Re: Survey of constructed cross section per Restoration Framework on Wind River, Fremont County, WY

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1 LeClair Irrigation District 1418 Cowboy Lane Riverton, WY (307) Re: Survey of constructed cross section per Restoration Framework on Wind River, Fremont County, WY Dear Mr. Hoelzen, We were approached by you last week to look at the Restoration Framework and survey a cross section just downstream (East) of the partially constructed South channel plug. I reviewed the Restoration Framework plan under the Master Settlement Agreement as presented to us. Sheet 3 of the plan shows the construction of a plug or dam with a rock revetment across the existing South channel. This dam, in effect, is supposed to restore the historical south bank of the Big Wind River prior to the construction of a minor dike which allegedly forced the river to use the South channel. A new North river channel including riffles and pools was to be constructed along some 2800 feet from the LeClair diversion structure. The stationing along the proposed new North channel begins at or near the LeClair Canal diversion structure with a station value of -600 feet. We were able to locate our surveyed cross section relative to some reference construction lath (set by Biota) still visible along the banks of the new North channel. We surveyed (using a survey grade GPS) a cross section about 76 feet downstream of a lath labeled 880 which puts our cross section (+965 station) in the middle of a designed riffle per the profile on sheet 4. We checked into the temporary benchmark pin set by Biota at the canal bridge (+1265 foot station elevation ). I have attached the plan view (with photo background) of the cross section we surveyed and a profile view of the surveyed cross section of the channel from the North edge of the canal road (0+00) to the right bank of the new channel (5+44). Holes were punched through the ice (8 to 12 inches thick) to survey the bottom of the channel. There was about 6 to 8 inches of snow on the ground outside of the channel. The ground shots were taken at the ground surface beneath the snow. The actual surveyed elevations of each cross section point are labeled under the channel bottom profile on our 956-Section Profile view. The apparent right and left design channel banks were determined by surveying an existing lath (set by Biota) marking that location. What we found, as summarized on the profile sheet, was that at the surveyed cross section the cross sectional area between the banks as constructed was ±281 square feet (sf). The distance between the two laths was ±180 feet as opposed to the design 193 foot. Page 1

2 On page 6 of the Wind River Restoration Framework Restoration Treatments states that; Design riffle geometry was developed based upon sub-reaches of the 2011 morphologic survey area that were identified as relatively stable and functional during field observation. These sub-reaches, utilized during restoration design as reference reaches, have average bankfull channel cross sectional area of 784 sq ft. The geometry of the subreach with lowest width/depth ratio was hydraulically scaled to achieve the design cross sectional area. This approach maintains the hydraulic geometry of the input reference reach while achieving cross sectional area typical of all reference conditions. The design channel geometry has bankfull area of 784 sq ft, bankfull width of 193 ft, mean depth of 4.1 ft, maximum depth of 5.8 ft, width depth ratio of 48, and channel capacity of 4,400 cfs at the existing channel slope of 0.24% (Figure 6). The design riffle cross section can be shifted vertically to identify design channel invert elevations throughout the project area based upon local floodplain elevation. This actual surveyed cross section does not appear to meet the required cross sectional area of 784 sf to pass the design flow of 4400 cubic feet per second (cfs). This requires a flow velocity of about 5.6 ft/sec. At that velocity the section we surveyed with a bankfull area of 281 sf will only carry about 1577 cfs. This is markedly smaller than the design 4400 cfs. We then looked at the available cross sectional area between the top of the right (south) bank and a water surface line extended north to intersect the ground surface North of the constructed left bank of the channel. The cross sectional area available at this level is ±610 sf. This is still less than the design required area of 784 sf. The water surface elevation that would need to be attained to provide the required 784 sf of flow area is ± The resulting water cross sectional area requires a water surface distance of ±272.5 feet across as compared to the design 193 foot wide cross section. Additionally, on our plan view sheet showing the cross section alignment, we surveyed a point just east of the 956 cross section. The existing elevation ( feet) here will allow water to spill over into the visible old channel running southeast at design flows of 4400 cfs. The anticipated flow (4400 cfs) will require the river to utilize the floodplain area between the reconstructed North channel and the bank of the LeClair canal road. The vegetated floodplain area will increase the friction coefficient. Flow of the river over the floodplain will potentially damage existing vegetation and cause a further rising of the level of the water surface. It is not desirable for the flow to reach the toe (± elevation) of the right bank of the LeClair canal at this or any other cross section along this reach. For anticipated flows above 4400 cfs (during spring runoff 10,000 cfs could occur), the reconstructed channel will not carry the total flow without serious erosion potential to the LeClair canal bank. Addendum No. 3 (Restoring Floodplain Connectivity on the South Bank) addresses the spoil piles that are opposite the LeClair headgate. The Addendum states; This addendum defines specific locations and approximate boundaries of spoil piles that would be eliminated or minimized to restore floodplain connectivity thereby allowing overbank flooding along the south bank upstream of the revetment shown on Sheet 3. There is concern as to whether the floodplain South of the Wind River, upstream of the constructed plug, will be able to carry enough of the 10 year peak runoff flow volume (possibly 12,078 cfs as referenced on page 10 of the Restoration Framework) to assure that Page 2

3 no damage to the LeClair canal bank will occur. Is there any analysis as to how much water can be expected to come down the North channel during flood events? LeClair also has concerns about water backing up enough to overtop the diversion structure (at -600 station) headwall at elevation Presently, the dam/plug is not yet complete and part of the south channel is still being used even with 450 cfs in the river. Apparently the West end of the dam was removed or breached concurrently with the end of the construction activity (December 2016) to relieve rising water levels upstream of the dam/plug. See dam breach in center of photo below. Once the breach is repaired, there are grave concerns as what extent flooding in the North channel will occur. While an exhaustive asbuilt survey of the completed channel has not been made available, evidence presented by this one surveyed cross section (956 section) should at least bring some attention to LeClair s concerns for the bigger picture. I would be available to answer any questions you might have on what we have provided. Terry Zenk, P.E. Apex Surveying, Inc. Page 3

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