Design of Grass Swales Norman W. Garrick

Overview Swales are designed to convey water as part of an OPEN storm water system – they are not meant to be continuously wet Typically the cross-section of swales are PARABOLIC in shape but other cross-sections are sometimes used Swales are commonly GRASS-LINED The type and height of grass affects the resistance to flow – an important factor for design

Overview Swale Design aims to 1.Limit erosion damage by making sure permissible velocity in not exceeded during peak flow 2.Ensure sufficient capacity to accommodate peak flow The maximum or permissible velocity for a given swale depends on The soil’s resistance to erosion Type of grass Height of grass Slope of swale

Overview The height of grass is an important design parameter but it changes with maintenance – the design should be based on the range of conditions expected Low grass – results in maximum velocity so low grass should be used to design for stability (erosion damage) High grass – results in minimum velocity so high grass should be used to design for capacity

Overview The swale should not be designed to slow the water that is entering it unless sedimentation is desired If sedimentation is desired, water can be slowed by Decreasing the slope of the swale Using wider swale Using taller vegetation As sedimentation accumulates, the character and capacity of the swale changes. Maintenance is required to remove sediment and to repair erosion

Swale Design Formulas Open channel flow, such as in a swale, is based on two formulas 1.Manning’s Equation 2.Continuity Equation

Swale Design Formulas Manning’s Equation Velocity of flow in an open channel is given by Manning’s Equation V = (1.486 R 2/3 S 1/2 ) / n V – flow velocity, ft/s N – Manning’s roughness coefficient for open channels R – hydraulic radius, ft S – channel slope, ft/ft

Swale Design Formulas R - Hydraulic Radius R = cross-section area of flow / wet perimeter Water Surface Wet Perimeter Area = 20 sq, ft WP = = 13 ft R = 20/13 = 1.54 ft Area = 20 sq, ft WP = = 22 ft R = 20/22 = 0.91 ft Larger R = less resistance to flow

Swale Design Formulas Continuity Equation Flow rate and velocity q = A V q – flow in cu. ft/s A – cross-section area for flow, sq, ft V – flow velocity, ft/s

Area and R of Parabolic Swales W2 W1 D1 D2 Area A = 2/3 WD Hydraulic Radius R = (W 2 D) / (1.5 W D 2 ) Width Ratio W2 = W1 (D2/D1) 0.5