Rapid assessment of geo-hydrological hazards in Antananarivo (Madagascar) historical centre for damage prevention

Figures & data

Figure 1. Twin cyclones Bansi and Chedza hitting Madagascar East coast on 16 January 2015 (a) (modified after European meteorological satellite EUMESAT image; available at: www.eumetsat.int); aerial pictures of the triggered shallow landslides within the city on March 5th event (b, c) (BNGRC - Bureau National de Gestion des Risques et des Catastrophes; http://www.bngrc.mg).

Figure 2. Location of Antananarivo on Madagascar Google Earth image (a); detail of the Antananarivo area on optical satellite image (b) (the red polygon represents the Unesco buffer zone). Source: Author

Figure 3. The Antananarivo Upper Town Cultural Heritage buildings: Rova palace complex lithographs and historical pictures (a–c; Courtesy of IMV-Institut des Métiers de la Ville-Tana City Lab; https://www.imvtana.org); current picture of the rebuilt structure following the 1995 fire; Rova palace (d), royal chapel (e). Andafiavaratra palace (f); Andohalo Cathedral (g). Source: Author

Figure 4. Spaceborne ortophoto (Pleiades satellite) of the town sectors showing the Upeer Town quarters (a) (in red the boundary of the Unesco Buffer zone). Panoramic views of the Upper-Middle town and Analamanga hill eastern (b) and western slopes (c). View of the Lower Town area (d). Source: IMV-Institut des Metiers de la Ville-Tana City Lab by its manager Tamara Teissedre-Philip ([email protected]).

Figure 5. Workflow of the applied methodology.

Figure 6. Analamanga hill DEM altitude map (the black boundary represents the UNESCO buffer zone), including with E-W (1–5) and N-S (6) profiles. Source: Author

Figure 7. Analamanga hill DEM slope map (a), and the related geology (arrows show the outcrops): (b) granite western slope rock wall; (c) laterite soils (d: detail); (e) slope eluvial-colluvial deposits (f: detail); (g) alluvial deposits. Source: Author

Figure 8. Geological-geomorphological map of Analamanga hill. Source: Author

Figure 9. Hydrographic network (in blue) and hydrographic basins (in red) of the Analamanga hill. Source: Author

Table 1. Characteristics of the mapped creeks.

Basin n°	Area (m^2)	Perimeter (m)	Mean slope (°)	Order	Geology	Running water	Notes	Accessible
1	139125	1857,18	16,56	1	Granite/laterite/eluvial-colluvial deposits	yes	Sheet erosion/scarps/ spring (buried Cathedral)	yes
2	47866,9	964,09	23,87	2	Granite/laterite/eluvial-colluvial deposits	yes	Sheet erosion/scarps	yes
3	165786	1703,47	19,57	1	Granite/laterite/eluvial-colluvial deposits	yes	Small pond (buried in parking lot)	yes
4	64636,5	1434,28	20,07	1	Granite/laterite/eluvial-colluvial deposits	no	Gully erosion/scarps	yes
5	101241	1438,05	24,08	2	Granite/laterite/eluvial-colluvial deposits	no	Scarps	No
6	99482,6	1482,91	23,91	2	Granite/laterite/eluvial-colluvial deposits	no	Gully erosion/scarps	yes
7	53325,5	1243,46	21,37	2	Granite/laterite/eluvial-colluvial deposits	no	Gully erosion/scarps/springs	yes
8	98294	1326,33	22,05	2	Granite/laterite/eluvial-colluvial deposits	no	Sheet erosion/scarps	yes
9	22258,6	647,52	14,64	1	Granite/laterite/eluvial-colluvial deposits	no	Sheet erosion	yes
10	10520,9	475,80	14,37	1	Granite/laterite/eluvial-colluvial deposits	no	–	yes
11	24774,6	833,48	14,96	1	Granite/laterite/eluvial-colluvial deposits	no	Sheet erosion	yes
12	51645	1360,18	14,96	1	Granite/laterite/eluvial-colluvial deposits	no	Scarps	No
13	48935,8	956,02	18,27	1	Granite/laterite/eluvial-colluvial deposits	no	Scarps/granite quarries	yes
14	65393,1	1061,97	20,64	2	Granite/eluvial-colluvial deposits	no	Scarps/granite quarries	yes
15	10482,5	522,48	14,15	1	Granite/eluvial-colluvial deposits	no	–	yes
16	47066,4	954,37	22,34	1	Granite/eluvial-colluvial deposits	no	Scarps/granite quarries	yes
17	95740,8	1536,01	18,61	2	Granite/eluvial-colluvial deposits	yes	Sheet erosion/scarps/3 springs	yes
18	27212,3	741,14	20,17	1	Granite/eluvial-colluvial deposits	no	Sheet erosion/scarps	yes
19	18295,5	740,29	12,20	1	Granite/eluvial-colluvial deposits	no	–	yes
20	40918,1	929,09	14,50	1	Granite/eluvial-colluvial deposits	no	Sheet erosion	yes
21	185145	1898,63	20,67	3	Granite/eluvial-colluvial deposits	yes	Sheet erosion/Cress field	yes
22	26741,2	711,35	22,68	1	Granite/eluvial-colluvial deposits	no	Scarps/sheet erosion	No
23	37678,4	1036,99	21,51	1	Granite/eluvial-colluvial deposits	no	Sheet erosion	No
24	54191,6	1152,39	15,92	1	Granite/eluvial-colluvial deposits	no	Sheet erosion	yes
25	130970	1502,19	25,41	3	Granite/eluvial-colluvial deposits	yes	Sheet erosion/spring	yes
26	42463,4	1120,8	21,63	1	Eluvial-colluvial deposits	no	Sheet erosion	yes
27	41279,7	1004,04	19,65	1	Eluvial-colluvial deposits	no	–	yes
28	62282,7	989,65	17,95	1	Eluvial-colluvial deposits	no	–	No
29	20529,7	699,64	17,97	1	Eluvial-colluvial deposits	no	–	yes
30	17136,2	659,97	17,65	1	Eluvial-colluvial deposits	no	–	yes

Table 2. Stream orders data.

Stream order	No. of streams (DEM)	No. of streams (LiDAR)
1	2161	7048
2	1033	3453
3	523	1764
4	204	886
5	–	28
6	–	7

Figure 10. Analamanga hill hydrographic map by using the 2 m resolution DEM (on the left) and the LiDAR (on the right). Source: Author

Figure 11. Maps of Basins 3–4 and 25. Green lines mark the roadways and paths. Source: Author

Figure 12. Geomorphological-erosional problems in basins 3–4. Hanging-like valley (a); damaged buildings (red square 1), retained roadway slope (red square 2). Creek 3 hydraulic narrowing (b). Creek 4 lavaka-like gully erosion (c) and hovels built within the creek bed (d). Basin 25 urbanization (e). Source: Author

Figure 13. Historical aerial (a–c) and terrestrial (d) pictures of the Rova complex (first half of the twentieth century) and the surrounding Analamanga hill slopes. The comparison with the current setting of the Analamanga hill (Figure 4) shows the evidence of the progressive deforestation and increase of urbanization of the Upper and Middle town. Source: Author