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Sugar Creek Monitoring SitesLooking south-southwest, downstream from the county bridge, approximately three miles north of Gracemont, Oklahoma. Bank heights are 20 plus (+) feet from channel bed to farm field. Saturation at the toe of the banks and a quick recession of peak flows lead to longitudinal cracks (parallel with flow) at the top of the banks, then, Mass Wasting of banks and channel widening.
Looking north, upstream from same county bridge as from previous photograph.
INTRODUCTIONA reach of Sugar Creek, in the "S-Curve". In 1983, a storm changed the straightened creeks channel pattern to a more sinuous one, but took out acres of productive farmland. The reach downstream from the S-Curve, is being monitored to track the loss of bank material on the western bank. 1994 & 1996 CHANNEL SURVEY RESULTSFive cross sections on Sugar Creek's main stem, from River Station 4500 (feet) to River Station 87,550 (feet), were surveyed in 1994 and 1996. Channel profile data also exists for some sections of the main stem; 1966 (Pre-construction) and 1967 (as-built) channel profile data extends from River Station 0 (feet) to River Station 72,419 (feet), past Cross Sections number (#) 4. November 1967 channel profile data extends from River Station 76,959 (feet) to River Station 129,379 (feet), whereas 1968 channel profile data extends only up to River Station 13,200 (feet), past Cross Section #2. This information is incorporated into the summaries below. Generally, channel bed slopes decrease with an increase in contributing drainage area or as flows accumulate. In geomorphic terms: slope is inversely proportional to water discharge. Sugar Creek generally follows this pattern. In 1967, Sugar Creek was constructed with channel slopes in the range of 7.5 to 8.7 feet/mile, greater than the average floodplain slope of 7.4 feet/mile in the lower valley and less than the average floodplain slope of 10.1 feet/mile in the upper valley. Since construction in 1967, Sugar Creek has downcut to reduce it's average channel bed slopes. This is a natural response to dissipating excessive energy that would naturally be absorbed by bank vegetation and channel meandering; In 1996, average channel bed slopes varied from 4.1 feet/mile (at Cross Section #1) to 7.6 feet/mile (at Cross Section #5). Although Sugar Creek has been straightened, it is estimated that Sugar Creek is adjusting to a lower valley channel gradient of approximately (~)5 to 5.2 feet/mile. A channel gradient equivalent to a meandering stream, in the same valley setting, with a sinuosity of ~1.4 to 1.5. One advantage to studying stream profiles using geomorphic principles can be viewed by plotting the bankfull water surface elevations with the channel bed elevations: Using the regional curve that relates bankfull cross-sectional area to drainage area, with a curve that plots contributing drainage area to channel stationing, bankfull cross-sectional areas can be determined for any channel stationing of the stream, in particular, at the five Cross Sections surveyed. From the Cross Section data, elevations can be determined for the bankfull cross-sectional area. Bankfull water slopes can then be compared with respect to average bed slopes. In the graphic below, bankfull water slopes, which is closely associated with the energy gradeline is plotted with average bed slopes. The energy gradient (bankfull water line) between Cross Sections #4 and #5 is steeper than the bed slope, and indicates that degradation is likely to occur. The steeper gradient is caused by a widening of the channel at Cross Section #4, which usually cannot be expressed in a profile view. Downstream, in the reaches between Cross Sections #1 and #2 and #2 and #3, the bed slope is steeper than the energy gradient, and indicates that these reaches are more likely to aggrade, which happens to be the case from analyzing the 1994 and 1996 Cross Section survey data.
CROSS SECTION 1: RIVER STATION 4500 FEET, WASHITA RIVER AT RIVER STATION 0Cross Section #1, is located ~120 feet upstream from the first bridge north of the Washita River that crosses Sugar Creek, 0.85 miles upstream of the Sugar Creek - Washita River Confluence. This cross section of Sugar Creek represents flows that drain 59,795 acres (93.4 square miles), even though the total drainage area above this cross section is 147,831 acres (231 square miles). Sixty percent of the upstream drainage area - 88,036 acres (137.6 square miles) is controlled by flood water retarding structures.
Cross Section 1 was surveyed in April and December, 1994 and November, 1996. Cross Section stationing was translated, in order to get better superposition of the three data sets. All cross section stations of the December, 1994 survey were translated 10 feet to the left and all cross section stations of the November, 1996 survey were translated 57 feet to the right. An anomaly in the data exists as far as alignment of floodplain elevations. Another site visit is warranted to check existing channel invert elevations and floodplain elevations. In 1994, the thalweg elevation was recorded at 1149.7 feet above mean sea level (amsl), 0.3 feet higher than the 1966 channel elevation before construction took place. Records do not indicate how far back, this equilibrium was attained. In 1966, prior to channel construction, the channel thalweg was measured at 1149.4 feet amsl. In 1967, the channel bottom was dredged out to an elevation of 1143.6 feet amsl. In June 1968, a thalweg survey was conducted to record effects from a storm on April 12, 1967 whose mean daily flow upstream at the Gracemont Gage was 1200 cubic feet per second (cfs). Channel bottom measurements were not conducted specifically at cross section #1, but downstream and upstream measurements indicate ~1.6 feet aggradation in the reach, (thalweg elevation of ~1145.2 feet amsl in 1968). In April and December, 1994, thalweg elevations were measured at 1148.6 and 1149.6 feet amsl respectively, approximately the same elevation as natural channel elevation in 1966. In 1996, the channel thalweg elevation was recorded at 1150.4 feet amsl. CROSS SECTION 2: RIVER STATION 12675 FEET, (WASHITA RIVER AT RIVER STATION 0)Cross Section #2, is located ~100 feet upstream from a blacktop county road bridge, 2.4 miles upstream of the Sugar Creek - Washita River Confluence. This cross section of Sugar Creek represents flows that drain 58,130 acres (90.8 square miles), even though the total drainage area above this cross section is 146,166 acres (228.4 square miles). Sixty percent of the upstream drainage area - 88,036 acres (137.6 square miles) is controlled by flood water retarding structures.
A review and comparison of April, 1994 and November, 1996 surveys indicate an anomaly in the data: Channel width in 1994 was wider than in 1996. It is very unlikely that deposition would occur to "fill in" one side of the bank uniformly from channel bottom to top of bank. It is more likely that measurements of channel bottom were not conducted perpendicular to flow. It was assumed that measurements across the channel were skewed from perpendicular. Adjustments to channel stationing were made by shortening channel stationing by a factor of Cosine (Cos) 37 = 0.7986. The adjustment factor is based on the width of floodplain features from 1994. The best practice is to have permanent bench marks/reference points on both sides of the channel, so that surveys over time can be duplicated as far as two elevations within the cross section and in alignment of cross section to the stream. In 1966, prior to channel construction, the ground elevation at station 126+75 was measured at 1164.6 feet amsl. In 1967, the channel bottom was dredged out to an elevation of 1155.8 feet amsl, and vegetation cleared. In June 1968, a thalweg survey was conducted to record effects from the April 12, 1967 storm, that had a mean daily flow recorded at the Gracemont Gage of 1,200 cfs. Thalweg measurements were not conducted specifically at cross section #2, downstream and upstream measurements indicate no aggradation or degradation in the reach, therefore, it is assumed that thalweg elevation remained at ~1155.7 feet amsl. In April, 1994, and November, 1996, thalweg elevations were recorded at 1156.6 and 1156.8 feet amsl respectively. The channel has aggraded approximately one foot since the 1967 design elevation was established. Cross Section #2 is located in the lower half of the valley, where the average overland (floodplain) slope is 7.4 feet/mile (0.001395 ft/ft). Assuming that Sugar Creek had a natural sinuosity of ~1.4, natural channel slope would be approximately (~)5.3 feet/mile (0.00100 ft/ft). The 1967 channel gradient was constructed to 7.9 feet/mile (0.00149 ft/ft). A storm over the watershed on April 12, 1967, showed that Sugar Creek was ready to readjust itself. Survey measurements in June, 1968 show aggradation below and degradation above Cross Section #2 while no changes were noted near Cross Section #2. By 1968, Sugar Creek adjusted it's bed slope, (between Cross Sections #1 and #2) to 6.8 feet/mile (0.00128 ft/ft). By 1994, Sugar Creek had adjusted it's bed slope to 4.5 feet/mile (0.00086 ft/ft). And by 1996, Sugar Creek had adjusted its bed slope to 4.1 feet/mile (0.00078 ft/ft). It is interesting to compare predicted bankfull width and mean depth to Sugar Creeks' actual channel width and mean depth at Cross Section #2; For a channel that drains 90.8 square miles of uncontrolled drainage area, the predicted channel width is 68 feet, Cross Section #2's channel width at bankfull flow is 69 feet; For the same channel, the predicted mean depth is 7.1 feet, Cross Section #2's mean depth at bankfull flow is 7.0 feet. Cross Section #2 appears true to bankfull predictions and has shown in the past, that it transports a range of sediments through its reach without substantially aggrading or degrading. It is also interesting to remember, that Sugar Creek, near Cross Section #1 had aggraded back to pre-construction conditions. CROSS SECTION 3: RIVER STATION 47907 FEET, (WASHITA RIVER AT RIVER STATION 0)Cross Section #3, is located ~100 feet upstream from the Gracemont Bridge, ~9.1 miles upstream of the Sugar Creek - Washita River Confluence. This cross section of Sugar Creek represents flows that drain 46,699 acres (72.9 square miles), even though the total drainage area above this cross section is 127,852 acres (199.8 square miles). Sixty-three percent of the upstream drainage area - 81,153 acres (126.8 square miles) is controlled by flood water retarding structures.
April, 1994 survey measurements were adjusted for similar reasons explained above for Cross Section #2 (Channel was wider in 1994 than in 1996). It is assumed that the extra width was caused from taking measurements at an angle other than 90 degrees to the direction of flow. It appears that stationing measurements across the channel were skewed from perpendicular and hence the adjustment factor to correct channel stationing is Cos(17.2) = 0.9552 or in other words, distances across the channel were shortened ~4.5 percent. All cross section stationing were translated 87 feet to the right to match edges of steep banks and berms. In 1966, prior to channel construction, the ground elevation at station 479+07 was measured at 1213.4 feet amsl. In 1967, Sugar Creek was realigned and the ground elevation was excavated from 1213.4 feet to 1206.2 feet amsl, and vegetation was cleared. In April, 1994, and November, 1996, thalweg elevations were recorded at 1194.8 and 1195 feet amsl respectively. The channel has downcut approximately eleven feet since the design elevation was established back in 1967. Cross Section #3 is located in the lower half of the valley, where the average overland (floodplain) slope is 7.4 feet/mile (0.001395 ft/ft). Assuming that Sugar Creek had a natural sinuosity of ~1.4, natural channel slope would be ~5.3 feet/mile (0.00100 ft/ft). The 1967 channel gradient was constructed to 7.6 feet/mile (0.00143 ft/ft). To dissipate this excessive energy, without the benefit of riparian vegetation, Sugar Creek downcut. Presently, Sugar Creek has adjusted its channel gradient to ~5.7 feet/mile (0.00108 ft/ft). CROSS SECTION 4: RIVER STATION 52907 FEET, (WASHITA RIVER AT RIVER STATION 0)Cross Section #4, is located ~100 feet upstream from Yellow Creek , ~10 miles upstream of the Sugar Creek - Washita River Confluence. This cross section of Sugar Creek represents flow that drains 45,580 acres (71.2 square miles), even though the total drainage area above this cross section is 123,792 acres (193.4 square miles) of which 78,212 acres (63 percent of the upstream drainage area) is controlled by flood water retarding structures.
April, 1994 cross section stationing were translated ten feet (+10 ft) to the right, to align floodplain features, in this case, a four foot berm, with survey conducted in November, 1994. Sugar Creek's thalweg elevations around cross section 4 remained nearly steady; April, 1994 and November, 1996 thalweg elevations were recorded at 1200.2 and 1200.4 feet amsl respectively. The channel width widened approximately eighteen feet, probably due to bank collapse. To determine the stream length of this bank collapse, will require a site visit. It has been noted that similar failures occur for approximately 30 to 40 feet. Cross Section #4 is located in the lower half of the valley, where the average overland (floodplain) slope is 7.4 feet/mile (0.001395 ft/ft). In 1966, prior to channel construction, the channel thalweg was measured at 1218.2 feet amsl. This puts the pre-constructed channel slope, between cross sections 3 and 4, at ~5.1 feet/mile (0.00096 ft/ft), the mildest slope of the four reaches studied. Assuming that a natural meander pattern existed, Sugar Creek had a sinuosity of ~1.45. From 1967 to 1996, Sugar Creek, near river mile 10, has down cut approximately twelve feet. In 1967, the constructed channel thalweg was established at 1212.7 feet amsl, the 1967 design channel slope was ~6.9 feet/mile, (0.0013 ft/ft). The current channel slope, as measured by the 1996 channel thalweg elevations from cross section's #3 and #4 is ~6.2 feet/mile, (0.00118 ft/ft). Sugar Creek will continue to adjust it's imposed gradient by first downcutting, then as the channel slope approaches it's equilibrium energy gradient (asymptotically), Sugar Creek is expected to widen, increasing it's flow path and resistance in order to match it's equilibrium energy gradient. Twenty-one years is a short period of time to a meandering stream system. If Sugar Creek adjusts it's channel slope back to pre-construction conditions (5.1 feet/mile, 0.00096 ft/ft channel gradient) without further downcutting, the reach length, between cross sections #3 and #4, will increase approximately 1150 feet, (from 5000 feet to 6150 feet). If Sugar Creek adjusts it's sinuosity back to pre-construction conditions (P = 1.45) while adjusting back to a slope of 5.1 feet/mile, the reach length, between cross sections #3 and #4, will increase approximately 2000 feet, (from 5000 feet to 7000 feet) and cross section #3 will downcut another 1.5 feet. CROSS SECTION 5: RIVER STATION 87,550 FEET, (WASHITA RIVER AT RIVER STATION 0)Cross Section 5, located approximately 16.58 miles upstream of the Sugar Creek - Washita River Confluence, represents Sugar Creek draining approximately 24,809 acres (38.7 square miles), even though the total drainage area above this cross section is 75,949 acres (118.7 square miles) of which 51,140 acres (67 percent of the upstream drainage area) is controlled by flood water retarding structures. Cross Section #5 crosses Medicine Creek, shown on the left (looking downstream) and Sugar Creek shown on the right. In April, 1994 and November, 1996 thalweg elevations on Sugar Creek were recorded at 1253.1 and 1250.6 feet amsl respectively. Comparison of surveys show that Sugar Creek has downcut 2.5 feet in two years and a total of 18.9 feet since 1967. 1966 profile surveys did not extend this far up on Sugar Creek and therefore, channel bed elevations were not recorded, prior to channelization. In 1967, the average bed slope from cross section #5 to cross section #4 was 8.6 feet/mile (0.00164 ft/ft). In 1994, average bed slope was 8.1 feet/mile (0.00153 ft/ft), and in 1996, average bed slope was 7.6 feet/mile (0.00145 ft/ft). Cross Section #5 is located in the upper half of the valley, where the average overland (floodplain) slope is 10.1 feet/mile (0.001924 ft/ft). Assuming that the natural meander pattern has a sinuosity of 1.4, the natural channel slope would be 7.2 feet/mile (0.00137 ft/ft). Overlaying the surveys indicate no channel widening, but as the channel slope approaches it's equilibrium energy gradient (asymptotically), Sugar Creek is expected to widen, increasing it's flow path and resistance in order to match it's equilibrium gradient.
PROFILE SURVEY - SLOPE AS AN INDICATOR OF CHANNEL CHANGESThe graphic below shows profiles of Sugar Creek's Channel Bed Elevations at the five monitored cross sections as measured in November, 1996. Based on the Regional Curves for Channel Geometry, Bankfull Cross Sectional Areas were determined for each cross section based on upstream uncontrolled drainage area. The corresponding bankfull water surface profile is plotted as an indication of the energy gradelines. Note the steep energy gradient from Cross Section #5 to Cross Section #4, as compared to the average bed slope. The steep gradient indicates an acceleration of the water, which will increase the sediment transport capacity. Cross Section #4 is almost twice as wide as a stable channel for the same amount of drainage area. It is likely that this reach will continue to widen. Likewise, from Cross Section #3 to Cross Section #1, the energy gradient is milder than the average bed slope. The mild gradient indicates a deceleration of the water, which should force the stream to aggrade. Top widths, cross-sectional areas and mean depths are shown in the table to show which reaches have adjusted and are continuing to adjust to imposed gradients.
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