DRILL PAD PRE- AND POST-DEVELOPMENT STORMWATER RUNOFF ANALYSIS
Diversified Engineering, Inc.
New Philadelphia, Ohio
DE was retained by a consulting firm to complete a series of pre- and post-development stormwater runoff analyses for the construction of nearly thirty drill pads used to recover oil and natural gas reserves in the Utica Shale formation in eastern Ohio.
DE’s stormwater runoff analyses were intended to demonstrate that the peak flows and velocities from the drill pads as designed by the consultant would not exceed the previously existing conditions. Often, initial results indicated the post-development flows and velocities were too high. In these cases, DE collaborated with the prime engineer to evaluate options and make changes in the design until the analysis produced a favorable result.
Upon receipt of the drill pad design, the drainage shed (or sheds) affected by the limit of disturbance were determined and the nearest downstream study points were identified. Modeling of the hydrology and hydraulics for the pre- and post-development conditions was completed using Hydraflow Hydrographs Extension for AutoCAD.
The Soil Conservation Service (SCS) Method and Technical Release (TR)-55: Urban Hydrology for Small Watersheds were used to calculate input variables for each sub-watershed in Hydraflow including, the watershed acreage, curve number or runoff coefficient, the storm distribution, and time of concentration. The sub-watersheds and other structures, such as detention ponds, scour holes with level spreader discharges, and defined channels both natural and designed swales, were then added to the model in Hydraflow. Model results from the shared study points in the pre-development model were compared to those in the post-development model to confirm neither peak discharges nor velocities from a 10-year, 24-hour frequency storm event are increased as a result of drill pad construction.