Physical dimensions and hydrologic effects of beaver ponds on Kuiu Island in southeast Alaska Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/td96k545q

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  • Dimensional characteristics of 44 beaver dams and ponds on Kuiu Island in Southeast Alaska were determined to evaluate the hydrologic effects of these structures on peak flows. The study area consisted of low gradient, incised streams in broad U-shaped valleys. Pond routing simulations were conducted using four return intervals (2-, 10-, 25-, and 50-year) and seven watershed sizes through medium- (752 m² of surface area) and large-sized (6002 m²) beaver ponds. The annual precipitation during field data collection was below normal resulting in a need to estimate winter pond conditions from summertime measurements. The average dam length and height was 32 m and 0.7 in, respectively. The average winter pond surface area and volume was 2,140 m² and 1,250 m³, respectively. Pond volume was significantly related (p = 0.05) to surface area (r² = 0.91). Dam and pond dimensions were influenced by local stream and landscape characteristics. Simulated peak flow routing through a beaver pond was accomplished using the Modified Puls method to calculate the theoretical percentage reduction in storm peak flows due to beaver damns and ponds. A triangular inflow hydrograph with the time to peak at 1/2 the total hydrograph duration was used. Thirty inflow hydrographs of various durations and peak discharges were routed through each pond to determine their effect on peak flows. Beaver damn surface profiles are very flat. This resulted in large outflow rates over a dam with little increase in pond head. Single, full beaver ponds were found to theoretical reduce peak flows by no more the 5.3%, regardless of the return interval or watershed size. The shape of the outflow hydrographs were the sante as the inflow hydrographs, with only a 10 or 15 minute delay in the time to peak and slightly increased duration. Reductions in peak flows became increasingly large as the number of ponds in a series increased. Five large-sized beaver ponds in series reduced the storm peak flow by 14% for a 2-year event, but only 4% for a 50-year event. Detention storage effects of stormwater discharging over a beaver dam and onto the floodplain was not addressed in this research.
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