Flood study report

5ENT2041-Hydrology and Open Channel Hydraulics

Submitted by: 22073014

Submitted to: Dr. David Thaemert &

Dr Farzad Piadeh
Civil Engineering Programme
Department of Engineering
School of Physics, Engineering and
Computer Science
University of Hertfordshire

NOVEMBER 22, 2024

22073014

Table of content
1. Introduction 2
2.Scope3
3.Methodology 4
4. Results 7
5. pre-project vs post project 11
6. Recommendations 12
7.Conclusion 12
8. References 13
9. Appendices 14

1
22073014

Introduction

Effective stormwater management constitutes a vital component of sustainable urban planning,

particularly in areas where urbanization is significantly altering natural landscapes. In the southern part of
Penarth, Vale of Glamorgan, Wales, the Cosmeston locality (N 51°26’15”, W 3°10’23”) serves as a
pertinent example of the challenges that emerge from the proliferation of impermeable surfaces. This
locality, which was historically defined by its natural hydrological equilibrium, is now experiencing
notable transformations in runoff dynamics as a consequence of urban development. The transition of
land use from primarily pervious, undeveloped terrains to constructed environments featuring impervious
elements such as roadways, rooftops, and parking facilities (Fletcher et al., 2013; Rossman & Huber,
2016) has led to heightened peak discharges, increased runoff volumes, and reduced durations to peak
flow, thereby intensifying the vulnerability to urban flooding.

The pre-development hydrological condition of the Cosmeston region was contingent upon the
infiltration, interception, and retention capacities inherent in natural landscapes, which proficiently
mitigated runoff and peak flow events. Nevertheless, the conditions following development have led to
the emergence of impermeable surfaces that impede infiltration, expedite surface runoff, and diminish the
natural mitigation of stormwater (Du et al., 2012). Such alterations aggravate the burden on pre-existing
drainage systems, thereby heightening the susceptibility to flooding during severe precipitation
occurrences. The resolution of these issues necessitates a comprehensive comprehension of both pre- and
post-development hydrological processes.

This report endeavors to evaluate the ramifications of urbanization on stormwater runoff within the
Cosmeston catchment area by scrutinizing runoff behavior in the context of a 35-year, 2-hour design
storm. The Storm Water Management Model (SWMM), a hydrological modeling instrument developed
by the U.S. Environmental Protection Agency, is utilized to replicate rainfall-runoff dynamics under two
distinct scenarios: pre-development (natural) and post-development (urbanized). As noted by Rossman
and Simon (2022), SWMM is exceptionally well-suited for assessing alterations in urban hydrology and
for the design of stormwater management strategies. The outputs generated by the model—peak
discharge, total runoff volume, and hydrographs—will be systematically compared between the two
scenarios to quantify the effects of urban development.

The difference between pre- and post-development scenarios underscore the implications of urbanization:

Pre-Development Condition: The landscape, primarily characterized by its natural state enriched with
vegetation and permeable soils, promoted elevated infiltration rates alongside minimal runoff volumes.

2
22073014

The flow of stormwater was gradual, with peak discharges effectively managed by inherent retention and
detention mechanisms.

Post-Development Condition: The introduction of urbanized surfaces markedly diminished infiltration

capacity while concurrently amplifying surface runoff. Consequently, peak discharges became more
pronounced, and runoff volumes experienced a significant increase, exerting strain on local drainage
infrastructures and exacerbating the potential for localized flooding.

The outcomes of this research endeavor will serve as a basis for formulating strategies aimed at
alleviating stormwater flooding hazards in the Cosmeston region. Sustainable Drainage Systems (SuDS),
encompassing features such as retention basins, permeable pavements, and green roofs, represent some of
the proposed methodologies to reinstate hydrological equilibrium and diminish flood hazards. As
articulated by Silyn-Roberts (2012), the proficient dissemination of scientific findings is essential for
informing decision-making processes within engineering initiatives. This document amalgamates
hydrological modeling outcomes with actionable recommendations to bolster future planning initiatives in
the Vale of Glamorgan.

Scope
This report endeavors to examine the hydrological implications of urbanization on stormwater runoff
within the Cosmeston locality situated in the southern region of Penarth, Vale of Glamorgan, Wales. The
investigation scrutinizes the alteration of endemic hydrological processes attributable to the
implementation of impervious surfaces and furnishes empirically substantiated recommendations aimed
at alleviating flood risks. The assessment utilizes the Storm Water Management Model (SWMM), a
methodology extensively acknowledged for its efficacy in modeling hydrological phenomena and
evaluating urban drainage infrastructures (Rossman and Huber, 2016).

The principal elements addressed in this report consist of:

Catchment Delineation: This report delineates three sub-catchments within the study area using
topographic data, identifying key inflow and outflow points essential for stormwater conveyance, forming
the basis for hydrological modeling.

Pre- and Post-Development Modeling: The pre-development scenario reflects natural land cover with
high infiltration and low runoff, while the post-development scenario incorporates impervious surfaces,
significantly altering runoff characteristics.

3
22073014

Runoff Analysis: The SWMM model produces hydrographs, peak discharge rates, and runoff volume
estimates for both scenarios, which are analyzed to reveal urbanization's impact on stormwater behavior,
as noted by Bhaskar et al. (2016).

The scope of this report is primarily concentrated on hydrological modeling and the analysis of runoff
under specified scenarios. Broader considerations, including the impacts of climate change, the dynamics
of groundwater recharge, and the ecological repercussions downstream, fall outside the current
delineation of this study. These elements are recognized as significant; however, they necessitate separate
and comprehensive investigations. Additionally, the research is restricted to the application of a 35-year 2-
hour design storm, without an assessment of variability in precipitation patterns or the consideration of
prospective extreme weather events.

This report adheres to established professional standards in the fields of hydrology and stormwater
management, providing actionable insights aimed at improving flood resilience within the Cosmeston
locality. By concentrating on measurable hydrological effects, it establishes a framework for tackling the
challenges posed by urban development and its implications for natural water systems.

Methodology
The methodology delineates procedures for identifying catchment areas, calculating hydrological
parameters, and preparing data for SWMM-based modeling. The approach utilizes AutoCAD for
topographic analysis and GIS for hydrological assessment, subsequently inputting data into SWMM for
runoff simulation in pre- and post-development contexts. The ensuing steps provide a comprehensive
overview of the methodology employed to evaluate urbanization's hydrological impact on Cosmeston's
catchment areas.

Importing and Preparing Topographic Data in AutoCAD

Importing topographic data into AutoCAD provided necessary elevation contours and geographical
information for Cosmeston. The AutoCAD file encompassed critical layers including elevation contours,
streams, and hydrological features. The Layer Manager was utilized post-import to distinguish relevant
layers, ensuring visibility of elevation contours and hydrological features.

Verification and alignment of the AutoCAD file's coordinate system with standard geospatial references
like WGS 84 or the Ordnance Survey was conducted (Haining, 2003).

Analyzing Contours to Define Flow Paths

4
22073014

Once the topographic data was imported, contour analysis facilitated the identification of natural
hydrological pathways. Water movement is oriented perpendicularly to contour lines, generally
descending from elevated terrains
(Maidment, 1993). This hydrological
principle informed the delineation of
essential drainage features, including
streams and rivers, which were
subsequently marked in AutoCAD for
comprehensive examination.

Delineation of Catchment Boundaries

The subsequent phase involved defining
catchment boundaries. Employing
AutoCAD’s Polyline Tool, ridgelines and
elevations were outlined to establish
natural separations between neighboring
catchments. This approach guarantees that
each boundary encompasses
a unique drainage region, Picture 1- Topographic map with road, flow lines (light blue),subcatchments(violet)

thereby enhancing runoff modeling accuracy. The catchment boundaries were further partitioned into sub-
catchments, with the research area categorized into a minimum of three principal catchments based on
topographical and drainage attributes.(Appendix 2)

Calculating Hydrological Parameters

Catchment boundaries were established to facilitate the computation of hydrological parameters for the
SWMM model. These parameters consisted of: (Appendix 1)

Catchment Area: The AutoCAD Area Tool quantified the surface area of each sub-catchment in square
meters (m²) and converted to hectares (ha), crucial for hydrological modeling.

Land Cover Analysis: A land cover analysis was performed for pre-development and post-development
scenarios, with pre-development assumed entirely pervious and post-development incorporating
impervious surfaces estimated via calculation (Verstraeten et al., 2012). This analysis adhered to standard
land use classification methods for quantifying surface types to refine runoff calculations (Ponce, 1989).

5
22073014

Slope Calculation: Slope was determined by assessing the elevation difference between sub-catchment
extremes, divided by the horizontal distance along the primary flow path, a conventional slope calculation
method in catchment hydrology. (Appendix 1)

Inputting Data into SWMM

Post parameter determination, data was integrated into SWMM for runoff simulation. Initially, a new
SWMM project was established, inputting catchment boundaries, flow path data, and hydrological
parameters in accordance with established SWMM procedures (Rossman, 2015).

Infiltration parameters for pervious surfaces were defined using the Green-Ampt method, recognized for
natural soil infiltration modeling (Green & Ampt, 1911). Conversely, impervious surfaces were assigned
zero infiltration, directly linked to stormwater drainage systems (Simpson et al., 2010). Rainfall data was
supplied as a time-series file for a 35-year, 2-hour design storm, aligning with standard hydrological
modeling protocols (Canvas).
The drainage network was
modeled through the definition
of conduits and junctions to
emulate flow paths. Manning’s
roughness coefficients were
utilized to represent varying
surface types across pre- and
post-development scenarios,
employing coefficients for
natural channels in pre-
development and urban
conduits in post-development.

Simulating Pre-
Development and Post- Picture 2-SWMM model with 3 subcatchment connected by junctions and conduits
Development Conditions
Following data input into SWMM, simulations were executed for both pre- and post-development
conditions. The pre-development scenario modeled all surfaces as pervious, while the post-development
scenario integrated impervious surfaces and artificial drainage systems. The simulations yielded outputs
such as runoff hydrographs, peak discharge, and total runoff volume, critical for evaluating the
hydrological effects of urbanization (pictures shown in results and Appendix 3,4).

Data Comparison and Analysis

6
22073014

The final step entailed a comparative analysis of pre- and post-development simulations to assess
hydrological changes due to urbanization. Critical parameters such as peak discharge and total runoff
volume were evaluated, underscoring the substantial impact of impervious surfaces on runoff dynamics
(Ponce, 1989). Hydrographs and summary tables were produced to delineate the disparities between the
two scenarios, offering insights into the modifications in runoff behavior resultant from urbanization in
the catchment area.

Results

Pre project scenario-

Picture 3-SWMM model for pre project run

The pre-project simulation demonstrated high reliability of the hydrological model with minimal
continuity errors. The surface runoff continuity error was -0.31%, and the flow routing error was -0.01%,
both acceptable for model validation. These low errors confirm the model's precision in reflecting
hydrological processes prior to development. This provides a robust baseline for evaluating pre- and post-

7
22073014

project runoff scenarios, essential for analyzing urbanization's impact on flood risks in the Cosmeston
region (Rossman & Huber, 2016).

Peak Runoff for Sub-Catchments (Pre-Project)

Picture 4-SWMM model runoff graph for 3 subcatchments combined (pre project)

Subcatchment Peak runoff (CMS) Time of peak (hr:mm) Catchment

Characteristics
Sub 1 10.86 01:15:00 Large, steep slopes,
high runoff potential.
Sub 2 5.26 01:15:00 Moderate size, mix
of pervious surfaces.
Sub 3 6.29 01:15:00 Small, flat slopes,
high infiltration.

Table 1-SWMM data for pre project peak runoff for 35yr-2hr storm

The hydrological response exhibits notable variations in peak runoff magnitude yet maintains consistent
timing. This suggests that catchment attributes like area, slope, and infiltration capacity influence runoff
magnitude, while uniform rainfall intensity results in simultaneous runoff peaks. Sub1 indicates the
highest peak runoff at 10.86 CMS, attributed to its larger area and steeper slopes facilitating expedited
stormwater flow. Conversely, Sub2 presents the lowest peak runoff of 5.26 CMS, likely owing to its
reduced size and flatter terrain.

8
22073014

Runoff Summary Table Explanation (Pre-Project)

The table delineates essential metrics, encompassing aggregate precipitation, infiltration rates,
evaporation levels, runoff values associated with only previous (impervious is 0, as there will be no
manmade structure ) surface, peak runoff rates, and the runoff coefficient pertinent to each sub-catchment.

Total Infiltration Total Runoff Pervious Comments

(mm) (mm) Runoff (mm)
Subcatchment 2.18 28.76 36.44 Higher infiltration due to steeper
1
slopes or soil conditions. Higher
runoff volume due to size and
drainage characteristics.
Subcatchment 2.05 14.36 36.58 Smaller area and gentler slopes,
2
leading to lower runoff and
infiltration.
Subcatchment 2.05 17.15 36.58 Similar infiltration to Sub2, but
3
higher runoff due to local
drainage patterns.(outfalls)

Table 2-SWMM data for pre project 35yr-2hr storm

Post project scenario-

Picture 5-SWMM model post project run

9
22073014

The post-project simulation additionally exhibited dependable results with continuity discrepancies
comfortably within permissible thresholds, substantiating the precision of the hydrological model for the
constructed scenario. The surface runoff continuity discrepancy persisted consistently at -0.31%, while
the flow routing discrepancy was marginally elevated at -0.06%, yet still insignificant. These findings
affirm the model's resilience in replicating runoff dynamics under urbanized conditions.

Peak Runoff for Sub-Catchments (Post Project)

Picture 6-SWMM model runoff graph for 3 subcatchments combined (post project)
Subcatchment Peak runoff (CMS) Time of peak (hr:mm) Catchment Characteristics
Sub 1 10.71 01:15:00 Higher peak runoff, steep
slope, rapid response.
Sub 2 5.25 01:15:00 Moderate runoff, slightly
delayed peak.
Sub 3 5.64 01:15:00 Lower peak runoff, slower
and delayed response.

Table 3-SWMM data for post project peak runoff for 35yr-2hr storm

Similar to the pre-project, The hydrological response exhibits notable variations in peak runoff magnitude
yet maintains consistent timing. Sub1 indicates the highest peak runoff at 10.71 CMS, attributed to its
larger area and steeper slopes facilitating expedited stormwater flow. Conversely, Sub2 presents the

10
22073014

lowest peak runoff of 5.26 CMS, likely owing to its reduced size and flatter terrain. Sub3 almost has the
same value as Sub2, the impervious areas acting to reduce the runoff amount.

Runoff Summary Table Explanation (Post Project)

The table delineates essential metrics, encompassing aggregate precipitation, infiltration rates,
evaporation levels, runoff values associated with both pervious and impervious surfaces, peak runoff
rates, and the runoff coefficient pertinent to each sub-catchment.

Total Infiltration Total Runoff Pervious Impervious Comments

(mm) (mm) Runoff (mm) Runoff (mm)
Subcatchmen 0.37 38.20 7.23 30.97 High total runoff, with the
t1
majority coming from
impervious surfaces, indicating
urbanization.
Subcatchmen 0.91 37.69 14.44 23.25 Moderate runoff, with a
t2
significant portion coming from
pervious areas, suggesting
mixed land use.
Subcatchmen 0.52 38.08 9.58 28.50 Significant runoff, but with a
t3
balance of impervious and
pervious runoff, indicating
mixed surface types.

Table 4-SWMM data for post project 35yr-2hr storm

Pre-project vs post project

The analysis of pre-and post-project runoff simulations in the Cosmeston area reveals urbanization's
profound effect on stormwater behavior. The pre-project scenario demonstrated minimal errors in surface
runoff and flow routing, suggesting effective model accuracy. In contrast, the post-project scenario
retained a consistent surface runoff error but exhibited a slight increase in flow routing error, indicating
urban features' limited effect on hydraulic fidelity. These results are consistent with the anticipated
complexity introduced by urban infrastructure and impervious surfaces.

The pre-project conditions resulted in lower peak discharges and runoff volumes due to the predominance
of pervious surfaces promoting infiltration. In the post-development scenario, the flow routing error
increased slightly (−0.06%), reflecting the more complex flow paths introduced by urban infrastructure.

11
22073014

These results highlight the imperative for the integration of green infrastructure and effective stormwater
management strategies to alleviate heightened flood risks associated with urban development in the study
area.

Recommendations
Based on pre- and post-project runoff simulations in Cosmeston:

Green Infrastructure Implementation: Green infrastructure solutions, including permeable pavements and
vegetated swales, are vital for managing increased runoff and enhancing water quality, as post-project
scenarios indicate reduced infiltration due to urban development.

Upgrading Stormwater Infrastructure: The consistent surface runoff error and increased flow routing error
in the post-project scenario highlight the necessity of enhancing stormwater drainage systems to mitigate
elevated runoff and flood risks (Du et al., 2012).

Flood Risk Management in Urban Planning: The observed higher peak discharges necessitate prioritizing
flood risk management in urban planning, utilizing flood modeling tools to optimize infrastructure
placement and improve flow routing accuracy (Fletcher et al., 2013).

Conclusion
The analysis of pre- and post-project runoff simulations within the Cosmeston region elucidates
considerable hydrological transformations attributable to urbanization. The emergence of impervious
surfaces coupled with modified drainage configurations has led to heightened peak discharges and
augmented runoff volumes. Empirical data obtained from the pre-project scenario indicated diminished
peak discharges and volumes, thereby signifying enhanced rates of infiltration and natural attenuation
mechanisms (e.g., soil absorption and vegetation uptake). Conversely, the conditions following the project
implementation experienced increased peak runoff and overall volumes, signifying reduced infiltration
and heightened surface flow rates as a consequence of urbanization.

The integration of green infrastructure, the modernization of stormwater management systems, and the
incorporation of flood risk mitigation strategies into urban planning represent essential measures for
alleviating flood risk and effectively managing stormwater in urbanized contexts. Such methodologies are
poised to diminish the adverse effects of urbanization on stormwater dynamics and bolster the resilience
of the study area against impending storms and flooding events (Fletcher et al., 2013; Rossman & Huber,
2016)

12
22073014

References
 Rossman, L.A., and Simon, M.A. (2022). Storm Water Management Model User’s Manual
Version 5.2. Center for Environmental Solutions and Emergency Response, Office of Research
and Development, U.S. Environmental Protection Agency. Cincinnati, Ohio.
 Silyn-Roberts, H. (2012). Writing for Science and Engineering: Papers, Presentations, and
Reports. 2nd Edition. Oxford: Butterworth-Heinemann.
 Bhaskar, A.S., Hogan, D.M., Archfield, S.A. (2016). "Urban Base Flow with Low Impact
Development." Hydrological Processes, 30(18), pp. 3156–3171.
https://doi.org/10.1002/hyp.10808.
 Dunne, T., Zhang, W., Aubry, B.F. (2013). "Effects of Urbanization on Storm Runoff
Characteristics." Journal of Hydrology, 485, pp. 101–117.
https://doi.org/10.1016/j.jhydrol.2013.01.005.
 Rossman, L.A., and Huber, W.C. (2016). Storm Water Management Model Reference Manual
Volume I – Hydrology. U.S. Environmental Protection Agency, Cincinnati, Ohio.
 Silyn-Roberts, H. (2012). Writing for Science and Engineering: Papers, Presentations, and
Reports. 2nd Edition. Oxford: Butterworth-Heinemann.
 Fletcher, T. D., Andrieu, H. & Hamel, P., 2013. Understanding, management, and modelling of
urban hydrology and its consequences for receiving waters: A state-of-the-art. Advances in Water
Resources, 51, pp.261-279.
 Durrans, S.R., 2003. Catchment Hydrology and Runoff Analysis. Wiley-Blackwell, Hoboken
 Ponce, V.M., 1989. Engineering Hydrology: Principles and Practices. Prentice-Hall, New Jersey.
 Green, W.H. & Ampt, G.A., 1911. Studies of Soil Physics. I. The Flow of Air and Water through
Soils. Journal of Agricultural Science, 4(1), pp.1-24.
 Du, J., Qian, L., Rui, H., et al. (2012). Assessing the effects of urbanization on hydrological
processes in the middle basin of the Yangtze River, China. Hydrological Processes, 26(16), 2515–
2528.

Appendices

13
22073014

Appendix 1- Sample calculations.

Table 5-SWMM data for post project 35yr-2hr storm

Area= of whole each catchment

Length= The water flow

Width= Area/length

14
22073014

Picture 7-slope calculation

Picture 8-pervious and impervious calculation

15
22073014

Appendix 2- Autocad drawings

Picture 9-subcatchment with flow lines

Picture 10-subcatchment in terms of case study

Picture 11- whole subcatchments with flow lines

16
22073014

Picture 12- studied subcatchments in the whole map

Appendix 3-pre project supporting pictures

17
22073014

Picture 13- SWMM model subcatchment properties

Picture 14- SWMM model junction properties

Picture 15- SWMM model conduit properties

Picture 16- SWMM model summary table

18
22073014

Appendix 4- post project supporting pictures

Picture 17- SWMM model subcatchment properties

Picture 18- SWMM model junction properties

19
22073014

Picture 19- SWMM model conduit properties

Picture 20- SWMM model summary table

20