FLOOD03848

• County: Denver
• County: Jefferson
• Community: Denver County
• Stream Name: South Platte River
• Basin: South Platte
• Title: Stream Stability Investigation South Platte River
• Date: 11/1/1983
• Prepared For: UDFCD Denver
• Prepared By: Michael Stevens
• Floodplain - Doc Type: Floodplain Report/Masterplan

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For a design discharge of 33,000 c~b1c feet per second (all in
the channel). the size of bed material needed to reduce the
bed-load discharge to zero 1n the existing channel is computed as
follows:

Hintnq

Gravel mining in the South Platte River 1n Metropolitan Denver,
especially in the downstream reach, 15 the source of much of the instability
fn the river. Hiners move the river channel sideways, make cutoffs,
excavate fn the bed of the channel and make artificial rlver banks of waste
material. The new alignments afe not 1n harmony with those needed by the
river to remain stable. If the channel 15 made straight. tile flow creates
alter~te bars around which the low-flow channel meanders. When cutoffs
are made, stability problems usually arise downstream. New meanders may
fonn or existing ones are eroded severely. When the riverbed is excavated,
the bed load of the stream is deposited in the mined area, causing add1.
tional degradation, mostly downstream. Similarly, if the river cuts into a
mined area, its bed load is deposited in the pit. In fact, a pit can be as
effective a gravel trap as the upstream flood control reservoirs. Gravel
pits would not trap much of the silt and clay load.
Uncontrolled exploitation of the gravel in the South Platte River
Valley will create continued problems with river stability. Vet, the
gravel is an fmportant resource. Gravel pftsclose to the market are
more valuable as trucking costs are an important component of the value of
the gravel. Improvementstothecurrents1tuationaresuggestedinthe
section on recommendations.

Zero Bed.load Transport
Present Unit Slope Between Required Size
Width Di~charge Drops of Bed Material
Reach ft ft Islft ft/mi '".
Upstream Rural 150 206 7.3 3.8
Upstream Suburban 150 206 12.0 5.6
Urban 120 275 7.4 4.7
Downstream
Suburban 250 132 6.7 2.6
Downstream Rural 250 132 6.8 2.7

Because the flow in the South Platte River is nefther straight
nor one-dimensional, the size of bed~aterial necessary to prevent
degradation 1s larger than that calculated above.
3. If the channel width is decreased from 250 feet to 120 feet, the
sizes of the ~~terials required for tile armor coat increases by a
factor of approxilr'.ately two.
4. The possibility exists to provide the arrmr coat for the river with~
out allowing degradation.
The amount of degradation z required to armor the bed with particles
of size II. is

2 p
z" '3 n f) {1

C
.njk

(6)

Here p " fraction of the bed covered by particles of size II. or larger
f " fraction (by weight) of the initial bed material whiCh is finer
tllansizek
C ~ ratio of the weight of a particle size II. to that of a spherical
particle of the same material and diameter
n . porosity of the bed material.
With very coarse gravel such as Sample 9 in Figure 10, the degrada.
tion required to armor the entire bed with one layer of particles 100
millimeters or larger in diameter is approximately 2 feet.