ML21070A130
| ML21070A130 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 02/11/2021 |
| From: | Xcel Energy, Northern States Power Company, Minnesota |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML21070A105 | List:
|
| References | |
| L-MT-21-004 | |
| Download: ML21070A130 (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 PAGE I INTRODUCTION G.1-1 Scope G.1-1 Definition G. l-1 Authorization G.1-2 Data G.1-2 Investigations G.1-2 Acknowledgements G.1-3 II CLIMATE AND HYDROLOGY OF THE STUDY AREA G.2-1 General G.2-1 Reference Data G.2-1 The Study Area G.2-1 Climate G.2-2.
Precipitation G.2-3 Snow G. 2-3 Temperature G.2-3 Wind G.2-5 Hydrology G.2-5 Annual Runoff G. 2-5 Floods G. 2-5 III PROBABLE MAXIMUM FLOOD DETERMINATION Probable Maximum Storm G.3-1 Snow Cover G.3-3 Temperature Sequence G.3-4 Snowmelt G.3-5 Infiltration and Retention G.3-5 Runoff Sequence G.3-6 Unit Hydrographs G.3-7 Flood Routing G.3-8 G-ii REV 4 12/85
MONTICELLO TABLE OF CONTENTS (Continued)
GB.APTER PAGE IV STAGE DISCHARGE RELATIONSHIP G,4-1 Procedure G.4-1 Computer Program G.4-2 Cross Sections G.4-2 Starting Elevations G.4-3 Manning's Roughness Coefficients G.4-4 Verification of Procedure G.4-4 Stage Discharge*curve ef ., =~. 4-5 V CONCLUSIONS G.5-1 G-iii REV 4 12/85
MONTICELLO LIST OF FIGURES Exhibit 1 General Location Map Exhibit 2 Isohyetal Map Spring Storm Orientation No. 2 Exhibit 3 Isohyetal Map Summer Storm Orientation No, 2 Exhibit 4 Depth-Duration Curve for 1s. ooo* Square Milespring Storm OR 1-15 Exhibit 5 Depth-Duration Curve for 15,000 Square Mile Summer Storm UMV 1-22 Exhibit 6 Basin Snow Cover (Water Equivalent in Inches) li.
Probability, March 16-31 Exhibit 7 24 - Hour Unit Hydrographs Basins 3, 4, 6 and Ba Exhibit 8 Discharge Rating Curve Mississippi River at Monticello, Minnesota Exhibit 9 Probable Maximum Flood Hydrograph Mississippi River at Monticello, Minnesota G-iv REV 4 12/85
Northern States Power Company Monticello Generating Plant Probable Maximwn Flood Study Cb.apter I INTRODUCTION Scope This report describes the determination of the probable maximum.
flood level on the Mississippi 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 north-west of Minneapolis-St. Paul.
The study area for the probable maxir.nwn flood includes the Mississippi River drainage 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 maximum 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 hypo-thetical flood that would result ii all the factors that contribute to the generation of the flood were to reach their .tnost critical values that could occur concurrently. The probable maximum floe:! is derived from G. l-1 REV 4 12/85
MONTICELLO hydrometeorological and hydrological studies and is independent of his-torical flood frequencies.
- It is the est:iniate of the boundary between possible floods and impossible floods. There.fore, it would have a re-turn period approaching infinity and a probability of occurrence, in any particular year, approaching zero.
Authorization Authorization to conduct this study was given by the Northern States Power Company, Minneapolis, :M:innes eta, by Purchase Order M-79613 dated May 1, 1969.
Data Data used in the study included U. S. Geological Survey maps, Northern States Power Co~ 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 meteorologi-cal data and Technical Bulletins of the University of Minnes eta Agricul-tural Expe :ri.rnent Station on the cl:iznate of Minnesota. ln addition, s oil maps of the basin were obta:ined from the U. S. Department of Agricul-ture.
Investigations Determination of the probable maximwn flood in~ 1 uded studies of the probable maximwn 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 fiood runoff were made !or each of the sub-basins comprising the drainage area. above tt.:: ;:-- ject site.
Acla:rowled gements The assistance of the adm.inistra.to:-s and engineers of the Northern States Power Company is gratefully acknowledged. Th.eir coop~;~ion 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 consulting engineering stafi were:
Project Sponsor K. E. Sorensen Project Manage:r L. L. Wang Chief Hydrologist R. W. Revell Overall Report Responsibility J. C. Ringenoldus Civil Eng:ineer A. A. MueIJ,er The report wa.s reviewed by Dr. R. A. Cla:rk, Professor in the Department of Meteorology, College of Geo-Sciences, Texas A&M University, as a consultant to Harza Engineer:ing Company.
REV 4 12/85
MONTICELLO Chapte'l:' II CLIMATE AND HYDROLOGY OF TH.E STUDY AREA General Data were taken from .:reports and technical papers to descl:':ii'be the general climatic and hydrologic conditions 0£ the basin. The de-gree of variance f:rom 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 1 000 with a 10-foot contour interval were u,sed in the study. In addition, de-tailed topographic maps near the project site were obtained from the Client. Special purpose maps were *also a~railable in many 0£ the re-po'l:'ts on the climate and hydrology of the bas in.
Technical bulletins on the "Climate of Minnesota, 11 published by the University of Minnesota, Ag"ricultural Expe*runent Station, were used extensively in desc:ribing the climate of the basin.
~ Studv Area
- The Mississippi River basin above the plant site has a drainage area of approximately 13,900 square miles and lies entirely in the state of Minnesota.
The topography of the basin is characterized by level to rolling prairie land interspersed with areas o:f glacial moraines whol'e hills rise from SO to 300 feet above the surrounding land.
REV 4 12/85
MONTICELLO Elevations in the basin range from 900 to 2100-ieet, msl. The elevation at the project site is about 930 feet above mean sea level, and the average elevation of the basin is about 1200 feet.
Most of the basin is covered with glacial deposits and the land sur.;.
face consists. of features derived from'the several different ice__ ....sheets that advanced and then retreated from the area. The principal feature, f:rom the hydrological standpoint, is the num.erous lakes that were form.ed in the sur!ace depressions created by i:he movem.ent of ice. As the ice retreated, depressions we:.:-e left, which filled with water to form. lakes.
The st:rea.milow characteristics of the Mississippi and of all its c.1iie:f' tributaries a.re largely determined by the natural storage provided by these lakes and the many swamps.
Clim.ate
'I'he study area lies within a zone of marked continental climate characterized by wide and rapid variations in temperature, ~ ager winter precipitation and usually am.ple swn.m.er ram.fall. lt has a. tendency to extre.r.nes in all climatic !ea.tu.res, although this is moderated somewhat by the large number of bodies of water in the area.
Atmospheric moisture mamly *nows mto the region along two water vapor st:::-eams: a strong southerly flow fro::i the Gulf of Mexico and a.
comparatively diffuse westerly movement from the Pacific Ocean. 0£ the two, Gulf moisture is the more im.portant, accounting for most of the precipitation in the study area. Du.ring the months when the s outhe:rly winds reach Minnesota, May through September, about 65 percent oi the annual rainfall is recorded. Because these ai:r masses must travel 1200 to 1500 miles before reaching Minnesota., minor wind changes 'can account
__ ) for large variations from norm.al 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 south-ern portion of the area. Minimum normal monthly precipitation i;-about 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 mast affected by snow conditions. Gradual melting of snow. 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
MONTICELLO on a diurnal bas is as well. The only nearby water body of sufficient size to modify dim.ate on more than an extremely localized basis is L Superior. However, its influence on the study area is restricted essen-t"ia.lly to coniines of the shoreline due essentially to prevailing westerly winds and also to the abrupt rise of the land from the lake shore.
Normal average daily temperatures at St. Cloud, which i~ approx-inl.ately 25 miles upsbeam of the plant site, range from about 10°F in late January to about 72°F in late July. Normal maximum and mini.mum.
daily temperatures for the same station are about 20°F and 0°F in Janu-ary and about S3°F and 59°F in July.
Norm.al temperatures over the basin for each season of the year are as follows:
Season Temoerature Winter (December, January and February}
Spring (March, April and May) 37°F Swnm.er (June, July, and August)
Fall {September, October, and 42°:F' November)
The great extremes of temperature in the area are apparent from the absolute range of l 73°F that has been recorded. The extreme max~-
mwn temperature recorded during the total record period was l 14°F in July while the extreme minimum recorded was -59°F in January. Ex-treme maximum and min:ilnum temperatures for each month o! the year
!ram St. Cloud are shown below. Tempe.ratures given are in degrees Fahrenheit.
REV 4 12/85
MONTICELLO J F M A M J J A s 0 N D Extreme Maximum 55 58 81 91 105 102 107 105 l 06 90 71 63 Extreme Minimtun 35 -32 2 18 33 41 34 18 6*-23 -32 Wind
, =--
Prevailing winds are from the northwest during the winter and early spring, and from the southeast during the sum.me:- and latter part of spring. Monthly mean wind speeds vary slightly over the basin. An-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 a:re attained in April.
Hydrology Annual Runoff The annual runoff from the rivers and streams throughout the basin is directly aifected 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 -with 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 \lil1.1ile open-water conditions prevail is caused by :runoff producing rains, or by melting snow, or by a combination of the hvo. Flooding because of backwater is usually caused by ice jams. The most serious flooding throughout the basin has been associated with exces-sive snowmelt and rainfall. The tune of occurrence of floods shows the greatest frequency in April during the spring th.aw. A second peak occurs G.2-5 REV 4 12/85
MONTICELLO in .June due to thunderstorms. A smaller peak occurs in the fall. Local flash floods occur in the smaller streams in the spring thaw and also in the warmer season from locally-intensive rainfall.
The maximum flood of record on the Mississippi River at the plant site was ~l,_9_00 cubic feet per second (elevation 916. 2 feet) in April, 1965. Records for the station at St. Paul indicate that this was probably the max:imwn 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 cli..TTiatic 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 ill PROBABLE MAXIM:UM FLOOD DETERMINATION The probable ,maximum flood at the plant site was determined by transposing an actual, critical-spring storm to the drainage basin and maximizing the precipitation for potential moisture:
- Potentfalsnow cover and a critical temperature sequence were developed for deter-mining snowmelt contribution to flood runoff. Flood runoff at the plant site was dete rmincd by developing u~it hydrographs for four sub- b<!.sin~,
applying rainfall and snowmelt excesses to the unit hydrographs and rout-ing the resultant hydrograph~s for the sub-basins to the project site.
A probable ma_~im.um sum,mer storm over the project area was also studied in. detail and the resulting flood at the project site deter-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 pro-ducing the more critical flood. Exhibit l shows the gerieral location of the study area.
Probable Maximum Storm A probable maximum spring storm and a probable maximu..T. Sl.L'cl-mer storm were determined by transposing and ma.-..::irn.izing*actual re-corded storms.
The storms selected were the March 23-27, 1913 storm centered at Bellefontaine, Ohio, (OR 1-15, U. S. Army Corps of Engineers, "Storm Rainfall in the United States") and the August 28-31, 1941 storm centered at Hayward, Wisconsin (UMV 1-22). These storms :represent
{;. 3-1 REV 4 12/85
MONTICELLO near maximum conditions of meteorological events for spring and sUin-mer conditions.
Maximization of these storms involved multiplying the observed rainfall values by the ratios of the ma.."Cim.um precipitable water in an air column over the study area to the obser.red precipitable water in an air colwn..n for the actual storm. Under the asswnption*of a sattir"ated pseudo-adiabatic atmosphere, the amount of moisture is a unique func-tion of the ground elevation and surface dewpoint. Precipitable water was thus determined from the inflm,-.r barriers to the storm centers and observed and maximum persisting l 2-hou:r dewpoints. Persisting 12-hour dewpoints for the actual storm were obtained from U. S. Weather Bureau data. In accordance with frontal theory, the storm dewpoints were measured in the warm air rather than at the point of raini-all.
Maximum persisting 12-hour dewpo:.nts for the study area were taken from the National Atlas __E! the United States, rrMaxim.um Persisting 12-Hour 1000-MB Dewpoints (°F), Monthly and of Record. 11 For the trans-posed storms, the maximum persisting 12-hour dewpoi,:;its 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 fiow 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 areas..between isohyetal lines. The precipitati_on was then adjusted for ma..*dmuzn moisture charge in accord-ance wit.1/2. the above criteria.
- Superposition of the observed s'torm pat-terns over the study area is justified because the areas are meteorologically homogeneous, and no major orographic diife:rences exist between the study G.3-2 REV 4 12/85
MONTICELLO area and the observed storm areas. Rotation of the transposed storm patterns was limited to 20 degrees from the observed storm.
The depth-duration relationships for 15,000 square miles from the recorded storms were used to determine rainfall increments for each sub- basin. The rainiall increments were then arranged into a sequence considered to be the most critical *that could reas oriably occur*. <-The re-sulting depth-duration curves for the spring and sum.mer storms are shown on Exhibits 4 and 5. Exhibits 2 and 3 show the transposed is ohyetal patterns and the ma.xirriized precipitation for each storm.
Following the determination of the flood resulting from the spring storm,the isohyetal pattern was re:--oriented over the study area to find the most critical rainfall pattern. Although an infinite number of ori-entations is possible, the effect on the resulting flood was fonnd to be-come negligible with additional orientations.
Snow Cover Snow cover over the basin was taken from the U. S. Weather Bureau, Technical Paper No. 50, "Frequency of Maximum. Water Equi-valent of March Snow Cover in North Cent:ral Uni:ed States. 11 For the purpose of the probable maximum flood study, maximum water equiva-lent (inches) for March 16- 31 having one percent probability was used.
Lines of equal snow cover, taken from the report, were superimposed ove:r the basin and the weighted average snow cover for each sub- basin determined by planimetering. Exhibit 6 sho*1:"!'I the assumed basin snow cover.
G.3-3 REV 4 12/85
MONTICELLO TemDerature Seauence For purposes of snowmelt computations, it was necessary to de-termine a c:ritical temperature sequence that could reasonably be ex-pected to occu:r while the snow cover was at a maxi.mum.. Weather re-cords for the Minneapolis station offered the longest record of observed
- ,L . . . .
temperatures near the basin (54 years) and this record was used to de-termine a critical temperatuJ."e sequence. As a large percentage of the total snow cover could be melted in about five days the maximum. histori-cal five-day mean daily temperature sequence occurring from April l to 15 was selected. This was the period April Z-6, 1921. It was assumed that this tempera.tu.re sequence could occur a.t any ti.me between April 1 and April 15 and tha.t it could occur following a period of extremely cold weather such that th.e snow cover would be at a maxim.um.
Since temperatures va:ry considerably over the basin, several sta-tions located throughout the basin we:re 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 lII-1 shows the re-co:rd five-day tempe:ratu:re sequence used for each sub- basin. Tem.pera-tures subsequent to the maximum five-day sequence were asswned to be the same as those recorded in 1921.
TABLE ill-1 Mean Daily Temoerature Seauence Sub-Basin Station l 2 3 4 s 3 Brainard 46.0 54. 5 57.0 57.5 57.0 4-8a St. Cloud 48.5 54.0 69.0 6to 57. 5 6 Pokagama 40.5 53.5 54.0 54.0 53.5
-G. 3-4 REV 4 12/85
MONTICELLO Snowmelt Snowmelt £or the probable maxim.um .flood study was computed using methods developed by the U. S. Army Corps of Engineers and de-scribed in their Manual EM 1110-2-1406, 5 January 1960, 11 Runoff from Snowmelt." These methods utilize basic data on temperature, precipi-tation, wind velocities, ins elation, snow albedo, basin expos~-;e and canopy cover, and a convection-condensation-melt factor which repre-sents the mean exposure of the basin to wind. Average monthly values of insolation and wind velocity were determined from Minnesota weather records for use in the c amputations. Ins elation oi 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 asswned to be 45 percent.at the start of the melting period. Basin exposure was assumed to be high due to the lack of large topographic variations and basin canaopy cover was determined fer each sub-basin by estimating the percentage of forested area :from map::; showing for~st cover. A mean relative huxnidity of 70 percent was used for converting air tem-peratures to dewpoint temperatures during the days of high ins elation melt.
Infiltration and Retention Infiltration and initial retention losses were asswned to be extremely low at the start of the runoff period. Documented flood events in the Upper Mi-:3sissippi 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 cov!:!r 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 tempera-
.__,res that ~ause ice to .form over the ground surface, and a heavy snow-pack accumulation. II these conditions are followed by an extremely warm period and rainfall there is almost no loss of free water a:l;ld run-off is maxiinized. Since records of frost depth indicate that three to five !eet of frozen ground at the end of March are not unusual, retention rates are not likely to increase significantly for some time after melting starts. Initial retention was assurn.ed to be zero in this study, and other losses were assumed to be O. 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 lea.ding to a major flood _wo~d 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 impervious 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 assUined to cover the study area on March 31. On April 1, the ma."C-imum 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 begin April 5.
Temperatures for the period following the maxi.Inum five-day sequence REV 4 12/85
MONTICELLO were asswned 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's method, which,is derived from the various physical basin characteristics. The equations which Snyder developed are:
- 1) t p
=C {LL t ca
,°* 3 time to peak
- 2) q_ = C x 640 peak rate of discharge
---P P T p
- 3) T =3 + (t + 8) duration of unit hydrograph p
Where: t = Lag time from center of rainfall period to peak oi P unit hydrograph in hours L = Length of river to the most remote portion of the bas in, in miles L = Distance along the water course to the geographical ca center of gravity oi the drainage basin
= Peak rate of discharge, in cubic feet per second per square mile T = Duration of unit hydrograph in days Ct and CP are constants The constants Ct and Cp are critical values in determining the 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 a.ctual* data. After these unit hydrographs were developed, known values were applied to Snyder's equations and a range of values for Ct and Cp were determined. The ave:ra.*ge value of Ct was 8. 0 and the average value of Cp was O. 75. These two values we:re then used in Snyder's equations to develop synthetic unit hyd:rog:raphs
!or the sub-basins used in the study. -.-
l;Jnit hydrograph peaks were increased by 25 percent and basin lag decreased by one-sixth in accordance with standard Corps of Engineers practice. Exhibit 7 shows the unit hydrographs £or each sub- basin.
Flood. R Quting; Snowmelt and rainfall excesses were a*pplied to the unit hydrographs and the resulting hydrographs we re determined for each sub- basin. Sub-bas in hydrographs were then :routed to th~ project site by computer pro-gram. using the modified Wilson method -- the equation used in the method is:
0 2.
= 0.
.1
+K (l 1
fl :- iO )
2 * .
- l.
t where K = 2 T + t
= Instantaneous discharge or outflow from a basin
= Instantaneous discharge or inflow to a basin t = Routing bterval - 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> for this study T- = Travel ti.me - hours G.3--8 REV 4 12/85
MONTICELLO Travel time T £or 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, Minnesota with a drainage area of 14,500 square miles. Examination of the records indicated that £or the months of March - April, a base flow of 5000 cfs was reasonable. The base flow of 5000 cfs was then added to the total of the
- routed flood hydrographs. The resultant probable maxirnwn flood hydro-graph is shown on Exhibit 9.
G.3-9 REV 4 12/85
MONTICELLO Chapter IV STAGE DISCHARGE RELATIONSHIP The stage-discharge curve at the nuclear plant site was extended above the ra1.. 6 e of historical experience by means of hydraulic computa-tions based on the river channel downstream. This was done by a series of back-water computations based on a range of discharges. The back-water computation procedure takes into account channel conditions over a reach of :river and tends to converge on the true stage at the *-u~tream end of,the Teach 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 derived by the 11 Standa:rd Step Method II of back-water calculations, using an electronic computer. The term 11backwater, 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. downstream depth greater th.an normal extends with diininishing effect upstream !rorn the control until the "backwater profile" becomes coincident with the normal depth pro!:ile.
The standard step method utilizes the Manning formula in a com-putational procedure designed to take into account the effects of gradually G.4-1 REV 4 12/85
MONTICELLO varied flow. As utilized in computations for a natural channel, this pro-cedure includes effects of downstream c!i.annel conditions and constric-tions in the determination of the water surface elevation at upstream points. The Bernoulli equation of conservation of energy is successively appl:i:ed for channel reach segments using the Manning formula for deter-mination of head lasses. * .. ~-
Comnuter Program Bacbvater computations were made on an IBM 1130 computer sys-tem, using a program prepared by the Hydrologic Engineering Center, Corps of Engineers, Sacramento, California. The effective cross-sections were completely described to the computer and Manningrs 11 n 11 values for left overbank, right ove:rbank and channel for each section were read in.
Discharge and starting water .surface elevations were read into the com-puter at the :first section and the computer carried out computations of water surface elevation for 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. Computa-tions were made of average slope, velocity, dis charge in left and right over bank as well as the channel, velocity head, eddy loss due to expansion or contraction, ene:-gy gradient, and water surface elevation.
Output data include discha:rge, 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 North-ern States Power Company and from smaller scale 10-foot contour inter-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 com-puted by asswned trapezoidal sections would not diifer from the true area by a significant amount.
Cross-sections were determined at average intervals of about one-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 area is about five and three-quarter miles below the generating plant site and about one mile below Monticello, Minnesota. A stage-discharge relation-ship was determined for this cross-section using Manning 1 s formula.
Q = 1. ~S-6 A R 2 I 3_ s 1/ 2 n
where Q = Discharge, cfs n = Manning'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, hydraulic radius and discharge Q were then determined for each value. The average channel slope, S, was obtained from topographic maps ..
G.4-J REV 4 12/85
MONTICELLO Mannbg 1 s Roughness Coefficients Values a. Manning's "n" coefficient used in the study were deter-mined from exaznination of detailed topographic maps of the plant site and of USGS IO-foot contour maps with.a scale of 1:24, 000. Average values of 11 n 11 determined were
- 032 for -the main channel,
- 050 for the left overba.nk and
- 045 for the right overbank. A ~omewha~ -!_l_!gher value of . 065 was used for the right overbank in Monticello and a value of
- 060 was used for tb.e island i.nunediately upstream of the plant site. These 11 n 11 values were determined by assuming a basic "n" value and then making adjustments £or irregularity, changes in shape, obstructions, vegetation, and meander.
Verification of Procedure The maxim.wn flood of record at the site occurred in April, 1965, with a discharge of approx:i.m.ately 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 computation yielded a water surface elevation at the plant site which agreed within O. 2 feet of the meas*.ired water surface elevation £or the flood of record.
For the dete:?:mination of the water surface elevation corresponding to the probable maximum flood, two different starting elevations were selected from the rating curve downstream of Mo:iticello. Elevations were selected £or stages somewhat higher, as well as somewhat lower, than that anticipated for the probable-maximum flood. Analysis was then made to determine backwater profi~es 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 p*eatly effected by adjustment of the sta.r"1..u.,b elevation. An average of their values was used for the probable max:unu.m flood stage.
As a further verification, it was decided to determine what maxi-rnwn stage would occur at the plant site ii the values for Manning 1 s roughness coefficient 11 n 11 were lowered for the overbank sections of the channel reach. Lowering Tin II for both the left and right over bank by
- 0. 005 yielded a probable rnax:imu.m flood stage elevation of 938. 9 or about O. 3 :feet lower than the elevations obtained by 'using the higher 11 n 11 values. It is believed that any further decrease in the roughness coef-ficient would be unrealistic in view o:f the channel and overbank char-acteristics.
Stage Dis chari.e 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. Us:i.ng the dis-charges and the water surface elevations determ.ined, a stage discharge 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. 2 feet MSL. The probable ma..**dmwn f.lood hyd-regraph is shown on Exhibit 9. The occurrence of the sequence of events de-scribed in Chapter m would cause the £load to reach its maximum. level about 12 days after the beginning of high temperatures and would remain above elevation 930. 0 for a.bout 11 days.
G.* 5-1 REV 4 12/85
MONTICELLO EXHIBIT 0
IO en
~ ..........
47°
,_)
46° Scale O 10 20 JO Miles j.. If,' I .I GENERAL LOCATION MAP
+IA~ZA ENIINE!'.RIN* COMPAN'I' DWI. NO,
- 13 F I MA'I', 1989 REV 4 12/85
MONTICELLO EXHIBIT 2 t
N ISOHYETAL MAP Sc:1111 0 10 20 301,Wes hn11tt111 t 1 SPRING STORM ORIENTATf ON N0.2.
REV 4 12/B5
MONTICELLO EXHI SIT 3
,---,::~..;;=;:;:;:,.,_r-:-_:-_-_-_-_5s"
- .-::::::::r::==:..:7~~ *- ~*-
l J
WONT1 C!:LLO Scali -0 10 20 30 Mll~s ISOHYETAL MAP SUMMER STORM
- --f wwl 1 I ORIENTATION NO.2 REV 4 12/85
MONTICELLO EXHIBIT 4 100
~
/
- so 80
/ ---
- /
I 70
-=-
I 60 a
a::
I I
0 0
}- 50 I 0
I C:
"'...u 40
., I I
I 0.
NOTE: I For flood studfc purposes O!surne I
100"/., of rain oil occurred in 4 days.
30 I
20 lO o.
0 2 3 4 TtlTlCI in Ooys DEPTH - DURATION CURVE FOR -~ s,ooo* SQ. Ml.
-i<AllU EtHIIIIEERil<I: C0!,11'1'-l'IY 0\11'$' NO. 4131' .4- IUY, 1'969 REV 4 12/85
MONTICELLO EXHIBIT 5
/
100 I
90 I
l I
80
--ot..-
I 70 60 I
- 0
-c:
0 CZ:
I I
50
) Q_ I
)'
0
-40 -
0 I
C
10. 413 I' 6, llAY, f9ft9 REV 4 12/85
MONT!CELtO EXHIBIT 6 Scala O IO 20 30 Milel BASIN SNOW COVER t, t t .. ,
(WATER EQUIVALENT IN INCHES) 1% PROBABILITY, MARCH 16-31 IIARZA ENOllfEERING COMPANY DWII ND. 411 f' I MA'r I IHI REV 4 12/85
MONTICELLO EXHIBIT 7 24-HOUR UNIT HYDROGRAPHS BASINS 3, 4, 6 AND Sa 25 - - - - - , - - - - - - - - -........----.----------
2.-0
- -.).
.....co
~-,.:.--
0 0
0 15 0
\
\
C CD-
!:'I Cl
.c 10
\
<.)
"' '\-
0 5
Time in days LEGEND:
- - - BASIN 3
- - - BASIN 4
- - - BASIN 6
- * - - BASIN ea
'NARZA ENllNEEIIINO CONP"-NY DWS, NO. 41;'1 F 7 MAY, 1969 REV 4
HONTICELLO EXHIBIT 8 1000 j I 990 980 970 960 950
,. /
'/
940
/!
939_2--..L..---v, V
9:SO
-* l I
I I
36.491
/
920
"; 919
,/' j I ti'.'. 918
/
1 I I I
'/ I I l
.!:. 917 g 916
-/1 1
.... I I
l I I I
~ 915 Q) w 914 I
I 913 912 I l 9 J '2 I 3 4 5 s 7 8 9 10 20 30 40 50 60 708090100 Oischaroe in 10,000 efs DISCHARGE RATING CURVE MISSISSIPPI RIVER AT MONTICELLO, MINN.
H.t..llZ;,, -ENOINE:ElllNO C-()ilPANY D\¥0 NO. 413 I' I MAT, 194, REV 4 12/85
MONTICELLO EXHIBIT 9 400r----,----,-----,------
A 350t-----+,i'~\--+-----1------1-----1.,~-~
300 i-------+1/---+----+------~
I C) 250
\
- 1 0
.1 0
-0 200 \
\
I I !5 O
\
ID b
_g u
l'1 Q
\
100 I
u \
50 00 10 20 30 40 50 Time In days PROBABLE MAXIMUM FLOOD HYDROGRAPH MISSISSIPPI RIVER AT MONTICELLO, MINN.
iwa:., C,.ell<UJIINII COIIFllln' DWG NO 41SF I YAY, I 111 REV 4 12/85