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.