appendix II, Job control 37 Appendix III, Model execution setup 39 Appendix IV, Diagnostic messages 40 FIGURES 1. Map showing Potomac River near Washington, D.C. 2 2. Map showing the tidal Potomac River system 3 3. Schematic diagram of the tidal Potomac River system for the branch-network flow model 6 4. Graph showing model-generated water-surface elevations versus measured water- surface elevations for the Potomac River near Broad Creek (A, fi), near Hains Point (C), near Key Bridge (D), and near National Airport (E) 10 5. Graph showing model-generated discharges versus measured discharges for the Potomac River near Broad Creek (A, fi), at Indian Head (C, D), and near National Airport (£) 11 6. Graph showing discharges computed for the Potomac River at Indian Head by use of 5- , 15- , and 60-minute time steps (Ar) 13 7. Graph showing discharges computed for the Potomac River at Indian Head by use of 0.6, 0.75, and 1.0 values for the spatial derivative weighting factor (6) 14 A Flow-Simulation Model of the Tidal Potomac River V 8. Graph showing measured water-surface elevations near Key Bridge versus computed by use of +10-cm and 10-cm datum adjustments at Indian Head 15 9. Graph showing discharges computed for the Potomac River at Indian Head by use of known, estimated, and hypothesized initial conditions 16 10. Graph showing water-surface elevations computed for the Potomac River near Key Bridge by use of known, estimated, and hypothesized initial conditions 16 11. Graph showing water-surface elevations recorded at Indian Head and near Key Bridge and wind data recorded at Indian Head for Sept. 21-27, 1981 17 12. Graph showing water-surface elevations recorded at Indian Head and near Wilson Bridge and wind data recorded at Indian Head for Nov. 18-24, 1981 18 13. Graph showing model-generated plot of computed versus measured water-surface elevations for the Potomac River near Key Bridge on Sept. 23, 1981 18 14. Graph showing model-generated plot of computed versus measured water-surface elevations for the Potomac River near Wilson Bridge on Nov. 21, 1981 19 15. Graph showing time-of-travel plot of injected particles in the Potomac River from Aug. 15, 1981, through Sept. 13, 1981 22 TABLES 1. Comparison of computed and measured flow volumes 13 2. Model performance using alternative computation-control parameter values 14 3. Potomac River cross-section locations for the branch-network flow model 20 4. Accumulated flow volumes for the Potomac River at Wilson Bridge from Aug. 15, 1981, through Sept. 13, 1981 23 SYMBOLS AND DEFINITIONS A Area of cross section L2 B Top width of cross section L Cd Water-surface drag coefficient g Gravitational acceleration LT~2 k Flow-resistance coefficient function T2L~2/3 q Lateral inflow per unit length of channel L2T-1 Q Flow discharge L3T-1 R Hydraulic radius of cross section L t Time T A? Time increment T u Point flow velocity LT~ J U Mean flow velocity LT" 1 Ua Wind velocity LT" 1 x Longitudinal channel distance L A* Length increment L Z Water-surface elevation L a Angle between wind direction and x-axis Degrees P Momentum coefficient n. Flow-resistance coefficient similar to Manning's n TL~ 1/3 0 Spatial-derivative weighting factor £ Wind-resistance coefficient p Water density ML~3 p Atmospheric density ML~3 VI A Water-Quality Study of the Tidal Potomac River and Estuary A WATER-QUALITY STUDY OF THE TIDAL POTOMAC RIVER AND ESTUARY A Flow-Simulation Model of the Tidal Potomac River By Raymond W.