Grade Control Structures

Grade Control Structures
CIVE 717: River Mechanics April 11, 2013
Ami Cobb Jonathan Rainwater

What is a Grade Control Structure (GCS)?

• An earthen, wooden, concrete, or other structure used to prevent gully development and bed erosion

• Typically built on minor streams or part of a dam spillway to pass water to a lower elevation while controlling the energy and velocity of the water as it passes over

Preconstruction condition energy diagram

Post construction modified energy diagram

Benefits of Grade Control Structure
• Stabilizes the banks and bed of channel by reducing stream slope and flow velocity  Controls erosion
• Prevents gully head cut formation and channel bed erosion by lowering water in a controlled manner
• Enhances environmental quality and reduces pollution hazards
• Manages channel flow line for non-erosion benefits, including fish passage, water table control, and reduced turbidity
• May provide water source and habitat for wildlife
• Protects existing structures that can be at risk from bed degradation

Grade Control Structure Hydraulics
• Continuity of water and sediment through the stream reach promotes channel stability
Lane’s Relationship: ∝

Type 1 GCS: Bed Control Structure

• Provides a hard point in the streambed which resists erosive forces of degradational zone

• Lanes Equation:

∝

• Bed control structure is analogous to locally increasing the size of bed material, thus an increased slope is offset by an increase in bed material size

• Structure is built at grade and does not change upstream or downstream flow conditions

Initial Condition with degradational zone between A and B

Stabilization with 3 bed control structures

Type 2 GCS: Hydraulic Control Structure
• Reduces the energy slope along the degradational zone so that the stream is no longer capable of scouring the bed
• Lanes Equation: ∝
• Structure is built above grade and causes a backwater effect to the upstream flow

Initial Condition with degradational zone between A and B

Stabilization with 3 hydraulic control structures

Variations of Grade Control Structures

• Material
• Riprap, concrete, sheet piling, treated lumber, soil cement, gabions, compacted earth fill, ect.

• Shape
• Sloping • Vertical drop

Chute Blocks

Concrete Sloping Sill

Baffle Blocks

• Appurtenances
• Chute Blocks • Baffle Blocks • End Sills

Vertical Drop Gabions

End Sill

Boulder Weir
• Imitates natural steps • Concentrates energy at the
crest and dissipates it through turbulence and bed scour • Bed scour can undermine the structure and outflanking is the most common mode of failure

Boulder Weir Formations
• Cross-Vane
• Decreases the energy near the bank but increases the energy in the center of the channel
• W-Weir
• Prevents bed and bank scour on large rivers by concentrating the spill at ¼ and ¾ channel widths
• Enhances fish habitat
• J-Hook Vane
• Reduces bank erosion but increases bed erosion in the center of the channel
• Directed upstream and on the outside of stream bends

Rigid Weir (log)

• Creates drops to raise the downstream water surface to the elevation of a culvert
• Used on narrow channels with moderate gradients
• Provides fish passage

Low-flow notch