FLOOD06925

• County: Larimer
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Master Floodway Planning
• Date: 3/11/1983
• Prepared For: Greeley
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydraulic, Erosion and Sedimentation Analysis of Indian School Road Bridge

~ the ~ Fria River, Phoenix, Arizona

SLA conducted a study for Maricopa County Flood Control District to

determine the causes of failure of the Indian School Road Bridge during the

February 20, 1980 flood on the Agua Fria River and to suggest stability

measures to prevent a recurrence of failure. The bridge is located on the

Agua Fria River approximately 20 miles northwest of Phoenix. During the

February 20, 1980 flood event, several of the bridge piers were undermined,

causing the failure of the bridge. Downstream of the Indian School Road

Bridge (ISRD), extensive flood plain gravel mining has occurred. Levees were

constructed to protect the flood plain gravel pits; however, the levees have

severely encroached the flow area in the Agua Fria directly downstream of the

bridge extending to the Roosevelt Irrigation District (RID) flume crossing

approximately 2,200 feet downstream of the ISRB.

The three-level analysis applied to the ISRB failure included: (1) a

qualitative geomorphic, (2) a quantitative engineering geomorphic and (3) an

application of a mathematical model to evaluate the potential local scour,

general regional scour, and potential aggradation/degradation at the ISRB and

the RID flume crossing. The results of the three-level analysis were compared

for river conditions as they existed previous to the constriction in the Agua

Fria due to gravel mining and as they exist with the levees and were used as

evidence in a litigation suit involving the gravel mining companies.

A three-level analysis was also utilized to evaluate the conditions as

they exist in 1982 and to compare the 1982 conditions with the protective

measures recommended by SLA. Protective measures suggested by SLA included:

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1. Channelization of approximately 7,000 feet.

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2.

Tranverse guide banks and a spur dike upstream of ISRB to align the
flows through the bridge.

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3.

Riprap protection of the bridge piers.

4. Burial depths of failed piers.

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5.

Increase in the opening of the Agua Fria at the RID flume crossing.

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6.

Protection of the piers at the RID flume crossing.

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Construction of a grade control structure directly downstream of the
RID flume crossing.

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