Short course for UNIVERSIDAD NACIONAL DE INGENIERIA January 26-29, 2016
PLANNING AND DESIGN FOR REHABILITATION OF RIVERS USING LARGE WOOD METODOLOGÍA PARA REFORESTAR RÍOS DEGRADADOS POR ACTIVIDADES HUMANAS USANDO TÉCNICAS DE BIOINGENIERÍA
5.0 Little Topashaw Creek Case Study
Shields et al. 2008. Ecology and Society. 13 (2): 54
Shields Engineering LLC
Course overview Day I (Jan 26)--Foundational topics
•
Constructability assessment
•
Introductions
•
Case study II—Trinity River, California
•
Review of information resources (design handbooks and spreadsheets) for large wood
•
Monitoring
•
Is wood appropriate for your site?—criteria for screening (Planning)
•
Three design approaches
•
Key issues for large wood design
Day 2 (Jan 27)—Designing large wood structures •
Case study I—Little Topashaw Creek, Mississippi
•
Design life for wood structures/selection of design event or condition
•
Types of wood structures
•
Findings of recent research on drag and lift coefficients
•
“Road testing” selected design spreadsheets
Day 3 (Jan 28)—Risk, uncertainty and construction •
Sensitivity and Monte Carlo analyses
Day 4 (Jan 29)--Field trip
2
PRE PROJECT CONDITIONS An incising, sand bed stream
Little Topashaw Creek Watershed Modified reach •4th-order stream •Watershed area = 37 km2 •Maximum relief ~ 65 m •Mixed cover land use •Warmwater stream •Incising channel network •Agricultural and forestry land uses
Channel constructed in 1913
1941
1996
Little Topashaw Creek •Bed slope = 0.0025 •Sinuosity = 2.1 •W~33m, D~3.6m •Mean width has increased 45x since ~1955 •Simon stage IV/V.
Incised channel CEM
% finer than
100 75 50 25 0 0.01
0.1
1
10
Grain size, mm
Bed material is sand 0.2-0.3 mm
Outcrops of cohesive material are common
Let’s take a look around…before adding wood
CH/MH SM SP-SM SC-SM CL-CH CL
62 m
Changes since 1999
2001
2002
Mean wavelength
2%
3%
Mean meander length
-7%
-5%
Mean amplitude Mean radius of curvature
-14% -13% 26%
23%
n = 23
1999
1999
Ecological status before rehabilitation • Vegetation dominated by opportunistic and invasive exotic
species with remnants of old floodplain forest. Succession within enlarged channel hindered by rapid sedimentation. • Flashy hydrology—almost no overbank flow • Water quality issues (sediments, pesticides, nutrients)
• Depressed levels of POM in bed sediments • Higher levels of LWD, but unstable • Shallow depths (5 cm), unstable substrate
• Fish and invertebrate collections typical of streams impacted
by erosion and sedimentation
Aquatic Habitat Water Depth, cm 30 25 20 15 10 5 0 Before
After
Reach modified by LW
Before
After
Downstream Reach
Large wood Density , m2/100 m 2 50 45 40 35 30 25 20 15 10 5 0
Water Width, m 10 8 6
4 2 0 Before
After
Reach modified by LW
Before
After
Downstream Reach
Before
After
Reach modified by LW
Before
After
Downstream Reach
Invertebrates before/without wood addition % of captures by substrate type 16
% composition by functional feeding group 3.2 2.8 1.9
6
10.7 Filtering collectors
78
Leaf Packs
Gathering Collectors
Sediment
Shredding herbivores
Wood
80.9
Scrapers Predators
Fish family composition Percent of biomass as centrarchids
Percent of biomass as cyprinids 60 50 40 30 20 10 0
100 80 60 40 20 0 Before
After
Reach modified by LW
Before
After
Downstream Reach
Before
After
Reach modified by LW
Before
After
Downstream Reach
REHABILITATION PROJECT This is where we added wood
Purpose of project “The purpose of large wood structures placed in an incised, sand-bed stream is to accelerate evolution toward a sinuous, two-stage channel with wooded berms….”
Rehabilitation • 2 km reach treated, but 4 km were
studied (1999-2004) • Large woody debris structures • Willow planting • Vegetated inlets—gully
stabilization by planting switchgrass sod
LW structure design
Steps in constructing LW structure
Anchor Installation 58 of the 72 LWS were anchored using four Duckbill Earth Anchors (138-DB1) each.
Each anchor was driven into the ground by pressing with a metal rod and a track hoe bucket and then load tested to 1000 lb (4.5 kN).
Top View of a Completed LWS (note the crossing cables)
Completed LWS and willow planting
Construction costs in 2000 (USD)
• Debris structures
$80 m-1
=(19% to 49% of recent costs for stone treatments)
• Willow planting
$7.42/cutting
Large wood structure dimensions • N = 72 • Crest elevation = 2.1 + 0.5 m • Dimensions: 14 m long by 5.3 m wide • 4.4 key members and 14.7 racked
members per structure • Diameter of key members = 45 cm • Spacing = 13.8 + 10.8 m • Cost = $80 m-1 treated bankline (~1/3 the
cost of stone toe)
Were promising!
INITIAL RESULTS
Little Topashaw Creek Discharge, m3/s
WY 2000 60 Construction of 72 LWS
0
WY 2001
60 68 LWS remaining
0 1-Oct
29-Jan
29-May
26-Sep