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

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

are stratified. The bank that does not erode is either away from the fast current or protected by
riprap or concrete and asphalt rubble.

In the period of record, the 106 banks at the 53 monitoring cross sections have for the most part
been stable. Of the nine eroding banks in 1996, six are in the north Rural reach and three are
in the north Suburban reach (Table 4). Also, six are gravel banks and the other three sandy silt.

Most of the Urban reach is completely protected by riprap or rubble. A lot of this aged
protection is no longer visible, having been covered up by soil and vegetation. No effort was
made to uncover the materials in these banks to identify them. None are eroding at this time at
the monitored cross sections, but there are several small areas of bank erosion between sections.
The scalloping of the left bank upstream from Cross Section 34 is an example (see Appendix A
for photograph).

All the banks of the cross sections in the Engineered reach are riprapped and have withstood the
flows with no apparent damage. These banks are undergoing the first stages of aging, with
willows coming up at the base of the protection and grass higher up. These banks are stable.

In the Rural reach, two monitored banks have riprap and seven have rubble protection. In the
Suburban reach, the numbers are seven and eight respectively. Two banks were eroding at the
base but not on top. Seven banks, two in the Rural, three in the Suburban stretch, had been
eroding, but by 1996, these had been stabilized with riprap.

The most severe erosion of the banks is not at the monitored cross sections but in between, in
the Rural reach. One example is the west bank just upstream from Cross Section 6 (see
Appendix A, Photo 6). Another is the west bank between Cross Sections 12 and 13 (see
Appendix B, Photo 14). Other banks are eroding adjacent to riprapped sections or hardpoints
(see Appendix B, Photos 7, 10, 13, 16,25).

Most banks have some type of vegetation growing on the them. Grasses do well except where
the bank material is very dry - only weeds colonize these. Trees can become established in
riprap to about mid-bank height. Older banks have trees up higher. Willows first appear at the
base of the banks. Grass, willows, and weeds are found on the riprapped and rubble banks. The
distinguishing factor for trees is their linearity. Trees grow in a row along the base of many
banks. Here, such growth is labelled Linear Vegetation.

GRADE-CONTROL STRUCTURES

The bed of the South Platte River through the Denver metropolitan area is punctuated with
structures which modify the smooth profile from that previously made by nature. The profile
now has ponds and drops. The structures include on.e fabric dam, and a number of weirs and
concrete encasements of utility lines. The functions vary and include diverting water out of the
river, arresting the downcutting of the thalweg, and protecting infrastructure. Their inadvertent
effect is to change the riverbed profile. In addition, the Urban Drainage and Flood Control
District plans and places grade-control structures across the river bed to arrest and prevent
degradation. Most of these are constructed with riprap obtained from quarries. A listing of the

11