Laserfiche WebLink
<br />DEPARTMENT OF NATURAL RESOURCES <br />COlORADO GEOlOGICAL SURVEY <br />JOHN W. ROLD, DIRECTOR <br /> <br />INTRODUCTION <br /> <br />The intense rain and flooding in the Bi~ <br />Thompson Canyon area on July 31-August 1, 1976, <br />activitated or accelerated several geologic and <br />geomorphic processes. The affected area included <br />not only the main canyon area but also several <br />tributaries and nearby drainages. Actual hazards <br />occur only when man's activities and structures <br />are situated in hazardous locations with no con- <br />sideration of the dangers, or when the danger is <br />recognized but protective measures (mitigation) <br />are inadequate. Some hazards may be so severe <br />that protective measures are impractical or pro- <br />hibitively costly. <br /> <br />The hazardous processes described in the map <br />units are the direct and indi rect results of down- <br />slope movement of water and solid earth materials <br />acting under the forces of gravity and/or running <br />water. Actual damages are caused not only by run- <br />ning water, but also by abrasion and impact from <br />moving flood debris, undercutting by erosion, and <br />rockfalls, landslides, and debris slides and de- <br />bris avalanches (Varnes, 1958) from adjacent <br />slopes. Additional damage can be caused by par- <br />tial or complete burial by deposited stream de- <br />bris. Stream deposits typica1'y include c1ay- to <br />boulder-sized rock materials and floating debris <br />consisting of uprooted trees, other vegetation <br />material, and sometimes the remains of man-made <br />structures. <br /> <br />Most places in the Big Thompson Canyon area <br />that have geologic hazards and high likelihood <br />for post-flood (re)deve10pment are located adja- <br />cent to major streams but out of their flood <br />plains. Moreover, many of these geologic-hazard <br />areas are esthetically attractive and offer the <br />apparent advantages of being outside legal flood <br />plains and on moderate slopes that offer ease of <br />access and relatively low development costs. Such <br />areas have included some of the preferred build- <br />ing sites in the past and presumably will contin- <br />ue to be under pressure for both seasonal and <br />year-round recreational-residential development. <br /> <br />The high-intensity rainfall that caused se- <br />vere damages during this storm involved a rela- <br />tively small (15-20 percent) area compared to the <br />total area of the Big Thompson drainage basin <br />west of the mountain front. Because of thi s, many <br />potentially hazardous areas were only sl ight1y af- <br />fected. Our studies in the area indicate that <br />such violent processes have occurred at many times <br />and places during the past; accordingly, it is <br />reasonable to believe that future damaging events <br />may occur in different parts of the area. Most <br />of the mapped hazards can also be caused by lo- <br />calized intense thunderstorms which occur much <br />more frequently than do storms of the magnitude <br />of the July 31-August 31, 1976, event. Thus, <br />this mappin9 delineates both localities where <br />hazardous processes took place during the July 31- <br />August 1, 1976, rainstorms and places where simi- <br />lar adverse events could reasonably be expected <br />to occur sometime in the foreseeable future. These <br />studies show ~here hazards may exist or could be <br />initiated by man's activities, not ~hen hazardous <br />events will take place. <br /> <br />WYOMING <br /> <br /> <br />------ r----------------------- <br /> <br />\ <br />'\ <br /> <br />------ <br /> <br />, <br />..... <br />J A C K SON 'l LA RIM E R <br />.Walden "" <br />\ <br />\ <br />\ <br />\ <br />l. <br />A <br />{ , <br />. <br />f" <br />,....,_,-1 <br />...... . <br /> <br />WELD <br /> <br />Greeley. <br /> <br /> <br />Grand \ <br />OLake ,------__ <br />GRAND I <br /> <br />..... <br />. <br /> <br />Longmont <br />BOULDER <br /> <br />I <br />, <br />r <br />. <br /> <br />o <br />. <br /> <br />SCALE OF MILES <br />51011:52025 <br />, . <br /> <br />INDEX <br /> <br /> <br />Scale in Miles <br />0'1.1 2 <br />, ., . <br /> <br />~ <br /> <br />-N- <br /> <br />~ Ld. <br />~ uvtl/tlnd <br /> <br />R'" ~ <br />~SOI1 V' <br />r~o..""", LtlKB Ci . . <br />$0" <br />-'P...... <br />..~ <br /> <br />m:rn:r:m:rn Mapped by William P. Rogers <br />I I Mapped by James M. Soule <br /> <br />PLATE 2 <br />Part of Glen Haven 7.5' QuodronQle <br />PLATE 3 <br />Part. of Drake a Mo.onville 7,5' Quodran9le. <br /> <br />TOTAL RAINFALL (July 31-August 2.1976) <br /> <br />Equal-Precipitation lines are in inches <br /> <br /> <br />Scale in Miles <br />0'1.1 2 <br />l I I <br /> <br />~ <br /> <br />-N- <br /> <br />~ Ld. <br /> <br />Lo...,..d <br /> <br />@j <br /> <br />From \),5, National WHlher Service, 1976 <br /> <br />REFERENCES <br /> <br />Colorado Water Conservation Board, 1976, Special <br />flood-plain information report, Big lhompson <br />River and tributaries: Englewood, Colo., Gin- <br />gery and Associates. <br /> <br />Soule, J.M., 1976, Geomorphology of geologic haz- <br />ards in the Big Thompson Canyon area, Larimer <br />County, Colorado, in Doehring, D.O., ed., The <br />Big Thompson F100dllf 1976: Geo1. Soc. Amer- <br />ica 1976 Ann. Mtg. Guidebook, p. 25-34. <br /> <br />Grozier, R.U., ~1cCain, J.F., Lang, L.F., and <br />Merriman, D.F., 1976, The Big Thompson <br />River Flood of July 31-August 1, 1976, <br />Larimer County, Colorado: Colorado Water <br />Conserv. Board Flood Inf. Rept., 78 p. <br /> <br />Rogers, W.P., Ladwig, L.R., Hornbaker, A.L., <br />Schwochow, S.D., Hart, S.S., Shelton, D.C., <br />Scroggs, D.L., and Soule, J.M., 1974, Guide- <br />lines and criteria for identification and <br />land use controls in geologic hazard and <br />mineral resource areas: Colorado Geol. Survey <br />Spec. Pub. 6. 146 p. <br /> <br />U.S. National Weather Service, 1976, Isohyetal <br />map of the Big Thompson River drainage basin <br />and adjacent areas, July 31-August 2, 1976 <br />(revised Nov. 16, 1976): Kansas City, Mo., <br />U.S. Dept. Commerce, National Oceanic and <br />Atmospheric Administration. <br /> <br />Varnes, D.J., 1958, Landslide types and processes, <br />in Eckel, E.B., ed., Landslides in engineer- <br />Tng practice: Highway Research Board Spec. <br />Rept. 29, p. 20-47. <br /> <br />Geologic <br /> <br /> <br /> <br /> <br />, <br /> <br />Hazards in the Big Thompson Canyon Area <br />Larimer County, Colorado <br />by <br />James M. Soule and William P. Rogers <br />December, 1976 <br /> <br />COLORADO GEOLOGICAL SURVEY <br />ENVIRONMENTAL GEOLOGY 10 <br />PLATE 1 of 4 <br /> <br />'1'1 Gi '1 <br /> <br />EXPLANATION <br /> <br />MAIN-STREAM FLOODING <br /> <br />DEBRIS <br /> <br />PROCESS MODELS FOR THE MOVEMENT OF <br />MATERIAL ON DEBRIS FANS <br /> <br />FANS <br /> <br />MAP UNIT <br /> <br />MAP UNIT <br /> <br />FLOOD LIMIT: Approximate extent of area inun- <br />dated by flooding caused by the July 31-August 1, <br />1976, rainstorms and resulting water runoff. <br />Shown only for the Big Thompson River; North Fork, <br />Big Thompson River from Drake to Glen Haven; and <br />Devils Gulch above its confluence with the North <br />Fork near Glen Haven. This unit indicates areas <br />subjected to both water and debris movement and <br />inundation. Recognition of the flood limit is <br />accomplished by location of deposits of water- <br />transported debris and sediment, by f1uvia11y <br />scoured and abraded bedrock adjacent to stream <br />channels, and by other evidence recognized on <br />aerial photographs of the area made soon after <br />the flOOding. The flood limit is included in <br />this geologic-hazards!lidentification study to <br />show its areal relationship to geologic-hazard <br />areas and to show places subjected to coincidental <br />flOOding by tributary streams. The flood limit <br />shown mayor may not be equivalent to a 100-year <br />or other statistically determined flood plain. <br />Moreover, it includes only areas inundated during <br />the July 31-August 1, 1976, rainstorms; thus, it <br />does not indicate flood hazards on large streams <br />that were essentially unaffected by these rain- <br />storms. <br /> <br />DEBRIS FANS: Debris fans are triangular-shaped <br />landforms that form by deposition of water-trans- <br />ported rock fragments and soil and vegetation de- <br />bris at the confluence of tributary streams with <br />a larger trunk stream. In the Big Thompson drain- <br />age basin, debris fans or vestiges of them are <br />found at nearly every stream confluence, including <br />those formed by ,"dry washes." In many places, <br />debris fans have been removed by man and used for <br />fill material or eroded away by main-stream flood- <br />ing. <br /> <br /> <br />TRIBUTARY STREAM OCCUPIES NEW <br />CHANNEL ON DEBRIS fAN. <br /> <br /> <br />(A) NO FLOOOING OF MAIN OR TRIBUTARY <br />STREAMS. <br /> <br />(6) MAJOR FLOOD ON TRIBUTARY STREAM; <br />NO OR MINOR FLOOD ON MAIN <br />STREAM. <br /> <br />FLOOD-PLAIN <br />LIMIT <br /> <br />FLOOD-PLAIN LIMIT <br />CHANGED MAIN-STREAM 6 <br /> <br />W"",, if ::,," "CO"" "'"0'" <br />ON DEBRIS FAN <br /> <br />RELATIVELY OLD OEBRIS <br />FAN INCISEO BY <br />TRIBUTARY STREAM <br /> <br /> <br />FLOOD-WATER FLOW MOVES <br />DEBRIS FROM UPPER <br />REACHES OF TRIBUTARY <br />DRAINAGE BASIN. <br /> <br />As indicated on the maps, debris movement or de- <br />position on debris fans during the July 31-August <br />1, 1976, rainstorms occurred primarily in the <br />Glen Haven quadrangle, as this area received the <br />most rainfall (U.S. National Weather Service) and <br />apparently was also subjected to the highest rain- <br />fall intensities. Debris fans are shown on the <br />maps by a pattern and the letter symbol (A), lhe <br />second I etter (A a) indicates that rock fragments <br />and soil\ and vegetation debris moved down the <br />associated lower-order drainage and moved across <br />or was deposited on the debris fan during the <br />July 31-August 1, 1976, rainstorms. The third <br />1etter(Aab) indicates the predominant size of <br />rock particles involved, irrespective of whether <br />the debris fan was active during the recent <br />storms: (s) sand and silt; (g) pebble to cobble <br />sizes, including a significant percentage of <br />boulders, (b) boulders with some pebb1e- to <br />cobble-size material. (sib) indicates a bimodal <br />distribution of sizes. <br /> <br />(DEBRIS FAN COMMONLY MAKES <br />ATTRACTIVE STREAM-SIDE <br />BUILDING SITE.) <br /> <br />(BUILDING = .) <br /> <br />(BUILOINGS ON DEBRIS <br />FAN ARE DESTROYED BY <br />DEBRIS MOVEMENT.) <br /> <br />(BUILDING ON MAIN-STREAM <br />BANK IS OESTROYED BY <br />DEBRIS IMPACT.) <br /> <br />(C) MAJOR FLOOD ON MAIN STREAM; NO <br />OR MINOR FLOOD ON TRIBUTARY STREAM. <br /> <br />(D) MAJOR FLOOD ON MAIN STREAM AND <br />FLOOD LIMIT TRIBUTARY STREAM. <br />r~AIN STREAM OCCUPIES ...---7 <br />NEW CHANNEL. ~' ~ FLOOD LIMIT <br />.:-1:( DEBRIS FAN TRUNCATED BY <br /> <br />~;/ NEW CHANNEL. <br /> <br />r :/1..: NOTE PARTIAL DESTRUCTION <br />DEBRb POSSIBLY ~~ / OF BUILDING <br />CARRIED AWAY <br />~ BY MAIN-STREAM POSSIBLE VESTIGES OF OLD <br />). FLOODING. DEBRIS FAN <br />:., . ~~ OBLITERATION OF OLD DEBRIS <br />)( : A Ji \ FAN; POSSIBLY A NEW DEBRIS <br />Y :; Aa DEPOSIT ON DEBRIS FAN. <br />(BUILDINGS ON TRUNCATED ))"') FLOOD- WATER FLOW <br />, AREA DF DEBRIS FAN AND <br />I ALONG MAIN-STREAM CHANNEL MOVES DEBRIS FROM <br />UPPER REACHES OF <br />ARE DESTROYED.) ! TRIBUTARY DRAINAGE BASIN. <br /> <br />EROSION <br /> <br />MAP UNIT <br /> <br />SHEET-EROSION AREAS: Areas where large amounts <br />of fine-grained surficial materials were removed <br />and/or transported by sheet wash during the rain- <br />storms of July 31-August 1, 1976. Sheet erosion <br />can occur during heavy rainstorms in essentially <br />all parts of the Big Thompson Canyon area where <br />vegetation is sparse, slopes are gentle to mod- <br />erate, and fine-grained, poorly consolidated ma- <br />terials are found at the surface. <br /> <br />As indicated on the process-model diagram, modes <br />of flooding and material movement on debris fans <br />generally can be placed in three classes: Ma- <br />jor flooding on a tributary stream without major <br />flooding of the main stream results in material <br />movement and associated damages being confined <br />to the area of the debris fan and possibly the <br />opposite main-stream bank. Major flooding of <br />the main stream with little, if any, flooding of <br />the tributary stream usually results in damages <br />on the debris fan being confined to that part of <br />the debris fan that lies within the main-stream <br />flood plain; erosion by main-stream flooding may <br />partially or completely remove the debris fan <br />(debris-fan truncation). In the case of flood- <br />ing of both main and tributary streams, material <br />moving across the debris fan may be carried away <br />by main-stream flooding, resulting in partial to <br />complete removal or modification of pre-existing <br />debris-fan deposits. <br /> <br />The mapped extent of debris fans shows areas sub- <br />ject to these processes and disregards present <br />absence of debris-fan deposits in some areas. <br /> <br />(MODIFIED FROM SOULE, 1976) <br /> <br />LAND - USE CONSIDERATIONS <br /> <br />SHEET-EROSION, GULLY EROSION, AND SHEET WASH: Sheet <br />erosion, gully erosion, and sheet wash cause pro- <br />blems for residential developments because of un- <br />desirable erosion of the substrate under structures; <br />deposition of eroded materials in natural and man- <br />made drainageways; obstruction of drainage-control <br />structures by sediment; and deposition of sediment <br />on roads, on building lots, and in buildings. Al- <br />though threat to life and destruction of struc- <br />tures are unusual in these areas, erosion and de- <br />position of fine-grained materials are usually a <br />costly nuisance. Typically, the most desirable <br />means of mitigating problems in sheet-erosion <br />areas is careful, well-planned control of surface <br />drainage around roads and structures and avoidance <br />of steeper cuts that are left barren of vegetation <br />or other protective cover. <br /> <br />FLOOD-WATER FLOW MOVES DEBRIS <br />DOWN MAIN-STREAM CHANNEL. <br /> <br />FLOOD- WATER FLOW MOVES <br />DEBRIS DOWN MAIN-STREAM CHANNEL. <br /> <br />NOTE: SUBSCRIPT <br />INDICATING I~ATERIAL <br />SIZE IN D~BRIS FAN(S) <br />IS OMITTED FROM DEBRIS- <br />FAN SYMBOLS ON THIS DIAGRAM. <br /> <br />(ESSENTIALLY ALL STRUCTURES <br />ARE DESTROYED.) <br /> <br />LAND-USE CONSIDERATIONS <br /> <br />DEBRIS FANS: Debris fans are moderate to severe <br />hazards for most residential developments. As <br />evidenced by debris fans that were active during <br />the recent storms, structures any place on debris <br />fans whose drainages received the largest amount <br />of rainfall or had the greatest rainfall inten- <br />sities were typically obliterated. In other pla- <br />ces, where runoff was less, structures adjacent <br />to stream channels on debris fans were most apt <br />to receive heavy damages. Thus it appears as if, <br />generall,y, chances for damage to structures on <br />debris fFans are less the farther a structure is <br />placed rrom active drainage channels. As recur- <br />rence of major events in these areas subject to <br />debris and water movement is not known, deter- <br />mination of risks for structures placed in these <br />areas is difficult. However, site-specific study <br />may indicate that for some land uses, relatively <br />safe sites may exist on some debris fans. <br /> <br />SLOPE <br /> <br />I NSTABI LITY <br /> <br />FLASH-FLOODING <br /> <br />MAP UNITS <br /> <br />UNSTABLE OR POTENTIALLY UNSTABLE SLOPES: Slopes <br />composed of earth materials that are undergoing, <br />or are susceptible to, mass downslope movements. <br />Slope stability is dependent on composition and <br />thickness of residual soil (regolith) and debris <br />(loose rock material) above bedrock, slope aspect <br />and inclination, vegetation cover, and local sea- <br />sonal changes in ground moisture. Related hazards <br />within these areas vary in severity from minimal <br />to very great. Generally, potential hazard in- <br />creases on steeper slopes. Types of mass downslope <br />movements include rockfalls, rockslides, landslides, <br />debris slides and debris avalanches, and accel- <br />erated creep. Predominantly south-facing slopes <br />are more susceptible to debris avalanches and <br />debris slides, whereas predominantly north-facing <br />slopes commonly experience 1ands1iding. Landslides <br />frequently occur where slopes are undercut by nat- <br />ural erosion or by man-made excavations. Irrespec- <br />tive of slope aspect, rockfall and rocks1ide areas <br />are typically located adjacent to sparsely vegeta- <br />ted, jointed bedrock cliffs; the lower slopes of <br />these areas consist of rock rubble strewn on steep <br />to very steep slopes. Unstable or potentially un- <br />stable Slopes are indicated very generally on the <br />maps. A more precise delineation of areas of <br />actual or potential slope movements would involve <br />much detailed field and laboratory work and de- <br />tailed topographic base maps that are beyond the <br />scope of or not available for this st~dy. However, <br />most of the steeper slopes in the Big Thompson <br />Canyon area are to some degree susceptible to these <br />processes. Arrows in the mapping symbol graphi- <br />cally indicate the general sense of material move- <br />ment downslope. The amount of potential movement <br />in each occurrence or series of occurrences is <br />usually difficult to determine. <br /> <br />J-" <br />,. <br /> <br />MAP UNIT <br /> <br />DOWNCUT STREAM CHANNELS: Channels in which large <br />volumes of high-velocity floodwater during the <br />rainstorms of July 31-August 1, 1976, were suffi- <br />cient to transport or remove some or all of the <br />rocks and vegetation in channels. Typically 3 to <br />10 ft (1 to 3 m) of downcutting occurred in upper, <br />steeper-gradient reaches of channels, whereas <br />lower, lesser-gradient reaches saw transport of <br />boulders 3 to 6 ft (1 to 2 m), and occasionally <br />larger, in diameter. In some channels essentially <br />all material was removed, leaving only a scoured <br />bedrock surface. Channel downcutting is most pro- <br />nounced in drainages that were subjected to the <br />highest rainfall intensities during the recent <br />rainstorms. Essentially all channels, including <br />sma 11 ephemeral dra i nageways ("dry washes"), in <br />the Big Thompson Canyon area have evidence of simi- <br />lar flooding and downcutting in the past. Channels <br />that were not downcut during the recent storms are <br />not mapped; mapping would merely produce a drainage <br />net that can be easily deduced from the topography <br />shown on the base map. <br /> <br />LANDSLIDES <br /> <br />I <br />I ' <br />I " <br />I Isa , <br />, ~J <br />" ' <br />, / <br />-" <br /> <br />MAP UNITS <br /> <br />LANDSLIDE AREAS: Slopes composed of materials <br />highly susceptible to 1ands1iding. Landslide <br />areas are differentiated from unstable/potentially <br />unstable slopes because of local severity and im- <br />portance of this kind of potential hazard for <br />some areas that may be (re)deve10ped following the <br />recent flooding. <br /> <br />LANDSLIDES: Landslides that occurred during <br />rainstorms of July 31-August 1, 1976. These land- <br />slides usually occurred where slopes composed of <br />relatively thick accumulations of colluvium were <br />eroded and undercut by flOOding of the Big Thomp- <br />son River or its tributaries. In a few local- <br />ities (noted on the maps), debris avalanches and <br />debris slides developed. <br /> <br />, <br /> <br />LAND - USE CONSIDERATIONS <br /> <br />STREAM CHANNELS: FLASH-FLOODING HAZARDS: Flood- <br />plain studies, whether routine or in the after- <br />math of a major flood, do not usually include all <br />potential flood-hazard areas in a drainage basin <br />such as the Big Thompson. The hundred-year flood <br />and larger events such as the Standard Project <br />Flood are usually mapped only for selected reaches <br />of a stream. These limits are then used as the <br />principal basis for flood-hazard planning. As a <br />consequence, "unexpected" f1 ash f1 oodi ng in smal- <br />ler tributary drainage basins and ephemeral (dry- <br />wash) channels is common in the mountainous areas <br />of Colorado and is usually not adequately consid- <br />ered in land-use planning. <br /> <br />The very intense rain that caused this flood oc- <br />curred over less than 20 percent of the Big Thomp- <br />son drainage basin west of the mountain front. <br />Because of this rainfall pattern, there was a <br />two-fold aspect to the flooding phenomena and the <br />damages that resulted. In the areas of greatest <br />rainfall intenSity, flooding was characterized <br />by spectacular flash-flood surges in relatively <br />small and steep tributaries that also carried <br />relatively large amounts of woody and rock debris. <br />Those parts of the Big Thompson drainage basin <br />that received moderate to no rainfall suffered <br />little damage from local runoff. Damages in <br />these areas were the result of main-stream f100d- <br />ing caused by heavy runoff and debris production <br />in upstream areas. <br /> <br />cD <br /> <br />LAND-USE CONSIDERATIONS <br /> <br />LANDSLIDE AREAS: Landslides cause severe problems <br />for most construction. Mapped landslide areas are <br />marginally stable at best, and loading of slopes <br />by structures or cutting of slopes in order to <br />construct roads will most likely cause slope move- <br />ments. Consequently, these areas should be con- <br />sidered to be severe hazards and probably should <br />not be developed. During the recent flooding, sev- <br />eral structures were lost owing to sliding into <br />the Big Thompson River after the slopes they were <br />on were cut by erosion caused by flood water. <br /> <br />LAN D - USE CONSIDERATIONS <br /> <br />UNSTABLE OR POTENTIALLY UNSTABLE SLOPES: The wide <br />ranges of-Severity and variety of geologic hazards <br />in these areas are such that in most cases site- <br />specific engineering and engineering-geologic in- <br />vestigations to determine feasibility of develop- <br />ment and construction are advisable. Safe 10w- to <br />moderate-intensity land uses are possible if the <br />potentially hazardous conditions are recognized <br />and seriOUSly considered in site and construction <br />plans. It should be anticipated that parts of <br />these areas will very likely not be amenable to <br />safe and economical development for many kinds of <br />land uses. <br /> <br />ROCKFAllS <br /> <br />MAP UNITS <br /> <br />ROCKFALL AREAS: Areas subject to frequent free- <br />falling andlor rolling and bounding masses of <br />coherent rock or individual, large rock blocks. <br />Rockfall areas are found on and below very steep, <br />n~ar1y barren bedrock cliffs. Jointing, foliation, <br />and weathering characteristics of the bedrock can <br />greatly affect the severity of the hazard. <br /> <br /> <br />DISCHARGES <br /> <br />The flash flooding during the July 31-August 1, <br />1976, rainstorms and the strong geologic evidence <br />for many past flash-flood events at differentp1a- <br />ces in the Big Thompson drainage basin indicate <br />that all tributary drainage basins and dry-wash <br />channels in this area are subject to periodic <br />flash flooding. Flash flooding may accompany a <br />large regional rainstorm or it may result from <br />an isolated thunderstorm which produces only lo- <br />cal effects. The frequency of flash flooding in <br />a given dry wash is difficult and perhaps impos- <br />sible to determine. It is clear, however, that <br />these local and potentially destructive events <br />are much more frequent than larger events of re- <br />gional impact. <br /> <br />PEAK DISCHARGE (CUBIC FEET PER SECOND) <br />AT LOCALITY INDICATED ON MAP <br /> <br />Station numoers correspond to those assigned by <br />the Colorado Water Conservation Board (1976). <br /> <br />Mapped rockfall areas include only those for <br />which potential for frequent rock movements is <br />considered to be so great that potential hazard <br />for most human activities is high. Some rela- <br />tively small areas of equivalent rockfall hazard <br />and larger areas of lesser hazard are included <br />in areas mapped as unstable or potentially un- <br />stable slopes. <br /> <br />ROCKFALLS: Rockfall events that occurred during <br />the rainstorms of July 31-August 1, 1976, are <br />shown separately to demonstrate the Similarity of <br />these localities to many other places susceptible <br />to rockfalls throughout the Big Thompson Canyon <br />area. Undoubtedly, many rockfalls that occurred <br />during the indicated periOd have not been recog- <br />nized. In many places, evidence for rockfall <br />was probably removed by stream erosion or recon- <br />struction before this study was completed. <br /> <br />. <br /> <br />o <br />o <br />o <br />o <br />o <br />o <br />e <br />e <br />e <br />e <br />@ <br />e <br />@J <br />e <br />€J <br />e <br /> <br />Drafted by Daniel J. Fair <br /> <br />4,460 <br /> <br />4,330 <br /> <br />The following land-use recommendations are based <br />on results of this study. We suggest strongly <br />that these recommendations be considered in as- <br />sessing the safety of existing as well as future <br />building sites in areas susceptible to flash <br />flooding. In any area not fully studied as to <br />flash-flood potential, keep homes, sewer systems, <br />vehicle parking areas, and other essential con- <br />struction at least 12 ft (4 m) above stream chan- <br />nels. Areas on the outside of sharp stream bends <br />are especially hazardous owing to supere1evation <br />of floodwater. Man-made constrictions of stream <br />channels can cause flooding by backwater upstream <br />from constrictions. <br /> <br />8,700 <br /> <br />7,210 <br /> <br />6,910 <br /> <br />LAND-USE CONSIDERATIONS <br /> <br />5,500 <br /> <br />ROCKFALL AREAS: Potential hazards in mapped <br />rockfall areas are severe in most places. Because <br />of technical difficulties associated with removing <br />or stabilizing large numbers of potentially mo- <br />bile rocks, corrective engineering to ensure ade- <br />quate safety for residences will be costly and, <br />in most cases, prohibitively expenSive. In some <br />places, site-specific, detailed evaluation of <br />rockfall potential may indicate that a few appro- <br />priately located and engineered structures are <br />feasible. <br /> <br />2,320 <br /> <br />28,200 <br /> <br />2,810 <br /> <br />9,670 <br /> <br />1,790 <br /> <br />2,060 <br /> <br />lIAccording to Colorado Statute 106-7-103 (8) <br />C.R.S., water flooding is not considered to be a <br />geologic hazard. For a discussion of the State <br />of Colorado's involvement in geologic-hazard area <br />identification and legal definitions of geologic <br />hazards, see Rogers and others (1974). For de- <br />tailed information about the legal (100-yr) flood- <br />plain of the Big Thompson River and North Fork, <br />Big Thompson River, studies by the Colorado <br />[State] Water Conservation Board (1976) should be <br />consulted. <br /> <br />3,240 <br /> <br />8,710 <br /> <br />30,100 <br /> <br />31,200 <br /> <br />, <br />