|
<br />,~
<br />
<br />:_~""..t~:;;n-",'iL.';';,..\.~~;:';~>: '}'M\~:~!(.';" ~- 't~{~.- >:"""''':,~lf.,~;...''~:''i!l('<;if'~''t~~ ~,:~; '. : (i' '" '~iiio'; .-, .
<br />
<br />FLOOD CONTROL
<br />
<br />
<br />Appli ed;:;;~i;i:~1~;tt~;"
<br />G h I. A',;;~ll.~~t,'~5d:-;~4~;'
<br />,~ ,"" _ ",~.n '.'
<br />
<br />eomorp 0 ogy n '
<br />Floodplain · '
<br />Managemenf:~;,
<br />
<br />
<br />by Mike Grimm, City of Fort Collins,
<br />Stormwater Utility
<br />
<br />Introduction
<br />mE delineation of floodplains has
<br />generally been based on the application
<br />of established methodologies of hydrol-
<br />ogy and hydraulics for channels with
<br />fixed boundaries, These studies do not
<br />acknowledge the potential for river sys-
<br />tems to move both laterally and vertical.
<br />ly, The role of sediment, and geomor-
<br />phic and geologic controls on channel
<br />form and stability, in general, has
<br />received minimal
<br />attention in
<br />floodplain man-
<br />agement and
<br />river engineering
<br />until relatively
<br />recently,
<br />Floodplain
<br />studies using
<br />computer pro-
<br />grams such as
<br />HEG-Z or. WSPRO have difficulty dealing
<br />with moveable boundaries, varying ero-
<br />siveness of bed and bank materials, ;")-d.
<br />the narural variability of sediment triils:
<br />port, which is often typical of alluvial
<br />streams (tributaries) in the western
<br />United States, such as along the Front
<br />Range and Plains of Colorado'.
<br />Therefore, additional interdisciplinary
<br />techniques based on the principals of
<br />fluvial geomorphology and hydraulic
<br />engineering must be considered in
<br />floodplain management studies, By
<br />linking the river morphology (present
<br />conditions) and the geomorphic history
<br />
<br />32oMarcb/Apri/1996
<br />
<br />,,:
<br />
<br />.:-)'
<br />
<br />(past conditions) of the system with
<br />hydrologic and hydraulic data, the accu-
<br />racy of prediction with regard to (furure)
<br />river behavior may be increased,
<br />The City of Fort Collins, Colorado,
<br />regulates floodplains according to the
<br />National Flood Insurance Program
<br />(NFIP) criteria, The City has also been
<br />incorporating channel stability studies
<br />into Floodplain Management and Master
<br />Planning in order to delineate minimum
<br />erosion buffer setbacks for proposed
<br />development. For example, the City has
<br />been conducting interdisciplinary chan-
<br />nel stability studies on Fossil Creek in
<br />
<br />.,' ','f .~"~,Ji, " ,,', ". ..- ".
<br />order to charaaerize the siability of the
<br />channel based on an evaluation of his-
<br />, ".'.;; --,,,,"
<br />. t2;i.5,,~hannel patterns, hydrologic,
<br />hydraulic, geomorphic, and sediment
<br />rrn.nsp?!t characteristics of the stream,
<br />'-'~'~{f~~~.~ti;:~i_, .
<br />
<br />Background and Field Area
<br />: c~,"t ',' , '.,,,,,,'
<br />uS Fossil Creek is an intermittent
<br />sti~~& _y.,hich flows easterly from the ,
<br />edge of the foothiils to the Cache La
<br />Poudre River and is characterized by a
<br />flashy hydrograph, high sediment ioad,
<br />entrenched channel, steep eroding
<br />banks, tight meanders and general chan-
<br />nel instability which is typical of many
<br />
<br />land and \v.lter
<br />
<br />
<br />alluvial streams along the Front Range of
<br />the Rocky Mountains,
<br />Entrenched channels, such as Fossil
<br />Creek, present special problems in the
<br />interpretation of the flood hazard,
<br />Entrenchment occurs from entrainment
<br />and transport of alluvial materials by
<br />channel erosion, Ai; a result, channels
<br />are frequently so deep that floods with
<br />recurrence intervals of greater than 50.-
<br />to lOG-years are required for overflow,
<br />Entrenchment of Fossil Creek prob-
<br />ably began in the last cenrury and may
<br />be attributable to land use practices such
<br />,
<br />as grazing and urban development. This
<br />conclusion is inferred by comparing the
<br />stream reach immediately upstream of
<br />the study reach, a Public Park, which
<br />exhibits substantially different channel
<br />characteristics, The Public Park channel
<br />reach is well vegetated and not nearly as
<br />incised as the downstream srudy reach,
<br />The study reach has been extensively
<br />grazed and/or farmed in the past.
<br />In addition, much of the land adja-
<br />cent to'the study reach has been devel-
<br />oped into subdivisions, Although the
<br />entrenclunent of the channel preceded
<br />the adjacent subdivision development,
<br />channel instability could be exacerbated
<br />by changes in runoff and sediment con-
<br />veyed to the channel associated with
<br />watershed development.
<br />The bankfull channel capacity in the'
<br />Public Park Reach is between the 5 to
<br />lO-year event, whereas the downstream
<br />entrenched srudy reach contains the 50.-
<br />year event as bankfull discharge,
<br />Vertical bank heights in the entrenched
<br />reach range from 10 to Z4 feet high,
<br />Banks are generally 4 to 8 feet high in
<br />the Park Reach,
<br />Within the study reach, areas near
<br />the channel that once were probably
<br />inundated are now safe from inunda-
<br />tion, However, they are at risk from erq-
<br />sion damage because the channel may
<br />be frequently occupied by flood flows
<br />with the competence necessary to
<br />entrain sediments in the bed and banks,
<br />For example, one of the primary bank
<br />failure mechanisms identified is under-
<br />cutting of the banks from low flows less
<br />than the Z-year event causing a critical
<br />threshold bank height to be reached;
<br />severe erosion and bank collapse result.
<br />Additionally, seepage from irrigation
<br />
<br />1
<br />
<br />~"
<br />
<br />~
<br />
<br />,~
<br />
<br />~<,
<br />,*'
<br />ii
<br />"~':'"'"
<br />4
<br />" .
<br />;'l
<br />".'~il::
<br />
<br />,l~.-, ',,,-
<br />~"
<br />':~~
<br />~,J,'
<br />~~I';
<br />.", .
<br />'!!l .
<br />'I;
<br />il'
<br />1l# '"
<br />
<br />!~~.
<br />,i.;.' ",;
<br />"i~-
<br />-qt/:1:if
<br />,11
<br />* '.
<br />...,
<br />,;t;. ,
<br />
<br />-.t,w'l--"
<br />~~
<br />~:
<br />~h
<br />!I
<br />
<br />
<br />~~ '
<br />
|