CVS 447_Surface Water_Part 2

Page 1

• MOI University: Department of Civil and Structural Engineering

• Course: CVS 447: Applied Hydrology (3 Units)

• Topic: Surface Water (Part 2)

Page 2: Surface Water - SCS Method for Abstractions

• Main abstractions from precipitation:

  • Initial abstraction before ponding (Ia)

  • Continuing abstraction after ponding (Fa)

• Depth of excess precipitation (Pe) ≤ depth of precipitation (P)

• Runoff begins after satisfying Ia

• Potential runoff = (P - Ia)

• Ratios of actual to potential quantities are assumed equal.

Page 3: Precipitation Rate

• Graph depicting relationship between precipitation rate and time:

  • P = P₁ + 1₁ + Fa

Page 4: Continuity Principle in SCS Method

• Excess precipitation (Pe) yields:

  • P = depth of precipitation (mm)

  • Ia = initial abstraction (mm)

  • S = maximum potential retention (mm)

Page 5: Estimating Retention Parameter (S)

• Retention parameter, S, is estimated as:

  • CN (curve number)

  • Composite CN calculated from various land uses and soil types:

    • CNW = weighted CN

    • CNi = CN for land use i

    • Ai = area for land use i

Page 6: Curve Number and Soil Groups

• Curve number (CN) based on hydrologic soil group and land use:

  • Group A: Low runoff potential, high infiltration (e.g., deep sand)

  • Group B: Sandy loam

  • Group C: Clay loams

  • Group D: High runoff potential (e.g., heavy plastic clays)

Page 7: Flow Depth and Velocity

• Water movement from rainfall categorized into:

  • Overland flow

  • Shallow concentrated flow

  • Streamflow

Page 8: Flow Analysis

• Consider flow into control volume from rainfall intensity (i) and infiltration rate (f) on a surface of length (Lo) with slope (Θ).

Page 9: Discharge per Unit Width

• Discharge per width (qo) equation:

  • V = velocity of flow

  • y = depth of flow

• Uniform laminar flow analysis with friction factor (F).

Page 10: Resistance Coefficient (CL)

• Resistance coefficient (CL) defined:

  • Based on rainfall intensity (i) in inches per hour

• Laminar flow assumption:

  • Hydraulic radius (R) for wide shallow flows

Page 11: The Drainage Basin

• Definition: Area draining into a common point (outlet)

• Water divide or watershed delineates the catchment.

Page 12: Travel Time in a Catchment

• Travel time necessary for flow aggregation to determine discharge at outlet:

  • Total travel time = sum of individual travel times

  • Longest travel time = time of concentration

• Hydraulic path defined for time of concentration estimate using empirical formulas (e.g., Kirpich formula).

Page 13: Stream Networks

• Catchments characterized by merging channels:

  • Order designation for channels: Order 1 for smallest, increasing with size.

  • Higher order retains the higher order number when combined.

Page 14: Stream Order Graph

• Visual key shows channel orders and designations.

Page 15: Bifurcation Ratio (RB)

• Ratio of number of channels of order i to order (i+1) is relatively constant:

  • Empirical bifurcation ratio (RB) typically lies between 2-5.

  • Average lengths of streams related by length ratio (RL).

Page 16: Drainage Density and Rate

• Law of Stream Areas (RA) describes average area drained by streams of successive order.

• Drainage density = total length of stream channels / catchment area.

• Indicates rate at which catchment conveys rain.