Water Resource Engineering (CE-603 (A)) - Unit 2 Short Notes

UNIT 2: WATER RESOURCE ENGINEERING

I. IRRIGATION ENGINEERING

A. Introduction to Irrigation

• Necessity: Provides assured water supply to crops in regions with insufficient/erratic rainfall, increases yield, enables multiple cropping, and stabilizes agriculture.

• Advantages:

  • Increases agricultural productivity & food security.

  • Enables cultivation in arid/semi-arid regions.

  • Provides drought protection.

  • Improves soil fertility through leaching.

  • Generates hydroelectric power & inland navigation (multipurpose).

• Disadvantages:

  • High initial cost & maintenance.

  • Waterlogging & salinity if not managed properly.

  • Potential for water-borne diseases (malaria).

  • Displacement of population during reservoir construction.

  • Can lead to groundwater depletion.

[!TIP] Exam Focus: Be prepared to list 4-5 specific points for both advantages and disadvantages. Link disadvantages directly to poor irrigation water management (e.g., waterlogging → poor drainage).

B. Methods of Irrigation

Sub-methods of Surface Irrigation:

1. Free Flooding: Uncontrolled flow from field channel. Suitable for close-growing crops on irregular land.

2. Border Flooding: Field divided into strips (borders) with low levees. Water advances as a sheet. Common for grain & forage crops.

3. Check Flooding: Field divided into small, level checks (ponds) surrounded by levees. Suitable for heavy soils & paddy.

4. Furrow Irrigation: Water flows in small channels (furrows) between crop rows. Suitable for row crops (cotton, maize). Saves water compared to flooding.

[!TIP] Exam Focus: Compare Sprinkler vs. Drip (water efficiency, cost, crop suitability). Sketch border and furrow irrigation systems.

C. Soil-Water-Plant Relationships

1. Soil Moisture Concepts:

   • Field Capacity (FC): Moisture content after free drainage ceases (~2-3 days after saturation). Definition: Water held against gravity by capillary forces.

   • Permanent Wilting Point (PWP): Moisture content at which plant cannot extract water & wilts permanently. Definition: Soil moisture tension ~15 bars.

   • Wilting Coefficient: The moisture content at which plants wilt temporarily but recover at night. Historically used, now largely replaced by PWP.

   • Available Soil Moisture (ASM): Water between FC and PWP that is plant-available.

$$ \text{ASM (\%)} = \left( \frac{\theta_{FC} - \theta_{PWP}}{1 - \theta_{PWP}} \right) \times 100 $$

(on mass/volume basis)

2. Root Zone Depth & Effective Depth: Depth of soil from which plant extracts most water (usually >80%). Determines volume of soil to be replenished.

3. Soil Properties:

   • Bulk Density (ρb): Mass of dry soil per unit volume (g/cm³). Inversely related to porosity.

   • Texture: Proportion of sand, silt, clay. Affects water holding capacity (clay > silt > sand).

[!TIP] Exam Focus: Differentiate FC vs. PWP vs. Wilting Coefficient. Calculate ASM given soil density, FC, PWP, and root depth (common numerical).

D. Crop Water Requirements

1. Consumptive Use (ET): Total water used by crop for transpiration + evaporation from soil + metabolic processes.

   • Direct Methods:

     • Soil Moisture Depletion: Measure change in ASM over time.

     • Lysimeter: Weighed/measured soil column with crop.

   • Indirect Methods (Empirical/FAO):

     • Blaney-Criddle: $$\displaystyle ET = K \cdot f $$ (where $f$ is seasonal % of daylight hours, $K$ is crop factor).

     • Penman Equation: Combines energy balance & aerodynamic transfer. Most accurate for large areas.

     • Modified Penman (Penman-Monteith): FAO-56 standard, uses net radiation, temperature, humidity, wind speed.

2. Irrigation Scheduling:

   • Irrigation Interval (Frequency): Time between irrigations until ASM is depleted to allowable depletion level.

$$ \text{Interval (days)} = \frac{\text{ASM (mm)}}{\text{Daily Consumptive Use (mm/day)}} $$

*   **Depth of Water Application (d):** Should replenish ASM to FC.

$$ d = \frac{(\theta_{FC} - \theta_{initial}) \times D_e}{E_a} $$

    Where $$\displaystyle D_e $$ = effective root depth, $$\displaystyle E_a $$ = application efficiency.

*   **Example:** Given FC=35%, PWP=15%, ρ=1.5 g/cm³, root depth=80 cm, daily CU=12 mm. Compute interval.

[!TIP] Exam Focus: Derive scheduling interval formula. Be comfortable with Penman equation variables (Rn, G, T, u2, es-ea). Numerical on interval & depth is very common.

E. Irrigation Planning and Terminology

Methods to Improve Duty (Increase Efficiency):

• Reduce conveyance losses (canal lining).

• Reduce application losses (sprinkler/drip, proper land leveling).

• Reduce evaporation & deep percolation (mulching, scheduling).

• Re-use of runoff water.

• Improve soil structure (organic matter).

[!TIP] Exam Focus: DERIVE Δ-D-B relationship (8.64 factor from unit conversion). Define all terms (GCA, CCA, Kor, Paleo). List 4-5 methods to improve duty.

F. Irrigation Water Management Efficiencies

1. Water Application Efficiency (ηa): Ratio of water stored in root zone to water delivered to field.

$$ \eta_a = \frac{\text{Water stored in root zone}}{\text{Water delivered to field}} \times 100\% $$

2. Conveyance Efficiency (ηc): Ratio of water delivered to field to water diverted from source.

$$ \eta_c = \frac{\text{Water delivered to field}}{\text{Water diverted from source}} \times 100\% $$

3. Overall/Project Efficiency (ηo): Product of application & conveyance efficiencies.

$$ \eta_o = \eta_a \times \eta_c / 100 $$

[!TIP] Exam Focus: Distinguish between ηa and ηc. Overall efficiency is always < individual efficiencies.

II. CANAL HYDRAULICS AND DESIGN

A. Classification of Irrigation Canals

1. Based on Function:

   • Main Canal: From headworks to branch canals. No direct irrigation.

   • Branch Canal: Off-takes from main, supplies distributaries.

   • Distributary: Supplies water to minor/field channels.

   • Field Channel/Minor: Directly supplies water to fields.

2. Based on Discharge & Importance:

   • Primary (Main): > 20 m³/s (large).

   • Secondary (Branch): 5-20 m³/s (medium).

   • Tertiary (Distributary/Minor): < 5 m³/s (small).

B. Canal Design Theories

Kennedy's Theory (Regime Flow):

• Concept: A channel is in regime if it can carry a given discharge without silting or scouring, maintaining a stable slope & section, with silt in suspended condition.

• Equation: $$\displaystyle V_o = 0.55 \cdot m \cdot D^{0.5} \cdot S^{0.25} $$ (for alluvial soils)

  Where $$\displaystyle V_o $$ = critical velocity, $m$ = critical velocity ratio (CVR, 1.0-1.5), $D$ = mean depth, $S$ = slope.

• Design Procedure: Assume $m$, $S$, side slope ($z$). Use continuity $$\displaystyle Q = A \cdot V $$ & Kennedy's $V$. Solve for $A$, $D$, $T$ (bottom width) iteratively. Check with Kutter's/Manning for $n$.

• Limitation: Based on limited data, assumes silt is in suspension, no equation for $n$.

Lacey's Theory (Regime Channels):

• Concept: Defines true regime where channel dimensions adjust so silt is just carried in suspension. Introduces Silt Factor (f).

$$ f = 1.76 \sqrt{d_{50}} $$

(mm) where $$\displaystyle d_{50} $$ is median silt size.

• Design Equations for Regime Channel:

  1. Velocity: $$\displaystyle V = \frac{Q}{A} = \sqrt{\frac{A \cdot S}{f}} $$

  2. Perimeter: $$\displaystyle P = 4.75 \sqrt{Q} $$ (for $$\displaystyle f=1 $$)

  3. Area: $$\displaystyle A = \frac{Q}{\sqrt{S \cdot f}} $$

• Design Procedure: Given $Q$, $f$, $S$ (or from $P$ equation), $z$. Use $A$, $P$, $z$ to find $T$ & $D$. Check $V$.

• Drawbacks of Lacey:

  1. Empirical, based on limited Indian data.

  2. $f$ depends on $$\displaystyle d_{50} $$, which varies along channel.

  3. No theory for non-regime (non-alluvial) channels.

  4. Perimeter equation ($P \propto \sqrt{Q}$) often not accurate for large $Q$.

  5. Does not account for silt grade or charge.

Comparison: Kennedy vs. Lacey

[!TIP] Exam Focus: State & explain Lacey's regime concept. Derive/state Lacey's equations. List 4 drawbacks of Lacey. Compare both theories in a table. Numerical: Design channel for given Q, S, z using either theory.

C. Canal Lining

• Objectives:

  • Reduce seepage loss (conveyance efficiency ↑).

  • Increase velocity (reduces canal cross-section).

  • Prevent weed growth & bank erosion.

  • Enable steeper slopes (shorter alignment).

  • Reduce maintenance cost.

• Types of Lining Materials:

  • Rigid: Concrete (precast/shotcrete), brick, stone masonry.

  • Flexible/Impermeable: Bitumen, asphalt, geomembranes (HDPE, PVC).

  • Semi-permeable: Soil-cement, bentonite.

• Advantages: Seepage control, area saving, higher velocity, durability.

• Disadvantages: High initial cost, requires good subgrade, difficult repairs, expansion joints needed.

D. Canal Alignment and Planning

• Factors Influencing Alignment:

  1. Topography: Follow contour to minimize excavation/fill. Avoid steep slopes.

  2. Soil & Geology: Stable foundations, avoid landslide/erosion zones, minimize seepage through permeable strata.

  3. Structures: Minimize number of cross-drainage works (aqueducts, syphons).

  4. Command Area: Should serve maximum area with shortest length.

  5. Existing Features: Avoid forests, towns, monuments.

  6. Drainage: Align along natural drainage lines where possible.

• Significance: Good alignment reduces cost, maintenance, water loss, and ensures equitable distribution.

E. Canal Hydraulic Design

• Manning's Formula (Most Common):

$$ V = \frac{1}{n} R^{2/3} S^{1/2} $$

Where $V$ = velocity, $n$ = roughness coefficient, $R$ = hydraulic radius ($A/P$), $S$ = slope.

• Kutter's Formula: Older, more complex. $$\displaystyle V = \frac{23 + \frac{0.00155}{S}}{N + \frac{0.0028}{R}} \cdot R^{2/3} S^{1/2} $$

• Selection of Section & Side Slopes:

  • Trapezoidal: Most common (stable, easy construction). Side slope ($z$) 1:1 to 1:2 based on soil.

  • Parabolic: Economical in earth, smooth flow, but difficult to construct.

  • Rectangular: Used in lined canals, tunnels.

  • Circular: For small conduits (pipes).

  • Selection: Based on material (earth vs. lined), discharge, velocity limits (non-erosive, non-silting).

F. Canal Structures

1. Regulation Structures:

   • Gates/Regulators: Control flow (head regulator at offtake, cross regulators in main canal).

   • Checks/Drop Structures: Create sudden drop to dissipate energy, maintain depth.

   • Escapes/Spillways: Safely discharge excess water.

2. Cross-Drainage Works (CDW): Cross canal over/under drainage.

   • Types:

     • Aqueduct: Canal over drainage (most common).

     • Syphon: Canal under drainage (pressurized flow).

     • Superpassage: Drainage over canal (rare).

     • Level Crossing: Canal & drainage at same level (with regulator).

     • Inlet & Outlet: Canal into/from drain.

   • Selection Factors: Relative levels, discharge, silt load, subsoil, cost.

3. Aqueduct Design: Design of waterway (irrigation channel) & waterway (drainage). Ensure critical flow in drain does not submerge canal.

4. Weirs & Barrages:

   • Weir: Low, overflow structure for measurement/regulation. Crest above bed.

   • Barrage: High, gated structure for large flow control & storage. Crest near bed. Gates regulate discharge.

[!TIP] Exam Focus: Sketch & label an aqueduct. Differentiate weir vs. barrage. List factors for CDW selection.

III. GROUNDWATER HYDROLOGY

A. Aquifers and Their Properties

1. Types of Aquifers:

   • Unconfined (Water Table): Upper surface is water table, open to atmosphere. Recharge direct.

   • Confined (Artesian): Sandwiched between impermeable layers (aquitards). Under pressure. Piezometric surface > top of aquifer.

   • Leaky (Semi-confined): Confined aquifer with semi-permeable over/underlying layers. Leakage occurs.

2. Aquifer Properties:

   • Porosity (n): $$\displaystyle n = \frac{V_v}{V_t} $$. Total void space. Not all available for flow.

   • Specific Yield (Sy): Volume of water drainable by gravity per unit aquifer volume. Key for unconfined. Sy < n.

   • Specific Retention (Sr): Volume of water held by capillary forces. $$\displaystyle n = S_y + S_r $$.

   • Coefficient of Permeability (K) / Hydraulic Conductivity: Rate of flow under unit hydraulic gradient. (m/day, cm/s). Darcy's Law: $$\displaystyle Q = K \cdot i \cdot A $$.

   • Storage Coefficient (S): For confined: Volume of water released per unit area per unit decline in head. Dimensionless (typically 0.0001-0.001). For unconfined ≈ Sy.

   • Transmissivity (T): $$\displaystyle T = K \cdot b $$ (b = aquifer thickness). Rate of flow through 1m width under unit gradient. (m²/day).

B. Wells and Well Hydraulics

1. Types of Wells:

   • Open/Dug Wells: Large diameter (1-3m), shallow, low yield.

   • Tube Wells: Small diameter (10-30 cm), deep, high yield.

     • Gravity (Non-artesian): Water flows by gravity.

     • Artesian: Water flows under pressure.

   • Infiltration Galleries: Horizontal collection system near surface/subsurface.

2. Well Discharge Equations:

   • Dupuit's Equation (Unconfined, steady-state):

$$ Q = \frac{\pi K (h_1^2 - h_2^2)}{\ln(r_2/r_1)} $$

    Where $$\displaystyle h_1, h_2 $$ = heads at radii $$\displaystyle r_1 $$ (well radius), $$\displaystyle r_2 $$ (radius of influence).

*   **Thiem's Equation (Confined, steady-state):**

$$ Q = \frac{2\pi K b (h_1 - h_2)}{\ln(r_2/r_1)} $$

    Where $b$ = aquifer thickness, $$\displaystyle h_1, h_2 $$ = piezometric heads.

*   **Radius of Influence (ro):** Distance from well where drawdown is zero. Estimated by $$\displaystyle r_o \approx 3000 \cdot s \cdot \sqrt{K} $$ (s in m, K in m/s).

3. Well Losses & Efficiency:

   • Losses: Entrance loss, friction loss in screen & casing.

   • Well Efficiency (ηw): $$\displaystyle \eta_w = \frac{\text{Ideal drawdown}}{\text{Actual drawdown}} \times 100\% $$. Actual > Ideal due to losses.

   • Specific Capacity: $$\displaystyle SC = \frac{Q}{s} $$ (m³/day per m drawdown). Index of well productivity.

[!TIP] Exam Focus: State & derive Dupuit's & Thiem's equations (assumptions: steady, homogeneous, isotropic, horizontal flow). Numerical: Given Q, K, b, s, find ro or K. Define specific yield vs. storage coefficient.

C. Groundwater Recharge

1. Natural Recharge: Infiltration from precipitation, seepage from rivers/lakes.

2. Artificial Recharge Methods:

   • Infiltration Ponds/Basins: Shallow ponds on permeable soil.

   • Recharge Wells: Direct injection into aquifer (for confined).

   • Infiltration Galleries: Horizontal trenches filled with gravel.

   • Check Dams/Percolation Tanks: Small barriers in streams to slow flow & increase infiltration.

3. Role of Infiltration Galleries: Collect shallow groundwater/surface water and convey to recharge area or directly to aquifer. Used in both recharge & extraction.

D. Waterlogging

1. Causes: High water table due to over-irrigation, poor drainage, canal seepage, flat terrain, heavy rainfall.

2. Effects: Soil salinity (capillary rise brings salts), reduced crop yield, land unsuited for construction, health hazards.

3. Prevention & Mitigation:

   • Surface Drainage: Open ditches, buried pipes.

   • Subsurface Drainage: Tile drains, mole drains.

   • Land Leveling: For uniform drainage.

   • Irrigation Management: Precise scheduling, avoid over-irrigation.

   • Canal Lining: To reduce seepage.

E. Salinity in Soils and Groundwater

1. Salt-Affected Lands:

   • Saline Soil: High soluble salts, pH < 8.5, good drainage.

   • Alkali (Sodic) Soil: High sodium, pH > 8.5, poor structure.

   • Saline-Alkali: Both high salts & sodium.

2. Reclamation Strategies:

   • Leaching: Apply excess water to flush salts below root zone. Requires good drainage.

   • Drainage: Essential to remove leached salts.

   • Soil Amendments: Gypsum (for alkali soils) replaces Na with Ca.

   • Crop Selection: Salt-tolerant crops (barley, sugarbeet).

3. Salt Efflorescence: Crystallization of salts on soil/structural surface. Causes disintegration of masonry/concrete by crystal growth (hygroscopic salts also cause dampness).

F. Groundwater Flow and Contour Mapping

1. Water Table: Top of saturated zone in unconfined aquifer.

2. Piezometric Surface: Level to which water rises in a well tapping confined aquifer.

3. Flow Direction & Gradient: Perpendicular to equipotential lines (contours of equal head). Gradient $$\displaystyle i = \frac{\Delta h}{\Delta l} $$.

4. Estimation of Water Table: Interpolation between known water table elevations (linear, inverse distance).

5. Flow Nets: Graphical representation (equipotential lines & flow lines) for 2D steady flow. Used to estimate flow rate ($$\displaystyle Q = k \cdot i \cdot A $$) or discharge per unit width.

[!TIP] Exam Focus: Sketch flow net for a simple dam/well problem. Calculate gradient & flow rate from given water table map.

IV. SURFACE HYDROLOGY

A. Hydrological Cycle

B. Precipitation

1. Measurement Techniques:

   • Non-Recording: Symon's Rain Gauge (standard). Manual measurement daily.

   • Recording:

     • Tipper (Siphon) Type: Automatic tipping bucket.

     • Float Type: Float in collecting chamber operates pen.

     • Weighing Type: Weighs collected water, most accurate.

   • Remote Sensing: Radar (estimates intensity/area), satellites (e.g., TRMM, GPM).

2. Estimation of Missing Data:

   • Arithmetic Mean: $$\displaystyle \bar{P} = \frac{\sum P_i}{n} $$. For uniform rainfall.

   • Thiessen Polygon: Weighted mean based on area of influence.

$$ \bar{P} = \frac{\sum (P_i \cdot A_i)}{\sum A_i} $$

*   **Isohyetal Method:** Contour map of rainfall. Planimeter area between contours.

$$ \bar{P} = \frac{\sum (P_{avg} \cdot A)}{\text{Total Area}} $$

3. Depth-Area-Duration (DAD) Curves:

   • Significance: Relates maximum average rainfall for a given area & duration. Used for design storm estimation (PMP, flood).

   • Construction: For a storm, plot max depth vs. area for fixed durations (e.g., 1-hr, 6-hr, 24-hr). For fixed area, plot depth vs. duration.

C. Infiltration

1. Definition: Process of water entering soil surface.

2. Infiltration Capacity (fₚ): Maximum rate at which soil can absorb water (mm/hr). Decreases with time (initially high, approaches constant).

3. Infiltration Rate (f): Actual rate of entry. $$\displaystyle f = \min(f_p, i) $$ where $i$ = rainfall intensity.

4. Factors Affecting Infiltration:

   • Soil: Texture (clay low, sand high), structure, initial moisture (wet soil lower), organic matter (increases).

   • Land Cover: Vegetation (increases by preventing crusting, but intercepts), mulch (increases).

   • Surface: Roughness (increases), slope (decreases).

   • Rainfall: Intensity (if > fₚ, Hortonian overland flow), duration, depth.

5. Infiltration Indices:

   • φ-index (Phi-index): Constant infiltration rate that produces exact observed runoff for a given storm. $$\displaystyle \phi = \frac{P - R}{t_r} $$ where $P$=total ppt, $R$=total runoff, $$\displaystyle t_r $$=duration of excess rainfall.

   • W-index: Average infiltration rate during time of actual infiltration (excludes initial & final periods where $$\displaystyle f > i $$). More physically based than φ.

D. Runoff and Hydrographs

1. Factors Affecting Hydrograph Shape:

   • Basin: Size (larger → lag ↑, peak ↓), shape (elongated → lag ↑), slope (steeper → peak ↑, lag ↓), soil (permeable → lower peak), vegetation (increases lag, decreases peak).

   • Storm: Intensity, duration, spatial distribution.

   • Channel: Storage, resistance.

2. Unit Hydrograph (UH) Theory:

   • Concept: Direct runoff hydrograph (DRH) from 1 cm (or 1 unit) of excess rainfall uniformly distributed over the basin in a specified duration ($$\displaystyle T_r $$).

   • Assumptions: Time invariance (linear system), superposition (additivity), rainfall uniform.

   • Derivation: From single storm hydrograph: Separate baseflow → get DRH → Divide DRH ordinates by total excess rainfall depth (cm).

3. Synthetic UH - SCS Dimensionless UH: Standardized shape based on lag time ($$\displaystyle T_{lag} $$) and time to peak ($$\displaystyle T_p $$). $$\displaystyle T_p = \frac{D}{2} + T_{lag} $$.

4. Deriving UH for Different Durations:

   • S-curve Method: Convolve (add with time lag) UH of duration $T$ with itself to get S-curve. Shift S-curve by $T$ and subtract to get UH of duration $nT$.

   • Example: Given 4-hr UH, derive 2-hr UH.

5. Streamflow Types:

   • Perennial: Flows year-round (groundwater fed).

   • Ephemeral: Flows only in direct response to rainfall (no baseflow).

   • Intermittent: Flows seasonally (dry for part of year).

E. Hydrological Routing - Muskingum Method

• Concept: Storage in reach is function of both inflow & outflow. $$\displaystyle S = K [x I_t + (1-x) O_t ] $$ where $x$ = weighting factor (0.0 to 0.5), $K$ = storage time constant.

• Routing Equation:

$$ O_{t+\Delta t} = C_0 I_{t+\Delta t} + C_1 I_t + C_2 O_t $$

Where coefficients depend on $K$, $x$, $\Delta t$.

$$ C_0 = \frac{-Kx + 0.5\Delta t}{K(1-x) + 0.5\Delta t}, \; C_1 = \frac{Kx + 0.5\Delta t}{K(1-x) + 0.5\Delta t}, \; C_2 = \frac{K(1-x) - 0.5\Delta t}{K(1-x) + 0.5\Delta t} $$

• Procedure:

  1. Determine $K$, $x$ from known hydrograph (storage-outflow relation).

  2. Choose $\Delta t$ (usually $ \Delta t \leq K $).

  3. Compute $$\displaystyle C_0, C_1, C_2 $$.

  4. Route inflow hydrograph step-by-step.

• Example: Route given inflow hydrograph.

[!TIP] Exam Focus: State Muskingum equation & storage concept. Derive routing coefficients. Numerical routing problem is common.

V. FLOOD ESTIMATION AND MANAGEMENT

A. Flood Frequency Analysis

• Objective: Estimate magnitude of flood for given return period ($T$) or exceedance probability ($$\displaystyle p=1/T $$).

• Data: Annual Maximum Series (AMS) or Partial Duration Series (PDS).

• Methods:

  1. Graphical (Gumbel Type I): Fit to extreme value distribution. $$\displaystyle y = -\ln[-\ln(1-\frac{1}{T})] $$. Plot $Q$ vs. $y$, fit line, estimate $$\displaystyle Q_T $$.

  2. Log-Pearson Type III (USGS): Fit to log-transformed data (3 parameters: mean, std dev, skew). Most common in practice (Recommended by ISI).

  3. Regional Frequency: Use data from similar catchments if local record short.

• Example: Given annual flood series, compute 50-yr flood using Log-Pearson III.

B. Rational Method for Peak Discharge

• Formula: $$\displaystyle Q_{peak} = \frac{1}{360} C \cdot i \cdot A $$

  • $Q$ = peak discharge (m³/s)

  • $C$ = runoff coefficient (dimensionless, 0.1-0.9)

  • $i$ = rainfall intensity (cm/hr) for duration = $$\displaystyle t_c $$ (time of concentration) & return period $T$.

  • $A$ = catchment area (km²).

• IDF Curves: Intensity-Duration-Frequency curves for location. $$\displaystyle i = f(t_c, T) $$. Use $$\displaystyle t_c $$ from empirical formula (e.g., $$\displaystyle t_c = L^{0.8}(S+1)^{0.7}/1140 $$ for natural catchments).

• Example: Given $A$, land use (get $C$), IDF, compute $$\displaystyle Q_{peak} $$.

C. SCS Curve Number Method (Brief)

• CN based on land use & hydrologic soil group (A, B, C, D).

• Runoff Depth: $$\displaystyle Q = \frac{(P - I_a)^2}{P - I_a + S} $$ where $$\displaystyle I_a = 0.2S $$, $$\displaystyle S = \frac{25400}{CN} - 254 $$ (mm).

• Used for total runoff volume, not peak.

D. Flood Control Measures

1. Structural:

   • Reservoirs/Dams: Store floodwater, release later. Multi-purpose.

   • Channel Improvements: Straightening, deepening, clearing to increase capacity & reduce travel time.

   • Levees/Floodwalls: Contain flow within channel.

   • Bypass Channels/Diversions: Redirect flood away from protected area.

2. Non-Structural:

   • Forecasting & Warning: Hydrometeorological networks, models.

   • Flood Plain Zoning: Regulate development in flood-prone areas.

   • Flood Insurance: Risk transfer.

   • Watershed Management: Afforestation, check dams to reduce runoff & increase infiltration.

E. Role of Reservoirs & Channels

• Reservoir Operation: Store flood peak, release at safe rate (controlled by rule curve). Requires forecasting.

• Channel Routing: Attenuate flood wave as it travels downstream due to storage in channel & floodplain. Methods: Muskingum, kinematic wave.

VI. EVAPORATION AND LAKE HYDROLOGY

A. Evaporation Process & Factors

• Process: Liquid water → water vapor. Driven by solar radiation, requires energy (latent heat of vaporization).

• Factors:

  • Climate: Temperature (↑T → ↑evap), humidity (↓RH → ↑evap), wind speed (↑wind → ↑evap), solar radiation (↑rad → ↑evap).

  • Water Body: Surface area (↑area → ↑total evap), depth (shallow → ↑evap), salinity (salt water < fresh).

  • Other: Atmospheric pressure, water quality.

B. Methods of Measuring Evaporation

1. Direct Methods:

   • Evaporation Pans: Class A Pan (standard, 120 cm diameter, 25 cm deep). Measure water loss, apply pan coefficient (Kp) to get lake/reservoir evaporation. $$\displaystyle E_{lake} = K_p \cdot E_{pan} $$.

   • Lysimeters: Weighed soil columns with vegetation. Measure evapotranspiration directly.

2. Analytical Methods:

   • Energy Balance: $$\displaystyle E = \frac{R_n - G - H}{\lambda} $$ (latent heat flux). Accurate but complex.

   • Aerodynamic: $$\displaystyle E = f(u) (e_s - e_a) $$. Based on vapor pressure gradient & wind.

   • Combined Methods:

     • Penman Equation: Combines energy balance & aerodynamic. Standard for large areas.

$$ E = \frac{0.408 \Delta (R_n - G) + \gamma \frac{900}{T+273} u_2 (e_s - e_a)}{\Delta + \gamma(1+0.34u_2)} $$

    *   **Penman-Monteith (FAO-56):** More accurate, uses canopy resistance.

C. Lake Evaporation Estimation

1. Pan Coefficient Method: Most common for reservoirs. $$\displaystyle K_p $$ depends on pan type, location, humidity, wind. Typical $$\displaystyle K_p $$ for Class A pan: 0.7 (humid) to 0.8 (arid).

2. Analytical Approaches: Use Penman/Monteith with open water surface parameters (no canopy resistance).

[!TIP] Exam Focus: Write Penman equation & define all terms. Explain pan coefficient & its range. Compare direct vs. analytical methods.