<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 />
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<br />o
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<br />o
<br />o
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<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 />
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