9.2.3 Incorporating Climate Change ... 9-4 9.2.4 Suggested Allowance for Climate Change ... 9-5 9.3 REFERENCES... 9-6
B.1 OVERVIEW ... 1
B.2 SEDIMENT CONTINUITY ... 2
B.3 SEDIMENT PROPERTIES ... 3
B.4 SEDIMENT TRANSPORT CONCEPTS ... 4
B.4.1 INITIATION OF MOTION ... 4
B.4.2 MODES OF SEDIMENT TRANSPORT ... 5
B.4.3 EFFECTS OF BED FORMS AT STREAM CROSSINGS ... 6
B.4.4 SEDIMENT TRANSPORT EQUATIONS ... 6
B.4.4.1 Meyer-Peter & Muller Equation ... 7
B.4.4.2 Einstein Method ... 9
B.4.4.3 Colby Method ... 9
Volumes
Volume 1 Introduction and Overview Volume 2A GeoHazard Assessment Volume 2B Engineering Surveys
Volume 2C Geological and Geotechnical Investigations Volume 3 Water Engineering Projects
Volume 4 Highway Design Volume 5 Bridge Design
Volume 6 Public Buildings and Other Related Structures
Annex
A Estimating Scour
Tables and Figures
Table 3-1 Values of 'c' Recommended for Rational Formula ... 3-8 Table 3-2 Kravens Formula ... 3-10 Table 3-3 Equations for Estimating the Time of Concentration in Urban ... 3-10 Table 3-4 Values of Horton's Roughness n* ... 3-10 Table 3-5 Constant (c) for Regional Specific Discharge Curve ... 3-14 Table 3-6 Runoff-Volume Models ... 3-18 Table 3-7 Direct-Runoff Models ... 3-19 Table 3-8 Baseflow Models ... 3-19 Table 3-9 Routing Models ... 3-19 Table 3-10 Information to be Provided on Parameters with Hydrological Models ... 3-20 Table 3-11 Minimum Hydrological Reporting Requirements ... 3-21 Table 4-1 Stream Types ... 4-4 Table 4-2 Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Natural Channels ... 4-12 Table 4-3 Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Floodplains ... 4-12 Table 4-4 Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) – Man-made Channels &
Ditches ... 4-13 Table 4-5 Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Pipes ... 4-13 Table 4-6 Local Losses Coefficient (K) ... 4-20 Table 4-7 Weir Coefficient, μ for Different Weir Shape ... 4-27 Table 4-8 GeoHazard Impacts on Hydraulics ... 4-34 Table 4-9 Overview of Different Model Types ... 4-35 Table 4-10 Advantages and Disadvantages of Model Types ... 4-36 Table 4-11 Overview of Different Software for Flood and Drainage Analysis ... 4-38 Table 4-12 Minimum Hydrological and Hydraulic Reporting Requirements ... 4-40 Table 5-1 Design Flood - Suggested Protection Levels ... 5-3 Table 5-2 Causes of Dike Damage and Potential Countermeasures ... 5-5 Table 5-3 Freeboard Allowance for Dikes ... 5-6 Table 5-4 Recommended Crest Widths for Dikes ... 5-8 Table 5-5 Overview of Different Slope Protection Works & Considerations ... 5-31 Table 5-6 Coefficient for Riprap Design ... 5-36 Table 5-7 Dry Boulder Rip Rap Sizing (D50 in mm) ... 5-37 Table 5-8 Minimum Diameter of Boulder (Riprap Type) ... 5-45 Table 5-9 Indicative Velocity Limits for Gabions and Gabion Mattress ... 5-45 Table 5-10 Potential Failure Mechanisms for Revetments ... 5-52 Table 5-11 ICOLD Definition of a Large Dam ... 5-57 Table 5-12 Minimum Freeboard for Small Dams ... 5-60 Table 5-13 Overview of Some Typical Outlet Control Structures ... 5-67 Table 5-14 Different Types of Floodway/ Road Embankment Protection ... 5-71
Table 6-1 Minimum Capacity of Drainage Infrastructure ... 6-2 Table 6-2 Channel Types and Examples ... 6-3 Table 6-3 Manning’s Roughness of Rock Lined Channels with Shallow Flow ... 6-5 Table 6-4 Manning’s Roughness for Grassed Channels (50–150 mm blade length)* ... 6-6 Table 6-5 Permissible Velocities for Different Channel Linings ... 6-6 Table 6-6 Permissible Velocities ... 6-8 Table 6-7 Recommended Side Slope Material... 6-11 Table 6-8 Typical Transition Losses ... 6-13 Table 6-9 Recommended Inclusions for Safety ... 6-14 Table 6-10 Blockage Factors to be Applied to Culverts ... 6-16 Table 6-11 Example Hydrograph Inputs ... 6-31 Table 6-12 Worked Example Detention Routing... 6-32 Table 6-13 Basin Freeboard Requirements ... 6-33 Table 7-1 Protection Levels for Coastal Structures ... 7-2 Table 7-2 Tidal Terminology ... 7-2 Table 7-3 Suggested Hudson Coefficient Values ... 7-7 Table 7-4 Dimensionless Breaker Parameter ... 7-8 Table 7-5 Relationship for Toe Protection ... 7-10 Table 9-1 Sea Level Rise Predictions (IPCC, 2013) ... 9-2 Table 9-2 Overview of Different Impacts of Climate Change ... 9-3 Table 9-3 Suggested Approach for Incorporating Changes to Extreme Rainfall ... 9-5 Table 9-4 Suggested Approach for Incorporating Sea Level Rise ... 9-6 Figure 3-1 Typical Catchment Configuration ... 3-2 Figure 3-2 Overview of Rational Formula Applicability* ... 3-7 Figure 3-3 Unit Hydrograph Method ... 3-12 Figure 3-4 Specific Discharge Curve ... 3-15 Figure 3-5 HEC-HMS Watershed Runoff Processes ... 3-18 Figure 4-1 Drainage Basin Zones ... 4-2 Figure 4-2 Typical Longitudinal River Profile ... 4-2 Figure 4-3 Sinuosity ... 4-4 Figure 4-4 Meandering Stream Processes (Source: Ohio DNR, undated)... 4-5 Figure 4-5 Energy Grade Line ... 4-7 Figure 4-6 Specific Energy Diagram ... 4-9 Figure 4-7 Hydraulic Jump Diagram ... 4-10 Figure 4-8 Non-Uniform Flow Profiles ... 4-15 Figure 4-9 Part-Full Flow Relationship for Circular Pipes ... 4-16 Figure 4-10 Hydraulic Gradeline and Energy Grade Line for Piped Drainage Systems ... 4-17 Figure 4-11 Commonly Used Culvert Shapes ... 4-22 Figure 4-12 Standard Inlet Types (Schematic) ... 4-22
Figure 4-14 Weir Coefficient with Tailwater Submergence ... 4-27 Figure 4-15 Tailwater Conditions for Submerged Overfall ... 4-28 Figure 4-16 Profile of Rectangular Sluiceway ... 4-28 Figure 4-17 Sluiceway Discharge Coefficient as a Function of h/a & hu/a ... 4-29 Figure 4-18 Free Discharge (Top) and Submerged Discharge (Bottom) ... 4-30 Figure 4-19 Limit between Free & Submerged Discharge ... 4-30 Figure 5-1 Example Countermeasure against Seepage ... 5-6 Figure 5-2 Key Components of a Dike ... 5-6 Figure 5-3 Dike Height ... 5-7 Figure 5-4 Freeboard due to Backwater Effects ... 5-7 Figure 5-5 Plan and Perspective of Dike Showing the Location of Access Road ... 5-8 Figure 5-6 Example of Crib-Wall used with Restricted Space ... 5-10 Figure 5-7 Arrangement of Berm ... 5-10 Figure 5-8 Incorporating Settlement into Design of Levee ... 5-11 Figure 5-9 Self-Standing Retaining Wall (Example) ... 5-15 Figure 5-10 Parapet Wall (Example) ... 5-16 Figure 5-11 Illustrative Example of Overflow Dike ... 5-17 Figure 5-12 Widening and Increasing the Height of Dike ... 5-17 Figure 5-13 Example of Spur dikes used to protect outer River Bank ... 5-18 Figure 5-14 Example of Spur dikes used with Bridge Design... 5-19 Figure 5-15 Example Permeable Spur Dike ... 5-20 Figure 5-16 Dimensions of Spur Dike – Impermeable Overflow Type ... 5-22 Figure 5-17 Effective Length of a Spur Dike ... 5-23 Figure 5-18 Scour Adjustment for Spur Orientation ... 5-24 Figure 5-19 Toe Protection Works for Spur Dike ... 5-25 Figure 5-20 Shape of Spur Dike ... 5-25 Figure 5-21 Location of Revetment at River Bend ... 5-26 Figure 5-22 Components of a Revetment ... 5-27 Figure 5-23 Components of a Revetment Cross-Section ... 5-28 Figure 5-24 Velocity Adjustment Factor ... 5-30 Figure 5-25 Sodding with Grass or Some Other Plans (Natural Type) ... 5-31 Figure 5-26 Wooden Pile Fence ... 5-32 Figure 5-27 Dry Boulder Rip Rap ... 5-32 Figure 5-28 Grouted Rip Rap, Spread Type ... 5-32 Figure 5-29 Grouted Riprap, Wall Type ... 5-33 Figure 5-30 Gabion Mattress, Spread Type ... 5-33 Figure 5-31 Gabion Mattress (Gabion Wall), Pile-up Type ... 5-33 Figure 5-32 Rubble Concrete, Spread Type ... 5-34 Figure 5-33 Rubble Concrete, Wall Type ... 5-34 Figure 5-34 Reinforced Concrete ... 5-35
Figure 5-35 Gravity Wall ... 5-35 Figure 5-36 Sheet Pile and Reinforced Concrete (Segment Combination) ... 5-35 Figure 5-37 Typical Forces Acting on a Gabion Wall ... 5-38 Figure 5-38 Example Vegetated Bank Protection ... 5-40 Figure 5-39 Height of Revetment ... 5-40 Figure 5-40 Provision of a Berm in a Revetment ... 5-41 Figure 5-41 Depth of Foundation ... 5-42 Figure 5-42 Foundation Work ... 5-43 Figure 5-43 Types of Foot Protection Works ... 5-44 Figure 5-44 Concrete Block Type - Orderly Pile Up - Single Unit ... 5-46 Figure 5-45 Concrete Block Type –Orderly and Random Types ... 5-46 Figure 5-46 Weight of Concrete Block ... 5-46 Figure 5-47 Width of Foot Protection Required ... 5-47 Figure 5-48 Riprap Revetment with Mounded Toe Approach ... 5-48 Figure 5-49 End Protection Works ... 5-49 Figure 5-50 Crest Protection ... 5-49 Figure 5-51 Development of Residual Hydraulic Pressure without Drainage Pipes/ Weep Holes ... 5-50 Figure 5-52 The Need for Filter Cloth/ Gravel ... 5-50 Figure 5-53 Typical Groundsill Layout ... 5-55 Figure 5-54 Groundsill Locations ... 5-57 Figure 5-55 Sluiceway for Drainage ... 5-64 Figure 5-56 Typical Detail for Overtopping at Bridge Approach/ Floodway ... 5-70 Figure 5-57 Typical Types of Roadway Embankment Protection ... 5-72 Figure 5-58 Typical Types of Roadway Embankment Protection ... 5-73 Figure 6-1 Turf Reinforcement Matting Profile ... 6-9 Figure 6-2 Open Channels and Freeboard (Source: QUDM, 2013) ... 6-11 Figure 6-3 Example Low Flow Channel for Dry Weather Flows ... 6-12 Figure 6-4 Maximum Rate of Expansion ... 6-12 Figure 6-5 Debris Deflector Walls ... 6-17 Figure 6-6 Typical Inlet Structures ... 6-17 Figure 6-7 Dry Boulder (Riprap) Outlet ... 6-18 Figure 6-8 Sizing of Dry Boulder Outlet Structures for Single Pipe or Box Culverts ... 6-19 Figure 6-9 Sizing of Dry Boulder Outlet Structures for Multiple Pipe or Box Culverts ... 6-20 Figure 6-10 Typical Rock Pad Outlet Configuration ... 6-20 Figure 6-11 Typical Orientation and Set-Back of Outlet ... 6-21 Figure 6-12 Grated Pit (in depression) Inflow Rating Curves ... 6-23 Figure 6-13 Side Opening Pit (in kerb or gutter) Inflow Rating Curves ... 6-24 Figure 6-14 Inlet Weir Flow Behavior ... 6-25 Figure 6-15 Inlet Orifice Flow Behavior ... 6-26 Figure 6-16 Typical Schematic of Detention Basin... 6-28
Figure 6-17 Example of Above Ground Detention System after Heavy Rain ... 6-28 Figure 6-18 Example Underground Storage System ... 6-29 Figure 6-19 Example Underground Detention System using Permeable Pipes ... 6-29 Figure 6-20 Example Underground Detention System ... 6-30 Figure 6-21 Basin Volume Estimation ... 6-33 Figure 6-22 Typical Spillway Design ... 6-35 Figure 6-23 Centrifugal Pump ... 6-38 Figure 6-24 Positive Displacement Pump ... 6-39 Figure 6-25 Estimated Required Pump Storage from Inflow Hydrograph ... 6-40 Figure 6-26 Typical Wet-Pit Pumping Station ... 6-41 Figure 6-27 Typical Dry-Pit Configuration ... 6-42 Figure 7-1 Example of Sea Wall ... 7-4 Figure 7-2 Example of Rock Sea Wall/ Revetment ... 7-4 Figure 7-3 Typical Revetment Section ... 7-5 Figure 7-4 Overview of Parameters for Wave Runup ... 7-8 Figure 7-5 Types of Waves ... 7-8 Figure 7-6 Example of Toe Protection Options ... 7-11 Figure 7-7 Example Scour Protection using Toe extending to Depth of Anticipated Scour in Moderate
Scour Environments ... 7-12 Figure 7-8 Example Scour Protection using Toe extending to Depth of Anticipated Scour in Severe Scour Environments ... 7-12 Figure 7-9 Example Sea Wall - Constructed to appear like a natural bluff ... 7-13 Figure 7-10 Example Detached Breakwaters ... 7-13 Figure 7-11 Example of Groynes as Shoreline Protection ... 7-14 Figure 8-1 Rainfall Distribution in the Philippines ... 8-2 Figure 8-2 Distribution System Classification ... 8-8 Figure 4-13 Definition Sketch - Triangular Section... 4-24
Abbreviations
Abbreviation DefinitionALS Airborne Laser Survey
AMWS Association of Massachusetts Wetland Scientists BF Blockage factor
BMPs Best Management Practices
CAD Computer Aided Design
CETMEF Centre d'Études Techniques Maritimes Et Fluviales CIRIA Construction Industry Research and Information Association CUR the Netherlands Centre for Civil Engineering Research and Codes DEM Digital Elevation Model
DENR Department of Environment and Natural Resources
DFL Design Flood Level
DGCS Design Guidelines Criteria and Standards DID Department of Irrigation and Drainage (Malaysia) DNR Department of Natural Resources (Ohio) DPWH Department of Public Works and Highways
DTMR Department of Transport and Main Roads (Queensland)
EGL Energy Grade Line
EO Executive Order
FCSEC Flood Control and Sabo Engineering Centre FHWA/FHA Federal Highway Administration
GIS Geographic Information System GPS Global Positioning System GPTs Gross Pollutant Traps
HEC23 Hydrologic Engineering Centre Circular 23
HEC-HMS Hydrologic Engineering Center-Hydrologic Modelling System HEC-RAS Hydrologic Engineering Center – River Analysis System HGL Hydraulic Grade Line
HWL High Water Level
ICOLD International Commission on Large Dams IPCC Intergovernmental Panel on Climate Change IRR Implementing Rules and Regulations LiDAR Light Detection and Ranging LLDA Laguna Lake Development Authority LWUA Local Water Utilities Administration MC Memorandum Circular
MMDA Metropolitan Manila Development Authority MO/DO Ministry Order/Department Order
MSMA Manual Saliran Mesra Alam (Urban Stormwater Management Manual for Malaysia) MWSS Metropolitan Waterworks and Sewerage System
N/A Not Applicable
Abbreviation Definition
NDCC National Disaster Coordination Council
NJDEC New Jersey Department of Environmental Conservation NRE Department of Natural Resources and Environment
OWL Ordinary Water Level
PD Presidential Decree
PAGASA Philippine Atmospheric, Geophysical and Astronomical Services Administration PNG DoW Papua New Guinea Department of Public Works
PPA Philippine Ports Authority
Project ENCA Enhancement of Capabilities in Flood Control and Sabo Engineering PUB Public Utilities Board (Singapore’s national water agency)
QUDM Queensland Urban Drainage Manual
Rep. Representative
RIDF Rainfall Intensity-Duration-Frequency SCS Soil Conservation Service
SMA Soil Moisture Accounting
SUDS Sustainable Urban Drainage Systems TRM Turf Reinforcement Matting
UDFCD Urban Drainage and Flood Control District
UH Unit Hydrograph
USACE United States Army Corps of Engineers USBR United States Bureau of Reclamation USDA United States Department of Agriculture WSUD Water Sensitive Urban Design
Glossary
Acronym Definition
Abutment Structure at the two ends of a bridge used for transferring the loads from the bridge superstructure to the foundation bed and giving lateral support to the embankment.
Afflux The upstream rise of water level above the normal surface of water in a channel caused by an obstruction in the waterway, such as a bridge or weir or by regulation. The increased amount of water which occurs upstream from a structure (dam) or obstruction in a stream channel, due to the existence of such obstruction and the raising of the water level to considerable distance upstream.
Alluvial Soil or earth material which has been deposited by running water. Alluvial Fan (alias Gravel
Wash) A fan shaped deposit formed where a stream emerges from an entrenched valley into a plain or flat. Alluvial Stream Stream flowing mainly in self-transported alluvial deposits.
Annual Risk of
Exceedance The chance or probability of a natural hazard event (usually a rainfall or flooding event) occurring annually and is usually expressed as a percentage. Apron A floor or lining of concrete, timber, or other resistant material at the toe of a dam, bottom of a spillway,
chute, etc. to protect the foundation from erosion and falling water or turbulent flow.
As-Built Plan A scaled drawing that shows a project and infrastructure components after completion of construction
Avulsion A sudden cutting off of land by floods, currents, or change in course of a body of water.
Backwater The rise of water level that occurs immediately upstream from a structure (eg.dam) or obstructions in a river to a considerable distance brought about by the presence of structure.
Bed Load Material moving on or near the stream bed by rolling, sliding, and sometimes making brief excursions into the flow of new diameters above the bed.
Bed Material The material of which the riverbed is composed.
Berm A horizontal strip or shelf built into an embankment or cut, to break the continuity of an otherwise long slope.
Bioengineering The use of mechanical elements in combination with biological elements (e.g.plants) particularly for control of erosion and prevention of slope failures.
Borrow Site An excavation source ouside the project area that is used to supply soils for earthwork construction (i.e. gravel pit).
Borrow Materials Filling materials acquired from a Borrow Site.
Bridge A structure carrying a road over a road, waterway or other feature, with a clear span over 3.0 meters along the centreline between the inside faces of supports. A bridge may have an independent deck supported on separate piers and abutments, or may have a deck constructed integral with supports. Catchment Area
(alias Catchment Basin, Watershed, Drainage Area, Drainage Basin, River Basin)
The area from which a lake, stream or waterway receives surface water which originates as precipitation.
Climate Change A long-term change in the statistical distribution of weather patterns over periods of time that range from decades to millions of years.
Coarse-grained Soils Soils with more than 50% by weight of grains retained on the number 200 sieve (0.075 mm). Cohesionless Soils Granular soils (sand and gravel type) with values of cohesion close to zero.
Cohesive Soils Clay type soils with angles of internal friction close to zero. Concrete A mixture of cement, fine aggregate, coarse aggregate and water. Cross Section
(alias Cross Section Plan) View generated by slicing an object at an angle perpendicular to its longer axis.
Culvert A structure in the form of a pipe or box, below road level, for conveying storm water runoff . Cutoff A wall or diaphragm of concrete or steel, or a trench filled with puddled clay or impervious earth.
Density The ratio of the total mass to the total volume of material.
Design Life Period assumed in the design for which the infrastructure is required to perform its function without replacement or major structural repair.
Detached Breakwaters A structure parallel, or close to parallel, to the coast, build inside or outside the surf zone. Digital Photogrammetry
(alias Photogrammetry) The art of using computers to obtain the measurements of objects in a photograph. It typically involves analyzing one or more existing photographs or videos with photogrammetric software to determine spatial relationships.
Ditch An artificial open channel or waterway constructed through earth or rock, for the purpose of carrying water.
Drawdown The magnitude of the lowering of a water table, usually near a well being pumped. Dredging Removal from beneath water and raising through water of soil rock and debris. Embankment A raised structure of soil aggregate, rock or a combination of the three.
Energy Grade Line A line joining the elevation of energy heads of a stream; a line drawn above the hydraulic grade line a distance equivalent to the velocity head of the flowing water at each cross section along a stream or channel reach or through a conduit.
Factor of Safety The ratio of a limiting value of a quantity or quality to the design value of that quantity or quality. Flood Control Detention or diversion of water for the purpose of reducing discharge for downstream inundation.
Flood Plain Flat land bordering a river and subject to flooding
Force A push or a pull in a given direction on a body that changes or tends to change its state or rest. (or its state of motion).
Free Water
(alias Phreatic Water, Gravitational Water)
Water that is free to move underground through a soil mass under the influence of gravity.
Gabion A basket or cage filled with earth or rocks and used especially in building a support or abutment. Grain Size Distribution
Curve A curve drawn on a log scale to represent the distribution of particle sizes in a soil.
Gravity Walls Retaining walls which depend upon their selfweight to provide stability against overturning and sliding; usually made of a high bulk structure
Grouted Riprap When the stones in the rip-rap are fastened together by grout of mortar. Groin
(alias Groyne) A wall, crib, row of piles, stone, jetty or other barrier projecting outward from the shore or bank into a stream or other body of water, for the purpose of protecting the shores or bank from erosion, arresting sand movement along the shore, concentrating the low flow of a stream into a smaller channel, etc. Hydraulic Grade Line Line connecting the points to which the liquid would rise in piezometer tubes if inserted at various
places along any pipe. It is the measure of the pressure head plus the elevation of the pipe at these various points.
Hydrofracturing A well stimulation process used to maximize the extraction of underground resources. In-situ Undisturbed, existing field conditions.
Land-use Map Maps that reflect the land resources and types of land use in the national economy. Levee
(alias ‘Dike’) An embankment, generally constructed on or parallel to the banks of a stream, lake or other body of water for the purpose of protecting the land side from inundation by flood water, or to confine the stream flow to its regular channel.
Light Detection and
Ranging (LiDAR) A remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light. Although thought by some to be an acronym of Light Detection And Ranging, the term lidar was actually created as a portmanteau of "light" and "radar".
Lining A protective covering over all, or over a portion of the perimeter of a conduit, canal, or reservoir, to prevent seepage losses, to withstand pressure, or to resist erosion.
Longitudinal Section View generated by slicing an object at an angle parallel to its longer axis
Manhole An opening through which a person may enter or leave a sewer, conduit, or other closed structure for inspection cleaning, and other maintenance operations, closed by a removable cover.
Matchline A line on a design drawing that projects a location or distance from one portion of the drawing to another portion of the drawing.
Maximum Flood Level The highest recorded flood level.
Mean Sea Level The average height of the sea for all stages of the tide. Mean sea level is obtained by averaging observed hourly heights of the sea on the open coast or in adjacent waters having free access to the sea, the average being taken over a considerable period of time.
Navigational Pertaining to, or used in, conducting ships or other vessels on the water from one place to another. Open Channel Any conduit in which water flows with a free surface. Channel in which the stream is not completely
enclosed by solid boundaries and therefore has a free surface subjected only to atmospheric pressure.
Ordinary Water Level The height of water in the river under normal condition.
Parcellary Survey A survey to determine and establish the legal boundary of real properties.
Pier A structure usually of concrete or stone masonry, which is used to transmit loads from the bridge superstructure to the foundation soil and provide intermediate supports between the abutments.
Pile A slender member that is driven (hammered), drilled or jetted into the ground. Piles are usually constructed of timber, steel or pre-stressed reinforced concrete.
Piping The movement of soil particles as a result of unbalanced seepage forces produced by percolating water.
Profile Series of elevation along a line.
Reinforced Concrete A composite material which utilizes the concrete in resisting compression forces and some other materials, usually steel bars or wires, to resist the tension forces.
Retaining Wall A structure usually made of stone masonry, concrete or reinforced concrete that provides lateral support for a mass of soil.
Riprap Rock or other material used to armor shorelines, streambeds, bridge abutments, piling and other shoreline structures against scour and water erosion.
River Training A group of engineering works built along a river or a section thereof in order to direct or lead the flow to a prescribed channel, with or without the construction of embankments.
Rubble Concrete Concrete in which large stones are added to the freshly placed concrete while it is still soft and plastic. Runoff Surface water of an area of land.
Sand Particles that pass through a number 4 sieve (4.75 mm), and retained on a number 200 sieve (0.075 mm).
Scour Lowering of stream-bed or undermining of foundations by erosive action of flowing water. Scoured Depth Total depth of water from surface to a scoured bed level.
Depth of Scour The depth of materials removed below the set datum.
Settlement The downward movement of soil, or the downward movement of a foundation.
Sheet Piles A long vertical earth retention and excavation support, steel, vinyl or reinforced concrete, driven into the ground with interlocking edges to form a continuous wall to resist water or earth pressure.
Stilling Basin A depression in a channel or reservoir deep enough to reduce the velocity or turbulence of the flow. Artificial Submerged
Reefs An alternative method of shoreline stabilization and beach erosion control, using a man-made underwater structure to mitigate the wave induced erosion. Time of Concentration The period of time for the stormwater or rainwater to flow from the most distant point to the point under
consideration.
Topographic Plan A graphic representation of horizontal and vertical positions of an area which uses contour lines to show mountains, valleys, and plains.
Topographic Survey
(alias Ground Survey) Collection of data to represent horizontal and vertical positions of an area, including features such as roads, bridges and bodies of water with contours, elevations and coordinates. Tributary A stream or other body of water, surface or underground, which contributes its water, either
continuously or intermittently, to another and larger stream or body of water.
Vertical Alignment The position or the layout of the highway on the ground which includes level and gradients. Wave Height The height of the wave from the wave top, called the wave crest to the bottom of the wave,
Wave Runup The maximum vertical extent of wave uprush on a beach or structure above the still water level (SWL). Weep Hole An opening provided during construction in retaining walls, aprons, canal linings, foundation, etc., to
permit drainage of water collecting behind and beneath such structures to reduce hydrostatic head. Weir A low dam built across a river to raise the level of water upstream or regulate its flow.
Wetlands
(alias Swamp, Marshes, Bogs)
Those areas that are inundated and saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions.
Wingwall A vertical wall located at both ends of the coping of the abutment or at both extreme wall of a reinforced concrete box culvert.
1
General Provisions
1.1
Scope and Application
This guideline aims to provide the Department of Public Works and Highways (DPWH) Engineers (including concerned Local Government Units and Government Consultants) with the basic knowledge and essential tools in undertaking design of water engineering projects specifically for flood control, water supply, coastal facilities and drainage infrastructures.
This guideline provides an overview of some of the key issues, considerations and items to be incorporated into design. As with the Guide, this is not meant to be an exclusive list of design criteria or a manual for the design of these infrastructures. Therefore, it is important that the designs of these infrastructures are undertaken by suitably qualified engineers with experience in undertaking this work.
The design of Sabo Engineering structures is not covered by this Guide. For the design of Sabo Engineering structures, reference should be made to the Flood Control and Sabo Engineering Center (FCSEC) Guideline.
1.2
Governing Laws, Codes, Memoranda, Circular and Department Orders
Water engineering projects are indispensable in the socio-economic development and the protection of lives, infrastructures, agricultural, and other resources of the country. To promote water engineering activities, laws, codes and department orders governing were formulated and executed, which include the following:
PD 1067. Water Code of the Philippines, thereby revising and consolidating the laws governing the ownership, appropriation, utilization, exploitation, development conservation and protection of water resources.
PD 296. Directing all persons, natural or juridical, to renounce possession and move out of portions of rivers, creeks, esteros, drainage channels and other similar waterways encroached upon by them and prescribing penalty for violation hereof.
PD 78 of 1972 creation of The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA).
Letter of Instruction (LOI) No. 19 dated Oct. 2, 1972 directed then Secretary of Public Works and Communications, to remove all illegal construction including buildings on and along esteros and riverbanks, and to relocate, assist in the relocation and determine sites for informal settlers and other persons to be displaced
PD No. 772 of 1972, for penalizing informal settlers and other similar act.
PD No. 198. The Provincial Water Utilities Act of 1973, for declaration of a national policy of local water utilities and for creating the Local Water Utilities Administration (LWUA).
PD 1149 of 1977 organized the National Flood Forecasting Office as one of the major organization units of the PAGASA. This P.D. amends certain sections of
P.D. No. 78 otherwise known as “The Atmospheric, Geophysical and Astronomical Science of 1972”. The present PAGASA is attached to the National Science and Technology Authority by Executive Order (EO) No. 128 in 1987.
PD 1566 of June 11, 1978 establishment of a National Program on Community Disaster Preparedness. The National Disaster Coordination Council (NDCC) issued the Calamity and Disaster Preparedness Plan in 1988. Flood fighting is undertaken nationwide by virtue of this PD.
Ministry Order No. 20, Series of 1982. Guidelines for the Preparation, Evaluation and Ranking of Flood Control and Drainage Projects.
PD 187 as amended by P.D. 748 and Batas Pambansa Blg. 8, An act defining the Metric System and its Units, providing for its implementation and for other purposes; and MPWH Memorandum Circular No.6, dated January 6, 1983, re Metric System (SI) Tables. Under the Local Government Code, a city or municipality may reclassify agricultural lands and provide the manner of their utilization and disposition.
Executive Order No. 192 of 1987 mandates Department of Environment and Natural Resources (DENR) for conservation, management, development and proper use of the country’s environment and natural resources including those in the watershed.
Republic Act No. 4850, creating the Laguna Lake Development Authority (LLDA).
Republic Act No. 6234, creating the Metropolitan Waterworks and Sewerage System (MWSS).
Executive Order No. 215 and 462, for private sector participation in hydrological endeavors.
Republic Act No. 7924 of 1994, for creating the Metropolitan Manila Development Authority (MMDA), defining its powers and functions, providing funding therefore and for other purposes.
Republic Act 9003 - Solid Waste Management Act, overall institutional framework of managing solid wastes including functions and responsibilities
IRR of Republic Act 9003 Section 6 – Creation of Local Solid Waste Management Committee (Creation of Barangay Solid Waste Management Committee)
Republic Act 7942 – Philippine Mining Act of 1995
PD No. 825 – Providing Penalty for Improper Disposal of Garbage
PD No. 856 – Sanitation Code
DMC 5-97 – Navigation Clearance for Road Bridges [(CGAO/CG-10)-HQ Philippine Coast Guard]
DO 50 of 1987 – Soil Investigation for Design of Foundation of Various Structures
DO 06 of 2012 – Coconet Bioengineering Solutions
DO 68 of 2012 – Guidelines in Design of Slope Protection Works
Memorandum of 1983 – Quarrying of Construction Aggregates and/ or Materials
1.3
Planning Process
The planning process can be undertaken in a number of different ways. A general procedure for planning process identifies three key stages:
Master Plan – a high level strategic plan that assesses existing constraints and issues, and identifies potential solutions at a large scale. For example, for flood control a river basin wide approach may be adopted where analysis is undertaken on the flooding issues and potential flood control alternatives are identified;
Feasibility Study – prioritization and selection of projects from the Master Plan;
Implementation Plan – a plan that specifies the works selected from the Feasibility Study, including the funds required and the estimated benefits. The above approach is based on Technical Standards and Guidelines for Planning of
Flood Control Structures (FCSEC, 2010), and generalized for water engineering
projects in general.
Within the context of DPWH, the Master Plan and Feasibility Study represent stages prior to, and during the Concept Development phase of the design process, while the Implementation Plan represents the Design Development and Detailed Design Phase.
This approach should be adopted, rather that targeting a specific problem in isolation.
Where it is not possible to undertake a master plan approach, Concept Development should still be undertaken to ensure that sufficient constraints are identified and that adequate budgets are allocated, prior to moving to the Design Development and Detailed Design Phase.
1.4
Structure of Volume 3
Volume 3 is divided into a number of key sections. An overview of these sections is provided below:
Section 2 – Data Requirements. This section identifies some of the key input data sources for water engineering projects. The focus of this section is on identifying data sources, providing an overview of these sources and key limitations and constraints, and providing guidance on scoping for collection of these data sources, where required.
Section 3 – Hydrology. This section provides an overview of current hydrological techniques that are typically applied in the Philippines, and the use
of these methods. This chapter is intended on being a key reference chapter for flood control, drainage, highway drainage and bridge design.
Section 4 – Hydraulic Analysis. This section provides an overview of basic hydraulic principles, as well as background on different river processes. As with the hydrological chapter, this chapter is intended on being a key reference chapter for flood control, drainage, highway drainage and bridge design. This section also provides general guidance on likely impacts of various geohazards on hydraulics.
Section 5 – Flood Control & Regulating Structures. This section provides guidance on flood control and regulating structures, including:
- Dikes/ Levees
- Spur Dikes
- Revetments
- Small Dams
- Groundsills
- Sluiceway and Conduits for Dikes/ Embankments
Section 6 – Drainage. This section provides guidance on the design of drainage infrastructure, including:
- Open Drains and Channels
- Pipe Networks, Inlet Manholes and Manholes
- Culverts
- Detention Basins
- Overland Flowpaths
- Pumping Stations
Section 7 – Coastal Structures. This section provides general guidance focusing on shoreline protection. It provides general guidance on revetment design.
Section 8 – Water Supply. This section provides the general guidance in the design of water supply system particularly in small water system or rural development.
Section 9 - Climate Change. This section provides a general overview of considerations for climate change when undertaking Water Projects.
Annex A – Estimating Scour. This section provides a description of stream stability and scouring mechanism.
Annex B – Sediment Transport Concepts. This section provides a brief introduction to key sediment transport concepts.
1.5
References
Flood Control & Sabo Engineering Center, June 2010, Technical Standards and
Guidelines for Planning of Flood Control Structures, Japan International Cooperation
2
Data Requirements
2.1
Survey
All survey should be collected based on the methods and requirements identified in Volume 2.
2.2
Scoping Survey
In defining the scope of the survey that is required, it is important to understand the requirements of the design that is being undertaken and define the area, detail and accuracy of the survey appropriately.
Key considerations in scoping of survey for water engineering projects include:
The survey collected should be sufficient to undertake the analysis, while also being sufficient to design any specific infrastructure (such as levees, revetments etc.).
Survey should be collected a sufficient distance upstream and downstream so that the hydraulic behavior of the study area can be adequately understood.
It may be appropriate to have a higher level of resolution in the survey within the immediate vicinity of the proposed works, while a lower resolution upstream and downstream of this area.
Consideration for specifying provision of the survey in an electronic format, without the need for drafted plans. This data should be provided as a three dimensions CAD file, which will allow direct interpretation by the designer. This may result in savings in time and cost of preparation of the survey data. Alternative formats, such as GIS, should also be considered where this is appropriate.
It is essential that a clear scope of works is prepared for the surveyors, to ensure that the survey collected meets with the requirements for the project. This scope of works should be prepared by the engineering team who is to undertake the design/ hydraulic analysis etc. This scope of works should include (where applicable):
- Locality of the project site, including key place names, road names and coordinates where available
- Plans or sketches showing the location of the cross sections to be collected, along with locations of topographic information required
- A project briefing document, identifying key requirements (e.g. accuracy, details required etc.)
- It may be appropriate to use photographs and other tools to assist in identifying location of survey details required, where it may not be clear
- The road network alignment and profile along the distribution system and transmission mains (i.e. from water source to distribution system)
- Location of houses, public building, utility facilities, treatment plant, water tanks
- Tidal level measurements
2.3
Other Data
2.3.1 Rainfall Data
Rainfall information is a key input to hydrological analysis. There are two key types of rainfall data relevant to the design process:
Recorded rainfall data – this data is recorded by rainfall gauges, and provides information on historical rainfall that has occurred. Historical rainfall can be used to verify design rainfall information or as an input to a hydrological model in order to calibrate it to a historical flood event. This data is typically available as a depth of rainfall over a specified time period. Rainfall gauges may collect at small time increments (e.g. 5 minutes) through to daily time increments.
The use of multiple recorded rainfall gauges may assist in understanding the path of a particular storm as well as the areal distribution of the rainfall.
Design rainfall information – this information is the predicted 100 year rainfall, 50 year rainfall etc. that is available from PAGASA.
2.3.2 Evaporation Data
Evaporation data may be required for water supply projects (continuous hydrological modelling, reservoir analysis etc). Evaporation data is typically collected alongside rainfall gauges, but will not be available with every rainfall gauge.
Measured evaporation data is also referred to as “pan evaporation” data, based on how the data is collected. For some continuous hydrological modelling software, evapotranspiration is required, and therefore a conversion factor is needed. This conversion factor should be confirmed based on local conditions – an indicative value of 70% may provide a reasonable preliminary estimate.
2.3.3 River or Channel Gauge Data
River or channel gauge data may be available for some flood control and drainage projects. The data may either be recorded water levels, or, where a rating curve is available, observed discharges as well. The following are key considerations:
Ensure that the datum used for the collection of the data matches the datum of the survey. Where this is not the case, a transformation may be required.
Rating curves to determine discharges have inherent inaccuracies, particularly with larger flows. It is best for the designer to understand the limitations and ensure that they are aware of the likely variance in flow estimates.
Changes in river profile over time in the vicinity of the gauge have the potential to impact on the observed water levels or discharge estimates.
2.3.4 Tidal Data
Tidal data can come in two forms:
Observed or measured data. This data is measured in the field and represents historical tidal measurements. Similarly with river gauges, the datum needs to be confirmed and ensure that this is consistent with the survey being used. A transformation may be required where this is not the case.
Predicted data. Predicted data is based on tidal constants that are available for a number of key ports and harbors. Predicted data does not represent “real” observed data, which may be influenced by factors such as storm surge and other weather at the time. It represents the expected tidal levels where these influences do not occur.
Tidal data is available from NAMRIA (National Mapping Resource Information Administration).
2.3.5 Wind Speed Data
Wind speed data is available for a number of locations around the country, such as airports and harbors. The information may include gust speed, average wind speed and direction over specified increments in time that are dependent on the measurement.
Most wind speed measurements are measured a certain height above the ground, and therefore a conversion factor may be required for some coastal modelling software, but this should be confirmed with the software manual.
It is also important to take into consideration the locality of the wind speed measurement in respect to the study area. For example, a gauge at an airport near the coast (with low vegetation) is unlikely to be representative of a lake 20km inland and surrounded by forest.
Wind data is available from PAGASA.
2.3.6 Land-use Mapping
Land-use mapping data can be used to:
Define catchment characteristics, both for existing land-uses and potential future land-uses;
Define roughness characteristics for hydraulic analysis.
2.3.7 Aerial Photos
Aerial photos provide useful information on catchment and floodplain characteristics. They may not be available in all study areas. They can be available in a range of scales and resolutions.
If aerial surveys such as LiDAR or photogrammetry are being collected, then it is usually possible to acquire aerial photos at the same time.
Some aerial photos are geo-referenced, which means that they can be uploaded into a GIS or CAD based software in the correct coordinated location.
It is important to understand the date that the aerial photography was taken, as changes may have occurred in the catchment since that time.
2.3.8 Soils Investigation
The stability and performance of a structure such as weir, gate, coastal revetment or dam, etc. founded on soil depend on the subsoil conditions, ground surface features, type of construction, and sometimes the meteorological changes. Subsoil conditions can be explored by drilling and sampling, seismic surveying, excavation of test pits, and by the study of existing data. These techniques are outlined in detail in Volume 2C.
The data required for soil investigation for structures is equivalent to the investigations outlined in Volume 4. It is recommended that data required for this volume is identified for areas where structures are proposed.
2.3.9 Riverbed Material
The riverbed material is important to understand the river characteristics, potential for scour, potential flood control options etc. Key information required is the grain size distribution and classification of the soil. This information is required for almost all flood control based projects, and drainage projects undertaken in combination with natural channels.
Information on the riverbed should be collected at representative locations Techniques to collect this information include:
Sieve analysis (typically for grain sizes less than 100 mm). This method is outlined in Volume 2.
On-Site measurements (for coarse bed streams and rivers, with grain sizes greater than 10 mm). These methods are outlined in Volume 2.
One-dimensional sampling method (for grain size greater than 200 mm)
Two-dimensional sampling method (for grain size less than 200 mm)
Samples should be collected based on an inspection of the river, and identification of any significant changes in riverbed material. However, as a guide, it is recommended that riverbed material be collected:
Master plan – collection of riverbed material information at minimum spacing as follows:
Minimum of 1 site to be sampled every 2 km, to be taken at same location of surveyed cross section.
Concept Development/ Design Development/ Detailed Design – one site every 200 to 500 m, depending on the riverbed characteristics. Samples should be taken at representative locations of riverbed material for that portion of the river. Samples should be taken at the same location as a surveyed cross section. Note that at each site for the riverbed sample, a sample should be taken at the centre (where access is possible), left and right banks.
Following a flood, finer sediments may be deposited on the riverbed surface. In order to obtain a representative sample, it may be necessary to extract the sample following removal of the surface material.
2.3.9.1 One-Dimensional Sampling Method
This on-site testing procedure is outlined below:
Find a representative sampling spot in the river where a sample of riverbed material is exposed and is representative of the study area or design area.
Within the sampling spot, find the biggest riverbed material and approximately determine its size. Measure 20 evenly spaced sampling point on the ground using a steel tape with interval the same as that of the biggest riverbed material. If the maximum riverbed diameter is 50 cm, then the sampling interval should also be 50 cm as shown in Figure 2-1.
Figure 2-1 One-Dimensional Sampling Method
Source: FCSEC, 2010
Pick the stones beneath the sampling interval point and arrange it in a straight line (Figure 2-2), from smallest to biggest. Select the stone size from the 12th smallest interval from the arrangement. This is the equivalent 60% of the riverbed material samples and the corresponding representative riverbed diameter (dr).
Figure 2-2 Representative Grain Size Sample
Source: FCSEC, 2010
This sample can be measured using a ruler, and dr can be computed using the formula:
Equation 2-1
𝑑𝑑
𝑟𝑟= (𝑋𝑋
1𝑌𝑌
1𝑍𝑍
1)
1⁄3An overview of the parameters is provided in Figure 2-3.
Figure 2-3 Measurement of Sample
Source: FCSEC, 2010
Using these diameters, percent finer (P(di)) corresponding to di (‘i’ is the smallest
diameter stone) can be obtained as follows: Equation 2-2
𝑃𝑃(𝑑𝑑
𝑖𝑖) =
𝑑𝑑
1 3+ 𝑑𝑑
2 3+ ⋯ + 𝑑𝑑
𝑖𝑖3𝑑𝑑
13+ 𝑑𝑑
23+ ⋯ + 𝑑𝑑
203. 100
where:
d1, d2, di ... = stone diameter
2.3.9.2 Two-Dimensional Sampling Method
An improvised screen (Figure 2-4) with equally spaced string on a 1 m square wooden frame is used for sampling.
Find the best sampling spot in the river where representative sample of riverbed material is exposed.
Get a sample riverbed material and approximately determine its size.
Within the sampling spot, find the biggest riverbed material and approximately determine its size. When maximum riverbed diameter D < 10 cm, use a 1.0 m x 1.0 m improvised screen with openings evenly spaced at 10 cm both ways. When maximum riverbed diameter D > 10 cm, use a 1.0 m x 1.0 m improvised screen with openings uniformly spaced at 20 cm both ways at the middle. Note that the any reasonable sized string may be used, as the string size is not important, provided it is strong enough to be strung tightly across the frame. Figure 2-4 Improvised Screen for Two-Dimensional Sampling Method
Source: FCSEC, 2010
Lay the improvised screen on the exposed ground making sure that representative riverbed materials are contained within the 1 m2 area (Figure 2-5).
Figure 2-5 Two-Dimensional Sampling Method
Source: FCSEC, 2010
Pick gravels just beneath of each intersection of strings of the improvised screen and arrange it in a straight line (Figure 2-6), from smallest to biggest. Select the 60% smallest sample from the arrangement. Say, the 15th sample in the 20 cm spacing strings (within 5 x 5 = 25 samples) or the 60th sample in 10 cm spacing strings (within 10 x 10 = 100 samples).
Figure 2-6 Representative Grain of Sample
Source: FCSEC, 2010
Measure the dimensions of the selected grain and calculate the representative grain diameter of the site. Calculation procedure is same as the One-Dimensional Sampling Method.
2.3.9.3 Coastal Bed Material
Coastal bed material is an important consideration in the design of coastal protection measures. The techniques, as identified in Section 2.3.8 and 2.3.9 can also be adopted to describe the characteristics of the bed material.
2.3.10 Specific Data for Water Supply Projects
The first step in designing a water system is to determine how much water is needed by the population to be covered. The water to be supplied should be sufficient to cover both the existing and future consumers. It must include provisions for domestic and other types of service connections. In addition to the projected consumptions, an allowance for non-revenue water (NRW) that may be caused by leakages and other losses should be included.
Water consumptions served by water utilities are commonly classified into domestic use, commercial use, Institutional use, or Industrial Use. In rural areas, water consumption is generally limited to domestic uses, i.e., drinking, cooking, cleaning, washing and bathing. Domestic consumption is further classified as either Level II consumption (public faucets) or Level III consumption (house connections).
2.3.10.1 Unit Consumptions
Unit consumption for domestic water demand is expressed in per capita consumption per day. The commonly used unit is liters per capita per day (lpcd). If no definitive data are available, the unit consumption assumptions recommended for Level II and Level III domestic usages in rural areas are as follows:
Level II Public Faucets: 50 - 60 lpcd (Each public faucet should serve 4 - 6 households)
Level III House Connections: 80 - 100 lpcd
If there are public schools and health centers in the area, they will be supplied from the start of systems operation and be classified as institutional connections. Commercial establishments can also be assumed to be served, after consultation with the stakeholders, within the design year. The unit consumptions of institutional and commercial connections are, in terms of daily consumption per connection, usually expressed in cubic meters per day (m3/d). Unless specific
information is available on the consumptions of these types of connections, the following unit consumptions for commercial and institutional connections can be used.
Institutional Connections: 1.0 m3/d
Commercial Connections: 0.8 m3/d
The total consumption is the sum of the domestic, institutional and commercial consumptions expressed in m3/d.
2.3.10.2 Design Population
The design population is the targeted number of people that the project will serve. The projection of served population and water demand is based on the assumption of design period (say 5 or 10 year) and the design year (or base year).
There are two ways of projecting the design population.
1. Estimate the population that can be served by the sources. In this case, the supply becomes the limiting factor in the service level, unless a good abundant and proximate source is available in the locality.
2. Project the community or barangay population, and determine the potential service area and the served population.
Population growth to be assumed will need to be determined in consultation with relevant government bodies. The latter projection method is most commonly adopted. First, the annual municipal and barangay growth rates are determined from previous population census as expressed in the following equation:
Equation 2-3 P2007 = P2000 (1 + GR)n or 1 P2007 n GR = - 1 P2000 where: P2007 = population in 2007 P2000 = population in 2000
GR = annual growth rate (multiply by 100 to get percent growth rate) n = number of years between the two census, in this case n = 7
The projected population is then estimated with the same basic population equation on a year to year projection starting from initial year population. After determining the projected population, the next step is to determine the actual population to be served. The primary factors in assessing the served population are socio-economic conditions of potential service area, level of acceptance of residents for proposed water system, availability of and abundance/scarcity of alternative water sources and potential development program in the municipality. Detailed discussion can be found in the Rural Water Supply Design Manual (WPP, 2012).
2.3.10.3 Water Quality
Water quality of the source water for supply is an important consideration in water supply projects. Water quality sampling should be undertaken by appropriate qualified personnel, and tested in appropriately certified laboratories.
The chemical, physical and microbiological water quality parameters should be tested as required by the end use. The physical aspects include water turbidity, color, taste, and odor. The chemical aspects are the hardness, alkalinity and acidity, carbon dioxide, dissolved oxygen, chemical oxygen demand, organic nitrogen, iron and manganese, toxic substances and phenolic compounds in the water sample. Microbial water testing should be for protozoa, helminths, and bacteria.
The Philippines National Standards for Drinking Water 2007 (PNSDW-2007) provide the minimum standards for quality of potable water. Per PNSDW, drinking water must be clear, colorless and free from objectionable taste and odor. All other standard values are contained in the PNSDW Administrative Order No. 2007-0012 or any other standards more recently issued by the Department of Health.
2.4
References
Flood Control & Sabo Engineering Center, June 2010, Technical Standards and
Guidelines for Planning of Flood Control Structures, Japan International
Cooperation Agency, Philippines.
Republic of the Philippines Department of Health (2007) [PNSDW]. Philippines
National Standards for Drinking Water 2007, Administration Order 2007-0012.
WPP (Water Partnership Program), 2012, Rural Water Supply Volume 1, Design
3
Hydrology
3.1
Introduction
This section of Volume 3 provides a broad outline of hydrological techniques. It outlines the following steps in the hydrological analysis process:
Catchment delineation
Design rainfall analysis
Choice and use of hydrological analysis techniques
3.2
Catchment Delineation
One of the basic data required in undertaking hydrological analysis is the catchment area.
The catchment area (Figure 3-1) is derived by delineating the basin boundary in a topographic map. Topographic maps may include:
1:50,000 or better mapping from National Mapping and Resource Information Administration (NAMRIA)
Topographical survey, which may assist particularly for smaller portions of the catchment and for drainage projects
Aerial survey, such as LiDAR or photogrammetry
Urban drainage layout, which provides an indication of the runoff characteristics
A discussion on these different data sources is provided in Section 2. The catchment area is then computed using the following:
Planimeter – subjected to regular calibration/maintenance to attain accurate result/reading
Triangulation
Cross-section millimeter paper, and
CAD / GIS software
CAD and GIS software are likely to be the most common method for delineating catchments in the coming years.
In addition to overall catchment delineation, further sub-catchment delineation is typically undertaken to:
Provide flow estimates at different points in a study area
Align with key inflow points to a hydraulic model
In drainage studies, to estimate the flows arriving at drainage inlets or culverts. Note that this might change with different drainage layout alternatives that might be considered
Provide sufficient resolution for models other than the Rational Method (Section 3.4.1).
The level of detail that the catchment is delineated into sub-catchments is highly dependent on the particular project and study area. For large river basins, sub-catchments may be in the order of 100 km2 to 200 km2, while for drainage studies
catchments could be less than 1ha.
More details on the procedure for delineation of catchment areas is provided in FCSEC (2010).
Figure 3-1 Typical Catchment Configuration
Source: FCSEC, 2010
3.3
Rainfall Analysis
Rainfall analysis includes the formation of design hyetographs for hydrological analysis, as well as the analysis of recorded rainfall data.
3.3.1 Methods to Establish Design Hyetograph
The characteristic of rainfall is expressed by three factors.
Amount of rainfall (or rainfall intensity)
Temporal distribution of rainfall
3.3.1.1 Design Rainfall Intensity
The Rainfall Intensity-duration-Frequency (RIDF) data prepared by the Hydrometeorological Investigation and Special Studies Section of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) is the technical data used in determining the intensity of rainfall in a particular place. The data is plotted to show values at various return periods.
RIDF is separated into short duration (10 min to 1 hour) and long duration (1 hour to 1 day). Various durations may need to be analyzed depending on the project and the application. For example, longer durations may be more important for storage based analysis.
PAGASA operates/maintains 52 Synoptic stations equipped with automatic rainfall gauges. Updates of runoff analysis may be secured from PAGASA.
When a station cannot be located or there is no station, the RIDF can be estimated from the specific coefficient shown in Iso-Specific Coefficient and the probable daily rainfall value shown in lsohyet of Probable 1-Day Rainfall.
In the unusual situation of more than one rainfall station in a catchment, the catchment average rainfall can be determined in accordance with the methods described in Section 3.3.4.
3.3.1.2 Establishing a Temporal Pattern
With the exception of the Rational Formula, the majority of hydrological analysis requires the establishment of a temporal pattern. In the absence of other information, the Alternating Block Method is an appropriate approach to determining the temporal pattern. This methodology is described in detail in FCSEC (2010) as well as many hydrological textbooks.
Note that an alternative approach is the analysis of recorded temporal patterns from a synoptic gauge within the catchment. This would involve the selection of a representative rainfall pattern based on analysis of historical rainfall events for large floods. This approach is suitable for larger river basins, and is described in Section 5.3.3 of FCSEC (2010).
3.3.1.3 Area Reduction Factor
Intense rainfall is unlikely to be distributed uniformly over a large river basin. The basin mean rainfall for specified frequency and duration is less than point rainfall. To account for this, Technical Standards and Guidelines for Planning of Flood
Control Structures (FCSEC,2010) recommends the use of Horton's formula to
convert point rainfall to basin mean rainfall. This is presented in Equation 3-1. Equation 3-1
𝐼𝐼 = 𝐼𝐼
𝑜𝑜𝑒𝑒
[−0.1(0.386𝐴𝐴)0.31]where:
I = basin mean rainfall (mm) Io = point rainfall (mm)
A = catchment area (km2)
Fa = I/Io area reduction factor
3.3.2 Effective Rainfall
The next step after determination of design hyetograph is to estimate the effective rainfall. The effective rainfall or excess rainfall is neither retained on the land surface nor infiltrated into the soil but becomes direct runoff to the outlet of the river basin. A lot of methods have been proposed to estimate effective rainfall; however, when data are available effective rainfall can be established by the relationship between rainfall and runoff.
3.3.3 Analysis of Recorded Rainfall
Recorded rainfall data can be an important tool in hydrological analysis. It can be used:
For validation of RIDF values, where the recording period is sufficiently long. Intensities from the recorded data series and the associated return period can be determined and compared with the RIDF values. This is particularly useful where there is no synoptic gauge within the catchment.
For use in calibration and verification of hydrological and hydraulic analysis. Recorded rainfall can be applied to the analysis, and compared with recorded flow data or water level data. This allows for calibration and verification of parameters adopted for the analysis.
For general information on the likely return period of a recorded flood event.
For comparison of the assumed temporal pattern from the design hyetograph with actual recorded temporal patterns.
An analysis technique for the determination of return period for recorded rainfall is presented in Section 5.3.2 and 5.3.3 of Technical Standards and Guidelines for
Planning of Flood Control Structures (FCSEC, 2010).
3.3.4 Average Rainfall in Catchment Area
There are three methods of determining the catchment area average rainfall, as described below. These are generally applied to the analysis of recorded rainfall data. However, where multiple RIDF gauges exist within or near a catchment, the Arithmetic Mean and Thiessen Method can also be adopted.
Detailed examples for undertaking these methods are presented in the Technical
Standards and Guidelines for Planning of Flood Control Structures (FCSEC, 2010).
3.3.4.1 Arithmetic-Mean Method
This is the simplest method by averaging the rainfall depths recorded at a number of gages. This method is satisfactory if the precipitation is almost uniformly distributed within the catchment area.
3.3.4.2 Thiessen Method
This method assumes that at any point in the catchment area, the rainfall is the same to the nearest rainfall gauge. The value recorded at a given rainfall gauge can be applied halfway of the next station in any direction.
The relative weights for each gauge are determined from the corresponding areas of application in a Thiessen polygon network, the boundaries of the polygons formed by the perpendicular bisectors of the lines connected to the adjacent gauges.
3.3.4.3 lsohyetal Method
This method takes into account the orographic influences (mountains, terrain, etc.) on rainfall by constructing isohyets, using observed depths at rain gauges and interpolation between adjacent rain gauges.
Once the isohyetal map is constructed, the area A, between isohyets, within the catchment, is measured and multiplied by the average rainfall depths P1 of the two adjacent isohyets to compute the average rainfall.
Information of the storm patterns can result in more accurate isohyets; however, a fairly dense network of rain gauges is needed to accurately construct the isohyetal map from a complex storm.
3.4
Runoff Analysis
There are many methods for runoff analysis. This Volume introduces the following:
Rational Formula
Unit Hydrograph Method
Storage Function Method
Flood Frequency Analysis
Specific Discharge Method
This is not an exhaustive list, and does not mean that other methods cannot be adopted where appropriate.
3.4.1 Rational Formula
The Rational Formula Method is one of the most commonly used for estimating flood peak discharge for small watersheds. It is widely applied in rivers where storage phenomena are not required, where the catchment is treated as rectangular, symmetrical about the river course and where the rainwater flows down the river course at a constant speed.
3.4.1.1 Basic Equation
The principle behind the Rational Formula Method is that a rainfall intensity (I) begins and continues indefinitely and then the rate of runoff increases until it reaches the time of concentration (tc), where all of the watersheds are contributing
)
to the flow at the outlet point or point under consideration. The Rational Formula is provided in Equation 3-2.
The Rational Formula is applicable to a rural or forested catchment area smaller than 20 km2.
For urban catchments, caution should be applied in the application of the Rational Formula for catchments greater than 5 km2. In urban catchments, the impacts of
local obstructions, hydraulic controls and localized storages can result in significant impacts on the peak flow estimate.
Equation 3-2
𝑄𝑄
𝑃𝑃=
𝑐𝑐𝑐𝑐𝑐𝑐
3.6
where:
QP = maximum flood discharge (m3/s)
c = dimensionless runoff coefficient
I = rainfall intensity within the time of flood concentration (mm/hr) A = catchment area (km2)
The key assumptions associated with the Rational Formula Method are:
The computed peak rate of runoff at the outlet point is a function of the average rainfall rate during the time of concentration, i.e., the peak discharge does not result from a more intense storm of shorter duration, during which only a portion of the watershed is contributing to the runoff at the outlet.
The time of concentration is the time for the runoff to become established and flow from the most remote part of the drainage area to the outlet point.
Rainfall intensity is constant throughout the rainfall duration.
A general overview of the decision of what runoff analysis to adopt is outlined Figure 3-2.
Figure 3-2 Overview of Rational Formula Applicability* Delineate Catchment Area Delineate Sub-Catchments Rational Formula Appropriate Is Catchment Area < 20km2? Is the catchment urban or rural Is Catchment Area < 5km2? urban Rural Are storage issues important? Y Y Other Hydrological Analysis Method required Other Hydrological Analysis Method required N N Y Y N
*Other hydrological analysis method may include the Unit Hydrograph or other computer based methods
3.4.1.2 Runoff Coefficient (c)
The runoff coefficient (c) is the least precise variable of the Rational Formula implying a fixed ratio of peak runoff rate to rainfall rate for the catchment area, which in reality is not the case. Proper selection of the runoff coefficient requires judgment and experience on the part of the hydrologist/engineer. The proportion of the total rainfall that will reach the river and/or storm drains depends on the percent imperviousness, the slope and the ponding characteristics of the surface. Impervious surface, such as asphalt pavements and roofs of buildings, will produce nearly 100% runoff after the surface has become thoroughly wet, regardless of the slope.
Some general guidance on potential ‘c’ values to adopt is provided in Table 3-1. Field inspection, aerial photographs, and present land use maps are useful in estimating the nature of the surface within the target basin. The runoff coefficient will increase with urbanization due to increased impervious surface and installation of drainage system. In a large-scale development, the projected runoff