Shafer, Kline & Warren (McClure Engineering)

2018

Western Cass County, Missouri’s, Village of Loch Lloyd, was concerned that increasing sedimentation would interfere with residents’ enjoyment of the 110-acre reservoir on a tributary of the Blue River. Prime contractor McClure Engineering contracted Water Resources Solutions (WRS) to, first, study the degree and patterns of sedimentation, and then to study the feasibility of alternative solutions.

WRS conducted statistical analysis of the watershed’s two sub-basins using HEC-HMS to develop discharge-frequency relationships, flow duration characteristics and volume-probability relationships for various hydrologic conditions including regulated and unregulated conditions and mixed population analysis Monte Carlo simulation. USDA-NRCS Technical Release 55 (TR-55) modeling calculated storm runoff volume, peak rate of discharge, hydrographs, and storage volumes required for floodwater reservoir.

WRS sampled the lake sediment in three locations to make a qualitative assessment of the sediment composition. Additional data collection included watershed information, precipitation information, runoff modeling resulting from precipitation events, suspended sediment data collection and analysis after each storm, bathymetric surveys to determine total sediment load, analysis to calibrate sediment load to precipitation

Water samples were collected from the tributary during rain events for one year and analyzed for sediment. This analysis used the sampling data, along with watershed information including topographic, soils and land-use information, to determine the amount of sediment entering the lake and to develop a prediction tool that could be used by Loch Lloyd to estimate the quantity of sediment entering the lake.

Based on the water quality data collected, WRS made revisions to calibrate the existing watershed sedimentation model using the collected suspended sediment data. The resulting sediment yield model of the MUSLE (modified universal soil loss equation) analysis provided an average concentration of sediment entering the lake. The concentration was converted to average weight and volume of sediment entering the lake based on the volume of runoff entering the lake. A total of 19 precipitation events were sampled and tested.

Bathymetric survey data was collected in September 2016 and September 2017. Those surveys were analyzed and compared to measure the increased sediment into the lake. This total volume of sediment calculation was used to determine the amount of bed material load entering the lake. The sediment load calibration was made by comparing the average sediment weight and volume to the total rainfall of each rainfall event.

WRS used the model to create an original web-based computer application written and compiled in Python to permit laymen to estimate sediment load and water quality implications based on rainfall events. The modeling was also used to guide three concept-level proposed alternative solutions to address sedimentation, including:

• A sediment forebay upstream of the south cove of Loch Lloyd lake

• Underwater jetties in the south cove of the lake

• Dredging maintenance around the bank.