1,Design of Open Channels and Culverts CE453 Lecture 25,Ref: Chapter 17 of your text and HYDRAULIC DESIGN OF HIGHWAY CULVERTS, Hydraulic Design Series Number 5, Federal Highway Administration, Publication No. FHWA-NHI-01-020, September 2001; available at http:/www.cflhd.gov/design/hyd/hds5_03r.pdf, accessed March 18, 2006,2,Design of Open Channels,3,Longitudinal Slopes,Gradient longitudinal direction of highway to facilitate movement of water along roadway,4,Drains,Along ROW Collect surface water,A typical intercepting drain placed in the impervious zone http:/www.big-o.com/constr/hel-cor.htm,5,Drainage Channels (Ditches),Design Adequate capacity Minimize hazard to traffic Hydraulic efficiency Ease of maintenance Desirable design (for safety): flat slopes, broad bottom, and liberal rounding,6,Ditch Shape,Trapezoidal generally preferred considering hydraulics, maintenance, and safety,Source: Fabriform1.com,7,Ditch Shape,V-shaped less desirable from safety point of view and maintenance,Source: Fabriform1.com,8,Terms,Steady Flow: rate of discharge does not vary with time (Mannings applies) Uniform: channel properties are constant along length of channel Slope Roughness Cross-section Water surface is parallel to slope of channel Non-uniform: properties vary,9,Terms,Unsteady flow: rate of discharge varies with time Critical depth a hydraulic control in design depth of water where flow changes from tranquil to rapid/shooting Critical velocity: velocity corresponding to critical depth Critical slope: slope corresponding to critical depth,10,11,Flow Velocity,Depends on lining type Should be high enough to prevent deposit of transported material (sedimentation) For most linings, problem if S 2 fps when full) Should be low enough to prevent erosion (scour) For most types of linings, problem if S 5%,12,Use spillway or chute if elev is large,13,Rip Rap for drainage over high slope,14,Riprap (TN Design Manual),15,Side Ditch/Open Channel Design-Basics,Estimate Q at point of interest Select ditch cross section Erosion control? Mannings formula used for design Assume steady flow in a uniform channel,16,Mannings Formula,V = R2/3*S1/2 (metric) V = 1.486 R2/3*S1/2 n n where: V = mean velocity (m/sec or ft/sec) R = hydraulic radius (m, ft) = area of the cross section of flow (m2, ft2) divided by wetted perimeter (m,ft) S = slope of channel n = Mannings roughness coefficient,17,18,Side Ditch/Open Channel Design-Basics,Q = VA Q = discharge (ft3/sec, m3/sec) A = area of flow cross section (ft2, m2) FHWA has developed charts to solve Mannings equation for different cross sections,19,Open Channel Example,Runoff = 340 ft3/sec (Q) Slope = 1% Mannings # = 0.015 Determine necessary cross-section to handle estimated runoff Use rectangular channel 6-feet wide,20,Open Channel Example,Q = 1.486 R2/3*S1/2 n Hydraulic radius, R = a/P a = area, P = wetted perimeter,P,21,Open Channel Example,Flow depth = d Area = 6 feet x d Wetted perimeter = 6 + 2d,6 feet,Flow depth (d),22,Example (continued),Q = 1.486 a R2/3*S1/2 n 340 ft3/sec = 1.486 (6d) (6d)2/3 (0.01)1/2 (6 + 2d) 0.015 d 4 feet Channel area needs to be at least 4 x 6,23,Example (continued),Find flow velocities. V = 1.486 R2/3*S1/2 n with R = a/P = 6 ft x 4 ft = 1.714 2(4ft) + 6ft so, V = 1.486(1.714)2/3 (0.01)1/2 = 14.2 ft/sec 0.015 If you already know Q, simpler just to do V=Q/A = 340/24 = 14.2),24,25,Example (continued),Find critical velocities. From chart along critical curve, vc 13 ft/sec Critical slope = 0.007 Find critical depth: yc = (q2/g)1/3 g = 32.2 ft/sec2 q = flow per foot of width = 340 ft3/sec /6 feet = 56.67ft2/sec yc = (56.672/32.2)1/3 = 4.64 feet depth of 4,26,Check lining for max depth of flow ,27,28,Source: FHWA Hydraulic Design Charts,29,A cut slope with ditch,30,A fill slope,31,Inlet or drain marker,32,Ditch treatment near a bridge US 30 should pier be protected?,33,A fill slope,34,Wheres the water going to end up?,Hidden Drain,35,36,37,Median drain,38,Design of Culverts,Source: Michigan Design Manual,39,Culvert Design - Basics,Top of culvert not used as pavement surface (unlike bridge), usually less than 20 foot span 20 feet use a bridge Three locations Bottom of Depression (no watercourse) Natural stream intersection with roadway (Majority) Locations where side ditch surface drainage must cross roadway,40,Hydrologic and Economic Considerations,Alignment and grade of culvert (wrt roadway) are important Similar to open channel Design flow rate based on storm with acceptable return period (frequency),41,Culvert Design Steps,Obtain site data Roadway cross section at culvert location (best is at existing channel) Establish inlet/outlet elevations, length, and slope of culvert,42,43,Sometimes you want a dam why?,44,Culvert Design Steps,Determine allowable headwater depth (and probable tailwater depth) during design flood control on design size f(topography and nearby land use) Select