Rock Riffle Design Course

Goals of Streambank Restoration Work With Nature Reduce Sediment Protect Cropland Protect infra-structure Improve Water Quality Improve Aquatic Habitat Make it Affordable

Lanes Balance

200+ years of Human Activity on the Landscape Cleared the Timber Plowed the Prairie Drained the Wetlands Straightened the Streams Leveed the Floodplains Built Cities with Large Areas of Concrete, Asphalt and Rooftops

Results of Human Activity on Stream dynamics Increased Runoff Increased Stream Slope Reduced Floodplain Width

Design Philosophy Work WITH the Natural Processes. Know Where Nature is Taking the Stream. Determine the Cause of The Problem Treat Only the Cause Let Nature Finish Healing The Banks Keep the Cost Down

Channel Evolution Model (CEM)

Stream Stabilization Techniques 4 Major Practices Used in Illinois =========================== Rock Riffle Grade Control Structures (stage II or III) Stone Toe Protection (stage III or IV) Bendway Weirs (stage IV) Stream Barbs (stage IV)

Rock Riffle Grade Control Structures (stage II/ III streams) Stabilize the Bed Creates Riffle/Pool Sequence Dissipates Energy Aerates Water Permit Fish Passage Aesthetically Attractive

Determine Stability Review I & E Data CEM Stage? Entrenchment Ratio?

McCray Stream Site Valley slope = 0.0028 Surveyed slope ave. 0.0024 (slightly lower) Bed in Riffle ---Clay (exposed in riffle bed) Width/Depth ratio = 10.3 (just over 10) Entrenchment Ratio = 1.28 (just under 1.4) 2 elements suggest Grade Control 2 elements outside suggested range (barely)

Channel Geometry Considerations Sine Wave Flow Anticipated Scour Depth Radius Curvature/Bankfull Width Ratio (should be more than 1.8) Existing Riffle Location Existing Riffle Spacing

Scour Depth Assume Scour Depth will equal Max. Bankfull Flow Depth Assumes bed material allows full Sine Wave Flow to develop over time

Radius of Curvature Ave radius = 2.3 times bankfull width (Leopold) Range generally from 1.6 to 4.5 times bankfull width (natural streams) 2.3 is the optimum for hydraulic efficiency 1.8 is the suggested minimum under guidelines

Locating Riffles 5 to 7 Bankfull widths (28 ft) Expect Spacing--140 to 196 ft. Located at Cross-over Points

Read The Channel Knickzones and Headcuts May be hundreds of feet long Little or no bedload in crossover pts. Will not be bedload material (sand wave) Increasing bank height downstream? Stable Points? (Bedrock, Culvert, etc)

Gather Survey Data Bed Profile Water Surface Profile Typical X-Section @ Riffles (detailed) Low Bank Profile Stream Cross-over Points (Stations) or surveyed planform

Plot profile Channel Grade (riffle to riffle) Riffle Spacing Compare low bank vs. channel grade Pool Depths Stable Points

Riffles

Existing Riffles? Stations: 0+00, 4+25, 5+90, 8+25, 10+20 11+50, 13+20, 13+75, 14+75 15+50,16+40, 20+20 12 riffles in 2020 ft. --Ave. Spacing 168 ft. Max. Spacing --425 ft. Min. Spacing-- 55 ft.

Low Bank Grade Line Profile

Bank Heights

Plot Cross-Section(s) Bankfull depths Riffle Crest Elev.

Determine New Gradeline Slope (will be greater than existing slope) Stable Upstream(flood out headcut/knickzone) Stable Downstream(Culvert/Stable Grade, etc) Blend into existing channel bed (generally 1.0 ft. height or less @ last riffle)

Locate Riffles and Crest Elev. Geometrically Correct Uniform Gradeline 6 Bankfull Width spacing? Backwater to Toe of Upstream Riffle

Planned Riffles

Riffle Spacing Existing Riffles range from 55 ft. to 425 ft. (2 to 15 bankfull widths) Ave. 168 ft. (6 bankfull widths) Planned Riffles range from 210 ft. to 370 ft. (7.5 to 13 bankfull widths) Ave. 233 ft. (8.3 bankfull widths) Too Wide?

Riffle Spacing (cont.) Concern is natural flow pattern and siltation problems if spacing is too wide. Experience has not shown spacing to be as critical as once thought (especially with silt and sand bed streams) Attempt to locate riffles near existing riffles at cross-over points and put backwater on tail of upstream riffle.

Check for increased Flooding (when necessary) Bankfull Specific Energy Line Critical Flow Depth Critical Specific Energy Line Allowable Riffle Height = Bankfull Specific Energy - Critical Specific Energy

Max. Riffle Ht. Calculation Specific Energy Line Energy Line = Bkf Depth + Velocity squared divided by 2 times gravity Energy Line = 2.7 + (5.1sq./ 2 x 32.2) Energy Line = 2.7 + (26.01/64.4) Energy Line = 2.7 + .40 = 3.1 ft.

Max. Riffle Ht. Calculation Critical Flow Depth Critical Depth = (Q sq. divided by gravity times width sq.) raised to the 1/3 power Critical Depth = (350 sq./32.2 x 28 sq.) raised to 1/3 power Critical Depth = (122,500/32.2 x 784) 1/3 Critical Depth = (122,500/28142) 1/3 Critical Depth = (4.35) 1/3power = 1.63 ft.

Max. Riffle Ht. Calculation Critical Energy Line Critical Energy Line = 1.5 times Critical Flow Depth Critical Energy Line = 1.5 x 1.63 Critical energy Line = 2.45 ft.

Max. Riffle Ht. Calculation Max. Riffle Ht. Allowable without raising water surface profile or creating backwater effect = Energy Line - Critical Energy Line Max. Riffle Ht = 3.1 - 2.45 Max. Riffle Ht. = 0.65 ft.

Max. Riffle Ht. Calculation Summary Specific Energy Line = 3.1 ft. Critical Flow Depth = 1.63 ft. Critical Specific Energy Line = 2.45 Max. Allowable Riffle Ht. W/O effecting water surface elev. = Specific Energy Line - Critical Specific Energy Line or 3.1 ft. - 2.45 ft. = 0.65 ft. max. ht.

Critical Velocity Estimate Velocity at Critical Flow(Vc) = (gravity x critical flow depth) raised to the 0.5 power Vc =32.2 x 1.63 (raised to 0.5 power) Vc = 52.5 (raised to 0.5 power) Vc = 7.24 ft/sec.

What if I Raise Flood Surface Profile? Will I increase Out of Bank Flooding? Will I effect instream pipe outlets, tile, etc. Is the increase in flood surface profile acceptable to all effected parties?

Compute Quantities and Cost Determine Stone Size from CriticalVelocity Calculations (double size for safety) Quantity Chart Average County Cost per Ton of Stone

Cost of RipRap Statewide Average is about $25 per ton installed Check with local quarries/truckers for delivered price of A-5 or A-6 stone---Add $3 to $5 per ton for installation “quarry run” or “shot-rock” CAN be acceptable--Check with designer for O.K. before using and plan on being at site during delivery.

ROCK RIFFLE CONSTRUCTION

Construction Layout Riffles must be located at stream crossover points where flow goes from one side to the other. (riffles) Riffles should be located slightly upstream of existing riffle so backslope can “blend” into existing riffle--also places maximum water depth closer to apex of bend and dissipates energy in pool.

Sequencing Build from Upstream to Downstream Riffles will not “seal” and pond water until voids are filled with sediment. Can speed up “pooling” by placing a few inches of fine silt/clay on front face of riffle.

Bed Key 4 ft. minimum width 3 ft. minimum depth/or to bedrock Extends across channel bottom and 3-4 ft. into outer bank. Doesn’t need to be larger material Do not over excavate! Easy to “lose” stone in bed key

Bank Key 4 ft. minimum width 5 ft. minimum depth Construct with a 1.5 on 1 slope into bank so that as “vertical banks”stabilize the ”key” is not exposed No need to “fill” key trench out to original bank. Top Elev. approx.1 ft. below top bank

Riffle Crest Build Crest to elevation before starting on backslope Use largest material in gradation Place crest stones individually with “thumb” if possible Build Crest with slight “V” shape using control elevation as low point in “v”.

Backslope Build with 20:1 backslope Place next largest stone available from gradation immediately downstream of Crest stones. Flare the backslope up the channel sides slightly (1.5 ft?) Leave backslope “rough” to aerate flow and provide resting places for fish passage.

Blending into Exit Channel H-4 dimension is depth to place material below existing channel bed at exit. Most streambeds are “soft” enough that material can be “pressed” into channel bed without excavating

Alignment Align Crest so that desired exit is at 90 degrees to Crest. If channel has a sharp bend near tail of riffle use Stone Toe Protection downstream to protect against increased velocity near riffle exit. (Riffle backslope has a 5% grade)

Follow-up and Maintenance Check each structure after first major storm (near bankfull flow) Some movement and of smaller stone and sorting of material is expected. If any crest stone moves it should be repaired immediately