Robert D. Jarrett

Paleodischarge of the late Pleistocene Bonneville Flood, Snake River, Idaho, computed from new evidence

The Bonneville Flood resulted from catastrophic outflow from Pleistocene Lake Bonneville about 15,000 yr ago, when the lake overtopped its rim at Red Rock Pass in southeastern Idaho and discharged a vast volume of water down the Snake River. This paper provides revised estimates of the paleodischarge, volume, and duration of the Bonneville Flood, based on new evidence of its height and on current understanding of the amount of lowering of Lake Bonneville. Evidence for the revised height of the flood is derived from the altitude of erosional features and flood deposits at the head of a constricted reach of the Snake River Canyon at the mouth of Sinker Creek and from the altitudes of flood deposits at several places about 53 km upstream. Using the step-backwater method, we estimate that peak discharge for the Bonneville Flood through the constricted reach was from 793,000 to 1,020,000 m 3 /s and most likely was 935,000 m 3 /s. This discharge is 2.2 times the discharge previously reported and is the second largest flood known to have occurred in the world. At this rate of discharge, the shear stress for the flood would have been 2,500 N/m 2 , and the unit stream power would have been 75,000 N/m/s, as compared with values of 6 to 10 N/m 2 and 12 N/m/s for recent floods on the Mississippi and the Amazon. Other recent studies of the history of Lake Bonneville show that the volume of water released was 4,700 km 3 , or about 3 times greater than the volume previously inferred. Although this volume indicates a flood duration of 8 weeks at constant peak discharge, an accurate estimate of the duration would require dam-break modeling at Red Rock Pass. From a dam-break model, flood hydrographs at Red Rock Pass and the hydraulics of the flood wave along the Snake River could be computed.

Determination of hydrologic pathways during snowmelt for alpine/subalpine basins, Rocky Mountain National Park, Colorado

Alpine/subalpine ecosystems in Rocky Mountain National Park may be sensitive to atmospherically derived acidic deposition. Two- and three-component hydrograph separation analyses and correlation analyses were performed for six basins to provide insight into streamflow generation during snowmelt and to assess basin sensitivity to acidic deposition. Three-component hydrograph separation results for five basins showed that streamflow contained from 42 to 57% direct snowmelt runoff, 37 to 54% subsurface water, and 4 to 13% direct rain runoff for the May through October 1994 study period. Subsurface contributions were 89% of total flow for the sixth basin. The reliability of hydrograph separation model assumptions was explored. Subsurface flow was positively correlated with the amount of surficial material in a basin and was negatively correlated with basin slope. Basins with extensive surficial material and shallow slopes are less susceptible to ecosystem changes due to acidic deposition than basins with less surficial material and steeper slopes. This study was initiated to expand the intensive hydrologic research that has been conducted in Loch Vale basin to a more regional scale.

Velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions

Abstract Keller [Keller, E.A., 1971. Areal sorting of bed-load material: the hypothesis of velocity reversal. Geological Society of America Bulletin 82, 753–756] hypothesized that at high flow, near-bed velocities in pools exceed velocities in riffles and create pool scour. Pools, however, typically have larger cross-sectional areas of flow at bankfull discharge. This condition raises an inconsistency with Kellers velocity reversal hypothesis and the one-dimensional continuity of mass equation. To address this problem, a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit-slopes of pools. According to the model, a narrow zone of high velocity occurs in the center of pools, creating scour. Along the downstream end of pools, an uphill climb of particles up the pool exit-slope promotes sediment deposition. The model is tested with field and flume measurements of velocity, water-surface elevation, and size of bed sediments in recirculating-eddy influenced pools. Local reversals of the water-surface gradient were measured in the field and a velocity reversal was created in the flume. The reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow. The distribution of bed sediments collected in the field also support the proposed model of pool maintenance.

Errors in slope-area computations of peak discharges in mountain streams

During an evaluation of 70 slope-area measurements on higher-gradient streams (stream slopes greater than 0.002) throughout the United States, peak discharge measurements were found to be affected by n values, scour, expansion and contraction losses, viscosity, unsteady flow, number of cross sections, state of flow and stream slope. Problems due to measurement error can often be as great as or greater than 100% and leads to overestimation of the actual peak discharge. This can result in misleading maximum flood values, erroneous flood-frequency analyses and overdesign of flood-plain structures. A brief discussion of these problems, tentative solutions and research needs is presented. The critical-depth method of computing peak discharge provides the most reasonable results in higher-gradient streams.

Regional interdisciplinary paleoflood approach to assess extreme flood potential

In the past decade, there has been a growing interest of dam safety officials to incorporate a risk-based analysis for design-flood hydrology. Extreme or rare floods, with probabilities in the range of about 10−3 to 10−7 chance of occurrence per year, are of continuing interest to the hydrologic and engineering communities for purposes of planning and design of structures such as dams [National Research Council, 1988]. The National Research Council stresses that as much information as possible about floods needs to be used for evaluation of the risk and consequences of any decision. A regional interdisciplinary paleoflood approach was developed to assist dam safety officials and floodplain managers in their assessments of the risk of large floods. The interdisciplinary components included documenting maximum paleofloods and a regional analyses of contemporary extreme rainfall and flood data to complement a site-specific probable maximum precipitation study [Tomlinson and Solak, 1997]. The cost-effective approach, which can be used in many other hydrometeorologic settings, was applied to Elkhead Reservoir in Elkhead Creek (531 km2) in northwestern Colorado; the regional study area was 10,900 km2. Paleoflood data using bouldery flood deposits and noninundation surfaces for 88 streams were used to document maximum flood discharges that have occurred during the Holocene. Several relative dating methods were used to determine the age of paleoflood deposits and noninundation surfaces. No evidence of substantial flooding was found in the study area. The maximum paleoflood of 135 m3 s−1 for Elkhead Creek is about 13% of the site-specific probable maximum flood of 1020 m3 s−1. Flood-frequency relations using the expected moments algorithm, which better incorporates paleoflood data, were developed to assess the risk of extreme floods. Envelope curves encompassing maximum rainfall (181 sites) and floods (218 sites) were developed for northwestern Colorado to help define maximum contemporary and Holocene flooding in Elkhead Creek and in a regional frequency context. Study results for Elkhead Reservoir were accepted by the Colorado State Engineer for dam safety certification.

Paleohydrologic techniques used to define the spatial occurrence of floods

Abstract Defining the cause and spatial characteristics of floods may be difficult because of limited streamflow and precipitation data. New paleohydrologic techniques that incorporate information from geomorphic, sedimentologic, and botanic studies provide important supplemental information to define homogeneous hydrologic regions. These techniques also help to define the spatial structure of rainstorms and floods and improve regional flood-frequency estimates. The occurrence and the non-occurrence of paleohydrologic evidence of floods, such as flood bars, alluvial fans, and tree scars, provide valuable hydrologic information. The paleohydrologic research to define the spatial characteristics of floods improves the understanding of flood hydrometeorology. This research was used to define the areal extent and contributing drainage area of flash floods in Colorado. Also, paleohydrologic evidence was used to define the spatial boundaries for the Colorado foothills region in terms of the meteorologic cause of flooding and elevation. In general, above 2300 m, peak flows are caused by snowmelt. Below 2300 m, peak flows primarily are caused by rainfall. The foothills region has an upper elevation limit of about 2300 m and a lower elevation limit of about 1500 m. Regional flood-frequency estimates that incorporate the paleohydrologic information indicate that the Big Thompson River flash flood of 1976 had a recurrence interval of approximately 10,000 years. This contrasts markedly with 100 to 300 years determined by using conventional hydrologic analyses. Flood-discharge estimates based on rainfall-runoff methods in the foothills of Colorado result in larger values than those estimated with regional flood-frequency relations, which are based on long-term streamflow data. Preliminary hydrologic and paleohydrologic research indicates that intense rainfall does not occur at higher elevations in other Rocky Mountain states and that the highest elevations for rainfall-producing floods vary by latitude. The study results have implications for floodplain management and design of hydraulic structures in the mountains of Colorado and other Rocky Mountain States.

A revised velocity-reversal and sediment-sorting model for a high-gradient, pool-riffle stream

Sediment-sorting processes related to varying channel-bed morphology were investigated from April to November 1993 along a 1-km pool-riffle and step-pool reach of North Saint Vrain Creek, a small mountain stream in the Rocky Mountains of northern Colorado. Measured cross-sectional areas of flow were used to suggest higher velocities in pools than in riffles at high flow. Three hundred and sixteen tracer particles, ranging in size from 16 mm to 256 mm, were placed in two separate pool-riffle-pool sequences and used to assess sediment-sorting patterns and sediment-transport competence variations. Tracer-particle depositional evidence indicated higher sediment-transport competence in pools than in riffles at high flow. Pool-riffle sediment sorting may be created by velocity reversals, and more localized sorting results from gravitational forces along the upstream sloping portion of the channel bed located at the downstream end of pools. [Key words: channel morphology, fluvial geomorphology, velocity reversal...

Coarse-sediment distribution as evidence of an elevation limit for flash flooding, Bear Creek, Colorado

Bear Creek is a tributary of the South Platte River in central Colorado. The stream flows east from an elevation of 4348 m at the Continental Divide to the mountain front at 1670 m. It thus encompasses the 2300 m elevation limit for substantial rainfall flooding in the Colorado Front Range proposed by Jarrett. Maximum paleoflood discharges estimated from flood deposits at four sites along Bear Creek demonstrate a consistent decrease in unit discharge with increasing elevation and support the hypothesis of an upper elevation limit for rainfall floods. The unit discharge values were used to explain coarse-sediment distribution along Bear Creek. Measurements of coarse-grained channel sediment at 19 sites along the creek indicate a decrease in particle size in flood deposits with increasing elevation, as well as a decrease in the size of clasts introduced to the main channel along tributaries. These changes in grain size are hypothesized to reflect changes in the competence of channel transport as a result of snowmelt-dominated versus rainfall-dominated discharge regimes above and below 2100 m elevation. Calculations of flow competence versus entrainment thresholds for the deposits may support this interpretation. One of the geomorphic implications of the elevation limit on flash flooding is a reversal of the usual downstream-fining trend in coarse channel sediments.

Data-based comparisons of moments estimators using historical and paleoflood data

Abstract This paper presents the first systematic comparison, using historical and paleoflood data, of moments-based flood frequency methods. Peak flow estimates were compiled from streamflow-gaging stations with historical and/or paleoflood data at 36 sites located in the United States, Argentina, United Kingdom and China, covering a diverse range of hydrologic conditions. The Expected Moments Algorithm (EMA) and the Bulletin 17B historical weighting procedure (B17H) were compared in terms of goodness of fit using 25 of the data sets. Results from this comparison indicate that EMA is a viable alternative to current B17H procedures from an operational perspective, and performed equal to or better than B17H for the data analyzed. We demonstrate satisfactory EMA performance for the remaining 11 sites with multiple thresholds and binomial censoring, which B17H cannot accommodate. It is shown that the EMA estimator readily incorporates these types of information and the LP-III distribution provided an adequate fit to the data in most cases. The results shown here are consistent with Monte Carlo simulation studies, and demonstrate that EMA is preferred overall to B17H. The Bulletin 17B document could be revised to include an option for EMA as an alternative to the existing historical weighting approach. These results are of practical relevance to hydrologists and water resources managers for applications in floodplain management, design of hydraulic structures, and risk analysis for dams.

Wading measurements of vertical velocity profiles

Abstract Increasingly, there is a recognized need to enhance research of hydraulic, geomorphic and sediment-transport processes in rivers. This research often includes studies of the velocity distribution in rivers. This paper describes the use of the topsetting wading rod and a table and an analytical expression to more easily obtain vertical velocity profiles in rivers.