Principal Spillway Research Articles

Simulation of Rainfall-Runoff Process in a Catchment with a Check-Dam System Equipped with a Perforated Riser Principal Spillway on the Loess Plateau of China

Check dams are applied worldwide as an effective approach for soil and water conservation. To improve the simulation accuracy of the hydrological processes in a catchment with a check-dam system, this study analyzed the applicability and accuracy of a formula for the drainage process of a perforated riser principal spillway based on observational experiments. The rainfall-runoff processes in a catchment with a check-dam system were also simulated with the recommended formulas for the drainage process of a perforated riser principal spillway. The deviations in the calculated discharge from the observed values of the experiment with the recommended formulas under normal and abnormal working conditions were within ±15% and ±5%, respectively. The hydrologic model used in this study needed only a few parameters to achieve a satisfactory simulation accuracy. The recommended formulas for the drainage process of a perforated riser principal spillway can improve the simulation accuracy of a flood peak by 7.42% and 19.58% compared with the accuracies of the technical code formula scenario and no drainage scenario, respectively. The results of this study are expected to provide a reference for flood warnings and safe operations of check-dam systems.

EFFECTIVENESS OF SEDIMENTATION BASINS THAT DO NOT TOTALLY IMPOUND A RUNOFF EVENT

Twelve simulated 35–mm, 2–yr, 24–hr runoff events from a denuded 0.4–ha construction site in Pennsylvania were introduced into a 51–m 3 sedimentation basin. Each event consisted of a 100–m 3 inflow hydrograph that contained a sedigraph with 454 kg of soil. The basin’s sediment–capture ability was evaluated during four sediment removal/dewatering control treatments, each designed to dewater the basin in 24 hrs. Additionally, these small–basin sediment capture results were compared to those obtained from a 142–m 3 large basin previously charged with identical hydrographs and sedigraphs. The perforated riser treatment in the small basin discharged 37.5 kg of the injected 454 kg of soil through the combined principal and emergency spillways, in comparison to 31.8 kg discharged from the large basin, where only the principal spillway was used. The skimmer treatment in the small basin discharged 26.4 kg of the injected 454 kg of soil through the combined principal and emergency spillways, in comparison to 15.0 kg discharged from the large basin. On average, the small basin discharged 150% more sediment than the large basin. The addition of trenched–in geotextile barriers oriented perpendicular to the primary flow direction in the basin did not cause a significant improvement in the capture efficiency of either basin or for either of the principal spillway configurations.

Sediment Retention Efficiencies of Sedimentation Basin Filtered Outlets

Principal spillway filter envelopes of expanded polystyrene chips (EPS) and 2-B gravel were compared to a no-filter control to assess their effectiveness in capturing sediment from a laboratory scale sedimentation basin. Each filter was evaluated with two perforated risers, one designed to dewater the basin in 1.5 h and the other in 3.0 h. The average sediment retention efficiencies for the no-filter treatments were 59.8 and 71.0% for the 1.5- and 3.0-h outlet configurations, respectively. Perforated risers encased in 508-mm-diameter envelopes of EPS chips and gravel had sediment retention efficiencies, averaged over dewatering time, of 84 and 82%, respectively. The 1.5- and 3.0-h dewatering times had sediment retention efficiencies, averaged over the EPS- and gravel-filter treatments, of 78.3 and 87.5%, respectively. Both EPS and gravel envelopes show promise in enhancing the sediment captured from sedimentation basins.

HGL Elevation at Pipe Exit of USBR Type VI Impact Basin

Model tests on a Soil Conservation Service principal spillway with a U.S. Bureau of Reclamation Type VI impact basin for the outlet structure provided an opportunity to establish a relationship to predict the hydraulic grade line (HGL) elevation at the pipe exit as a function of the tailwater (TW) elevation. The relationship is \IHGL\N/\ID\N = 0.85 + 0.76(\ITW\N/\ID\N); \ITW\N/\ID\N > 0.65 or \IHGL\N/\ID\N > = 1.31, where \IHGL\N is the HGL elevation relative to the pipe-exit invert; \ID\N is the pipe diameter; and \ITW\N is the tailwater elevation downstream of the basin relative to the pipe-exit invert. This relationship permits accurate determination of the head for flow computations. The equation is only valid at design discharge for impact basins designed according to \ITechnical Release No. 49,\N U.S. Dept. of Agriculture, Soil Conservation Service. Because the results showed that a \ITW\N/\ID\N = 0.65 had no adverse effect on the HGL elevation, a minimum \ITW\N/\ID\N = 0.65 is recommended for design because the tailwater reduces water surface roughness and bed erosion.

Evaluation of Principal Spillway Conduit Life

Visual inspection of soil and water sampling near 38 principal spillway conduits in Kansas PL-566 watershed dams were used to determine overall condition and obtain estimates of the expected average life of various conduit materials. Corrugated metal and C-76 reinforced concrete pipes had the lowest ratings and shortest expected average life of all materials in the sample. Expected life of corrugated metal was limited primarily by corrosion and leakage at joints. Most C-76 pipes were cracked. Lined welded steel and C-301 concrete steel cylinder pipes had the longest expected average life. Unlined welded steel pipes were usually corroded, while welded steel pipes lined with cement mortar or coal tar enamel were less corroded. C-301 steel cylinder concrete pipe was generally in excellent condition except for minor cracking of the lining and minor corrosion of the steel at a few joints. Some relationships between the expected life of conduits and environmental conditions were also found.

Model Study for Non-Standard Principal Spillway

Model tests were conducted on a non-standard principal spillway to evaluate the performance of the proposed drop inlet and impact basin and to evaluate the riprap size and placement downstream of the impact basin. An undistorted 1:24 model scale was used. The prototype pipe was 2.13 m (84 in.) in diameter. The outlet structure was a USBR Type VI impact basin. The design discharge was 43.9 m/s (1550 cfs). The spillway was a 1 3, two-stage, covered drop inlet with baffle trash racks and four low-stage orifices that occupied half the length of each side. The proposed design performed poorly with flow in the orifices. The trash rack was modified, performed very well, and was recommended for design. With the drop inlet in a berm [with the berm 0.610 m (2.0 ft) below the orifice invert elevations] the entrance loss coefficients were 0.28 and 0.26, respectively, with and without orifices. Locating the drop inlet in a berm will require riprap around the drop inlet entrance to prevent erosion of the berm. The outlet impact basin performed very well. There was minor scour over a considerable area downstream of the basin, but the riprap design is adequate.

Visual Inspection System for Principal Spillway Conduits

A system which consists of a portable, self-propelled vehicle to transport a video camera to record a tape of the inside of circular conduits is described. A rating system to evaluate the condition of the conduit based upon the visual aspects observed of cracks, corrosion, lining, and joints is presented. The equipment and rating procedure provide a method to evaluate the condition of principal spillway conduits with diameters 38 cm (15 in.) and larger.

Approximate Sizing of Reservoirs for Detention Time

To meet the detention time requirements of the Office of Surface Mining, sediment control reservoirs must be sized to give a certain detention time, as defined by the time difference between the center of mass of the inflow and outflow hydrographs. Designing a reservoir to give a certain detention time is time consuming as a principal spillway size must be selected, the routing accomplished, and the detention time calculated. Often several pipe sizes might be tried before the necessary detention time is attained. To minimize the trial and error, a procedure using triangular hydrographs is developed to give a first approximation of the peak outflow and storage volume necessary to give a desired detention time. The procedures are tested against detailed routings and are found to be good first approximations. The procedures are applicable to reservoirs with principal spillway crests at the top of the sediment storage pool.

Floodwater Retarding Structure Yield Impact

ABSTRACT KNOWLEDGE of the magnitude of impact of the Soil Conserva-tion Service's flood retention program on downstream runoff yield in water-scarce areas is very important. The paper examines published yield im-pact research for general information, usable in making impact research for general information, usable in mak-ing impact statements. No relation-ship or conclusions have been found that adequately support preparation of yield impact statements. The upstream watershed-protection and flood-prevention program of the Soil Conservation Service is very im-portant to agriculture. Directed pri-marily toward flood and sediment re-duction, it also affects agricultural water supply and irrigation. Sloggett (1974), Smith and Mills (1972), and McGill and Whisenant (1967) have covered needs, scope, authorizations under law, and progress. One of the most important com-ponents of the program is the flood-water-retarding structure (FRS), a small dam with a principal and em-ergency spillway. The FRS impounds runoff from the drainage area above the dam and releases it over a period of days, reducing downstream flood peaks and sedimentation.