Horizon Six Mohave County Flood Control Infrastructure Design

Mohave County, Arizona

Services Provided:  Civil Engineering, Land Planning, Storm Water Management, Landscape Architecture

Project Description: The Horizon Six is a 320 acre mature residential development located in Mohave County and is contiguous to the Lake Havasu City Limits. No provisions were made to control or channelize the existing Mockingbird Wash that bisects the site from the northeast to the southwest.

Uncontrolled grading activities have occurred along the wash by adjacent homeowners, which has consequently caused adverse flooding impacts. As a result, Mohave County has embarked on a program to construct drainage improvements throughout the development. CVL has been assisting Mohave County by providing recommendations for desirable drainage improvements after completing a comprehensive existing conditions analysis that included hydrology, hydraulics, and sediment transport. CVL used the HEC-1 model to determine design event flows from a 3.2 square mile watershed. CVL also used the FLO-2D model to define the floodplain throughout the development. Based on this analysis, two (2) projects have emerged from this effort, which consists of an upstream detention basin and an engineered channel.

The detention basin was proposed to attenuate design event flows before entering into the development. This recommended concept took shape, because CVL was able to demonstrate two (2) benefits: (1) a significant reduction in the engineered channel cross section and culverts for five (5) road crossings, which (2) led to a more convenient approach to acquiring Right-of Way property for the engineered channel. The detention basin consists of a cascading basin system with three main components: (1) inline basins/sediment traps, (2) offline basins, and (3) and an inline channel, which was modeled using HEC-RAS unsteady flow models. CVL selected the offline strategy to increase the attenuation benefit while reducing the area needed for the basin facility. As a result, CVL was able to reduce the upstream design flows by approximately 50% consequently reducing the material costs of the channel. The selection of the offline basin approach and the reduced engineered channel cross section resulted in a savings of approximately $2 million.

Other features of the project include CLOMR/LOMR submittals to FEMA and preliminary jurisdictional determination submittals to the USACOE for construction permitting.