Author Archives: Conrad

Rational Method for Peak Flow

The rational method determines the max flow (Q) at a point after a storm event.

$$ Q = CiA $$

Q, peak flow

C, runoff coefficient

i, storm intensity ($$\frac{in}{hr} $$)

A, area (acres)

To use the rational method you need to divide the watershed (area contributing flow to the point) up into separate areas depending on the runoff coefficients (C). The runoff coefficient changes based on different land types such as concrete, bare earch, turf meadow, residential etc. If the entire area is under one runoff coefficient then lucky you. If not, determine the area of each subregion and it’s accompanying C value. Tables for C values are provided in both the AIO and CERM.

You should end up with (or be given) data similar to this:

Get the Average Runoff Coefficient

You can only enter one C value into the equation. It should be the weighted average of all of the C values.

$$ C_{avg} = \frac{ \sum{CA}}{ \sum{A} } $$

With the data above this would be:

$$ C_{avg} = \frac{25*0.2 + 15*0.3 + 2*0.4}{25+15+2} = 0.245$$

On a short question like on the breadth, I am thinking that the intensity MAY be provided to you, it is sort of a process to get it on your own. Either way, get the intensity. Once you have all three values, multiply for max Q!

Let’s just say I solved for intensity (using information that has not been provided in this example) and it is 1.25 $$\frac{in}{hr}$$ (on the less-intense end):

$$ Q = C_{avg} * i * A $$

$$ Q = 1.25 * 0.245 * 42 $$

$$ Q = 12.875 \frac{ft^3}{s}$$

There you have it! This seems like a high flow to me, but it’s a huge area. And it’s an example problem, I made all these values up.

Speed Tip

You may have noticed that to solve for the $$C_{avg}$$ value you have to divide by the total area. Why would you divide by the total area to get that and then turn around and multiply by the total area when you solve or Q?

To save some time, just multiply the sum of the region areas and coefficients by the intensity. It could be rewritten as $$Q=\left( \sum{ CA_{region}  }\right)*i$$.

Budgeting a Civil Engineering Project


Civil Engineering projects (Public Works) cost a lot! The big scale of them is a part of what makes the career fun.

You get to deal with big budgets, design huge stuff, watch the dirt move, see it built, and hopefully see it used. I have had the lucky experience of dealing with some semi-large scale projects first hand through my position at work these past two years.

I have been working as a consultant on the Program Management team for a rail corridor that has ~22 projects in progress. The total cost to build all of the projects is in the high $700 million region depending on who you ask. Each individual project has its own budget within the public agency’s overall budget and as part of my job I get to have a direct hand in setting these budgets in their yearly update process.

Here is the magical sequence we use to budget a project.

Note: We follow the Design-Bid-Build method of project execution and this is for the general Southern California region.

Construction Cost (Hard Costs)

First, we obtain the raw construction cost of the project. This is provided to us by another consultant who has been hired to design the project. The cost estimates we receive vary from being at the <0% (planning) to 100% (design is complete) levels. They usually look something like what is pictured to the left (this is from a 30% level project, and anything project specific has been blurred out and several values changed.)

We take this total cost estimate and make it as “raw” as possible. This means we strip out any design costs or management fees that the other consultant may have included in their estimate, leaving only the cost for a contractor to do the work.

We call this stripped down estimate the Base Construction Cost (BCC) or Base Construction Estimate (BCE) and use this number to “derive” other costs of the project. These derived costs are generally referred to as “soft costs” versus the construction “hard costs”.

Project Cost Formula’s:

Administration (8% BCE)

Design (8-12% BCE)

Design During Construction (2-4% BCE)

Environmental Clearance (4% BCE)

Construction Management (13-15% BCE)

Construction Change Orders (10% BCE)

Construction Cost (BCE)

Adding these percentages up exactly gives you a rough rule of thumb for the total cost of about 1.45 to 1.53 times the Base Construction Estimate. People toss around 1.5xBCE as the accepted rule of thumb pretty often.

 Total Project Cost = 1.5 x BCE

What do each of these budget categories entail?


Administration is the cost of the project manager (client/owner or another consultant) and also the program manager (us) both during the design and construction period of the project.


This is the cost to design the project. Typically one “prime” design consultant is chosen to design the project, and all other design related tasks that may involve other consultants go through them. This budget is for ALL of those related tasks.

Design During Construction

You may be asking, what is left to be designed? In my experience after a finished set of plans has been delivered to a construction contractor, they almost always want to make changes to it. Whether it is to reduce costs, or improve the design based on their field experience, and to compile the As-Built plans (the plans after the work has been completed). Typically the same designer that worked on the project earlier remains the “prime” designer during construction.

Environmental Clearance

These big projects nearly always impact the environment in some way. Whether it is major and they need to mitigate their environmental damage, or it is minor and they just need to state their minor impacts, environmental studies need to be done, and somebody needs to manage that work. We use environmental sub-consultants that are managed by an environmental program manager and they complete all of this work, which is good, because I sure don’t want to deal with the environmental agencies!

Construction Management

The Construction Managers deal with the Contractor on behalf of the owner/client. They are usually a consultant. Other things I have seen falling under construction management are support items like dealing with local traffic agencies, traffic safety measures etc.

Construction Change Orders

It is inevitable that the site conditions will be different than what the designers thought. Contractors use this to their advantage to get some extra work, extra work that they get paid for. That is charged to this budget item. Also if the Construction Manager, on behalf of the owner, decides some extra features should be added to the project, that also comes out of this budget. Typically 10%, though we budget for this, it is never shown to the contractor, otherwise they would know they can go for it. Of course, they probably know anyway since it is so common.


This is the cost of all of the Construction Items.

That is how budgets seem to be calculated in my general area in Southern California. The percentages vary slightly but I am using some ballparks.

I am curious to know how it differs in other areas.

Slope Stability Basics

The single biggest thing to know about slope stability, the breadth PE, is the factor of safety numbers:

1.5 – Permanent conditions

2.0 – Slope is supporting a building foundation

1.3 – temporary slope conditions

These will probably be in a word problem.

What is the factor of safety?

The factor of safety is a ratio of how stable the slope is given soil characteristics and geometry of the slope (angle and height etc).

The methods of determining the factor of safety for slope stability are somewhat advanced. I think they will definitely appear on the depth exam but likely not on the breadth. I suggest knowing they exist and where they are (Ch 210 Goswami, 40-7 CERM).

What Causes Slope Failure?

All of these things cause failure, eventually:

  • Steepening of slopes
  • Increase in groundwater pressure
  • Weathering (which may be related to steepness)
  • Vibrations, especially repetitive vibrations (traffic), or extreme vibrations (earthquakes)
  • Extra Loading above the slope
  • Excavation at the toe of the slope

 What prevents Slope Failure?

  • Making it less steep through grading
  • Retaining structures
  • Weight at the tow
  • Less weight above the slope
  • Reduce or remove vibrations
  • basically reverse the causes….


Soil Permeability

Soil permeability is a measure of fluids flowing through the soil. Aside from the soil, ease of flow also depends on the fluid (oil is slower than water etc) but you only need to worry about the soil end of things for the geotech breadth.

Coefficient of Permeability (K)

The coefficient of permeability is the characteristic that measures how permeable a soil is. The units of K are the same as velocity (ft/s), but theoretically it is volume per area per time ($$\frac{ft^3}{ft^2 t}$$).

Typical values for K are probably in your reference of choice, and probably also in any permeability problem prompts. In the CERM the values are on page 21-3 and are provided for many different unit types.

 Darcy’s Law

Darcy’s Law will calculate the groundwater flow (Q) through a soil given a hydraulic conductivity (K) a hydraulic grade (i, units are $$\frac{in}{in}$$ or $$\frac{ft}{mi}$$ etc.) and an area covered (A). The units of area should match the units for grade.

$$ Q = -KiA$$

Some references show the negative sign, others don’t. I think this is to account for the hydraulic grade and direction of flow. In most cases the point you are measuring the flow at is roughly the lowest point in the area (A). The water table grade/slope from any other point will be negative to end at the point, so to make the flow positive they throw that negative in to reverse the grade negative. Anyway, regardless of the negative, the absolute value for flow doesn’t change and you should be able to tell what direction the flow is going in.