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DEPARTMENT OF NATURAL RESOURCES COLORADO GEOLOGICAL SURVEY JOHN W. ROLD, DIRECTOR ~-~ ~... ..................... ~ qf/). .. -, .',':~ .". ... :,.~:':',~;.;.<:.:[:. "',;_0,..., ."..,... - '.__,:0. ;<.~.'>.':" ~ Geomorphic Features River Lower Big Thompson Formed by Flooding on July 31 - August 1 , by David C. Shelton the 1976 COLORADO GEOLOGICAL SURVEY ENVIRONMENTAL GEOLOGY 10 PLATE 4 OF 4 '1'l~ '1 T 6 R 70W .J / \ \ J J ,J'" , . + " , '''''' , , ., '" -It! I / / ~ 1"- ','. ~ ":~V \\V(v/ \ II (G~ ~ T ~~~ /f 'l (/t2I-~~ &:: '\ () 'f'_,/~y """ < ,~.".......,.{\\,i ~,.. \' ~~, o. ,v., v ::) ,.' ~~/ ~,,/, ))~~~'j)i l ~~\ I~< I" \ ~~!(i" ~ ... '~~c,X'..,,~~Z ii /. v--, ~~t\~o- ''1, : ~- -.-.,- - -->;.'5~79_.. / C~ -'0"'.\\ [ ?r' 'li ~td? ~ . ,~~~ c' II - -1=, ' V, . . . \', 0 '"-" ) , I" \Jo.J. \ I .' .~ ?# :\/ "e~~\ ~ fc: V" L (.", ::::::"j ~ ,~ '\ ( . 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(j( -L~k=--_~ ~ f (~)r ';\ "\ /'\ \ ". \ \, 0.5~~. ,/ It{ (/ \5/36+,\ ,r~ (? ~~"'h" ~'. ~'i('~. ~O Vr ..0 ~-.-f. 3. 'v---1 \ II ) ~ " \ \ \ \ '-J 7/ / (/ / ( v ~ EXPLANATION DISCUSSION OF MAP UNITS ,~ \\~ ~ ~"" .;~~C,i~\~<..\:~.. .\ !~1( '.~ .~. E:k.. .0 f ~~.:.~~...::~:./c.:~::~ Limit of floodin~~~ngU~~:~1-AU9Ust 1, 1976, The extent of flooding and pattern of damages Stream-bank erosion also occurred where a flood- Note I: In the so~t~::l: of the flooded area, Braddock, W,A" :a~:~t~:~H~,E:warecki' S,J" r \) ~. ~I \ \ \ \-,_:-...:~,__'" <;' ~ '.~ h ~ ' ".' .:: ~.;r.;-.. c=-::---. _ ,_c~.~. --' =:J.'>'" flood: Based on field observation and aerial- were determined largely by the dynamic processes channel bend existed, and the flow was contained, the river deposited 2-5 ft (0,6-1.5 m) of ooulders and Nutalaya, Prinya, 1970, Geologic map of thei;) ,. = 1,/.0._ \c, ="". =. ',..,.' ""-'~ , H _ -:'" h--- ~...~':::-:::: _ H _ - ~_ '. . '------~- , ." '_ <,'.' ,_ Dr:..... " .......... . photograph interpretation. of water movement. New channels formed by erosion causing erosion on the outside of the bend. This up to 5 ft (1.5 m) in diameter. Equivalent or Masonville Quadrangle, Larimer County, Colorado: I \\ .-- -- .-____ ~ - ------, ~ r...---' outside the main channel result in changed pat- process can cause flooding and/or severe damage in greater thicknesses of sand, silt, and debris were V,S. Geol. Survey Geo1. Quad, Map GQ-832, . I "'.; ~~ ~~ .~, "'r=-,_",_"--~, '. ~ ... terns of deep, high-velocity flOOding. Such f100d- areas that, prior to the flond, were outside of deposited on the north half of the flooded area, '., 'i .~ ------. . -"": -,-, , ~ --""...-----.--.,-..,'..' . '~ '"",.' ~ Channel displacement: line represents the loca- Y i ( ) ~ ~ ~;_::~--., ----, ---- .~ ing modifies water flow, extent of flaodinl', and and above the anticipated flood limits. Structures Aggradat on deposition occurred as the velocity Cor:hran, D.M.. 1975, Environmental and engineering \". \ I.." ( '.,-'. ........... tion of a new channel that formed during flooding' d d h f h d h c' related damages. An example of this occurred in and roads in such areas can be damaged or de- an ept 0 t e water ecreased, t ereby de- geologic map for land use, Masonv;l1e quadrangle, ~ .....~/"V ~." ~ ~ but did not replace the preexisting channel. . th t "\ \ ~ -..::....._ / ~ 0 ~ ' the vicinity of the Sulzer Gulch alluvial fan strayed by erosion and undercutting. creaslng e s ream competence. Larimer County, Colorado: Golden, Colorado, (I -......r- '.. -.- -, ~ ~ ~~~~~i~~ =~~r~a~;a~~o~x~:s: ~~'\~~~n~~jc~a~~~~d Significant deposition of sediment corrmonly oc- Note 2: Although it did not flood in this event, Charles S. Robinson & Associates, Inc. \' .' _'" 'i-:-V /10-7. '~~ ./~.~~ have occurred had no new channel formed curred where there was a decrease in stream com- the alluvial fan of Sulzer Gulch and the adjacent Follansbee, R.. and Sawyer, L.R., 1948, Floods in I I -- " --- '" 1 -...::::...J ~~ __" . petence. This usually resulted from a velocity small creek were overrun by the flood water of the Colorado: U.S. Geol. Survey Water-Supply Paper 5/76 1,,18 ----...\ ~ '11': 1\ ~._'I ~ V'. 'iJ "~,. --' .. ~ Bank erOS10n lncreased the area of flood1ng by decrease caused by decrease 1n water depth or Big Thompson River. Pre-flood aerial photography 997 155 P ,~ J" '- - . -~ widening ~he ~lOOd plain as the river er~ded lat- stream gradient, widening of'the flood plain, a indicates that the formation of the alluvial fan ,. I. II' ", y. ~ ~ 1 / .:.:: 1 f erally, The most obvious examples of this oc- constriction, or a local variation of flood-plain had forced the Big Thompson River channel against \ I ( curred on the south bank of the river (Note 1) geometry, Damages associated with these deposits its southeast bank, Erosion during the flood j I "-1,,0,, .../~~ ,j , inrnediately downstream from the Big Thompson Power include partial or complete burial of structures, removed the lower part of this fan as well as part , I " ...~.' ~ /. Plant, where damages to roads and houses were roads, agricultural fields,and irrigation ditches. of the upper fan where the new channel is shown . /' II ~""I"" . caused by erosion of fills and materials support- on the map. Future land-use decisions in this \ II ~ \Il" ria na . ing structures, area should consider both flooding by the Big \\ ! ~ ", --....... ~ _ - ~ \\:::..J ~,'>t:t Debris accumulates where it is caught on obstruc- Thompson River and alluvial-fan processes at its I.........r<... 6 1,. \\ " /r~7 e " Oebris accumulations affected the limits of tions such as trees, bridges, buildings, and other confluence with Sulzer Gulch" J. I C. N" ee f\I II ~ 0 flooding and damages by partially darrming and structures, Smaller deposits of debris occur r---~ ' I deflecting water. Evidence for these processes where depth of water decreases and can no longer Note 3: Green Ridge Glade drainage was inactive / '( .~ were observed in sec. 11, T. 5 N" R, 70 W" where carry its load. Damage patterns are altered due during this flood. An alluvial fan is located at ( : : Highway 34 crosses the river, At this location, to the changes in flow depth and velocity. Debris its confluence with the Big Thompson River, This I " debris caught in trees and on the highway bridge caught in trees and in or on structures may pro- potentially hazardous condition should be consid- I ' /,.--;/ constricted the flow and caused a backwater effect teet an area irrmediate1y downstream by decreasing ered prior to development between the Loveland I ::,,~.~ .,...? -' that increased the severity of flOOding upstream, water velocity. Oeflections of water by debris may Filtration plant and the Big Thompson River. ' , -,,:....,',, locally increase velocity and cause severe damage. \".,..7 Accumulations of debris on bridges and houses may Note 4: According to a local resident (Roben ~ 0 cause them to fail owing to the increased cross- Lindsay, 1976, pers. corrm,), Buckhorn Creek ,~ section area on which the flood water pushes. In flooded two days after the Big Thompson flood of ~lr //J- \ ---... the case of bridges, temporary damming followed July 31-August 1, 1976. Field evidence observed by failure can cause flood surges that can in- on August 13, 1976, indicated approximately 10 ft crease flooding and damage downstream. of water in Buckhorn Creek at the south line of sec. 1, T. 5 N" R, 70 W. It is uncertain whether this depth of water was caused by the Bi9 Thompson River or by flooding on the Buckhorn Creek two days later. Historically Buckhorn Creek has ex- perienced at least two large floods--June, 1923 and August, 1951 (Follansbee and Sawyer, 1948). A dam failure was a factor contributing to the later flood. December 1976 R69W ,,", " "-, -N- .i . SCALE 1: 12000 t I MILE o 500 .................... ........ 2000 4000 FEET , 1000 3000 o ". I KILOMETER CONTOUR INTERVAL 40 FEET ~ ';'. . y--~ ~ '. . Pronounced strea~bank erosion: Arrows indicate direction of lateral erosion, Significant flood deposit of boulders, cobbles, sands, and silts: Generally greater than 2 ft (0.6 m) thick and typically 1 acre (0,4 ha) or greater in extent; grain size generally decreases downstream; boundaries are gradational. t::Jo Significant flood-debris accumulation: Consists of intermixed vegetation, mobile homes, remains of buildings, sediment, and other materials. GENERAL DISCUSSION .." '"'' 7/ . ':'". ,{(, .~'!'..- i~ Buckinflham glL ..Lake~ "508~\ A9" ...... " On \... ';, "" -- " '. '0 '\ This map depicts geomorphic features resulting from the July 3l-August 1, 1976, flood on the' lower Big Thompson River west of Loveland, Lari- mer County, Colorado, This area is part of the Front Range foothills belt and is included in the Masonville 7~-minute topographic map and geologic-quadrangle map (Braddock and others, 1970), The geomorphic features were selected for their prominence and their importance for future land-use decisions. Other environmental and engineering-geologic features in the Masonville quadrangle have been mapped for Larimer County by Charles S, Robinson and Associates, Inc. (Cochran, 1976). This study includes only the area inundated by the flood of July 31-August 1, 1976, ~~ 8M "'-- '4-t.~. Oeposition of sediment is the most widespread geo- morphic process and cause of damage in the lower Big Thompson River flood plain. This map unit includes only those areas of relatively thick (2 ft or greater) deposits. Deposition occurred to some degree throughout the flood plain except in those few areas indicated as experiencing eros i on. / ~ -..... 080" " . - CONCLUSIONS , " . ./ Several geomorphic processes that produced ob- servable features were associated with flooding of the lower Big Thompson River. The locations and rates of the processes change with time dur- ing a flood event causing changes in the locations and types of damage produced. Study of these pro- cesses and the features prOduced gives consider-- able inSight into factors that should be consid- ered in land-use planning for flood-prone areas, Flooding is a Changing and dynamic process during which channel size, shape, and location; water flow patterns; and damage patterns change. The interactions between man1s use of the land and these changing processes should be understood if we expect to make safe and wise use of flood plains, Base from U. S. Geologica I Sur vey , 7.5 topographic quadrangle: Masonville High-intensity rainfall over approximately 60 square miles of mountainous terrain west of this study area caused severe flooding of the Big Thompson River and several of its tributaries. In addition to the large area inundated by the flood water, the dynamic processes of flooding caused erosion, deposition, and formation of new channels. These geomorphic processes and re- sulting features greatly influenced the damage caused by this flood and should be considered in planning the reconstruction of this area, These processes are not unique to the Big Thompson River, and thus this discussion is applicable to other drainages along the Colorado Front Range. MAPPING OF GEOMORPHIC FEATURES AND LOCATIONS OF ASSOCIATED DAMAGES In general, the locations of the geomorphic fea- tures formed, and associated processes and damages that occurred during this flood were 'predictable, New channels were formed where the preexisting channel was bent; whereas deep, higher-velocity flood waters continued directly over the old bank and eroded an additional channel, Oamage increased in such new channel areas because of erosion and increased water velocity.