Sealing flood “cracks”: A service for urban authorities.

There is a Swahili proverb, “Usipoziba ufa, utajenga ukuta“. Its translation in English is that if you do not repair the cracks on the wall, you eventually, will have to build a new wall. In many contexts, the crack is an indication of weakness.

Usipoziba ufa, utajenga ukuta

Swahili proverb

Urban Flood “cracks”

If you live in an urban area, you may have experienced urban floods commonly characterized by flooded streets and other low-lying areas. These floods develop quickly due to faster flow times over the impervious surfaces. These result in loss and damage to vital infrastructure and livelihoods while the secondary effects of pollution linger long after the flood event. With weather events expected to get more extreme, the impacts will only get worse. The stormwater infrastructure in many urban areas, at present, is insufficient to deal with current and future rainfall events. The unmanaged stormwater is akin to cracks in a wall that are bound to get worse if not addressed, with the ability to paralyze activities in urban areas. A case in point is the 2021 flooding that resulted from the remnants of Hurricane Ida in New York, where up to 3 inches of rain an hour resulted in devastating flash floods in the city.

The case of Gowanus!

Authorities in New York have however been learning from previous flooding events and are implementing measures to control flooding and pollution in New York. This is best exemplified in Gowanus. Gowanus is a neighborhood in Brooklyn, New York that was recently rezoned and is set to see a lot of new development in the coming years. To protect the neighborhood from loss and damage, the neighborhood will adopt green infrastructure to control flooding and pollution. There is also a new unified stormwater rule requiring large lots to retain rainwater with technical guidelines provided in a stormwater manual.

However,

• the city notes that all the planned measures to control flooding and pollution will still be inadequate leaving 115 million gallons of Combined Sewer Overflow (CSO) unmanaged.
• Moreover, space is also limited with developers likely to maximize on real estate, leaving no room for more green infrastructure.

Our solution: the choice, number, capacity, and location of green infrastructure installations

So, how do we manage the excess stormwater where space is a scarce resource?

We provide a service solution for urbanized areas by identifying the excess stormwater runoff paths as well as volume and velocity to minimize the concentration of stormwater in the flood prone areas. In an urban area such as Gowanus, where it is already determined what green infrastructure will be adopted (as provided for in the New York City Stormwater Manual), our solution provides an objective method for determining the choice, number, capacity, and location of green infrastructure installations. In addition, since space is limited, we propose installations that are well distributed in the upslope areas to “seal” the runoff before it is concentrated into a devastating hazard.

The approach

The solution is based on two technologies, GIS and video analytics to determine the contribution of upslope areas to the flooding experienced downslope. We summarize the solution here and in our video (at the bottom of the page);

In brief, the following is our solution

a) Identify flood-prone areas. Places at greater risk such as subway stations and basement apartments should be considered.

b) Next, determine all the area upslope that would drain its waters through the flood-prone location

c) Sub-divide the contributing area into sub-catchments using existing drainage networks. I.e. identify different sub-catchments according to the junctions of the drainage network.

d) Evaluate the characteristics (geomorphology) of these sub-catchments. For example, different shapes, sizes, and drainage densities would result in different rates of flow.

e) In addition to the characteristics, we would also employ video analytics to determine the velocity of water along the streets. We will use existing footage from Closed Circuit Television (CCTV) cameras of the flooding events to detect and track objects carried by the runoff.

f) With this information, we would identify the source and volumes of runoff and be better placed in deciding which measure would be sufficient to manage such rates and volume.

Benefits of our solution

There are benefits to such an approach where the green infrastructure is distributed over an entire watershed rather than concentrated in low-lying areas. The benefits include;

• Any additional infrastructure would require space for implementation which would be difficult to come by in one location e.g. low-lying areas only. A distributed approach would ensure that even smaller installations would be effective where a rainfall event is distributed.
• The volume and velocity of stormwater runoff reaching the low-lying areas are reduced enabling the existing infrastructure to comfortably convey any stormwater within it.
• Our service is complimentary to hazard mapping commonly achieved through hydrologic and hydraulic modeling. We will provide information on excess stormwater within the streets as the flood develops. Urban areas are dynamic and little developments such as putting up a flood barrier would affect the volume and rate of flow to low-lying areas. Such dynamics can be accounted for by monitoring the actual flow on the streets through CCTV cameras.
• Different ways of managing the stormwater can be implemented at once. Depending on the location, stormwater can either be treated and re-used, allowed to infiltrate, released slowly to stormwater infrastructure, etc.
• We also consider readily available datasets such as LiDAR-derived Digital Elevation Models (DEMs) and CCTV footage of previous rainfall events. Huge investments have gone into generating LiDAR and establishing the CCTV monitoring network, thus we would be unlocking more value without requiring additional investment in data collection. In developing countries, where such data may be scarce or unavailable, high-resolution data can be captured using low-cost technology such as drones.

Our value proposition

We are offering a unique, objective, and accurate way of determining actual excess stormwater from previous rainfall events such as the high-intensity rainfall experienced as a result of Hurricane Ida. This is through video analytics to capture the actual velocity and volume of stormwater. We will make use of available footage from CCTV cameras to detect, track and extract information that would help in understanding and managing the excess stormwater. For a different scenario involving higher intensity rainfall events, we can further scale up the volumes and velocity and provide suitable recommendations for the scenario. As such, we can provide valuable information on excess stormwater within a short duration of time to enable response and mitigation of future risks.

Who is this solution meant for?

The current solution is intended for urban authorities who are responsible for the Environmental Protection and Management of Urban areas. We have considered readily available datasets for this solution. We, therefore, intend to only charge for the analysis within a short duration (5 to 7 months) with deliverables including a detailed report on the runoff paths, sub-catchment characteristics, volume, velocity, and height of previous flood events. We will also include recommendations for green infrastructure to put to manage the stormwater.

The consultancy team

We are a start-up consultancy from Kenya currently comprised of four members. Dennis is a Geographical Information Systems (GIS) and digital image analysis expert, Evans is an Urban Planner, while Emmanuel and Phillip are specialists in Soil, Water, and Environmental Engineering.

So, how did we begin this journey?

We all receive an email, followed by a phone call from one intelligent Professor whom I will call John (actually, that is his real name). So, Prof., John feels that we would benefit from taking part in the 2022 Next Generation Water Action (NGWA). He puts us in the same team to take on the challenge of tackling flood risks in a new urban district, namely, Gowanus. The challenge is proposed by a partner from Ramboll Group. We also benefited from boot camps and keynotes organized by the NGWA team that led us to this point.

You can reach us through our team lead, Dennis @ theurigitundu@gmail.com.