---
title: "Flood routing and Clark IUH method"
subject: "Engineering Hydrology"
chapter: 7
key: "semester_5/engineering_hydrology:flood-routing-clark-iuh"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/flood-routing-clark-iuh
site: BCT Engineering
---

# Flood routing and Clark IUH method

Engineering Hydrology · Chapter 7: Flow Routing

- Asked in: 2082 Baisakh, 4 marks
- Video: [Solving IUH with Clark's Method - IOE 2074 Aswin | Hydrology](https://www.youtube.com/watch?v=5q0ULBwi9OI) — Hydropower
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/flood-routing-clark-iuh
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-7

## Question

What is flood routing? Explain about Clark IUH method of flood routing.

## Short answer

Flood routing predicts hydrograph transformation through a reach/reservoir using continuity and storage relation. Clark IUH: excess rainfall is translated via time–area inflow Iᵢ=2.78Aᵢ/Δt, then routed through a linear reservoir (S=KO) yielding IUH; averaging consecutive ordinates gives Δt‑h UH, and superposition with rainfall excess produces the flood hydrograph.

## Answer

**Flood routing** is the procedure for determining how the hydrograph of a flood changes in shape, peak, and timing as it passes through a river reach or reservoir. It uses continuity ($I - O = dS/dt$) and a storage-discharge relation.

#### Clark IUH method

Clark's method transforms excess rainfall into a catchment outlet hydrograph, using two steps.

1. **Translation:** isochrones divide the catchment into strips of area $A_i$ between lines of equal travel time. For 1 cm of excess rain, the inflow to the outlet is $I_i = 2.78\,A_i/\Delta t$ (m³/s).
2. **Storage:** this inflow is routed through a single linear reservoir at the outlet ($S = KO$):
$$
O_2 = C_0 (I_1 + I_2) + C_2 O_1,\quad C_0 = \frac{\Delta t}{2K + \Delta t},\quad C_2 = \frac{2K - \Delta t}{2K + \Delta t}
$$

The outflow ordinates give the **instantaneous unit hydrograph (IUH)**, and the average of two consecutive ordinates gives the $\Delta t$-hour unit hydrograph. Using the excess rainfall depths and superposition, the flood hydrograph is obtained. The parameters needed are $t_c$, $K$ and the isochrone areas.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Flood routing is best described as:
   A. Predicting rainfall from storm tracks
   B. Determining how a flood hydrograph changes in shape, peak and timing as it passes through a reach or reservoir
   C. Measuring discharge at a gauge
   D. Computing infiltration losses
   Answer: B. Routing uses continuity and a storage-discharge relation to find outflow from inflow. (Key point: Flood routing definition)

2. Clark's method has which two steps?
   A. Infiltration and evaporation
   B. Translation by isochrones, then storage by a single linear reservoir
   C. Peak estimation and recession fitting
   D. Rainfall averaging and loss subtraction
   Answer: B. Translation inflow is first found from strip areas, then routed through one linear reservoir S = K O. (Key point: Two steps of Clark's method)

3. Which parameters does Clark's method need?
   A. Only catchment area
   B. tc, K and the isochrone areas
   C. Only K
   D. Rainfall intensity and slope only
   Answer: B. tc and isochrone areas define the translation; K defines storage. (Key point: Parameters needed)

## Related in Engineering Hydrology

- [Clark's 2-h IUH, 110 km2](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aclark-2h-iuh-110km2) — Engineering Hydrology, chapter 7, asked 2×
- [Principles of IUH development by Clark's method](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aclark-iuh-principles) — Engineering Hydrology, chapter 7
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