FLOOD06873

• County: Statewide
• Basin: Statewide
• Title: Hydraulic Engineering volume 1
• Date: 1/1/1994
• Prepared By: American Society of Civil Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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IIYlJR^lJl.I(' EN(jINEERIN(j '94

Boulder-bed, steep gradient open-channel Oow is not wen understood by the
average design engineer. Flood estimates arc often ha<red on slope~area methods
where the roughness factors (such as Manning's n values) are the weak link in the
calculations (Jarrell, 1984). An error in estimating Manning's n can cause errors in
the computation of the magnitude of past floods. This is also true for estimates of
Manning's n values for constructed channels where lhe n tends to be lower than for
a similar natural channel. The hydropower engineers. in seuing draft tuhe and
powerhouse floor elevations. must compule the waler surface profile elevations in the
downstream channel for a selected series of flood frequencies. The hydropower
engineer must often estimate the downstream channel roughness factors for steep
armored stream beds. Needed data and' methods would be from the technology and
research of the disciplines of geomorphology, hydrology and open-channel hydraulics.
The average consulling civil engineer should be able to rely on the availability of
user-friendly and accurate hydraulic, hydrologic. and geomorphologic data and
technology for steep sncam channel and chute analyses.

While technology transfer is important for steep channel and chute desiJ!.lI. il
is also important for the selection and design of gritdil~1l1 conuol structures which can
have a direct impact on river boating. and public safely (Klumpp, 1989). SafelY is an
important design criterion (Jansen, 1980), With the increased interest in river
rafling. kayaking and canoeing during the last two decades. particularly in and near
urban areas, "drop structures" with reverse rollers and other forms of "dangerous
hydraulics" must be avoided (Taggart. 1984: Centerline, 1992). Enmples are
described below.

Bil! ThOlnoson River at R..tes Park. Colorado

Channel modifications were made to the steep and bouldery Big Thompson
River through lhe town of Estes Park, Colorado 10 eliminate over-bank now for the
t percenl nood (62 ems) and to create a whitewater visual ameni1y for a new river
walk corridor. Design objectives included enlargement of the channel cross section.
maintenance of peak desig.n nows al critical velocities or less, a chule to overcome
earlier head CUlling of the channel downstream of the Elkhorn Street culvert, and
creation of a natural appearing stream even after enlargement of the channel cross
section. The channel enlargemenl was accomplished by removing the natural bed
armoring. excavating the channel bonom by up to 1.2 m in depth. and replacing the
natural armoring stones and boulders. Additional large boulders up to 1.4 m in
diameter were placed to form the chute configuration, Stones having a median
diameter of 0.310 0.5 m were Ulilil.oo for the banks at slopes of up to O.S mtm wilh
larger boulders placed at strategic locations for energy dissipation and the creation
of special whitewater visual affects.

Six months after the completion of the construction, 32 cubic meters 01
smaller diameter stones were missing. both above and below the high water line. due

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STEt:I' STRI:^M ('II^NN1;' S
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10 vandalism. Missing stones were re Iced.
and larger. During the following 5no:~elt ru:~f new sto,nes of 05 m .in diameter
of the waler surface profiles at variou d' h peak dISCharge, detailed survey!';
of the new channel to calculale n v~u:f:g~~ were performed for the fU."lengi,;
surveyed crO!.~ sections and reliable " cons~ted channel. USlOg field
R I . ow rates provided by U S B
ec amallOlI (USBR) permanent parshall n . , .. a .. ureau of
were derived. The reconslructed channel ,;me,. th~ Manmng S n~roughness faclors
was determined 10 be 0 036 12 I annlOg s n yalue for the I percelll nood
(Jarrett. 1985). Using' the'car~~~ntm:u~:' the esti~te of O'<l4 I using NCALC
rough~ess factor. the water-surface Y profile for ~~auh;:arameters and derived
establIShed for suhmj.tal 10 the Federal E pe ol"ood 01 62 ems was
refer to Figure I for 3 comparison of .... merg:ncy Management Agency. Please
W3u;r sur.ace profiles ror this example.

0.<>5

Computed Roughness
n=0.34 SO.Jll R-O.l
(Jarrett, R.D., 1985)

1X r100d
62 ems

C
M
'"
.~ 0.04
c
c
!i

field survey~ ""'-
of WSP, Resources ~___
0.036 0 --=--:.:.~-
1 X r100d

0.03
o

Big Thompson River
Estes Pork. CO
0.25

1.50

0.50

Figure 1,

0,75
R (meters)

Comparison 01 computed channel Mannings n with
meosured roughness of reconstrucled channel.

1.0

1.25

Mill Creek. Vail~. Colorad2

Relocalioo of Mill Creek " Vail CoI d' .
channel (0,12 m/m), two channel curves ~ ;ra ~m 1 990 ~nvolved a very steep
now within the channel banks A ' e to contam the I percent nood
maintenance of a natural and env::::1 goal in ~ design process was the
Design objeclives included a series of h. h Y :sacuve channel configuration,
visually attractive and varied slope and I~. .roug s steep chutes to provide a
with base flows. Due 10 dj~ panicular =a:: ~=ighborh~ waler ~~~iIY even
channel relocalion, coupled with the inhe .the associated sensnlVny or this
a five.slep engineering process was ad ~~t :nce~mty of.th~ ~~ghly lurbu~enl now,
rough construction. field measwemen:" final ble: I~I~~. IDllIal hydraulIC design,
modifICations. Water-surface rofiie so y au Ie eslgn, and final construction
appro~imately t.Ocms. Final oo":truction ;~~:~ ::;:'ledthal,O' aob~ow: of
were met Ie jectlves