ML18017A422
| ML18017A422 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 01/11/2018 |
| From: | Xcel Energy, Northern States Power Co |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML18017A380 | List:
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| References | |
| L-MT-17-075 | |
| Download: ML18017A422 (37) | |
Text
MONTICELLO APPENDIX G ---PROBABLE MAXIMUM FLOOD MISSISSIPPI AT MONTICELLO MINNESOTA MAY 26, 1969 G-i REV 4 12/85 MONTICELLO APPENDIX G PROBABLE MAXIMUM FLOOD MISSISSIPPI RIVER AT MONTICELLO, MINNESOTA TABLE OF CONTENTS CHAPTER I II INTRODUCTION Scope Definition Authorization Data Investigations Acknowledgements CLIMATE AND HYDROLOGY OF THE STUDY AREA General Reference Data The Study Area Climate Precipitation Snow Temperature Wind Hydrology Annual Runoff Floods III PROBABLE MAXIMUM FLOOD DETERMINATION Probable Maximum Storm Snow Cover Temperature Sequence Snowmelt Infiltration and Retention Runoff Sequence Unit Hydrographs Flood Routing G-ii PAGE G .. l-l G .. l-2 G.l-2 G.l-3 G.2-l G.2-1 G.o2-1 G.:2-2 -G:.2-3 G.,2-3 G.2-3 G,.2:-5 G.2-5 G .. 2-5 G.2-5 G.3,...1 G.3-1 G.3-3 G .. 3-4 G.3-5 G.3-5 G.3--6 G.3-7 G. 3-8 REV 4 12/85 MONTICELLO TABLE OF CONTENTS (Continued)
IV STAGE DISCHARGE RELATIONSHIP v Procedure Computer Program Cross Sections Starting Elevations Manning's Roughness Coefficients Verification of Procedure Stage Discharge'Curve CONCLUSIONS G-iii PAGE G .. 4-l G.4-1 G.4-2 G.4-2 G.4-3 G.4-4 G.4-4 ..
G. 5-1 REV 4 12/85 MONTICELLO LIST OF FIGURES Exhibit 1 Exhibit 2 Exhibit 3 Exhibit 4 Exhibit 5 Exhibit 6 Exhibit 7 Exhibit 8 Exhibit 9 G-iv General Location Map Isohyetal Map Spring Storm Orientation No. 2 Isohyetal Map Summer Storm Orientation No. 2 Depth-Duration Curve for 15, ooo* square Storm OR 1-15 Depth-Duration Curve for 15,000 Square Mile Summer Storm UMV 1-22 Basin Snow Cover (Water Equivalent in Inches) lio Probability, March 16-31 24 -Hour Unit Hydrographs Basins 3, 4, 6 and Ba Discharge Rating Curve Mississippi River at Monticello, Minnesota Probable Maximum Flood Hydrograph Mississippi River at Monticello, Minnesota REV 4 12/85 Northern States Power Company Monticello Generat:ing Plant Probable Maximum Flood Study Chapter ! INTRODUCTION Scope This report describes the determination of the probable maximum flood level on the Miss is sippi River at the Monticello Nuclear Generating Plant. The plant site is on the right bank of the Mississippi River, about 3. 5 miles upstream of Monticello and about 70 river miles west of Millnea.polis-St.
Paul. The study area for the probable ma:ci.mum.
flood includes the Mississippi River dra:inage above the. plant site; about 13, 900 square miles. Hydrologic and meteo:-ologic data developed by Harza for the Prairie 'Island nuclear generating plant site have been used extensively to develop the probable ma.xim.um flood for the Monticello site whose drainage area lies entirely within the boundaries of the northern portion of the Prairie Island drainage area. Definition The term 11 probable maximum. flood, n as used herein, is the thetical flood that would :result i.f all the factors that contribute to the generation of the flood were to reach their tnost critical values that could occur concurrently.
The probable maxiinum floc;! is derived from G.l-1 REV 4 12/85 MONTICELLO hydrometeorological and hydrological studies and is independent of torical flood frequencies.
- It is the estiinate of the boundary between possible floods and iinpossible floods. Therefore, it would have turn period approaching infinity and a probability of occurrence, m any particular year, approaching zero. Authorization Authorization to conduct this study was given by the Northern States Power Company, Minneapolis, Minnesota, by Purchase Order M-79613 dated May 1, 1969. Data Data used in the study included U. S. Geological Survey maps, Northern States Power detailed topographic maps for the area near the site and publications on water supply and .floods in the study area, U. S. Army Corps of Engineers reports on river hydraulics and storm data, U .. S. Weather Bureau reports and technical pape;""S on data and Technical Bulletins of the University of Minnesota tural Experiment Station on the climate of Minnesota.
In addition, soil m.aps of the basin were obtained from the U. S. Department of ture. Investigations Determination o£ the probable maximwn flood in"'" 1 uded studies of the probable max:Unum precipitation for both spring and swnme't' storms, infiltration rates for various soil conditions, snowfall and snow cover, and historical temperature sequences and snowmelt rates. Unit hydrographs REV 4 12/85 MONTICELLO were developed and studies of flood runoff were made for each of the . sub-basins comprising the drainage area. above tt.:: ject site. Acknowledgements The assistance of the administra.to::-s and engmeers of the Northern ... c.* ...... States Power Company is gratefully acknowledged.
Th.eir cooperation and provision of materials used in the study were very helpful. The assistance of the U. S. Army Corps o£ Engineers, the U. S. Weather Burea".l and the U. S. Geological Survey, who provided valuable hydrological and meteorological information, is greatly appreciated.
Principal participants of th.e c onsu.lting engineering staH were: Project Sponsor K. E. Sorensen Project Manager L. L. Wang Chief Hydrologist R. W. Revell Overall Report Responsibility J. C. Ringenoldus Civil Eng:ineer A. A.
The report was reviewed by D:r. R. A. Clark, Professor in the Department o£ Meteorology, College of Geo-Sciences, Texas A&:M University, as a consultant to Harza Engineering Company. REV 4 12/85 MONTICELLO Chapter II CLllviATE AND HYDROLCGY OF THE STUDY AREA General Data were taken £rom .reports and technical papers to the general climatic and hydrologic conditions of the basin. The gree o£ variance from normal conditions of climatic events was studied to determine the .range of expected values under reas enable but very :rare conditions.
Reference Data U. S. Geological Survey maps of the basin at a scale of 1 to 24,000 with a 10-foot contour interval were u.sed in the study. In addition, tailed topographic maps near the project site were obtained from the Client. Special purpose maps were *also a"railable in .many o£ the ports on the climate and hydrology of the bas in. Technical bulletins on the "Climate of Minnesota," published by the University of Minnesota, Agricultural Expe*ri.Inent Station, were used extensively in describing the climate of the basin. The Studv Area *The Mississippi River basin above the plant site has a drainage area of appro:rir.nately 13 1 900 square miles and lies entirely in the state o£ Minne s ota. The topography of the basin is characterized by level to rolling prairie land interspersed with areas of glacial moraines whoe-e hills rise from 50 to 300 feet above the surrounding land. REV 4 12/85
) MONTICELLO Elevations in the basin range from 900 to 2100-feet, msl. The elevation at the project site is about 930 feet above mean sea level, and the average elevation o£ the basin is about 1200 feet. Most of the basin is covered with glacial deposits and the land face consists.
of features derived from' the several different ice sheets . -*-that ad'V'a.nced and then retreated .from the area. The principal feature, from the hydrological standpoint, is the nu.m.e reus lakes that were formed m the surface depressions created by the movement of ice. As the ice ret:reated, depressions were le:ft, which filled with water to form lakes. The st:ream.flow c...l:l.aracteristics of the !vfississippi and of all its tributaries are largely determined by the natural storage provided by these lakes and the many swamps. Clim.ate The study area lies within a zone of marked continental clim.ate characterized by wide and rapid variations in tempe:-ature, ager winter precipitation and usually am.ple summ.e r raln.fall.
lt has a tendency to extremes m all climatic features, although this is moderated somewhat by the large numbe:-of bodies of water in the area. Atmospheric moisture mainly *flows :into the region along tv:lo water vapor st:reams:
a strong southerly D:ow fro::1 the Gulf of Mexico and a comparatively diifuse westerly movement from the Pacific Ocean. Of the two, Guli moisture is the more important, ace ounting for most of the precipitation in the study area. During the months when the southerly winds reach Minnesota, May through September, about 65 percent of the annual rainiall is :recorded.
Because these air masses must travel 1200 to 1500 miles before reaching Minnesota, minor wind changes 'can account for large variations from normal precipitation
.. REV 4 12/85 MONTICELLO Precipitation Annual normal precipitation over the study area ranges from about 28 inches in the east to about 24 inches in the west. Extremes of annual precipitation recorded at Minneapolis
- St. Paul range from 49.7 inches to 10.2 inches. Monthly normal precipitation also ranges widely across the study area but reaches a high of about 5.5 inches in June in the ern portion of the area. Minimum normal monthly precip.itation one-half inch in.January and December in the western part of the area. The maximum 24-hour precipitation recorded was 8.07 inches at Marshall, which is near the study area, in Julx 1909. Most winter precipitation occurs as snow, which largely is stored on the ground until the spring thaw. Normal annual snowfall in the study area averages about 50 inches. A maximum annual fall of 107 inches was recorded about one hundred miles to the northeast of the study area, in Cook County, Minnesota.
Accumulations of three to four feet of snow within the study area are not unusual. Runoff is most affected by snow conditions.
Gradual melting of on unfrozen ground may result in much moisture entering the soil and sub soil, but sudden thaws in the spring may cause rapid runoff of the entire winter accumulation of snow, especially with deep frost penetration.
Temperature The study area, lying within the heart of the North American land mass, displays a typically continental climate. It has great extremes in temperature, not only from season to season and month to month, but G.2-3 REV 11 12/91 l .. .. *'8 I MONTICELLO on a diurnal basis as well. The only nearby water body of sufficient size to modify clilnate on more than an extremely localized basis is L * -Superior.
However, its UU'luence on the study area is restricted tially to coniines of the shoreline due essentially to prevailing westerly winds and also to the abrupt rise of-the land from the lakeshore.
Normal average daily te-mperatures at St. Cloud, which i.m.ately 25 miles upstream of the plant site, range from about 10°F in late January to about 72°F in late July. Normal maximum a::1.d minimum daily temperatures for the same station are about 20°F and 0°F in ary and about 83°F and 5 9°F in July. Normal temperatures over the basin for each season of the year are as follows: Season Winter (December, January and February)
Spring (March, April and May) Sum.mer (June, July, and August) Fall (September, October, and November)
Temperature . 7°F The great extremes of temperature in the area. are apparent from the absolute range of 173°F .that has been recorded.
The extreme mwn temperature recorded during the total record period was 114°F in July while the extreme minimum recorded was -59°F in January. treme maximum and minimum temperil.tures for each month of the year from St. Cloud are shown below. Temperatures given are in degrees Fahrenheit.
REV 4 12/85 MONTICELLO J F M A M J .r A s 0 N Extreme Maximum 55 58 81 91 105 102 107 105 106 90 71 Extreme Minimum 35 -32 2 18 33 41 34 18 6*-23 Wind -i;--a Prevailing winds are from the northwest during the winter and early spring., and from the southeast during the sum.me::-and latter part of spr:ing. Montbly.mean wind speeds vary slightly over the basin. nual averages are from about 10 to 13 miles per hour with mean monthly variation from about 9 to 15 miles per hour. The highest monthly mean winds are attained in April. Hydrology Annual Runofi The annual runoff from the rivers and streams throughout the basin is directly affected by the amount of lake or swamp are*a as evaporation losses reduce the yield. Then long-term mean annual runoff for the basin is approximately
- 5. 0 :inches '\Ni th a range in mean annual runoff from 1. 22 to 8. 93 inches. Floods Two types of flooding occur :in the basin--open-water flooding and backwater .flooding.
Flooding w!1ile open-water conditions prevail is caused by runoff producing rains, or by melting snow, or by a combination o{ the two. Flooding because of backwater is usually caused by ice jams. The most serious flooding throughout the basin has been associated with sive snowmelt and rainfall.
The tim.e of occurrence of floods shows the greatest frequency in April during the spring thaw. A second peak occurs D 63 -32 G.Z-5 REV 4 12/85 MONTICELLO in June due to thunderstorms..
A smaller peak occurs in the fall. Local flash floods occur in the streams in the spring thaw and also in the warmer season from locally-intensive rainfall.
The maxim.u.m flood of record on the Mississippi River at the plant site was cubic feet per second (elevation 916.2 feet) in 1965.. Records for the station at St. Paul indicate that this was probably the maximum flood since 1851. This flood which established record high stages at many stations in the Upper Mississippi Basin resulted from a severe winter and a combination of' clLorna.tic events that led to a deep snow cover on top of an ice layer. Moderate to heavy rainfall and a return to normal temperatures during April produced rapid melting and extremely high runoff. G.2-6 REV 13 4/95 MONTICELLO Chapter III PROBABLE MAXIMUM FLOOD DETERMINATION The probable .maximum flood at the plant site was determined by transpos:ing an actual, critical-spring storm to the drainage basin and maximizing the precipitation for potential moisture:
- -Potential snow cover and a critical temperature sequence were developed for deter-.. mining snowmelt contribution to flood runoff. Flood runoff at the plant site was determined by developing hydrographs for four applying rainfall and snowmelt excesses to the unit hydrographs and ing the resultant hydrog:raph_s the sub-basins to the project site. A probable ma...**dm.mn sum,mer storm over the project area was also studied in. detail and the resulting flood at the project site mined. Although the summer storm was much larger than the spring storm, the much lower retention rates under ordinary spring conditions, and the snowmelt contribution to runoff, resulted in the spring storm ducing the more critical flood. Exhibit 1 shows the gerie:ral location of the study area. Probable Maximum. Storm A probable maximum spring storm and a probable maximu.."n mer storm were determined by transposing and ma..ximizing*actual corded storms. The storms selected were the March 23-27, 1913 storm centered at Bellefontaine, Ohio, (OR 1-15, U.S. Army Corps ofEngineers, "Storm Rainfall :in the United States") and the August 28-311 1941 storm centered at Hayw-ard, Wisconsin (UMV 1-22). These storms :represent G.3-l REV 4 12/85 MONTICELLO near maximum conditions of meteorological events for spring and mer conditions.
Maximization of these storms involved multiplying the observed rainfall values. by the ratios of the ma.."Cirnu.m.
precipitable water in an air colu.m..n over the study area to the observed precipitable water in an air colwn.n for the actual storm. Under the assumption
- of a sattirated pseudo-adiabatic atmosphere, the amount of moisture is a unique tion of the ground elevation and surface dewpoint.
Precipitable water was thus determined f:rom the inflm.'7 barriers to the storm centers and observed and maximum persisting 12-hour dewpoints.
Persisting 12-hour dewpoints for the actual storm were obtained from U. S. Weather Bureau data. In accordance with frontal theory, the storm dewpoi:nts were measured in the warm air rather than at the point of rainiall.
Ma.xim.u.m persisting 12-hour dewpo:.nts for the study area were taken from the National Atlas _5!! the United States, 11 Maximum Persisting 12-Hour 1000-MB Dewpoints
(°F), Monthly and of Record. " For the posed storms, the maximum persisting 12-hour dewpoi't;ts were taken from the atlas at a point equally distant from the center of the study area as the point at which the observed dewpoints.were from the recorded storm centers. The distances were measured in a direction into the general path of air i1ow from the Gulf of Mexico. The original observed storm patterns were superimposed over the study area and the weighted average precipitation over each sub-basin determined by planimetering the isohyetallines.
The precipitation was then adjusted for ma.."'Cimum moisture charge in ance the above criteria.
- Superposition of the observed s'torm terns over the study area is justified because the areas are meteorologically homogeneous, and no major orographic differences exist betw'een the study REV 4 12/85 MONTICELLO area and the observed storm areas. Rotation of the transposed storm patterns was limited to 20 degrees !:rom the observed storm. The depth-duration relationships for 15,000 square miles from the recorded storms were used to determine rain!all increments for each sub-basin. The :rainfall increments were then arranged into a sequence considered to be the most critical-that could reasonably occur*.<-The sulting depth-duration curves for the spring and sum.m.er storms are shown on Exhibits 4 and 5. Exhibits 2 and 3 show the transposed is ohyetal patterns and the maxiinized precipitation
!or each storm. Following the determination of the flood :resultL'rlg from the spring storm, the is ohyetal pattern was re:-oriented over the study area to find the most critical rainfall pattern. Although an inf:inite number of entations is possible, the effect on the resulting flood was found to come negligible with additional orientations.
Snow Cover Snow cover over the basin was taken from the U. S. Weather BUJ:"eau, Technical Paper No. 50, "Frequency of Maximum. Water valent of March Snow Cover in North Central Uni:ed States. 11 For the purpose of the probable maximum flood study, maximum water lent (inches) for March 16-31 having one percent probability was used. Lines of equal snow cover, taken from the :report, were superi.Inposed over the basin weighted average snow cover for each sub-basin determined by planinletering.
Exhibit 6 the assumed basin snow cover.
REV 4 12/85 MONTICELLO Temnerature Seauence For purposes of snowmelt computations, it was necessary to termine a critical temperature sequence *that could reasonably be pected to occur while the snow cover was at a rnaxirn.urn..
Weather cords for the Minneapolis station offered the longest record of observed temperatures near the basin (54 years) and this record was used to termine a c:ritica.l temperature sequence.
As a large percentage of the total snow cover could be melted in about five days the maximwn cal five-day mean daily temperatul'e sequence occurring from April 1 to 15 was selected.
This was the period April 2-6, 1921. It was assumed that this temperature sequence could occur at any time bet-ween April 1 and A*pril 15 and that it could OCC:Ul' following a period of extremely cold weather such that the snow cover would be a.t a maximum. Since temperatures vary considerably over the basin, several tions located throughout.the basin were selected and the observed April 2-6, 1921, temperature sequence recorded.
These temperatures were assumed to be representative of the sub-basin which they*were nearest and were used in the snowmelt computations.
- Table III-1 shows the cord five-day temperature sequence used for each sub-basin.
tu.:res subsequent to the maximum five-day sequence were assu.r.ned to be the same as those :recorded in 1921. TABLE ill-1 Mean Daily Tem-oe:rature Seauence Sub-Basin Station 1 2 3 4 5 3 B:ra:inard 46.0 54.5 57.0 57.5 57.0 4-8a St. Cloud 48.5 54 .. 0 69. 0 6i .. o 57.5 6 Pokagama 40.5 53.5 54.0 54.0 53.5 REV 4 12/85 MONTICELLO Snowmelt Snowmelt for the probable maximum flood study was computed using methods developed by the U. S. Army Corps of Engineers and scribed in their Manual EM 1110-2-1406, 5 January 1960, Runoff from Snowmelt.
11 These methods utilize basic data on temperature, tation, wind velocities, ins alation, snow albedo, ha.sin and canopy cover, and a convection-condensation-melt factor which sents the mean exposure o£ the basin to wind. Average monthly values of insolation anc; wind velocity were from Minnesota weather records for use in the c amputations.
Ins elation of 45 0 Langleys was used throughout, and average wind velocities were determined to be 12 miles per hour for the snowmelt period preceding precipitation and 20 miles per .hour during precipitation.
Snow surface albedo was as su.m.ed to be 45 percent.a.t the start of the melting period. Basin exposure was asswned to be high due to the lack of large topographic variations and basin ca.naopy cover was determined for each sub-basin by estimating the percentage of forested area from map:-:; showing cover. A mean relative hUII1idity of 70 percent was used for converting air peratures to dewpoint temperatures during the days of high ins elation melt. Iniiltration and Retention Infiltration and initial retention losses were assumed to be extremely lew at the start of the runoff period. Documented flood events in the Upper Mi-::>sissippi Basin indicate that it is not unusual to have very high runoff in the early spring due to surface conditions at this time o! year. Commonly, early warm spells will cause melting of a light snow cover with the free water percolating through the soil to fill surface voids and G.3-5 REV 4 12/85 MONTICELLO depressions.
This is often followed by the return of freezing ,__;oes that cause ice to form over the ground surface, and a heavy pack accumulation.
If these conditions are followed by an extremely warm period and rainfall there is almost no loss of free water and off is max:i.Inized.
Since records of frost depth indicate that three to five .feet of frozen ground at the end of March are not unusual, retention rates are not likely to :increase significantly for some tiin.e after melting : starts. Initial retention was assUined to be zero in this study, and other losses were assumed to be 0. 02 inches per hour during the snowmelt period and 0. 03 inches per hour during the period following the beginning of rainfall.
Runoff Seouence The most critical sequence of. events leading to a major :flood be to have an unusually heavy spring snowfall and low temperatures after a period of intermittent warm spells and sub-freezing temperatures has formed an inlpervious ground surface and then a period -of extremely high temperatures followed by a major storm. This sequence of events is not unusual in the study area and the maximization of rainfall, snow-cover, and temperature would produce a probable maximum flood. For the purposes of this study, antecedent conditions were assumed to be such that extremely high runoff rates would result from sno-..v melt and precipitation.
Snow water equivalent having a one percent probability, was assurned to cover the study area on March 31. On April 1, the imwn historical temperature sequence was started *. By the fifth day the high temperatures were below the dewpoint temperatures of the storm and the probable maximum spring precipitation was assumed to beg:in April 5. Temperatures Jar the pe:dod following the maximum five-day sequence REV 4 12/85 MONTICELLO were assumed to be the same as those recorded for the April 7-16, 1921, period. Unit Hydrograohs The study area w.as divided into four major sub-basins and syn---.........
thetic unit hydrographs were developed for each using Snyder 1 s method, which .. is derived from the various physical basin characteristics.
The equations which Snyder developed are: 1} 2) 3) Where: L L ca. T t = C (LL )0" 3 p t *
- ca q_ '-= C X 640 -p P r p T = 3 + (t + 8) p time to peak peak rate of discharge duration of unit hydrograph t = Lag tim.e from center of rainfall period to peak of P unit hydrograph in hours = Length of river to the most remote portion of the bas in, in miles = Distance along the water course to the geographical center of gravity o£ the drainage bas in = Peak rate of discharge, in cubic feet per second per square mile = Duration of unit hydrograph in days C t and C p are constants The constants C and C are critical values in determining the t p basin lag and the peak discharge.
Data exists at several gaging stations in Minnesota on the Missis.sippi and Minnesota rivers, so unit hydrographs G.3-7 REV 4 12/85 MONTICELLO could* be constructed based on actual* data. After these unit hydrographs were developed, known values were applied to Snyder's equat1ons and a range of values !or Ct and Cp were determined.
The ave:ra*ge value of Ct was B. 0 and the average value of Cp was 0. 75. These two values were then used in Snyder's equations to develop synthetic unit hydrog:raphs for the sub-basins used in the study. l)nit hydrograph peaks were increased by 25 percent and basin lag decreased by one-sixth in accordance with standard Corps of Engineers practice.
E*ibit 7 shows the unit hydrog:raphs for each sub-basin. Flood Rquting Snowmelt and rainiall excesses were a*pplied to the unit hydrog:raphs and the resulting hydrographs were determined for each sub-basin.. basin hyd:rographs were then :routed to pl"oject site by computer gram using the modified Wilson method --the equation used in the method is: t where K = .2T + t 01' 02 Il, Iz t 'I'* = = = = Instantaneous discharge or outflow from a basin Instantaneous dis charge or inflow to a basin Routing L"lte :rval -24 hours for this study Travel time -hours G.3--8 REV 4 12/85 MONTICELLO Travel time T for flood routing were taken from Corps of Engineers recorded travel times for large floods. Base flow was determined from long-term.
USGS records for the stream gage at Elk River, Mmnes ota with a drainage area. of 14, 500 square miles. Examination of the records indicated that for the months of March -April,. a base flow of 5000 cfs was ...............
reasonable.
The base flow of 5000 cis was then added to the total of the routed flood hyd:rographs.
The resultant probable maximum flood graph is shown on Exhibit 9. 3-9 REV 4 12/85
) _ _,.. MONTICELLO Chapter IV STAGE DISCHARGE RELA.TIONSHIP The stage-discharge curve at the nuclear plant site was extended above the of historical experience by means of hydraulic tions based on the river channel downstream.
This was done* by a series of backwater computations based on a range of discharges.
The water computation procedure takes into account channel conditions over a reach of river and tends to converge on the true stage at the **uPstream end of.the reach even though the starting elevation at the downstream end of the reach cannot be established.
- selection of an adequately long reach of river for analysis will provide the correct upstream stage regardless of errors in starting elevation.
Procedure River profiles were by the "Standard Step Method 11 of water calculations, using an electronic computer.
The term 1 1backw'ater, 11 as applied in the discussion of natural channel (mild slope} hydraulics, generally refers to a depth of flow, or water surface elevation, which is greater than normal.depth because of a downstream control such as a dam or channel condition.
Since the depth of flow is greater than normal under these conditions, the cross-sectional area is greater than normal, and the velocity is less than normal. Normal velocity corresponds to a rate of head loss equal to channel gradient, so the rate of head loss must be less than the channel gradient.
Thus, the water surface slope is less than the bed slope. This slope-reduction caused by a downstrea.In depth greater than normal extends with dim.inishing effect upstream from the control until the "backwater profile If becomes coincident with the normal depth profile .. The standard step method utilizes the Manning formula in a putational procedure designed to take into account the effects of gradually G.4-l REV 4 12/85 MONTICELLO varied flow. As utilized in computations for a natural channel, this c*edure includes effects of downstream channel conditions and constric-tions in the determination of the water surface elevation at upstream points. The Bernoulli equation of conservation of energy is successively appli:ed for channel reach segments using the Manning formula for mination of head losses. * ... """ .... Comnuter Progra...'TI Backwater computations were made on an IBM 1130 computer tem, using a program prepared by the Hydrologic Engineering Center, Corps of Engineers, Sacramento, Caliiornia.
The effective cross-sections were completely described to the computer and Manning's 11 n 11 values for left overbank, right overbank and channel for each section were read in. Discharge and starting water .surface elevations were read into the puter at the first section and the computer carried out computations of water surface elevation fo:r each section*in upstream order. Elevations at each succeeding upstream section were determined by the computer by assuming a water surface elevation equal to that of the last section raised by the product of the slope and the distance between sections. tions were made of average slope, velocity, discharge in left and right overbank as well as the channel, velocity head, eddy loss due to expansion or contraction, ene:-gy gradient, and water surface elevation.
Output data include discharge, velocity and area for each overbank and channel section, slope, head loss and water surface elevation.
Cross Sections Channel and overbank cross-sections were determined from large scale two-foot topographic maps near the plant site furnished by ern States Power Company and from smaller scale 10-foot contour val maps prepared by the USGS. Points in the cross-section were G.4-2 REV 4 12/85 MONTICELLO described at each major break in the side slope so that sub-areas puted by assumed trapezoidal sections would not differ from the true area by a signiiicant amount. Cross-sections were determined at average intervals of about fourth mile near the power station and about one and one-half miles in the downstream reaches of the river near Monticello.
.. , .... Starting Elevations The first cross-section on the Mississippi River in the study a=ea is about five and three-quarter miles below the generating plant site and about one mile below Monticello, Minnesota.
A stage-discharge ship was determined for this cross-section using Manning 1 s formula. Q = 1. 48:6 A R 2/3 , S l /Z n where Q = Discharge, cfs, n = Manning 1 s coefficient of roughness A = Area of the cross-section R = Hydraulic radius which is equal the Area divided by the wetted perimeter S = Average channel slope The rating curve was developed by assuming various values for water surface elevation.
The area 1 hydraulic radius and dis charge 0 were then determined for each value. The average channel slope, S, was obtained from topographic maps.* G.4-J REV 4 12/85 MONTICELLO Manning's Roughness Coefficients Values ci Manning's "nrr coefficient used in the study were mined from e::cunination of detailed topographic maps of the plant site and o£ USGS 10-foot contour maps with. a scale of 1:24,000.
Average values of "n 11 determined were
- 032 for -the main channel,
- 050 for the left overbank and
- 045 for the right overbank.
A
!_l_!gher value of . 065 was used for the right overbank in Monticello and a value of .-060 was used for the island imm.ediately upstream of the plant site. These "n 11 values were determined by assUIIJ.ing a basic trn" value and then making adjustments for irregularity, changes in shape, obstructions, vegetation, and meander. Verification of Procedure The maximum flood of record at the site occurred in April, 1965, with a discharge of approximately 51 .. 000 cubic feet per second. For this flood, records of water surface elevation exist at several points along the river near the plant site. A trial backwater.computation was made starting from a point of known water surface elevation slightly more than a mile downstream from the plant site. The trial c amputation yielded a water surface elevation at the plant site which agreed within 0. 2 feet of the water surface elevation for the flood of record. For the determination of the water surface elevation corresponding , to the probable maximum flood, two different starting elevations were selected from the rating curve downstream of Elevations were selected for stages somewhat higher, as well as somewhat lower, than that anticipated for the probable-maxinlwn flood. Analysis was then made to determine backwater prof:ife s for the river reach extending to the plant. The profiles converged to within 0. 4 feet at the plant site, G.4-4 REV 13 4/95 MONTICELLO indicating that the river stage at Monticello plant would not be greatly effected by ad"justment of the stan.u.1b elevation.
An average of their values was used for the probable max:Untun flood stage. As a further verification, it was decided to determine what mum. stage would occur at the plant site if the values for Manning's
..........
roughness coefficient 11 n 11 were lowered for the overbank sections of the channel reach. Lmve:ring "n" for both the left and right overbank by 0. 005 yielded a probable rnax:imum flood stage elevation of 938. 9 or about 0. 3 feet lower than the elevations obtained by 'using the higher "n" values. It is believed that any further decrease in the roughness ficient would be unrealistic in view of the channel and overbank char-a.cteristics.
Stage Curve Several sets of backwater computations were made using water *surface elevations and their corresponding discharges as determined from the :rating curve downstream from Monticello.
Using the charges and the water surface elevations determined, a stage dis charge curve was then constructed for the plant site (Exhibit 8). REV 4 12/85 MONTICELLO Chapter V CONCLUSIONS The probable maximum flood at the project site was determined to be 364, 900 cubic feet per second and to have a corresponding peak stage of 939. Z feet The probable ma..**dmum f.l.ood hyd-rograph is shown on Exhibit 9. The occurrence of the sequence of events scribed in Chapter m would cause the flood to reach its maxim.um.
level about 12 days after the beginning of high temperatures and would remain above elevation 930. 0 for about 11 days. G. 5-1 REV 4 12/85
Scale 0 J.., 0 10 en 10 20 30 Miles _, MONTICELLO GENERAL HARZA ENIINEERIN; COMPANY DWG NO, -41:!\F I MAY, 1988 EXHIBIT LOCATION MAP REV 4 12/85 MONTICELLO Seclo 0 10 20 30Milu hzquuf1 f 1 EXHIBIT 2 ISOHYETAL MAP SPRING STORM ORIENTATION N0.2 t N ' REV 4 12/85 MONTICELLO Seal* 0 10 20 30Miles --! .. ,, r r
- EXHIBIT 3 ISOHYETAL MAP SUMMER STORM ORIENTATION N0.2 REV 4 12/85 c -a a:: a 0 }--! 0 -c: CD u ... Cjl Q.. 100 '90 80 70 60 50 40 I I 30 I I l 20 tO o. 0 MONTICELLO EXHIBIT 4 /l v */ . -* ._._ .I I I I j NOTE: I l For flood studfc purposes IOOo/a of rain oil occurred in 4 doys. 2 4 T1me in DQ)'S DEPTH-DURATION CURVE FOR 5,000 SQ. MI .. SPRING STORM 0 R l-.15 REV 4 12/85
,_ 100 90 80 70 GO I c 0 a:. 50 I I Q_ 0 1--0 40 -c u '-* Q. 30 20 I JO ;* I 0 a MONTICELLO EXHIBIT 5 I / lf I I I I -..,.._--q:
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1-I I . 2 3 4 Time in Days DEPTH -DURATION . CURVE FOR. 15,000 SQ. MI. SUMMER STORM UMV 1-22 HARZA EHGIHE'f:RING COioiPANY I)Wll NO. I' 5. llA'I', T9U REV 4 12/85 12 Scola 0 fO 2. 0 30 Mile$ t I MONTICELLO EXHlBIT 6 14 13 BASIN SNOW COVER {WATER EQUIVALENT IN INCHES) lcyo PROBABILITY, MARCH 16-31 HARZA EHOINEERIHGI COWPANY DW8 NO. '11411 F' 6 WAY 1 1868 REV 4 12/85 MONTICELLO 24-HOUR UNIT HYDROGRAPHS BASINS 3, 4, 6 AND Sa EXHIBIT 7 REV 4 MONTICELLO EXHIBIT 8 1000 990 980 970 960 950 . -*-** <= . / / v 940 I 939.2--0C/1
' v I I I I L l .... v 36.491 920 I ';'919 I J v ' I 918 .:. 917 v I / _g 916 I .... I' 915 2. I J I UJ 9 (4 913 I I I 912 I i 9 J 12 I 3 4 5 6 7 8 9 10 20 30 40 50 60 70 8090 )00 Discharge in 10,000 cfs DISCHARGE RATING .CURVE MISSISSIPPI RIVER AT MONTICELLO, MlNN. -
EHGINE£R1NQ C0141"1LNY OWG HO. 413 r 8 MA1', /94li! REV 4 12/85 MONTICELLO EXHIBIT 9 17 \ ..... -. . / \ :
.5 2 00 t---+----11 I / 0 0 10 20 30 40 50 Tlme In days PROBABLE MAXIMUM FLOOD HYDROGRAPH MISSISSIPPI RIVER AT MONTICELLO, MINN. REV 4 12/85