--- title: "River morphology; porosity, aquifer, piezometric level" subject: "Engineering Geology I" chapter: 6 key: "semester_3/engineering_geology_i:river-morphology-porosity-aquifer-piezometric" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/river-morphology-porosity-aquifer-piezometric site: BCT Engineering --- # River morphology; porosity, aquifer, piezometric level Engineering Geology I · Chapter 6: Physical Geology - Asked in: 2059 Chaitra (old course), 3+1+1+1+1 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/river-morphology-porosity-aquifer-piezometric - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6 ## Question Describe the morphology of river channel and define porosity, permeability, aquifer, aquiclude and piezometric levels. ## Short answer River channel morphology covers planform (straight, meandering, braided, anastomosing) and long profile from steep upper to gentle lower. Porosity: void ratio; permeability: ability to transmit fluid; aquifer: water-yielding formation; aquiclude: impermeable; piezometric level: level to which water rises in a confined aquifer. ## Answer #### Morphology of river channel River channel morphology is the form of the channel: its cross-section, long-profile, slope and plan form. By plan form, channels are straight, meandering, braided or anastomosing. In the long profile, a river changes along its length from a steep, narrow, V-shaped upper course (erosion) through a middle course to a gentle, wide lower course with meanders and floodplain (deposition). River channels are classified by their plan shape (pattern) as: ```text Straight: ---------------------- ====================== Meandering: ___ ___ / \ / \ __/ \__/ \__ Braided: ___ ___ ___ / \_/ \_/ \ \___/ \___/ \___/ ``` 1. **Straight:** nearly straight, steep, in rocky hills. 2. **Meandering:** sinuous loops on gentle slopes; outer bank erosion, inner bank deposition. 3. **Braided:** many channels around bars with heavy bed load, e.g. Koshi in the Terai. 4. **Anastomosing:** stable multiple channels with vegetated islands. #### Definitions - **Porosity ($n$):** ratio of the volume of voids to the total volume of the rock or soil, $n = V_v/V$. It measures the water storage capacity. - **Permeability:** the ability of a material to transmit fluid through its interconnected pores. Gravel is highly permeable; clay is almost impermeable. - **Aquifer:** a saturated rock or soil formation that stores and yields useful amounts of water, e.g. sand, gravel, fractured rock. - **Aquiclude:** a saturated formation that stores water but cannot transmit it in useful amounts, e.g. clay, shale. - **Piezometric level:** the height to which water rises in a well drilled into a confined aquifer; the imaginary surface joining these levels is the piezometric (potentiometric) surface. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which set lists the plan forms of river channels? A. Youthful, mature, old and rejuvenated B. Antecedent, consequent and subsequent C. Straight, meandering, braided and anastomosing D. Erosional, depositional and transport Answer: C. Plan form describes the pattern of the channel viewed from above. (Key point: Plan form) 2. Braided channels, like the Koshi in the Terai, are characterised by: A. A single deep sinuous channel B. Stable vegetated islands with little bed load C. A steep straight channel in rock D. Many channels around bars with heavy bed load Answer: D. Heavy bed load forms bars that split the flow into many channels. (Key point: Braided channel) 3. How does a river change along its length? A. From a steep, narrow, V-shaped upper course (erosion) to a gentle, wide lower course with meanders (deposition) B. From a gentle wide upper course to a steep narrow lower course C. It stays the same along its length D. Deposition dominates the upper course and erosion the lower Answer: A. Erosion dominates upstream and deposition dominates downstream. (Key point: Long profile) 4. The piezometric level is: A. The depth of the water table in an unconfined aquifer B. The height to which water rises in a well drilled into a confined aquifer C. The ratio of voids to total volume D. The rate at which water passes through clay Answer: B. The imaginary surface joining these levels is the piezometric surface. (Key point: Piezometric level) 5. Which statement about permeability is correct? A. Clay is highly permeable since it has many small pores B. Permeability measures only the volume of voids C. Gravel is highly permeable while clay is almost impermeable D. Permeability is the same as porosity Answer: C. Permeability depends on interconnected pores, not just void volume. 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