Hydrologic evaluation research has recently been completed by the Prairie Farm Rehabilitation Administration (1996) regarding the effectiveness of Deerwood's small dam network.  

Watershed modeling work was undertaken by Jim Yarotski, of PFRA Regina and based on extensive field work and "real time" hydrologic monitoring carried out by Deerwood and PFRA at the monitoring sites mentioned above.  

Modeling Methodology   

As of January 1995, 26 small dams have been installed by the Deerwood Association. Collectively, these basins control approximately 23 km2 or 30% of the total South Tobacco Creek drainage area upstream of Miami  

The HEC-1 model was used for the study because of its assumptions regarding basin hydrology; HEC-1 represents a watershed as in interconnected system of measurable hydrologic and hydraulic components. HEC-1 assisted in two aspects of the Pilot Project:  

1. To adjust historical spring and summer runoff affected by the dams in order to estimate " natural" peak flows within the watershed system; and  

2. To utilize these "natural flows" in a flood frequency analysis of the South Tobacco Creek systems.  

Prior to modeling, 60 South Tobacco Creek sub-basins were identified for HEC-1 inclusion (click for sub-watershed map (92K) and Table 4). In accordance with standard streamflow hydrology concepts, each sub-basin was linked with others to portray a "natural" progression of runoff collection within the basin. As the storage capacity of each Deerwood dam was known, the HEC-1 model could be run to exclude or include dam structures.  

Baseline Conditions  

Spring (snowmelt) and summer (rainfall) runoff peak flows were reviewed according to historical and reconstructed information. This permitted the establishment of baseline conditions for various frequency events for the South Tobacco Creek watershed (i.e. prior to dam network installation). (Table 5)  

Rainfall and Snowmelt Runoff with Dams  

The HEC-1 model was then used to assess the impact of Deerwood's 26 dams during summer rainstorms and/or spring snowmelt. Runoff peaks ranging from a 1:2 year frequency up to 1:500 year frequency were analyzed, with corresponding % reductions in peak flows resulting in each case (Table 1).  

Significant reductions in peak flow were observed in the model, both for Spring melt and Summer rainfall runoff.  

The small dams are less effective in reducing spring runoff peaks because snowmelt runoff events generally take several days to peak, and the entire runoff may last a week or more. In contrast, summer rainfall runoff events peaks within several hours, and the entire event typically lasts less than a day. The rapid increase and decrease in rainfall runoff, representing a relatively small amount of water in total, increases the potential for the flow to be stored within a small dam.  

Within this period, there have been several major summer rainfall and spring snowmelt events to test individual dam effectiveness and integrity. The hydrological effectiveness of some Deerwood dams has been proven to be substantial; peak flow reductions of up to 90% have been recorded (PFRA, 1992). Table 2 outlines the measured impacts of these structures. (insert clickable charts for each dam, and story of rainfall event, need help on this)  

Cumulative Impacts of Individual Dams  

The HEC-1 model predicted that the cumulative effect of Deerwood's 26 dams could reduce peak flow impacts within the entire South Tobacco Creek watershed by as much as 25% for a summer rainstorm, and by up 19% during the spring melt. In order to cross-reference this impact predicted by the HEC-1 model, "real time" peak flow reduction results for individual Deerwood dams were analyzed for their actual likely contributions toward overall peak flow reductions within the South Tobacco Creek Watershed.  

Comparison to One Large Dam  

The HEC-1 model was again used to compare the effectiveness of Deerwood's small dam network to one larger structure located upstream from Miami, MB. This reservoir is in fact the same one which was designed in 1971. The results of this analysis indicate that the reduction in summer peak flows ranged from 9% to 38%, with the greatest reduction occurring during the 1:100 runoff event. Spring peak flow reductions ranged from 0% to 27%. These results compared favourably to the small dam network results (Table X ), and indicate the viability of the small dam option.  

Projected peak flow impacts indicate similar effects, particularly with more common runoff intensities (i.e. 1:2 - 1:50 events). Trends of: reduced flooding and soil erosion, improved water quality, and lengthened runoff periods would be expected according to the PFRA model, which predicts Deerwood's dams to be effective in reducing peak flows throughout the watershed by up to 25% and 19% in Summer storms and Spring snowmelts respectively (Table X) (PFRA, 1995).  
  

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