Record Details
page 32
Digital Collections at BYU
| Field | Value |
|---|---|
| Title | page 32 Diamond Fork System Final Supplement to the Final Environmental Impact Statement, page 32 |
| Coverage | Electronic reproduction; |
| Format | 32 text/PDF |
| Rights | Brigham Young University; http://lib.byu.edu/about/copyright/generic.php Public Domain Public |
| Language | English; eng; en |
| Relation | Central Utah Project; Western Waters Digital Library; comment letter no 13 page 32 of 36 the analysis is in error when it takes a relationship developed for stable rivers under natural conditions of hydrology and sediment transport and applies it to a stream diamond fork which is clearly not in equilibrium and is strongly influenced by an artificial flow regime using the current artificial flow regime as a reference from which to gauge stream recovery is also an error because it is not based on an assessment of the hydrologic conditions under which the stream would be expected to be stable under the proposed flow regime annual runoff at the red hollow gage age on diamond fork creek would be reduced approximately 30 percent from 90000 acre ft per year to 60000 acre ft per year the proposed flow regime reduces the magnitude of summer streamflows stream flows in diamond fork creek below monks hollow but increases the magnitude and duration of minimum flows this point is critical as especially in the braided section of lower diamond fork creek the proposed low flows are capable of transporting considerable bedload material to verify this point the forest has collected a limited set of bedload samples from the braided section of diamond fork creek which confirm bedload movement at flows as low as 170 cfs cs at this flow particles considerable larger 45 mm than the d50 or median particle size are in motion the average sediment transport on an annual basis can be estimated by the product of flow magnitude and duration expressed as either the number of days per year or the average percent of time that a given flow is exceeded wolman and miller 1960 this is method used by andrews 1980 in defining the effective discharge or the flow that moves the most sediment over time there is ample evidence to suggest Z that the initiation of bedload movement in this section of diamond fork begins at flows of 80 to 100 cfs cs or less for example at 170 cfs cs particles as large as 3 times the diameter of the d50 or median particle size of 13 mm are in motion under natural conditions it would be expected that flows of this magnitude would occur for only 10 to 15 days per year andrews 1980 found that the effective discharges for the 15 streams included in his study occurred between 1.5 15 15 days per year 04 0.4 04 of the time to 11 days per year 30 3.0 30 of the time for comparison the natural mean annual flood the average of annual flood peaks as determined from a pre dre preliminary liminga reconstruction of gaging station records for the red hollow and thistle near brimhall gages on diamond fork creek are roughly 230 and 240 cfs cs respectively these flows are only estimates at this time as some of the data have not been fully corrected for the effects of irrigation flows the actual averages will likely be lower once the correction is made the reconstructed flood peaks were ranked and flood frequencies estimated using a weibull distribution using the 1.5 15 year flood as a first approximation of the bankfull flow the resulting values are 134 cfs cs for diamond fork below red hollow and 153 cfs cs for diamond fork near thistle note that the ratio of the mean annual flood to the bankfull flood is approximately 1.6 16 ig lg for diamond fork and that this is consistent with the data for streams in central utah and surrounding areas table 1 if as observed by andrews and nankervis 1995 sediment transport begins at roughly 60 percent of the natural bankfull stage then it would be expected that sediment motion could be observed at flows of 80 cfs cs near red hollow and about 90 cfs cs in lower diamond fork this is consistent with what we have observed with our sediment transport measurements to date for the streams included in the andrews 1984 and andrews and nankervis 1995 study an average of 80 of the total sediment load was transported by flows between 0.8 08 08 and 1.6 16 lg ig times the bankfull discharge and these flows occurred an average of 15.6 156 156 days per year for lower diamond fork gecy and gecy 19971998 1997 1998 using data only for the preirrigation pre irrigation period of 1908 to 1915 estimated that flows in 31 |
| Identifier | http://cdm15999.contentdm.oclc.org/cdm/ref/collection/WesternWatersProject/id/9353 |
