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

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SUMMARY OF FINDINGS

Since monitoring of the South Platte River in the Denver metropolitan area began in 1986, the
river has remained stable for the most part, not degrading or aggrading its bed significantly, nor
eroding its banks but at a few locations. The stability has been achieved in part by new grade-
control structures placed across the river and by protecting some more banks with riprap or
dumped concrete and asphalt rubble. The river has been able to supply itself with new sediment
from eroding banks, and the new hydrologic regime established with the flood-control dams is
less prone to cause erosion. The anticipated degradation has not yet appeared in the downstream
reaches of the Metro South Platte River.

In general, the river corridor is much more pleasant now than nine years ago, and much more
accessible. With the exception of a small segment between 50th Avenue and Franklin Street,
there is a bike path running continuously from a mile north of 88th Avenue to Highway C-470
near the Chatfield Darn. In places, the path is on both sides of the river. Parks, rest areas, and
planted trees are more numerous. There appears to be many more beavers.

As a result of the prolonged spring runoff in 1995, the river bed in the Urban reach between
Interstate 270 and 31st Street degraded approximately one foot Ten other cross sections spread
about the monitored South Platte experienced degradation since 1986, the most being 2.7 feet
Aggradation of approximately two feet has appeared over a period of nine years at four cross
sections. Three of these are between 104th Avenue and Interstate 270, a reach affected by in-
river construction during the 1995 spring flood season. A significant amount of erosion occurred
upstream from 88th Avenue because the construction project was flooded for a period exceeding
three months.

In 1996, there are only nine eroding banks in a total of 106 banks being monitored. Six of these
are in the north Rural reach between 1 04th Avenue and Brighton and three in the north Suburban
reach between Interstate 270 and 104th Avenue. The most serious bank erosion is in between
monitored cross sections in the north Rural reach and can be seen on aerial photographs taken
in the winter of 1995-96.

A review of the 1984 Major Drainageway Planning, South Platte River Report (Master Plan) is
in order and changes should be made to accommodate the existing infrastructure as well as the
needs of the river. The findings on river morphology changes presented in this report are based
on observations of the river between 1983 and 1996, and the annual survey, starting in 1986, of
53 marked cross sections between Baseline Road and Chatfield Dam. These same cross sections
provide part of the much needed data for the review. The ideals set forth for river morphology
in 1983 are still considered valid, but changes may be in order to create a river with different
environmental qualities. Also, the review should reevaluate the effect that grade-control
structures have had on the river's vertical and horizontal stability.