CVEN90051 CIVIL HYDRAULICS Module 1: Channel Hydraulics and Hydraulic Structures 1Java

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Java Python CVEN90051 CIVIL HYDRAULICS

Module 1: Channel Hydraulics and Hydraulic Structures

Assignment 1 (20 Points)

deadline: 11.59pm on August 25th, 2024

General information & Instructions

The aim of this exercise is to assess your understanding of Module #1 through the capacity to: (i) evaluate a problem and identify fundamental as well as advanced issues that need to be solved (problem solving); (ii) select the most appropriate method and input conditions; (iii) apply and execute the appropriate method correctly; (iv) present results using both textual and graphical means in a clear and concise manner; and (v) discuss/interpret the results using critical thinking.

The tasks specified in the assignment can be solved entirely using the learning material for the module, i.e. reading material, lectures and lecture notes, topic guides, discussion topics, and exercise solutions.

Marking of the assignment will also rely on learning material. However, it is possible that some aspects of the topic need to be looked up over the internet to complement the learning material.

Once you have reached a solution for all the tasks, summarise your work in a clear and concise report. We expect that you will need about 2,000 words. It may be convenient to summarise the results into tables and graphs, to reduce word count. If you use tables, add a column for comments where you provide a brief explanation of the specific result/step/raw. Note that tables are useful to summarise general results, such as normal depth; graphs are more convenient for water profiles. The mark will be based on what we understand from your document. Therefore, we will only consider what is presented in the report without adding any personal interpretation of your figures/tables. So, make sure to have short and clear statements (in the body of text or tables) telling us what we should take away from your results.

Important: try to follow the headings of the rubric (e.g. for task 1: 1.1 flood risk analysis and 1.2 identify problems) when compiling the report. This streamlines marking and feedback.

NOTE #1: World limit. You have some flexibility on the number of words. So, do no stress if your report is a bit longer. Overall, we can accept reports with up to 3,000 words (as provided by Turnitin – note that Turnitin would probably count all words including titles, headings, etc…). A 10% penalty (i.e. 2 points) may be applied for longer reports.

NOTE #2: If you like to work with your peers to solve the assignment tasks, please feel free to do so.

However, you are required to submit an individual report which is an original document that MUST be different from those submitted by your peers. This is because we are assessing your individual capacity and not ability to work in a group.

NOTE #3: We run consultation session weekly. While we cannot provide direct answers that solve the tasks (or part of them), we will offer supervision and discuss your work with the intent to guide you towards the  right answer. This means we will help you reasoning, but we will not provide any direct answer to your problems/questions.

Assignment #1

The context

Over 25 years ago, the local council completed a housing development to cope with an increase in population. This resulted in the creation of seven new suburbs along the 22-metre wide river Chalk (see Figure 1, where the suburbs are referred to as Dev#1, Dev#2, etc…). To facilitate accessibility across the river, two culverts were also constructed at the time; their characteristics are reported in Table 1 (as an example, culvert #2 looks like the one in Figure 2). The distance between these two infrastructures is 6 km, while the distance of the various suburbs from the most downstream culvert (i.e culvert #2) are reported in Table 2.

Table 1: Geometrical characteristics of the culverts (the freeboard is the distance between  the upper level of the culvert and the road level).

Culvert	No. Barrels	Barrel height [m]	Barrel width [m]	Barrel length [m]	Barrel slope	Manning Coef	Freeboard (distance btw obvert and road level) [m]	Road level relative to riverbed [m]
#1	1	3	12	28	1/800	0.025	1.5	4.5
#2	2	3	5	26	1/800	0.025	1.2	4.2

During this development, the council commissioned a hydraulic study to calculate the design discharge  (the  100-year  return period value) in the river and assess flooding risks. The catchment of the river Chalk was, at that time, not monitored by any observation stations, although it was common knowledge that water depth in average conditions was about 1.5- 1.8m. Therefore, the civil engineer in charge had to rely on water discharge data from a nearby catchment that was deemed “statistically” similar. The declared design discharge was 110 m3/s and, based on this information, the riverbanks were adjusted. The results of a recent survey at some key cross-sections providing data on e.g. bank heights, and roughness are reported in Table 2. The width is constant throughout the river in this region. 

Figure 1: Map of the development, highlighting the seven suburbs (Dev#1, Dev#2, etc…), the two culverts, the proposed location of the barrage and a nearby irrigation channel [not in scale].

Table 2: Cross-sections

 Figure 2: Example of culvert with tw CVEN90051 CIVIL HYDRAULICS Module 1: Channel Hydraulics and Hydraulic Structures Assignment 1Java o barrels. This resembles culvert #2 in this assignment.

The two squared opening are the two barrels.

The problem

Since the completion of the development, a number of flooding events were reported across the various suburbs, a couple of which with serious damages especially in Developments #1- 3.  While  these  were  deemed  as  extraordinary  events  in  the  first  instance,  the  new government requested a more careful assessment of past floodings. Results indicated that these  events  occurred  with  a  frequency  of  once  per  year,  with  the  most  severe  ones happening once every five years. It was therefore argued that the design discharge used for the original design and flood analysis was not adequate.

One of the civil engineers at the council noted that the local University installed a couple of observation stations in the catchment of river Chalk to monitor atmospheric and hydraulic conditions about 20 years ago (five years after the development was completed). Although 20-year of data is not enough to underpin an accurate statistical analysis, an estimate of the 100-year return period discharge in the Chalk catchment resulted to be QD_New = 160 m3/s. This is about 28% higher than the original design value. Furthermore, records of the maximum recorded discharge during the 20-year period and the average annual maxima, reveal values above the discharge used in the original design phase. Various discharges obtained from the 20-year data analysis are reported Table 3.

Table 3: Various discharges reported from the 20-year data analysis.

Result from 20-year data analysis	Discharge [m3/s]
New Design discharge (QD_new)	160
Maximum recorded discharge (Qmx)	152
Average annual maxima (Qa_avg)	146

The tasks

The council  has  discussed flood  risk  during their  last  technical  meeting  and  agreed  on  a solution to mitigate severe flood events for developments #1-4. To accomplish this task, the council is proposing the construction of a barrage (a small dam), which constrains the flow discharge. Specifically, it is suggested to split storm discharge discharge between the river downstream and a new service flume that diverts water to a nearby irrigational channel as follows: 80% of the new 100-year return value for the Chalk catchment to flow downstream the  barrage;  the  remaining  20%  of  the  new  100-year  return  value  to  be  diverted  to  an irrigational channel through the service flume. The inlet to the service flume is enabled by a sluice gate.

A survey of the region indicates that the best location for the structure is 4.5 km upstream culvert #2 (see Figure 1). There are several reasons for this location:

(1) the barrage would be sufficiently far from any significant infrastructures;

(2) the barrage is at a suitable distance (not too long but not too short either) to enable an efficient link with a nearby irrigation channel;

(3) the riverbanks between the barrage and the culvert #1 are deep;

(4) the portion of the river between the barrage and the culvert #1 is surrounded by fields allocated to agricultural purposes so that there are no significant constraints to any flood mitigation strategies that might be required;

(5) the East side of the river between the barrage and the culvert #1 is about 1,800m from the closest settlement (Società Agricola Maracuja, see Figure 1); and

(6) the West side of the river between the barrage and the culvert #1 is about 300m from the closest infrastructure (a main road servicing Dev. #6, see Figure 1).

After an animated discussion, the council agreed that the barrage may not present any risks at developments #5-7. However, no evidence was provided to support this speculation and some engineers at the technical office submitted a written statement warning of potential problems if no further mitigation strategies are adopted.

Moving on with the design  phase, the council  has  commissioned a civil engineering firm specialised in river hydraulics to design and assess the environmental impact of the barrage. You are the civil/environmental engineer in charge of this assignment. Your tasks are the followings:

Task 1 (6/20 points):

Check flood risks in the River Chalk under the new hydraulic conditions (riverbed’s slope is 1/800); do not consider the barrage for this task (i.e. discharges through culverts #1 and #2 are the same). Identify and briefly discuss any problem(s) that may justify the need for flood mitigation strategies.

NOTE – To simplify the calculations, assume the cross section is rectangular.

HINT – Culverts will be introduced in Topic 4. For the sake of this task (Task #1), the headwater at the culverts (i.e. the water level immediately upstream the structure, which is needed to initiate the calculations of water profiles) can be obtained using the equation

Q = CdAB[2gH1]1/2         (1)

where:

•   Q is the flow discharge;

•   Cd is the coefficient of discharge: Cd = 0.48 for culvert #1 and Cd = 0.51 for culvert #2;

•   AB is the total cross-sectional area of the barrel system (this is total area of the opening allowed by the culvert: barrel height times barrel width);

•    H1  is the water depth just upstream the barrel (the headwater).

Task 2 (6/20 points):

Design all components of the barrage (weir, spillway and stilling basin), the sluice gate for the overflow, and the cross section of the service channel. (Note – do not consider the water profiles upstream the barrage for this task; this will be the aim of Task 3). The following design conditions apply:

Barrage:

(1) the bank height is 4.8m;

(2) the width of the weir is 86% of the river’s width;

(3) the freeboard for headwater should be between 0.3 – 0.5 m at design discharge

(4)  Note that the maximum discharge allowed by the weir is 80% of QD_new (this is the value to be used in the headwater calculation).

Sluice gate:

(5) the sluice gate should operate in free flow mode with a nominal discharge coefficient of Cd_g = 0.52 as provided by the manufacturer;

(6) the width of one sluice gate is 2.5 m (i.e. the total length of the gate’s opening depends on how many units you decide to install), and it can rise up to a maximum of 1.8m;

(7)  NOTE: the headwater for the sluice gate is the one produced by the barrage/weir.

Service flume:

(8) the service flume is long enough to ensure that the M1 profile emerging from the junction with the irrigation channel (opposite end of the flume) does not affect the sluice gate  (i.e.  downstream the  sluice  data the water  depth  is  always  in  normal conditions);

(9) the service flume has a natural surface of type “Earth, straight” with a manning coefficient n = 0.018 (see reading material for Topic 1) and a slope of 1/500;

(10)             the service flume’s width must not exceed 10 m as requested by owners of neighbouring fields;

(11)             the flume will be hidden by vegetation planted along its edges.

Task 3 (8/20 points):

Re-assess flood risk after the construction of the barrage. If further problems arise, provide one appropriate solution or mitigation strategy – the barrage and sluice gate cannot be re- designed. Elaborate your solution with some numerical details that support the effectiveness of your solution. If assumptions are needed to facilitate calculations, clearly state and briefly justify them.

Note to Task 3:  the equation (1) implies that the outlet is free, i.e. the water depth downstream the culvert is lower that the barrel’s height. If the water level downstream is greater than the culvert (the outlet is drawn/submerged), other equations should be used. Refer to Topic #4 for extra details, if this condition applies         

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