FLOOD03181

• County: Weld
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Master Floodway Plan
• Date: 8/1/1983
• Prepared For: Greeley, CWCB
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

I

18

I

19

I

IV. EROSION AND SEDIMENTATION ANALYSIS

Qualitative Geomorphic Analysis

A qualitatiye geomorphic analysis was conducted to determine the general

characteristics of the Cache la Poudre River system. A river can be aggrading

(depositing sediment) degrading (eroding away sediment) or at equilibrium

depending on sediment particle size and flow rate. The long-term response of

the river system must be known when designing improvements to the river chan-

nel because channel capacity is gradually reduced in an aggrading system and

erosion caused by a degrading system can undercut structures and channel

banks. River channels can migrate laterally as well as aggrade or degrade.

Lateral migration is generally caused at river bends by eroding the outside of

the bend and depositing material on the inside of the bend.

The underlying strata of the Cache la Poudre River valley consists of

alluvial deposits. The age of the deposits ranges from modern alluvium to

older gravel deposits laid down during periods of glacier melting in

Pleistocene time. The gravel deposits have been commercially mined near the

study area. The depth of gravel deposits exceeds 30 feet in some locations.

Underlying the alluvial deposits are the Laramie formation and Fox Hills

Sandstone. The Laramie formation consists of shale, claystone, sandstone and

some coal. No major bedrock outcrops were identified in the study area.

A field investigation of the study area was conducted. Sediment samples

were collected from channel banks and from sand bar deposits. The average

size of materials sampled ranged from three to ten millimeters (0.12 to 0.39

inches). The largest material present is in the two- to three-inch diameter

size range. The existing slope of channel banks is fairly steep, 1 horizontal

to 1 yertical (1h:1v). The river has been straightened and channelized within

the study area. Many sand bars are seen in the vicinity of bridges.

Aerial photographs and maps were examined to identify general charac-

teristics of the river system; much evidence of channel migration is seen in

aerial photographs. The corporate boundary of the City of Greeley was

established as the centerline of the Cache la Poudre River and the fact that

the river channel has migrated in Some locations is clearly seen (see

Figure 1).

The relationship of river length to valley length is termed sinuosity and

can indicate the tendency of a channel to migrate. The sinuosity of the river

in the study reach is about 1.1 indicating a fairly straight river. However,

I

the river has a sinuosity of about 1.4 upstream and downstream of the study

area. The sinuosity within the study area was probably very close to 1.4

prior to channelization. Since rivers tend to revert to their natural state,

lateral migration must be considered in any improvement alternative.

Rivers are dynamic systems in that sediment is eroded at some location

and deposited at some downstream location. The rate at which materials are

transported is dependent upon many factors such as flow velocity, size of

transported materials, channel slope and geometry, and depth of flow among

others. If the river is transporting less material than its capacity, erosion

of the river bed or banks will occur. Conversely, if the transport capacity

is exceeded, materials will be deposited. The sediment transport capacity of

the Cache la Poudre River in the study area was compared to the sediment ca-

pacity immediately upstream of the City. The results of this analysis indi-

cate that the river is capable of transporting more sediment upstream of the

City than in the study reach. This will result in deposition of sediment in

the study reach. This tendency for aggradation is confirmed by the fact that

the City is required to dredge the river channel periodically. Approximately

2,500 to 3,000 tons of material is dredged annually. A summary of the City's

dredging activities is given in Table 3.

Erosion of bank materials occurs in the study area. The City places

approximately 500 tons of riprap annually to control bank erosion. Bank ero-

sion generally begins at the toe of the slope and undercuts the bank resulting

in bank sloughing. This process can repeat itself during high-flow periods

and result in lateral migration.

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

Bridge Scour

Scour at bridges can undermine structure supports and cause bridge

collapse. Scouring can occur locally at piers or abutments or general scour

can occur across the entire width of the bridge. Local scour at piers or

abutments is caused by vortex action as the flow passes around the pier and

forms eddies on the downstream face. General scour is caused by a reduction

in flow area at a bridge constriction which causes an increase in flow

velocity.

The velocities calculated for the

I

I

I

I

I

I

bridges in the Greeley reach of the
i
Cache la Poudre River range from three feet per second (fps) at 8th Street to
eight fps at 11th Avenue. Scour depths are estimated to reach four to five

I

I

I

I