Drainage Planning · Hydrologic Modeling · Alternatives Analysis · Stormwater Design · Cost Estimating

JR Engineering prepared an Outfall Systems Plan (OSP) for the previously unstudied watershed located between the St. Charles River and Six Mile Creek, including the Town of Vineland, in Pueblo County, Colorado.  The 12.5 square mile basin is predominantly agricultural, but certain areas in the watershed are prone to flooding, and there are numerous instances of poor stormwater drainage.  Pueblo County sought to analyze the hydrology and prepare conceptual design solutions to manage stormwater runoff, with conceptual costs.

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Drainage Assessment & Problem Areas

JR Engineering met with the County and inspected areas prone to flooding and poor drainage.  JR prepared a preliminary Problem Area Assessment with mapping and photo logs and worked with the County to prioritize these problem areas with the intent of solving the drainage issues at the most critical of them with the conceptual design.

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Watershed Hydrologic Analysis

JR Engineering prepared a hydrologic analysis for both the natural pre-developed condition and the existing/future condition.  The natural conditions hydrology was prepared to understand the peak discharges for the design storms at various locations as a basis for comparison for stormwater detention options.  The existing conditions analysis took into account the hydrologic routing created by the roadway grid with roadside ditches and culverts.  Since there are no current plans for development in the watershed, the existing conditions analysis was termed ‘existing/future conditions’.  JR Engineering utilized a 2-hour rainfall distribution with NOAA Atlas 14-point precipitation depths for the area.  Despite the size of the watershed, no Depth Area Reduction Factors (DARF) were used to provide a conservative estimate of peak-routed discharges.

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Alternatives Analysis & Recommendations

The goals of any improvements were to reduce discharges at U.S. Highway 50 Business, create positive drainage outfalls to the river, and maintain existing irrigation ditch infrastructure.  JR Engineering then developed several concepts for handling runoff in the basin, starting with traditional options such as open channels, detention ponds, and retention ponds.  With County input, conceptual pond locations were selected and evaluated.  The required pond volumes and channel dimensions to achieve any measurable benefit were found to be too large and costly for further consideration.  Nine options were evaluated.  With County input, JR Engineering began looking at infiltration options.  A Multi-Criteria Decision Analysis (MDCA) Matrix was prepared to evaluate conceptual alternatives with weights assigned to many parameters.  JR Engineering evaluated four additional infiltration options using varying lengths of “exfiltration trench” in the public roadway as a means of peak attenuation.​​

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JR Engineering prepared hydrologic modeling and developed conceptual cost estimates for each alternative.  A recommended alternative was selected from the available options and considering the MCDA scores, which used an exfiltration trench to significantly reduce routed peak discharges and achieve the project goals.

A future analysis, the Outfall Systems Plan Phase B, will provide preliminary design drawings and phasing.