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    I.. J . Kremer2

    ABSTRACT: A synthetic hydrograph method was utilized in the development of a watershed model for a small urbanizing watershed. The model was applied to the watershed and the largest flood of record was accurately reproduced. Because the model would be utilized for design of flood control plans with complete urbanization, the method was also applied to an urbanized watershed and reproduced a measured event with good results. The method does not require extensive hydrologic data for its implementation, can be applied to watersheds in various stages of urbanization, and permits consideration of natural or potential floodwater storage. (KEY TERMS: Hydrographs, Hydrologic Models, Hydrology, Watershed)

    Typically, there is a lack of stream flow records and other hydraulic data for small watersheds, requiring use of general empirical formulas t o determine peak flow and evaluate other runoff parameters. Bassett Creek Watershed is n o excpetion in this respect with only meager stream flow records available. In addition, a preliminiary review of the watershed indicated that extensive natural and potential floodwater storage was available throughout the watershed and would have to be accounted for in the hydrologic analysis. An important additional characteristic of the watershed is its development pattern. Approximately 60% of the watershed is completely urbanized while the remainder is essentially rural.


    The design of the Bassett Creek Watershed model was based on a synthetic hydrograph method developed by Barr Engineering Co. in 1957 for the design of trunk storm sewer systems involving temporary storage. The method was especially designed for use in areas in transition from rural to urban development where extensive data was not available. The application of the method requires an analysis of the characteristics of the watershed including soils, proposed and existing development patterns, approximate land surface slopes, channel slopes, and some knowledge of the hydrology of the general area. Since 1957, the method has been continuously refined and is now completely computerized.

    Paper No. 73166 of the Wufer Resources Bulkfin. Discussion are open until F'ebruary 1, 1975. Project Engineer, B a r Engineering Co., Minneapolis, Minnesota 55435.


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    Discussions of the method and its implementation were presented by Molsather (1972) at the University of Minnesota, Water Resources Seminar, and by Hayden (1972) at Colorado State University, National Symposium on Watersheds in Transition. Its unique feature is the implementation of four separate basic hydrographs for each subwatershed area. One hydrograph is developed for impervious areas and three hydrographs are developed for pervious areas dependent on slope, thereby permitting the application of the model to watersheds in various stages of development. Channel routing or reservoir routing submodels are utilized for combining subwatershed hydrographs in the determination of outflow hydrographs. The method has been the basis for the implementation of several comprehensive watershed management plans including Nine Mile Creek Watershed (Barr, 1961) and Riley-Purgatory Creek Watershed (Barr, 1972), both in southwestern Hennepin County. It also has been used in the design of many overall storm drainage systems in the metropolitan area of Minneapolis and St. Paul and surrounding areas in Minnesota.


    This 43 square mile watershed shown on figure 1 includes part of Minneapolis and several suburban municipalities west of Minneapolis. The western boundary of the watershed rises from 980 to 1,010 feet above mean sea level, then drops irregularly along the northern and southern boundaries to about 800 feet where Bassett Creek enters the



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    Figure 1 . Location of Bassett Creek Watershed.


    Mississippi River. The unurbanized portions o f the watershed are characterized by undeveloped drainage patterns and marshlands interspersed with irregular hills.

    In order t o determine critical discharges a t various points along the 28 miles of channel, the watershed was subdivided into 1 2 0 subwatershed areas. In areas without storm sewer systems, it was assumed that the future storm sewer discharges would be directed approximately along natural drainage patterns.

    Land use for each subwatershed was determined from topographic maps, aerial photos, and municipal land use and zoning maps for the various stages of urbanization reviewed.


    Flooding events in the Bassett Creek Watershed have been recorded in little or n o detail in the past, with the most recent incidence of severe flooding on Bassett Creek occurring in June, 1942. As a result of that storm, the developed areas of Minneapolis near the outlet of Bassett Creek experienced major flood damage. Several 1942 highwater marks were located and because the development pattern of the watershed in 1942 could be determined from aerial photos, the model was applied in an attempt t o reproduce those highwater elevations.

    From an analysis of rainfall data, the St . Paul District Corps of Engineers estimated that the 1942 flood approximated a 50-year frequency storm. However, accurate rainfall data for the 1942 storm in the area of the watershed was not available. Because preliminary watershed analysis indicated that short duration storms would be critical in the area where 1942 highwater elevations were available, the 50-year, one-hour and two-hour duration rainfall events were selected from Technical Paper 40, for preliminary investigation. The calculated critical peak discharge at the outlet was 1,530 cfs, producing a flood level of approximately elevation 810.2. The 1942 highwater mark was elevation 810.5 a t the outlet. Approximately one mile upstream a t Fruen Mill, the 1942 highwater mark was elevation 819.5. The peak discharge determined at this point using the model was 600 cfs, producing a flood level of approximately elevation 819.3. The hydrographs at these two points are shown in figure 2.

    Because the design of the final flood control plan for the Bassett Creek Watershed would be based on the analysis of the watershed with anticipated ultimate urbanization, it was also decided t o test the method with completely urbanized conditions. Research completed by the University of Cincinnati on the Oakdale Avenue Basin in Chicago provided sufficient data (Papadakis, 1972). This 12.9 acre, urbanized watershed is approximately 55% pervious and 45% impervious. The storm of July 7 , 1964 was analyzed using the basic method developed by Barr Engineering Co.

    Peak discharges for the computed two-peaked runoff hydrograph were 4.9 cfs and 10.1 cfs, respectively, compared to 4.2 cfs and 9.6 cfs for the recorded event. The computed and recorded hydrographs are shown in figure 3.


    In the next phase of the Bassett Creek study, the model will be implemented t o determine the effect on the flood profile of various alternate methods of flood control including additional storage areas, channel improvements, changes in various outlet structures and other combinations of drainage improvements. The results of these analyses will be reviewed t o determine which combination produces the most benefits a t the least cost and provides the flood protection level desired by the citizens of the district.

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    0 2 4 6 8 10 12 14 16 T I M E I N HOURS

    Figure 2. Calculated Runoff Hydrographs, Basset Creek Watershed, Minneapolis - SO-Year Storm, 1942 Urbanization

    0 20 40 60 T I M E I N MINUTES

    Figure 3. Calculated and Observed Runoff Hydrographs, Oakdale Avenue Basin, Chicago - Storm of July 7 , 1964.



    Most of the various methods currently in use for the hydrologic analysis of small watersheds have features which restrict their general application. The method utilized for developing the Bassett Creek Watershed model permits:

    1. The evaluation of watersheds at various stages of development without significant

    2. Implementation without reliance on the existence of an extensive hydrologic data

    3. An analysis of the effect of natural or potential floodwater storage. 4. Computerization of the method for the evaluation of various alternative flood

    5. An evaluation of the effect of alternate land use on runoff. 6. The determination of runoff hydrographs in relation to rainfall frequency.

    alteration of the model.

    or the initiation of expensive data collection programs.

    control plans.


    The development of the Flood Control Plan for Bassett Creek Watershed is being done under a contract with the Bassett Creek Flood Control Commission, a joint powers agreement between the communities of Crystal, Golden Valley, Medicine Lake, Minneapolis, Minnetonka, New Hope, Plymouth, Robbinsdale, and St. Louis Park.


    Barr Engineering Co. 1961. Nine Mile Creek Watershed District Overall Plan. Ban Engineering Co. 1972. Riley-Purgatory Creek Watershed District Overall Plan. Hayden, J. W., Dickson, J. D., Wisniewski, D. 1972. Water Resources Management For a Small

    Watershed in Urbanizing Area. Transactions National Symposium on Watersheds in Transition, American Water Resources Association and Colorado State Universi


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