FLOOD02337

• County: Statewide
• Community: State
• Stream Name: South Platte River
• Basin: South Platte
• Title: Geomorphic Assessment at Surveyed Cross-sections South Platte River
• Date: 6/1/1996
• Prepared For: UDFCD
• Prepared By: Michael Stevens
• Floodplain - Doc Type: Floodplain Report/Masterplan

o
o

5

10

15

20 25 30 35 40
Cross Section Number

45

50

55

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

The monitored South Platte River is 41 miles long. Most of the cross sections are in the Rural
( 11 miles) and Suburban (8 miles) reaches where most of the instability is anticipated to occur.
The Urban reach (17 miles) is most stable.

Width. The main geomorphic feature that illustrates the different South Platte River fonns is
river width.. The width is defined as the distance across the river between top of banks. For the
South Platte, this is easier than determining the bottom width. Shown in Fig. I. the top widths
vary greatly, being largest, up to 550 ft near Brighton. The Urban and Engineered reaches are
between 100 and 200 ft wide, with two exceptions. A short piece of river near the Chatfield
Dam is Rural, and is approximately 350 feet wide. The Suburban reach, with "intennediate"
widths, has become narrower since 1983. A long reach of Suburban river through Littleton was
converted to Engineered river in the late 1980's by the U.S. Corps of Engineers.

100

~Lburba~ +T ! I I I I
Urban
. 1 T1 II
, Engineered _ t- Rural
,
Suburban
.
, ~
j \ T' T
J i J l~ +- fi
..n, r\ /\ I
-i .. V :v -\JI.
- Rural ~ /
..
--.-

600

500

400

...
-

-5 300
'0
~

200

Figure 1.

The top bank width of the South Platte River at the monitoring cross sections
between Brighton (Cross Section I) and Highway C-470 (Cross Section 53).

Out of economic necessity, weirs and some bridges do not extend over the entire width of the
river. The narrower bridges and weirs affect the river downstream and sometimes upstream,
confining all waters to one. channel. This is called the bridge or weir effect. Often. the
monitoring cross sections are placed just up or downstream from bridges to get an early warning
of degradation that uncovers steel bridge piles, leaving them prone to corrosion. Cross Sections
I (State Highway 7) and 18 (Fulton Ditch diversion weir) are examples. The trend is that newer
bridges are longer. Riprapped grade-control structures extend across the active river bed and
do not narrow the low-flow zone.

4