Introduction

Introduction

Stoneman Lake is one of 2 natural lakes in Arizona, and in 1998 was placed on Arizona’s 305(b) report for impaired waters due to excessive levels of nutrients, dissolved oxygen and pH¹. In 2000, Malcolm Pirnie and the Arizona Department of Environmental Quality (ADEQ) performed a Total Maximum Daily Load (TMDL) analysis and proposed a few solutions to the problem¹. One of their proposals included reopening an existing ditch, originally built by the Civilian Conservation Corps (CCC), that would contribute flow from an adjacent watershed; the effect being to dilute the concentration of pollutants, allowing nutrient and dissolved oxygen levels to be within compliance.

The Stoneman Lake Property Owners Association (SLPOA) is in favor of the suggestion to reopen the ditch, because the lake is currently dry (in January of 1982, the U.S. Forest Service installed 4 diversion structures to prevent inflow to Stoneman Lake due to flooding concerns). Increasing the water yield to the lake has many advantages: it will provide habitat to endangered species, restore recreational activities (fishing), and most likely increase lake property values when full. In addition, the US Forest Service owns half of the Stoneman Lake watershed. The Forest Service has agreed to let the homeowners proceed with the plan to reopen the ditch, but will not assume any liability for control of the inflow. The homeowners are responsible for placing a control structure at the outlet of the ditch in order to control runoff.

The SLPOA has some concerns about reopening the ditch, and a few questions remain:

How will the additional watershed runoff contribute to lake elevation?
Will a 50-year or 100-year precipitation event flood homeowner property (when the ditch is open)?
What is the carrying capacity of the ditch (necessary for design of the control structure)?

The homeowners asked me to predict the 50-year and 100-year event storm ramifications, as well as calculate the maximum flow rate for the ditch. In addition, I have also modeled the 2-year storm. The storms were modeled using the Army Corps of Engineers Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS)², while ditch flow was calculated using the Army Corps of Engineers Hydrologic Engineering Center-River Analysis System (HEC-RAS)³.