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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49

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The experience gained from the Bureau of Reclamation's studies of
degradation downstream from Glen Canyon Dam demonstrates that only a few
gravel or cobble bars are needed to stabilize lOng reaches of degrading
rherbed. In this case, there were 10 such COntrols in 15 miles. COinci-
dentally, there are 15 crossings in the 15-mile reach. On the basis of this
knowledge, the following progra~ for the development of the South Platte
River 1n the uncontrolled reaches of the Denver Metropolitan area is
recorrmended.

In the existing river system. all bends within the above ranges
of values have either point bars or no bars at all. lnather
ranges, the bars are not at the appropriate location 1n some of
the bends. That 1s, there is some combination of point bars,
middle bars, side bars on the outside of the bend or alter-
nating bars 1n many of the bends.
d. The length of the crossing bet~een bends should not be more than
700 feet. OtherwIse, unwanted bars may form in the crossing.
Long crossings are suitable downstream of long.radius bends and
short crossings downstream from sharp bends.

Proqram for Development
Without any constraints due to political or economic concerns, one
can postulate an ideal development program for the riverine environment
of the South Platte River t~rough Metropolitan Denver. Then any criticism
of the program can be accoll"lOOdated as a compromise of the ideal. In this
manner, an optimum program should result--one that obliges the desires and
the resources of the people within the framewor~ of what nature wants to do
with the river. It is most desirable to have the needs of the people and
the forces of nature aligned to produce the new river at the least possible
cost.

e. The meander length to channel width ratio should be in the range
of 7 to 15 (see Figure 15). Two consecutive bends make one
meander length.

J. Using groins, di~es, or vanes, direct the river to excavate the
desired alignment with its own forces. Man-made excavations will be
required in some places to accelerate the movement of the river.
4. Stabilize the outside of the bends with a suitable construction mate -
rial or with vegetation. The Corps of Engineers (1981) has conducted
a national field program to identify materials and vegetation which
protect the ban~line from erosion ~ enhance the terrestrial and
aquatic environment. Their publications contain the contract drawing
showing typical sections for various types of protection. The insides
of the bends will be in an dre<l tending towards deposition and not
erosion. Therefore, no protection is required on the insides of
bends. The banks on some crossings Il'(IY need some protection.

In sequential order, this proposed program for the uncontrolled
reaches is as follows.
1. Choose the desired bed width for the river channel. Presently,the
average width 1s 160 feet in the upstream portion near Chatfield Dam
and 250 feet downstream from Sand Creek. A suitable choice is 200
feet for both reaches. Within this width, gravel bars will form and
become sparsely vegetated and the depth in the low-flow channel will
vary somewhat from the pools in the bends to the shallows between
bends.
2. Fita sinuous alignment (see Figure 15) to the rfver to follow the
existing alignment as closely as possible and yet be within these
constraints:

5. Ifleveesareneeded,theycanbeconstructedafterthealignmenthas
beenobt<lined.ltshouldbeplannedthatmostofthefloodplain
inside the levees will become vegetated in the future. Material
excav<lted from the river to improve its aligment can be used to con-
struct the levees.
6. Allow the riverbed to degrade down to the desired level. Degrada-
tion norm<l11y occurs upstream first and then moves downstream.
Consequently, in any long reach, the degradation process will be ter-
minated initially at the upstream end.
7. Monitor the level of the crossings after each flood to determine the
status of the degradation process. Sample the armor coat which is
forming naturally and determine its size, density, and durability.
8. When degradation reaches the desired level in any sUbreach, stabilize
those crossings which control the bed profile of the river and which
have not developed a suitably armored bed yet. This will be done by
covering the bars and bed on the crossings with 8 to 16 inches of
4- to 8-inch di<lmeter cobbles. The cobbles will have a natural
appearance in the riverine environment and therefore will not be
out of place. The areal extent of the cobble layer should be deter.
mined by studying the local slope in the field. A typical case is
shown in Figure 15.

a. The deflection angles should be in ther<lnge between 55 and 75
degrees.
b. Theradifofcurvatureshouldnotbelessthan400feetor
gre<lter than 1500 feet.
c. longer radius bends can deflect mo~ than short radius bf'nds. A
suitable relation is

p

R ~ 50
'"

in which 6 ~ deflection angle, degrees

R c radius of curvature, feet.

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