NCI performed a supplemental design study for Llagas Creek under the Upper Llagas Creek Flood Control Project. The scope included a field reconnaissance investigation and bed material sampling; stable low-flow channel design; sediment budget analysis; evaluation of the diversion channel initial conditions; and floodplain mapping. The effective discharge approach was applied to the low-flow channel design for various reaches. The effective discharge was determined using the bed material load histogram method, with the flow duration curves and the sediment transport rating curves being computed with SAM. The stable channel design module in SAM was used to size the stable low-flow channel for the sub-reaches where stable channel may potentially exist. Other sub-reaches either remained unchanged due to project constraints or environmental considerations, or were re-designed to be more consistent with adjacent low-flow channels. The sediment budget was computed using SAM based on the updated channel design. The floodplains were determined using HEC-RAS, and were mapped using GIS and HEC-GeoRAS for different flood events.

A potential sedimentation concern was identified for the preliminary diversion channel design. Several mitigation options were proposed, including re-aligning the diversion channel, and/or re-grading the diversion channel and adjacent reaches. These mitigation measures were investigated and the optimal option was recommended.