Hydraulic Radius Interactive Calculator

Sizing a storm sewer, irrigation canal, or culvert without knowing hydraulic radius means guessing at flow velocity and discharge capacity — and that's how designs fail. Use this Hydraulic Radius Calculator to calculate flow area, wetted perimeter, and hydraulic radius (R_h) using channel width, depth, side slope, or pipe diameter depending on your section geometry. Getting R_h right matters in civil drainage design, agricultural irrigation, and environmental stream analysis — it's the number that feeds directly into Manning's equation and determines whether your channel actually conveys the design flow. This page covers the full formula set, a worked trapezoidal canal example, flow theory, and an FAQ.

What is hydraulic radius?

Hydraulic radius is a number that describes how efficiently a channel cross-section carries flow. It equals the flow area divided by the wetted perimeter — the length of channel boundary actually in contact with water. A larger hydraulic radius means less friction per unit of flow, so water moves faster.

Simple Explanation

Think of hydraulic radius like a ratio of "how much water" to "how much wall friction." A wide deep river has a lot of water and relatively little bank contact, so it flows fast. A thin shallow stream has a lot of bank contact relative to its volume, so it drags along slowly. Hydraulic radius puts a single number on that relationship — bigger is better for moving water efficiently.

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Cross-Section Diagram

How to Use This Calculator

1. Select your calculation mode from the dropdown — rectangular channel, trapezoidal channel, circular pipe (partially full), find depth or width from R_h, or Manning velocity.
2. Enter your channel geometry inputs: width, depth, bottom width, side slope, pipe diameter, or hydraulic radius depending on the selected mode.
3. For the Manning velocity mode, also enter the channel slope (S) and Manning's roughness coefficient (n).
4. Click Calculate to see your result.

Interactive Hydraulic Radius Calculator

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Equations & Variables

Use the formula below to calculate hydraulic radius for any channel or pipe cross-section.

Simple Example

Rectangular channel: width B = 4 m, flow depth y = 2 m.

Flow area: A = 4 × 2 = 8 m²

Wetted perimeter: P = 4 + (2 × 2) = 8 m

Hydraulic radius: R_h = 8 / 8 = 1.00 m

Theory & Practical Applications

Fundamental Concept and Physical Significance

The hydraulic radius represents the ratio of flow area to wetted perimeter and serves as a characteristic length scale that governs flow resistance in open channels and partially filled pipes. Unlike the geometric radius of a circular pipe, hydraulic radius applies universally to any cross-sectional shape and directly influences the shear stress distribution at channel boundaries. A larger hydraulic radius indicates more efficient flow—the same cross-sectional area generates less friction when distributed to minimize wetted perimeter contact. This efficiency principle explains why semicircular channels outperform rectangular channels of equal area for gravity-driven flow systems.

The hydraulic radius appears as the characteristic length in the Reynolds number for open channel flow (Re = VR_h/ν) and in the Manning and Darcy-Weisbach friction factor relationships. For wide shallow channels where width greatly exceeds depth, the hydraulic radius approximates the flow depth (R_h ≈ y), simplifying many agricultural and floodplain calculations. Conversely, for deep narrow channels, hydraulic radius remains significantly less than depth due to the proportionally large wetted perimeter contribution from the sidewalls.

Channel Geometry and Hydraulic Efficiency

Different cross-sectional shapes exhibit markedly different hydraulic efficiencies. For a given flow area, the semicircular cross-section provides the maximum hydraulic radius and therefore minimum flow resistance—this is why natural streams tend toward semicircular geometry through erosion processes. Rectangular channels are common in constructed systems due to ease of forming with concrete or masonry, but their hydraulic radius is always less than half the depth unless the width is infinite. Trapezoidal channels with side slopes matching the natural angle of repose of earthen materials (typically 1.5:1 to 2:1 horizontal:vertical) represent a compromise between construction practicality and hydraulic efficiency for irrigation and drainage canals.

The concept of "best hydraulic section" refers to the channel geometry that maximizes hydraulic radius for a given cross-sectional area and construction constraints. For rectangular channels, the best hydraulic section occurs when width equals twice the depth (B = 2y), yielding R_h = y/2. For trapezoidal channels with given side slope z, the optimal geometry satisfies the relationship that half the top width equals the sloped side length. These principles guide initial sizing in canal design before site-specific constraints modify the geometry. However, real channels must also consider sedimentation, maintenance access, structural stability, and vegetation control—factors that often override pure hydraulic efficiency.

Manning Equation and Flow Velocity Prediction

The Manning equation relates mean flow velocity to hydraulic radius through the empirical relationship V = (1/n)R_h^2/3S^1/2, where the 2/3 exponent on hydraulic radius reflects the empirical observation that velocity increases with channel size but at a diminishing rate. The Manning roughness coefficient n accounts for boundary friction effects ranging from n = 0.010 for smooth concrete to n = 0.035 for natural channels with vegetation. This equation dominates civil engineering practice for open channel design despite its empirical origins, as it provides adequate accuracy for most practical applications with much simpler application than theoretical turbulent flow solutions.

A critical insight often overlooked is that Manning's equation assumes uniform flow conditions—constant depth and velocity along the channel reach. In reality, flow profiles vary with channel transitions, controls, and changes in slope or roughness. The hydraulic radius calculated at one cross-section may not represent conditions even a short distance upstream or downstream when flow is rapidly varied. For storm sewer design, engineers typically apply Manning's equation to individual pipe segments between manholes, recalculating hydraulic radius for each segment based on predicted flow depth from gradually varied flow analysis or rational method estimates.

Partially Full Circular Pipe Flow

Circular pipes flowing partially full present a non-intuitive relationship between depth and hydraulic radius. Maximum hydraulic radius in a circular pipe does NOT occur at half-full depth but rather at approximately 81% depth (y/D ≈ 0.81), where the increase in flow area with rising depth finally is overcome by the more rapid increase in wetted perimeter. This means that for a given slope and roughness, maximum velocity occurs at 81% depth, not full pipe flow. At full pipe flow, the hydraulic radius equals D/4, but the wetted perimeter includes the entire circumference πD, reducing efficiency compared to the optimal partially full condition.

This phenomenon has important implications for storm sewer sizing. Designing sewers to flow exactly full at design storm flow creates a hydraulic capacity deficiency—if flow exceeds design by even a small margin, the pipe surcharges, potentially causing flooding at upstream structures. Best practice involves designing for 80-85% full at peak design flow, maintaining freeboard for surges while operating near maximum hydraulic efficiency. The central angle θ in radians provides the key geometric parameter: θ = 2arccos[(r-y)/r], from which both flow area (r²/2)(θ - sinθ) and wetted perimeter (rθ) derive. For depths below D/2, simplified approximations introduce significant error, making the full geometric solution necessary.

Worked Example: Irrigation Canal Design

An earthen trapezoidal irrigation canal must deliver Q = 8.5 m³/s through agricultural land with a longitudinal slope of S = 0.0008 m/m. Soil conditions limit side slopes to z = 2:1 (horizontal:vertical) for stability. The canal will be excavated in natural earth with sparse vegetation, giving Manning's n = 0.028. Determine the required channel dimensions using best hydraulic section principles, then calculate the resulting hydraulic radius and mean flow velocity.

Step 1: Apply Best Hydraulic Section Theory

For a trapezoidal channel, the best hydraulic section satisfies: half the top width equals the sloped side length. The top width T = b + 2zy, and sloped side length = y√(1+z²). Therefore: (b + 2zy)/2 = y√(1+z²). Solving for bottom width: b = 2y√(1+z²) - 2zy = 2y[√(1+z²) - z].

With z = 2: b = 2y[√(1+4) - 2] = 2y[√5 - 2] = 2y[2.236 - 2] = 0.472y.

Step 2: Express Area and Hydraulic Radius in Terms of Depth

Flow area: A = (b + zy)y = (0.472y + 2y)y = 2.472y².

Wetted perimeter: P = b + 2y√(1+z²) = 0.472y + 2y√5 = 0.472y + 4.472y = 4.944y.

Hydraulic radius: R_h = A/P = 2.472y²/4.944y = 0.5y.

Note that for the best hydraulic section of a trapezoidal channel, R_h = 0.5y regardless of side slope z—this is a characteristic property of optimal geometry.

Step 3: Apply Manning Equation to Solve for Depth

Discharge: Q = AV = A × (1/n)R_h^2/3S^1/2.

Substituting: 8.5 = 2.472y² × (1/0.028) × (0.5y)^2/3 × (0.0008)^1/2.

Simplifying: 8.5 = 2.472y² × 35.714 × (0.5y)^2/3 × 0.02828.

8.5 = 2.496y² × (0.5y)^2/3 = 2.496y² × 0.63 × y^2/3 = 1.572y^8/3.

y^8/3 = 8.5/1.572 = 5.407.

y = 5.407^3/8 = 2.14 m.

Step 4: Calculate Channel Dimensions

Bottom width: b = 0.472 × 2.14 = 1.01 m.

Flow area: A = 2.472 × 2.14² = 11.32 m².

Wetted perimeter: P = 4.944 × 2.14 = 10.58 m.

Hydraulic radius: R_h = 11.32/10.58 = 1.07 m (confirming R_h = 0.5y = 1.07 m).

Step 5: Verify Flow Velocity

V = Q/A = 8.5/11.32 = 0.751 m/s.

Check using Manning: V = (1/0.028) × 1.07^2/3 × 0.0008^1/2 = 35.714 × 1.023 × 0.02828 = 1.033 m/s.

Discrepancy suggests calculation error; rechecking Manning application: V = (1/n)R_h^2/3S^1/2 must yield Q when multiplied by A. Direct verification: Q = A × (1/n)R_h^2/3S^1/2 = 11.32 × 35.714 × 1.023 × 0.02828 = 11.69 m³/s, which exceeds the target 8.5 m³/s. Adjusting depth downward: trying y = 1.85 m gives A = 8.46 m², R_h = 0.925 m, V = (1/0.028) × 0.925^0.667 × 0.02828 = 0.999 m/s, Q = 8.46 m³/s—acceptably close to target.

Final Design: Flow depth y = 1.85 m, bottom width b = 0.87 m, hydraulic radius R_h = 0.93 m, mean velocity V = 1.00 m/s. This example demonstrates that even with best hydraulic section theory, iterative solution is often necessary due to the nonlinear relationship between depth and discharge in Manning's equation.

Critical Depth and Hydraulic Radius in Supercritical Flow

In open channel flow, the Froude number Fr = V/√(gy) determines flow regime, but for non-rectangular channels, the appropriate depth parameter becomes hydraulic depth D_h = A/T (where T is top width), and the Froude number is Fr = V/√(gD_h). Hydraulic radius differs from hydraulic depth but both serve as characteristic lengths—R_h governs friction resistance while D_h governs gravitational wave propagation. At critical flow (Fr = 1), specific energy is minimized and flow transitions between subcritical and supercritical regimes. Hydraulic jumps, which dissipate energy when supercritical flow transitions to subcritical, depend on sequent depths related through momentum equations involving hydraulic radius indirectly through the cross-sectional area and centroid depth relationships.

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