The Lugeon Test: Methodology, Interpretation, and Applications in Geotechnical Engineering

 A check dam is a small barrier or dam constructed across a swale, drainage ditch, or other area of concentrated flow, typically in shallow rivers and streams. Its primary function is to reduce the velocity of concentrated stormwater flows, thereby minimizing channel erosion and facilitating water conservation through groundwater recharge and improved irrigation.

Overview and Importance of Check Dams

Check dams are versatile structures crucial for erosion control and water resource management. By reducing flow velocity, they effectively lessen the erosive power of stormwater runoff, preventing degradation of channels and ditches. While not their main purpose, check dams also trap small amounts of sediment, though this sediment requires periodic removal.

Beyond erosion control, check dams play a significant role in water conservation and augmentation. They are primarily designed to facilitate lift irrigation for nearby agricultural lands and to firm up irrigation under existing wells by promoting percolation. This helps in recharging depleted aquifers by storing water upstream, which then infiltrates into the ground. This "managed aquifer recharge" (MAR) is vital in semi-arid regions where rainfall is seasonal, providing water security and enhancing climate resilience. The infiltration process from check dams can also be significantly increased by "induced recharge," which occurs due to groundwater withdrawals from nearby wells, creating a drawdown that further encourages water infiltration. In some cases, check dams can even facilitate small-scale electricity production.

Check dams are typically applied in small open channels, swales, or drainageways that drain 10 acres (4 ha) or less. They are especially useful on sloping sites where the gradient of waterways is close to the maximum for a grass lining. They should not be used in a live stream.

Components of a Check Dam

Check dams can be constructed from various materials and consist of several key structural elements:

• Materials: Common materials include rock (loose stone, boulders), logs, straw bales, filter fence, sandbags, concrete, steel, and other manufactured products or composites. Geocells, which are strong, lightweight three-dimensional cellular systems, can also be used, infilled with granular material and potentially concrete for outer cells.
• Structure:
  • Embankment/Body-wall: The main barrier constructed across the flow.
  • Spillway/Weir: A section designed to allow excess water to flow over or through the dam, usually at a lower elevation than the outer edges to concentrate flow. Spillways can be trapezoidal, rectangular, or include pipes.
  • Abutments/Flanks: The ends of the dam that extend into the channel banks to prevent water from flowing around the structure.
  • Foundation: The base upon which the dam rests, which should be stable and at a reasonable depth, sometimes requiring keying into the stream bed or banks.
  • Energy Dissipation: Measures like a water cushion chamber or exposed rock downstream of the dam to absorb the energy of falling water and prevent scour. For low height dams, a water cushion is often sufficient.
  • Wooden Needles: In some designs, if banks are not high enough, piers with grooves for inserting wooden needles (e.g., 1.8 meters long, 7.5 cm thick) can be used to raise water levels, with a clay-filled gap to prevent leakage.
  • Weep Holes: Pipes provided on the downstream side to relieve any possible build-up of pore water pressures within the dam body.
  • Geotextiles: Nonwoven geotextile fabric may be used in the keyway or as a separator between riprap/rock and soil to prevent soil migration and support the structure.

Design Procedure

Designing a check dam involves several steps, from initial investigation to detailed calculations and drawing preparation. No formal design may be required for temporary check dams, but specific criteria must be followed. For more permanent structures, a qualified design professional should be consulted.

1. Site Selection:

   • Check dams are generally chosen when larger irrigation tanks or bandharas are not feasible.
   • The stream should have a straight reach of at least 300 meters upstream and downstream of the proposed site.
   • Banks of the stream should be fairly high and stable.
   • A good foundation must be available at a reasonable depth.
   • Irrigable land and wells should be present on both banks within an 800-meter radius.
   • The site should be accessible for construction, operation, and maintenance.
   • The drainage area for temporary check dams should not exceed 10 acres (4 ha). Permanent structures may cater to larger areas.
   • Avoid curves, junctions of gullies/streams, and areas with mass soil movement.

2. Investigation and Data Collection:

   • Catchment Area: Determine the catchment area at the proposed site.
   • Stream Cross-Section: Obtain the cross-section of the nalla (stream) at the site.
   • Maximum Flood Level (MFL): Inquire from local residents or observe flood marks to determine the maximum flood level.
   • Contour Survey: Extend this survey up to 0.5 meters above the expected affluxed High Flood Level (HFL) upstream and 300 meters downstream.
   • Longitudinal Section (L.S.) of the Stream: Extend L.S. upstream up to 0.5 meters beyond the affluxed HFL and 300 meters downstream.
   • Strata: Identify the available strata for the foundation.
   • Wells and Irrigable Area: Note the number of existing wells and irrigable land within an 800-meter radius, including ground level, water level fluctuations, and strata in wells.
   • Trial Pits: Dig a minimum of three trial pits: one in the stream's center and one on each bank where the dam's wall or abutment will be located.
   • Discharge Data: Collect stream discharge data for at least the previous three years; if less, correlate with normal year rainfall.
   • Rainfall Figures: Gather data from nearby rain gauge stations or from geologically similar rivers/nallas if unavailable locally.

3. Design Calculations and Criteria:

   • Design Discharge (Q):
     • Determine the expected discharge in a normal year from available data.
     • Fix the design flood judiciously, typically using the higher value between the observed maximum discharge and the discharge calculated using Dicken's formula.
     • Observed Flood Discharge (Q = AV): Calculated by multiplying the cross-sectional area (A) of the stream by the velocity (V), which is determined using Manning's Formula (V = (1/N) * R^(2/3) * S^(1/2) in M.K.S. system, or V = (1.486/N) * R^(2/3) * S^(1/2) in F.P.S. system).
     • Dicken's Formula (Q = C × A^(3/4)): An empirical formula where Q is discharge in cusecs, A is catchment area in square miles, and C is a constant (e.g., 2400 for below 30 sq. miles, 1500 for 300-1000 sq. miles).
     • Ensure that encroachment on the freeboard (when using Inglis formula Q = (7000A) / sqrt(A + 4)) does not exceed 25%.
   • Length of Check Dam (L):
     • Fixed to minimize afflux (rise in water level due to obstruction).
     • Molesworth's equation for afflux (h = (V^2 / 58.6 + 0.05) × [(A/a)^2 - 1]): Where h = afflux in ft, V = upstream velocity in ft/sec, A = unobstructed waterway, a = obstructed waterway.
   • Spillway Dimensions:
     • Maximum height of the check dam should be 2 feet (60 cm) for temporary structures. Larger heights up to 3.8m are seen in case studies.
     • The center of the check dam must be at least 6 inches (15 cm) lower than the outer edges to concentrate flow.
     • Spillway discharge formula (Q = CLD^(3/2)): Where C is a coefficient (3.0 for loose rock/log/brushwood; 1.8 for gabion/cement masonry), L is length of spillway (m), and D is depth of spillway (m).
     • Spillway crests should be level.
   • Stability Analysis:
     • Check for water up to crest upstream with no downstream water.
     • Check for maximum water levels on both upstream and downstream sides.
     • Check for water level up to crest downstream with corresponding upstream level.
     • An uplift factor of 50% must be considered in design calculations. Safety factors for sliding and overturning are typically 1.5 and 2.0, respectively.
   • Spacing Between Check Dams (S):
     • The toe of the upstream dam should be at the same elevation as the top of the downstream dam.
     • Formula (S = h/s): Where h = height of check dam (ft) and s = channel slope (ft/ft).
     • The spacing should create a "compensation gradient" (3-5% of slope for general practice) that yields non-erosive flow velocity.
     • Horizontal distance (b = h*100 / (So - Se)): Where b=spacing, h=height up to notch, So=existing bed slope, Se=establishing bed slope.
   • Energy Dissipation: A water cushion chamber is usually sufficient for low-height dams. If rock is exposed within 0.5 meters, no cushion chamber is needed. Outlet protection (e.g., riprap-lined apron) is critical to reduce flow velocity and dissipate energy where high-velocity discharge is released on erodible material.
   • Keyway: For rock or log check dams, keying into the channel bottom and abutments (12-24″ deep, at least 12″ wide) prevents erosion around and beneath the dam. Hay bale check dams should be embedded at least 3″.
   • Side Slopes: Typically 2:1 or flatter.
   • Geotextiles: When using riprap or other granular materials, geotextiles (e.g., Class I nonwoven) should be placed between the stone and subgrade to prevent soil migration and support the structure.
   • Earthen Flanks: Masonry is normally keyed into flanks; if earthen bunds are needed, follow percolation tank standards.
   • Maintenance Considerations: Sediment should be removed when it accumulates to half of the dam's original height. Regular inspections should ensure the center remains lower than edges and correct erosion around the dam. Check dams must be removed when their useful life is completed or when permanent stabilization (e.g., grass lining) is established. The disturbed area should be re-seeded or sodded immediately.

4. Drawing Requirements:

   • Index Plan (1″-1 mile): Showing proposed site, catchment, nearest village/town, roads.
   • Contour Plan (1 cm = 10-30 mts.): Extending 1.5 meters above anticipated affluxed HFL upstream and 300 meters downstream.
   • Village Plan (1:330 ft. or 1:660 ft.): Showing dam location, submerged areas, existing/proposed wells, and benefited areas.
   • Plan, Longitudinal Section (L.S.), and Cross-Section (C.S.) of the Check Dam: Including strata details from trial pits. Specific scales are recommended (e.g., Plan: 1 cm = 5-20 mts.; L.S. horizontal: 1 cm = 5-20 mts., vertical: 1 cm = 0.5-1 mt.; C.S. horizontal: 1 cm = 1-5 mts., vertical: 1 cm = 0.5-1 mt.).
   • L.S., C.S., and Plan of the Stream: Showing dam location, trial pit details, HFL, etc..

5. Cost Criteria: Check dams are primarily protective works, so the benefit-cost ratio is less critical. The cost should generally not exceed Rs. 3.00 lakhs; for higher costs, government permission is needed. Maintenance and repair (M&R) costs are typically borne by the local Panchayat.

Example Design Calculation

Let's illustrate the design process using a scenario similar to the provided sources.

Given Data:

• Catchment Area (A): 15.68 sq. km (equivalent to 6.127 sq. miles).
• Nature of Catchment: Good.
• Average Annual Rainfall: 825 mm.
• 65% Dependable Rainfall: 717 mm.
• Gauge-Discharge Table is available.
• Assume Friction Factor (f) = 1.
• Assume Coefficient of End Contraction (K) = 0.1.
• Number of End Contractions (n) = 2.
• Assumed total head over spillway crest (H) = 1.05 m (including velocity head of 0.05 m).
• Assumed weir foundation coefficient (C) = 4.

Design Steps:

1. Yield from Catchment:

   • From Strange's Table (not provided, but implied as a reference tool), the yield per square kilometer for 717 mm rainfall is 26.08% of rainfall, which equals 0.187 MCM (Million Cubic Meters).
   • Total Yield from Catchment = Catchment Area × Yield/sq. km = 15.68 sq. km × 0.187 MCM/sq. km = 2.93 MCM.

2. Design Flood (using Dicken's Formula):

   • Dicken's Formula: Q = 1000 A^(3/4).
   • Q = 1000 × (6.127)^(3/4).
   • Q = 3894 cusecs.
   • Convert to cumecs: 3894 cusecs × 0.0283 m³/s/cusec ≈ 110.37 cumecs.

3. Design of Sharp Crested Weir:

   • Weir Discharge Formula: Q = 1.84 (L − KnH) H^(3/2).
   • Rearrange to solve for L (Length of weir):
     • 110.37 = 1.84 (L − 0.1 × 2 × 1.05) (1.05)^(3/2).
     • 110.37 = 1.84 (L − 0.21) × 1.076.
     • 110.37 = 1.97984 (L − 0.21)
     • L − 0.21 = 110.37 / 1.97984 ≈ 55.747
     • L = 55.747 + 0.21 = 55.957 meters.
   • Length of Weir (L) ≈ 56 meters (Say 56 m).

4. Discharge Intensity (q):

   • q = Q / L = 110.37 cumecs / 56 m = 1.97 cumecs/m.

5. Normal Scour Depth (R):

   • R = 1.35 (q^2 / f)^(1/3).
   • R = 1.35 (1.97^2 / 1)^(1/3).
   • R = 1.35 (3.8809)^(1/3) = 1.35 × 1.5714 ≈ 2.12 meters below the maximum flood level.

6. Flood Level and Crest Level:

   • Computed flood level at weir site (corresponding to 110.37 cumecs) = 99.75 m.
   • Keeping the crest level = 99.00 m.
   • Maximum Water Level = Crest Level + Total Head (H) = 99.00 + 1.05 = 100.05 meters.
   • Net Flood Lift at Weir Site = Maximum Water Level − Computed Flood Level = 100.05 − 99.75 = 0.3 meters.

7. Downstream Cutoff Depth and Level:

   • Depth of downstream cutoff = 1.5 × R = 1.5 × 2.12 = 3.18 meters.
   • Desired R.L. (Reduced Level) of cutoff = Maximum Water Level − Downstream Cutoff Depth = 100.05 − 3.18 = 96.87 meters.
   • Average bed level of deep channel = 97.30 m.
   • Providing a minimum depth of 1 m for cutoff.
   • Actual R.L. of cutoff = Average Bed Level − Provided Cutoff Depth = 97.30 − 1.00 = 96.30 meters (This is against the desired level of 96.87 m, but acceptable if minimum depth is met).

8. Design of Weir Floor:

   • Design Flood = 110.37 cumecs.
   • Length of Weir = 56 m.
   • Height of Weir above bed = Crest Level − Average Bed Level = 99.00 − 97.30 = 1.7 meters.
   • Bottom Width of Weir = 1.6 m (assumed value from diagram).
   • Total Maximum Head (H) for creep length calculation = 1.7 m.
   • Total Creep Length Required (L) = C × H (using Lane's Weighted Creep Theory).
     • L = 4 × 1.7 = 7.22 meters (Say 7.25 m).
   • Length of Downstream Floor (Ld) = 2.21 C × (1/13 H) (specific formula for downstream floor).
     • Ld = 2.21 × 4 × (1/13 × 1.7) = 8.84 × 0.1307 ≈ 1.156 meters (The source's calculation uses (13/H)^(-1) in a different form, resulting in 3.19 m, which is different from direct application of the formula provided. Following the source's result: Ld = 3.19 meters (Say 3.20 m)).
   • Provide a length of 6.0 m for the downstream floor and a wearing coat for 3.20 m.
   • Assuming bottom level of Upstream Cutoff = 96.60 m.
   • Provide floor thickness = 0.3 m.
   • Actual Creep Length: This is calculated by summing horizontal and vertical segments of the weir's contact with the foundation (based on the diagram in the source):
     • Actual L = (U/S cutoff depth) + (U/S floor horizontal length) + (weir base width) + (D/S floor horizontal length) + (D/S cutoff depth)
     • From Diagram: Vertical U/S cutoff (1.0m from 97.60 to 96.60), Horizontal U/S floor (0.5m), Vertical weir (99.00-97.60 = 1.4m), Horizontal weir base (1.6m), Vertical D/S wall (97.60-96.30 = 1.3m), Horizontal D/S floor (3.2m), Vertical D/S cutoff (1.0m from 97.30 to 96.30).
     • The diagram in shows vertical segments: 99.00-97.60 = 1.4m + 97.60-96.60 = 1.0m + 97.60-96.30 = 1.3m (at D/S corner) + (D/S cutoff vertical section) + horizontal segments (0.5m + 1.6m + 3.2m).
     • Actual Creep Length (from source's example calculation values) = 1.0 + 0.5 + 1.6 + 3.2 + 2.8 + 1.3 = 10.4 meters.
     • Compare: Actual creep length (10.4 m) is greater than the required (7.25 m). Hence, the design is O.K..

This detailed design calculation covers the hydrological and structural aspects of a check dam, illustrating how various parameters are determined and checked for stability and functionality.