CHAPTER 13 SECTION 6

Transcription

1 SEPTEMBER 30, 2008 CH13-600

2 TABLE OF CONTENTS 6.1 INTRODUCTION CHANNEL STABILIZATION APPROACHES TO RIVER RESTORATION ENERGY DISSIPATION CHANNEL GRADE CONTROL TYPES OF S BOULDER GROUTED SLOPING BOULDER FORMED AND SCULTED CONCRETE RIFFLE VERTICAL HARD BASIN BASIC APPROACH TO STRUCTURE SELECTION AESTHETICS AND ENVIRONMENTAL IMPACT LOW-FLOW CHECK AND WETLAND MAINTENANCE DETAILED ANALYSIS CREST AND UPSTREAM S WATER SURFACE PROFILE DOWNSTREAM OF THE CREST Critical Depth Along a Drop Structure Hydraulic Analysis Manning s n for Concrete, Boulders and Grouted Boulders Avoid Low Froude Number Jumps in Grass-Lined Channels JUMP Hydraulic Jump Jump and Basin Length SEEPAGE ANALYSIS FORCE ANALYSIS Shear Stress Buoyant Weight of Structure Impact, Drag and Hydrodynamic Lift Forces Turning Force Friction Frost Heave Seepage Uplift Pressure Dynamic Pressure Fluctuations Overall Analysis SEEPAGE ANALYSIS METHODS FOUNDATION/SEEPAGE CONTROL SYSTEMS FOUNDATION/SEEPAGE CONTROL CHECKLIST 649 SEPTEMBER 30, 2008 CH13-601

3 ANAYLSIS AND Grouted sloping boulder drops can be built in series to create pleasing amenities and provide stable and long-lived grade control structures. A stepped boulder drop structure provides dissipation of energy at each step for low flows. This chapter was developed with gracious cooperation from the Urban Drainage & Flood Control District. Although the information has been reformatted, much of this section s content (text and graphics) are directly from the Urban Storm Drainage Criteria Manual, Volume 2: Hydraulic Structures, Chapter 2: Channel Grade Control Structures, revised January Additional sections were added for Boulder Drop Design, Formed and Sculpted Concrete Drops, and Riffle Drops. Pictures from around the State of Colorado of different drop structures have been added, along with supplemental information in the Introduction, Stabilization, Approaches to River Restoration and Energy Dissipation. 6.1 INTRODUCTION Hydraulic structures are used to guide and control water flow velocity, direction and depth, the elevation and slope of the streambed, the general configuration of the waterway, and its stability and maintenance characteristics. Hydraulic structures for grade control include channel drops, low-flow checks and general energy dissipators. Their shape, size, and other features vary widely for different projects, depending upon the discharge and the function to be accomplished. Hydraulic design procedures must govern the final design of all structures. A boulder drop structure on the Blue River downstream of the Town of Breckenridge. SEPTEMBER 30, 2008 CH13-602

4 To provide vertical stability for a river, drop structures may be used. Drop structures are used for erosion control, grade control, and energy dissipation. The drops are designed to provide a hard control to keep the river from incising or eroding. They can also be instrumented in creating pool riffle sequences for trout habitat. Careful and thorough hydraulic engineering is required for the design of hydraulic structures. Consideration of environmental, ecological, and public safety objectives should be integrated with hydraulic engineering design and floodplain management. The proper application of hydraulic structures can reduce initial and future maintenance costs by managing the character of the flow to fit the environmental and project needs. Engineering is needed in the design of drop structures to appropriately address the following issues at a minimum: Adequately pass all flows up to and including the 100-year (1% probability) flood flow without causing adverse impacts to upstream, downstream or adjacent property owners. Verification of no adverse impact can be demonstrated through the use of a CWCB approved step-backwater hydraulics model such as HEC-RAS. Adequately withstand stream forces for all flows up to and including the 100-year flood so that damage and failure of the boating course is avoided. Where applicable, the proposed drop structure must be analyzed for flood impacts by modeling the existing conditions (pre-facility) scenario and the proposed conditions (with-facility) scenario. The hydraulic computer modeling must be performed by a registered Professional Engineer (Colorado Registration) having expertise in surface water hydraulic modeling. Impacts that should be analyzed include increases in water surface elevations for the range of flows experienced at the proposed recreational facility, but especially the 100-year flood event. According to federal regulations, water surface elevations at the proposed drop structure may not increase at all ( zero rise ) in the event that the proposed recreational facility is within a stream reach having a designated Floodway (the most hazardous portion within the 100-year floodplain). For reaches that are not within a designated Floodway, any increases in the 100-year flood profile for the proposed drop structure (up to a maximum of one foot or a lesser amount specified by local floodplain ordinances or codes, whichever is more restrictive) must be documented. Proper notification of adverse impacts (increased water surface elevations) must be provided to affected property owners and a Letter of Map Revision (LOMR) should be obtained as required by FEMA for National Flood Insurance Program purposes. It is recommended that the drop structure consist of appropriate materials and installation to endure velocities, shear stresses, other erosive conditions, sedimentation, and debris that would be experienced during the range of expected flows and high flow (flood) conditions. For example, a boulder drop structure should be constructed with material having suitable size, hardness, shape and gradation and should be installed in such a manner that it is anchored into the stream bank and streambed to survive long-term operation and flood events. Proper durability of the facility will assist the applicant in avoiding costly repairs and maintenance. SEPTEMBER 30, 2008 CH13-603

5 A structure introduced into the waterway can impact the dynamics of the stream, including its flow and configuration. The goal is to use stabilization structures that work with the stream s natural dynamics to enhance the waterway for habitat and flood control. Accomplishing this goal requires planning, expert assistance and a maintenance program. 6.2 CHANNEL STABILIZATION Channel stabilization reestablishes a river s pattern and geometry to resist long-term erosion or sediment deposition which can alter habitat and impact land use along a drainage corridor. Restoration efforts may involve stabilizing a river by adjusting the channel slope and strengthening the banks to resist overbank flooding. Controlling structures will eventually fail without regular maintenance. Nature will find the weak point and continually work to overcome the control and reestablish equilibrium. Maintenance is critical to the long-term success of the structures. 6.3 APPROACHES TO RIVER RESTORATION The basic design principles are shown on the following graphics. The structures should be moderately angled into a V pattern pointing upstream to focus the river s energy into the center of the channel and away from the banks to reduce bank erosion. The downstream angle inside the V should be 120 to 180. The drop structure should be constructed with a slight dip in the center of 4 to 18 inches to concentrate low flows to the center of the channel. A second minor sill wall installed downstream from the first primary drop is designed to back-up water and create a pool between the structures. The pool dissipates the energy of the river over the first drop and minimizes the chances the structure will be undercut by scour. The angle of the second drop should be 135 to 180, though frequently the second structure is constructed at 180 (straight across the river). The spacing (S) of the drop and sill wall is typically 0.3 to 0.6 times the channel width (W). A vertical water SEPTEMBER 30, 2008 CH13-604

6 surface drop less than 18 inches allows fish migration and safe boating passage. Shallow drops minimize the amount of scour occurring downstream which can undermine the structure. For aesthetics and availability of material drop structures can be constructed of rock boulders, although they are also frequently built of concrete and/or sheet piling. Boulders work best where the river is small enough to control. The primary causes of failure in rock drop structures are piping, sliding, and undermining. Piping occurs when the water upstream of the structure seeps through the foundation and the upstream side of the structure develops a sinkhole. As this continues the water can work its way under the drop structure and create a tunnel effect until the structure collapses. This can be avoided by constructing a watertight foundation under the structure through the use of cutoff walls. Sliding develops when the river moves the rocks in the downstream direction. To avoid sliding, the rock in the structure should be heavy enough to counter the drag forces due to the water flow. Also, a firm foundation attached to the structure will counter sliding. Undermining occurs when a scour hole on the downstream side of the drop structure forms and the boulders roll into the hole. This can be avoided by armoring the downstream bed of the drop structure. 6.4 ENERGY DISSIPATION The energy of moving water is known as kinetic energy, while the stored energy due to elevation is potential energy. A properly sloped open channel will use up the potential energy in a uniform manner through channel roughness without the flow being accelerated. A grade control structure (i.e., drop and check) converts potential energy to kinetic energy under controlled conditions. Selection of the optimum spacing and SEPTEMBER 30, 2008 CH13-605

7 vertical drop is the work of the hydraulic engineer. Many hydraulic structures deal with managing kinetic energy to dissipate it in a reasonable manner, to conserve it at structures such as transitions and bridges, or occasionally to convert kinetic to potential energy using a hydraulic jump. Thus, managing energy involves understanding and managing the total energy grade line of flowing water. 6.5 CHANNEL GRADE CONTROL Grade control structures, such as check structures and drop structures, provide for energy dissipation and thereby result in a mild slope in the upstream channel reaches. The geometry at the crest of these structures can effectively control the upstream channel stability and, to an extent, its ultimate configuration. A drop structure traverses the entire waterway, including the portion that carries the major flood. A check structure is similar, but is constructed to stabilize the low-flow channel (i.e., one carrying the minor or lesser flood) in artificial or natural drainageways. It crosses only the low-flow portion of the waterway or floodplain. During a major flood, portions of the flow will circumvent the check. Overall channel stability is maintained because degradation of the low-flow channel is prevented. Typically, the 2-year flows are contained in the protected zone so that the low-flow channel does not degrade downward, potentially undermining the entire waterway. These naturally occurring drops dissipate energy and promote channel stability. 6.6 TYPES OF S Drop Structures are broadly defined. They establish a stable stream grade and hydraulic condition. Included are structures built to restore damaged channels, those that prevent accelerated erosion caused by increased runoff, and grade control drops in new channels. Drop structures provide special hydraulic conditions that allow a drop in water surface and/or channel grade. The supercritical flow may go through a hydraulic jump and then return to subcritical flow. There are two general categories of drops: sloping and vertical. For safety reasons, vertical drops should be avoided under urban conditions for public safety reasons. SEPTEMBER 30, 2008 CH13-606

8 Performance of vertical or smooth sloping drops into a hard basin is relatively well documented. Specific design guidance is presented for the following basic categories of drop structures: Sloping Drops o Boulder Drops o Grouted Sloping Boulder Drops o Sculpted Concrete Drops o Riffle Drops Vertical Hard Basin Drops o Sheet Pile o Concrete Wall Baffle Chute Drops are another type of structure and are a concrete flume design with blocks for energy dissipation. This type of design has specific application to steep slopes (2:1) needing a high degree of control and protection. One advantage of this type of drop is that it does not require tailwater control. Consult U.S. Bureau of Reclamation (USBR) design manuals for further guidance on this type of structure. The USBR has developed design standards for a reinforced concrete chute with baffle blocks on the sloping face of the drop, commonly referred to as baffled apron or baffle chute drops. There are references such as Hydraulic Design of Stilling Basins and Energy Dissipators (Peterka 1984) and Design of Small Canal Structures (Aisenbrey, et al. 1978) that should be used for the design of these structures. Baffle Chute Drop All drop structures should be evaluated after construction. Bank and bottom protection and adjustments may be needed when secondary erosion tendencies are revealed. It is advisable to establish construction contracts and budgets with this in mind. Use of standardized design methods for the types of drops suggested herein will reduce the need for secondary design refinements. 6.7 BOULDER This type of structure has gained acceptance in the Rocky Mountain region due to close proximity to high-quality rock sources, design aesthetics, and successful applications. SEPTEMBER 30, 2008 CH13-607

9 The size, quality, durability, angularity and density of rock used are very important to the structural integrity. Boulder drop structures have been commonly used for irrigation diversions to control the elevation of the low flow water surface. The water surface drop height is typically limited to 3-feet for ungrouted boulder structures. Grouted structures are recommended over non-grouted structures for stability against flood forces. The Urban Drainage & Flood Control District requires grouting of all boulder drop structures. However, the option to allow non-grouted structures can be decided locally. Ungrouted boulders may unravel during a flood event and therefore require well integrated larger rocks to maintain structural integrity. Reducing the slope of the drop reduces the minimum required boulder size. Reducing the depth of flow over the drop by increasing the width also reduces the minimum required boulder size. In no case shall the boulders be less than 18-inch diameter. The following Table 6.7 illustrates the boulder sizing technique: Table 6.7 Boulder Sizes for Various Rock Sizing Parameters Ungrouted Boulders Grouted Boulders * Rock Sizing Parameter, R p Minimum Dimensions of Boulder, D r Boulder Classification Minimum Dimensions of Boulder, D r Boulder Classification Less than inches B18 18 inches B to inches B24 18 inches B to inches B30 24 inches B to inches B36 30 inches B to inches B42 36 inches B to inches B48 42 inches B to 8.00 n/a n/a 48 inches B48 * Grouted to 1 / 3 to ½ the boulder height. The following detailed design procedure is needed to determine R p for the previous table: 1. Determine the critical velocity, V c, using drawdown calculations to establish the 100-year flow depth at the toe of the drop. V c = (gy c ) 0.5 ; where g=32.2 ft/s 2 2. Calculate rock-sizing parameter, R p, for the trapezoidal channel cross-section segment using the critical velocity estimated for this segment of the cross section: R p = V 0.17 ( S 1) s in which: c S S = longitudinal slope of the drop along direction of flow in ft/ft S s = Specific gravity of the rock. Assume 2.55 unless the quarry certifies higher specific gravity. SEPTEMBER 30, 2008 CH13-608

10 COLORADO A boulder drop structure provides stable and predictable water levels at irrigation diversion turnouts. The drop, located downstream of a bridge, reduces local channel degradation at the bridge abutments and enhances aquatic habitat. Smooth rocks are placed in the center of the drop to improve safety for boaters. The water surface drop should be kept less than 3 feet, and preferably only 18 inches to protect the integrity of the structure and maintain fish migration. Boulder drop structures must be anchored into the bank with large boulders and a dip should be constructed in the center to focus the river s energy into the center of the channel. These large angular boulders on the Blue River worked well to establish grade control and resist movement during a flood. Small drops provide vertical stability and can protect buried utility lines upstream of the drop. Properly designed drop structures can also provide recreational value. SEPTEMBER 30, 2008 CH13-609

11 6.8 GROUTED SLOPING BOULDER Construct sloping grouted boulder drops using uniform-height boulders with a minimum height specified in Table 6.8. Grout all boulders to a depth of 1/2 or 1/3 of their height through the approach, sloping face, and basin areas, except at the upstream crest where it needs to extend the full depth of the rock in order to provide stability of the approach channel. The spacing and integration of rock used and proper grouting procedure are very important to the structural integrity. There is no maximum height limit; however, the rock sizing procedure is more complex for grouted sloping drops 6-feet or more in height. Adequate seepage control with underdrains is important for a successful design whenever drop height exceeds 5-feet. The following figures illustrate the general configuration of three types of grouted sloping drops which include the following: A trickle channel for intermittent drainageway (Figure 6.8A), A low-flow channel for ephemeral and perennial streams (Figure 6.8B), Channels in erosive soils or unstable conditions (Figure 6.8C). Table 6.8 Grouted Sloping Boulder Drops: Minimum Design Criteria for Grass-Lined Channels Design Parameter Drop Height (H d ) 6 Feet or Less Drop Height (H d ) Greater Than 6 Feet Maximum longitudinal slope 4H to 1V 4H to 1V Minimum boulder depth Use V c to size* Use V n to size** Grout thickness D g ½ to 1 / 3 D r except at the upstream crest of the structure where full grout depth is needed ½ D r to 1 / 3 D r except at the upstream crest of the structure where full grout depth is needed Basin depression 1 to 2 feet Do sequential depth analysis Grouted boulder approach L a 5 feet (min.) 8 feet Basin length L b ** Erosive (sandy channel) Non-erosive Basin width B Trickle and low-flow zone provisions 20 feet 15 feet Same as crest width 20 feet 15 feet Install large boulders in center basin zone to break up high flow stream Other provisions A buried riprap zone should be installed for 2H d (10 feet minimum) downstream of the basin. Do not locate a drop within a channel curve or immediately downstream of one. * Use critical velocity in low-flow and main channels to size boulders. ** Use drawdown velocity at H d to size low-flow and main channel section boulders. SEPTEMBER 30, 2008 CH13-610

12 Placement of grout around the boulders during the construction of a grouted boulder drop structure on Sand Creek at Quebec Street in Denver, CO. Construction of grouted boulder drop on Fountain Creek in Colorado Springs, CO. SEPTEMBER 30, 2008 CH13-611

13 For typical channels the drop is designed with a hydraulic jump dissipator basin, although some energy loss is incurred due to the roughness of the grouted rock slope. In sandy soil channels the design provides for a scour at the toe and does not require an energy-dissipating basin. Structure integrity and containment of the erosive turbulence within the basin area are the main design objectives. The following outlines the fundamental design steps and guidelines. 1. Hydraulics should be completed whenever the drop height exceeds 6 feet. Otherwise, use critical depth to size the boulders, using the boulder sizing procedure described below. 2. Grouted boulders must cover the crest and cutoff and extend downstream through the energy-dissipating basin when there is one, or through the imbedded toe of the drop when not present. 3. The vertical cutoff should be located at the upstream face of the crest, at a minimum depth of 0.8H d or 4 feet, whichever is deeper. Evaluate specific site soils for use in seepage analysis and foundation suitability. 4. The trickle or low-flow channel should extend through the drop crest section. Downstream, the trickle or low-flow channel protection should extend past the main channel protection, or large boulders and curves in the trickle or low-flow channel can be used in the basin area to help dissipate the energy. 5. Grout thickness, D g, and rock thickness, D r, should be determined based upon a minimum safety surplus net downward force of 30 pounds. The rocks must be carefully placed to create a stepped appearance, which helps to increase roughness. 6. The main stilling basin should be depressed 1 to 2 feet deep in order to stabilize the jump. A row of boulders should be located at the basin end to create a sill transition to the downstream invert elevation. It is advisable to bury riprap for a distance of 10 feet downstream of the sill to minimize any erosion that may occur due to secondary currents. When the drop is located in sandy soils and in channels with lesser stability, the stilling basin is eliminated and the sloping face extended to where the top of the boulders are five feet (5 ) below the projected (i.e., after accounting for downstream degradation) downstream channel invert. 7. Do not use longitudinal slopes steeper than 4:1. Longitudinal slopes flatter than 4:1 improve appearance and safety while steeper slopes reduce structural stability. With high public usage, very flat longitudinal slopes (i.e., flatter than 8H:1V) help to mitigate reverse roller formation at higher tailwater depths that can cause submerged hydraulic jump formation and create keepers. SEPTEMBER 30, 2008 CH13-612

14 To determine the required boulder sizing for the drops, a detailed design procedure has to be used consisting of the following: 1. Determine the critical velocities using drawdown calculations to establish the 100-year flow depth at the toe of the drop. a. For a composite channel, find critical velocity, V c, for the channel crosssection segment outside the low-flow section. b. For a composite channel, find critical velocity, V mc, for the low-flow channel cross-section segment. c. For a simple trapezoidal or wetland bottom channel, find critical velocity, V c, for the channel cross section. 2. Calculate rock-sizing parameter, R p, for the channel cross-section segment outside the low-flow section or for a simple trapezoidal channel section using the critical velocity estimated for this segment of the cross section: R p = V 0.17 ( S 1) s in which: c S S = longitudinal slope along direction of flow in ft/ft S s = Specific gravity of the rock. Assume 2.55 unless the quarry certifies higher specific gravity. 3. Calculate rock-sizing parameter, R pl, for the channel cross-section segment within the low-flow section using the critical velocity for drops 6-feet in height (the draw-down velocity estimates at bottom of the drop for taller structures ): R pl = V 0.17 ( S 1) s mc S 4. Select minimum boulder sizes for the cross-section segments within and outside the low-flow channel cross-section from Table 6.8. If the boulder sizes for the low-flow channel and the overbank segments differ, decide to use only the larger sized boulders throughout the entire structure, or to specify two sizes, namely, one for the low-flow channel and the other for the overbank segments of the cross section. Consider the complexity of specifying two different sizes on the design drawings and in the construction of the structure before deciding. Regardless of the design procedure used above, all boulders shall be grouted in accordance with the specifications Figure 6.8D. All grouted boulders outside of the lowflow channel shall be buried with topsoil to a depth of no less than 4 inches (6 inches or more preferred for successful grass growth) above the top of the highest boulder and the surface vegetated with native grasses on the overbank bench and native grasses and dryland shrubs on the overbank channel s side slopes. SEPTEMBER 30, 2008 CH13-613

15 B C D CREST GROUTED BOULDERS TOE SILL E B Z*Yn Z*Yn 2 36" MIN. GROUTED BOULDER SILL WEEP DRAIN SYSTEM TYPE M RIPRAP (BURIED) FLOW TRICKLE CHANNEL B A STRUCTURE CL TRICKLE CHANNEL BASIN Z*Yn Z*Yn 15 S:1 S:1 S:1 BURIED RIPRAP RAISE GROUT LEVEL IN THE BASIN TO 3 4 THE BOULDER DEPTH SO BASIN CAN DRAIN DOWNSTREAM. 15 S:1 15' MIN. Z*Yn Z*Yn 2 S:1 S:1 STRUCTURE PLAN NTS MAIN CHANNEL INVERT 1.0' FLOW BURIED TYPE M RIPRAP > 8' So S o=0.0 La > 8' Lf = S*(Hd+Db) 15' MIN. FACE BASIN CREST TOP OF ROCK TOE 2*Hd (10' MIN.) 36'' MIN. GROUTED BOULDER SILL AT END OF BASIN CHINK BETWEEN SILL BOULDERS WITH RIPRAP TO PREVENT SMALLER ROCK AND TOPSOIL FROM FALLING THROUGH BETWEEN BOULDERS TRICKLE CHANNEL INVERT BEYOND TRICKLE CHANNEL INVERT ELEV. AT CREST DEPTH VARIES, SEE SECTION C 15" MIN. NTS SEEPAGE CUTOFF. SEE DETAIL 1 WEEP DRAIN SYSTEM. SEE DETAIL GROUTED BOULDERS SEE DETAIL 3 STRUCTURE PROFILE TYPE M RIPRAP SEPTEMBER 30, 2008 CH D b Hd TRICKLE CHANNEL INVERT ELEV. AT SILL, SEE SECTION Figure 6.8A Grouted Sloping Boulder Drop with Trickle Channel for Stabilized Channels in Erosion Resistant Soils (Figure 1 of 2) A E 1.0' So GRANULAR BEDDING MATERIAL (PER UDFCD REQUIREMENTS) MAIN CHANNEL INVERT TRICKLE CHANNEL INVERT

16 STRUCTURE C L 4' MIN. WIDTH TRICKLE CHANNEL 100-YR W.S. Z>4 Z> % Y n<5' 1% TRICKLE CHANNEL TYPICAL CHANNEL SECTION (UPSTREAM AND DOWNSTREAM OF ) NTS B STRUCTURE C L TOP OF ROCK ELEVATION Z*Y n TRICKLE CHANNEL ELEV. AT CREST TOP OF 6' Z>4 ROCK Z>4 1 1' 1 1% 1% B Z*Y n TOP OF ROCK ELEVATION GROUTED BOULDERS DEPTH OF CUTOFF TO BE DETERMINED BASED UPON GEOTECHNICAL INVESTIGATIONS AND SEEPAGE ANALYSIS. IN THE ABSENCE OF A SEEPAGE ANALYSIS, SEE SEEPAGE CUTOFF DETAIL 1 SEEPAGE CUTOFF SECTION NTS C SEEPAGE CUTOFF STRUCTURE C L Z*Y n B Z*Y n TOP OF ROCK ELEV. VARIES TOP OF ROCK ELEV. VARIES Z>4 TOP OF Z>4 1 ROCK 1 GROUTED BOULDERS UNDISTURBED SOIL, OR COMPACTED MATERIAL TYPICAL FACE SECTION NTS D STRUCTURE CL TOP OF ROCK ELEV. Z*Y n TRICKLE CHANNEL ELEV. AT SILL GROUTED TOP OF BOULDER SILL ROCK RAISE GROUT LEVEL IN THE TRICKLE ZONE SO Z>4 Z>4 BASIN CAN DRAIN DOWNSTREAM 1 D B =0.5*D r 1 1% (1.0' MIN.) 1% B Z*Y n TOP OF ROCK ELEV. UNDISTURBED SOIL, OR COMPACTED MATERIAL TYPICAL BASIN SECTION AND SILL NTS BASIN BOTTOM WIDTH VARIES GROUTED BOULDERS Figure 6.8A Grouted Sloping Boulder Drop with Trickle Channel for Stabilized Channels and Erosion Resistant Soils (Figure 2 of 2) SEPTEMBER 30, 2008 CH E

17 B C CREST D GROUTED BOULDERS TOE SILL E B Z*Yn Z*Yn 2 A 36'' MIN. GROUTED BOULDER SILL TYPE M RIPRAP S:1 B L B Z :1 L Z :1 L FLOW LC Z :1 L Z :1 L WEEP DRAIN SYSTEM S:1 S:1 STRUCTURE C L S:1 1% 1% 1% 1% Z :1 L Z :1 L Z :1 L BASIN Z :1 L Z :1 Z :1 L L B L B 15' MIN. 1% 1% 1% 1% Z*Yn Z*Yn 2 STRUCTURE PLAN NTS FLOW LOW FLOW CHANNEL INVERT ELEV. AT CREST BURIED TYPE M RIPRAP > 8' S o S o= 0.0 La > 8' Lf = S*(Hd+Db) 15' MIN. FACE BASIN CREST TOP OF ROCK S > 4 1 TOE Db Hd 2*Hd (10' MIN.) SILL 36'' MIN. GROUTED BOULDER SILL AT END OF BASIN SEE SECTION E CHINK BETWEEN SILL BOULDERS WITH RIPRAP TO PREVENT SMALLER ROCK AND TOPSOIL FROM FALLING THROUGH BETWEEN BOULDERS So DEPTH VARIES, SEE SECTION C SEEPAGE CUTOFF SEE DETAIL 1 WEEP DRAIN SYSTEM. SEE DETAIL 2 STRUCTURE PROFILE NTS GROUTED BOULDERS SEE DETAIL SEPTEMBER 30, 2008 CH A 3 TYPE M RIPRAP Figure 6.8B Grouted Sloping Boulder Drop with Low-Flow Channel for Stabilized Channels in Erosion Resistant Soils (Figure 1 of 2) LOW FLOW CHANNEL INVERT GRANULAR BEDDING MATERIAL (PER UDFCD REQUIREMENTS)

18 STRUCTURE CL * Z > 4 * Z L > 2.5 B L B = MAIN CHANNEL WIDTH 9' B L = LOW FLOW WIDTH B R Z* Z* 1 1% 1% 1 * ZL Y Z * L 1 Y L = 3'-5' 1 3' MIN. 12" THICKNESS TYPE VL RIPRAP, TYP. BOTH SIDES TYPICAL CHANNEL SECTION (UPSTREAM AND DOWNSTREAM OF NTS B B L STRUCTURE CL B L = LOW FLOW WIDTH B R TOP OF ROCK ELEVATION * Z > 4 * Z L > 2.5 TOP OF ROCK ELEVATION GROUTED BOULDERS Z* 1 1% TOP OF ROCK Y L = 3'-5' LOW FLOW CHANNEL ELEV. AT CREST 1% Z* 1 DEPTH OF CUTOFF TO BE DETERMINED BASED UPON GEOTECHNICAL INVESTIGATIONS AND SEEPAGE ANALYSIS. IN THE ABSENCE OF A SEEPAGE ANALYSIS, SEE SEEPAGE CUTOFF DETAIL 1 SEEPAGE CUTOFF SECTION NTS C SEEPAGE CUTOFF STRUCTURE CL B L B L = LOW FLOW WIDTH B R TOP OF ROCK ELEV. VARIES * Z > 4 * Z L > 2.5 TOP OF ROCK ELEV. VARIES Z* 1 1% Z * L 1 TOP OF ROCK Y L = 3'-5' Z* L 1 1% Z* 1 UNDISTURBED SOIL, OR COMPACTED MATERIAL GROUTED BOULDERS TYPICAL FACE SECTION NTS D STRUCTURE CL B = MAIN CHANNEL WIDTH TOP OF ROCK ELEV. * Z > 4 * Z L > 2.5 B L B L = LOW FLOW WIDTH B R TOP OF ROCK ELEV. Z* 1 TOP OF ROCK 1% * ZL 1 LOW FLOW CHANNEL ELEV. AT SILL D B =0.75*D (1.0' MIN.) r GROUTED BOULDER SILL * ZL 1 1% 1 Z* UNDISTURBED SOIL, OR COMPACTED MATERIAL TYPICAL BASIN SECTION AND SILL NTS GROUTED BOULDERS Figure 6.8B Grouted Sloping Boulder Drop With Low-Flow Channel For Stabilized Channels and Erosion Resistant Soils (Figure 2 of 2) SEPTEMBER 30, 2008 CH E

19 B C D CREST GROUTED BOULDERS TOE B A B FLOW Z*Yn LC BURIED RIPRAP Z*Yn Z*Yn Z*Yn WEEP DRAIN SYSTEM S:1 S:1 S:1 S:1 S:1 S:1 STRUCTURE CENTERLINE SCOUR STILLING BASIN STRUCTURE PLAN NTS FLOW TRICKLE CHANNEL INVERT BEYOND MAIN CHANNEL INVERT 1.0' TRICKLE CHANNEL INVERT ELEV. AT CREST DEPTH VARIES, SEE SECTION C BURIED TYPE M RIPRAP > 8' 15" MIN. La > 8' CREST So o S =0.0 SEEPAGE CUTOFF. SEE DETAIL 1 WEEP DRAIN SYSTEM. SEE DETAIL 2 Lf = S*(Hd+Db) FACE TOP OF ROCK S > 4 * Hd * 10H (MIN) : 1V FOR BOATABLE S So 5' MIN. MAIN CHANNEL INVERT NTS GROUTED BOULDERS SEE DETAIL 3 STRUCTURE PROFILE Figure 6.8C Grouted Sloping Boulder Drop for Unstable Channels in Erosive Soils (Figure 1 of 2) SEPTEMBER 30, 2008 CH A

20 STRUCTURE C L 100-YR W.S. Z>4 Z>4 1 1% Y < 5' n 1% 1 NATURAL LOW-FLOW ZONE TYPICAL CHANNEL SECTION (UPSTREAM AND DOWNSTREAM OF ) B NTS TOP OF ROCK ELEVATION Z n * Y STRUCTURE C L B LOWER CREST TO MATCH LOW-FLOW ZONE (6" MIN., 12" MAX.) Z n * Y TOP OF ROCK AT CREST Z>4 Z >4 1 1% 1% 1 TOP OF ROCK ELEVATION GROUTED BOULDERS DEPTH OF CUTOFF TO BE DETERMINED BASED UPON GEOTECHNICAL INVESTIGATIONS AND SEEPAGE ANALYSIS. IN THE ABSENCE OF A SEEPAGE ANALYSIS, SEE SEEPAGE CUTOFF DETAIL 1 SEEPAGE CUTOFF SECTION NTS C SEEPAGE CUTOFF STRUCTURE C L TOP OF ROCK ELEV. VARIES Z n * Y B Z n * Y TOP OF ROCK ELEV. VARIES TOP OF Z>4 ROCK Z>4 1 1 GROUTED BOULDERS UNDISTURBED SOIL, OR COMPACTED MATERIAL TYPICAL FACE SECTION NTS D Figure 6.8C Grouted Sloping Boulder Drop for Unstable Channels in Erosive Soils. (Figure 2 of 2) SEPTEMBER 30, 2008 CH13-619

21 PROPOSED GRADE 4" GROUT LIMITS (APPROX.) GROUTED BOULDERS 0.8*Hd OR 4', WHICHEVER IS GREATER TOPSOIL IN OVERBANKS ONLY GROUT TO EXTEND UPSTREAM OF BOULDERS THOROUGHLY CLEAN SURFACE OF SEEPAGE CUTOFF PRIOR TO GROUTING BOULDERS ROUGHEN TOP SURFACE OF GROUT CUTOFF 6" WIDE BY 4" DEEP KEYWAY GROUT OF CONCRETE SEEPAGE CUTOFF NTS 15" MIN. SEEPAGE CUTOFF DETAIL 1 SEE DETAIL FOR GROUTED BOULDER CONSTRUCTION 4" NON-PERFORATED LATERAL PIPES SPACED 10' O.C., MAXIMUM. SLOPE AT 1% TO DAYLIGHT AT MAIN CHANNEL INVERT ELEVATION. PIPE ALIGNMENT MAY BE CURVED SLIGHTLY TO FIT BETWEEN BOULDERS. 4" MIN. TOPSOIL COVER OVER HIGHEST ROCK IN AREAS OUTSIDE THE LOW FLOW CHANNEL. 6" TO 12" COVER PROVIDES MORE RUBUST MEDIA FOR VEGETATION SURVIVAL. PLACE APPROVED GEOTEXTILE FILTER FABRIC OVER AND UNDER 3/4" ANGULAR ROCK TO PREVENT CONTAMINATION BY GROUT AND SOILS 1 1 6" 6" FILL SPACES BETWEEN BOULDERS ABOVE GROUT WITH LIGHLY COMPACTED TOP SOIL. TRIM PIPE END TO MINIMIZE PROTRUSION 4'' PERFORATED MANIFOLD PIPE. PROVIDE 4'' TEES TO LATERAL PIPES, AND END CAPS AS REQUIRED. REFER TO PLAN. 3/4" ANGULAR ROCK WEEP DRAIN FILTER MATERIAL. MINIMUM 6" THICKNESS SURROUNDING PIPE SYSTEM AT ALL POINTS NTS WEEP DRAIN SYSTEM DETAILS USE ONLY IN S HIGHER THAN 5 FEET 2 PLACE BOULDERS AS TIGHTLY AS POSSIBLE WITH THE REQUIRED BOULDER HEIGHT VERTICAL TO MINIMIZE VOIDS AND GROUT TOP 1/2 OF BOULDERS TO REMAIN CLEAN AND FREE OF GROUT TOP OF ROCK LAYER SHOWN ON PROFILE AND SECTIONS TOP OF GROUT GROUT 1/2 D r D r NTS PREPARE SUBGRADE PER THE SPECIFICATIONS PLACE GROUT IN A MANNER THAT FILLS ALL VOIDS TO THE SPECIFIED GROUT THICKNESS BEFORE GROUTING, CLEAN ALL DIRT AND MATERIALS FROM ROCK THAT COULD PREVENT THE GROUT FROM BONDING TO ROCK. GROUTED BOULDER PLACEMENT DETAIL Figure 6.8D Grouted Sloping Boulder Drop Details. SEPTEMBER 30, 2008 CH

22 GROUT NOTES Material Specifications Placement Specifications All grout shall have a minimum 28-day compressive strength equal to 3200 psi. One cubic yard of grout shall have a minimum of six (6) sacks of Type II Portland cement. A maximum of 25% Type F Fly Ash may be substituted for the Portland cement. For Type A grout, the aggregate shall be comprised of 70% natural sand (fines) and 30% 3 8 -inch rock (coarse) All Type A grout shall be delivered by means of a low pressure (less than 10 psi) grout pump using a 2-inch diameter nozzle. All Type B grout shall be delivered by means of a low pressure (less than 10 psi) concrete pump using a 3-inch diameter nozzle Full depth penetration of the grout into the boulder voids shall be achieved by injecting grout starting with the nozzle near the bottom and raising it as grout fills, while vibrating grout into place using a pencil vibrator For Type B grout, the aggregate shall be comprised of 3 4 -inch maximum gravel, structural concrete aggregate. Type B grout shall be used in streams with significant perennial flows. The grout slump shall be 4-inches to 6-inches After grout placement, exposed boulder faces shall be cleaned with a wet broom. All grout between boulders shall be treated with a broom finish. All finished grout surfaces shall be sprayed with a clear liquid membrane curing compound as specified in ASTM C Air entrainment shall be 5.5%-7.5%. To control shrinkage and cracking, 1.5 pounds of Fibermesh, or equivalent, shall be used per cubic yard of grout. Color additive in required amounts shall be used when so specified by contract Special procedures shall be required for grout placement when the air temperatures are less than 40 F or greater than 90 F. design engineer of the procedures to be used for protecting the grout. Clean Boulders by brushing and washing before grouting. Figure 6.8E Specifications and Placement Instructions for Grout in Sloping Boulder Drops. SEPTEMBER 30, 2008 CH13-621

23 6.9 FORMED AND SCULTED CONCRETE Although rock has been the past material of choice to resist the scour forces of water over a drop, different forms of concrete have also performed well. Sculpted concrete is a variation of the standard grouted boulder drop by modifying the surface material. The basic shape remains essentially the same. The need for cutoff walls and weep drains also remain the same. However, rather than natural stone, the surface can be man-made. Formed or sculpted concrete can be constructed with structural concrete, shotcrete, soil cement, roller compacted concrete, or event glass fiber reinforced concrete. Special consideration must be given to: Size and configuration Stilling basins Weep drains Cutoffs and seepage control Currently, the State of Colorado nor the Urban Drainage & Flood Control District does not have design guidelines for these unique structures. Each drop must be individually designed and certified by an engineer. The concrete is strengthened with reinforcing steel to minimize cracking. These structures are labor and material intensive. They require careful consideration to the finish and texturing. Care must be given to avoiding voids in the surface finish. Typically, these structures are completed in one or two pours. They can either be constructed by pouring the foundation and then a second pour for the surface, or completed in one full-depth pour. SEPTEMBER 30, 2008 CH13-622

24 COLORADO Sculpted Concrete Drop Structures SEPTEMBER 30, 2008 CH13-623

25 6.10 RIFFLE Riffles occur in nature at bedrock outcrops, or in gravel bed streams at the toe of a pulse or wave where bed material becomes deposited. Riffles created by mobile bed material may change or move during a flood event. Therefore, if the riffle drops are designed to promote channel stability, they must be able to withstand flood forces. Riffle drops are used to mimic natural systems and provide vertical channel stability. Riffles are appropriate on small channels with relatively low design flows, or on channels with vegetated broad floodplains to dissipate the energy of flood flows. Concentrated flood flows can generate high shear stress on the channel bottom and cause the riffle to fail. The failure of the riffle can then cause headcutting to propagate upstream, causing the entire channel reach to become unstable. Shear stress is a function of flood depth and channel slope, so riffles are best suited on drops with shallow slopes and low flood depth. Riffle drops have been used with variable success on channel restoration/stabilization projects. A riffle has the flattest slope of the design drops, usually at a 20:1 or flatter. They are also used in areas where only a shallow drop is necessary, usually 18-inches or less. The drop is constructed with interlocking void-filled riprap, smaller than the boulders recommended for sloping boulder drops to achieve the riffle appearance. Since the rock is relatively small, care must be taken during design and construction to create a structure that prevents mobilization of the placed rock. The design should include an anchor trench of larger rock at the toe of the drop, well below the channel bottom, to provide a firm foundation for the drop. Channel degradation can expose the toe of the drop and cause it to fail. A Fluvial Geomorphologist or designer with similar expertise should comment on the general channel degradation of the stream reach where the drop is proposed to help with the design of the riffle drop. SEPTEMBER 30, 2008 CH13-624

26 6.11 VERTICAL HARD BASIN The vertical hard basin drops include a wide variety of structure designs, but they are not generally recommended for use in urban areas because of concerns for public safety, during wet and dry weather periods. In addition, vertical hard basin drops are to be avoided due to impingement energy, related maintenance and turbulent hydraulic potential. Whenever used, it is recommended their drop height, upstream invert to downstream channel invert, be limited to 3-feet. The hydraulic phenomenon provided by this type of drop is a jet of water that overflows the crest wall into the basin below. The jet hits the hard basin and is redirected horizontally. With sufficient tailwater, a hydraulic jump is initiated. Otherwise, the flow continues horizontally in a supercritical mode until the specific force of the tailwater is sufficient to force the jump. Energy is dissipated through turbulence in the hydraulic jump. The basin is sized to contain the supercritical flow and the erosive turbulent zone. Figure 6.11A shows a vertical drop with a grouted boulder basin. The rock-lined approach length ends abruptly at a structural retaining crest wall that has trickle channel section. Figure 6.11A Vertical Drop Hydraulic System Basic design steps are as follows: 1. The design approach uses the unit discharge in the main and trickle channel to determine separately the water surface profile and jump location in these zones. The overall jump hydraulic problems are the same as previously described. Chow (1959) presents the hydraulic analysis for the Straight Drop Spillway. Add subscript ( t ) for the trickle channel area and subscript ( m ) for the main channel area in the following equations. The drop number, D n, is defined as: SEPTEMBER 30, 2008 CH13-625

27 D = n in which: COLORADO q 2 3 ( gy ) f q = unit discharge (cfs/ft) Y f = effective fall height from the crest to the basin floor (ft) g = acceleration of gravity = 32.2 ft/sec 2 For hydraulic conditions at a point immediately downstream of where the nappe hits the basin floor, the following variables are defined as illustrated in Figure 6.11B: L d = Y f Dn Y p = Dn Y f Y l = Y f Dn Y 2 = Dn Y in which: f Y f = effective fall height from the crest to the basin floor (ft) L d = length from the crest wall to the point of impingement of the jet on the floor or the nappe length (ft) Y p = pool depth under the nappe just downstream of the crest (ft) Y 1 = flow depth on the basin floor just below where the nappe contacts the basin (ft) Y 2 = tailwater depth (sequent depth) required to cause the jump to form at the point evaluated (ft) In the case where the tailwater does not provide a depth equivalent to or greater than Y 2, the jet will wash downstream as supercritical flow until its specific force is sufficiently reduced to allow the jump to occur. Determination of the distance to the hydraulic jump, D j, requires a separate water surface profile analysis for the SEPTEMBER 30, 2008 CH13-626

28 main and low-flow zones as described herein for sloping drops. Any change in tailwater affects the stability of the jump in both locations. 2. The hydraulic jump length, L j, is approximated as 6 times the sequent depth, Y 2. The design basin length, L b, includes nappe length, L d, the distance to the jump, D j, and 60% of the jump length, L j. (The subscripts "m" and "t" refer to the main and trickle channel zones, respectively.) At the main channel zone: ( 6Y ) Lbm = Ldm + D jm + 60% 2m At the trickle channel flow zone, without baffles or boulders to break up the jet: ( 6Y ) Lbt = Ldt + D jt + 60% 2t 3. Caution is advised regarding the higher unit flow condition in the low-flow zone. Large boulders and meanders in the trickle zone of the basin may help dissipate the jet and may reduce downstream if riprap extended downstream along the lowflow channel. When large boulders are used as baffles in the impingement area of the low-flow zone, the low-flow basin length L bt, may be reduced, but not less than L bm. Boulders should project into the flow 0.6 to 0.8 times the critical depth. They should be located between the point where the nape hits the basin and no closer than 10 feet from the basin end. 4. The basin floor elevation should be depressed in depth, and variable with drop height. Note that the basin depth adds to the effective tailwater depth for jump control. The basin can be constructed of concrete or grouted rock. Use of either material must be evaluated for hydraulic forces and seepage uplift. There should be a sill at the basin end to bring the invert elevation to that of the downstream channel and sidewalls extending from the crest wall to the sill. The sill is important in causing the hydraulic jump to form in the basin. Buried riprap should be used downstream of the sill to minimize any local scour caused by the lift over the sill. 5. Caution is advised to avoid flow impinging on the channel side slopes of the basin. 6. Crest wall and footer dimensions should be determined by conventional structural methods. Underdrain requirements should be determined from seepage analysis. 7. Seepage uplift conditions require evaluations for each use. Thus, seepage analysis should be completed to provide for control and weight/size of components. 8. Simplified design criteria are provided in Table 6.11 for vertical hard basin (grouted boulder) drops. These criteria are valid only where the drop does not exceed 3-feet in height. 9. Drops with reinforced concrete basins will have slab thickness and drop lengths that vary, depending upon hydraulic and seepage considerations. SEPTEMBER 30, 2008 CH13-627

29 Table 6.11 Vertical Drops with Grouted Boulder Basin: Simplified Design Criteria for Small Vertical Drops in Grass-Lined Channels Design Parameter Maximum Drop Height Boulder size D r * Grout thickness D g Basin depression B (see Figure 6.11A) Basin length L b (see Figure 6.11A) Approach length L a Trickle flow zone provisions Other provisions Criterion 3 feet, invert to invert 18 inch minimum dimension 10 inches** 1.5 ft 25 ft 10 ft buried riprap Install large boulder or baffles in center zone to break up high flow stream, or apply separate water surface analysis A buried riprap zone should be installed for 10 ft minimum downstream of the drop basin Consider the possible hazard to public when selecting this type of drop for use in urban areas. * Boulder size refers to the minimum dimension of all boulders measured in any direction. ** Bury all grouted boulders on side slopes by filling all gaps and depressions to top of boulders with lightly compacted topsoil and capping with at least 4 inches of top soil; however, capping it with 6 to 12 inches of topsoil will insure a much more robust conditions the native grasses to be seeded on the soil cap. SEPTEMBER 30, 2008 CH13-628

30 Figure 6.11B Vertical Hard Basin Drop SEPTEMBER 30, 2008 CH13-629

31 6.12 BASIC APPROACH TO STRUCTURE The basic approach to design of drop structures includes the following steps: 1. Boatable Channel: Determine if the channel is, or will be, a boatable channel. If boatable, the drop or check structure should use a standard of care consistent with adequate public safety to provide for boater passage (refer to Section 7 for Boatable Structures). 2. Design Discharge: Define the representative maximum channel design discharge (often the 100-year) and other discharges appropriate for analysis, (e.g., low or trickle flows and other discharges expected to occur on a more frequent basis) which may behave differently. All channels need to be designed for stability by limiting their erosion and degradation potential and for longevity by analyzing all the effects on channel stability at levels of flow, including the 100-year flood. 3. Channel Geometry and Slope: Approximate the channel dimensions and flow parameters including longitudinal slope. Identify the probable range of drop choices and heights. 4. Drop Structure Type: Select drop structure alternatives to be considered. 5. Drop Parameters: Decide if channel performance at maximum allowable criteria (i.e., velocity, depths, etc.) for grass-lined channels is practical or desirable. If the design flow is less than 7,500 cfs, the simplified design charts in Table 6.8 may be used to size the basic configuration of the crest. The designer should review the precautions given and the limits of application with respect to site conditions. The crest section and upstream channel transition will need to be refined for incorporation of the trickle or low-flow channel. This requires review of the upstream water surface profile and the supercritical flow downstream of the crest through the dissipation zone of the drop. Under conditions of a submerged jump due to a high tailwater elevation, steps to mitigate the reverse roller should be evaluated. If measures are taken to provide baffles or large boulders to break up the jet, then extensive analysis of the trickle zone hydraulics is not necessary. 6. Seepage Analysis: Perform soils and seepage analyses as necessary to obtain foundation design information. 7. Hydraulic Analysis: Provide a complete hydraulic analysis documenting the performance and design for the type of drop or other type of channel being considered. For channels with alluvial beds that present an erosion/degradation risk, a complete stability and scour analysis should be completed, accompanied by a geotechnical investigation and seepage analysis. 8. Design Details: Use specific design criteria and guidelines to determine the final drop structure flow characteristics, dimensions, material requirements, and construction methods. 9. Permitting: Obtain necessary environmental permits, such as a Section 404 permit. SEPTEMBER 30, 2008 CH13-630

32 6.13 SELECTION CONSIDERATIONS The primary concerns in selection of the type of drop structure should be functional hydraulic performance and public safety. Other considerations include land uses, cost, ecology, aesthetics, and maintenance, and environmental permitting. Site conditions, such as public safety, and aesthetics may weight the selection of a drop structure type. Whenever public access is likely to occur, the use of sloping drops is preferred for safety reasons over the use of vertical ones. Environmental permitting (particularly Section 404 of the Clean Water Act under the jurisdiction of the U. S. Army Corps of Engineers) can also influence the drop selection. Sometimes a shallow (10:1) sloping drop is recommended for fish passage. Other times, a vertical drop is recommended to reduce the footprint area that may impact wetlands and Waters of the U.S.. From an engineering design standpoint, there are two fundamental components of a drop structure: 1. Hydraulic Surface of the drop system, and 2. Foundation and Seepage control system. The selection of the best components for design of the surface drop system based on: Project objectives, Channel stability, Approach hydraulics, Downstream tailwater conditions, Height of drop, Public safety, Aesthetics, and Maintenance considerations. Future downstream channel degradation can destroy a drop structure if adequate precautions are not provided. The material components that can be used for the foundation and seepage control system are a function of on-site soils and groundwater conditions. Thus, foundation and seepage control system considerations are discussed separately. One factor that influences both systems is the extent of future downstream channel degradation that is anticipated. Such degradation can destroy a drop structure if adequate precautions are not provided AESTHETICS AND ENVIRONMENTAL IMPACT Natural materials, rock, and vegetation can be used for bank stability and erosion protection while providing unusual interest, spatial character, and diversity. The placement and type of the rock can provide poor or pleasing appearance. A stepped boulder arrangement for drops, where there is a larger top horizontal surface, is usually an appealing placement that also improves hydraulics. SEPTEMBER 30, 2008 CH13-631