Hydraulic Evaluation of Discharge Over Rock Closing Dams on the Upper Mississippi River

Transcription

1 ydraulic Evaluation of Discarge Over Rock Closing Dams on te Upper Mississippi River Jon endrickson, P.E. Senior ydraulic Engineer, St Paul District Introduction Prototype data was used for calibrating a weir equation to improve predictions of discarge over closing dams on te Upper Mississippi River (UMR). Closing dams are weir-like rock structures constructed across secondary cannels to reduce flow break-outs from te main cannel to backwater areas. Most of te closing dams in te St. Paul District reac of te UMR were constructed over 100 years ago as part of inland navigation system improvements. Originally constructed as emerged structures, tey were permanently submerged wen te locks and dams were constructed in te mid 1930 s. In te last decade, some of te original closing dams ave been modified, and new ones ave been constructed, bot for navigation cannel improvement and for abitat reabilitation. Being able to accurately predict te discarge over closing dams, will result in better management decisions and project designs. Variable ist = Dept of eadwater over structure crest = Dept of tailwater over structure crest = ead loss across structure (ie. = - ) /= submergence ratio = etted area of structure (based on Tailwater) = Crest lengt of structure (ie. lateral dimension) = Crest widt (ie. upstream to downstream measurement) Q = Discarge Cf = eir coefficient for free flow Cs = eir coefficient for submerged flow g = acceleration of gravity R = ydraulic Radius of rock structure vailable Prototype Data ydrodynamic and geometric data for several closing dams (or rock weirs) are listed in Table 1. Tis information was obtained troug te St. Paul Districts, River Monitoring Program.

2 Table 1. ydrodynamic and geometric data at closing dams. Benover Sloug Rock eir, Pool 8, UMR River Mile /D 8 Date of Measurements /21/ /04/ /02/93 isconsin Cannel Closing Dam, Pool 4, UMR River Mile /D 3 Date of Measurement /21/92 Belvidere Sloug, Closing Dam 2, Pool 5, UMR River Mile /D 5 Date of Measurement /22/92 Peterson ake Rock eir, 8, Pool 4, UMR River Mile /D 4 Date of Measurement /17/96 Sommers Cute Rock eir, Pool 7, UMR River Mile /D 7 Date of Measurements /21/ /27/ /06/ /08/ /18/95

3 Review of eir Equations and Coefficients Closing dams are essentially broad crested weirs, and if a weir coefficient can be determined, and oter pysical caracteristics calculated or measured, discarge over te structure can be predicted. Te discarge caracteristics at weirs are satisfied by te following two equations: Free : Q = Cf * * 3/2 Submerged : Q = Cs * * * (2g*) 1/2 Te criteria for free versus submerged flow is based on te relationsip between tailwater and eadwater dept (Reference 3, rkansas River Study). plot of tis is sown on Figure 1, along wit te prototype data measured at closing dams in te St. Paul District. In all cases, flow was submerged. Because of tis, te focus of tis study will be on determining representative values of Cs for use in te submerged flow equation. Relationsips between te discarge coefficient and flow caracteristics at te structure are available from references 1 troug 4. In te rkansas River Study (Reference 3), Cs varied from 1.2 to 2, depending on te submergence ratio /. In a similar aterways Experiment Station study on cannel control structures for te Souris River, Minot, Nort Dakota (Reference 2) a weir coefficient of 1.25 was used to predict flow over rock weirs. Yarnell (Reference 4), used te free flow equation for all flow conditions and found a wide range of weir coefficients. In te rkansas River study, it was concluded tat te rougness of te four stone gradations didn t ave an appreciable effect on discarge caracteristics. Gradations investigated were bot coarser and finer tan tose typically used by te St. Paul District in closing dam construction. owever, in te 1964 USGS report "Discarge Caracteristics of Embankment Saped eirs" it was concluded tat boundary rougness affected te coefficient of discarge and tat because of tis te embankment widt was important also. Calculated eir Coefficient Based on Field Data Figure 1 - Prototype data from St Paul District Closing Dams Plotted on Plate 40 of aterways Experiment Sation Tecnical Report (reference 3) Using te available prototype data, te equation for submerged flow was solved for Cs (Table 2). Te

4 average value of Cs of 0.99 (range of 1.22 to 0.82) is less tan tose found in te literature. For example, Plate 44 of te rkansas River Report gives values of Cs of 1.3 to 2.0 for te same range of submergence ratios. Plots of Cs versus ydrodynamic and structure caracteristics are sown on Figure 2. Tere appears to be weak trends of increasing weir coefficient value wit increasing submergence ratio and ydraulic radius (R 2 = 0.14 and 0.53 respectively). Best-fit lines were drawn sowing tis trend. No trend was apparent between weir coefficient and weir widt. Table 2. ydrodynamic data, geometric data, and calculated values of Cs at closing dams. Date Cs R / Benover Sloug 04/21/ Benover Sloug 05/04/ Benover Sloug 06/02/ isconsin Cannel 10/21/ Belvidere Sloug 10/22/ Sommers Cute 03/21/ Sommers Cute 03/27/ Sommers Cute 06/06/ Sommers Cute 07/08/ Sommers Cute 09/18/ Peterson ake, 8 04/17/ verages 0.99 Figures 2a,2b, and 2c: Submerged eir Coefficent, Cs, versus Closing Dam Prototype Data

5 Conclusions a. over closing dams on te Upper Mississippi River is usually submerged and te equation for submerged flow sould be used wit a weir coefficient of /2

6 Q = Cs * * * (2 g * ) 1/2 Cs = 0.99 b. Te submerged flow weir coefficient increases sligtly wit increasing submergence ratio and ydraulic radius (Figure 2). No trend was apparent between te weir coefficient and weir widt. References: 1. U.S Geological Survey ater Supply Paper "Discarge Caracteristics of Embankment Saped eirs" aterways Experiment Station Report "Cannel Control Structures for Souris River, Minot,ND" aterways Experiment Station Tecnical Report 2-650, "Stability of Riprap and Discarge Caracteristics, Overflow Embankments, rkansas River, rkansas" Yarnell, D.. and F.. Nagler (1930) of Flood ater Over Railway and igway Embankments. Public Roads, Vol 11, Number 2.