Stormwater and Surface Drainage

Introduction

Stormwater is precipitation that flows over roofs, paved areas, soil and other surfaces when rainfall exceeds infiltration and temporary surface storage. In urban areas, construction replaces permeable ground with impervious surfaces, increasing runoff volume, peak discharge and speed. If this water is not safely collected and conveyed, it can cause flooding, erosion, traffic disruption, structural damage, groundwater contamination and public-health hazards.

For a civil draftsman, stormwater work involves more than drawing pipes. It requires interpreting site levels, estimating runoff, selecting suitable drainage components, preparing clear layouts and coordinating hydraulic requirements with roads, buildings, utilities and environmental objectives. The draftsman normally works from rainfall data and engineering design criteria supplied by a civil or drainage engineer, but must understand the underlying principles sufficiently to prepare accurate and constructible drawings.

This chapter presents basic methods for estimating runoff and introduces the layout and detailing of roof drainage, open drains, culverts, catch basins, detention systems and sustainable urban drainage systems (SuDS). Local authority standards, rainfall-frequency data, flood regulations and construction specifications must always take precedence over generic values in this chapter.

Learning objectives

After studying this chapter, the learner should be able to:

1. Explain the hydrologic movement of rainfall from a catchment to an outlet.
2. Estimate peak runoff using the Rational Method and estimate runoff volume using a simple rainfall–loss approach.
3. Apply Manning’s equation to basic open-channel and pipe-flow checks.
4. Prepare preliminary layouts for roof drainage, surface drains, catch basins and culverts.
5. Explain the purpose and basic sizing logic of detention and infiltration systems.
6. Integrate sustainable drainage measures into site plans.
7. Identify drafting, safety, maintenance, environmental and regulatory requirements.

1. The drainage system as a catchment-to-outfall network

A drainage system is a connected sequence of surfaces, inlets, conveyance elements, storage facilities and receiving waters. The principal parts are:

• Source areas: roofs, roads, parking areas, yards and landscaped ground.
• Collection elements: gutters, downpipes, grated inlets, catch basins and channels.
• Conveyance elements: pipes, open drains, culverts, kerbs and swales.
• Storage or treatment elements: detention basins, ponds, tanks, wetlands, infiltration trenches and bioretention areas.
• Outfall: a controlled discharge point to a watercourse, sewer, lake, soakaway or other approved receiving system.

The hydrologic sequence is rainfall → interception and depression storage → infiltration and evaporation → surface runoff → collection → conveyance or storage → discharge. The balance between these processes changes with rainfall intensity, duration, soil, slope, vegetation, antecedent wetness and development density.

Important terminology includes:

• Catchment: land contributing runoff to a specified outlet.
• Time of concentration, tᶜ: the travel time from the hydraulically most distant point of a catchment to the outlet.
• Peak discharge: the maximum flow rate during a storm.
• Runoff coefficient, C: a dimensionless representation of the fraction of rainfall contributing promptly to runoff in the Rational Method.
• Return period: the average statistical interval associated with a selected rainfall or flood magnitude; it is not a guarantee that events occur regularly.
• Freeboard: vertical distance between the design water level and the top of a bank, kerb or structure.
• Minor system: ordinary drainage infrastructure for frequent design storms.
• Major system: roads, overland routes, storage areas and flood pathways that operate when the minor system is exceeded.

A sound plan should show both systems. Blocking a gully should not force water into a building entrance if safe overland flow routes have been considered.

2. Rainfall and runoff estimation

2.1 Design rainfall

Drainage design commonly uses an intensity–duration–frequency (IDF) relationship or a design-storm hyetograph. Rainfall intensity must correspond approximately to the catchment’s time of concentration and selected return period. The designer should obtain values from the responsible meteorological agency or authority.

A rainfall intensity is commonly expressed in millimetres per hour (mm/h). Rainfall depth is commonly expressed in millimetres (mm). Intensity and depth are not interchangeable unless duration is specified:

where P is rainfall depth, i is average rainfall intensity, and t is duration, with consistent time units. This relationship describes average depth over the duration; actual storms may have variable intensity.

2.2 The Rational Method

For small, relatively homogeneous catchments, the Rational Method estimates peak runoff as:

where:

• Q = peak discharge in m³/s;
• C = dimensionless runoff coefficient;
• i = design rainfall intensity in mm/h;
• A = catchment area in hectares (ha);
• 0.00278 = unit-conversion factor.

The method assumes that rainfall intensity is reasonably uniform over the catchment, the selected intensity corresponds to approximately tᶜ, and the catchment response is sufficiently simple for a single coefficient. It is most suitable for preliminary design and small urban catchments. It should not be used uncritically for large, highly varied or strongly storage-controlled basins.

Typical indicative coefficients are shown below. Local standards should be used in preference to these ranges.

For a mixed catchment, use an area-weighted coefficient:

Here Cⱼ and Aⱼ are the coefficient and area of surface j. The result is dimensionless and should be checked against the actual land cover and drainage connectivity.

#### Worked example 1: peak runoff

A site contains 0.40 ha of roof with C = 0.90, 0.25 ha of asphalt with C = 0.85 and 0.35 ha of landscaped area with C = 0.30. The design rainfall intensity for the selected duration and return period is 75 mm/h.

The weighted coefficient is:

Cʷ = [(0.90 × 0.40) + (0.85 × 0.25) + (0.30 × 0.35)] ÷ 1.00

Cʷ = (0.360 + 0.2125 + 0.105) ÷ 1.00 = 0.6775 ≈ 0.68

Therefore:

Q = 0.00278 × 0.6775 × 75 × 1.00 = 0.141 m³/s

The preliminary peak runoff is approximately 0.141 m³/s, or 141 L/s. The calculation does not determine pipe size by itself; hydraulic grade, inlet capacity, blockage allowance, permissible velocity and downstream conditions must also be checked.

2.3 Runoff volume

For a simple event estimate, runoff volume may be approximated by:

when P is in metres and A is in square metres. V is then in cubic metres. This is a simplified event-volume relationship. It does not represent infiltration variation, initial losses, depression storage or changing runoff coefficients during a storm.

A more explicit loss approximation is:

where Vᵣ is runoff volume, P is rainfall depth, L is an adopted effective loss depth, and P > L. If P ≤ L, the calculated direct runoff is taken as zero in this simplified model. The value of L must be justified by soil, land cover and local guidance.

2.4 Time of concentration

Time of concentration is important because a short, intense rainfall may not produce simultaneous contribution from every point in a large catchment. It is normally estimated by adding travel times through overland flow, gutters, channels and pipes:

Each travel time is distance divided by representative flow velocity. This is a conceptual relationship; the velocity is not known until the drainage arrangement and hydraulic conditions are assessed. The shortest credible tᶜ should not be selected merely because it produces a larger intensity. Use the method prescribed by the relevant authority.

3. Hydraulic principles for surface drainage

3.1 Continuity

The continuity principle expresses conservation of flow. For steady incompressible flow:

where Q is discharge in m³/s, Aᶠ is flow cross-sectional area in m² and v is mean velocity in m/s. For storage systems, the more useful form is:

where S is stored volume, I is inflow and O is outflow, all in m³ or m³/s as appropriate. A detention basin fills when inflow exceeds outflow and empties when outflow exceeds inflow.

3.2 Manning’s equation

For uniform or approximately uniform open-channel flow, Manning’s equation is:

where n is Manning’s roughness coefficient, Rₕ is hydraulic radius in metres, and S₀ is the energy or bed slope, normally approximated by channel slope for uniform flow. Hydraulic radius is:

where Pʷ is wetted perimeter in metres. The equation assumes steady flow, a prismatic or reasonably regular section, fully rough turbulent conditions represented by n, and a suitable slope. It is not a substitute for checking transitions, inlet control, outlet control, backwater, debris or surcharging.

For a rectangular channel of width b and flow depth y:

#### Worked example 2: trapezoidal open drain

A trapezoidal drain has bottom width b = 0.60 m, flow depth y = 0.40 m, side slope 2 horizontal to 1 vertical, bed slope S₀ = 0.005 and n = 0.030. The horizontal projection of each side is 2y = 0.80 m, and each sloping side length is √(0.40² + 0.80²) = 0.894 m.

Top width = 0.60 + 2(0.80) = 2.20 m.

Aᶠ = [(0.60 + 2.20) ÷ 2] × 0.40 = 0.56 m²

Pʷ = 0.60 + 2(0.894) = 2.388 m

Rₕ = 0.56 ÷ 2.388 = 0.234 m

Q = (1 ÷ 0.030) × 0.56 × 0.234^(2/3) × 0.005^(1/2)

Q ≈ 0.381 m³/s.

The mean velocity is Q ÷ Aᶠ = 0.381 ÷ 0.56 ≈ 0.68 m/s. This result is only a preliminary capacity estimate. Lining, permissible velocity, sediment deposition, freeboard and maintenance access must be reviewed.

3.3 Pipes and hydraulic grade

A pipe may flow partially full as an open channel or full under pressure. Layout drawings should show invert levels, gradients, pipe diameter, material, manholes and connection details. Avoid unnecessary sharp bends and provide access at changes in direction, grade, diameter and junctions. A pipe that has adequate theoretical capacity may still fail because its outlet is submerged, its inlet is blocked or its hydraulic grade rises above ground level.

4. Roof drainage

Roof drainage begins with dividing the roof into drainage zones. Each zone should have a defined fall toward a gutter, internal outlet or valley. The layout should identify roof areas, arrows showing falls, gutters, downpipes, overflow paths and discharge points.

For preliminary roof design, the Rational Method may be applied to each roof catchment. Since roofs are highly impervious, C is usually high. Downpipes should be distributed to avoid excessive gutter lengths and should discharge to a safe surface drain, storm sewer, rainwater tank, soakaway or approved outlet. Water should not be discharged against foundations or across pedestrian routes.

Important drafting details include:

• gutter direction and minimum practical fall;
• downpipe diameter and spacing as specified by local plumbing and building codes;
• leaf guards where appropriate;
• overflow scuppers or secondary drainage for enclosed roofs;
• cleanouts and access points;
• expansion and movement provisions;
• connection to stormwater infrastructure without cross-connection to sanitary sewage;
• splash blocks, erosion protection or trapped connections where required.

Internal roof drains require especially careful coordination because blockage can cause ponding and structural overloading. Finished floor levels, parapets, thresholds and emergency overflow levels should be shown clearly.

5. Open drains, kerb-and-gutter systems and swales

Open drains may be earth channels, grassed swales, lined concrete channels, masonry drains or engineered roadside channels. Selection depends on discharge, available width, slope, soil erodibility, maintenance capacity, pedestrian safety and appearance.

A drainage section should show bed width, side slopes, design water depth, freeboard, lining thickness, longitudinal slope and adjacent ground levels. Side slopes must be stable and safe. Deep or fast-flowing channels may require fencing, barriers or restricted access.

A swale is a shallow vegetated channel that conveys, slows and sometimes infiltrates runoff. Its advantages include low-energy flow, sediment capture and landscape integration. Its limitations include land requirement, vegetation management, low performance in saturated or poorly drained soils and possible erosion at concentrated inflows.

Kerb-and-gutter systems should be graded so that water reaches inlets without ponding at driveways or crossings. Inlets should be placed at sag points, before intersections, at pedestrian crossings where appropriate, and at regular intervals based on hydraulic calculations. A sag inlet should have an overflow route because blockage or excessive storm flow can exceed its capture capacity.

6. Catch basins and inlets

A catch basin is a chamber with an inlet grate or opening, usually connected to a stormwater pipe. Some include a sump intended to trap sediment; others are simple junction chambers. A sump is not a substitute for maintenance. Accumulated sediment reduces capacity and may be washed downstream.

A catch-basin layout should show:

• grate type and dimensions;
• chamber internal size and depth;
• inlet and outlet pipe sizes and invert levels;
• sump depth where specified;
• benching or channel formation through the chamber;
• access cover and safe load rating;
• surface levels and flow arrows;
• connection to downstream structures.

Inlets should be located where they intercept flow while avoiding conflict with wheel paths, accessible routes and utilities. Grates should be selected to reduce blockage and hazards to pedestrians and cyclists. In flood-prone areas, dual drainage and emergency overland flow should be documented.

7. Culverts

A culvert conveys water beneath a road, railway, driveway or embankment. It consists of an inlet, barrel, outlet and protection works. Design conditions may be controlled by inlet geometry, barrel friction, outlet tailwater or downstream flooding. A draftsman must not assume that a larger barrel alone solves a culvert problem.

A culvert plan and profile should show alignment, diameter or box dimensions, material, slope, inlet and outlet elevations, headwalls, wing walls, aprons, scour protection, cover depth, embankment and maintenance access. The inlet and outlet should be aligned with the natural channel where practical. Sudden changes in direction or elevation can create erosion and hydraulic losses.

Safety and environmental provisions may include trash racks, debris control, fish passage, sediment management and fencing. Trash racks can themselves become dangerous blockage points, so they require safe access and a maintenance plan. The design must comply with road authority and watercourse requirements.

8. Detention and retention systems

A detention system temporarily stores stormwater and releases it at a controlled rate. A retention or wet system holds water for longer periods, sometimes permanently. The essential design relationship is the inflow–outflow balance:

where ΔS is change in storage volume and I and O are inflow and outflow rates. In a simplified level-pool routing calculation over a time interval Δt:

This approximation is useful for preliminary routing when inflow and outflow vary approximately linearly during the interval. More detailed designs use approved hydrologic and hydraulic software and must check emergency overflow, embankment stability, freeboard, water quality, mosquito control, safety and maintenance.

A detention basin normally includes a forebay or sediment zone, main storage, outlet control, emergency spillway, access track, side slopes and maintenance area. The outlet may be an orifice, weir, perforated riser or controlled pipe. Small outlets are vulnerable to blockage; screens and accessible maintenance arrangements are essential.

Illustrative detention example

Suppose a preliminary analysis gives a required temporary storage volume of 420 m³. A basin is proposed with an average plan area of 700 m² and an average effective water depth of 0.60 m. The approximate storage is:

V = plan area × average depth = 700 × 0.60 = 420 m³.

This is only a geometric check. Actual basin storage must be calculated from surveyed contours or level–area–volume data. Sloping sides mean that plan area changes with depth, and freeboard cannot be counted as active storage unless permitted by the design criteria.

9. Sustainable urban drainage

Sustainable urban drainage seeks to manage runoff close to where rain falls, imitate natural hydrology and improve water quality. Typical measures include:

• rain gardens and bioretention cells;
• infiltration trenches and soakaways;
• permeable pavement;
• green roofs;
• vegetated swales;
• rainwater tanks;
• constructed wetlands;
• detention basins and constructed ponds.

The treatment train principle combines measures in sequence: source control → local infiltration or storage → conveyance treatment → detention → controlled discharge. No single device is universally suitable. Infiltration systems require soil permeability testing, groundwater separation, setback from foundations and consideration of contaminated land. In clay soils or high groundwater, lined systems with controlled overflow may be more appropriate.

A SuDS layout should show contributing areas, pretreatment, flow direction, overflow route, underdrain if used, inspection points, filter media, vegetation zones, high-water level and maintenance access. Sustainable drainage is not simply landscaping; it is infrastructure with hydraulic, geotechnical and operational requirements.

10. Applied case: illustrative small institutional site

Background

An illustrative 1.00 ha institutional site is being redeveloped. The site includes a 0.30 ha roof, 0.25 ha paved court and 0.45 ha landscaped area. The existing downstream drain has limited capacity, so the authority requires controlled discharge and safe major-system routing.

Problem

The preliminary design storm has an intensity of 80 mm/h. The existing site produces a high peak flow after redevelopment, and uncontrolled discharge could overload the downstream drain.

Analysis

Using C values of 0.90, 0.85 and 0.30 respectively:

Cʷ = [(0.30 × 0.90) + (0.25 × 0.85) + (0.45 × 0.30)] ÷ 1.00

Cʷ = (0.270 + 0.2125 + 0.135) = 0.6175.

The peak runoff is:

Q = 0.00278 × 0.6175 × 80 × 1.00 = 0.137 m³/s.

The design team proposes roof tanks, a vegetated swale along the parking edge, a catch basin at the low point and a detention basin before the legal outfall. The draftsman prepares a drainage plan, long section and details. Roof areas are divided into four zones, each with a downpipe route. The swale receives sheet flow through level spreaders rather than concentrated uncontrolled outlets. The detention basin has an emergency spillway directed toward a public open-space corridor, away from buildings.

Approach and solution

The plan includes:

1. roof gutters and downpipes connected to tanks and overflow pipes;
2. permeable paving in selected parking bays, subject to subgrade testing;
3. a sediment forebay before the detention basin;
4. a controlled outlet sized by the drainage engineer;
5. catch basins at the road sag and upstream of the pedestrian crossing;
6. invert levels and arrows on every pipe run;
7. a major overland flow path shown with a dashed line;
8. maintenance access and inspection points.

Outcome

The proposal reduces direct runoff, delays the peak and limits discharge to the approved downstream rate. It also provides a visible treatment train and a documented emergency route. Final acceptance remains conditional on detailed rainfall modelling, geotechnical investigation, structural design, authority review and a maintenance schedule.

Learning points

Runoff estimation, grading and hydraulic layout must be developed together. A sustainable feature is successful only when its inlet, overflow, access and maintenance requirements are drawn and specified. The drainage plan must communicate not only normal operation but also blockage and exceedance behavior.

11. Drafting and professional practice

A complete stormwater drawing package commonly includes a location plan, existing and proposed contours, drainage layout, pipe schedule, long sections, typical sections, inlet and chamber details, roof drainage plan, detention or SuDS details, erosion-control notes and construction specifications.

Use consistent symbols and labels. Every pipe should have a direction arrow, diameter, material, gradient and invert levels at relevant points. Use a benchmark and clearly identify datum. Coordinate stormwater lines with water supply, sewerage, electricity, telecommunications, foundations and tree roots. Distinguish existing, proposed and abandoned works using the project’s standard line types.

The draftsman should check:

• whether all low points have a defined drainage route;
• whether the proposed outlet is legally and hydraulically acceptable;
• whether access is available for cleaning and inspection;
• whether slopes and cover are constructible;
• whether the drawing includes tolerances, interfaces and protection works;
• whether overflow paths avoid buildings and unsafe locations;
• whether the design accounts for climate-change allowances required by the authority.

Computer-aided design and digital terrain models can improve grading and quantity calculations. GIS can identify catchments and flood-prone areas. Hydraulic software can route design storms and test alternatives. AI may assist with drawing checks, feature recognition or scenario comparison, but it must not replace verified rainfall data, engineering judgment, authority criteria or professional sign-off. Input assumptions, software versions and review decisions should be recorded.

12. Limitations, safety and governance

Basic methods involve uncertainty. Runoff coefficients are averages, rainfall is spatially variable, surfaces change over time, sediment reduces capacity, vegetation develops and climate conditions may differ from historical records. The Rational Method does not provide a complete flood model, and Manning’s equation does not automatically account for complex junctions or pressurised transients.

Public safety is a design obligation. Open water, deep channels, steep embankments, grates and culvert entrances can be hazardous. Designs should address child safety, drowning risk, traffic loading, slip hazards, accessibility and maintenance worker protection. Stormwater should not be cross-connected with sanitary sewage, and polluted runoff from industrial or contaminated areas may require treatment before discharge.

Regulatory requirements commonly address planning approval, waterway protection, discharge permits, road crossings, environmental impact, erosion and sediment control, accessibility and construction safety. Requirements differ by jurisdiction. The responsible authority and qualified engineer must confirm design storms, allowable discharge, minimum pipe sizes, cover, materials, freeboard and inspection requirements.

Chapter Summary

Stormwater drainage conveys rainfall safely from catchment surfaces to an approved outfall while controlling flooding, erosion, pollution and public risk. The Rational Method provides a basic peak-flow estimate for small catchments: Q = 0.00278 C i A. Weighted coefficients are needed for mixed surfaces, and runoff volume requires separate consideration. Manning’s equation supports preliminary checks of open drains and partially full conduits, while continuity governs conveyance and storage.

Effective layouts divide roof areas, grade surfaces toward inlets, show pipe direction and levels, provide maintainable catch basins, and detail culvert inlets, outlets and scour protection. Detention systems temporarily store water and release it through a controlled outlet; SuDS manage runoff near its source through infiltration, filtration, vegetation and storage. All proposals require safe exceedance routes, maintenance access, regulatory compliance and coordination with other infrastructure. Calculations and drawings are preliminary until verified against local standards, survey, geotechnical information and professional engineering review.

Key Terms

Catchment: Area that contributes runoff to a defined outlet.

Runoff coefficient: Dimensionless coefficient representing prompt runoff response in the Rational Method.

Peak discharge: Maximum flow rate during a design event.

Time of concentration: Travel time from the hydraulically most distant point to the outlet.

IDF curve: Relationship between rainfall intensity, duration and frequency.

Minor system: Drainage infrastructure intended for ordinary design storms.

Major system: Overland flow and storage routes used when the minor system is exceeded.

Catch basin: Chamber that receives surface water and connects it to drainage infrastructure.

Sump: Lower chamber space intended to collect sediment.

Culvert: Conduit carrying water beneath an obstruction such as a road.

Freeboard: Vertical safety distance above the design water level.

Detention: Temporary storage followed by controlled release.

Retention: Storage in which water remains for an extended period or permanently.

Swale: Shallow vegetated channel for conveyance, slowing and treatment.

Bioretention: Vegetated filter system that treats runoff through soil and plant processes.

Hydraulic radius: Flow area divided by wetted perimeter.

Manning roughness: Coefficient representing resistance to flow in Manning’s equation.

Outfall: Approved point where drainage water leaves the system.

SuDS: Sustainable urban drainage systems designed to manage runoff near its source and reduce adverse impacts.

Review Questions

Short Answer Questions

1. What is the purpose of a runoff coefficient?
2. Define time of concentration.
3. State the Rational Method equation and its principal variables.
4. What is the function of freeboard in a detention basin?
5. Name four components that should be shown on a stormwater drainage drawing.

Long Answer Questions

1. Explain the hydrologic sequence from rainfall to outfall in an urban catchment.
2. Describe the assumptions and limitations of the Rational Method.
3. Explain how a catch basin, pipe network and open overland route should work together.
4. Describe the principal planning and drafting requirements for a culvert.
5. Compare conventional drainage with a SuDS treatment train.

Analytical / Application Questions

1. A 0.80 ha mixed catchment has Cʷ = 0.65 and design intensity 90 mm/h. Calculate the preliminary peak discharge using the Rational Method and state two checks that remain necessary.
2. A proposed detention basin has a required active storage of 300 m³. Explain why multiplying plan area by average depth is only a preliminary estimate and identify the survey information needed for final storage calculations.
3. Prepare a conceptual drainage strategy for a sloping institutional site containing roofs, a parking area, landscaped ground and a downstream watercourse. Identify collection points, conveyance, treatment, detention, emergency overflow and maintenance provisions.

Multiple Choice Questions with Answers

Question 1

Which equation is the basic Rational Method relationship for peak runoff when area is in hectares and intensity is in millimetres per hour?

A. Q = 0.00278 C i A

B. Q = C ÷ (iA)

C. Q = 1. Manning A Rₕ S₀

D. Q = P − L

Correct Answer: A. Q = 0.00278 C i A

Question 2

A catchment has area 2 ha, runoff coefficient 0.50 and rainfall intensity 60 mm/h. What is the preliminary peak discharge?

A. 0.0167 m³/s

B. 0.0834 m³/s

C. 0.167 m³/s

D. 16.7 m³/s

Correct Answer: B. 0.0834 m³/s

Question 3

Which quantity is divided by wetted perimeter to obtain hydraulic radius?

A. Rainfall depth

B. Flow area

C. Peak intensity

D. Storage volume

Correct Answer: B. Flow area

Question 4

What is the principal purpose of a detention basin?

A. To permanently increase peak discharge

B. To temporarily store runoff and release it at a controlled rate

C. To eliminate the need for an outfall

D. To replace all maintenance operations

Correct Answer: B. To temporarily store runoff and release it at a controlled rate

Question 5

Which feature is most directly associated with a swale?

A. A deep pressurised sewer

B. A shallow vegetated drainage channel

C. A roof structural beam

D. A sealed sanitary tank

Correct Answer: B. A shallow vegetated drainage channel

Question 6

For a mixed catchment, why is an area-weighted runoff coefficient used?

A. Because all surfaces have identical runoff behavior

B. Because different surface areas have different runoff responses

C. Because rainfall intensity has no units

D. Because it calculates pipe roughness

Correct Answer: B. Because different surface areas have different runoff responses

Question 7

Which item is essential on a stormwater pipe layout?

A. Only the property owner’s name

B. Pipe direction, diameter, gradient and relevant invert levels

C. Only landscaping symbols

D. Only the roof colour

Correct Answer: B. Pipe direction, diameter, gradient and relevant invert levels

Question 8

What does freeboard provide in a basin or channel?

A. A safety allowance above the design water level

B. A method of increasing soil permeability

C. A replacement for hydraulic calculations

D. A permanent reduction in rainfall

Correct Answer: A. A safety allowance above the design water level

Question 9

Which sequence best represents a SuDS treatment train?

A. Source control, treatment, storage and controlled discharge

B. Roof removal, sewer isolation and uncontrolled discharge

C. Culvert blockage, ponding and overflow to buildings

D. Only a larger downstream pipe

Correct Answer: A. Source control, treatment, storage and controlled discharge

Question 10

In the simple storage balance dS/dt = I − O, what happens when inflow exceeds outflow?

A. Storage decreases

B. Storage remains exactly constant

C. Storage increases

D. Rainfall becomes zero

Correct Answer: C. Storage increases