Abstract

In Le Sueur County, Minnesota, the hydrology of a gravel inlet system was explored using curves of water height, surface velocity, rainfall rate for major rainfall events in 2002. The stage-discharge relationship of the gravel inlet was presented based on water height and discharge rate measured in the field. Three types of flow were identified in the gravel inlet system: ponded, edge, and sidewall. The first type of flow dominated in most of rainfall events. This location had a surface inlet system before being converted to a gravel inlet system. The difference of flow capacity between the two systems was evaluated with average flow rate from two similar events in June of 1998 and June of 2002. The flow system of the surrounding area influenced the flow capacity more than the equipment field setup for either surface or gravel inlet by generating back-pressure in the outlet pipe. Contaminant loadings were examined by monitoring their above and below gravel concentrations. The gravel inlet reduced to some extent the concentrations of total solids, chemical oxygen demand, particulate phosphorus, and total phosphorus. Trapping efficiency of the gravel inlet ranged from 14% to 32% with respect to previously listed contaminants. Concentration of soluble contaminants such as soluble phosphorus and nitrate increased over the duration of each rainfall event. Ponding of water in the depression played a major role in releasing soluble phosphorus via oxygen depletion and decreased redox potential. Nitrate concentration increased significantly toward the end of one rainfall event. An estimation of gravel longevity was attempted based on the assumed uniform deposition of total solids over the void volume in the gravel trench and on the quantity of total solids deposited. The gravel matrix was filled with sediment at 28% compared to its initial void volume following the rainfall events of summer 2002.

Keywords

gravel inlet, blind inlet, surface inlet, flow types, back-pressure, pollutant loading, soluble and particulate contaminant, deposition, trapping efficiency