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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than D.S feet. The variation in bank height between the inside and outside
of the bends is also not large because the sinuosity is low.
Iolfdth_to_DepthRatio.lf1t1sassumedthatthebankfulldepth
(average distance from bankfull level to thalweg level) is 2 fe~t greater
than the depths given in Table 10, then the ratio of bankfull wldth to
bankfull depth 1s approximate1y 30 to 40 in the reaches in which the,river
is still somewhat free to move laterally. In the Urban reach the ratlO
decreases to 12 due to the narrowing of the channel and degradation of
the riverbed.
Bends. In a normal a"uvfal rfverbend. the flow erodes theoutsfde
of the bend and deposits sediment on the inside forming a point bar, part
of which may be vegetated. The outside bank is known as the concave bank;
the inside is the convex bank. In the study reach of the South Platte
River most unprotected concave banks are eroding.
An unusual bend is shown in Figure 12. This bend 1s located ;n the
downstream Rural reach at River Station 397 ~ 00. Here there are new
deposits on the outside of the bend as well as on the point bar. The
radfus of curvature is 800 feet and the deflectlon angle is 71 degrees.
A study was made of all undeformed bends fn the study reach to
deternline which bends had point bars and an eroding or protected concave
bank. The results are presented in Figure 13. Those bends that have
deflectfon angles and radii of curvatfon designated by the solid circ~es
have either normal point bars or no bars at all. The other bendS des1g-
nated by open circles have middle bars, bars on the concave bank or very
asymmetrical point bars,
The region outlined by the closed curve in Figure 13 contains only
bends for whiCh there are symmetrical point bars or no bars at all. Most
of the bends with deflection angles smaller than 50 degrees are in the U~ban
reach. Some have this same planview geometry but there.are also bends.w1th
other types of bars. The bends with very large deflect10n angles are 1n the
Rural and Suburban reaches.

8y 1953, the bend upstream had m1grated down-river and two new
bends. one upstream and the other downstream, had started to form.
Downstream from Henderson Road. the river had become nearly strai9ht with
two long straight sections and half as many bends as in 1937. In the lower
end, a long cutoff was made, probably artlficfally.
By 1983, the prominent bend upstream from Henderson Road had moved
downstream 1000 feet from its positfon fn 1937. The other two newer bends
had grown in amplitude since 1953. Also, three new bends are being created.
Downstream, new bends are being formed and others had enlarged since 1953.
Only the cutoff remained unchanged. Overall, the rlver reach is more
sinuous in 1983 than in 1953.
In 1963, there were no gravel pits in the Henderson Road reach. Now,
in 1983. the gravel mining area fs extensfve downstream from the road. The
largest pit is between the cutoff and State Highway 85.
The outline of the traces of r1vermeanderswhichare stlll visible
on the aerial photographs are also shown on Figure 14. These scars mark the
fonnerlocationsoftherfverChannel,priorto1937. In its pristine
environment, the river meandered from valley wall to valley wall. Between
1937 and 19B3, the right bank (looking downstream) moved 1800 feet laterally
at Henderson Island. Overall, it can be said that the Henderson Road reach
has behaved as an unstable river, first straightening itself and then
creating new bends. However, since 1937 it has retained its slightly
sinuous thalweg channel.

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Bars and Veqetation
Gravel and sand bars form in the South Platte River at regular
intervals in some reaches and at irregular intervals in others. Once a
bar grows to some threshold height above the low-water level, it becomes
colonized wfth vegetation. The pioneer species are willow. cottonwood. and
some types of grasses. If the vegetation causes the deposition of more
sediment on the bar, the bar will grow fn height to the level of the flood-
plain, and over a period of time, the willows die out leaving only the
pioneer cottonwoods and any new species which can survive in this secondary
stage of vegetative development. In general. most of the side bars (those
attached to either bank) have some vegetation, usually next to the bank.
Often middle bars are barren.
In April 1983, there were 265 bars exposed on the bed of the South
Platte River from Chatfield Dam to Baseline Road immediately north of
Brighton. On the average, that is slightly ITl.Ire than 6 bars per mile. Only
17 percent were covered with vegetation.
The bars are not distributed equally in all reaches. The Rural
and Suburban reaches immediately downstream from Chatfield Dam had, respec-
tively,10and9barspermi1e. In the Urban reach there are only four bars
per mile on the average and sli9htly more than SO percent of these are
completely vegetated. However, the bars in the Urban reach are much

River Alignment
In the study reach, the South Platte River channel moves laterally
where it is free to do so. It migrates by eroding the c~ncave banks on fts
bends and by depositing sediment on the convex banks. M1g~ation is arrested
by protecting the concave banks against erosion, usually w1th concrete and
asphalt rubble.
Shown in Figure 14, the reach of Rural river at Henderson Road
(River Station 357 + SO) is an example of how the South Platte channel has
Jroved in the past. In 1937, the channel was very straight. upstream from
Henderson Road; there being only one prominent bend in a d1stance of 1.5
miles. Downstream, there were 12 bends in 3 miles. The first four bends
are in the reach known as Henderson Island.

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