ML25107A244
Text
MONTICELLO APPENDIX G PROBABLE MAXIMUM FLOOD MISSISSIPPI AT MONTICELLO MINNESOTA MAY 26, 1969 REV 4 12/85
CHAPTER MONTICELLO APPENDIX G PROBABLE MAXIMUM FLOOD MISSISSIPPI RIVER AT MONTICELLO, MINNESOTA TABLE OF CONTENTS I
INTRODUCTION Scope Definition Authorization Data Investigations Acknowledgements II 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
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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'Cqrve CONCLUSIONS G-iii PAGE G,.'4~i-G.4.;..l G.4-2 *
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Exhibit l Exhibit 2 Exhibit. 3 Exhibit 4 Exhibit 5 Exhibit 6 Exhibit 7 Exhibit 8 Exhibit 9 MONTICELLO LIST OF FIGURES G-iv General Location Map Isohyetal Map Spring Storm Orientation No. 2 Isohyetal Map Summer Storm Orientation No. 2 Depth-Duration Curve for 1s,ooo* Square Mil;-spring Storm OR 1-15 Depth-Duration Curve for 15,000 Square Mile Summer Storm UMV 1-22 Basin Snow Cover (Water Equivalent in Inches) li.
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 Gene rating Plant Probable Maximum Flood Study Chapter 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-1 ** '
west of Minnea.polis-St. Paul.
~*2:.:)
The study area £or the probable ma:dmum. flood includes the Mississippi River dram.age above the. plant site: about 13 1 900 square
.miles.
Hydrologic and m.ete o:-ologic data developed by Harza for the Prairie
- Island nuclear genera.ting plant site have been used extensively to develop the probable maximum flood for the Monticello site whose drainage area lies entirely within the boundaries 0£ the northern portion of the Prairie Island dram.age area.
Definition The term 11proba.ble ma.ximwn flood, 11 as used herein, is the hypo-thetical flood that would result ii all the factors that contribute to the generation of the flood we:re to :reach their inost critical values that could occur concur:rently.
The probable maximum. £loo: is derived from G.l-l REV 4 12/85
MONTICELLO hydrometeorological and hydrological studies and is independent of his-torical flood frequencies. ' It is the est:im.ate of the boundary between possible floods and impossible floods.
Therefore., 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., Jvfinnes ota, by Purchase Order M-79613 dated May 1, 1969.
Data Data used in the study included U. S. Geoldgical 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-c~ data and Technical Bulletins of the University of Minnesota Agricul-tural Experiment Station on the clim.ate of Minnesota.
In addition, soil maps of the basin were obta:ined from the U. S. Department of Agricul-ture.
Investigations Determination of the probable maximwn flood in"'1uded studies of the probable ma.xhnum precipitation for both spring and summe 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 !or each of the I
sub-basins comprising the drainage area above tL,:; r-ject site.
Ackn:qwled ge*ments The assistance of the administrato::-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 of Engineers, the U. S..
Weather Bureau and the U. S. Geological Survey, who provided valuable hydrological and meteorological in.iormation, is greatly appreciated.
Principal participants of th.e consulting engineering staif were:
Project Spans or K. E. Sorensen Project Manager L. L. Wang Chief Hydrologist R. W. Revell Overall Report Responsibility J. C. Ringenoldus Civil Eng:ineer A. A. Muel~e:r The report was reviewed by Dr. R. A. Clark, Professor in the Department 0£ Meteorology, College of Geo-Sciences, Texas A8z:M University, as a consultant to Harza Engineering Company.
REV 4 12/85
MONTICELLO' Chapter II CLIMATE AND HYDROLCGY OF THE STUDY AREA General Data we:re taken £:rom,reports and technical papers to desc:rie-e the general climatic and hydrologic conditions of the basin.
The de-gree 0£ variance from normal conditions of clim.atic events was studied to determine the.range of expected values under reas ona.ble 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 ~sed in the study.
In addition, de-tailed topographic: maps near the project site Wel'e obtained from the Client.
Special purpos~ maps were *also a"railable in many of the re-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*rim.ent Station, were used extensively in des c :ribing the climate of the bas in.
The Studv Area
- The Mississippi River basin above the plant site has a drainage area of appro:ximately 13 1 900 square miles and lies entirely in the state o£ Minne s eta.
The topography of the basin is characterized by level to rolling prairie land interspersed with areas of glacial moraines whoe-c 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-feet, 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 ~erived from'the several different ice sheets that advanced and then retreated from the area.
The principal feature, from the hydrological standpoint, is the num.e:rous lakes th.at were formed i:n the surface depressions created by i:he movem.en't of ice.
As the ice
- retreated, depressions we:re left, which *filled with water to form lakes.
The strea.m.flow c.hara.cteristic:s of the.Mississippi and of all its c...11.ie.f t:ributa:ries a.re largely deter.mint?d by the natural storage provided by these lakes and the many swamps.
Clim.ate The study area lies within a. zone of marked continental climate characterized by wide and rapid variations in te.m.pera.tu:re 1 n:e ager winter precipitation and usually am.ple sum.m.e :r ra:iniall.
lt has a tendency to ext:rem.es in all climatic !eatu:res 1 although this is moderated somewhat by the la:rge number of bodies of water :in the area.
Atmospheric moisture ma.inly *nows mto the region along t\\vo water
-vapor st=ea.ms:
- a. strong s outhe.tly fl:ow fro::-:i the Guli of Mexico and a comparatively d:iifuse westerly movement from the Pacific Ocean.
0£ the two, Gulf moisture is the more im.portant, accounting £or most of the precipitation in the study area.
Du.ring the monfas when the southerly winds reach }v1:i.nnesota, May through September, about 65 percent oi the annual rainfall is recorded.
Because these air masses must travel 1200 to 1500 miles before reaching Minnesota, minor wind changes 'can account
£.or large variations from norm.al precipitation.
REV 4 12/85
MONTICELLO Precipitatlon 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 Minneapoli~ - 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 precip.i tation l~-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 JulY. 1909.
Snow 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 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 REV 11 12/91
MONTICELLO on a diurnal basis as well. The only nearby water body of sufficient size to modify cl:imate on more than an extremely localized basis is r_. *
- Superior.
However, its influence on the study area is restricted essen-tially to confines 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;n_peratures at St. Cloud, which i:~ approx-imately 25 miles upstream of the plant site, range from about 10°F in late January to a.bout 72°F in late July.
Normal maximum a..'"ld minimum.
daily temperatures for the same station are about 20°F and 0°F in Janu-ary and about 83°F and 59°F in July.
Norm.al temperatures over the basin for each seas on of the year are as follows:
Season Winter (December., January and February}
Spring {March, April and May)
Swnmer (June, July, and August)
Fall (September, October, and November)
Temperature The great extremes of temperature in the area are apparent from the absolute range of l 73°F.that has been recorded. The extreme maxi-mum 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 minim.um. temperatures for each month of the year from St. Cloud are shO\\Vn below.
Tempe;ratures given are in degrees Fahrenheit.
REV 4 12/85
MONTICELLO J
F M
A M
- r
- 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
~t -.:t-...
Prevailing winds are from the northwest during the winter and early spring, and from the southeast du:ring the sum.me:- and latter part of spring.
Month.ly*mean wind speeds vary slightly over the basin.. An-nual averages are f:rom 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,Rune££ The annual runoff from the rivers and streams throughout the basin is directly afiected by the a.mount 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 £rom 1. 22 to 8. 93 inches.
Floods Two types of flooding occur in the basin -- open-water flooding and backwater flooding. Flooding while open-water conditions prevail is caused by runoff producing rams, 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 exces-sive snowmelt and rainfall.
'Ihe tim.e of occurrence of floods shows the greatest frequency in April during the spring thaw.
A second peak occurs D
63
-32 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 s~aller 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.,Q_OO cubic feet per second (elevation 916. 2 feet) in April, 1965..
Records £or the station at St. Paul indicate that this was probably IC,.....
the maxim.um 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' cl:L,natic 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.
-REV. B 4:/95
MONTICELLO Chapter m PROBABLE MAXIMUM FLOOD DETERMINATION The probable _maximum.flood at the plant site was determ:i:c.ed by transposing an actual, c:ritical-spring storm to the drainage basin and maximizing the precipitation for potential moisture: ** Potentfalsnow cover and a critical temperature sequence were developed for deter-m.in:ing snow melt contribution to flood runoff.
Flood runoff' at the plant site was determined by developing u~it hydrographs for four sub-basin~ 1 applying rainfall and snowmelt excesses to the unit hydrographs and rout-
. ing the resultant hydrograph_s fo~ the sub-basins to the project site.
A probable ma..**d.1nurn sum,m.er storm over the project area was also studied in. detail and the resulting flood at the pr eject site deter-mined.
Although the summer storm was m:uch 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 general location of the study area.
Probable Maxim.um. Storm A probable maximum spring storm and a probable ma.:d.mu..T.. su_"'!'l-mer storm were determined by transposing and ma.."'Cimizing*ac.tual 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, 11Storm Rainfall in the United States 11 ) and the August 28-31, 1941 storm centered at Hayward, Wisconsin (UMV 1-22).
These storms represent G.3-1 REV 4 12/85
MONTICELLO near maxim.um conditions of mete o:i:ological events for spring and s.um-mer conditions.
Maximization of these storms involved multiplying the observed rainfall values. by the ratios of the ma..**dmum. precipitable water in an air colu.m.n over the study area to the observed precipitable water in an air colWllll for the actual storm.
Under the assumption.. of a satti'ra.ted 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 inflow barriers to the storm centers and observed and maxm1um persisting 12-hou:r dewpoint~.
Persisting 12-hour dewpoints for the actual storm were obtained f:rom U. S. Weather Bureau data.
In accordance with.frontal theory, the storm dewpoints were measured in the wa:rm air rather than at the point of rainfall.
Maxim.um persisting 12-hour dewpo::.nts !or the study area were taken from the National Atlas_.!:! the United States, "Maximum Persisting 12-Hour 1000-MB Dewpoints (°F) 1 Monthly a.nd of Record. 11 For the t:rans-posed storms, the maximum persisting 12-hour dewpo~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 cen:te:rs.
The distances were measured in a. direction into the general path of air z'J.ow from the Gulf of Mexico.
The original observed storm patterns were supcrirnpos ed over the study area and the weighted average precipitation over each sub-basin determined by planimetedng the areas... between isohyetal lines.
The precipitati.on was th.en adjusted for ma..""Cimu.m 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 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 1 000 square miles from the recorded storms were used to determine raWall increments for each sub-basin.
The rainfall increments were then arranged into a sequence considered to be the most critical -th.at could reasoriably occur*.<_ The re-sulting depth-duration curves for the spring and su.m.mer storms are
~
shown on Exhibits 4 and 5.
.Exhibits 2 and 3 shaw the transposed is ohyetal patterns and the maximized precipitation !or each storm.
Following the determination of the flood resulting from the spring storm,the is ohyetal pattern was re:- oriented over the study area to find the most critical rainfall pattern.
Although an infinite num.ber of ori-entations is possible, the effect on the resulting flood was found 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. SO, 11Frequency of Maximum Wa.ter Equi-valem: of March Snow Cover in North Cent:ral United States.
11 For the purpose of the probable maxixnum flood study1 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 super:iJnposed over the basin ~nd th~ weighted average snOW' cover £or each sub-basin determined by planimetering.
Exhibit 6 show!; the assumed basin snow cover.
G~3-3 REV 4 12/85
MONTICELLO Temnerature Secuence Fo:r purposes 0£ snowmelt computations, it was necessary to de-termine a critical temperature sequence *that could reasonably be ex-pected to occur while the snow cover was at a maxim.urn.
Wea.th.er re-cords £or the Minneapolis station offered the longest record of observed temperatures near the basin {54 years) and this record was used to de-termine a critica). temperature sequence.
As a large percentage 0£ the total snow cover could be melted i:o. about five days the ma:ximwn histori-cal five-day mean daily temperature 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 between April 1 and April 15 and that it could occur £chowing a period of extremely cold weather such th.at th.e snow cover would be a.ta ma.x:irnum.
Since temperatures vary considerably over the basin, several sta-tions located throughout.the basin were selected and the observed April 2-6, 1921., temperature sequence recorded.
These ternpe:ratures were assumed to be representative of the sub-basin which they*were nearest and were used in the snowmelt computations.
- Table IJl-1 shows the re-cord five-day temperatul'e sequence used for each sub-basin.
Tempera-tures subsequent to the maximum Iive-day sequence were assumed to be the same as those recorded in 1921.
TABLE ill-1 Mean Daily Temoerature Secuence Sub-Basin Station 1
2 3
4 5
3 Brainard 46.0 54~ -5 S7.0 s1.*s.
5*1. 0 4-8a St. Cloud 48*.. 5 54.0
~*
6'9. -
62~0
- 57. 5 6
Pokagama 40;.5 *
.S 3. 5
.54. 0 54.0
-53~ 5
-G.3-4 REV 4 12/85
MONTICELLO Snowmelt SnOW"melt !or the probable maxim.wn.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 1 11Runo£f from Snowmelt."
These methods utilize basic data on temperature, precipi-tation, wind velocities, ins olation 1 snow albedo 1 has in expos~-;e and canopy cover, and a convection-condensation-melt !actor which repre-sents the mean exposure of the basin to wind.
Average monthly values of insolation anq wind velocity were determi.;ied 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 2 0 miles per.hour during precipitation.
Snow surface albedo wa.s assumed 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 for each sub-basin by estimating the percentage of forested area frorn map:;; showing for~st cover.
A mean relative hwnidity of 70 percent was used £or converting air tem-peratures to dewpoint temperatures during the days of high ins olation 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~sissippi Basin indicate that it is not unus~l 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 depress ions.
This is often followed by the return of freezing tempera-i.-.res that cause ice to form over the ground surface, and a heavy snow-pack accumulation.
- If these conditions are followed by an extremely warm period and rainfall th.ere is almost no loss of free water al;ld run-off is maxim.ized.
Since records of frost depth :indicate that three to five feet of.frozen ground at the end of March are not unusua.1 1 retenfion rates are not likely to increase significantly for s om.e time aiter melting starts.
Initial retention was asswned to be zero in this study1 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 leading to a major ;flood.wo~d be to have an unusually heayy spring snowfall and low temperatures after a period of intermittent warm spells and sub-freez:ing 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 assu.rned to cover the study area on March 31.
On April 1., the ma."'1:-
imwn historical temperature sequence was started *. By the fifth day the high temperatures were ~elow the dewpoint temperatures of the storm and the probable maximum spring precipitation was asswned to begin April 5.
Temperatures Jor the period following the max::im.ur.n five-day sequence REV 4 12/85
MONTICELLO were asswned to be the same as those recorded £or the April 7-16, 1921, period.
Unit Hydrograohs The study area w.as divided into four major sub-basins and syn-th.etic 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
1:
- c (LL /~3
.. p_
i;
- *
- ca..
4p ~ fp x;4o p
T = 3 + (t + 8) p time to peak peak rate of discharge duration of unit hydrograph t = Lag time from center 0£ rainfall period to peak oi P
unit hydrograph in hours
= Length 0£ river to the most remote portion of the bas in, in miles
= Distance along the water co11rse to the geographical center of gravity oi the drainage basin
= Peak rate of discharge, in cubic feet per second per square mile
= Duration of unit hydrograph in days Ct and Cp are constants The constants Ct and Gp 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 actual*data.
After these unit hydrographs wer~ developed, known values were applied to Snyder's equations and a range of values for Gt and Gp were determined.
The ave:ra*ge value 0£ Ct was 8.. 0 and the average value of Cp was O. 75.
These two values were then used in Snyder's equations to develop synthetic unit hydrog:raphs
!or the sub-basins used in the study.
'O'nit hydrograph peaks were increased by 25 percent and basin lag decreased by one-sixth in accordance with standard Corps of Engineers p:ractice.
Exhibit 7 shows the unit hydrographs £or each sub-basm.
Flood. R qutmg_
Snowmelt and rainfall excesses were ~."pplied to the unit hydrographs and the resulting hyd:rographs were determined for each sub-bas:in..
Sub-basin hydrographs were then :routed to th~ project site by computer pro-gram using the modified Wilson method -- the equation used in the method is:
t...
- where K = Z'I' + t 01., 02 Il, lz t
'I.
=
=
=
=
Instantaneous discharge or outilo\\v from a basin Instantaneous discharge or inflow to a basin R outing fate rval - 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> for this study
'Ira vel ti.me - hours G.. 3--:8 REV 4 12/85
MONTICELLO Travel tiine T for flood routing were taken from Corps 0£ Engineers recorded travel times for large floods.
Base £low was determined from long-term. USGS records for the stream. gage at Elk River, Minn.es 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 cfs was then added to the total of the routed flood hydrographs.
The resultant probable maxim.um.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 ra1.1.5e of historical experien~e by means of hydraulic computa-tions based on the river channel downstream.
This was done* by a series 0£ backwater computations based on a range of dis charges.
The back-water computation procedure takes into account cha:ruiel conditions over a reach. ~f river and tends to converge on the true stage at the **u~tream.
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 upst:ream stage regardless of e r:r ors in starting elevation.
Procedure River profiles were derived by the "Standard Step Method II of back-water calculations, using an electronic computer. The term. 1'backwa.ter,"
as applied in the discuss ion of natural channel (mild slope) hydraulics 1 generally refers to a depth 0£ flow, or water surface elevation, which is greater than norm.al.depth because of a downstream. control such as a dam o:r channel c' ondition.
Since the depth o! flow is greater than normal unde:r 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 1 the water surface slope is less than the bed slope.
This slope-reduction caused by a downstrea.In depth greater than normal extends with d:irninishing efiect upstream. from the control until the 11back\\vater profile tr becomes coincident with the normal depth profile.
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 £or a natural channel, this pro-c-edure includes effects of downstream channel conditions and constric-tions in the determination of the water surface elevation at upst:ream points.
The Bernoulli equation of conservation 0£ energy is suc:ces sively applied for channel reach segments using the Manning formula for deter-mination of head losses. *
<C.....
C on:rnute r Program.
Backwater computations were made on an IBM 1130 computer sys-tem, using a program prepared by the Hydrologic Engineering Center, Corps of Engineers 1 Sacramento, Cali!ornia.
The effective cross-sections were completely described to the computer and Manning's "n" values for left overbank, right overbank 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 1 velocity., dis charge in left and right overbank as well as the channel, velocity head, eddy loss due to expansion or contraction, ene :-gy g:radie nt, 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 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 assumed trapezoidal sections would not differ 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 tb..e river near Monticello.
..'C.....
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-discharg~ relation-ship was determined for this cross-section using Manning 1s formula.
Q -
.L :48:6
- n:*
where Q
= Discharge, cfs, n
= Manning1s coefficient of roughness A
= Area 0£ the cross-section It
= Hydraulic radius which is equal the Area divided by the wetted perimeter s
= Average channel slope The rating curve was develo_ped by asswning 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 fr om topographic maps.
- REV 4 12/85
MONTICELLO Manning's Roughness Coefficients Values d Manning's "n" coefficient used in the study were deter-mined from examination of detailed topographic maps o! the plant site and o! USGS 10-foot contour maps with.a scale of 1 :24, 000.
Average values of "n11 determined were
- 032 :for -the main channel,
- 050 for the left overbank and. 045 :for the right ove:rbank.
A ~omewha~. !1_!gher value of. 065 was used for the right overbank in Monticello and a value of *. 060 was used for the island immediately upstream of the plant site.
These "n" values were determined by assu.m.ing a basic "n" value and then making adjustments for irregularity, changes in shape, obstructions, vegetation, and meander.
Verification of Procedure The maxiinu.rn 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 e.:x:ist 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 O. 2 feet of the meas*.ired water surface elevation for the flood of record.
For the determination o! 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 for stages somewhat higher, as well as somewhat lower, than that anticipated for 'the probable-maxi.Inum 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 site1 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.10 elevation.
An average 0£ their values was used for the probable maxim:wn flood stage.
As a further verification, it was decided to determine what maxi-mum. stage would occur at the plant site i£ the values for Manning's
- roughness coefficient 11n" were lowered for the overbank sections of the channel reach..
Lowering "n" for both the left and right overbank by O. 005 yielded a probable maximum. flood stage elevation of 938. 9 or about O. 3 feet lower than the elevations obtained by 'using the higher "n" values.
It is believed that any further decrease in the roughness coe£-
- ficient would be unrealistic in view 0£ the channel and ove rbank char-a.cte :ristics.
S.tage.Dischar~e. Curye Several sets of backwater computations were m.ade using water
- surface elevations and their corresponding discharges as determined from the rating curve do'"11.stream from Monticello.
Using the dis-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 max:ir.num 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 MS~.
The probable ma..**dmum. flood hydT-egraph is shown on Exhibit 9.
The occurrence of the sequence of events de-scribed in Chapter ID would cause the flood to reach its maximum level about 12 days after the beginning of high temperatures and would remain above elevation 930. 0 for about l l days..
G.* 5-1 REV 4 12/85
0 IC) m MONTICELLO II
~
. en EXHIBIT 0,..,
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o 1111 1 IO 20 30 Mllu GENERAL HARZA ENIINEE'.RIM; CDMPAN'I' DWG NO, 413 F I MAY 1 19119 47° 46° LOCATION MAP REV 4 12/85
MONTTCELLO Set1lo 0 10 20 30Mllu luutttffi J
1 EXHIBIT 2 ISOHYETAJ,.. MAP SPRING STORM ORIENTATION N0.2 t
N '
REV 4 12/B5
MONTICELLO
- 30Mll1:s
(**'*******
E.Xl{IBIT 3 fa*~*
ISOHYETAL MAP SUMMER STORM ORIENTATION N0.2
.REV 4 11/8.5.
MONTICELLO EXHIB.IT 4 JOO,--;----~-"T------,-------r-------------
0,
. ~-
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.NOTE,:
For flood study pu(pos_es o~su-me.*.
- 30 i----t-----+--...;...._----+---10_0
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- 2
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- 0 R 'J-.15 REV 4 12/85
MONTICELLO JXHIBIT 5 GO o* --
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- SU}iMER STORM 'u.MV*f~*22
~ala O ro. 20 I eel 30 Miles j
MONT!.CELLO EXHIBIT 6 BASIN SNOW COVER (WATER EQUIVALENT IN INCHES) 1% PROBABILITY, MARCH 16-31 HAfUA EHOINEERINQ COMPANY DWlil HO, llS F 8 MAY 1 19811 REV 4:
12/85.
---~--
MONTICELLO 24-HOUR UNIT HYDROGRAPHS BASINS 3, 4, 6 AND Sa EXHIBIT 7 25 --------------,;..-----~~---..;._-
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--- BASIN 3
--- BASIN 4
--- BASIN 6
BASIN Sa Time in days
'HARZA EN81HEEAIHO COMPANY 0'<<8, NO, 4111 F 7 REV 4
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91 *1 /
2 a
4 5
6 7 8 9 JO 20
- 30
-40 50 60 708090100 Discharge in 10,000 ~ f s DISCHARGE RATING.C.UHVE
- MiSSISSIPPI RlVER AT MONTICELLO, MINN.
- ,
- ;j)
C) 0 0
.Q
.5
- 41) 01
'l,.o
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IO E MONUGELLO EXHIBIT 9 400r-----,-----r-----.-------
250 200 150 100 10 20 30 40 50 Tltne In ' dQ)'I PROBABLE MAXIMUM FLOOD HYDROGRAPH MISSISSIPPI RIVER AT MONTICELLO, MINN.
twlZ:I. tN*INUJIIHI ~
DWG NO 41:SF t 11.f.Y, 1,ee