Category Archives: Updates

Peak Discharge Under Different Rain Events

The peak discharge of the watershed area under 2 years, 10 years, 25 years, 50 years and 100 years rain were simulated in the HEC-HMS program, using NOAA’s Point Precipitation Frequency Estimates for the Pierre region of South Dakota. The storm duration in this analysis is 1 day under 1, 2,3,6,12 and 24 hours increment. The SCS curve number method was used to determine loss while SCS Unit Hydrograph method was used to transform the unit hydrographs in the analysis. From the previous study, a curve number of 87 was determined for the model. The area of the watershed was calculated to be 0.61 square miles while the average basin slope was calculated to be 2.08. A lag time of 57 minutes was also identified.

Rain EventPeak Flow
2 years123.5 CFS
10 years298.5 CFS
25 years409.4 CFS
50 years497.8 CFS
100 years589.9 CFS

Determination of Curve Number

The latitude and longitude of the project site at Fort Thompson, South Dakota was located using Google Map. Knowing the exact location of the project site allow StreamStat, South Dakota to identify the watersheds area at the site. Three watersheds were identified within the boundary of the site along highway 47.

The identified watersheds were then imported to NRCS’s Web Soil Survey Website where the different soil groups within the watersheds were identified. The soil in the area was classified as soil group D. The project site showed evidence of cultivated agricultural lands and other agricultural land. Using the information on land use and soil group, according to USDA guideline for Urban Hydrology for Small Watershed , a curve number of 87 was assigned to the project site.

Update on Geotechnical Report

The twelve soil samples, six from each boring hole, were combined and divided into three layers for grain size analysis. The boring log of each boring hole would consist of three layers: top soil which consist of a thin layer of organic soil and clay, the middle layer which consist mainly of clay and the bottom sand layer.

The sieve analysis of the top two layers from both boring locations indicated that the soil consist of more than 90% of clay which was sufficient to classify the soil as clayey soil.

Because of the cohesive nature of the soil, the Atterberg limit tests are required for further classification.