North India Floods of October 2023

Natural Disasters
Development Seed
Open Data
A quick turn analysis to visually compare events caused by the Glacial Lake Outburst Flood in the Indian Himalayan Region of Sikkim during early October of 2023.

Vincent Clemson


October 12, 2023


February 18, 2024

Figure 1: A split web map displaying avalanche damage to South Lhonak Lake (a glacier lake) in the Indian Himalayan Region.
The pre & post event satellite imagery was obtained from Maxar’s Open Data Program.


I quickly visualize Maxar’s recent release in response to the flooding events in the northern Himalayan region of Sikkim, India in October of 2023. Figure 4 displays a demo of this. You can use Figure 3 to interact with the web map for yourself.

My previous post is a technical deep dive that covers the tooling used behind these techniques.


On the morning of October 12th, 2023, Maxar released an activation alert within their Open Data Program on the recent flooding that devastated Sikkim, India in the Indian Himalayan Region last week on Wednesday October 4th. It has been reported that the flooding was caused by a Glacial Lake Outburst Flood (GLOF) from a melting glacier at South Lhonak Lake. Ultimately, the flooding triggered from the glacier lake area destroyed the Chungthang Dam and it’s surrounding town miles away downstream.

Figure 2 captures the area between the South Lhonak Lake (the glacier lake) and the Chungthang Dam.

Figure 2: Google Maps view from South Lhonak Lake (upper left) to Chungthang Dam (bottom right). Source

Here I use open source technologies to build a web map that displays the released satellite imagery over the region. In particular, Figure 3 is a web map that showcases before and after the GLOF took place to help make visual comparisons of the disaster.

Additionally, Dan Shugar previously released an in-depth thread of the GLOF that triggered the flooding using Planet Labs imagery from the SuperDove satellites. It’s a great resource (below) to understand the chain of events that happened during the floods.

Data Aggregation

North India Flood Event Image Tile Aggregation - Maxar’s Open Data Program
import time
import numpy as np
import pandas as pd
import geopandas as gpd

start = time.time()

p_datasets = ''
d_maxar = pd.read_csv(p_datasets)
p_top = ''
p_bottom = '.geojson'

event = 'India-Floods-Oct-2023'
d_event = d_maxar[d_maxar.dataset == event]
l_gdf = [gpd.read_file(p_top+d+p_bottom).assign(event = d) for d in d_event['dataset'].to_list()]
d_tiles = (pd.concat(l_gdf, ignore_index=True))
# changes column order - puts event & geometry first
d_tiles = d_tiles[d_tiles.columns[-2::][::-1].append(d_tiles.columns[:-2])]

dict_event = {
    'India-Floods-Oct-2023': ['2023-10-04 00:00:00', ''],

d_event = pd.DataFrame.from_dict(dict_event, orient='index', columns=['t_event', 'source'])
d_event = (d_event
 .assign(t_event = pd.to_datetime(d_event.t_event, utc=True).to_list())
 .sort_values('t_event', ascending=False)

# right join removes old datasets stored in opengeos/maxar-open-data
d_tiles = pd.merge(left=d_tiles, right=d_event, how='right', left_on='event', right_on='event')
d_tiles = d_tiles.assign(pre_post = np.where(d_tiles.datetime < d_tiles.t_event, 'pre', 'post'))
d_tiles.insert(1, 'pre_post', d_tiles.pop('pre_post'))
# sort by latest event date 
d_tiles = (d_tiles
 .assign(latest = d_tiles.groupby('event')['t_event'].transform('max'))
 .sort_values(['latest','datetime'], ascending=False)
 .drop('latest', axis=1)

end = time.time()

print(str(round(end-start, 2))+' seconds')
0.29 seconds
Pre & Post Event MosaicJSON Creation
import os
from cogeo_mosaic.mosaic import MosaicJSON
from cogeo_mosaic.backends import MosaicBackend

for e in np.unique(d_tiles.event):
    start = time.time()
    d = d_tiles[d_tiles.event == e].reset_index(drop=True)
    # write output
    os.makedirs('data/'+e, exist_ok=True)
    for i in ['pre', 'post']:
        images = d[d.pre_post == i].reset_index(drop=True).visual.tolist()
        if len(images) > 0:
            mosaic = MosaicJSON.from_urls(images)
            with MosaicBackend('data/'+e+'/mosaic-tiles-'+i+'.json', mosaic_def=mosaic) as f:
    end = time.time()
    print(e+' Pre/Post MosaicJSON Time: '+str(round(end-start, 2))+'s')
India-Floods-Oct-2023 Pre/Post MosaicJSON Time: 66.61s
Image Footprint Metadata Aggregation
cols_tile = [
    'visual', 'tile:data_area', 'tile:clouds_area', 'tile:clouds_percent',
    'view:off_nadir', 'view:azimuth', 'view:incidence_angle', 'view:sun_azimuth', 'view:sun_elevation',
    'gsd', 'proj:bbox', 'quadkey', 'utm_zone', 'grid:code'
# create bounding box from image footprint instead of these
cols_drop = ['proj:bbox', 'quadkey', 'grid:code']

cols_tile_unique = ['utm_zone', 'tile:clouds_area']
cols_tile_mean = ['gsd', 'tile:clouds_percent', 'view:off_nadir', 'view:azimuth', 'view:incidence_angle', 'view:sun_azimuth', 'view:sun_elevation']
cols_tile_footprint = ['event', 'pre_post', 'catalog_id']

d_agg = (d_tiles[cols_tile_footprint+cols_tile_unique]
     'utm_zone': lambda x: np.unique(x).tolist(),
     'tile:clouds_area': lambda x: round(sum(x), 2),
d_agg_mean = (d_tiles[cols_tile_footprint+cols_tile_mean]
 .agg(lambda x: round(np.mean(x), 2))

d_foot = (d_tiles[d_tiles.columns.difference(cols_drop+cols_tile_unique+cols_tile_mean).to_list()]
 .dissolve(cols_tile_footprint, aggfunc = 'first')
 # same index - can left join aggregates on index by default
 # add index back to dataframe
# split because `d` carries from original when chaining
d_foot = (d_foot
     # uses equal area projection - converts km^2 original units 
     footprint_data_area = lambda d: round(d.geometry.to_crs(epsg=6933).area/1e6, 2),
     bbox = d_foot.geometry.bounds.applymap(lambda x: round(np.mean(x), 2)).values.tolist()
 .sort_values(['t_event', 'datetime'], ascending=False)
     'tile:clouds_area': 'footprint:clouds_area', 'tile:clouds_percent': 'footprint:clouds_percent',
     'footprint_data_area': 'footprint:data_area'


We see that there are currently 5 images acquisitions (image footprints) released from before the flooding and 1 image acquisition taken after the flooding.

d = pd.DataFrame(
  .groupby(['event', 'pre_post'])
d = (d[d.event == event]
event pre_post count
India-Floods-Oct-2023 post 1
India-Floods-Oct-2023 pre 5

Web Map

Split Web Map of North India Floods
import re
import leafmap
import leafmap.foliumap
from folium import Element
from leafmap2 import add_geojson2, add_gdf2
leafmap.foliumap.Map.add_geojson2 = add_geojson2
leafmap.foliumap.Map.add_gdf2 = add_gdf2

# 'p' for path | 'm' for mosaic | 'f' for footprint
event = 'India-Floods-Oct-2023'
pm_top = ''
pm_end_pre = '/mosaic-tiles-pre.json'
pm_end_post = '/mosaic-tiles-post.json'

cols_drop = ['source', 'proj:epsg', 't_event']
d = d_foot[d_foot.event == event]
d.drop(cols_drop, axis=1, inplace=True)

m = leafmap.foliumap.Map()
def style_footprints(color, fillOpacity, weight):
    style = {
        'color': color, 'fillOpacity': fillOpacity, 'weight': weight
    return style
basemap = {  
  "url": "{x}&y={y}&z={z}",
  "attribution": "Google",
  "name": "Google Satellite",
m.add_tile_layer(**basemap, shown=True)
m.add_stac_layer(pm_top+event+pm_end_pre, name="Pre-Event - Mosaic", attribution="Maxar Technologies", shown=True)
m.add_stac_layer(pm_top+event+pm_end_post, name="Post-Event - Mosaic", attribution="Maxar Technologies")
    d[d.pre_post == 'pre'], "Pre Event - Image Footprints",
    style=style_footprints('blue', 0, 1.5), info_mode='both',
    tooltip_fields_remove=['visual'], kwargs_tooltip={'sticky': False}
    d[d.pre_post == 'post'], "Post Event - Image Footprints",
    style=style_footprints('red', 0, 1.5), info_mode='both',
    tooltip_fields_remove=['visual'], kwargs_tooltip={'sticky': False}
with open('./popup-fix.js') as f: js_po =
with open('./side-by-side.js') as f: js_sbs =
with open('./split-map-layout.css') as f: css_sbs_l =
with open('./split-map-range.css') as f: css_sbs_r =
#with open('./split-map.js') as f: js_sm =
# must render 1st to add script after - otherwise adds JS before Leaflet L.Map assignment
# add map & geojson layer variables to script
var_map = m.get_name()
var_geojson_lgs = str([i for i in m._children.keys() if'^geo_json_', i) is not None]).replace("'", "")
tile_layers = [i for i in m._children.keys() if'^tile_layer_', i) is not None]
# need to know the order of your tiles for adding as split map
# an area of multiple layers can be passed to left & right arguments of L.control.sideBySide
# however, it's pretty buggy when passing more than one layer
i_all = [0,1,2] # a basemap, pre, & post event
i_left = [2]
i_right = [0]
l_lleft = str([j for i, j in enumerate(tile_layers) if i in i_left]).replace("'", "")
l_lright = str([j for i, j in enumerate(tile_layers) if i in i_right]).replace("'", "")
m.get_root().script.add_child(Element(js_po.format(m=var_map, lg_geo=var_geojson_lgs)))
m.get_root().script.add_child(Element('L.control.sideBySide({l_lleft}, {l_lright}).addTo({m});'.format(l_lleft=l_lleft, l_lright=l_lright, m=var_map)))
Figure 3: A split web map of Pre/Post Event Imagery after the Flooding in Northern India.
Sources: opengeos/maxar-open-data Github & Maxar Open Data Program STAC


If you have any thoughts or questions, feel free to let me know what you think in the comments below. You’ll need to sign in to your GitHub account to do so. Like my work? Feel free to reach out.

We only have one rock, and it’s a beautiful one. Thanks for reading! ✌️🌍

Figure 4: 2023 South Lhonak Lake Sikkim, India - Maxar Open Data Program Leaflet Split Map
Back to top


© 2024 Vincent Clemson | This post is licensed under <a href='' target='_blank'>CC BY-NC-SA 4.0</a>


For attribution, please cite this work as:
Clemson, Vincent. 2023. “North India Floods of October 2023.” October 12, 2023.